INTL SYMP. ON NEXT GENERATION MAGNESIUM ALLOYS AND THEIR APPLICATIONS FOR SUSTAINABLE DEVELOPMENT

The activity of individual deformation mechanisms during deformation tests of wrought Mg alloys was investigated using the acoustic emission (AE) technique. The investigated alloys exhibited a typical basal texture with basal planes oriented nearly parallel to the extrusion or rolling direction. Deformation tests were performed at room temperature and the obtained results are supported by microstructure evolution analysis provided by electron backscattered diffraction (EBSD). The AE signal analysis correlates the microstructure and the stress-strain curves to the active deformation mechanisms quite well. To determine the dominant deformation mechanism in a given time period, the adaptive sequential k-means (ASK) clustering was applied. X-ray diffraction was used to characterize the deformation texture before and after the test in order to obtain comprehensive data for texture characterization.

It is well known that the principle slip system of magnesium is a basal slip. Activation of non-basal slip systems, however, is necessary to show good ductility. Recently, the effect of yttrium on ductility of magnesium and discussion for activity of (c+a) pyramidal slips has been reported [1]. In this study, to investigate the effects of yttrium and lithium for non-basal slips, pure magnesium and magnesium alloy single crystals were stretched parallel to the basal plane in various temperatures, and polycrystalline magnesium alloys were also tested to estimate contribution of non-basal slips to their tensile deformation behavior.
In pure magnesium and Mg - (7-14)at%Li single crystals, second order pyramidal (c+a) slips (SPCSs) were observed at 77-298K as CRSS of the SPCS was decreased. Above room temperature, the first order pyramidal (c+a) slip (FPCS) was active in pure magnesium. In the Mg-(0.6-0.9) at%Y alloy single crystals, FPCS was observed at 77K to 298K while yield stress of the Mg-Y alloy single crystals was higher than that of pure magnesium [2]. In tensile tests of polycrystalline pure magnesium, Mg-(0.5-1.2)at%Y and Mg-(6-12)at%Li, slip lines of non-basal slip systems such as the SPCS, FPCS and prismatic slip were observed even at yielding in addition to basal slip lines. Among the non-basal slips, activities of FPCS and prismatic slips were increased with increasing strain in magnesium - yttrium and magnesium- lithium alloys. Our study suggested that the active non-basal slip system in tension parallel to the basal plane is a (c+a) pyramidal slip and enhanced ductility of magnesium alloys would be caused from increased activity of FPCS by alloying.

Grain boundaries play a critical role in plastic deformation and ultimately, in the control of mechanical properties and formability of magnesium alloys. Defects in such boundaries, especially segregated solute atoms, are often small in scale and their detection and characterization are beyond the capability of conventional electron microscopy techniques. Consequently, gaining fundamental insights into such defects has proved elusive. Advances in aberration-corrected scanning transmission electron microscopy (STEM), especially the techniques of high-angle annular dark-field STEM and energy-dispersive X-ray spectroscopy STEM, provide an opportunity to reveal the distribution and identity of solutes at the atomic scale. This presentation will review recent progress on the characterization of segregated solute atoms in the boundaries of some magnesium alloys. Implications of such observations on intelligent design for achieving improved properties will be discussed.

Recently, a new generation of Mg alloys with an LPSO phase have received considerable attention due to their enhanced mechanical and promising high-temperature properties compared to the conventional Mg alloys. Nevertheless, those alloys still suffer from anisotropy of mechanical properties. It is generally agreed that besides the dislocation slip, deformation kinking and twinning contribute to the plastic deformation of those alloys. The materials’ parameters (shape and orientation of LPSO phase, grain size, texture) as well as the experimental conditions (loading direction, temperature etc.) are factors on which deformation kinking depends for it to be considered common for Mg/LPSO alloys. The conditions for kink formation in Mg-LPSO alloys and their dependence on temperature, however, are still under consideration.
In this present work, a directionally solidified Mg -24 wt.% Y- 12wt.% Zn alloy having a lamellar structure elongated along the solidification direction was investigated. In order to reveal the effect of orientation on deformation behavior, uniaxial compression tests were performed parallel and perpendicular to the LSPO lamellae. Active deformation mechanisms were revealed by combination of two advanced in-situ techniques: acoustic emission and neutron diffraction. Detailed microscopy observations by optical and scanning electron microscopy (including EBSD, BSD imaging and IGMA) were performed for getting information about microstructure changes (e.g. twin and kink formation) with respect to a lamellar structure and crystallographic orientation.
Kinking was found to be a dominant deformation mechanism during compression along the lamellar structure, resulting in high yield strength. In the case of loading perpendicular to lamellae, kinking was limited to well oriented lamellae and rather higher activity of the dislocation slip was observed. IGMA analysis has shown that all observed kinks in both orientations were found to be of <1-100>-and <1-210>-rotation types.

