Improvement of dynamic compression properties and strengthening mechanism of Mg-Y-Nd-Gd-Zr alloy by pre-twinning

The effects of pre-twinning on the microstructure and dynamic compression properties of non-aged and artificially aged Mg-Y-Nd-Gd-Zr alloys were investigated in this study. Previous studies have shown that in the pre-deformed T3 alloy, precompression along the extrusion direction (ED) generates high-density a > and < c + a > dislocations and deformation twins, such as the {10 (1) over bar2} < 10 (11) over bar > extension, the {10 (1) over bar1} - {10 (1) over bar2} double, and {10 (1) over bar1} < 10 (12) over bar > compression twins at 86 degrees, 38 degrees, and 56 degrees, respectively. In the pre-deformed T8 alloy artificially aged at 200 degrees C for 12 h, the dislocation density was found to decrease, with beta" (Mg3Y0.85Nd0.15) and beta' (Mg12NdY) precipitates appearing at both ends of the twin boundary. Subsequently, the precipitates extended toward [0001](Mg) and [0001](T) in the matrix region and the twin, respectively. The orientation relationships (ORs) between the beta" and beta' precipitates and the matrix/twin were determined to be (01 (1) over bar0)(beta") //(0001)(Mg/T), [2 (11) over bar0](beta")//[2 (11) over bar0](Mg/T) and (001)beta'//(0001)(Mg/T), [2 (11) over bar0](beta')//[2 (11) over bar0](Mg/T), respectively, in contrast to the counterpart ORs between the beta" and beta' precipitates and the matrix/twin in the non-deformed T61 alloy. Evaluation of the dynamic compression properties showed that compared with the compressive yield strength (CYS) of the T61 alloy, that of the T8 alloy considerably increased from 220 to 339 MPa. However, their ultimate compressive strengths (UCSs) of 363 and 360 MPa, respectively, were similar owing to the synergistic effect between the dislocations, the twin boundaries, and the precipitates. These results suggest that the pre-twinning approach should be adopted to improve the strengths of magnesium alloys.