Phase and Microstructure Stability of BCC-V/MAX Two-Phase Alloys With Quaternary Additives

Max phases have attracted attentions because of its high strength, high melting point and low density [1]. The present authors conducted a broad reserach for establishing the BCC-V based materials with Max phase V₂AlC as a dispresoid for strengthening. It was found that BCC-V matrix alleviates the delamination tendency of MAX phase during plastic deformation even at relatively lower temperature, however, the microstoructure stability of V-Al-C ternary alloys is not enough. In this study effect of various quaternary elements on the microstructure stabilization is investigated.<br/>Based on the V-Al-C ternary phase diagram [2], we prepared alloy ingots with and without quaternary additives such as Cr, Ti, Mo and W. An Ar-arc melting machine was used to melt the sample several times, and a part of ingot sealed in a evacuated silica tube was subjected to a heat-treatment at 1473 K for 1 week. Microstructure observation using FE-SEM and WDS analysis for determining the composition of each phase is conducted, and the EBSD analysis was also conducted to understand the crystallographic orientation relationship between BCC-V and MAX phase.<br/>It was confirmed that the alloys with and without the heat-treatment have two-phase microstructure, however, plate-like V₂AlC-MAX phase in as-cast alloys with Ti spheroidized during the heat-treatment. On the other hand, the MAX phase in alloys with Cr, Mo or W keep its plate-like structure even after the heat-treatment. It was confirmed that V has large solubility of Cr, Mo and W, while Ti tends to be in MAX phase because Ti forms MAX phase (Ti₂AlC). In our previous study it was found that the BCC-V and MAC phase has a certain crystallographic orientation relationship. The lattice mismatch between BCC-V and MAX in ternary alloys are evaluated to be about 9%, and the value increases in alloys with Ti because Ti increases the lattice constant of MAX by replacng smaller V atom. On the other hand either Mo and W increase the lattice constant of BCC-V solid solution, resulting in decreasing the lattice mismatch between BCC-V phase and MAX phase. Smaller inter-phase boundary energy between BCC-V and MAX phase caused by smaller lattice mismatch might be a reason why Mo and W stabilize the microstructure of alloys. In the presentation a part of the mechanical testing will be presented.<br/>Acknowledgement<br/>This work was supported by a grant from JSPS KAKENHI for Scientific Research on Innovative Areas “MFS Materials Science (Grant Numbers JP18H05482)".<br/>References<br/>[1] M. W. Barsoum and T. El-Raghy: Am. Sci., 2001, 89, 334-343.<br/>[2] B.Hallstedt, CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry, 2013, 41, 156-159.