The effect of macrozones in Ti-6Al-4V on the strain localisation behaviour

UoM administered thesis: Phd

Abstract

Ti-6Al-4V is the most widely used titanium alloy and is typically used in stages of gas turbine engines, due to its high strength-to-weight ratio, corrosion resistance and high strength at moderate temperatures. However, the alloy is susceptible to the development of strong textures during thermomechanical processing that leads to a preferred crystallographic orientation. These are referred to as macrozones and are thought to develop during the β to alpha phase transformation, as a result of the retention of large prior β grains during processing and variant selection. Macrozones are clusters of neighbouring grains with a common crystallographic orientation that may act as one single grain during loading and have been shown to cause scatter in the fatigue life.The focus of the current work was based on the analysing the strain behaviour of soft, hard and no macrozones within the microstructure, during various loading conditions. The local strain behaviour was studied at a micro and nanoscale, using the digital image correlation (DIC) technique, which utilises microstructural images recorded during mechanical loading. On a microscale, the no-macrozone and strong-macrozone condition loaded at 0% exhibited homogeneous strain behaviour. The strong-macrozone condition loaded at 45% and 90% to the extrusion direction, respectively, developed pronounced high strain bands correlating to regions that were favourably oriented for prismatic and basal slip, respectively. Characterisation of the slip bands provided a detailed understanding of the deformation behaviour at the nanoscale and the slip system was subsequently determined for each grain using slip trace analysis. Prismatic slip was the dominant slip system in all conditions, particularly in the soft-oriented macrozone regions of the strong-macrozone condition loaded at 45 degrees. Shear strains of 10 times the appliedstrain were observed. Further investigations on the strong-macrozone condition loaded at 45 degrees to ED during standard and dwell fatigue demonstrated early failure in the dwell sample, with higher strain accumulation for dwell.

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Original languageEnglish
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Award date1 Aug 2015