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AMERICAN INSTITUTE OF MINING AND METALLURGICAL ENGINEERS Technical Publication No. 1979

(CLASS E. INSTITUTE OF METALS DIVISION. NO. 497) DISCUSSION OF THIS PAPER IS INVITED. Discussion in writing (z co ies) may be sent to the Secre-

tary. American Institute of Mining and Metallurgical Engineers, a9 West jgtK Street. Sew.York 18. N. Y. Unless special arrangement is made. discussion of this paper will close May I . 1946. Any d~scuss~on offered thereafter should preferably be in the form of a new paper.

Electron Metallographic Methods and Some Results for Magnesium Alloys

(Chicago Meeting. February 1946)

TIIE electron microscope techniques and their application to magnesium alloys that are to be discussed in this paper are the result of research a t The Dow Chemical Co. over the past three years. The viewpoint underlying the work is not wholly metallurgical, but has evolved through a variety of problems related to the physics and chemistry of surfaces. I t can be said that the interest and stimulus were closely associated with a desire to employ effec- tively the electron microscope in research concerning the properties of solids as related to their physical structure. Only a portion of this work will be described here, and, consequently, many points of interest and debate are of necessity either only briefly recognized or are completely omitted.

As has been pointed out by Desch,' a great deal of the knowledge of the structure of metals has been obtained through microscopic studies of the surfaces of metallic specimens prepared in a particular manner. This information, when added to that obtained by means of thermal, elec- trical, X-ray, dilatometric and other methods, has been of great value both from a theoretical and a practical viewpoint. With the introduction of new instru-

ments such as the electron diffraction camera and the electron microscope, it would be expected that still more information could be obtained. These two instru- ments have in many instances been very effective when directed toward the investi- gation of metallic surfaces, provided the proper methods of surface preparation were employed. Experience shows that the problems of surface preparation greatly limit the general application of these instru- ments to metallographic studies.

I t was found necessary to discard the standard metallographic procedures in applying the electron microscope to magnesium alloys and to work out new methods, which were required to satisfy certain conditions. These methods are unique to magnesium only in the specific chemical reactions occurring during etching.

The presentation of the methods and results will be ordered as follows:

I. Electron Microscopy of Surfaces. z . Surface Preparation of Magnesium

Alloys. 3. Discussion of Microstructures.

a. Pure Magnesium and Single-crystal Studies.

b. Types of Precipitation. G. Some Common Structures. d . Special Micrographs and Tech-

niques. Manuscript received a t t h e office of the

Institute Nov . 13, 1945. e. Structure of Mg-A1 Solid Solutions. The D o w Chemical Co. . Midland. 4. Correlation of "Fine Structure" with

Michigan. References are a t the end of t h e paper. Corrosion Behavior of Magnesium Alloys.

Copyright. 1946. by the American Institute of Mining and Metallurgical Engineers. Inc. METALS TECHNOLOGY, April 1946. Printed in U. S. A.

ELECTRON METALLOGRAPHIC METHODS

ELECTRON MICROSCOPY OF SURFACES such replica films, only the polystyrene-

I t seems unnecessary to present more silica replica will be considered for this than a very brief recapitulation of the work. Fig. I illustrates the principle of features of the present-day electron micro- such a replica, while the actual stepsZ

( Polystyrene I 1 Polyetyrene 1 -200A

-'-J9 Evaporates -& Silica

Silico Film Replica

scope and the problem of applying i t to solid surfaces. Numerous articles and papers deal with the design, construction and principles of this instrument.* Its specific application to surface-contour investigations has been described2-& in detail.

The use of an electron beam as illuminating radiation imposes severe restrictions on the type of specimen that may be examined. Among the restrictions are:

I. The most satisfactory images are obtained with transmission specimens. The penetrating power of a stream of electrons is small, and 60-kv. electrons do notpro- duce images of high quality after penetrating more than about 3oohj of matter.

2. I t is necessary that the specimen be in an evacuated space (about 10-4 mm. Hg), since the presence of air or other gases causes ionization.

Point I is the major consideration in metallographic applications. If the surfaces of matter in bulk are to be examined with a transmission-type electron microscope, i t is necessary that not more than about joohj. be penetrated in forming the image. The present solution to this difficulty is the use of a surface replica, or thin film of material that has impressed into it all of the contours of the original surface. Of the several suggested methods2.' of producing

For discussion and references see A . F . Prebus: Alexander's Colloid Chemistry. 5 , 152. Reinhold Pub. Co . New York (1945).

required to produce the final film are enumerated below:

I. Onto the prepared surface is pressure- molded a thermoplastic, polystyrene, which produces a first impression of the surface contours.

2. The polystyrene molding is separated from the original surface mechanically or, if necessary, by dissolution of the sample in an appropriate reagent.

3. Silica is vaporized onto the surface of the polystyrene molding to produce the final replica film.

4. The silica film, still on the molding, is scribed .into 36-in. squares and the whole is then placed in a dish containing a solvent consisting of ethyl bromide with 10 per cent benzene. The silica squares are released from the polystyrene within a few minutes and are thereupon transferred to fresh solvent for rinsing. They are then picked up from the rinse solution on %-in. disks of zoo-mesh stainless steel, which serve as specimen supports for the electron microscope. The portions of the a m stretched over the holes in the screen are imaged by the electron microscope and photographed directly. The intensity variations in the final image result from thickness variations in the replica film, which in turn depend on the contours of the original surface as illustrated in Fig. I.

Although such a process may appear complicated and clumsy, a replica may be

R. D. HEIDENREICH, C. H. GEROULD AND R. E. McNULTY

turned out in less than 45 min. when the equipment and technique have been organ- ized. In general. more time is spent examining the fields in the microscope than in

1 making the replica. Many hundreds of surfaces have been examined in these laboratories by this method.

The faithfulness of reproduction of surface contours has been inve~t iga ted ,~ with the following results:

I. The electron images of the silica replica are very similar in general appearance to ordinary micrographs of the original surface (see ref. 3).

