A New Ni-Based Metallic Glass with High Thermal Stability and Hardness

Glass forming ability (GFA), thermal stability and microhardness of Ni51−xCuxW31.6B17.4 (x = 0, 5) metallic glasses have been investigated. For each alloy, thin sheets of samples having thickness of 20 μm and 100 μm were synthesized by piston and anvil method in a vacuum arc furnace. Also, 400 μm thick samples of the alloys were synthesized by suction casting method. The samples were investigated by X-ray diffractometry (XRD) and differential scanning calorimetry (DSC). Crystallization temperature of the base alloy, Ni51W31.6B17.4, is found to be 996 K and 5 at.% copper substitution for nickel increases the crystallization temperature to 1063 K, which is the highest value reported for Ni-based metallic glasses up to the present. In addition, critical casting thickness of alloy Ni51W31.6B17.4 is 100 μm and copper substitution does not have any effect on critical casting thickness of the alloys. Also, microhardness of the alloys are found to be around 1200 Hv, which is one of the highest microhardness values reported for a Ni-based metallic glass until now.


Introduction
For the last two decades, multicomponent bulk metallic glasses (BMGs) have attracted great attention because of their unusual physical, chemical and mechanical properties [1].The major factor which restricts the utilization of the bulk metallic glasses at high temperatures is their crystallization temperatures, above which they crystallize and lose their excellent properties.In general, if a metallic glass alloy contains elements having high melting point, it is expected to have high crystallization temperature.For this reason, number of metallic glass alloys containing high amount of refractory metals, such as tungsten, ruthenium, rhenium, iridium, osmium, tantalum and niobium, have been studied in order to develop metallic glasses having high crystallization temperatures [2][3][4][5][6][7][8][9][10].In these studies, metallic glasses which have crystallization temperatures higher than 1100-1200 K have been developed.In addition, microhardnesses of the refractory based metallic glasses determined to be between 1200-2000 Hv, which are much higher than almost all of the non-refractory metal based metallic glasses.Examinations of the compositions of the refractory metal based metallic glasses show that most of these alloys contain high amount of boron in addition to refractory metals [2][3][4][5][6][7][8][9][10].This indicates that these alloys owe their attractive mechanical and thermal properties to strong bonds form between refractory metals and boron.
Although refractory metal based metallic glasses are superior to other metallic glasses in terms of mechanical properties and thermal stability, their critical casting thicknesses are quite low, which are less than 30 µm [2][3][4][5][6][7][8][9][10].Such low critical casting thickness values prevent them from being used in industrial applications.The reason of these low critical casting values is that their liquidus temperatures are quite high because of high refractory metal contents.As known, increasing the liquidus temperature of an alloy for a constant glass transition temperature increases the cooling rate required for glass formation without crystallization.As a result, critical casting thickness decreases.In fact, most of the refractory metal based metallic glasses have such high liquidus temperatures that they can not be measured with thermal analysis equipment.
Therefore, in order to develop refractory metal based metallic glasses which have high critical casting thickness, alloy compositions having sufficiently low liquidus temperatures must be found.Unfortunately, there is very limited information about RM-B-X (X: other elements) systems in the literature.In a theoretical study on Ni-W-B system, it has been reported that one of the eutectic compositions is Ni51W31.6B17.4 and its euctectic temperature is 1622 K [11].This composition contains significantly high amount of tungsten and boron and has a quite low liquidus temperature with respect to other refractory metal based metallic glasses.Therefore, we believe that it deserves an investigation to determine its glass forming ability (GFA), thermal stability and microhardness.
In this study, GFA, thermal stability and microhardness of Ni51−xCuxW31.6B17.4(x = 0, 5) alloys are investigated.The base alloy, Ni51W31.6B17.4, is modified with copper due to the fact that copper has such a low solubility with boron and tungsten that it is supposed not to form any phase with them.Therefore, copper substitution for nickel is expected to improve GFA of the alloy by hindering formation of precipitating phases during solidification.

