Structural and diffusional phase transformations in liquid-quenched Mg₈₅Y₉Zn₆ ribbons below the bifurcation temperature

In-situ and simultaneous small- and wide-angle scattering measurements of liquid-quenched Mg₈₅Y₉Zn₆ alloy ribbons were performed to examine phase transition during isothermal annealing at the temperatures below the bifurcation temperature, where long-period stacking ordered (LPSO) structures are not formed yet in the previous work. Even below the temperature, sluggish structural transformation destroying hcp stacking sequence was observed, and development of in-plane and inter plane cluster structures, as imperfect LPSO structure, became noticeable at long time annealing. The characteristic time for structural transformation showed well-defined thermally activated process with Q = 173.6 +/- 14.4 kJ/mol

Strengthening of Mg-based long-period stacking ordered (LPSO) phase with deformation kink bands

The mechanical properties of the Mg-based LPSO-phase are expected to be strongly affected by the microstructure due to its anisotropic crystal structure. However, the fine details have not been sufficiently understood yet. This study first clarified the detailed microstructural factors that govern the strength of the LPSO-phase by examining alloys with microstructures that were significantly varied via directional solidification and extrusion processes. Refining the microstructure is significantly effective for strengthening LPSO-phase alloys. The yield stress of LPSO-phase alloys with random texture was previously reported to be increased by reducing the “length” of plate-like LPSO-phase grains. In addition, it was found in this study that the formation stress in the deformation kink band, which is a unique deformation mode in an LPSO-phase alloy, can be increased by decreasing the “thickness” of the grains. Furthermore, the study used directionally solidified crystals provided direct evidence that the introduction of the deformation kink band effectively increases the yield stress and work-hardening rate of alloys by hindering the motion of basal dislocations. This “kink-band strengthening” was found to have considerable temperature dependence. The strengthening is significant at or below 200 °C, but the effect gradually decreases above 300 °C and is accompanied by the operation of non-basal slip. The results quantitatively clarified that kink-band strengthening is one predominant reason why the LPSO-phase extruded alloy exhibits an unusually high yield stress at any loading orientation.Hagihara K., Yamasaki M., Kawamura Y., et al. Strengthening of Mg-based long-period stacking ordered (LPSO) phase with deformation kink bands. Materials Science and Engineering A, 763, 138163. https://doi.org/10.1016/j.msea.2019.138163

Development of microstructures in rapidly-quenched Mg85Y9Zn6 alloy ribbons during heating at a constant speed examined by simultaneous small- and wide angle scattering measurements

Developments of microstructures during heating Mg85Y9Zn6 amorphous ribbons have been examined by in-situ synchrotron radiation small- and wide-angle scattering measurements. The samples show sharp crystallization peak in a DSC measurement at 450 K for a heating rate of 10 K/min. During and just after crystallization, clustering occurred first within supersaturated hcp crystallites, with concomitant hcp grain growth. Above 550 K, the spatial arrangements of the cluster became anisotropic, eventually lead to 18R LPSO structures accompanied by introduction of stacking faults

Deformation Behavior and Workability of Supercooled Liquid in Zr_<65>Al_<10>Ni_<10>Cu_<15> Metallic Glass

The tensile deformation behavior and the workability of Zr_Al_Ni_Cu_ Metallic Glass with a wide supercooled liquid have been examined. The glassy solid exhibited the homogeneous deformation at higher temperatures and lower strain rates. The supercooled liquid revealed a homogeneous deformation even at high strain rates above 0.5 s^. The strength in the homogeneous deformation region was lower at higher temperatures and lower strain rates. The supercooled liquid exhibited similar deformation behavior to the other high-strain-rate superplastic materials. Using this superplastic-like deformation in the supercooled liquid, complex-shaped glassy component with original strength was produced through an extrusion process

Strain-rate dependence of deformation behavior of LPSO-phases

This is the first report clarifying the influence of the strain rate on the deformation behavior of Mg-based long-period stacking ordered (LPSO) phases with 14H, 18R, and 10H structures. The flow stress by basal slip showed a weakly positive or negligible strain-rate dependence, while the flow stress accompanied by the formation of deformation kink bands showed a unique negative strain-rate dependence. These results give the first experimental evidence on the recent proposal that Zn and Y atoms segregate at the kink band boundaries and hinder their migration, from the viewpoint of the mechanical properties.Hagihara K., Li Z., Yamasaki M., et al. Strain-rate dependence of deformation behavior of LPSO-phases. Materials Letters 214, 119 (2018); https://doi.org/https://doi.org/10.1016/j.matlet.2017.11.117

Surprising increase in yield stress of Mg single crystal using long-period stacking ordered nanoplates

Mg–Zn–Y ternary alloys containing the long-period stacking ordered (LPSO) phase exhibit superior mechanical properties. This is believed to be originating from the LPSO phase acting as the strengthening phase. However, we first clarify that the mechanical properties of the matrix Mg solid solution in the Mg/LPSO two-phase alloy are significantly different from those of pure Mg. The yield stress of a Mg99.2Zn0.2Y0.6 single crystal (matrix Mg solid solution) is almost the same as that of an LPSO single-phase alloy. This is ascribed to the formation of thin stacking-fault-like defects, named “LPSO nanoplate”. In Mg99.2Zn0.2Y0.6, kink-band formation is induced in the same manner as that in the LPSO phase in deformation, resulting in high strength accompanied with increased ductility. Our results suggest that the strengthening mechanism of the Mg/LPSO two-phase alloy must be reconsidered depending on the microstructure. Furthermore, the results suggest that new ultrahigh-strength Mg alloys, which have much lower Zn and Y contents but the mechanical properties are comparable or superior than the present Mg/LPSO two-phase alloys, are expected to be developed via the appropriate control of LPSO nanoplate microstructures.Hagihara K., Ueyama R., Yamasaki M., et al. Surprising increase in yield stress of Mg single crystal using long-period stacking ordered nanoplates. Acta Materialia, 209, 116797. https://doi.org/10.1016/j.actamat.2021.116797