2. Structural details or contour changes of less than IOOA (ref. 2) in size can be detected using the silica replica. Although the resolution of the magnetic electron microscope is estimated to be about one third of this figure, the resolution obtain- able with the replica is still some 30 times better than that of the light metallographic microscope. The depth of focus of the electron microscope is of the order of 10 times that of the light microscope, which means that quite large elevation changes can be accommodated.

All of the electron micrographs of this , paper were made as just described, using

an RCA electron microscope, type B. Electron micrographs of the surface con-

tours of a specimen are only a part of the information desired. Such images give no more information than can be deduced from size and shape. With properly prepared surfaces, however, further information as to the crystallographic features of the surface can be obtained by means of electron diffraction.6s7 By the so-called reflection method, the identity of the surface projections normal to the incident beam, and sufficiently thin to allow transmission, can often be ascertained; that is, the slow-etching phases in a metal specimen will produce the greater portion of the diffraction pattern. This matter is discussed in ref. 7 and several applications will be found there.

SURFACE PREPARATION OF

MAGNESIUM ALLOYS

The method used at present for pre- paring magnesium specimens for both electron microscope examination and electron diffraction identification of phases has been found to be uniquely successful. No effort will be made to trace the extensive development, which was initiated when standard etchants for light microscope work were found quite inadequate (see p. 16) for electron microscopy, Rather, the conditions and method are first outlined and then compared with standard metallographic results.

The requirements for a metallographic surface preparation suitable for electron microscope and electron diffraction work are considered to be the following:

I. Sufficient metal must be removed from the ground surface by chemical methods to ensure the absence of any dis- torted or fragmented metal in the regions to be studied.

2. The etching reagent must be selective for the structures in question; i.e., the rates of dissolution of different phases or com- ponents must differ sufficiently to produce the required surface contours. The order of attack of the various phases present should be known. 3. The etching reagent should not

produce surface tilms or stains that cannot be removed by subsequent rinsing.

4. The chemical behavior during etching must be controlled so that no component of the alloy is dissolved and then rede- posited on the etched surface (see ref. 7).

5. The rinsing procedure employed in conjunction with the etchant must remove any reaction products deposited and produce a surface that is free of oxide or hydroxide films and grease. The criterion for cleanliness is simply that electron diffraction reflection from the surface shall yield a pattern for the metallic constituents.

This set of requirements is not restricted to magnesium alloys, but applies to any

4 ELECTRON METALLOGRAPHIC METHODS

alloy system. The first requirement for most alloys is usually met by electrolytic polishing methods. Requirements 2, 3 and 4 are specific to the alloy being investigated. Requirement 5 has been satisfied for several alloy systems by a series of rinses described in ref. 7. These rinses were first employed for magnesium surfaces.

For nearly all magnesium alloys, requirements I, 2, 3 and 4 are satisfied by a single chemical reagent, which removes sufficient metal in a short time to eliminate the damage done by grinding and also produces a selective etch attack. This reagent is an anhydrous, organic etchant consisting of a mixture of one part methyl iodide and one part dry methyl alcohol to which is added a crystal or two of iodine to initiate the reaction.

The actual procedure employing the methyl iodide reagent and the subsequent rinses is as follows:

I. The specimen is abraded on No. I

metallographic paper under dry benzene. 2. I t is immediately transferred to the

etchant, which is generally used in 25-C.C. amounts for samples roughly M in. square. A small glass vessel about 5 cm. in diameter and 10 cm. deep is convenient. The mixture usually is heated slightly in warm water to aid in starting the reaction. The etchant is mixed as used.

3. After the reaction has continued vig- orously for 2 to 3 min., the specimen is rapidly transferred to the first of a series of three rinse solutions. The purpose of this sequence of rinses is to displace the etching solution with an inert liquid without de- position of foreign materials. The samples are agitated for about 5 sec. in each and transferred rapidly to the next.

Rinse I consists of I part methanol: I part acetone + 0.5 per cent formic acid.

Rinse 2 is the same as rinse I but without formic acid.

The 25 C.C. of etchant and 50-C.C. batches of each rinse may be employed for several samples before it is necessary to mix a new set.

The chemistry involved in the etching f i reaction has not been investigated. Meth- oxy magnesium iodide is a possible prod- uct, although probably it is unstable. The . reaction is very vigorous when once started and may heat the etchant sufficiently to boil away. the methyl iodide. Fortunately, the rate is quite sensitive to temperature and hence can be easily controlled by heat- ing or cooling in a stream of water.

Caution must be exercised in using the etchant. The hands should be kept away from the top of the vessel during etching, .

by the use of long-handled Monel-metal tweezers with which to hold the sample. The specimen must be held under the surface of the etchant, because of the copious gas evolution. Both the reagent and the fumes given off will produce blisters on the hands, followed by drying andpainful cracking. Gloves of any kind are not

I ' recommended. The tweezers and frequent washing of the hands are the safest means for preventing burns.

Numerous organic halides have been - tried, but methyl iodide is generally the most satisfactory. Alcohols higher than methyl reduce the rate of attack.* In general, the rate decreases as the amount . of hydrocarbon, either as alcohol or halide, increases. The rate of attack also decreases in the order iodide, bromide, and chloride.

The initiation of the etching reaction deserves some discussion, since some early difficulties were experienced here. I t was found that for pure magnesium the reaction could be converted from an isolated pit attack to a general surface reaction only when the specimen was prepared in a certain way. The presence of films of hydroxide or oxide on the surface is sufficient to allow

Rinse 3 is clean, dry benzene. Ethanol is recommended for electron

4. Dry in a blast of clean, warm air. diffraction studies on magnesium-zinc alloys.

R. D. HEIDENREICH, C. H. GEROULD AND R. E. NcNULTY 5

only a few isolated spots to undergo reac- methyl iodide reagent than does the bulk tion with the etchant. metal.

