Effects of multimodal microstructure on fracture toughness and its anisotropy of LPSO-type extruded Mg-1Zn-2Y alloys

The fracture toughness of extruded Mg-1Zn-2Y(at.%)alloys,featuring a multimodal microstructure containing fine dynamically recrystallized(DRXed)grains with random crystallographic orientation and coarse-worked grains with a strong fiber texture,was investigated.The DRXed grains comprised randomly oriented equiaxedα-Mg grains.In contrast,the worked grains includedα-Mg and long-period stacking ordered(LPSO)phases that extended in the extrusion direction(ED).Both types displayed a strong texture,aligning the10.10direction parallel to the ED.The volume fractions of the DRXed and worked grains were controlled by adjusting the extrusion temperature.In the longitudinal-transverse(L-T)orientation,where the loading direction was aligned parallel to the ED,there was a tendency for the conditional fracture toughness,KQ,tended to increase as the volume fraction of the worked grains increased.However,the KQ values in the T-L orientation,where the loading direction was perpendicular to the ED,decreased with an increase in the volume fraction of the worked grains.This suggests strong anisotropy in the fracture toughness of the specimen with a high volume fraction of the worked grains,relative to the test direction.The worked grains,which included the LPSO phase and were elongated perpendicular to the initial crack plane,suppressed the straight crack extension,causing crack deflection,and generating secondary cracks.Thus,these worked grains significantly contributed to the fracture toughness of the extruded Mg-1Zn-2Y alloys in the L-T orientation.

Kink-band formation in directionally solidified Mg/Mg_(2)Yb and Mg/Mg_(2)Ca eutectic alloys with Mg/Laves-phase lamellar microstructure

For the development of high-strength Mg alloys,active use of Laves phases such as C14-type Mg_(2)Yb and Mg_(2)Ca is strongly expected.However,the brittleness of the Laves phases is the biggest obstacle to it.We first found that kink-band formation can be induced in directionally solidified Mg/Mg_(2)Yb and Mg/Mg_(2)Ca eutectic lamellar alloys when a stress is applied parallel to the lamellar interface,leading to a high yield stress accompanied with ductility.That is,microstructural control can induce a new deformation mode that is not activated in the constituent phases,thereby inducing ductility.It was clarified that the geometric relationship between the operative slip plane in the constituent phases and the lamellar interface,and the microstructural features that provide kink-band nucleation sites are important factors for controlling kink-band formation.The obtained results show a possibility to open the new door for the development of novel high-strength structural material using the kink bands.

Effect of short-range order on the high-temperature plastic deformation behavior of Ni-Cr-W,Ni-W,and Ni-Re single crystals

The development of a Ni-based superalloy that consists of only theγ-solid-solution phase is highly desired for some applications such as high-temperature gas-cooled reactors in power plants.In this study,the temperature dependence of the plastic deformation behavior of Ni-22Cr-8W(at%)alloy was examined for the first time by using a single crystal,focusing on the influence of the short-range order(SRO),especially upon high-temperature deformation behavior.The critical resolved shear stress(CRSS)for(111)[10¯1]slip exhibited a rapid decrease as the temperature increased at low temperatures but an almost constant value of∼45 MPa between 500 and 900℃.In addition,a slight yield stress anomaly(YSA)was observed,reaching a maximum value at 1000℃.To clarify the influence of the SRO on the YSA behavior,the mechanical properties of Ni-8W and Ni-5Re single crystals were also examined.Similar temperature dependence for the CRSS,including the occurrence of slight YSA,was observed for Ni-8W,in which SRO developed.In the Ni-5Re single crystal in which SRO did not significantly develop,however,the CRSS monotonically decreased as the temperature increased.The results suggest that the presence of SRO in a Ni-Cr-W alloy contributes to the strengthening of the alloy not only at low temperatures but also at high temperatures up to∼1000℃.The results strongly suggested that the pseudo-PLC effect is related to the origin of YSA appearing in Ni-basedγ-solid-solution alloys at high temperatures.

Change in elastic properties during room-temperature aging in body-centered cubic Mg-Li and Mg-Li-Al single crystals

Biodegradable materials,with many advantages over conventionally used non-biodegradable materials,have received much attention owing to the elimination of the need for a secondary surgery for the removal of implants after implantation.Compared with other biodegradable ceramics and/or polymers,Mg alloys are considered to be the most promising biodegradable orthopedic materials because they feature not only high specific strength and compressive yield strength but also a low Young’s modulus,which is comparable to that of human bones(10-30 GPa)[1,2].