Constitutive Modeling And Computation Of High Temperature Viscoplastic Deformation Of β Titanium Alloy

Many high temperature mechanical tests ofβtitanium alloys show a common feature that the stress-strain curves exhibit a sharp initial peak stress followed by constant flow stress or flow softening. In this paper, high temperature deformation behavior of beta titanium alloy Ti-20V-4Al-1Sn sheet is studied by performing uniaxial tension experiments at three different strain rates at high temperatures of 700°C, 750°C and 800°C. The stress-strain curves at all temperature cases show strain rate sensitivity, yield point phenomena and continuous flow softening patterns. Microstructures of deformed specimens at several representative deformation stages and different strain rates are studied using optical microscope. Dynamic recovery does not occur at the early stage of deformation including yield-point and the subsequent yield drop regime, but it is activated at large deformation stage, where it is affected by strain rate and strain.As a first approach, a viscoplastic constitutive model based on the assumption of rapid dislocation multiplication is proposed to describe such high temperature yield-point phenomena. In this modeling, the softening effect due to dynamic recovery is also considered. The stress-strain responses predicted by the constitutive model well capture the yield-point phenomena, strain rate sensitivity and subsequent continuous flow softening behavior of the beta titanium alloy.The constitutive model is further implemented into commercial FE code by using implicit return mapping algorithm. The numerical implementation is constructed in a corotational frame under hypoelastic hypothesis to describe large deformation behaviors. The tangent matrix is also deduced in this paper to make the numerical calculation giving excellent convergence and stable results. The FE simulation results fit well with analytical calculation even at large time step. The generality of the model and the implicit algorithm makes it possible to extend the theory to other titanium alloys and the materials that have yield-point phenomena.