Immediately aftergrinding, a magnesium As mentioned, a crystal or so of iodine specimen abraded in air will yield an elec- is employed in the reagent. I t does not

TIME IN MINUTES FIG. 2.-LOSS IN WEIGHT OF PURE MAGNESIUM IN METEYL IODIDE ETCHANT AT 7.5' TO 85'F

tron diffraction pattern for magnesium seem to affect the general nature of the oxide. Contact with liquid water will re- etch attack, but does greatly aid in sensi- sult in the formation of magnesium hydrox- tizing the reaction. The sensitizing by ide. Grinding under a medium such as dry, iodine is demonstrated for pure magnesium distilled benzene greatly reduces the by the curves of Fig. 2, where weight loss tendency to form oxides or hydroxides. per unit apparent area is plotted against

The state of the surface, although free of time. With no iodine present, the surface is films, greatly influences the starting of the badly pitted after one minute in the reagent reaction. A pure magnesium surface formed and the attack does not become general by actual polishing on broadcloth under until 3 to 4 min. have elapsed. Withiodine benzene will not be uniformly attacked, present, however, the reaction fust starts but again will be pitted severely. This is a t a few scattered spots on the surface and attributed to the formation of a polish then rapidly spreads until the entire sur- layer (ref. 6, chap. 13) which has been face takes part. shown to possess considerably different With magnesium-aluminum solid soh- physical and chemical properties from the tions, it was found to be nearly impossible bulk material. This layer on magnesium to start the reaction without iodine appears much less reactive with the present. The attack of the various com-


ponents in a magnesium-aluminum alloy are, in the order of decreasing rate: (I) pure magnesium, (2) Mg-A1 solid solution, (3) intermetallic compound Mg17A112. The solid solutions and intermetallic compounds are generally more slowly attacked, except the magnesium-cadmium system, in which the rate is in excess of that for pure magnesium. None of the metals, zinc, aluminum, cadmium, beryllium, are themselves reactive, although their solid solutions in magnesium are attacked.

Etchants other than methyl iodide can be employed and clean surfaces obtained if the rinsing schedule is maintained. The type of attack is not so desirable, however, and the methyl iodide reagent is far more sensitive to structural heterogeneities, as will be seen in the following pages.

The chief disadvantage of the methyl iodide reagent is the macroroughness of the finished surfaces. They are generally quite rolling or wavy, as a result of the gas evolution during etching, and consequently are not satisfactory for light microscopy. The depth of the etch is generally greater than that recommended for high-magnification light microscopy. For electron microscopy and electron diffraction, however, such surfaces are quite suitable. This, again, is a matter of the focal depth of the two types of microscopes.

To illustrate more clearly the character of the etchant used, two light micrographs, two electron micrographs and two electron diffraction patterns are given comparing the conventional acetic glycol etch with the methyl iodide etch. Fig. ga is a light micrograph of a Mg + 8 per cent A1 alloy in the heat-treated and aged condition. The etchant is the acetic glycol used generally on magnes i~m.~ A very fine pearlitic type of precipitation is evident, with apparently some background within the grains, the details of which are not resolvable. There is a line structure within some of the grains, as though possibly the result of grain refine- ment, but the true significance of these

lines is not evident. Fig. gc is an electron micrograph of the same specimen with the same etch as in Fig. 3a. The pearlitic type of precipitation is seen to be coarse platelets. There is a structure within the grain consisting of a very fine distribution of platelets with a line of coarser platelets through the center of the grain. This is evidently one of the intragranular lines seen in Fig. ga.

Fig. 3b is a light micrograph of the same specimen, prepared with the methyl iodide etchant. Note the extreme waviness of the surface and the inability of the light microscope to maintain focus throughout the field. Fig. gd is of the same surface as Fig. gb viewed with the electron microscope. The greater depth of focus of the latter instrument is clearly apparent. The same general features are present in both Figs. gc and gd, but a greater sharpness and clarity of outline is seen in Fig. gd. More fine detail is also present. Fig. ge is an electron diffraction pattern from the surface of Fig. 3a showing a pattern for Mg(OH)2, while Fig. 3f is from the surface of Fig. gb, showing the pattern of Mg17All2. Thus, with the conventional metallographic techniques, a surface film is formed, which obscures the details. An etchant such as the methyl iodide, by contrast, leaves no film and the true metallic details can be seen in sharp outline.

No one field of magnesium metallurgy will be covered in detail, but rather a short survey will be given, illustrative of the accomplishments of the electron microscope as used in these laboratories.

Pure Magnesium and Single-crystal Studies

The high resolution of the electron microscope can be used to good advantage in studies of small amounts of impurities in pure metal and in research in crystal mechanics. Such things as slip-line spacing,

R. D. HEIDENR.EICR, C. H. GEROULD AND R. E. McNULTY 7

FIG. 3.-ALLOY O F MAGNESnJM PLUS 8 P E R CENT ALUMINUM I N T H E HEAT-TREATED AND AGED CONDITION.

Acetic Glycol Etchant Methyl Iodide Etchant a. Light micrograph. Original magnification b. Light micrograph. Original magnification

250. 2 5 0 . G. Electron micrograph. Original magnification d. Electron micrograph. Original magnification

5000. 5000 . e . Electron diffraction reflection pattern for j. Electron diffraction reflection pattern for

a magnesium hydroxide. Mg1iA112. Reduced approximately one third in reproduction.


twin formation, crystal habit, etch-pit The appearance of slip is illustrated in characteristics can be examined in detail. Fig. 6. The micrograph shows a pure mag- In this way information beyond that so far nesium specimen, as cast, etched, then possible by other techniques may be squeezed in a vice, with no furthertreat- gained. ment of the surface. True slip lines are

... . i

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FIG. 4.-HIGH-PURITY A N D COMMERCIAL CELL MAGNESIUM. a. Distilled magnesium crystal. Original magnification 6500. b. Commercial cell magnesium with inclusions indicating impurities (0.02 per cent Fe, 0.05

Mn). Original magnification 5500. Reduced approximately one-third in reproduction.

