Bo Shi

Cryogenic thermal cycling (CTC) treatment is a promising technique to rejuvenate metallic glasses (MGs), but whose effect is usually highly dependent on the original structure. Spanning shear bands and nonspanning shear bands in deformed MGs not only have different structures, but also are situated in different strain fields, so their responses to CTC are expected to be different. In the present work, the structure, energy state, and property evolutions of the spanning and nonspanning shear bands in deformed MGs during the CTC process were investigated by experiments and molecular dynamics simulations. It was found that CTC increased the proportion of atoms with inelastic strain and induced a rejuvenation in the immature shear band zones that nevertheless led a relaxation in the mature shear band zones. This work not only deepens the understanding of the rejuvenation mechanism of deformed MGs, but also provides a unique perspective for improving their mechanical properties.

The Mg-6Sn-3Al-1Zn alloys treated with the solid solution were induced 25%, 45%, 65% and 85% rolling deformation at 400 °C, respectively. The microstructures of as-rolled magnesium alloys were analyzed by XRD, OM, SEM and TEM, the mechanical properties were also measured. The results revealed that the rolled Mg-6Sn-3Al-1Zn alloys were composed of α-Mg matrix phase and Mg₂Sn second phase, Mg₂Sn distributed at the grain boundaries and intra-grains. With the increasing of the rolling deformation, the volume fraction of Mg₂Sn phase distributed at the grain boundaries firstly decreased and then stabilized, the average size of Mg₂Sn phase firstly increased and then decreased, the Mg₂Sn phase distributed inside the grains gradually transformed from a rod shape to a spherical shape, the volume fraction of the recrystallized grains gradually increased and the average size of the recrystallized grains early decreased and eventually almost unchanged. When the rolling deformation exceeded 25%, the value of tensile strength and Vickers hardness of the Mg-6Sn-3Al-1Zn alloys increased with the rolling deformation. However, when the rolling deformation exceeded 65%, the elongation of the magnesium alloys diminished, however, the strength of as-rolled magnesium alloys did not increase significantly when the amount of rolling deformation increased from 65% to 85%. The mechanical properties were the best when the amount of rolling deformation reached 65%. Meanwhile, the volume fraction and area of Mg₂Sn phase distributed on the grain boundaries were 2.9%, 105.5 μm², the volume fraction and average grain size of recrystallized grains were 87% and 2.2 μm, respectively. In addition, it was worth noting that the Mg₂Sn phase distributed inside the grains was radically converted into a spherical shape, and the tensile strength, elongation, hardness of the alloy were 317 MPa, 13% and 77.52 HV, respectively.

The mechanical properties of Mg‐6Sn‐3Al‐1Zn alloy were enhanced with bimodal grain size disturbed in the microstructure uniformly; the Mg‐6Sn‐3Al‐1Zn alloys were rolled with 60% thickness reduction at different rolling temperatures. The results have shown that the Mg‐6Sn‐3Al‐1Zn alloys are composed of Mg₂Sn phase and α‐Mg matrix phase. When the rolling temperature was less than or equal to 400°C, with the rolling temperature increasing, the average size and volume fraction of Mg₂Sn phase and the average grain size of small grains remained unchanged, the average grain size of large grains decreased, the volume fraction of small grains increased, and the yield strength of the alloy increased. When the rolling temperature reached 450°C, the average size and volume fraction of Mg₂Sn phase and the average grain size of large grains increased, and the volume fraction of small grains and the yield strength of the alloy decreased. The elongation increased with the rolling temperature increasing, but the change trend of hardness was just opposite. When the alloy was rolled at 400°C, the average sizes of small grains, large grains, and Mg₂Sn phases were 3.66 μm, 9.24 μm, and 19.5 μm, respectively. The volume fractions of small grains, large grains, and Mg₂Sn phases were 18.6%, 77.6%, and 3.8%, respectively. And the tensile properties reached the optimum; for example, the tensile strength, yield strength, elongation, and Vickers hardness were 361 MPa, 289.5 MPa, 20.5%, and 76.3 HV, respectively.