THEORY OF WORK SOFTENING IN HIGH-PERFORMANCE ALLOYS

In conventional alloys ductility decreases with rising flow stress to negligible values at some maximum yield stress (tau(m)), whereas high-performance alloys, e.g. formed through mechanical alloying or nano-powders, can have much higher yield stresses with ductility, but tend to worksoften. A simple theory is presented, based on the LEDS concept, to account for both of these behaviors: The flow stress is tau = tau-0 + const square-root-rho with rho the dislocation density, and the workhardening coefficient is theta almost-equal-to d(tau-0)/d(gamma) + C-beta with beta the rate of glide dislocation trapping. Beta depends on the specific LEDS formed but always decreases with stress and can become negative for tau > tau(m) at artificially high-rho. Worksoftening results when unconventional manufacturing methods have produced LEDS with metastable-rho and/or tau-0 values that are higher than conform to the LEDS generated through the conventional straining conditions in testing or use.