Glass Formation in Ni-Zr-( Al ) Alloy Systems

Structural and thermal properties of binary Ni 100−x Zr x (30 < x < 75) alloys obtained by melt spinning and copper mold casting methods were investigated. The fully amorphous samples in a bulk form cannot be obtained in the binary Ni-Zr alloys over a wide composition range, though they haveT g /T l and γ values close to or even higher than those of the binary Cu-Zr bulkmetallic glasses (BMGs).The low thermal stability of the supercooled liquid against crystallization and the formation of the equilibrium crystalline phases with a high growth rate are responsible for their low glass-forming abilities (GFAs). Relatively low thermal conductivities of Ni-based alloys are also considered to be another factor to limit their GFAs. The GFA of the binary Ni 65.5 Zr 34.5 alloy alloyed with 4% or 5% Al was enhanced, and a fully glassy rod with a diameter of 0.5mm was formed.


Introduction
Bulk metallic glasses (BMGs) have attracted much attention due to their excellent properties compared with their crystalline counterparts and potential applications as structural materials [1][2][3].A variety of BMGs with high glass-forming ability (GFA) based on multiple components have been developed over the past two decades or so [4][5][6][7][8][9][10].Among these BMGs, ordinary Ni-based BMGs are very attractive for engineering applications because of their ultrahigh strength and relatively low cost.Ni-based amorphous alloys with diameters of more than 1 mm have also been produced since 1999 [11][12][13][14][15][16].These Ni-based BMGs belong mainly to LTM-ETM (LTM-late transition metals, ETM-early transition metals) group of alloys.Now, their critical value of thickness is up to 15 mm, but metalloids included in some alloy systems have tampered their manufacturability.So, it is necessary to develop new Ni-based BMGs including only common metallic elements.Developing new Ni-based BMGs is expected to expand engineering applications and is important for understanding the long-standing issues of glass formation mechanism.
Various empirical methods have been proposed to guide the discovery of BMGs with higher GFA [1,10,17].The  rg criterion widely used as a long-standing guideline has predicted that deep eutectics are preferable for glass formation upon cooling a liquid, so the compositions near the eutectics have been chosen preferentially to obtain BMG in either simple binary or multiple component systems.It is generally known that binary alloys must have a very high critical cooling rate required to avoid crystallization upon cooling from liquid state because of the simplicity of their chemical compositions.However, some binary alloys such as Ni-Nb, Cu-Zr, and Cu-Hf can be bulk glass formers [18][19][20][21][22], though nanocrystalline phases are included.These binary metallic glasses provided an important guidance to search for ordinary multicomponent BMGs with extremely good GFA [14][15][16][17].As well as Ni-Nb and Cu-Zr binary alloy systems, the Ni-Zr one has been also known to be a system in which the amorphous phase can be formed over wide composition ranges by rapid quenching techniques.However, no systematic studies were preformed on the mold casting of this system.In the current work, the alloys with near eutectic composition (Ni 64 Zr 36 ) have been chosen by analogy with the similar Cu-Zr bulk glassy alloys.The GFA and thermal properties of the Ni 100− Zr  (30 <  < 75) alloys have been investigated.Besides, the influence of Al addition on the GFA and thermal stability of the binary Ni-Zr alloys was studied.

Experimental
The alloy ingots studied in this work were prepared by arc melting mixtures of Ni, Zr, and Al with purities of 99.97 wt.%, 99.9 wt.%, and 99.99 wt.%, respectively, in a Ti-gettered high purity argon atmosphere.The ingots were remelted at least three times to ensure the homogeneity of the samples.The alloyed ingots were then remelted in an evacuated state in a quartz tube using an induction heating coil followed by injection through a nozzle with a diameter of 0.5 or 1 mm.The ribbon samples were fabricated using a singleroller melt spinning apparatus.The structures of the as-cast samples were studied using X-ray diffraction (XRD) with Cu-K radiation.The thermal properties were examined by a differential scanning calorimeter (DSC) at a heating rate of 0.67 K/s and differential thermal analysis (DTA) at a heating rate of 0.33 K/s.

Result and Discussion
In the Ni-Zr alloy system, the compositions that we have primarily focused on are close to the eutectic point  The reasons for the lower GFA of binary Ni-Zr alloys might be explained as follows.Recently, thermodynamic simulations of the Ni-Zr and Cu-Zr alloy systems have been assessed in the relation with short range ordering in the liquid [24].The critical cooling rates in the Cu-Zr alloys are predicted to be smaller than those in Ni-Zr alloys, suggesting that Cu-Zr alloys have higher GFA than Ni-Zr alloys.While the driving force for crystallization in the Ni-Zr alloys may be much higher than those in the Cu-Zr and Ni-Nb alloys, this means that crystalline phases in Ni-Zr alloys can nucleate and grow up more easily than those in Cu-Zr and Ni-Nb alloys in an undercooled alloy melt.The difference in the atomic configuration of Ni-Zr and Cu-Zr metallic glasses also plays an important role in their GFAs.An icosahedron-like structure in Cu-Zr BMGs makes glassy phase more stable, but Ni-Zr metallic glasses do not have the same structure [25].Otherwise, the thermal conductivity of the molten alloy, indicating its ability to transfer heat upon cooling, also affects the GFA of the alloy.It has been reported that Ni-based alloys have much lower thermal conductivity than Cu-based alloys [26].
To improve the GFA of the binary Ni-Zr alloys, Al element as the third component was added in binary Ni-Zr alloy system.Since the atomic radius of Zr is 18% larger than that of Ni, it is predicted that maximum packing density in the undercooled liquid is obtained at around 35 at.%Zr in binary Ni-Zr alloys.So, the composition Ni 65.5 Zr 34.5 has been selected as the starting point.In contrast, the off-eutectic composition Ni 67 Zr 33 has also been selected.

