Work hardening and dislocation structure in Ta and Ta-base alloys

Abstract

The work-hardening characteristics of Ta and Ta-base alloy single crystals were investigated as a function of temperature, orientation and interstitial concentration; associated dislocation structures were characterized parallel and perpendicular to the primary slip vector. The presence or absence of stage II hardening can be related to the magnitude of the effective stress τ*. When τ*≃0, stage II hardening occurs and the work-hardening slope θII is a maximum. Asτ* increases, θII decreases, and in the limit only stage I is observed. In comparison to f.c.c. metals, there is a large degree of overshoot (∼10°), and it appears that secondary slip is a necessary condition for the existence of stage II hardening. An explanation for the dependency of stage II and the magnitude of θII on the level of τ* is advanced, based on two factors: (1) the probability of the stable reaction α/2[111] + α/2[111] = α[100] occurring increases with decreasing τ*; (2) the increase in internal stress required for secondary slip is lower at small effective stresses. These concepts are shown to be consistent with the associated structures. Increasing the solute concentration either eliminates the three stages of work hardening or reduces the rate of work hardening in stage II. Interstitial impurities lock the grown-in dislocations and/or lead to precipitates; both give rise to barriers and induce secondary slip at low strains with an associated parabolic form of stress-strain curve. Similarly substitutional atoms can lock the grown-in dislocations such that these act as barriers (e.g. tungsten additions) or reduce the number of operative sources on secondary slip systems (e.g. niobium additions).