Fig. qa illustrates a crystal of distilled visible in only one of the three crystallites. magnesium with only negligibleimpurities . The wavy lines in one of the grains are due detectable by spectroscopic analysis. The not to slip, but to the etch attack. black spot in the center is a dust particle, A striking picture of twins in a single useful for focusing. Even in this pure mag- crystal of a M g + 1.5 per cent M n alloy nesium, however, there is some faint is shown in Fig. 7. An interestingfeature structure in the center. Fig. qb is a n electron is a discontinuous change of direction on micrograph of normal commercial cell one side of a twin band, but a n apparently magnesium, containing 0.002 per cent Al, continuous change on the other side. 0.001 Cu, 0.02 Fe, 0.05 Mn, 0.003 Si and Fig. 8 is another electron micrograph of a 0.001 Cd, Ni, Sn and Zn. The effect of single crystal of Mg + 1.5 per cent Mn these normally contained impurities on the alloy, this time showing precipitation. microstructure is apparent. X-ray diffraction indicated that the nor-

I n Fig. 5 is shown the unetched surface mal to the surface shown is 8" from the pole of a single crystal of magnesium grown by. of the basal plane. Thus, precipitation is condensation of the vapor. Great care was occurring on the basal plane, along the taken to avoid working the crystal, never- three principal directions. theless i t is possible that the prominent Fig. 9 is of a Mg + 6 per cent Al alloy lines are slip lines. On the other hand, the in the heat-treated and quenched condi- lines could be evidence of growth charac- tion. For some unknown reason a crystallo- teristics of magnesium, possibly traces of graphic etch pattern was produced. This the basal plane. pattern is not obtained if any precipitation

R. D. HEIDENREICH, C. H. GEROULD AND R. E. McNULTY 9

has occurred. An interesting feature of this fromsolid solution. I t may be expected that picture is the manner in which the two initial stages in the precipitation process grains merge into one another. can be observed, that directional effects

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FIG. 5.-UNETCUED SURFACE OF MAGNESIUM CRYSTAL CONDENSED FROM THE VAPOR STATF Lines may be due to either slip or growth characteristics in the crystal. Original magnification

6500; reduced approximately one third in reproduction. FIG. 6.-SLIP LINES I N PURE MAGNESIUM, AS CAST.

Specimen deformed in compression after etching with true slip lines appearing in the lower crystal only. Original magnification 6000; reduced approximately one-third in reproduction. FIG. 7.-TWINS IN A SLNGLE CRYSTAL OF AN ALLOY OF MAGNESIUM PLUS 1.5 PER CENT MANGANESE.

Original magnification 6000; reduced approximately one-third in reproduction. FIG. 8.-PRECIPITATION I N A SINGLE CRYSTAL O F AN ALLOY OF MAGNESIUM PLUS 1.5 PER CENT

MANGANESE. The precipitation occurs as plate-shaped particles on the basal plane along the three principal

directions. Original magnification 7000; reduced approximately one third in reproduction.

Types of Precipitation may be clearly defined and that structure

A second basic field for careful study by within ,. the precipitated particles them-

electron microscope technique is that of selves may . . be found.

precipitation of second-phase material One example of striking orientation of

I 0 ELECTRON METALLOGRAPHIC METHODS

precipitation in a Mg + 1.5 per cent Mn + 1 .5 per cent Mn alloy, as cast. In the alloy was given in Fig. 8. I n general, three light microscope, they appear as random distinct types of precipitation are found in dots, not resolvable particles. Note the

FIG. 9.-CRYSTALLOGRAPHIC ETCH ATTACK I N A SOLID SOLUTION O F AIAGNESIUJI PLUS 6 PER CENT ALUMINUM.

Original magnification 6500; reduced approximately one third in reproduction. FIG. 10.-GLOBULAR PARTICLE PRECIPITATION IN A N ALLOY OF MAGNESIUM PLUS 1.5 PER CENT

MANGANESE A S CAST. T h e presence of such particles a s these is evident in the light microscope but they cannot

be individually resolved. Original magnification 5500; reduced approximately one third in reproduction. FIG. I I.-COARSE LAMELLAR PRECIPITATION IN AN AGED ALLOY OF MAGNESIUM PLUS 10 PER CENT

ALUiUINuM. Original magnification 6000; reduced approximately one third in reproduction.

FIG. 12.-FINE LAMELLAR PRECIPITATION IN AN AGED ALLOY OF MAGNESIUM PLUS 7 PER CENT ALUMINUM AND 3 PER CENT ZINC.

Original magnification 6500; reduced approximately one third in reproduction.

magnesium alloys. The first of these (Fig. structure within the particles. Several 10) is a series of globules appearing a t stages of coagulation of two or more par - random throughout the grain in a Mg ticles are also present.

R. D. HEIDENEEICH, C. .H. GEROULU AND E. E. M c N U L T Y I I

FIG. 1 3 . - W I D ~ ~ A N S T A T T E ~ ' T Y P E PRECIPITATION I N AN AGED ALLOY 01; MAGNESIUM P L U S 3 P E R C E N T ALUALINUM AND 7 P E R C E N T ZINC.

The directional platelike particles change direction a t a grain boundary. Original magnification 6500; reduced approximately one third in reproduction. FIG. 14.-HEAT-TREATED, QUENCHED AND AGED ALLOY O F bfAGNESIUM P L U S Q P E R C E N T ALUMINUM,

2 P E R C E N T ZINC A N D 0 . 2 MANGANESE. A massive piece of undissolved Mg1,Alls with surrounding Widmanstatten type precipitation

and beds of lamellar precipitation. Original magnification 4500; reduced approximately one third in reproduction. FIG. 15.-CROSS-ROLLED ALLOY OF MAGNESIUM PLUS 6 PER CENT ALUMINUM, I PER CENT ZINC AND

0 . 2 P E R C E N T MANGANESE, AFTER AGING, SHOWING T W I N N I N G AND VERY F I N E PRECIPITATION. Small black globular particles widely scattered over the surface are presumably actual particles

of MgllAll2 transferred from the metal specimen to the silica replica. Figs. 2 0 and 21 are further examples of this phenomenon. Original magnification 6500; reduced approximately one third in reproduction. FIG. 16.-GLOBULAR PARTICLE PRECIPITATION I N A ROLLED ALLOY O F MAGNESIUM P L U S I .5 P E R

C E N T MANGANESE P L U S 0.3 P E R C E N T CERIUM. Scattered coarse particles are resolvable with the light microscope but the fine particles are com-

pletely invisible. Original magnification 7000; reduced approximately one third in reproduction.


