Orderly nucleation and competitive growth behaviors of Ti-Al intermetallic compounds in Ti/TiAl3 diffusion couple under high temperature

Abstract

Precipitation and growth of intermetallic phases during thermal processing play an important role in the formation and evolution of heterogeneous microstructure of Ti-Al intermetallic based alloys, which have not been adequately understood. Herein, we study the nucleation and growth behaviors of Ti-Al intermetallic compounds in Ti/TiAl3 diffusion couple based on high temperature annealing. We combine scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron back-scatter diffraction (EBSD) to analyze the microstructure and crystallization behavior of newly-formed intermetallic phases. The experimental results showed that phase transformations of α-Ti → Ti3Al → TiAl → TiAl2 and TiAl3 → Ti2Al5 → TiAl2 occurred orderly during annealing at temperatures from 950 ℃ to 1150 ℃. Intermetallic compounds containing Ti3Al, TiAl, TiAl2 and Ti2Al5 were formed at Ti/TiAl3 interface and comprised four distinct single-phase layers. The newly-precipitated intermetallic grains usually presented certain crystallographic orientation relationships with the adjacent phases. Growth kinetics analysis indicated that the growth of intermetallic layers was governed by volume diffusion and grain boundary diffusion, respectively, during annealing at 950 ℃ and 1050 ℃. Both interfacial reaction and volume diffusion contributed to the growth of intermetallic layers at 1150 ℃. Besides, the competitive growth between different intermetallic phases was revealed. To provide more guidance for the design of high-performance Ti-Al intermetallic based alloys, the hardness and elastic modulus of the above intermetallic compounds were determined preliminarily in the current work.