Contributions to Transformation Superplasticity of Titanium from Rigid Particles and Pressurized Pores

Polymorphic materials which are subjected to an external stress while their temperature is repeatedly cycled about their phase transformation range experience, on average, Newtonian flow, which is driven by the biasing of internal transformation mismatch stresses by the external stress (1). This phenomenon, termed transformation superplasticity, is well documented for metals (as reviewed in Ref. (2)) and has recently been demonstrated for metal-matrix composites (MMCs) with a polymorphic matrix and a discontinuous, non-transforming reinforcement phase (3–5). The first report of transformation superplasticity of a MMC was that published by Dunand and Bedell (D&B) (3), who examined commercialpurity titanium (CP-Ti) reinforced with 10 vol. % particulate TiC. Furthermore, as compared to unreinforced CP-Ti specimens at the same stress, they observed an enhancement of the superplastic strain increment produced upon each thermal cycle for the composite. They attributed this effect to increased mismatch produced during the phase transformation of the matrix due to the presence of the rigid, non-transforming reinforcement phase. In our current investigation of transformation superplasticity of materials with the same nominal composition as those used in the original study of D&B (3), we have become aware of a microstructural feature of the experimental material which affects the superplastic strain increments, and which results in differences between the data of the two studies. In the present paper, we report the details of this effect and critically re-examine the original conclusions of D&B (3).