Design of dual-phase refractory multi-principal element alloy

Abstract

Refractory multi-principal element alloys (MPEAs) have gained attention as a next generation high temperature structural material for their outstanding mechanical properties at elevated temperature. Recent research showed that lack of ductility and toughness of refractory MPEAs at room temperature, one of their major application drawbacks, could be overcome in expense of strength by manipulating valence electron concentration (VEC). To further improve their mechanical properties at elevated temperature, precipitation hardening by coherent precipitate is promising approach. It is reported that addition of Al induces formation of coherent B2 / BCC dual phase microstructure in number of refractory MPEAs. Most of reported B2 / BCC dual phase refractory MPEAs have continuous B2 matrix with BCC precipitates, which is not beneficial in ductility and toughness at room temperature. In achieving combined property of room temperature ductility and high strength at elevated temperature, alloy with inverted microstructure of BCC matrix and B2 precipitates should be designed.

Currently, there exists two major obstacles in designing refractory MPEAs with such microstructure. Compositional complexity of refractory MPEAs make it cost-consuming to find composition with desired characteristics by conventional trial-and-error approach. Microstructure with B2 matrix / BCC precipitate is known to be formed by series of spinodal decomposition during slow cooling after homogenization, which requires alternative heat treatment method to be avoided. These obstacles suggest demand for novel design method, capable of manipulating desired microstructure with higher efficiency. Here we suggest partitioning behavior based alloy design approach, a novel method consisting of composition design by mixing base alloys and ageing at fixed temperature. In purpose of validating the design method, refractory MPEA with BCC matrix / B2 precipitate microstructure and room temperature ductility was designed using base alloys, namely Ti-25 at. % Nb and HfAl.

As a result, novel $Ti_{52.5}Nb_{17.5}Hf_{15}Al_{15}$ (TN70) alloy with desired BCC / B2 phase forming ability and proper phase stability was found out of 3 compositions investigated. By systematic ageing condition optimization, it was found that TN70 alloy is forming discontinuous and coherent B2 precipitate with size of ~0.8nm evenly distributed BCC matrix after ageing over 24 hours at $900^\circ C$. Under compressive stress at room temperature, BCC / B2 dual phase TN70 alloy exhibited excellent ductility of >60% combined with yield strength of 1070MPa, comparable to 1058MPa of previous known brittle refractory MPEA NbMoTaW. In addition, triple phase microstructure of HCP / BCC / B2 phase was found to be formed at lower ageing temperature. This microstructure, known as alpha / beta microstructure in Ti alloys is known to be beneficial in mechanical strength and creep resistance. Systematic ageing condition investigation and thermodynamic calculation showed that phase stability of B2 and HCP phases could be manipulated by controlling composition of each base alloys, and mixing ratio between them.