Material characterization, constitutive modelling, and processing map for superplastic deformation region in Ti-6Al-4V alloy

Abstract

Superplastic deformation behavior plays a significant role in the manufacturing of light and complex shaped components, and particularly, the superplastic behavior of Ti-6Al-4V alloy has different fields of applications such as hollow fan blades used in a gas turbine engine and high-performance heat exchangers. To study this, uniaxial tensile tests have been conducted within a temperature range of 700 to 900 °C at different strain rates, 0.01/s, 0.001/s, and 0.0001/s. The test results show more than 50% elongation in general and more than 200% elongation from 750 to 900 °C at 0.0001/s strain rate, representing the superplastic deformation behavior in Ti-6Al-4V alloy. The fractured specimens have been characterized by means of an optical microscope, scanning electron microscope, and X-ray diffraction techniques. Microstructure analysis confirms coarsening of grain size and variation in volume fraction of β with temperature, while SEM study clearly indicates ductile fracture with improved amount of dimples and flow lines at elevated temperatures. X-ray diffraction results indicate that the basic peaks position remains the same, but parameters vary due to superplastic deformation behavior. To accurately estimate the flow stress behavior, modified Arrhenius model has been developed and found to have the correlation coefficient (R) as 0.9939 when compared with experimental flow stress. Furthermore, by using the flow stress data, processing maps have been developed for analyzing the superplastic deformation behavior based on the efficiency and flow instability region at different elevated temperatures and strain rates. Processing maps clearly show excellent efficiency of power dissipation without any presence of flow instability in the superplastic deformation domain, i.e., from 770 to 900 °C temperature range and at 0.01–0.0001/s strain rate.