The spark plasma sintering (SPS) technique was used to prepare a bulk WN43 magnesium alloy from the gas atomized powder. Compression tests were performed to investigate the effect of different sintering regimes on the resulting mechanical properties of the material. It was shown that by increasing the sintering temperature, the ultimate compressive strength and ductility can be significantly improved. Moreover, complementary in-situ acoustic emission (AE) recording was employed to provide insights into microstructural changes during the deformation. Recent advances in the AE signal analysis in the frequency domain, together with microstructure observations, allowed us to reveal the evolution of different deformation mechanisms. It was shown that pronounced twin nucleation takes place around the yield point whereas twin growth and dislocation slip are the dominant deformation mechanisms in the later stages of deformation in this material.

Mg alloys have been attracting keen attention as promising lightweight materials for aerospace, automobile, and railway applications. On the other hand, it is often pointed out that Mg alloys have poor oxidation resistance and burn easily. The flammability of Mg alloys is a problem to be solved when we consider using Mg alloys as structural materials of mass transportation vessels. In fact, the Federal Aviation Administration (FAA) in the USA has banned the use of Mg alloys for aircraft cabins. From the point of view of reducing aircraft weight, however, the FAA decided to lift the ban on using the Mg alloy in an aircraft cabin and set up a flammability test for Mg alloys. As a part of the development of non-flammable Mg alloys, some metal elements have been added into Mg. It has long been known that the addition of rare earth elements can improve incombustibility of the surface of the oxide film on Mg alloys [1-6]. Among the RE-containing Mg alloys, Mg-Zn-Y with a long period stacking order (LPSO) phase has excellent mechanical properties and is expected to be used in aircraft components. Mg-Zn-Y alloys produced by rapid solidification powder metallurgy have extremely high yield strengths of ~600 MPa. Mg-Zn-Y alloys produced by ingot metallurgy and extrusion have a multimodal microstructure and high yield strengths of ~340 MPa [7, 8]. The Mg₉₇Zn₁Y₂ alloy, however, exhibits an ignition temperature of ~1150 K. This ignition temperature is lower than the flame temperature (~1200 K) of the oil burner of the FAA flammability test. Therefore, to use this alloy safely in an aircraft cabin, it is necessary to increase the ignition temperature of the Mg-Zn-Y alloy. In this study, to increase the ignition temperature, a fourth element was added in the Mg-Zn-Y alloy.
Mg-Zn-Y alloys were prepared using high-frequency induction melting in Ar atmosphere. Specimens were heated at 973 K in a muffle furnace in the air. For investigating the structure of oxide films, XRD measurement, SEM, and TEM observations were conducted on the cross section of the film formed on the Mg-Zn-Y alloys.
XRD measurement and SEM observation revealed that the surface film of the Mg-Zn-Y alloy was mainly composed of Y₂O₃. An inhomogeneous and thick Y₂O₃ layer was formed by internal oxidation of Y. Cracks were often observed in the inhomogeneous Y₂O₃. Furthermore, the metallic Mg was observed in gaps between the coarse Y₂O₃ crystal gains. Therefore, suppression of internal oxidation of Y will help to form a uniform and thin Y₂O₃ film on the surface of the Mg-Zn-Y alloy and prevent crack formation in the Y₂O₃ layer. On the other hand, Mg-Zn-Y alloys with fourth elements exhibit an ignition temperature of ~1320 K. Furthermore, the thin and homogeneous Y₂O₃ film is formed on the surface of Mg-Zn-Y alloys with the fourth element.

