Flow stress of niobium-oxygen and niobium-nitrogen alloys

Abstract

The effect of oxygen and nitrogen concentration on the room temperature flow stress of niobium has been studied at strain rates from 5 × 10 −5 to 2 × 10 −1 sec −1. The flow stress increases linearly with increasing solute content. The solution-hardening coefficients (α) for both oxygen and nitrogen are strain-rate independent, but vary with plastic strain (ε) according to the equation α = Aε a , where A and a have different values for each solute. As in the case of hardness and yield stress, nitrogen is twice as effective as oxygen as a solution hardener. The factor of two decreases with increasing plastic strain. Over the interstitial solute concentration range for direct solution hardening the dislocation velocity exponent ( m′), as calculated from the strain rate dependence of flow stress using the equation log σ = log B + ( 1 m′ ) log ε ̇ , varies with plastic strain and oxygen and nitrogen concentrations.