M. Jackson, M. Thomas, University of Sheffield, Sheffield, United Kingdom; T. Lindley, Imperial College London, London, United Kingdom; S. Turner, University of Sheffield, Rotherham, United Kingdom
The majority of aerospace titanium alloy components will be high performance machined and/or peened using laser shock or mild steel shot. Such techniques impart a high degree of surface deformation at very high strain rates. Peening, for example, is an established technique designed to impart compressive residual stresses that provide enhanced damage tolerance. However, recent work at Sheffield and Imperial has shown it to be deleterious to the surface microstructure of the component, providing enhanced oxygen diffusion kinetics and leading to a loss of creep strength. For example, the generation of deformation twins in Ti-834 during peening has been shown, for the first time, to provide short-circuit diffusional pathways for oxygen diffusion. The effect of shot peening and high performance machining on the surface microstructure and subsequent damage tolerance for a range of titanium alloys will be presented. The deformation mechanism with regard to alpha-beta morphology and texture has been investigated using scanning electron microscopy and electron backscatter diffraction. The effect of thermal cycling during service on the oxygen diffusion kinetics has also been determined and measured using secondary ion mass spectrometry.
Summary: The microstructural response of widely used surface conditioning processes such as shot peening and high performance machining (HPM) on a range of aerospace titanium alloys will be presented. The subsequent effects of the different processes on the rate of oxygen diffusion kinetics and alpha case formation will also be presented. The paper will demonstrate how the original forged alpha beta morphology and microtexture can have a direct impact on service properties of shot peened and HPM titanium alloys.
