Amorphous Metallic Alloys. 1.General features 2.Structure 3.Producing 4. Properties

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1 Amorphous Metallic Alloys 1.General features 2.Structure 3.Producing 4. Properties

2 Amorphous metallic alloys or metallic glasses have emerged as a new class of engineering materials after vitrficationof 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

3 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

4 HISTORY The first metallic glass was an alloy (Au80Si20) produced at Caltech by Pol Duwez in This and other early glass-forming alloys had to be cooled extremely rapidly (on the order of one megakelvin per second, 10 6 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 to thicknesses 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.

5 HISTORY In 1976, H. Liebermann and C. Graham developed a new method of manufacturing thin ribbons of amorphous metal on a supercooled fastspinning 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 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.

6 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.

7 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 themselves into the equilibrium crystalline state before their mobility is stopped. In this way, the random disordered state of the atoms is "locked in".

8 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.

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16 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.

17 PROCESSING OF METALLIC GLASSES

18 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 were discovered 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.

19 SINGLE-ROLLER MELT SPINNING 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.

20 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 remove impurities, 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.

21 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.

22 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. The material 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.

23 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 coercivity and 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.

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25 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.

26 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 noncrystalline structure, which does not have any of the defects (such as dislocations) that limit the strength of crystalline alloys.

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

28 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.

29 Nanoscale structure

30 Clusters in disordered systems Experimental studies: X-ray, neutron diffraction physical properties measurements; computer simulation theoretical studies

31 Cluster in melt (icosahedra,oktahedra, fcc, hcp, etc.)

32 Cluster in melt (icosahedra,oktahedra, fcc, hcp, etc.)

33 Methods: Thermal evaporation and aggregation. Chemical reactions Thermal annealing

34 Molten alloys r 1, Z 1 D 3 2π 2,5 k = 2 Amorphous solids r 1, Z 1 D = 2π k

35 Structure factors of amorphous+crystalline Al 87 Ni 8 Y 5 alloy. Temperature dependence of nanocrystal size of Al at crystallization. (1-amorphous-crystalline; 2-crystalline) Time-dependence of Al-volume fraction at continuous heating. (1-amorphous; 2-amorphous-crystalline; 3,4- crystalline) Temperature dependence of Alnanocrystals density at crystallization

36 Temperature dependence of nanoparticle size (1- Al, 2- Al 3 Ni, 3- Al 23 Ni 6 Y 4 ) Small angle scattering at annealing of Al 87 Ni 8 Y 5 amorphous alloy. Temperature dependence of Fe 3 Si nanoparticles

37 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 Cu Sn Bi Cu + Bi Sn Ni Technology parameters: temperature powder ( d 1 3µ m) duration of mixing process electronegativity difference outside influence (acoustic treatment, magnetic field applying).

38 Sn-Cu+15at%Ni Bi-Cu+15at%Ni Bi-Cu+15at%Ni

39 Influence on nanocluster formation H=0 H=360 ka/m Diffraction patterns of Sn-Cu eutectic alloy

40 Vibration treatment

41 Nanocrystallizationof Al-based amorphous alloys

42 Amorphousphase structure

43 Amorphous aluminiumalloys Al-RE, Al-TM-RE RE = Sm,Gd,Tb,Dy,Y...; TM = Fe,Ni,Co,... High strength-to-weight ratio Good ductility

44 Microstructure

45 I(s), (a.u) 500 T, K s, nm -1 Fig.1. Diffraction patterns of Al87Ni8Dy5 amorphous alloys

46 175 Principal peak 150 Intensity (a.u.) Pre-peak Additional peak s, nm -1 Fig.2. Profile of diffraction curve

47 T=408 K Intensity (a.u.) (111)Al AF-1 AF-2 (200)Al Fig.3.Principal peak interpretation s, nm -1

48 Intensity (a.u.) T=433 K T=473 K s, nm s, nm -1

49 Intensity (a.u.) T=523 K T= 533 K s, nm Structure evolution in nanocrystalline alloy s, nm -1

50 L, nm 60 II I T, K Temperature change of grain size for Al nanocrystals I- primary and II- second stage of crystallization

51 X c, (a.u.) T, K Volume fraction of crystalline phase

52 X-ray Structure of Fe 73.7 Nb 2.4 Cu 1.0 Si 15.5 B 7.4 ribbons

53 X-Ray analysis of irradiated ribbons Irradiated at 0,197mJ Irradiated at 0,245mJ Grain size, nm Fe 3 Si Fe 3 B Fe 23 B 6 Percent, Grain Percent, Grain % size, nm % size, nm Percent, %

54 Temperature dependence of magnetization for amorphous initial ribbon

55 irradiated

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

57 Laser irradiation of amorphous alloys leads to atomic structure changes with formation of nanocrystallineclusters 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