Plasticity of Ni3(Al, Hf) above the stress peak temperature

In an attempt to understand and control the stress decrease in Ni3Al at high temperatures, monotonic deformation tests, stress relaxation experiments and TEM observations have been performed. Preliminary results show that the yield stress of 〈1 2 3〉 Hf containing single crystals (γ̇=10−4 s−1) obeys two successive thermally activated mechanisms. Between 800 and 1050 K, screw dislocations glide on the cube planes, overcoming friction forces. The corresponding activation volume is lower than 70b3 and the athermal stress is close to 320 MPa. Between 1050 and 1280 K, a loosening of the dislocation network is thought to occur by edges gliding on {0 0 1} with an activation enthalpy close to 3.26 eV/atom and an activation volume smaller than 20b3.