Room temperature nanoindentation creep behavior of TiZrHfBeCu(Ni) high entropy bulk metallic glasses
Introduction
High entropy alloys (HEAs) [1], [2] and bulk metallic glasses (BMGs) [3], [4] have both attracted extensive research attention due to their outstanding mechanical properties. HEAs were defined as multicomponent alloys with at least five constituent elements in equiatomic or near equiatomic (5–35 at%) concentrations. Because of the high mixing entropy effect, HEAs always consists of simple solid solution structures (e.g. face centred cubic (FCC) [5], [6], body centred cubic (BCC) [7], [8], hexagonal close packed (HCP) [9], [10] or their mixture [11], [12]). However, as the high mixing entropy is also a necessary condition for glass formation, high entropy bulk metallic glasses (HE-BMGs) have successfully prepared in some alloy systems (e.g. Ti-Zr-Hf-Cu-Ni [13], Sr-Ca-Yb-Mg-Zn [14], Pd-Pt-Cu-Ni-P [15], Ti-Zr-(Hf)-Cu-Ni-Be [16], [17]) by rapid cooling. HE-BMGs possess characteristics of both HEAs and BMGs, which make them possible to be used as high performance structural materials.
For the service safety, it is necessary to investigate the creep resistance of structural materials. The conventional standard method to determine the creep parameters of materials is uniaxial tensile or compressive creep tests, which need lots of large-sized samples and consume much time. In recent years indentation creep testing technique has been adopted to study the time-dependent deformation behaviors of small BMG samples. Till now, scholars have studied the indentation creep behavior of various HEAs (e.g. CoCrFeCuNi [18] and AlCoCrFeNi [19]) and BMGs (e.g. Ce- [20], Fe- [21], Cu- [22], Mg- [23] and Co-based BMGs [24]). However, nanoindentation creep studies of HE-BMGs are rare.
Till now, only a limited number of HE-BMGs have been developed and a large proportion of these alloys exhibit poor glass-forming ability (GFA). In our previous study [25], Ti20Zr20Hf20Be20Cu20 HE-BMG was found to be the best glass former among the available quinary HE-BMGs with a critical size of 12 mm. By the substitution of 10 at% Ni for Cu, the critical size can be further increased to 25 mm [26]. In this paper, we chose Ti20Zr20Hf20Be20Cu20 and Ti20Zr20Hf20Be20Cu10Ni10 HE-BMGs as model materials to study the creep behavior at room temperature using the nanoindentation technique. The creep mechanisms were analyzed and discussed. The effect of Ni alloying on the creep behavior of Ti20Zr20Hf20Be20Cu20 HE-BMG was also interpreted.
Section snippets
Experimental procedure
Pre-alloyed ingots with nominal compositions of Ti20Zr20Hf20Be20(Cu20−xNix) (x=0 and 10 at%) were prepared by arc-melting a mixture of pure elements (purity≥99.9%) in a Ti-gettered high purity argon atmosphere. The ingots were flipped and re-melted several times to ensure compositional homogeneity. Cylindrical rods with 5 mm in diameter and 30 mm in length were produced by copper mold suction casting. The microstructure of the as-cast samples was examined by X-ray diffraction (XRD, Rigaku
Results and discussion
Fig. 1(a) shows the XRD traces of as-cast Ti20Zr20Hf20Be20(Cu20−xNix) (x=0 and 10 at%) alloy rods with a diameter of 5 mm. No sharp crystalline peak is present, indicating the structure of the rod samples is fully amorphous. Fig. 1(b) exhibits the DSC curves of the as-cast ϕ5 mm Ti20Zr20Hf20Be20(Cu20−xNix) (x=0 and 10 at%) alloy rods at a heating rate of 20 K/min. The glass transition temperature (Tg), onset crystallization temperature (Tx), supercooled liquid region (ΔTx=Tx−Tg), melting temperature
Conclusions
In summary, the room temperature creep behavior of Ti20Zr20Hf20Be20Cu20 and Ti20Zr20Hf20Be20Cu10Ni10 HE-BMGs has been studied by nanoindentation tests. The indentation creep behavior of the studied HE-BMGs can be well described by a generalized Kelvin model. Both of Ti20Zr20Hf20Be20Cu20 and Ti20Zr20Hf20Be20Cu10Ni10 HE-BMGs shows relatively small strain rate sensitivity parameter. The creep compliance shows that the Young's modulus of Ti20Zr20Hf20Be20Cu10Ni10 HE-BMG is higher than that of Ti20Zr
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51601063, 51271095 and 51575522). The authors are also grateful to the State Key Laboratory of Materials Processing and Die & Mould Technology and the Analytical and Testing Center, Huazhong University of Science and Technology for technical assistance. One of the authors (Pan Gong) would like to thank Dr. Zhengzhi Wang, Mr. Pan Liu, Ms. Xiao-ai Cheng, and Ms. Peng Zhou for technical support and
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