FIG. 17.-EUTECTIC STRUCTURE I N AN ALLOY OF hlAGNESIUY PLUS 3 2 . 2 P E R CENT ALLMINUM AS CAST.

Rodlike Mg solid solution, characteristic of this eutectic composition, has been sectioned transversely in the region shown. The circular structure thus consists of the pits from which the Mg phase has been removed by the etchant. The surrounding matrix or ridges consist of MgI7AlL2. Original magnification 4500; reduced approximately one third in reproduction. FIG. 18.-EXTRUSION OF FINE-GRAINED ALLOY O F MAGNESIUM PLUS 1.5 P E R CENT MANGANESE

WITH F I N E PARTICLE PRECIPITATION. Original magnification 6500; reduced approximately one third in reproduction.

FIG. 19.-HEAT-TREATED AND AGED FORGING OF AN ALLOY OF MAGNESIUM PLUS 8.5 P E R CENT ALUMINUM, 0.5 PER CENT ZINC AND 0 .2 PER CENT MANGANESE.

A few scattered spherical particles of massive compound may be seen with fine globular precipitation and lamellar precipitation. Original magnification 6500; reduced approximately one third in reproduction. FIG. 20.-PURE MAGNESIUM AS CAST, WITH GRAPHITE PLATELETS ON SILICA REPLICA.

Mg containing carbon in which some of the graphite inclusions were transferred from the etched surface of the metal to the silica replica in the form of very thin platelets. Original magnification 6500; reduced approximately one third in reproduction.

R. D. HEIDENREICH, C. H. GEROULD AND R. E. McNULTY '3

A second type, lamellar precipitation, is illustrated in Fig. 11. I n the light microscope, this aged Mg + 10 per cent A1 alloy closely resembles pearlite and in the electron microscope it is seen to consist of a series of coarse plates. A very muchher distribution of lamellar precipitation is shown in Fig. 12. The details shownhere are not resolvable, of course, in the light microscope.

A third type of precipitation is a Wid- manstatten arrangement of particles, appearing in the light microscope as parallel lines within a grain. Their appearance in the electron microscope (Fig. 13) from an aged Mg + 3 per cent Al + 7 Zn alloy is a series of platelike particles, ordered in direction within a given grain, and chang- ing this direction at a grain boundary.

Some Common Structures

Fig. 14 is of an alloy of Mg + 9 per cent A1 + 2 Zn + 0.2 Mn, in the heat-treated, quenched, and aged state. A piece of massive, undissolved Mg17A112, Widmanstatten precipitation and pearlitic-type precipitation are all shown. The similarity between this electron micrograph and a light micrograph is striking.

An alloy of Mg + 6 per cent Al + I Zn + 0.2 Mn cross-rolled and aged is illustrated in Fig. 15, showing twinning and some fine precipitation within the twins.

A heavily cold-rolled Mg + 1.5 per cent Mn + 0.3 Ce alloy is shown in Fig. 16. A few large particles and many small ones are..visible. This micrograph is interesting because it reveals the small particles that are invisible with the light microscope.

The appearance of a eutectic is given in Fig. 17, which shows a Mg + 32.2 per cent Al alloy, as cast. There is a pronounced three-dimensional effect in this electron micrograph, with the Mgl7A112 standing in relief. The Mg phase has been etched away, leaving the deep pits between the MgnAl~a ridges.

An example of a very fine-grained metal is given in Fig. 18, representing Mg + 1.5 per cent Mn alloy extrusion. Very fine particle precipitation is also exhibited.

The appearance of a forging of an alloy of Mg + 8.5 per cent A1 + 0.5 Zn + 0.2 Mn heat-treated and aged is shown in Fig. 19. A great deal of detail is shown, none of which is clearly evident in the light microscope. Globular precipitation, lamellar precipitation, and some massive compound are all present.

Special Micrographs and Techniques

During the course of research on a variety of subjects using the electron microscope, a number of interesting specialized techniques were developed. Most of these emphasize the importance of close col- laboration among various investigational methods for obtaining maximum results. Examples of the most interesting of these techniques are given.

Occasionally a small inclusion in the metal sample will stick to the polystyrene replica and then be transferred to the silica replica. When this happens, the inclusion itself, rather than a replica of the inclusion, is seen in the microscope. A striking example of this is shown in Fig. 20, of a replica of a magnesium sample containing carbon. Electron diffraction studies of the silica replica revealed that the dark platelets are graphite. Apparently flakes of free graphite were suspended in the melt before freezing.

Another example of the same effect is shown in Fig. 21, a Mg + 5 per cent Ni alloy hot-rolled + cold-rolled + aged. In this electron micrograph, transferred inclusions of MgzNi and replicas of the same inclusions are seen. The faithfulness of reproduction of the shape of the inclusionsis readily seen.

A n interesting use of the methyl iodide etchant is illustrated in Fig. 22, showing a Mg + 6 per cent Al + Be alloy. The sample was taken from the bottomof a billet, where


FIG. 21.-ALLOY OF MAGNESIUX PLUS 5 PER CENT NICKEL, HOT-ROLLED f COLD ROLLED f ACED, WITH GLOBULAR PARTICLES OF MgzNi ON THE SILICA REPLICA.

This micrograph not only shows a replica of the globular IllgzNi particles but also the actual particles themselves on the silica replica. The faithfulness of the replica technique in reproducing the size and shape of the particles is evident.

Faint grain boundaries appear in the background, showing the alloy to have very h e grains. Original magnification 6500; reduced approximately one third in reproduction. FIG. 22.-MASSIVE PARTICLE OF BERYLLIUM I N AS-CAST ALLOY OF MAGNESIUH PLUS 6 PER CENT

ALUMINUY PLUS BERYLLIUM. Illustrates preferential etch attack of methyl iodide etchant. This particle has been only very

slightly attacked by the etchant. Original magnification 7000; reduced approximately one third In reproduction. FIG. 23.-STRESS-CORROSION CRACK I N ANNEALED SHEET O F ALLOY OF MAGNESIUM PLUS 3 PER

CENT ALUMINUM, I P E R CENT ZINC AND 0.3 PER CENT MANGANESE. This alloy was etched following the laboratory stress-corrosion cracking. The replica shows

the side walls of this very minute transcrystalline crack. Original magnification 4500; reduced approximately one third in reproduction.

