Amorphous Metallic Alloys

• 1.General features

• 2.Structure

• 3.Producing

• 4. Properties

• Amorphous metallic alloys or metallic glasses have emerged as a new

• class of engineering materials after vitrfication of metallic alloys by using

• the technique of ultra-rapid quenching of molten alloys has become

• possible.

• These materials, which do not have any long range crystalline order but

• retain metallic bonding, exhibit several interesting properties emanating

• from their unique structure which is isotropic and homogeneous in the

• microscopic scale.

• Extremely high hardness and tensile strength, exceptionally good

• corrosion resistance and very low magnetic losses in some soft magnetic

• materials are some of the attractive properties associated with amorphous

• metallic alloys.

• Usually poor tensile ductility - prevents structural applications

amorphous

crystalline

There are three technologically

important classes of amorphous

alloys,

namely,1. the metal - metalloid alloys, such as Fe-B,

Fe-Ni-P-B and Pd-Si,

2. the rare earth - transition metal alloys, such

as La-Ni and Gd-Fe,

3. the alloys made up of a combination of

early and late transition

metals such as Ti-Cu, Zr-Cu, Zr-Ni and Nb-Ni.

Important issues:

• glass forming abilities

• diffusion mechanisms

• modes and kinetics of crystallization

• formation of bulk metallic glasses

HISTORY

The first metallic glass was an alloy (Au80Si20) produced at Caltech by Pol Duwez in 1957. This and other early glass-forming alloys had to be cooled extremely rapidly (on the order of one megakelvin per second, 106 K·s-1) to avoid crystallization.

An important consequence of this was that metallic glasses could only be produced in a limited number of forms (typically ribbons, foils, or wires) in which one dimension was small so that heat could be extracted quickly enough to achieve the necessary cooling rate. As a result, metallic glass specimens (with a few exceptions) were limited tothicknesses of less than one hundred micrometres.

In 1969, an alloy of 77.5% palladium, 6% copper, and 16.5% silicon was found to have critical cooling rate between 100 K/s to 1000 K/s.

HISTORY

In 1976, H. Liebermann and C. Graham developed a new method of manufacturing thin ribbons of amorphous metal on a supercooled fast-spinning wheel. This was an alloy of iron, nickel, phosphorus and boron. The material, known as Metglas , was commercialized in early 1980s and used for low-loss power distribution transformers (Amorphous metal transformer). Metglas-2605 is composed of 80% iron and 20% boron, has Curie temperature of 373 °C and a room temperature saturation magnetization of 125.7 milliteslas.

In the early 1980s, glassy ingots with 5 mm diameter were produced from the alloy of 55% palladium, 22.5% lead, and 22.5% antimony, by surface etching followed with heating-cooling cycles. Using boron oxide flux, the achievable thickness was increased to a centimeter.

HISTORY

The research in Tohoku University and Caltech yielded multicomponent alloys based on lanthanum, magnesium, zirconium, palladium, iron, copper, and titanium, with critical cooling rate between 1 K/s to 100 K/s, comparable to oxide glasses.

In 1988, alloys of lanthanum, aluminium, and copper ore were found to be highly glass-forming.

In the 1990s, however, new alloys were developed that form glasses at cooling rates as low as one kelvin per second. These cooling rates can be achieved by simple casting into metallic molds. These "bulk" amorphous alloys can be cast into parts of up to several centimeters in thickness (the maximum thickness depending on the alloy) while retaining an amorphous structure. The best glass-forming alloys are based on zirconium and palladium, but alloys based on iron, titanium, copper, magnesium, and other metals are also known.

HISTORY

Many amorphous alloys are formed by exploiting a phenomenon called the "confusion effect". Such alloys contain so many different elements (often a dozen or more) that upon cooling at sufficiently fast rates, the constituent atoms simply cannot coordinate themselvesinto the equilibrium crystalline state before their mobility is stopped. In this way, the random disordered state of the atoms is "locked in".

HISTORY

In 1992, the first commercial amorphous alloy, Vitreloy 1 (41.2% Zr, 13.8% Ti, 12.5% Cu, 10% Ni, and 22.5% Be), was developed at Caltech, as a part of Department of Energy and NASA research of new aerospace materials. More variants followed.

In 2004, two groups succeeded in producing bulk amorphous steel, one at Oak Ridge National Laboratory, the other at University of Virginia. The Oak Ridge group refers to their product as "glassy steel". The product is non-magnetic at room temperature and significantly stronger than conventional steel, though a long research and development process remains before the introduction of the material into public or military use.

PROCESSING OF METALLIC GLASSES

Virtually any liquid can be turned into a glass if it is cooled quickly enough to avoid crystallization. The question is, how fast does the cooling need to be?

Common oxide glasses (such as ordinary window glass) are quite resistant to crystallization, so they can be formed even if the liquid is cooled very slowly. For instance, the mirror for the 200" telescope at the Palomar Observatory weighed 20 tons and was cooled over a period of eight months, but did not crystallize.

