YIELDING OF AN ALLOY HARDENED BY COHERENT AND ORDERED PRECIPITATION

Hardening of a Ni3(TiAl) alloy with ordered coherent particles, free of interface strains, has been studied as a function of γ′ particle size. There is a definite average size ̄gFvc below which particles are sheared by dislocations and above which dislocations pass around the particles and form a loop Tensile curves analysis and activation volume measurements showed that in both cases, hardening caused by precipitation is essentially athermal. This result is in good agreement with the theoretical evaluation of the activation energy necessary to pass the particles, which has been carried out. A model was developed, which involves the average size of the particles as well as their size distribution. This model accounts for the rapid increase of the yield stress relative to the first case; and for the second case, it predicts a yield stress decrease slower than that given by Orowan's law. © 1969.