Micromechanics of fully lamellar TiAl alloys: effect of lamellar orientation and width

Abstract

Titanium aluminide alloys with Al concentrations between 35 and 49 at.% (γ-TiAl alloys) exhibit a wide range of microstructures formed by the two intermetallic phases γ(TiAl) and α2 (Ti3Al). Among these microstructures the so-called fully lamellar, consisting on the arrangement of γ and α2 lamellae into colonies, have the potential to replace heavier materials in high temperature structural applications such as automotive and aerospace engine components due to their excellent mechanical response. The increasing demands on modern turbines require microstructural optimisation of γ-TiAl alloys. However, this is a complex task, as the mechanical behaviour of these materials is determined by multiple microstructural parameters such as the colony size, the lamellar size and orientation, the domain size, etc. Isolating the influence of individual microstructural parameters has, to date, not been achieved successfully. The main scope of this thesis is to understand both the e ect that lamellar orientation and thickness have on the response of isolated colonies, as well as dislocation-interface interactions in a Ti-45Al-2Nb-2Mn (at.%) + 0.8(vol.%) TiB2 (Ti4522XD). With this aim, first, micropillars with lamellae oriented at 0°, 45° and 90° with respect to the loading direction were compressed at room temperature. The results revealed a large plastic anisotropy, that was rationalised, based on slip/twinning trace analysis, according to the relative orientation of the main operative deformation modes with respect to the lamellar interfaces. Loading at 45° resulted in the activation of soft longitudinal modes, where both the slip plane and the slip direction were parallel to the interfaces, and therefore, little interaction of dislocations with lamellar interfaces take place. At 0° loading, deformation was mainly accommodated by harder mixed deformation modes (with an oblique slip plane but a slip direction parallel to the lamellar interfaces), although the lamellar interfaces seemed to be relatively transparent to slip transfer. 90° loading represented the hardest mode and deformation was accommodated by the activation of transverse deformation mechanisms, confined to individual lamellae, together with longitudinal ones that were activated, due to their softer nature, despite their very small Schmid factors. In addition, a thorough study of pillar size efects revealed that the results were insensitive to pillar size for dimensions above 5 μm. The results can therefore be successfully applied for developing mesoscale plasticity models that capture the micromechanics of fully lamellar TiAl microstructures at larger length scales. Second, a fully lamellar microstructure with a nanoscale lamellae width was developed by, rst, heat treating the as-received material within the α domain, followed by rapid cooling at cooling rates of 2000 oC/s in a Gleeble system, and finally low temperature ageing. The strengthening induced by the nanoscale lamellar thickness, investigated as a function of lamellar orientations by micropillar compression, was found to be even higher than what can be achieved by high Nb alloying. Finally, a microtensile device, which allows to test specimens as small as 5 μm in length, was designed for the present thesis research. A microtensile specimen of the Ti4522XD alloy was tested in tension and the deformation mechanisms were investigated. The results revealed that lamellar interfaces are relatively transparent to dislocation propagation. Order variant interfaces present fully geometrically compatible slip systems that do not require a change in the Burgers vector of the transmitted dislocation. In contrast, in true twin and pseudo-twin interfaces, the geometrical alignment between incoming and outgoing dislocations is not perfect, but plays an important role on determining which slip systems are activated, being the preferred outgoing system that with the maximum geometrical alignment. The work presented in this thesis served to successfully isolate the effect of lamellar orientation and width in a γ-TiAl alloy, and to quantify their implication on the mechanical behaviour of isolated colonies. The methodology developed in this work can be further extended to study the effect of other microstructural parameters, whose results can be applied for developing mesoscale plasticity models that capture the micromechanics of fully lamellar TiAl microstructures at larger length scales.