Many polymer liquids can also be turned into glasses; in fact, many polymers cannot be crystallized at all.

For both oxides and polymers, the key to glass formation is that the liquid structure cannot be rearranged to the more ordered crystalline structure in the time available.

PROCESSING OF METALLIC GLASSES

Metallic glasses are another story. Because the structural units are individual atoms (as opposed to polymer chains or the network structure of an oxide), in most alloys it is relatively easy for crystals to nucleate and grow. As a result, the earliest metallic glasses (which werediscovered at Caltech in the late1950s) required very rapid cooling -around one million degrees Celsius per second - to avoid crystallization. One way to do it is by single-roller melt spinning, as shown here:

SINGLE-ROLLER MELT SPINNING

In this process, the alloy is melted (typically in a quartz tube) by induction heating, and then forced out through a narrow nozzle onto the edge of a rapidly rotating chill wheel (typically made of copper). The melt spreads to form a thin ribbon, which cools rapidly because it is in contact with the copper wheel.

Melt spinning and other rapid solidification techniques have been used to make a wide variety of amorphous and nanocrystalline metals from the 1960s to the present. However, materials produced in this way have a key limitation: At least one dimension must be very small, so that heat can be extracted quickly enough to achieve the necessary cooling rate. As a result, the early glass-forming alloys could only be produced as thin ribbons (typically around 50 µm thick), wires, foils, or powders.

Although some applications (notably those that made use of the magnetic properties of iron- and nickel-based alloys) could use metallic glasses in these forms, structural applications were obviously impractical.

SINGLE-ROLLER MELT SPINNING

The earliest demonstration of a bulk metallic glass came from Harvard University in the 1980s, where is was shown that by using a flux to removeimpurities, a palladium-based glass could be produced in thickness of more than a millimeter. More rapid progress was made in the early 1990s (notably at Caltech and Tohoku University) on developing alloys that could form glasses at much lower cooling rates, down to one degree Celsius per second or less.

Today, a wide range of glass-forming alloys are known, based on common elements including iron, copper, titanium, magnesium, zirconium, and platinum. These alloys can be produced using variations on standard metallurgical casting techniques, although in most cases processing must be done in vacuum or under an inert atmosphere to prevent contamination. One technique (common in research laboratories) is suction casting.

An ingot in the upper chamber under an inert atmosphere is melted with an electric arc (much like in arc welding) and then sucked into a mold when the lower chamber is opened to vacuum. One of the potentially useful properties of metallic glasses is that they do not melt abruptly at a fixed temperature. Instead, like ordinary oxide glasses, they gradually soften and flow over a range of temperatures. By careful control of temperature, the viscosity of the softened glass can be precisely controlled. This ability can be used to form metallic glasses into complex shapes by techniques similar to those used for molding polymers.

SUCTION CASTING.

PROPERTIES

Amorphous metal is usually an alloy rather than a pure metal. The alloys contain atoms of significantly different sizes, leading to low free volume (and therefore up to orders of magnitude higher viscosity than other metals and alloys) in molten state. The viscosity prevents the atoms moving enough to form an ordered lattice. Thematerial structure also results in low shrinkage during cooling, and resistance to plastic deformation.

The absence of grain boundaries, the weak spots of crystalline materials, leads to better resistance to wear and corrosion. Amorphous metals, while technically glasses, are also much tougher and less brittle than oxide glasses and ceramics.

Thermal conductivity of amorphous materials is lower than of crystals. As formation of amorphous structure relies on fast cooling, this limits the maximum achievable thickness of amorphous structures.

PROPERTIES

To achieve formation of amorphous structure even during slower cooling, the alloy has to be made of three or more components, leading to complex crystal units with higher potential energy and lower chance of formation.

The atomic radius of the components has to be significantly different (over 12%), to achieve high packing density and low free volume.

The combination of components should have negative heat of mixing, inhibiting crystal nucleation and prolongs the time the molten metal stays in supercooled state.

The alloys of boron, silicon, phosphorus, and other glass formers with magnetic metals (iron, cobalt, nickel) are magnetic, with low coercivityand high electrical resistance. The high resistance leads to low losses by eddy currents when subjected to alternating magnetic fields, a property useful for eg. transformer magnetic cores.

The critical casting thickness versus the year in which alloys were discovered. Over 40 years, the critical casting thickness has increased by more than three orders of magnitude.

PROPERTIES

Amorphous alloys have a variety of potentially useful properties. In particular, they tend to be stronger than crystalline alloys of similar chemical composition, and they can sustain larger reversible ("elastic") deformations than crystalline alloys.

Amorphous metals derive their strength directly from their non-crystalline structure, which does not have any of the defects (such as dislocations) that limit the strength of crystalline alloys.

Amorphous metallic alloys combine higher strength than crystalline metal alloys with the elasticity of polymers.

PROPERTIES

One modern amorphous metal, known as Vitreloy, has a tensile strength that is almost twice that of high-grade titanium. However, metallic glasses at room temperature are not ductile and tend to fail suddenly when loaded in tension, which limits the material applicability in reliability-critical applications, as the impending failure is not evident. Therefore, there is considerable interest in producing metal matrix composite materials consisting of a metallic glass matrix containing dendritic particles or fibers of a ductile crystalline metal.