R. D. HEIDENREICH, C. H. GEROULD AND R. E. McNULTY I5

most of the beryllium had settled. The A further fruitful field of application ap- particle shown in the figure stands in bold peared to be that concerned with solid relief with very little attack by the etchant. solutions and pre-precipitation phenomena. These particles were in such sharp relief At least one class of age-hardening alloys

a b FIG. 24.-SOLID SOLUTION OF MAGNESIUM PLUS 5 PER CENT ALUMINUY.

a. Conventional metallographic polish and glycol etch. Structure is independent of composition of the solid solution. Electron diEraction indicates the presence of a film on the metal surface.

b. Methyl iodide etch showing typical "fine structure." Electron diffraction of such a surface shows bare metal.


that it was found possible to scrape a owes its properties to the intermediate sufficient quantity of them from the sur- stage between complete solid solution and fade, using a low-power microscope for' fully formed precipitate. The technique viewing, to obtain X-ray diffraction, was therefore further developed to investi- spectroscopic and chemical analyses. gate the processes taking place when a solid

Fig. 23 is a replica of a stress-corrosion solution transforms to yield a precipitated crack in a sheet of Mg + 3 per cent A1 + I phase. per cent Zn + 0.3 per cent Mn alloy. The The first studies were conducted on a metal structure along the vertical edge series of binary Mg-A1 alloys ranging from of the crack can be seen. This technique is I to 10 per cent Al. Small specimens were useful in examining minutely the imme- solution-heat-treated in helium-filled glass diate region of failure and, together with ampules for several days a t 82s°F (440°C.) correlated work found in the following and then quenched. The electron micro- pages, has aided in a n understanding of the graphs of what were supposedly complete stress corrosion of magnesium alloys. solid solutions always exhibited consider-

able etch structure, which did not have the of IMg-*' appearance of etch figures and which was

The previous sections have covered the absent in pure magnesium. more or less usual microstructures and bave A series of experiments was carried out shown that the methyl iodide etchant not to determine the cause of this structure. It only gives results that are consistent with was found first that the structure (termed those obtained by standard methods, but "fine structure" for want of a better name) provides much additional detail. was obtained only with the methyl iodide

16 ELECTRON METALI .OGRAPHIC METHODS

etchant. If ordinary etchants such as glycol, nital, hydrochloric acid, acetic acid, acetic picral, or an electrolytic polish are used, no difference in the detailed microstructures of solid solutions of different properties is noted, even though (Fig. 3) gross phases are still found for other states. Fig. 24 shows a comparison between a normal metallographic polish and glycol etch and the methyl iodide etch on a Mg + 5 per cent A1 solid solution. The s t ~ c t u r e in the glycol-etched sample of Fig. 24a is now known to be due to a surface film and is independent of the composition of the solid solution. "Fine s t ~ c t u r e " and other details of similar magnitude observed in Fig. 24b are lost.

The following possible explanations for the "fine structure " were experimentally tested:

I. Peculiarity of the methyl iodide etch attack.

2. Precipitation of Mg17AllZ. 3. Formation of Guinier-Preston zones. 4. Stresses resulting from the quench. 5. A transformation similar to that of the

formation of martensite in steel. 6. Preferential segregations of impurity

atoms. All of these possibilities were satisfac-

torily eliminated except the last, and it was finally concluded that an impurity or impurities were responsible for the "fine s t~cture ." From complete analyses of the specimens and consideration of the atomic radii of the impurity metals present, iron was suggested as a possibility, since it was common to all of the samples in which the structure was observed. Further, stress- corrosion results with commercial alloys indicated that the alloys that exhibited the "fine structure" were more susceptible to stress corrosion. The influence of iron on the corrosion behavior of magnesium is well known.

More evidence was desired, however, and a series of alloys of varying compositions was prepared in order to ascertain the

effect of both composition and heat- treatment. These alloys were prepared, using sublimed magnesium crystals and Super Purity aluminum. The results for two alloys containing Mg + 6 per cent Al I but different iron contents are shown in Fig. 25. The far greater intensity of "fine structure" in the 0.021 per cent Fe alloy as compared with that with 0.001 per cent Fe may be presumed to be due to an iron phase, and has been confirmed many times.

Theeffect of actual precipitation of Mg17- A112 on the " h e structure" was rather illuminating. A specimen of an alloy of Mg + 6 per cent A1 + I Zn + 0.2 Mn was solution-heat-treated at 8 ~ 5 ~ F . (440°C.) and quenched to yield the structure shown in Fig. 26a. A similar specimen cut from the same sheet was furnace-cooled from 82s°F. (440°C.) to yield a precipitate of Mg17AIl2 particles shown in Fig. 26b. Another specimen from the same sheet was quenched from 82s°F. (440°C.), cold- worked by 10 per cent reduction in rolling and then aged 3 hr. at 480°F. (250°C.) (Fig. 266). I t is evident that the precipitation Mg17Allz in this alloy has eliminated the ' ' h e structure" from the solid solution. The conclusion is that the impurity is much more soluble in the compound and has concentrated there. (Direct experiment shows iron to be much more soluble in Mg17- Ails than in pure magnesium.) If the precipitated specimen is again solution-heat- treated and quenched, the "fine structure" is again observed (Fig. 264, so that the cycle is completed.

