EXPERIMENTAL STUDY ON VARIATION OF MICROSTRUCTURAL PARAMETERS IN NEAR ALPHA TITANIUM ALLOYS UNDER NON-STATIONARY PROCESS OF SUPERPLASTIC DEFORMATION

Abstract

Superplasticity is generally achieved in titanium alloys with a fine grained microstructure when deformation is carried out under narrow ranges of strain rates and temperature. Under these conditions, titanium alloys show large elongations (i.e. optimal superplasticity) with a relatively stable microstructure. In superplastic deformation (SPD) process, strain-rate sensitivity exponent (m) is a critical parameter, which value quantifies the optimal superplasticity. In real industrial applications, maintaining these narrow ranges of condition required for optimal superplastic flow from beginning to end is a very difficult and challenging task. When the aforesaid conditions lie beyond a certain limit (i.e. at near superplastic regime) during deformation process, non-stationary process of superplastic deformation is observed. The microstructure changes actively during the non-stationary process of superplastic deformation. Near alpha titanium alloy has been used in this study to find out those parameters of microstructure, which are varying significantly during non-stationary process of superplastic deformation. Tensile tests have been carried out at 930 degrees C with a constant strain rate of 5*10(-4) s(-1) and a jump wise varying strain rate of 1*10(-4) s(-1) & 5*10(-4) s(-1). Results indicated that microstructural parameters i.e. percentages of alpha phase, number of alpha grains per unit area and size of alpha grain varied significantly during non-stationary process of superplastic deformation. Deformation induced phase transformation is also observed in near alpha titanium alloys during non-stationary process of superplastic deformation. Optical microscope, Scanning electron microscope, micro Vickers hardness test, XRD, etc have been used in order to characterize the microstructure of the material.