Mg alloys are attractive for use in aircraft components primarily because of their low density and high specific strength. But current commercial Mg alloys, e.g., AZ31, have low yield strength and unacceptably low ignition temperature. Moreover, several years ago America’s FAA (Federal Aviation Administration) has lifted the ban on the use of some Mg alloy forms in the payload area and has set up a standardized testing method of flammability for Mg alloys [1].
We have developed two kinds of high-strength Mg alloys with a high ignition temperature: 1) Mg-Zn-Y alloys consisted of alpha Mg phase and a long period stacking ordered (LPSO) phase [2,3,4], and Mg-Al-Ca alloys consisted of alpha Mg phase and a C36-type intermetallic compound [5].
The LPSO-type Mg₉₆.₇₅Zn₁Y₂Al₀.₂₅ alloy, which was produced by hot extrusion of cast ingot, exhibited very high, symmetrical yield strength in both tension and compression, high heat resistance, and great flame resistance. Its corrosion resistance is the same as AZ31. The LPSO phase, which has a periodical stacking structure of the 4 atomic layers consisting of in-plane ordering of L1₂ Zn₆Y₈ clusters and the 1-4 atomic layer(s) of 2H Mg, is strengthened by kinking, which was formed during the hot extrusion. This kink strengthening is a brand-new concept for strengthening mechanism of materials. On the other hand, the C36-type Mg₈₄.₉₇Al₁₀Ca₅Mn₀.₀₃ alloy, which was also produced by hot extrusion of cast ingot, exhibited high, symmetrical yield strength, high corrosion resistance, and nonflammability; the ignition temperature is higher than the boiling temperature of pure magnesium. These advanced Mg alloys have passed the FAA flammability test for Mg alloys.
The development of more sustainable and more affordable manufacturing technology for these next-generation Mg alloys has been conducted via an integrated and comprehensive collaboration between academia and industry. Moreover, the applications and commercialization of these advanced Mg alloys have been under serious investigation and study for automobile, aircraft, and biomedical industries.

Mg alloys are attractive for use in aircraft components primarily because of their low density and high specific strength. But current commercial Mg alloys, e.g., AZ31, have low yield strength and unacceptably low ignition temperature. Moreover, several years ago America’s FAA (Federal Aviation Administration) has lifted the ban on the use of some Mg alloy forms in the payload area and has set up a standardized testing method of flammability for Mg alloys [1].
We have developed two kinds of high-strength Mg alloys with a high ignition temperature: 1) Mg-Zn-Y alloys consisted of alpha Mg phase and a long period stacking ordered (LPSO) phase [2,3,4], and Mg-Al-Ca alloys consisted of alpha Mg phase and a C36-type intermetallic compound [5].
The LPSO-type Mg₉₆.₇₅Zn₁Y₂Al₀.₂₅ alloy, which was produced by hot extrusion of cast ingot, exhibited very high, symmetrical yield strength in both tension and compression, high heat resistance, and great flame resistance. Its corrosion resistance is the same as AZ31. The LPSO phase, which has a periodical stacking structure of the 4 atomic layers consisting of in-plane ordering of L1₂ Zn₆Y₈ clusters and the 1-4 atomic layer(s) of 2H Mg, is strengthened by kinking, which was formed during the hot extrusion. This kink strengthening is a brand-new concept for strengthening mechanism of materials. On the other hand, the C36-type Mg₈₄.₉₇Al₁₀Ca₅Mn₀.₀₃alloy, which was also produced by hot extrusion of cast ingot, exhibited high, symmetrical yield strength, high corrosion resistance, and nonflammability; the ignition temperature is higher than the boiling temperature of pure magnesium. These advanced Mg alloys have passed the FAA flammability test for Mg alloys.
The development of more sustainable and more affordable manufacturing technology for these next-generation Mg alloys has been conducted via an integrated and comprehensive collaboration between academia and industry. Moreover, the applications and commercialization of these advanced Mg alloys have been under serious investigation and study for automobile, aircraft, and biomedical industries.