XRD Results
XRD patterns of 100 µm thick samples of the alloys are given in Figure 1a.In both of the XRD patterns, a broad diffraction peak, which is typical of amorphous structure, is observed.Also, no crystalline peak is visible in the patterns, which indicates that 100 µm thick samples of the alloys are fully amorphous.However, for the casting thickness of 400 µm, alloys almost completely crystallize during solidification (Figure 1b).Also, it should be noted that 400 µm thick sample of Ni51W31.6B17.4alloy contains some amount of amorphous phase.Examination of the XRD patterns revealed that for both of the alloys, Ni (space group Fm3m) and W2B (space group I4/mcm) phases form in the samples having casting thickness of 400 µm.Lattice parameter of nickel is found to be 3.59 Å, which is higher than the lattice parameter of pure nickel, 3.5238 Å (JCPDS-PDF-4-850).This result clearly shows that this phase contains some amount of tungsten, so it is actually a Ni-W solid solution rather than pure nickel.In addition, lattice parameters of W2B phase are also determined.They are found to be a

Thermal Stability
DSC scans of the alloys are shown in Figure 2.Each DSC scan exhibits exothermic reactions corresponding to crystallization of the undercooled liquid.However, glass transition temperatures of the alloys cannot be well defined.Only tentatively determined Tg values of the alloys are shown in Figure 2a.Glass transition temperatures of Ni51W31.6B17.4 and Ni46Cu5W31.6B17.4alloys are determined as 929 and 971 K, respectively.Also, crystallization temperatures of Ni51W31.6B17.4 and Ni46Cu5W31.6B17.4alloys are found to be 996 and 1063 K, respectively.In addition, solidus temperatures of Ni51W31.6B17.4,and Ni46Cu5W31.6B17.4alloys are measured as 1610 and 1620 K, respectively.Moreover, liquidus temperatures of Ni51W31.6B17.4 and Ni46Cu5W31.6B17.4alloys are found to be 1784 and 1780 K, respectively.Thermal properties of the alloys are summarized in Table 1.In addition, XRD patterns of the samples of Ni46Cu5W31.6B17.4alloy which were annealed at 950 and 1025 K are given in Figure 3. XRD patterns show that after annealing at 950 K small amount of nickel precipitates.Annealing at 1025 K results in significant increase in the amount of nickel which forms in the structure.These results show that Tg values of the alloys are lower than those determined tentatively.

Discussion
XRD results clearly show that critical casting thicknesses of the alloys are at least 100 µm but less than 400 µm.This critical casting thickness value is higher than critical casting thicknesses of most of the refractory metal based metallic glasses [2][3][4][5][6][7][8][9][10].This is due to the fact that liquidus temperatures of the alloys investigated in this study are much lower than those of other refractory metal based metallic glasses.Because of lower liquidus temperatures, Ni51W31.6B17.4 and Ni46Cu5W31.6B17.4alloys have lower reduced glass transition temperatures, Tg/Tl; as a result, they have higher GFA values.
Glass transition and crystallization temperatures of the alloys are increasing with copper addition.Copper is not soluble with tungsten in even liquid state [12], but completely soluble with nickel.Also, nickel and tungsten are completely soluble in liquid state and tungsten is partially soluble in nickel in solid state.Therefore, copper substitution for nickel must be lowering solubility of tungsten in nickel in liquid state.For this reason, in Ni46Cu5W31.6B17.4alloy, the number of Ni-W neighbors is decreasing and the number of W-W and W-B neighbors is increasing.Because of the reduction in the number of Ni-W pairs, formation of the first phase, nickel-tungsten solid solution, becomes more difficult during heating.In other words, crystallization of nickel takes places at a higher temperature (Figure 5).On the other hand, due to the increase in number of W-B neighbors, formation of the second phase, W2B, becomes easier.In fact, the second crystallization temperature of Ni46Cu5W31.6B17.4alloy is lower than that of Ni51W31.6B17.4alloy.Crystallization temperature of Ni46Cu5W31.6B17.4alloy, 1063 K, is higher than those of many of the refractory metal based metallic glasses [2-10] and the highest value reported for a Ni-based metallic glass until now (Table 2).For a constant Tl, increasing Tg and Tx is expected to result in improved critical casting thickness.However, as mentioned, since formation of W2B phase becomes easier because of copper addition, minimum cooling rate, Rmin, required for the formation of glass phase is still high (Figure 5).As a result no change in critical casting thickness is observed.In order to improve GFA of Ni-W-B metallic glasses, Rmin must be reduced, which can be accomplished by decreasing the liquidus temperature of the alloy and hindering the formation of precipitating phases, W2B and nickel.These can be achieved by decreasing the tungsten and boron contents of the alloy.However, it should be considered that decreasing tungsten and boron contents of the alloy also decreases Tg and Tx, which causes deterioration of thermal stability.Therefore, reduction of tungsten and boron contents of the alloy should be carried out systematically to obtain optimum critical casting thickness-thermal stability combination.In addition, if element(s) which will be substituted for tungsten and boron have low solubility with nickel in solid state, formation of nickel solid solution is hindered.As a result, Rmin decreases further, which causes more improvement in GFA.
Microhardness values of the alloys are found to be around 1200 Hv (11.7 GPa), which is one of the highest values reported for a Ni-based metallic glass (Table 3).Also, tensile yield strengths of the alloys can be estimated by using the equation σy = Hv/3 [19].Based on the microhardness values, the tensile yield strengths of the alloys are determined to be about 3.9 GPa, which is higher than yield strengths of other Ni-based metallic glasses [1].In addition to mechanical properties, one of the advantages of Ni-W-B metallic glasses over those reported in Table 3 is that Ni-W-B metallic glasses have higher thermal stability.In fact, crystallization temperature of Ni59.35Nb34.45Sn6.2alloy is 930 K, which is about 130 K less than that of Ni46Cu5W31.6B17.4alloy.The other advantage of Ni-W-B metallic glasses is their lower cost with respect to costs of those containing high amount of Nb, Ta and Hf.Although the alloys synthesized in this study have low critical casting thicknesses, 100 µm, there are still some potential applications where they can be utilized because of their high thermal stabilities and microhardnesses.Also, since the alloys contain significantly high amount of nickel, they are expected to have high corrosion resistance.For example, alloys can be coated on substrates by thermal spraying techniques to obtain hard, wear resistant and corrosion resistant coatings [29,30].Besides, powders of the alloys having diameter less than 100 µm can be produced by gas atomization, and then they can be sintered by spark plasma sintering to manufacture components with desired shape and dimensions [31].In addition, considering the fact that nickel and W2B phases precipitates, the alloys can be used as precursors to develop Ni-W2B metal matrix composite coatings having high hardness and high toughness.

