Microstructural stability of γ′ strengthened Co–Cr–Ti alloys

Abstract

A study has been made of the microstructural stability of various Co–Cr–Ti alloys. The alloy Co–16Cr–5Ti, when solution treated and cooled, forms a microstructure consisting of a homogeneous precipitation of Co₃Ti (γ′) particles in an fcc (γ) matrix. This structure remains relatively stable during heat treatment between 600 and 800°C. The addition of 3%Mo to the alloy causes transformation of the structure in this temperature range both by an fcc → hcp transformation and by discontinuous reprecipitation. The kinetics of these transformations are reported. The addition of 11%Ni to the Mo containing alloy suppresses the fcc → hcp transformation but not the discontinuous transformation. The fcc → hcp transformation, though martensitic in appearance, exhibits thermally activated kinetics. Possible explanations for this are discussed and possible reasons for the enhancement of the discontinuous transformation by Mo are also examined.