i.

.

LITERATURE CITED

I. S. Kozlovskii, V. A. Olovyanishnikov, and V. M. Zinchenko, "Criteria for evaluating and bases for rational selection of carburized and nitrided steels," Metalloved. Term. Obrab. Met., No. 3, 2-9 (1981). S. A. Yurasov, "Structural criteria for evaluating the quality of carburized and nitrided cases," Metalloved. Term. Obrab. Met., No. 3, 9-10 (1981).

A NUCLEUS-FREE MECHANISM OF IRREGULAR GRAIN GROWTH IN METALS

V. Pshetakevich (W. Przetakiewicz) UDC 620.186.8

An opinion was expressed in [i] that the mechanism of irregular grain growth after a critical amount of deformation has not been studied sufficiently. Some authors, e.g., [2, 3], assert that rapid boundary movement of the least deformed grains is the reason for grain growth after cold working with a critical amount of deformation, while others consider that the process of irregular growth proceeds due to the presence of a large number of nuclei in the steel structure [4 etc.]. The authors of [i, 5, etc.] connect this phenomenon with grain coalescence.

The marked difference in points of view about irregular grain growth is caused mainly by the fact that technological aspects of critical cold working for metals and alloys have been considered in the past. Basic questions, in particular the process of metal disloca- tion substructure evolution during heating after a critical amount of deformation, has not been especially studied. In this connection, the effect of lattice dislocations on irregu- lar grain growth has not been studied.

The present work contains an analysis of the effect of thermal activation processes in the formation of critical cold working based on the results of studying the change in the condition of the dislocation substructure, grain size, boundary structure, and also stored energy in nickel (99.85% Ni), copper (99.93% Cu), and s-brass (L68) differing significantly in the value of packing defect energy (PDE). Studies carried out showed that an increase in the grain size after a critical amount of deformation is not due to primary recrysta!liza- tion.

Dislocation density after a critical amount of deformation does not reach the value necessary to form new grain boundaries with considerable misorientation during heating. An insignificant excess of similar dislocations hinders an increase in misorientation angle e and transformation of subgrains into high-angle boundaries. So that subgrains are trans- formed into nuclei they should have a boundary with a misorientation greater than 18 ° [6]. Angle 0 depends directly on dislocation density. According to calculations carried out [7], a misorientation angle of i0-15 ° in a foil i ~m thick is obtained only with a dislocation density of 0 > i0 Is m-2. The thinner the foil, then the greater should be dislocation density with the same value of misorientation angle. Dislocation density in bcc metals after a critical amount of deformation reaches 1012-1013 m -2 [8, 9].

Considering the data provided, one can be certain that with a foil thickness of 0.3 ~m, formation of recrystallization nuclei is impossible. Formation of nuclei must be considered as a deviation from the normal mechanism and it should be connected with very marked local deformation inhomogeneity.

Grain growth with high-temperature heating (>0.6Tm) after a critical amount of deforma- tion also confirms the suggestion of a nucleus-free mechanism for the process being analyzed.

It is well known that the boundaries of a nucleus are in a nonequilibrium state [i0], and as a result of this it is possible for extrinsic grain boundary dislocations (EGBD), formed as a result of absorption across a boundary of lattice dislocations from the deformed

Military-Technical Academy, Warsaw, Poland. Translated from Metailovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 60-62, February, 1982.

0026-0673/82/0102- 0151507.50 © 1982 Plenum Publishing Corporation 151

Fig. i. Equilibrium structure of a gain boundary typical of bcc metals preliminarily annealed at a tempera- ture above 0.ST m.

Fig. i

Fig. 2 Fig. 3

Fig. 2. Dislocation cellular structure of copper after a criti- cal amount of deformation.

Fig. 3. Substructure of copper after completion of irregular grain growth.

Fig. 4 Fig. 5

Fig. 4. Stable dislocation walls in nickel annealed at 0.8T m after a critical amount of deformation.

Fig. 5. Planar location of dislocations observed in s-brass after a critical amount of deformation and during annealing after critical cold working.

matrix, to spread easily (starting at a lower temperature) within a nucleus. This is facilitated by low-temperature migration of nuclei since it is well known that displacement of a boundary surface containing EGBD is possible after spreading of these defects [ii]. Boundary movement caused by a marked free energy gradient occurs due to the "ideal" struc- ture of a nucleus in the zone adjacent to the boundary.

Primary grains of the metals studied after preliminary high-temperature annealing (above 0.STm) have a dislocation-free equilibrium structure (Fig. i). In view of this, spreading of EGBD in primary grains is initiated at high temperature significantly exceeding the normal recrystallization temperature. The actual presence of extrinsic dislocations at a boundary does not affect its energy state. It only changes as a result of EGBD spreading. Thus, movement of original grain boundaries after a critical amount of deformation occurs particularly easily at high homologous temperatures.

From the results obtained by studying the kinetics of change in grain size for nickel, copper, and s-brass, it follows that the degree of critical grain growth depends mainly on the activity of lattice dislocation reaction with primary boundaries during their migration.

152

Fig. 6 Fig. 7

Fig. 6. Local transformation of a large-angle boundary into a separate boundary as a result of annealing twin formation (nickel). x250.

Fig. 7. Grain boundary fragmentation in s-brass (dark field). x250.

The dislocation substructure forming in a metal after cold working with ecr (Fig. 2), not used previously to build nuclei, imparts to a moving boundary a reserve of energy for further movement with steady absorption of all new lattice dislocations and EGBD formed. In addition, it is well known that EGBD arising'in the process of boundary migration do not disappear until such time as previously formed extrinsic dislocations are completely annihilated [ii].

