On the characterisation of subsurface deformation microstructures in aerostructural titanium alloys

A research programme has been undertaken to investigate the subsurface deformation microstructures in aerostructural titanium alloys that result from industrial surface finishing processes. Microstructural analysis of the region immediately below the treated surface has been performed for high speed machined and shot peened material, with emphasis placed on characterising the mode of plastic deformation through high resolution scanning electron microscopy and electron backscatter diffraction. Both shot peening and high-speed machining result in a plastically deformed subsurface layer, with experimental evidence collected through electron backscatter diffraction suggesting that the mode of plastic deformation is influenced by local textural heterogeneities (microtexture) in addition to alloy chemistry and substrate temperature. The stability of the deformed subsurface microstructure following exposure to elevated temperature is investigated for the near-alpha titanium alloy Ti-834. Here, shot peening leads to an increase in oxygen uptake kinetics during exposure to laboratory air in the temperature range 600°C to 700°C. The increased levels of subsurface oxygen versus the non-shot peened condition were measured by secondary ion mass spectrometry and fatigue testing has suggested a lowering of the high cycle endurance limit in shot peened Ti-834 following prolonged thermal exposure in air. The outcomes of this research suggest that the mode (and magnitude) of the residual plastic strain introduced by shot peening and high speed machining may be predicted; should an improved understanding of microtexture evolution during the primary processing (such as hot-working) of titanium alloys be achieved.