Long period stacking-ordered (LPSO) magnesium and related alloys frequently show kink deformation under compressive loading. Recent experimental studies revealed that the kink microstructure improves the strength of Mg-based alloys [1]. The mechanism of the kink formation as well as the resulting strengthening, however, is still unclear and further investigation is required. In the present study, we conduct dislocation-based modeling and numerical analysis on the formation of kink bands using extended isogeometric analysis (XIGA). Our modeling is based on the growth of dislocation loops in the basal planes of hexagonal type elastic mediums. Dislocation loops or plastic displacements are introduced into the medium using the Peierls-Nabarro model, and the resulting elastic stress field is solved numerically using IGA. Here, the compressive loading is applied parallel to the Burgers vector of the dislocation loops. Present numerical analysis revealed that, under the uniform growth condition, screw components of the dislocation loop spread-out from the elastic medium since the side surfaces are traction-free. On the other hand, the edge components remained in the medium due to the compressive external loading. The edge dislocations form a polygonization microstructure that stabilizes the elastic strain energy. Although the edge dislocations form localized stress fields around the core of dislocations, the resulting macroscopic displacement induces the kink band.

Dynamics of kink formation behavior under compressive stress on the 18R Mg-based LPSO alloy, prepared by the one-directional solidification technique, will be presented. This technique uses materials of high strength dual phase Mg-based LPSO alloys [1]. This is observed at room temperature via a hybrid measurement of in situ neutron diffraction and acoustic emission (AE).
By in situ neutron diffraction, we revealed the relationship between the basal plane strains and kink formation. On the other hand, by AE measurement, we obtained statistics about kink formation size by analysis of AE absolute energy which consists of flat behavior in low energy regions and power law behavior in higher energy regions. This is basic knowledge for introducing kink structure efficiently. In the AE data plot, we can clearly observe the emergence of an amoeba which is defined by the image of the logarithm of the absolute complex coordinates of the plane complex algebraic curve in the mathematics context. This excludes the scale factor. Specifically, a plot of the energy-difference between (n+1) and n events versus energy-difference between n and (n-1) events indicates an amoeba whose Newton polygon consists of the coordinates (0,0), (1,0) and (0,1). This characteristic has deep meaning regarding the dynamics of kink formation. This reveals that the statistical sequence of kink formation is governed by a simple complex algebraic curve.

A class of dilute magnesium alloys in which solute atoms (Zn, Y) are enriched in a periodic stacking fault to the (0001) plane of the hcp structure show high-strength and ductility [1]. Much attention has been paid to the studies of materials that are expected to be involved in the strengthening of mechanisms. For the class of Mg - transition-metal (M) - rare-earth (RE) metal alloys, transmission electron microscopy (TEM) found various polytype structures such as 12R, 24R, 10H, 18R, and 14H, which have different numbers of Mg layers between solute atoms (M and RE) concentrated layers [2]. Such a unique atomic structure is referred to as a long-period stacking ordered (LPSO) structure. Furthermore, recent structural analysis using TEM and first-principles calculations reveal that solute elements form M₆RE8 (L1₂ type) cluster in the concentrated layer of the LPSO phase [3]. It is experimentally found that the degree of regularity of solute atoms depends on the choice of M and the difference in the amount (composition) of solute atoms, which is correlated to the observed type of LPSO structure [4]. The origin of the phase stabilities and formation mechanisms of the LPSO structure, however, has not been clarified yet. In this study, to understand the microscopic origin of the phase stabilities of the structural polymorphs with different compositions of solute elements, we performed first-principle density-functional theory (DFT) calculations for Mg –M - Y alloys (M = Co, Ni, Cu, Zn) with 12R, 18R and 10H structures. We found that the structural distortion of the L1₂ cluster is uniquely determined by the choice of M atom. We explain how the geometries of the L1₂ solute cluster affects the electronic state near the Fermi level that crucially determines the stabilities of the LPSO phases.

After a brief introduction of the current status of gradient theory and its implications to elasticity, plasticity and diffusion, a discussion on applications to Mg alloys is provided. An effort is made to focus on the novel Mg alloy discovered by Professor Kawamura and the projects LPSO & FRAMED.

