ORALS

Dilute Mg alloys containing a few atomic percent of transition-metals and rare-earth elements have attracted increasing attention because of their excellent mechanical properties. The remarkable microstructural feature common for all of these Mg alloys is formation of a novel type of long-period stacking/order (LPSO) structures, which reveal a remarkable strength through the warm-extrusion process. During the process, the LPSO crystals are deformed not by simple dislocation migrations, but by kink-type, that is, the direct relevance to realize excellent properties of the alloys. From the extensive studies of the LPSO-structured Mg alloys for more than a decade, it has become apparent that the kink regions indeed play a critical role for effective strain storage of the alloys, but its detailed mechanism is not fully understood yet. In order to deepen our understanding of the veiled work-hardening mechanism related to kink, we have just launched the new project aiming the establishment of the “Kink strengthening phenomenon” as a universal strengthen principle. In the meantime, the LPSO structure can be generally viewed as a “Mille-feuille structure (MFS)”, in the sense that they are constructed by alternate stacking of microscopic hard-layers and soft-layers. Our preliminary studies have confirmed that the MFS Mg alloys indeed reveal the kink strengthening, whose effect seems to be more prominent than LPSO Mg alloys. Therefore, solving the critical condition and universality on the kink-strengthening phenomenon will certainly lead to a further development of lightweight structural materials, including novel Al and Ti alloys, and even polymer materials in the future.

Magnesium (Mg) alloys containing long-period stacking/ordered (LPSO) phases have been gathering increasing attention owing to their superior strength and unique deformation mode: "kink" [1]. Kink is a type of plastic deformation that introduces rotation of crystal which has been reported as a deformation mode that is activated in highly anisotropic materials [2]. Since high strength of the LPSO type Mg alloys are realized after high temperature processing, i.e., introducing kink deformed microstructure, kink deformation is believed to play an important role in mechanical properties [3]. In the present study, we investigated microstructures of kinks while especially focusing on solute segregations around boundaries which are known to significantly affect mechanical properties of Mg alloys [4]. Atomic structure and solute segregations at the kink boundaries were directly observed by scanning transmission electron microscopy (STEM). STEM observations clearly show that kink boundaries consist of arrays of basal dislocations extended into Shockley-type partial dislocations. In addition, solute elements segregated within stacking faults were introduced by the extended basal dislocations. The solute segregations around kink boundaries, which can be understood as Suzuki effect, would improve thermodynamic stability of kink microstructure.

References:
[1] Y. Kawamura et al., Mater Trans. 48 (2007) 2986-2992.\n[2] E. Orowan, Nature, 149 (1942) 643-644. \n[3] K. Hagihara et al., Intermetallics, 18 (2010) 1079-1085.\n[4] M. Bugnet et al., Acta Mater., 79 (2014) 66-73.