Deformation Twinning in Ti and Zr

James R. Morris, Y. Y. Ye and Kai-Ming Ho,
Ames Laboratory

Man Yoo, Oak Ridge National Laboratory

In hcp metals and alloys, deformation twinning may determine whether a material is brittle or ductile, due to the limited number of slip modes in these materials. Twinning makes Ti and Zr very ductile, and makes them stronger with increasing temperature -- a very desirable property for alloys. However, as the factors which control either twinning or slip are not well understood, it is difficult to design alloys that will be ductile.

Under c-axis compression, Ti and Zr deform almost exclusively by twinning at low temperatures. At high temperatures, c+a slip plus other twinning modes dominate. We are using large scale molecular dynamic simulations, combined with accurate first principles calculations, in order to determine the structure and energy of these twin boundaries.

We are currently examining the competing slip process, for these metals as well as for Mg, which does not show the same twinning behavior. We have used new approaches to using molecular dynamic simulations to predict stable stacking faults, and supported these calculations using first-principles calculations. Large scale simulations of dislocations, involving 9600 atoms simulated for 400,000 time steps, support our prediction for the stacking fault structure.

Phil. Mag. Letters 69, 189 (1994); Phil. Mag. A 72, 4138 (1995).