Conclusions
The fully glassy sample in a bulk form cannot be obtained in the Ni-Zr alloys over a wide composition range, though they have   /  and  values close or even higher than those of the binary Cu-Zr BMGs studied earlier.The low stability of the supercooled liquid against crystallization and the formation of crystalline phases with large nucleation and growth rate of Ni-Zr alloys have led to their low GFAs compared with those of the Cu-Zr counterparts.Otherwise, their considerably lower thermal conductivities compared with Cu-Zr alloys are also responsible for their low GFAs.The minor addition of Al can improve the GFAs of binary Ni-Zr alloys.The GFA of the binary Ni 65.5 Zr 34.5 alloy alloyed with 4 or 5 at.%Al was enhanced and a fully glassy rod with a diameter of 0.5 mm was formed.

Table 1 :
Ni 64 Zr 36 by analogy with Cu-Zr bulk glassy alloys.In addition, other compositions like Ni 40 Zr 60 , Ni 35 Zr 65 , and Ni 26 Zr 74 were also investigated.Figure 1 shows XRD patterns of the as-cast Ni 100− Zr  (30 <  < 75) alloy rods with a diameter of 1 mm.It is seen that all these binary alloy specimens exhibit crystalline Bragg peaks, corresponding to different crystalline phases such as Ni 21 Zr 8 , Ni 10 Zr 7 , Ni 11 Zr 9 , and Ni 7 Zr 2 , indicating that they cannot have a fully amorphous structure in a bulk form.The equilibrium Ni 10 Zr 7 compound, which is typical in some Ni-based alloys [23], and eutectic-type Ni 21 Zr 8 phase can be observed in those alloys near eutectic Figure 2: XRD patterns of the as-cast Ni 67 Zr 33 melt spun ribbons at different spinning velocities.The inset is DSC curve of Ni 67 Zr 33 melt spun ribbon at 4000 rpm.Critical thickness   , reduced glass transition temperature  rg , supercooled liquid region width Δ, and  of typical binary BMGs and Ni 67 Zr 33 metallic glasses.theas-castribbon exhibits typical broad diffraction maxima of amorphous structure, and no obvious crystalline peaks can be observed with the XRD resolution limits.When the spinning velocity decreases from 4000 rpm to 2500 rpm, corresponding to the thickness of about 35 m, some small crystalline peaks appear in the XRD pattern, indicating the formation of nanocrystalline phases in glassy matrix.This shows that the formation of these crystalline phases is very sensitive to the cooling rate.The inset is the DSC curve of the Ni 67 Zr 33 ribbon prepared at 4000 rpm.It is noted that a distinct endothermic characteristic of the glass transition followed by a sharp crystallization peak is clearly exhibited.The values of the glass transition temperature   and onset temperature of crystallization   are measured to be 835 K and 860 K, respectively.Figure3presents DTA curve of the as-cast Ni 100− Zr  (65.5 <  < 68) alloy rods.With the addition of Ni, the onset melting temperature   of Ni 100− Zr  alloys also increases, but not the values of the liquid temperature   (  is somewhat overestimated as it is obtained on heating at 0.33 K/s).Table 1 lists the parameters denoting GFA of typical binary BMGs and Ni 67 Zr 33 metallic glasses.It is seen that the   /  and  values of binary Ni 67 Zr 33 are close or even higher than those of Cu-Zr and Ni-Nb BMGs, though no bulk glassy alloys were obtained in the studied binary Ni-Zr alloys.
compositions.If the nucleation and growth of these intermetallic compounds can be restrained, a fully amorphous structure will be obtained in Ni-Zr alloy system.Figure2presents XRD patterns of the Ni 67 Zr 33 melt spun ribbons prepared at different spinning velocities.At the spinning velocity of 4000 rpm, corresponding to the thickness of about rate of 40 K/min.It is found that only (Ni 65.5 Zr 34.5 ) 96 Al 4 and (Ni 65.5 Zr 34.5 ) 95 Al 5 alloys exhibit an obvious endothermic characteristic of the glass transition followed by a characteristic exothermic heat event indicating the successive stepwise transformations from supercooled liquid state to crystalline phases.The DTA curves of the as-cast (Ni 65.5 Zr 34.5 ) 100− Al  ( ≤ 8) alloys at a heating rate of 10 K/min are shown in Figure6.With the addition of Al in these alloys, the values of   and   will decrease.Figure7presents a DSC curve of the as-cast (Ni 65.5 Zr 34.5 ) 96 Al 4 alloy rod with a diameter of 0.5 mm scanned at a heating rate of 40 K/min, and the inset is a DTA curve of this alloy at a heating rate of 20 K/min.The ternary