COHERENCY STRESSES IN LAMELLAR TI-AL

General formulas are given for the coherency strains and stresses in a multilayer far from the free surfaces. The multilayer is assumed to b e a periodic stack of different elastically isotropic materials, but t here may be any number of layers in the stack and they may each have a ny thickness and any elastic constants. The results are applied to lam ellar Ti-Al alloys, in which there are shear misfits between different gamma layers and both shear and biaxial misfits between the gamma and alpha(2) layers. In a fully coherent multilayer, the stresses would b e large, in the GPa range, and in high strength, thin lamella alloys, the coherency stresses are a substantial fraction of a GPa. The shear stresses act principally on hard mode deformation systems, and the bia xial stresses place every alpha(2) lamella in biaxial compression. Thi s biaxial compression, which, for dislocation glide, is equivalent to a uniaxial tension normal to the lamella, is particularly large when t he alpha(2) volume fraction is small.