Grain boundary diffusion creep in magnesium

Abstract

The creep of pure polycrystalline magnesium has been investigated at stresses beluw 6 MPa and temperatures from 150 to 325°C using both spring and tensile specimens. Examination of the grain size and temperature dependence of the creep rate revealed that, at the lower stresses, deformation occurred by the grain boundary diffusion creep mechanism of Coble, although the rate depended upon stress in a Bingham rather than a Newtonian manner, i.e. there was a threshold stress below which no diffusion creep occurred. At the lowest temperatures, threshold stresses ∼ 1 MPa were found in fine-grained material, decreasing to ∼0.1 MPa as the grain size and temperature were increased. Existing theories were unable to account for the magnitude of the threshold stress. Calculation of magnesium grain boundary self-diffusivity from the creep data gives a value of wDgb = 8 × 10⁻¹⁰ exp -(l05 × 10³/RT) m³ S⁻¹ where R is in J mol⁻¹ K^−l and w is the grain boundary width.