Bulk Metallic Glasses: Materials of Future - DRDO

DRDO SCIENCE SPECTRUM 2009 DRDO Science Spectrum, March 2009, pp. 212-218 © 2009, DESIDOC

Bulk Metallic Glasses: Materials of Future B. Ramakrishna Rao Naval Materials Research Laboratory, ShilBadalapur Road, Ambernath-421 506

1.

INTRODUCTION

Metallic liquids are extremely unstable and crystallise almost immediately at temperatures lower than their melting point owing to high heat of fusion and low melt viscosity. Since recorded history, practical metallic materials have been made by taking advantage of this crystallisation principle. However, progress with this approach was limited because of the requirements of the thermodynamic equilibrium conditions as well as the necessity of translational and rotational symmetry. Presence of the crystalline defects has put limitations on properties of metallic materials. One of the approaches to improve metallic materials was to produce whisker’s exhibiting strength levels close to theoretical strength of metallic bond. Alternatively, crystalline structure of metals can be destabilised to obtain amorphous materials with no long range periodicity, as shown in Fig. 1. Such materials have been envisaged to exhibit high strength, corrosion resistance and unique physical properties due to absence of any crystalline defects. Metallic materials in amorphous state can be obtained by vapour condensation and inert gas sputtering techniques. Quantity of amorphous metals produced using these techniques were limited for any practical applications. Subsequently, rapid solidification techniques were developed to obtain ribbons of amorphous materials to be called metallic glasses. Detailed study of thermodynamic and kinetic principles related to crystallisation led to development of fluxing techniques to obtain metallic glasses in bulk (BMGs). Later on, importance of constitution, mixing enthalpy, and size mis-match among constituent elements was realised and these factors were described by Inoue’s empirical rules and Greer’s confusion principle. These principles finally led to synthesis of bulk metallic glasses in Fe-Zr-Mg- and Cu-based alloy systems. Present paper describes scientific endeavor to develop BMGs, properties of BMGs and their possible application areas. 2.

RAPID SOLIDIFICATION TECHNIQUES

The formation of the first metallic glass of Au 75Si 25 was reported by Duwez at Caltech, USA, in 1960 1. They developed a technique in which a gaseous shock wave 212

(a)

(b)

(c) Figure 1. Atomic arrangement in: (a) conventional crystalline material, (b) in amorphous material, (c) comparison of XRD patterns.

RAO: BULK METALLIC GLASSES–MATERIALS OF FUTURE

atomises the molten metal into droplets(~ 10 mg) which are forced against a Cu chill block to form thin foils called splats. Through this splat- quenching method, the molten metal can be subjected to cooling rates in excess of 10 5– 10 6 K/s. Their work showed that the process of nucleation and growth of crystalline phase could be kinetically bypassed in some alloy melts to yield a frozen liquid configuration, that is metallic glass. Subsequently, several other rapid solidification techniques like drop-smasher, melt spinning, pendant-drop melt-extraction, twin-roller quenching devices, etc. were developed. (Fig. 2).

amorphous-forming ability of metallic melts. Turnbull et al. illustrated the similarities between metallic glasses and other non-metallic glasses such as silicates, ceramic glasses, and polymers. They showed that, a glass transition manifested in conventional glass-forming melts could also be observed in rapid quenched metallic glasses. From thermodynamic and kinetic considerations of crystallisation involving nucleation growth, Turnbull predicted that a ratio, referred to as the reduced glass transition temperature T rg= T g/T m (of the glass transition temperature T g to the melting point T m of alloy) can be used as a criterion for determining the glass-forming ability (GFA) of an alloy3. According to Turnbull’s criterion , a liquid with T g/Tm = 2/3 becomes very sluggish. Such liquid can thus be easily under-cooled at a lowcooling rate into the glassy state. 4.

Figure 2. Rapid solidification techniques; (a) Drop-smasher, (b) melt-spinning, (c) pendent-drop melt extraction, (d) twin roller quenching [2]

The significance of Duwez’s work was that their method permits large quantities of an alloy to be made into glassy state comparing to other methods, for instance, vapur condensation. With this development, it was possible to produce a whole class of new materials with virtually no limitations to the number of components, but only the tailored excellent physical properties in mind. Different rapid solidification processing methods were developed to obtain amorphous structure in metallic systems. Metglass (an alloy of Fe-Ni-P-B) was the first metallic glassy ribbon exploited commercially for use in transformer core due to its soft magnetic properties, leading to low-core losses. However, most of the rapid solidification techniques had one serious drawback in that the material produced could be only of thickness of the order of a few microns, thus limiting the scope of applications. 3.

