Evolution of microstructure and mechanical properties of a Ti80(CoFeNi)20 ultrafine eutectic composite during thermal processing

In this study, the influence of thermal processing on phase transformation and microstructural evolution of Ti80(CoFeNi)20 ultrafine eutectic composites was investigated. The as-cast alloy exhibited the typical hypoeutectic microstructure consisting of a number of β-Ti dendrites and the β-Ti + Ti2Ni-type ultrafine eutectic matrix. In comparison with the as-cast alloy, the alloy annealed at 1173 K for 1 h exhibited significant microstructural change from a hypoeutectic structure to a microscale dual-phase structure comprising the dispersed Ti2Ni-type phase and β-Ti matrix domain. The alloy aged at 873 K for 1 h exhibited a significant phase transformation from the β-Ti matrix to α-Ti and Ti2Ni-type phases by the eutectoid decomposition. These microstructural evolutions induced by heat treatment significantly influenced to the mechanical properties. Optimized mechanical properties (yield strength of 1530 MPa and plasticity of 5.7%) were achieved for the as-annealed alloy.