Experimental study on microstructural variation in near alpha titanium alloy during non-optimal regime 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 rate and temperature. Under these conditions, titanium alloys show large elongation with a relatively stable microstructure. When the aforesaid conditions lie beyond a certain limit during deformation process, non-optimal process of superplastic deformation is observed. The microstructure changes actively during the non-optimal process of superplastic deformation. Near alpha titanium alloy has been used in the present study to find out those parameters of microstructure, which are varying significantly during non-optimal process of superplastic deformation. Superplasticity tests have been carried out at 930 degrees C with constant strain rates of 1 x 10(-4) and 5 x 10(-4) s(-1) and jump wise varying strain rates between 1 x 10(-4) and 5 x 10(-4) s(-1). Results indicated that microstructural parameters, i.e. percentage of alpha phase, number of alpha grains per unit area, average size of alpha grain, parameter of non-uniaxiality of alpha grain and grain boundary area of alpha grain, varied significantly during non-optimal process of superplastic deformation. Strain induced grain growth and deformation induced phase transformation are also observed in near alpha titanium alloy during non-optimal process of superplastic deformation. Optical microscope, micro-Vickers hardness test and X-ray diffraction have been used to characterise the microstructure of the near alpha titanium alloy.