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Title: Plasticity of γ-TiAl alloys
Author: Edwards, Thomas Edward James
ISNI:       0000 0004 7229 8010
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2018
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Gamma titanium aluminide alloys are emerging as a lightweight replacement to nickel superalloys, with current application in turbine stages of aero-engines, as well as in high performance automobiles and potentially the nuclear industry. The lack of toughness of its two constitutive intermetallic phases, γ-TiAl and α2-Ti3Al, prevents a conventional damage tolerant approach to fatigue lifing. To gain confidence in the use of γ-TiAl alloys and extend the temperature-stress envelope of applicability, the present work aims to achieve an understanding of the development of plasticity and flaw formation during cyclic loading. The general plasticity of a γ-TiAl alloy, Ti-45Al-2Nb-2Mn(at.%)-0.8vol.%TiB2, in compression was investigated by mapping the development of localised strain at the specimen surface. Methods were developed to produce speckle patterns for high resolution digital image correlation that were stable at test temperatures of 700 °C in air, in order to study the extent of plasticity generated by differing deformation mechanisms at application-relevant temperatures, with nano-scale resolution. At the colony scale (i.e. single stacks of co-planar α2-Ti3Al and γ-TiAl lamellae, where each stack is formed from a single high temperature disordered α-TiAl grain), macroscopic deformation bands were observed to develop at only a few percent strain. Within such bands, which propagated across many colonies of differing lamellar orientations, considerable lattice curvature and localised slip and twin operation occurred. This correlated with colony boundary failure in such bands. Twinning of the γ-TiAl phase parallel to the lamellar interfaces, longitudinal twinning, has rarely been studied, despite generalised twinning in equiaxed γ-TiAl grains being known to cause boundary decohesion. Here, the occurrence of longitudinal twinning in both microcompression and polycrystalline testpieces was investigated up to 700 °C by electron backscatter diffraction. The strength of constraint by surrounding lamellar domains was found to be the determining factor in the increased prominence of twinning at 700 °C, and hence determined whether twinning shear-induced flaws formed at colony boundaries. Using the high temperature digital image correlation strain mapping and electron backscatter diffraction techniques developed, along with transmission electron microscopy, the onset of plasticity at temperatures up to 700 °C was studied in both micro-scale and macro-scale test specimens for different lamellar thicknesses. Testpieces were loaded below the macroscopic yield stress in both monotonic and high cycle fatigue regimes, to 107 cycles, at a tensile stress ratio of R = 0.1. Longitudinal plasticity occurred in most colonies with soft mode lamellar orientations, and was located just 30 - 50 nm from lamellar interfaces. Lamellar refinement caused an increased number of slip bands to develop. In most cases, plastic strains decreased to zero by the colony boundary and strain transfer across such boundaries in high cycle fatigue was rare. At room temperature, the maximum applied stress was found to influence the number of slip bands more than the number of loading cycles.
Supervisor: Clegg, William John Sponsor: EPSRC ; Rolls-Royce ; Cambridge Commonwealth ; European and International Trust (CHESS) ; Cambridge Philosophical Society ; Worshipful Company of Armourers and Brasiers
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Titanium aluminide ; Digital image correlation ; Strain mapping ; Twinning