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.