Maximum m superplasticity deformation for Ti-6AI-4V titanium alloy

Material superplasticity is a state in which solid crystalline material is deformed well beyond its usual breaking point, usually with the elongation greater than 200% in tensile test. Such a state is generally achieved under specific microstructure and deformation conditions. In superplastic deformation (SPD) process, strain-rate sensitivity exponent (in) is a critical parameter, which represents, articulates and quantifies the superplasticity level. The higher the in value, the better the superplasticity of the materials. In this paper, a totally novel concept of maximum m superplasticity deformation (MaxmSPD) is proposed and realized through simultaneous measurment of m value and concurrent control of deformation strain rate in such a way that the maximum in is always maintained in the deformation process, through dynamic control and adjustment of deformation strain rate. To prove and realize this concept, the MaxmSPD of Ti-6Al-4V titanium alloy is conducted. The experimental results show that without grain refinement processing, the superplasticity of Ti-6Al-4V alloy is significantly enhanced compared to conventional SPD process in which the deformation strain rate is kept constant. In details, the elongation for Ti-6Al-4V in MaxmSPD is increased from 421 to 523%. Furthermore, the elongation of Ti-6Al-4V at 900 degrees C is 641%, which is almost double the value in conventional SPD process. Therefore, these interesting findings and the development of MaxmSPD not only simplify the requirements on grain refinement processing for SPD process, it also pioneers a totally new approach for realization of superplastic deformation and extends the application scenarios of SPD process. (C) 2007 Elsevier B.V. All rights reserved.