Mg₉₇Zn₁Y₂ alloy is a magnesium alloy with a duplex microstructure consisting of a long period stacking ordered (LPSO) phase and an alpha-Mg phase [1]. Kink deformation was observed in an as-cast Mg₉₇Zn₁Y₂ alloy subjected to hot compression. The refinement of LPSO via kinking was found to be the reason for strengthening of the material from microscopy analyses [2]. Direct evidence, however, has shown that increase in strength via kinking has not been observed so far. In this work, in situ neutron diffraction was used to investigate the anisotropic deformation behavior of LPSO and alpha-Mg phases during uniaxial compression or tension in an as-cast Mg₉₇Zn₁Y₂ alloy and an extruded Mg₉₇Zn₁Y₂ alloy. The evolutions of phase stresses in both the LPSO and alpha-Mg phases were evaluated and discussed with the occurrences of twinning and kinking during compression or tension.
The as-cast Mg₉₇Zn₁Y₂ alloy was prepared by high frequency induction melting in a carbon crucible. The extruded Mg₉₇Zn₁Y₂ alloy was prepared by hot extrusion at 623 K of a round bar of an as-cast Mg₉₇Zn₁Y₂ alloy at an extrusion ratio of 10 and a ram speed of 2.5 mm/s in the air. The in-situ neutron diffraction experiment during compression was carried out using TAKUMI of J-PARC and a cylindrical test piece having a length of 16 mm and a diameter of 8 mm. The peak positions and integrated peak intensities were evaluated from the obtained diffraction patterns and the evolutions of lattice strains. The texture was then estimated and the phase stresses were subsequently evaluated. The response of phase stress to the applied stress of alpha-Mg deviated from the linearity which describes a smaller value at the applied stress that is lower than the macroscopic yield stress for the as-cast alloy. The response obtained pretended to keep the linearity up to the macroscopic yield stress for the extruded alloy. The details will be presented.

Magnesium and its alloys are one of the lightest metals. The plasticity and formability of magnesium at room and lower temperatures, however, are poor because of its closely packed hexagonal crystal structure. Therefore, the deformation of magnesium by SPD methods is carried out at temperatures above 150 °C [1-3]. On the other hand, cold severe plastic deformation (SPD) of magnesium could lead to significant grain refinement and improvement of mechanical properties. At room and lower temperatures, however, the deformation of magnesium takes place mainly due to basal slip that causes formation of a sharp basal texture (0001) and prevents successful processing of magnesium at a high deformation strain.
In this work, an attempt was made to deform magnesium at room and lower (cryogenic) temperatures. Our SPD method included lateral extrusion (LE) and cold rolling (CR). At first, cylindrical Mg-workpieces without basal texture (0001) were subjected by LE at room temperature (deformation strain is ε~3.9). As a result, 1-mm plates were obtained. It was found that the initial grain size was significantly reduced from 7 mm to 1-3 μm and weak basal texture was formed after LE [4].
Plates demonstrated high plasticity and were rolled to foils of 150-µm (ε~6) and 50 µm (ε~7) at room and cryogenic temperatures. CR did lead to nanostructure formation and the average grain size of thin foils deformed at low temperatures was about 5 – 7 µm. The main feature of the foils after CR was that their microstructure had a large number of new recrystallized grains and had areas of fine-grained and cellular substructures. New grains had non-equilibrium grain boundaries. After cryogenic deformation, a higher dislocation density in grains was seen in foils in comparison with room-temperature rolled foils. As for texture, basal texture (0001) became stronger after CR.
The results of the work are of particular interest and they can be useful for practical application to create magnesium membranes for biotechnology and improve the mechanical properties of magnesium alloys.
This work was carried out with support of the Russian Foundation for Basic Research (the RFBR project no. 18-33-00474) and within the framework of the State task (theme: Pressure No. АААА-А18-118020190104-3).

Porous metals are used in a wide range of applications because of the several advantages they have over non-porous metals which include low density, high energy absorption capability, and so on [1]. Among the porous metals, porous magnesium (Mg) with parallel cylindrical pores exhibits a higher energy absorption capability compared to porous light metals with isotropic pores [2, 3]. The microstructure of this porous Mg shows several distinctive features: elongated pores in the solidification direction, elongated coarse grains in the solidification direction, and a peculiar crystallographic texture where one of the normal directions of the {10-13} planes is closely oriented to the solidification direction. In the previous research [2], crystal plasticity analysis estimated that the different deformation modes were activated depending on loading direction which led to anisotropic deformation behavior. Furthermore, more detailed numerical investigations were performed to clarify the underlying mechanism for high energy absorption capability of this porous Mg [3]. This suggested significant contribution of texture development triggered by intra-granular misorientations. Based on the understanding of the deformation mechanism from the viewpoint of crystal plasticity, a possible strategy for further improvement of energy absorption properties by pre-loading was also proposed [3].
In this study, the influence of the initial texture on energy absorption capability in porous Mg with oriented pores is summarized based on a series of crystal plasticity finite element calculations. Implemented deformation modes in the present numerical method are the basal slip, prismatic slip, first order pyramidal <a> slip, second order pyramidal <c+a> slip, and {10-12} tensile twinning systems. The analysis models, which reproduce elongated pore structure, grain morphology, and initial texture, were constructed based on microstructural observations [2, 3]. The results of numerical calculations showed significant dependence of initial texture on energy absorption capability.

