Allotropic transformation induced stacking faults and discontinuous coarsening in a γ-γ′ Co-base alloy

• High temperature long-term stability of Co–Cr–Ti–Mo alloy has been studied.
• The alloy initially comprised of an A1–Co matrix and L12-Co3Ti precipitates.
• Microstructural instabilities caused by continuous and discontinuous reactions.

The stability of a Co-based alloy designed to possess a microstructure comprising of L12, γ′ Co3Ti-type precipitates embedded in an A1, γ Co solid solution matrix has been investigated. The alloy showed acute microstructural instabilities upon ageing at 700 °C, resulting in the degeneration of the γ-γ′ aggregate into i) a faulted Co-based martensite and Co3Ti and ii) a lamellar aggregate of A3-Co and Co3Ti. The faulted Co-based phase was formed by isothermal diffusionless transformation of the metastable A1-phase, whilst the lamellar aggregate was a discontinuous reaction product.