Abstract
Titanium aluminides find application in modern light-weight, high-temperature turbines, such as aircraft engines, but suffer from poor plasticity during manufacturing and processing. Huge forging presses enable materials processing in the 10-GPa range, and hence, it is necessary to investigate the phase diagrams of candidate materials under these extreme conditions. Here, we report on an in situ synchrotron X-ray diffraction study in a large-volume press of a modern (α2 + γ) two-phase material, Ti-45Al-7.5Nb-0.25C, under pressures up to 9.6 GPa and temperatures up to 1686 K. At room temperature, the volume response to pressure is accommodated by the transformation γ → α2, rather than volumetric strain, expressed by the apparently high bulk moduli of both constituent phases. Crystallographic aspects, specifically lattice strain and atomic order, are discussed in detail. It is interesting to note that this transformation takes place despite an increase in atomic volume, which is due to the high ordering energy of γ. Upon heating under high pressure, both the eutectoid and γ-solvus transition temperatures are elevated, and a third, cubic β-phase is stabilized above 1350 K. Earlier research has shown that this β-phase is very ductile during plastic deformation, essential in near-conventional forging processes. Here, we were able to identify an ideal processing window for near-conventional forging, while the presence of the detrimental β-phase is not present under operating conditions. Novel processing routes can be defined from these findings.
Highlights
Titanium aluminides exhibit significant potential as a low specific weight structural material for high-temperature automotive and aerospace propulsive applications [1,2,3,4]
Diffraction patterns was accurately determined by the use of the equation of state of the pressure markers [51,52], using were taken while holding at 0, 3.2 and 9.6 GPa, respectively, before the system was heated under the PDIndexer software (V4.32, Yusuke Seto, Kobe University, Kobe, Japan, 2016) [53]
From a thermodynamic point of view, response to high pressure is given by the equation of state, relating the volume or density of the matter to pressure p, temperature T and potentially other variables
Summary
Titanium aluminides exhibit significant potential as a low specific weight structural material for high-temperature automotive and aerospace propulsive applications [1,2,3,4]. Heating into a multi-phase field results in sluggish grain growth, while the augmented fraction of β-phase allows for near-conventional forging [7] This has been partly achieved by alloying γ-based TiAl alloys with β-stabilizing elements, such as Nb and Mo; some residual β/β0 -phase is still present at the operating temperature [4,8,9,10]. We investigate the influence of pressure on the formation of the ductile β-phase in γ-based TiAl alloys, which is of fundamental interest, but is most relevant to modelling high-pressure deformation techniques, such as high-pressure torsion, in order to achieve severe plastic deformation [11,12,13,14,15] and high-pressure near-net-shape forging [16,17,18,19] These deformation processes operate at pressures up to 7 GPa and forces exceeding 1 GN, respectively. The chemistry of titanium alloys may change at the surface layer, such as the depletion of aluminium, as observed in a vacuum [40] or influenced by the pressure medium, as well as uni-axial stress components [48]
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