Increased structural stability in twin-roll cast AZ31 magnesium alloy processed by equal channel angular pressing

The present paper reports an effect of ECAP on the microstructure of the AZ31 magnesium alloy prepared in two conditions: conventionally cast and twin-roll cast. Subsequently, the thermal stability of fine-grained conditions was investigated with a special regard to microhardness, grain structure and dislocation density changes. Similar processing conditions of ECAP resulted in achieving similar average grain size for both initial conditions regardless differences in the initial microstructure. The only difference in the microstructure of a fine-grained condition was a distribution of beta-Al12Mg17 secondary phase particles. Isochronal annealing in the temperature range 160-500 degrees C showed differences in the response of individual fine-grained samples to the temperature increase. It was proven that these differences primarily originated from the different distribution of secondary phase particles, which significantly affected static recovery and grain growth. Consequently, the thermal stability of fine-grained structure was much better in twin-roll cast samples in the temperature range 220-340 degrees C. Exceeding 340 degrees C, accelerated dissolution of beta-Al12Mg17 phase resulted in a similar evolution of all studied parameters showing that distribution of secondary phase particles is a crucial parameter of thermal stability of fine-grained AZ31 magnesium alloy.