Solid solution creep behavior of Sn-xBi alloys

Abstract

This study details the steady-state creep properties of Sn-1 wt pct Bi, Sn-2 wt pct Bi, and Sn-5 wt pct Bi as a function of stress and temperature. All data, including previous work on pure Sn, are described by the following empirical equation: [1] $$\dot \varepsilon _{ss} = \frac{{CE}}{{RT}}\sinh \left( {\frac{{\alpha \sigma }}{E}} \right)\exp \left( {\frac{{ - Q}}{{RT}}} \right)$$ Equation [1] describes steady-state creep where at low strain rates there is linear stress dependence and at high strain rates there is an exponential stress dependence. The transition in creep behavior occurs at a critical, breakaway stress, σ c =E/α. This stress is compared to the breakaway stresses proposed by Friedel and by Cottrell and Jaswon. There is good agreement at low solute concentrations to the breakaway stress proposed by Friedel, but σ c is significantly lower than the breakaway stress predicted by Cottrell and Jaswon. Several observations suggest that for Sn-xBi alloys, dislocation climb is the rate-limiting mechanism in the nonlinear region. First, the stress sensitivity of the steady-state strain rate data is similar to that of pure Sn, where dislocation climb is known to be the rate-limiting mechanism. Second, primary creep is observed throughout the tested stress range. Third, incremental additions of Bi decrease the steady-state creep rates, even though Bi has a higher diffusivity in Sn than Sn by self-diffusion.