Changes in precipitate distribution during the low-stress creep of a hydrided magnesium-zirconium alloy

Microstructural changes occurring in hydrided Mg-Zr alloy (ZR55) during creep in the stress range 1.7-9.5 MPa at temperatures between 673 and 733 K have been critically examined by optical microscopy and by scanning electron microscopy. Particular attention has been given to the distribution of precipitates with respect to the grain boundaries to elucidate the operative mechanism of deformation. Geometrical features of the formation of precipitate-denuded zones and of the concurrent accumulation of precipitates have been explored in relation to the conditions of creep and the direction of tensile stress. Additional information has been obtained from microanalysis within the grains and near grain boundaries. Clear evidence is provided that, at the lower stress levels, all such microstructural changes are consistent with those expected from a deformation mechanism controlled by diffusional flow. The measured strain rates, however, exceeded those estimated from diffusional creep theory in this alloy and the microstructural observations provide some indication of reasons for this discrepancy.