Dislocation-creep-controlled superplasticity at high strain rates in the ultrafine-grained quasi-eutectoid Ti10Co4Al alloy

Abstract

The ultrafine-grained two-phase Ti10Co4Al alloy, consisting of α-Ti(Al) solid solution with a fine dispersion of 23 vol.% of the intermetallic Ti 2Co compound, exhibits superior superplastic properties at relatively low deformation temperatures between 650 and 750 °C and high strain rates up to ε ̇ ≈ 5 × 10 −2, s −1 . Maximum m values of about 0.5 and elongations of more than 1000% were achieved. The activation energy of the rate-controlling deformation mechanism at high strain rates ( ε ̇ ⩾ 10 −3, s −1 ) is in good agreement with that reported for lattice self-diffusion of titanium in α-Ti. A grain size exponent of about p = 2 was determined at high strain rates which decreases in the regime of power-law creep. A deformation model is presented to explain the superplastic behaviour of this alloy at high strain rates. This model, proposed by Fukuyo et al., is based on grain boundary sliding, accommodated by sequential steps of dislocation glide and climb. A general flow equation for dislocation-creep-controlled superplasticity involving dislocation glide competing with climb processes is discussed. In the slightly solid-solution-hardened α-Ti(Co, Al) matrix (solid solution class II) dislocation climb becomes the rate-controlling step. The maximum m value, the grain size exponent and the activation energy for superplastic flow of Ti10Co4Al are related to the predictions of solid solution class II alloys in the above model.