CRITERIA FOR FORMATION OF GLASSES

Necessity to produce metallic glasses in bulk form led researchers to understand the scientific principles underlying

DEVELOPMENT OF BULK METALLIC GLASSES

Based on above criterion, efforts were focused to produce metallic glasses in bulk. If millimeter scale is defined as bulk, the first bulk metallic glass was the ternary Pd– Cu–Si alloy prepared by simple suction-casting methods at a significantly lower cooling rate of 10 3 K/s 4. In 1982, Turnbull 5 et al. successfully prepared the well-known Pd– Ni–P BMG using boron oxide fluxing method to purify the melt and to eliminate heterogeneous nucleation. The fluxing experiments showed that the value of T rg of the alloy could reach 2/3 when the heterogeneous nucleation was suppressed, and the bulk glass ingot of centimeter size solidified at cooling rates in the 10 K/s region. Parallel to liquid metallurgy routes, a variety of solidstate amorphisation techniques, based on completely different mechanism from rapid quenching, such as mechanical alloying, diffusion-induced amorphisation in multilayers, ion beam mixing, hydrogen absorption, and inverse melting, had been developed 6. In the late 1980s, Inoue, et al. in Tohoku University of Japan systematically investigated the GFA of ternary alloys of rare-earth materials with Al and Fe metals, they observed exceptional GFA in the rare-earth-based alloys, for example, La–Al–Ni and La–Al–Cu 7. By casting the alloy melt in water-cooling Cu molds, they obtained fully glassy rods and bars with thicknesses of several millimeters. Based on this work, several quaternary and quinary amorphous alloys were developed (e.g., La–Al–Cu–Ni and La–Al– Cu–Ni–Co BMGs) at cooling rates under 100 K/s and the critical casting thicknesses could reach several centimeters8. Inoue’s efforts highlighted the fact that the correct choice of elemental constituents would lead to amorphous alloys even at cooling rates employed in conventional industrial practices. These slower cooling rates mean that large pieces of metallic glasses can be fabricated. For the new types of metallic glass- forming alloys, the intrinsic factors of the alloys (such as the number, purities and the atomic size of the constituent elements, composition, cohesion among the metals, etc.) instead of external factors (such as cooling rate, etc.) play key roles in the glass formation. In general, the GFA in BMGs tends to increase as more 213

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components are added to the alloy. That is called the confusion principle9, which implies that larger number of components in an alloy system destabilises competing crystalline phases which may form during cooling. This effect frustrates the tendency of the alloy to crystallise by making the melt more stable relative to the crystalline phases. Inoue summarised the results of glass formation in multi-component alloys and proposed three empirical rules 10. 1) multi-component systems consisting of more than three elements. 2) significant difference in atomic sizes with the size ratios above about 12% among the three main constituent elements; and. 3) negative heats of mixing among the three main constituent elements. They claimed that the alloys satisfying the three empirical rules have special atomic configurations in the liquid state which are significantly different from those of the corresponding crystalline phases and shift time-temperaturetransformation(TTT) curves for crystallisation to right as shown in Fig. 3. The atomic configurations favour the glass formation in terms of thermodynamics, kinetics as well as the microstructure development. Figure 4.

Cast vitreloy 1cast componets. 12

strength in the range of 3000 MPa. Fe-based BMGs developed were christened as amorphous steel. These steels exhibit very high super-cooled regime where it can be formed in to any shape due to superplastic behaviour and can be cooled to room temperature. Ships, submarines made of new steel will be-time stronger at the same thickness and will not be detected by underwater magnetic sensors. Research is currently focused to develop BMGs using cheaper raw materials and inexpensive techniques to exploit them 14 commercially . 5.

Figure 3. TTT diagrams for metallic glasses. critical cooling rates for newly designed bulk metallic glass forming liquids are quite low compared to ordinary amorphous forming alloy melts which require RSP techniques.

With this understanding several alloy systems were devloped to have good glass forming ability (GFA) in Zrbased, Cu based and Mg based multi-component systems. In 1992 first commercial BMG Vitalloy 1 (Zr41Ti14Cu12.5 Ni10Be22.5) was developed, which could be cast in to 5-10mm dia rods at a critical cooling rate of 1K/s 11. Formation of the BMGs in this family requires no fluxing or special processing treatments and can form bulk glass by conventional metallurgical casting methods. Some cast Vitreloy 1 components are shown in Fig 4. Recently, Fe-based BMG was developed by adding Yittrium to an earlier developed Fe-Cr-Mo-Mn-C-B-based BMG with 13 a typical composition of (Fe44.3Cr5Co5Mo12.8Mn11.2 C15.8B5.9)98.5Y1.5 , which was non-magnetic at room temperature and exhibited high hardness, high corrosion resistance, had a compressive 214

PREDICTION OF GLASS-FORMING ABILITY

There is no single predictive model to assess the GFA beforehand in many alloy systems due to complexity arising out of several components. For example, bulk glass forming alloys in Fe-based systems contain more than 7 elements. Starting from Turnbull’s Trg criterion, several other parameters such as DTx(Tx “Tg) 15, ³(Tx/(Tg + Tl)) 16, ± (Tx/Tl) 17 have been used to estimate GFA. However, all the above parameters need the alloy to be first prepared in glassy form to be able to measure the glass transition temperature TD and/ or the crystallisation temperature TD. Hence, they are not predictive in nature. There have been considerable efforts focused in the direction of predicting GFA from thermodynamic parameters calculated based various models.18,19 Recent studies on BMGs to determine local configuration of close packed clusters indicated that multi-component systems behave like quasi-ternary or quasi-quarternary systems. Several constituent elements topologically occupy the same sites in local clusters depending on their relative atomic sizes 20. It was proposed that specific combinations of these topologically identical solutes provide a dramatic difference in glass stability. For example (Cu+Ni) in Zr and Pd glasses, (Ti+Al) in Zr glasses, (C+B) in Fe glass. How,