Further evidence of the preference of aluminum over magnesium for iron was obtained by electron diffraction studies of a series of evaporated films. The films were prepared by evaporating simultaneously the metals Fc, Mg and Al onto a silica substrate to form a mixed crystal film. When such a film is heat-treated in vacuum, it is found from the diffraction patterns that the iron interacts with thealuminum to form FeAl (or FeAls) but no Mg17Al12


is found when the amount of iron is high. take up sufficient aluminum to form a com- If iron is absent, Mgl,Al12 is formed. pound, leaving only the excess aluminum Apparently the mixture FeAl + Mg is to form MgnAlli. more stable than Fe + Mg17AlIZ. An ex- With these results in mind, Fe and A1

FIG. 25.-EFFECT OF IRON CONTENT O N "FINE STRUCTURE" IN MAGNESIUX PLUS 6 PER CENT ALUMINUX.

Low iron, 0.001 per cent. a. Solution-heat-treated and water-quenched. b. Solution-heat-treated and furnace-cooled.

High iron, 0.021 per cent. G. Solution-heat-treated and water-quenched. d. Solution-heat-treated and furnace-cooled.


periment in which a piece of iron was dis- were evaporated simultaneously onto an solved in molten Mgl2Al12 corroborated etched magnesium single crystal and the this, for large crystals of FeAl could be specimen was then heat-treated in vacuum isolated from the alloy. I n the presence of to bring about diffusion. This surface was both magnesium and aluminum, iron will etched successively deeper and electron


diffraction patterns and micrographs were However, the electron diffraction pattern obtained a t various depths through the obtained from the diffusion zone was a spot diffusion zone. The appearance of the sur- pattern for magnesium with rings for FeAI. lace at a depth where a pattern for both The actual "fine structure" in the alloys

FIG. EFFECT OF n'Igt7A112 PRECIPITATION ON "PINE STRUCTURE" I N A N ALLOY O F M A G N E S I U M PLUS 6 PER CENT ALUMINUM, I PER CENT ZINC AND 0 . 2 PER CENT MANGANESE.

a. Solution-heat-treated at 82s°F. and quenched, showing considerable "fine structure." b. Solution-heat-treated at 825°F. and furnace-cooled. Mg~~Al l t particles are observed with the

"fine structure" greatly diminished. c. Solution-heat-treated a t 82s°F. and quenched to give "fine structure" and then cold-

worked and aged. MgllAll~ particles are evident with little "fine structure." d. Aged sample from c , showing little "fine structure," is again solution-heat-treated at

82s°F. and quenched. "Fine structure" has returned with the solution of the Mgl7Al11. Original magnification 6000; reduced approximately one third in reproduction.

magnesium and FeAl was obtained is always gives a ring pattern for magnesium shown in Fig. 2 7 . The similarity between along with rings that agree with the FeAl this structure and that found in a solution- spacings. Not enough lines are obtained heat-treated and quenched Mg + 6 per for positive identification as FeAl in this cent A1 binary (Fig. 28) is pointed out. case.


However, electron diffraction examina- Examination of a large number of alloys tion of a large number of alloys containing has indicated that the presence of both ['fine structure" has yielded the conclusion iron and aluminum in magnesium is re- that the pattern developed is consistent quired to produce the "fine structure." I n

FIG. 27. FIG. 28. FIG. 27.-SYNTHESIZED "PINE STRUCTURE."

Iron and aluminum were evaporated simultaneously onto a magnesium single crystal and heat-treated to bring about diffusion. The specimen was then etched into the diffusion zone. Electron diffraction showed both magnesium and FeAl present. Note similarity in the "fine structure" here and that observed in Fig. 28. Original magnification 6500; reduced approximately one third in reproduction.

FIG. 28.-SOLID SOLUTION OP MAGNESIUM AND 6 PER CENT ALUMINUM. Original magnification 7000; reduced approximately one third in reproduction.

with FeA1. The present conception of this structure is, then, that it consists of a segregation of FeAl,* perhaps along flaws or boundaries between mosaic blocks of the magnesium crystal. This segregation has often changed the magnesium diffractions from single-crystal spots to a ring pattern, indicating that the magnesium crystal is partially fragmented as a result. X-ray patterns from large-grained specimens will give single-crystal patterns indicating that only a small portion of the magnesium crystal is fragmented. This is doubtless in the regions near the FeAl deposits. Further work on the details of the structure is called for not only through fundamental interest but from a practical viewpoint as well.

* The term FeAl here includes any iron- aluminum compound isomorphous with FeA1.

the absence of aluminum, the structure is not found unless the iron content is very high. Systematic investigations of the aluminum-iron ratio for which the structure occurs have not been undertaken. I n a single series, however, the structure was absent for aluminum contents u p to 0.15 per cent. At 0.53 per cent A1 the structure was quite evident and increased in density up through I per cent. This series will be discussed in the next section.

Tha t the "fine structure" discovered in the magnesium-aluminum alloys is quite important from a practical standpoint will be shown in the next section. The great sensitivity of the electron microscope and electron diffraction to minute amounts of impurities is emphasized by this work. The selectivity of the methyl iodideetchant makes this typeof analysis possible.

2 0 ELECTRON METALLOGRAPHIC METHODS

CORRELATION OF "FINE STRUCTURE" The corrosion of magnesium-aluminum WITH CORROSION PROPERTIES alloys has been quite extensively studied9

The etch structure and conclusions re- as regards the effects of certain impurities garding its origin discussed in the fore- such as iron, nickel and copper. The general going pages are not only of fundamental conclusion has been that when theimpurity

FIG. 29.-RELATIONSHIP BETWEEX STRESS-CORROSION FACTOR, GENERAL CORROSION RATE AND ALUMINUbf CONTENT.

a. Stress-corrosion factor is delined as the ratio of stress for failure in salt chromate solution to the yield stress. Sharp increase in stress-corrosion sensitivity between 0.15 and 0.53 per cent Al correlates with the threshold of "line structure" as observed in Fig. 30.

b. Corrosion rate, (mg. per sq. cm. per day) in 3 per cent salt water. Correlation can also be observed between the increasing corrosion rate above 0.15 and 0.53 per cent A1 and the presence of "fine structure" as observed in Fig. 30.

interest but prove to bear a close relation to the corrosion properties of magnesium alloys. This relation was indicated before any work concerning its origin had been done. I t was iirst observed in alloys of Mg + 6 per cent A1 + I Zn + 0.2 Mn that were being tested for accelerated stress-corrosion properties. When it be- came apparent that there might be such a correlation, a program was undertaken to investigate the possibilities. This paper is concerned chiefly with metallography and any detailed consideration of corrosion is not warranted. However, sufficient corrosion data will be included to illustrate the importance of the electron microscope and electron diffraction findings in this connection.

content exceeds a "tolerance limit" (about 0 .002 per cent for iron) a discontinuity occurs in the corrosion-composition curves beyond which the corrosion rate is much greater than it is below. The increased corrosion was correlated with a critical number of discrete particles of an iron phase serving as cathodes during corrosion.

