Deformation Mechanims of Two-Phase Ttitanium Alloys

UoM administered thesis: Phd

  • Authors:
  • Rebecca Sandala

Abstract

Two-phase Ti6246 alloy is a light weight material exhibiting very high strength at higher temperatures compared to the commonly used Ti64 alloy. This particular alloy is used at the later stages of compressor discs within the aero engines. However, compressor discs undergo a number of cyclic stresses, which could eventually lead to fatigue failure. In order to optimize the microstructure for design and lifing models, an improved understanding of the localised deformation mechanisms is crucial, particularly at the surface, as cracks can be initiated leading to failure and in turn affect the life expectancy of the component. Two-phase alloys in use have very complex lamellar microstructures comprising of a mixture of coarse and fine phases and their role in deformation can be very complex and difficult to understand. The focus of this study was particularly based on the importance of the beta phase in strengthening two-phase microstructures. Therefore, this study has been simplified to compare model lamellar microstructures, which have particular sizes of beta phase in between alpha lamellae. Digital Image Correlation along with high resolution imaging was used to develop a detailed understanding of the localised deformation in these microstructures. Widening the beta phase in-between alpha lamellae caused a more homogenous deformation, while ageing the beta phase with fine secondary alpha strengthened the microstructure. However, all microstructures showed that the single continuous alpha layer at beta grain boundaries depicted the highest amount of deformation, which can be detrimental for the life of the component. The behaviour of slip at the alpha/β interface not only depended on the size of the phases but also depended on the neighbouring crystallographic orientations and the relationship of the two phases, specifically the alignment of the close packed slip directions. Strain maps of these microstructures were subsequently related to corresponding Schmid factor maps and crystal plasticity models to improve this understanding.

Details

Original languageEnglish
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Award date1 Aug 2014