RAO: BULK METALLIC GLASSES–MATERIALS OF FUTURE

these topologically equivalent but chemically distinct solutes stabilise glass has not been understood yet. It was proposed that the electronic structures present fundamental understanding on the structural stability of a solid phase, regardless of the crystalline or disordered nature. Dong, et al. successfully utilised e/a based criterion for estimating high GFA alloys 21. Independently, each of above methods may be successful in predicting GFA in some selected systems. However, other criterion must be used in conjunction to pin-point the composition with largest GFA in given multi-component system. These studies indicate that there can be ample scope for optimising good glass forming compositions by utilising22,23 combination of GFA predicting models. Recently Bhat, et al. combined thermodynamic and topological models to estimate glass forming compositions in Zr-Cu-Al system successfully. 6.

BMG WORK AT NMRL

Work has been initiated at NMRL to develop iron-and copper-based BMG alloy formulations for marine applications. Efforts are under progress to arrive at promising alloy system with GFA. Combined22 thermodynamic and topological model proposed by Bhatt, et al. 22 was used to predict glass forming ability in Fe-Zr-B ternary alloy system. In this

method, isometric contours of the enthalpy of chemical mixing (DHchem), the mis-match entropy normalised by Boltzman’s constant (Ss/k B) and configurational entropy normalised by gas constant (S config/R) were used to identify BMG compositions. Superimposed plots of DH chem, S config/R and Ss/kB is shown in fig. 5. It was demonstrated that compositions with DHchemX Ss/kB ³ 5.0 give good BMGs in Zr-based system. Following similar approach, various compositions at the intersection of DH chem and S s/k B contours in a known S config/R regime (0.8-1) were identified and presented in Table 1. For a fixed atomic % of B (B ~ 20%), compositions with Zr>10 found to haveDH chemX S s/k B ³ 5.0 and can be good glass formers. Table1. ∆ H chemX S s/k B values for selected compositions Composition (Fe80-xZrxB20)

ΔHchem, kJ/mol

Sconfig/R

Sσ /kB

ΔHchemX Sσ/kB kJ/mol

x=5

-11

0.68

0.2-0.4 -3.26

x = 10

-13

0.8

0.4

-5.25

x = 15

-16

0.88

0.43

-6.88

x = 20

-17

0.95

0.5

-8.50

Figure 5. Superposition of different contours. Compositions selected for mechanical alloying are also superimposed on ternary diagram.

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than steel, and more consistently manufacturable, since they are produced from a single mold (with microscale casting accuracy) ready for use. Other edged tool applications include knives and razor blades. Table 2.Comparison of mechanical properties of Vitreloy 1 with conventional metallic materials Properties

Vitreloy1 aluminum titanium alloys alloys

steel alloys

Density (g/cm3)

6.0

2.6-2.9

4.3-5.1

Figure 6. XRD patterns of Fe 80-x Zr x B 20 (x=5, 10, 15, 20) after 20h of milling.

Yield strength (GPa)

1.90

0.10-0.63

0.18-1.32

For model validation, mechanical alloying route was selected. Compositions indicated in Table 1 were milled in a high energy planetary ball mill at 300 rpm. XRD patterns of ball milled powders are given in fig. 6. Powder compositions with Zr >10 at% have shown faster amorphisation kinetics confirming the model predictions.

Elastic strain limit

2%

~ 0.5%

~ 0.5%

~ 0.5% 50-154

7.

strength (GPa/g/cm 3)

PROPERTIES AND APPLICATIONS OF BULK METALLIC GLASSES

BMGs exhibit unique set of properties not seen in conventional crystalline materials. They show very high strength, high elastic limit, corrosion resistance, wear resistance due to the absence of crystalline defects. Comparison of properties of glassy alloys with conventional materials are shown in Fig. 7.

Fracture toughness (MPa m 1/2 )

20-140

23-45

55-115

Specific

0.32

< 0.24

< 0.31

Typical properties of Vitreloy 1 are given in Table 2. BMGs are the ideal precursor materials for obtaining bulk nano-crystalline metallic materials. BMGs based on Pd, Zr, Mg were developed commercially to be used in consumer electronic industries, soft magnetic transformer cores, sporting goods industries and in various defence, aerospace applications [24] . BMGs are also found to be ideal materials for prosthetic implants and surgical instruments. They are of higher quality but less expensive than diamond, sharper and longer lasting 216

0.50-1.60