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The aim of this study was to gain an insight into the development of near-surface stress, which has been proposed to play a role in environmental embrittlement, as a function of depth after exposure. Investigations were performed, using both energy and angle-dispersive X-ray diffraction, to determine the residual stress as a function of depth for flat specimens made from two γ-based TiAl alloys in as-polished and exposed (700 °C/1 h/air) conditions. It has been found that large compressive stresses are present at the surface of as-polished specimens due to sub-surface deformation that remains despite careful grinding/polishing. After exposure the compressive residual stresses at depths below 0.6 μm are significantly reduced, probably due to the annealing out of deformation induced defects, but remain compressive in nature. However, within 0.6 μm from the outer surface, tensile stresses were present and reached a maximum of around +350 MPa at 0.15 μm depth. The extent to which the development of superficial tensile residual stress and reduced deeper compressive stress contribute to embrittlement remains unclear. [ABSTRACT FROM AUTHOR]

Abstract: Shot peening is assessed as a potential method for improving the fatigue strength of titanium aluminide alloys based on γ(TiAl). Metallographic characterization was performed with regard to surface roughness, microhardness, residual stress profiles, and structural changes occurring in the subsurface region. The fatigue performance that can be achieved by shot peening mainly depends on the compressive stresses left in the surface region, but is also governed by the fine details of the microstructure. In view of the anticipated high-temperature application of TiAl alloys, particular emphasis is placed on the thermal stability of the surface hardening developed by shot peening. [Copyright &y& Elsevier]

Abstract: The quality of wrought discs made from large Ti-45Al-5Nb-0.2B-0.2C (at. %) ingots has been assessed at various stages of processing. Such components suffer from different types of defects that can usually be traced back to structural and chemical inhomogeneities that are present in the starting ingot. In conventional manufacturing routes the imparted mechanical strain is often insufficient for complete dynamic recrystallisation, particularly for large parts. In an attempt to overcome these problems, a new multi-stage method has been developed, which has the advantages that: i) the strain is significantly higher than that imparted using conventional routes, ii) the flow direction of the material is inverted between the first and second processing stages in the sense that the length of the ingot is first increased via extrusion, and then the resulting extruded bar is shortened via axial forging in the subsequent processing stages, and iii) the geometrical constraints associated with the production of large components from ingots can be overcome. The reliability of the method has been assessed by metallographic and mechanical characterisation. The challenges that have to be addressed for the new technique to be employed for the manufacture of large components are discussed. [Copyright &y& Elsevier]