The results obtained make it possible to suggest that the phenomenon of critical grain growth is a continuous, rapidly repeated process of structural changes occurring by the following scheme:

i) absorption of lattice dislocations andformation of EGBD (a reduction in energy for the system);

2) spreading of EGBD (a growth in boundary energy with a further reduction in energy for the system);

3) rapid boundary migration (a sharp reduction in energy for the system).

It might also be supposed that the absorption of lattice dislocations, which continues until the boundary is saturated, induces the process of EGBD spreading, preceding further boundary migration. Spreading the EGBD should increase boundary porosity [ii, 12], and therefore the rate of boundary movement v increases according to the relationship [13]

v=MG'f,

where M G is boundary mobility coefficient, being a function of its structure (including porosity); f is driving force related to a unit of grain boundary surface.

It has been shown [14] that the effect of a nonequilibrium boundary structure on its migration rate is connected mainly with a change in boundary mobility M G since it depends exponentially on the increase in boundary energy, and linearly on driving force f. The strengthening gradient, revealed very clearly after a critical amount of deformation, and also the energy state of moving boundary surfaces during annealing cause a high rate of boundary movement. As a result of this, at high homologous temperatures intense irregular grain growth commences, and this continues up to the moment of absorption through the boundary of nonequilibrium dislocation substructural elements.

In copper having an average value of PDE (0.070 J/m2), grain growth precedes recovery accompanied by an increase in the dimensions of dislocation cells whose walls are non- equilibrium interlacings of dislocations. This type of substructure completely disappears with rapid boundary movement (Fig. 3).

In nickel distinguished by a high value of PDE (0.150 J/m 2) there is typical poly- gonization hindered by boundary migration. Weak interaction of boundary surfaces and lattice dislocations forming stable walls (Fig. 4) causes much less grain growth. In high-zinc u-brass characterized by a low PDE (0.015 J/m 2) a stable combination of extended disloca- tions with primary slip planes (Fig. 5) hinders active interaction of substructure disloca- tions with boundaries and simultaneously it makes grain growth more difficult.

153

Fig. 8. ×250.

Fig. 9. (copper) .

Fig. 8 Fig. 9

Dissociation of large-angle grain boundaries (nickel).

Pseudotwinned grains with symmetrical separate boundaries xlO.

The data provided make it possible to conclude that in nickel and a-brass, and more rarely in copper, at high homologous temperature additional thermally activated processes begin leading to a reduction in internal energy for a polycrystal and an increase in grain dimensions.

At temperatures above 0.6Tm fragmentation is observed and sometimes there is complete disappearance of certain large-angle boundaries which is accompanied by formation of many annealing twins. An area of an arbitrary boundary in which a twin forms is converted to a low-energy separate boundary (a partially conjugated boundary) as can be seen in Fig. 6. Further transformation (an increase in the level of ordering) of the grain boundary struc- ture may lead in many cases to its complete disappearance (Fig. 7).

In the last phase of critical growth together with changes in the amount of boundary surface, there are other grain boundary changes. They should be classed as particular cases of splitting (dissociation) of a wide-angle boundary into two component low-energy separate boundaries (Fig. 8).

There is also transformation of certain large-angle boundaries into symmetrical sepa- rate boundaries, as a result of which in an area of coarsened grains there remain fine pseudotwinned crystals [15] bounded by low-energy, weakly mobile separation surfaces (Fig. 9).

Thus, the results of tests carried out make it possible to verify that irregular grain growth in metals after a critical amount of deformation is a nucleus-free process. The mechanism of the process analyzed is complex. The amount of critical grain coarsening de- pends on the possibility for movement of primary (original) grain boundaries and also their structural changes which are specified by the thermal stability of the dislocation substruc- ture depending on packing defect energy for the material.

LITERATURE CITED

i. V. N. Gurashev and I. E. Kurov, "Effect of grain boundary condition on recrystalliza- tion with critical amounts of deformation in Armco iron," Metalloved. Term. Obrab. Met., No. i, 2 (1980).

2. S. S. Gorelik, "Mechanism of recrystallization after small amounts of deformation," Metalloved. Term. Obrab. Met., No. 3, 12 (1961).

3. B. Ciszewski, Defekty Struktury Krystalicznej, Warsaw (1973). 4. Ya. R. Rauzin, "Some irregularities in the initial stage of plastic deformation,"

Metalloved. Term. Obrab. Met., No. 9, 23 (1959). 5. A. P. Gulyaev and R. P. Leshchinskaya, "Naphthalene fracture of a fast-cutting steel,"

Metalloved. Term. Obrab. Met., No. 9, 22 (1963). 6. R. Doherty, Metall. Odlewnictwo, AGH, Krakow, Vol. 5, No. 2, 179 (1979). 7. G. V. Kopetskii and L. S. Kokhanchik, Fiz. Met. Metalloved., 48, No. 2, 367 (1979). 8. M. N. Bodyako, S. A. Astapchik, and G. B. Yaroshevich, Termokinetics of Recrystalliza-

tion [in Russian], Minsk (1968). 9. I. I. Novikov, Theory of Metal Heat Treatment [in Russian], Metallurgiya, Moscow

(1978). i0. R. Doherty, Met. Sci., No. 8, 132 (1974).

154

ii. P. Pumphrey and H. Gleiter~ Phil. Mag., 32, 881 (1975). 12. P. Pumphrey and H. Gleiter, Phil. Mag., 30, 593 (1974). 13. M. Grabski, Struktura Granic Ziarn Metalach, Kotowiee (1969). 14. R. Valiev, W. Giercman, and D. Kaibyszew, Phys. Status Solidi, 61, K95 (1980) o 15. W. Przetakiewicz and H. Ziencik, Biul. WAT, No. 6, 103 (1980).

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