Byeong-Chan Suh

Twinning-mediated formability in Mg alloys

Mg alloys are promising candidates for automotive applications due to their low density and high specific strength. However, their widespread applications have not been realized mainly because of poor formability at room temperature, arising from limited number of active deformation systems and strong basal texture. It has been recently shown that Mg-Zn-Ca alloys have excellent stretch formability, which has been ascribed to their weak basal texture. However, the distribution of basal poles is orthotropic, which might result in anisotropy during deformation and have adverse effect on formability. Here, we show that tension twinning is mainly responsible for enhanced formability of Mg-Zn-Ca alloys. We found that tension twinning is quite active during both uniaxial deformation and biaxial deformation of Mg-Zn-Ca alloy even under the stress conditions unfavourable for the formation of tensile twins. Our results provide new insights into the development of Mg alloys having high formability.

Deformation behavior of duplex austenite and ε-martensite high-Mn steel

Abstract Deformation and work hardening behavior of Fe–17Mn–0.02C steel containing ε-martensite within the austenite matrix have been investigated by means of in situ microstructural observations and x-ray diffraction analysis. During deformation, the steel shows the deformation-induced transformation of austenite → ε-martensite → α′-martensite as well as the direct transformation of austenite → α′-martensite. Based on the calculation of changes in the fraction of each constituent phase, we found that the phase transformation of austenite → ε-martensite is more effective in work hardening than that of ε-martensite → α′-martensite. Moreover, reverse transformation of ε-martensite → austenite has also been observed during deformation. It originates from the formation of stacking faults within the deformed ε-martensite, resulting in the formation of 6H-long periodic ordered structure.

Effect of differential speed rolling on the texture evolution of Mg-4Zn-1Gd alloy

The microstructural and texture evolution during differential speed rolling process of Mg 4Zn-1Gd (wt%) alloy have been investigated by means of electron backscatter diffraction observation and texture analysis. The angular distribution of basal poles are inclined about 10° from the normal direction towards the rolling direction and the maximum intensities of basal poles are decreased, compared to the conventional rolling process. Such an inclination of angular distribution of basal poles can be induced by the operation of shear stress along the rolling direction, as much as one quarter of tensile stress along the RD and one quarter of compressive stress along the ND. When the reduction ratios in differential speed rolling increase, there is no difference in texture evolution although there is a significant change in activated twinning systems. In addition, the engineering stresses after differential speed rolling are also similar to that after conventional rolling process, while ductility and stretch formability in the former are worse than those in the latter.

Role of Zr in the Microstructure Evolution in Mg-Zn-Zr Based Wrought Alloys

We investigated the role of Zr in the microstructure evolution in Mg-Zn-Zr based wrought alloys fabricated by twin-roll casting and hot rolling (TRC-HR) process and extrusion process. In the as-cast condition, the Mg(Zn, Zr) precipitates are observed in the as-cast Mg-6.2Zn-0.5Zr (ZK60) alloys indicating that the Mg(Zn, Zr) precipitates are formed during melting, solidification or cooling process. The ZK60 alloy exhibited finer grain structure compared to the Mg-6.2Zn (Z6) alloy in both as-cast and solution treated conditions since the Mg(Zn, Zr) inhibited the grain growth. The Mg(Zn, Zr) particle also contributed to the fine-grain structure formation during the extrusion and TRC-HR processes. During the wrought processing, the recrystallization starts to occur from the grain boundaries of the homogenized sample, and spread to the grain interior. Since the Mg(Zn, Zr) particle inhibited the grain growth, the fine recrystallized grain structure is formed in the ZK60 wrought alloy.

Effects of Texture and Alloying Elements on Stretch Formability of Mg Alloy Sheets

A study has been made on the effects of texture and alloying elements on stretch formability of Mg alloys. AZ31, AT31, and ZX11 alloys having different textures have been subjected to uniaxial tensile loading and bi-axial stretch forming tests, and their microstructures and textures before and after deformation have been analyzed. It shows that the deformation of AT31 alloy is mainly controlled by the activation of prismatic slip besides the basal slip. On the other hand, there is a significantly high activity of tension twinning in ZX11 alloy regardless of tensile loading directions, resulting in the smallest Lankford values in ZX11 alloy among three alloys, followed by AT31 and AZ31 alloys. Nevertheless, the results of Erichsen cup tests in these alloys show the different behaviors such that AT31 alloy has the best stretch formability, followed by ZX11 and AZ31 alloys. It has been shown that the excellent stretch formability of AT31 alloy comes from an increased activity of prismatic slip and its characteristic texture counterbalanced by broadened distribution of basal poles along the transverse direction and split basal poles along the rolling direction. Although ZX11 alloy shows an increased activity of tension twinning during stretch forming, its orthotropic texture would lead to significant anisotropic deformation at a late stage of stretch forming, resulting in a premature failure.