Phase separation and ω transformation in binary V-Ti and ternary V-Ti-Cr alloys

Abstract

V-Ti-Cr alloys have emerged as the potential candidate for many advanced engineering applications e.g. first wall blanket for fusion reactors, alternative energy technologies and hydrogen storage. Phase transformation behaviour of these alloys is not completely understood and the binary V-Ti phase diagram is still debated. The system becomes even more complicated with the addition of Cr as a ternary addition. Thermodynamic calculations indicate that within certain composition range in the V-Ti system, a prominent miscibility gap exists and the propensity for phase separation decreases with the addition of Cr in the binary system. Alloys with nominal composition of V-50Ti and V-40Ti-20Cr have been prepared through vacuum arc melting and detailed microstructural and microchemical studies have been carried out. Diffraction contrast and phase contrast microscopy studies confirm the occurrence of phase separation in both the alloys. The lamellar and interconnected nanostructured domains with coherent interfaces as seen in the phase contrast mode are indicative of possible phase separation in the alloys. It has further been confirmed by High-angle annular dark field (HAADF) studies associated with STEM-XEDS and STEM-EELS. Further, microscopic studies on the as melted and the heat treated alloys confirm that the phase separation propagates through a spinodal decomposition mechanism in this alloy and is not limited by nucleation and growth kinetics. Another interesting aspect of this study is the formation of nanostructured ω phase in the phase separated regions. Phase contrast microscopy studies in selected orientations confirm the presence of distorted hexagonal zones of ω phase in the phase separated domains of the alloys. © 2016 Acta Materialia Inc.