Evaluation of superplastic deformation mechanism for near-α TA15 Titanium Alloy: Microstructure Evolution and Constitutive Modelling
Creators: name: Yasmeen, Tabassam
Publisher: name: Imperial College London
Year: 2020

description: TA15 (Ti–6Al–2Zr–1Mo–1V) is a type of near-α titanium alloy. It is famous for its application in the aerospace industry because of its high strength to mass ratio, high weldability, superior creep, and excellent creep resistance at high temperatures up to 550 °C. However, TA15 is challenging to be deformed and processed due to its low plasticity and narrow processing window. Hence, to exploit the superplastic deformation behaviour of TA15 in an effective manner, more fundamental research is necessary. There are limited comprehensive studies on the microstructure evolution in superplastic deformation of titanium alloys, and most studies are devoted to α/β type alloys. Until now, investigations focusing on the superplastic behaviour of a near-α alloy have not been reported in detail, especially regarding its hot deformation mechanism at a temperature above 900 °C with a low strain rate range of 0.0005-0.01s-1 . This has necessitated a more detailed microstructural study to understand the superplastic deformation mechanism of TA15 and provides important details for the constitutive modelling of the superplastic blow forming of the alloy. An attempt has been made in this investigation to achieve this employing microstructural characterization of specimens deformed in tension with temperature and strain rate conditions of 880°C/0.01s-1 , 900°C/0.01s-1 , 880°C/0.001s-1 , and 920°C/0.0005s1 . Results show that TA15 alloy exhibits excellent superplastic behaviour for all the temperature and the corresponding strain rates. The microstructure remained equiaxed at all deformation conditions, which is a typical attribute of superplastic deformation. The maximum tensile elongation of 1450% is achieved for 880 °C and a strain rate of 0.001s-1 . Flow softening is observed the low elongations at temperature 880 °C and 900 °C with a strain rate of 0.01s-1 while strain hardening is observed for the high elongations at deformation conditions 880 °C/0.001s-1 and 920 °C/0.0005s-1 . The probable deformation mechanisms such as grain boundary sliding (GBS) under different deformation conditions are discussed in terms of grain changes, geometrically necessary dislocations (GNDs) and texture evolution. It is observed that the fraction of β-phase is accelerated during the hot deformation and contributed to the enhancement of superplasticity. The results show that the dominant mode of deformation changes at initial, intermediate, and final stages of deformation. Based on the obtained deformation behaviors, a physically based constitutive model combining a series of parameters is established, including grain size evolution, iii dislocation density, void fraction, and dynamic recrystallization (DRX). The material constants for the proposed constitutive equations are determined using a Gradient-based optimisation technique. Finally, to validate the accuracy of the established constitutive model, the finite element simulation of the superplastic gas-blow forming of TA15 sheet of the industrially relevant problem is performed.