Yield stress in niobium-nitrogen solid solutions

Abstract

The yield point and associated parameters have been studied in electron-beam-refined niobium containing amounts of nitrogen ranging from 60 to 850 wt. p.p.m., tested at room temperature and strain rates from 1 × 10 −4 to 1.3 × 10 −1 sec −1. The upper and lower yield stresses and the lattice friction stress show three stages of solution hardening, whilst the yield point drop and the k y parameter show one stage only. A comparison of solution-hardening coefficients of the present niobium-nitrogen alloys with those for niobium-oxygen alloys 6 shows that nitrogen is twice as effective as oxygen as a solution hardener. The solution-hardening coefficients for the upper- and lower yield stresses and the lattice friction stress decrease, whilst those for the yield point drop and the k y parameter increase, with increasing strain rate. The yield point and associated parameters exhibit similar strain rate dependence to that observed in niobium-oxygen alloys 6. The dislocation velocity exponent ( m), as calculated from the strain-rate dependence of the upper and lower yield stresses using the equation log σ = log B + ( i m ) log ε ̇ , shows a relationship with nitrogen concentration ( C) of the form, m = m 0 + β C 1.3.