Low-Temperature Coarsening and Plastic Flow Behavior of an Alpha/Beta Titanium Billet Material with an Ultrafine Microstructure

Abstract

The influence of microstructure evolution on the low-temperature superplasticity of ultrafine alpha/beta titanium alloys was established. For this purpose, the static and dynamic coarsening response and plastic flow behavior of Ti-6Al-4V with a submicrocrystalline microstructure were determined via a series of heat treatments and uniaxaial compression tests at temperatures of 650 °C, 775 °C, and 815 °C. At all test temperatures, static coarsening exhibited diffusion-controlled (r ³  vs time) kinetics and followed a dependence on phase composition and volume fraction qualitatively similar to previous observations at 850 °C to 950 °C. Dynamic coarsening at 775 °C and 815 °C and strain rates of 10⁻⁴ and 10⁻³ s⁻¹ were similar to prior higher-temperature observations as well in that the kinetics were approximately one order of magnitude faster than the corresponding static behaviors. The increase in coarsening rate with superimposed deformation was attributed to the enhancement of diffusion by dislocations generated in the softer beta phase. With respect to deformation response, plastic flow was superplastic with m values of ∼0.6 at 650 °C, 775 °C, and 815 °C and strain rates of 10⁻⁴ and 10⁻³ s⁻¹. Dynamic coarsening resulted in flow hardening at both temperatures and strain rates for a short preheat time (15 minutes) but was noticeably reduced when a longer preheat time (1 hour) was used prior to testing at 10⁻³ s⁻¹. The latter behavior was largely attributed to noticeable static coarsening during preheating. A generalized constitutive relation based on a single stress exponent and the instantaneous alpha particle size was shown to describe the superplastic flow of ultrafine Ti-6Al-4V at low and high temperatures.