ON STATIC STRAIN AGING

The phenomenon of static strain ageing (SSA) is reconsidered in order to clear otu contradictory experimental results and controversies with their interpretation. The constitutive model used includes the evolutionary behaviour of the dislocation densities and the transient character of ageing kinetics. The numerical solutions for the ageing peak recover all the different types of the strain and ageing time dependence reported in the literature. The SSA properties of b.c.c. alloys, f.c.c. alloys, in particular interstitial Ni-C, and the h.c.p. Ti alloys containing oxygen can be rationalized. It is concluded that there is no need to assume a vacancy mechanism to explain the observed SSA behaviour. It is rather the evolution of the mobile and forest dislocation densities which determines the strain dependence of static as well as of dynamic strain ageing.