Experimental Section
Ni-Cu-W-B alloy ingots with compositions of Ni51−xCuxW31.6B17.4(x = 0, 5) were prepared by arc melting the mixtures of Ni (99.9 mass%), W (99.9 mass%) and Cu (99.9 mass%) metals and crystalline B (98 mass%) in a Ti-gettered high purity argon atmosphere.In order to obtain homogeneous master alloys, samples were melted three times.Compositions of the alloys represent nominal atomic percentages.20 µm and 100 µm thick thin foils of the alloys were produced by piston and anvil method in an arc furnace.For the production of the thin foils, a molten sample of each alloy was squeezed between two copper plates pushed by pneumatic pistons.Velocity of each piston was about 400 mm/s and 50 mm/s for 20 µm and 100 µm thick samples, respectively.Thicknesses of the foils were determined by optical microscopy.Also, samples having thickness of 400 µm were produced by suction casting method.In addition, samples of copper containing alloy, Ni46Cu5W31.6B17.4,were annealed at 950 and 1025 K for 5 min.under high purity argon atmosphere to determine the thermal stability of the alloy.The structures of the samples were examined by X-ray diffraction (XRD) (Bruker Karlsruhe, Germany, D8 Advance equipped with Vantec-1 detector) with Cu-Kα radiation.The glass transition temperature (Tg), crystallization temperature (Tx), solidus temperature (Tm) and liquidus temperatures (Tl) of the alloys were measured by differential scanning calorimetry (DSC) (Netzsch Selb, Germany, STA 449 F3) at a heating rate of 0.33 K/s.Vickers hardnesses of the samples were measured with a Vickers hardness tester (Shimadzu Kyoto, Japan, HMV 2L) under a load of 1.96 N. Images of the indents obtained after microhardness measurements were acquired with scanning electron microscopy (SEM) (Leo Cambridge, UK, 1430 VP) under secondary electron (SE) imaging mode.

Conclusions
GFA, thermal stabilities and microhardnesses of Ni51−xCuxW31.6B17.4(x = 0, 5) metallic glass alloys are investigated.It is found that copper substitutions for nickel improves the glass transition and crystallization temperatures, which are comparable to those of other refractory metal based metallic glasses and higher than those of other Ni-based metallic glasses.However, copper substitutions for nickel do not have any effect on critical casting thicknesses of the alloys studied.Critical casting thicknesses of the alloys are found to be 100 µm.Although this value is quite low, it is still higher than critical casting thicknesses of most of the refractory metal based metallic glasses.Also, microhardness of the alloys are found to be around 1200 Hv and this value is higher than microhardness of other Ni-based metallic glasses.Furthermore, alloys can be used as a precursor to manufacture Ni-W2B metal matrix composites.

Table 2 .
Thermal properties (Tg,Tx) and microhardnesses of various Ni-based metallic glasses.
Alloy D (mm) T g (K) T x (K)Ref.

Table 3 .
Microhardnesses of various Ni-based metallic glasses.