13.1 CRYSTALLOGRAPHIC BASIS

The primary cause of anisotropy of plastic properties is preferred orientation of grains (i.e., statistical tendency for grains to have certain orientations). ^[*] Mechanical working of metals produces preferred orientations or crystallographic textures. Recrystallization during annealing usually changes the crystallographic texture but doesn t cause randomness. (Repeated heating and cooling through the ????? transformation may be a possible exception.) How crystallographic textures develop and how texture influences the anisotropy are not treated in this text. The anisotropy of textured titanium sheet will be considered, however, because of the insight it gives to the problem.

Alpha-titanium alloys have a hexagonal close-packed crystal structure. Deformation occurs primarily by slip in close-packed directions, which lie in the basal plane parallel to the edges of the hexagonal cell as shown in Figure 13.1. Whether slip occurs on the (0001) basal planes or on the prism planes, there is no strain parallel to the c-axis because the slip direction is normal to the c-axis. Not even slip on the pyramidal planes will cause c-axis strain.

Figure 13.1: Idealized texture of an ?-titanium sheet with the (0001) plane parallel to the plane of the sheet. Although there are several possible slip planes, slip is restricted to the family of slip directions, all of which lie in the (0001) plane, so slip can cause no thickening or thinning of the sheet. From W.F. Hosford and W.A.Backofen in Fundamentals of Deformation Processing (Syracuse University Press,...