Combinatorial alloy design of heat-resistant refractory high entropy alloys and their basic materials properties characterization

Abstract

A preliminary study of feasibility of high-entropy alloys (HEA) for application in high-temperature environment, for instance, in nuclear industry and aerospace engineering, is conducted based on the theoretical knowledge and experimental results. At the outset of our research work refractory elements were chosen as the constituents of an alloy system. There is a need to propose an efficient technique for the experimental design since there are infinite combinations of the fabricable alloys. Various methods such as artificial neural network, genetic algorithm, Taguchi method, combinatorial synthesis and other approaches were recently used to explore new materials in different disciplines. In this study a combinatorial technique was chosen to assess the refractory high entropy alloy system, $Al_x Cr_y Mo_z NbTiZr$, and narrow down until we get the candidate alloys in terms of high-temperature strength and oxidation resistance. The combinatorial library consists of 12 different alloys with variation of Al,Cr and Mo. It is believed that this variation will lead to examine all the alloys in view of the above-mentioned material properties. For example, changes in Al- and Cr-content enable us to see the trends in oxidation resistance, while alteration of Mo-content may forward one to observe tendency in heat resistanceof the alloy system. NbTiZr (60at%) is fixed in an equiatomic proportion (20at%) because of their thermodynamic stability within each pair. It was found out that the combinatorial library consists of BCC crystal structure and intermetallic phases such as B2 and C14-Laves phases. At room temperature (RT) the yield strength of the library are in the range of 268.7 and 1286 MPa while at T=1000°C it is between 101.0 and 486.0 MPa. Additionally, at RT Alloy-11 (Al,Cr,Mo) = (0,20,20at%) has a plastic strain of $\varepsilon_{p}$>5% and at T=1000$^\circ C$ its yield strength was highlighted to be the largest in the combinatorial library (486.0 MPa) which is comparable with current superalloys. Moreover, a higher Mo-content (30at%) and/or absence of Al/Cr-content lead to the formation of volatile oxides (i.e. MoO3) after a prolonged oxidation test (t=20h) in a static air condition at T=1000$^\circ C$. This work revealed that combinatorial technique is the potential strategy to examine mechanical properties such as Vickers Hardness and compression strength at different temperatures along with oxidation resistance for suitability as a structural material for turbine blades, engines and GEN-IV nuclear power plant component.