Textures and Anisotropy of Titanium Alloys

Abstract

The present paper deals with the texture, deformation mechanisms and mechanical properties of α and near α alloys (i.e. with hexagonal symmetry matrix). For items related aspects are reviewed: 1) texture formation during thermomechanical treatments depending on the composition of the alloys 2) determination of active deformation mechanisms 3) mechanical properties depending on the texture and deformation mechanisms 4) modelling and prediction of mechanical properties. For all the alloys, the texture evolution during the whole process must be considered especially during the hot rolling taking into account phase transformations. In the same way, the texture evolutions observed during cold rolling will be explained in terms of the evolution of the microstructures and deformation mechanisms. Finally, a classification of the sheet textures is made as a function of the composition of the alloys. In titanium α or near α alloys, the deformation can be accommodated by both glide and twinning. The selection of the different mechanisms activated during plastic deformation depends of the alloying elements, the grain size, grain orientations and the condition of the test (T,e,e). We present a method to determine different types of glide (prismatic, basal, pyramidal slips) and twinning systems activated as well as their relative critical shear stress ratios. The initial textures of the sheets are generally sharp after thermomechanical treatments and many properties such as yield stress are anisotropic. In addition, yield stress and ductility can be directly correlated with the deformation mechanisms activated in the sample to accommodate the deformation. The present work demonstrates the qualitative correlations between textures, deformation mechanisms and mechanical properties for all the alloys studied. Using different approaches (Taylor or Self - Consistent models), it is possible to model the texture evolution during the deformation taking into account the initial texture and the deformation mechanisms actived, and also to predict the mechanical properties of the studied alloys. Predicted and experimental curves are compared and a discussion of the most important parameters emphasises the great importance of the behavior parameters (i.e. the deformation mechanisms for successful modelling. Insufficient knowledge of the critical resolved shear stresses remains the main obstacle and explains why some results may be questionable. In conclusion the review identifies the next steps which are necessary in order to make progress when establishing relationships between textures -microstructures - mechanical properties.