A second conclusion in the work of Hanawalt and his collaborators~~~0 was that even below the "tolerance limit" residual impurities still control the corrosion process. However, the concept of discrete particles of an iron phase was be- lieved to be inadequate to explain stress- corrosion behavior, since several alloys with iron contents well below the "tolerance limits" were found to be quite

R. D. HEIDENREICH, C. H. GEROULD AND R. E. McNULTY 2 1

susceptible to stress cracking. It was con- I n Fig. 29a the "stress-corrosion factor" sidered that the transgranular "fine struc- (deiined as the ratio of stress for failurein ture" may well represent the manner in salt-chromate solution to the yield stress) which the residual impurities are dis- is plotted against aluminum content below

FIG. 30.-COJ.IPOSITE OF LIGHT MICROGRAPHS, ELECTRON MICROGRAPHS, AND GENERAL CORROSION SPECINENS O F ALLOYS O F MAGNESIUX PLUS 0.0 TO 1.0 PER CENT ALUMINUM.

Note correlation between "fine structure" in! electron micrographs of the 0.53 and 0.97 per cent A1 alloys and the high corrosion rate of the corresponding metal specimens in 3 per cent salt water.

a, light micrographs; b, electron micrographs; c, general corrosion samples.

tributed. Such a dispersion could account for the typical transcrystalline stress- corrosion cracks observed in wrought Mg- A1 alloys.

The corrosion data* are conveniently summarized for one particular series of Mg-A1 binary alloys by Fig. 29a and 2 9 b .

* A full report of the methods and results will appear in a separate paper. The effect of heat-treatment is of considerable importance in considering corrosion results.

the solid solubility limit of aluminum in magnesium. Fig. 2 9 b shows the general corrosion rate in 3 per cent salt water as a function of aluminum content for this same series of alloys. All of the impurities Fe, Ni, Cu, M n were present to less than 0.001

per centeach. Fig. 30 is a composite showing light

micrographs, electron micrographs and the general corrosion specimens of this same

2 2 ELECTRON METALLOGRAPHIC METHODS '

series. Close examination of the light light microscopic and X-ray methods is micrographs yields nothing that can clearly demonstrated and i t is possible that account for the corrosion behavior. The such investigations should prove fruitful electron micrographs, however, indicate the in many alloy systems. onset of the "fine structure" above 0.15 I~

per cent Al. The threshold is difficult to SUMMARY determine, but is certainly somewhere between 0.15 and 0.53 per cent Al in this I . A new etching (methyl iodide) and

rinse procedure has been developed for series. The effect of the "fine structure" on the general corrosion is strikingly magnesium alloys, which yields, in con-

shown by the pronounced pit attack, or trast to standard etchants, a surface free

"worm tracks," in the corrosion specimens of films and mechanical distortion.

(Fig. 30). The effect on stress corrosion is 2. The surface resulting from this

quite evident in Fig. 29a. etchant may be examined in the electron

I t is concluded from this and other simi- use the polystyrene-

lar data that the presence of the "fine silica replica technique, or by reflection

structure" is a large factor in the corrosion electron diffraction methods.

behavior of magnesium-aluminum alloys 3. The methyl iodide etchant gives, in

with iron contents below the tolerance the electron microscope, results that are

limit. not only consistent with those observed in

The iron content alone, however, is not the light microscope, but also much addi-

sufficient, but requires a minimum of alumi- tional detail.

num in order to produce the structure. 4. In a study of

The solution potential differences be- solid solutions, a new dispersion of an iron-

tween iron-aluminum compounds and the aluminum phase has been discovered.

matrixB are very much greater than be- This dispersion, called "fine structure," is

tween Mg17Al18 and the solid solution. important in understanding both stress and

Further, the precipitation of Mg17Al12 has general corrosion processes formagnesium-

been found to decrease the stress-corrosion of purity' susceptibility and general corrosion rate in accordance with the behavior of "fine REFERENCES structure" upon heat-treatment, as de- I . C. H. Desch: Metallography, Ed. 5. New scribed in the previous section. York. 1942. Longmans. Green and Co.

2. R. D. Heidenreich: Jnl. Optical Soc. In addition to the corrosion results, it has Amer. (1945) 35, 139.

3. R. D. Heidenreich: Jnl. Applied Physics been observed that magnesium-aluminum (1943) 14, 312. alloys exhibiting Itfine structuren tarnish 4. V. S. Schaefer and D. Harker: Jnl. Applied

Physics (1942) 13. 427. much more rapidly than do alloys in which 5. D. Harker and M. J. Murphy: Trans.

A.I.M.E. (1945) 161, 75. the structure is absent' The tarnishing 6 . G. p. Thornson and W. Cochrane: Theory

reaction has not been sufficiently well and Practice of Electron Diffraction investigated to lead to any conclusions as 137. Macmillan London, 1939.

7. R. D. Heidenreich, L. Sturkey, and H. L. to reaction products in the two cases. Woods: Jnl. Applied Physics (in process

of publication). There are many questions be investi- 8. P. F. George: Bull. Amer. Soc. Test Mat. gated concerning the occurrence of the (Aug. 1944) 35. "fine structure" and the physical and 9. J. D. Hanawalt, C. E. Nelson, and J. A.

Peloubet: Trans. A.I.M.E. (1942) 147, 273. behavior of the The im- 10. R. E. McNulty and J. D. Hanawalt: Trans.

portance of structures not detected by Electrochem. SOC. (1942) 81, 423.

electron metallographic methods some results for …library.aimehq.org/library/books/metals...

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