Nanoscale structure

Experimental studies: X-ray, neutron diffraction

physical properties measurements;

computer simulation

theoretical studies

Clusters in disordered Clusters in disordered systemssystems

Cluster in melt

(icosahedra,oktahedra, fcc, hcp, etc.)

Cluster in melt

(icosahedra,oktahedra, fcc, hcp, etc.)

Methods: Thermal evaporation and aggregation.

Chemical reactions

Thermal annealing

Molten alloys

AmorphousAmorphous solidssolids

rr11, Z, Z11

kD

∆= 2

3

5,22π

kD

∆= π2

rr11, Z, Z11

Temperature dependence of nanocrystal size of Al at crystallization.

(1-amorphous-crystalline; 2-crystalline)

Structure factors ofamorphous+crystalline Al87Ni8Y5 alloy.

Time-dependence of Al-volume fraction at continuous heating.

(1-amorphous; 2-amorphous-crystalline; 3,4-crystalline)

Temperature dependence of Al-nanocrystals density at

crystallization

Temperature dependence of nanoparticle size

(1- Al, 2- Al3Ni, 3- Al23Ni6Y4)

Temperature dependence of Fe3Si nanoparticles

Small angle scattering at annealing of Al87Ni8Y5

amorphous alloy.

Eutectic alloys (Pb-free solders, matrix of liquid magnetic composites, temperature reference points, coolants for nuclear power stations, casting alloys).

Sn-Cu, Bi-Sn, In-Ga-Sn, Li-Pb, Pb-Mg, Bi-Pb

)31( mdpowderNi

SnBi

CuBi

SnCu

µ÷≈−+

−−−

Technology parameters:Technology parameters:

�temperature�duration of mixing process�electronegativity difference�outside influence (acoustic treatment, magnetic field applying).

Sn-Cu+15at%Ni BiBi--Cu+15at%NiCu+15at%Ni

BiBi--Cu+15at%NiCu+15at%Ni

Influence on nanocluster formation

DiffractionDiffraction patternspatterns ofof SnSn--CuCueutecticeutectic alloyalloy

H=0 H=360 kA/m

Vibration treatment

Nanocrystallization of

Al-based amorphous alloys

Amorphous phase structure

Amorphous aluminium alloys

Al-RE, Al-TM-RE

RE = Sm,Gd,Tb,Dy,Y...;

TM = Fe,Ni,Co,...

• High strength-to-weight ratio

• Good ductility

Microstructure

10 20 30 40 50 60 700

100

200

300

400

500

I(s), (a .u)

s, nm -1

T, K

398

373

348

303

Fig.1. Diffraction patterns of Al87Ni8Dy5 amorphous alloys

5 10 15 20 25 30 35 400

25

50

75

100

125

150

175

Pre-peak

Inte

nsity

(a.

u.)

s, nm-1

Principal peak

Additional peak

Fig.2. Profile of diffraction curve

Fig.3 .Principal peak interpretation

15 20 25 30 35 4050

75

100

125

150

175

200

225

250

AF-2

T=408 K

AF-1

(111)Al

(200)Al

s, nm-1

Inte

nsity

(a.

u.)

15 20 25 30 35 4050

100

150

200

250

300

s, nm-1

Inte

nsity

(a.

u.)

T=433 K

15 20 25 30 35 4050

100

150

200

250

300

350

s, nm-1

T=473 K

15 20 25 30 35 4050

100

150

200

250

300

350

400

450

s, nm-1

Inte

nsity

(a.

u.)

T=523 K

Structure evolution in nanocrystalline alloy

15 20 25 30 35 4050

100

150

200

250

300

350

400

s, nm-1

T= 533 K

400 450 500 550 600 650

10

20

30

40

50

60 III

L, nm

T, K

Temperature change of grain size for Al nanocrystals

I- primary and II- second stage of crystallization

400 425 450 475 500 525 550 575 6000.0

0.2

0.4

0.6

0.8

1.0

T, K

Xc , (a.u.)

Volume fraction of crystalline phase

Structure of Fe73.7Nb2.4Cu1.0Si15.5B7.4

ribbons

X-ray

X-Ray analysis of irradiated ribbons

Fe3Si Fe3B Fe23B6

Grain size, nm

Percent, %

Grain size, nm

Percent, %

Grain size, nm

Percent, %

Irradiated at 0,197mJ

60 55 40 7 9 38

Irradiated at 0,245mJ

130 49 11 29 120 22

Temperature dependence of

magnetization for amorphous

ribbon• initial

• irradiated

AFM- images for initial (right) and irradiated (left) sample.

• Laser irradiation of amorphous alloys leads to

atomic structure changes with formation of

nanocrystalline clusters of different size.

Occurring of such changes is accompanied by

changes of magnetic properties that permit to

control the magnetic properties by means of

laser irradiation