A kinematical model of kink microstructure in a material which causes slip deformation on a specific plane is proposed based on the continuity of deformation at interfaces [1]. A kink band was regarded as a domain with a homogeneous shear, being rank-1 connected to the matrix. Ridge and ortho kink were modeled as rank-1 connections between the kink bands. Owing to the simple geometry of the kinks, the kink plane and the crystallographic rotation of the kink band were obtained in analytic forms as functions of the magnitude of the shears inside kinks. It was found that positive and negative partial wedge disclinations are inevitably formed in any kink band connection when the junction plane of the kink bands does not reach the surface of the body. In addition, it is possible for disclinations to annihilate by connection of suitable kink bands to a microstructure with zero elastic energy. A universal mechanism of kink strengthening in the LPSO Mg alloy is discussed.

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.

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.

A steel ball indenter was impressed on low index planes in Mg alloy single crystals with different CRSSs: Mg-Al, Mg-Zn, and Mg-Y alloy single crystals. Effects of alloying elements on indentation behavior and roles of slip and twins on indentation size were investigated. Pure Zn single crystals, whose loading direction causing {10-12} twins is opposite to pure Mg, were also prepared. Indentations showed a circular shape on (0001) in the Mg alloys and pure Zn single crystals. Also, slip lines and twins were hardly observed around (0001) indentations. On the other hand, (10-10) and (1-210) indentations showed an elliptical morphology which elongated to [0001]. Basal slip lines and {10-12} twins were observed around (0001) indentations in Mg-Al and Mg-Zn alloys. Twins, however, were hardly observed in Mg-Y alloy single crystals. Indentation sizes of (0001) of the Mg alloys were smaller than that of pure Mg. Also, indentation sizes of pure Mg and Zn were nearly the same. Indentation sizes decreased with increasing CRSS for the basal slip. Therefore, the size would be determined by CRSS for the basal slip. CRSSs for the basal slip of pure Mg and Zn were nearly the same. Therefore, the size gap would be caused by the presence of {10-12} twins. In Mg alloys, indentation sizes decreased as alloy elements were added. The size differences between pure Mg and the Mg alloys would be determined by ratios of basal slip and twinning activities.

Magnesium (Mg) and its alloys have received particular interest, especially in the automotive industry, for potential lightweight structural applications due to their low density and high specific strength. Most Mg alloys, however, suffer from poor mechanical properties at ambient and elevated temperatures. They also lack strong plastic anisotropy due to the low-symmetry of the hexagonal crystal structure. The strength of heterogeneity of the different deformation mechanisms and the thermo-mechanical processes often lead to strong crystallographic textures. Synthetic microstructures generated from statistical data obtained from EBSD measurements was generated based on a developed approach, and numerical simulations based on the Crystal Plasticity Finite Element Method (CPFEM) were also proposed [1, 2]. Crystal plasticity which takes into account the dislocation slip and twinning is considered and fitted against experimental cyclic stress-strain hysteresis curves. Fatigue simulations are conducted with different loading conditions to quantify and correlate the local plastic strain with the crack initiation sites observed experimentally in various Mg alloys, including LPSO-Mg.

The long-period stacking ordered phase, the so-called LPSO phase, is strongly focused as a suitable strengthening phase of Mg alloys[1-4]. One of the hot topics found in Mg-alloys containing a large amount of LPSO phase is the unusual increase in strength by the extrusion [5]. Concerning this, we recently clarified the mechanisms which induce the drastic strengthening of the LPSO-phase alloys by extrusion, on the basis of the quantitative analysis [6]. This is one of the main purposes in this presentation and the details of this are discussed.
In order to achieve this, the temperature and loading orientation dependence of the deformation behavior of the Mg₈₈Zn₄Y₇ extruded alloy, which contains a ~86 vol.% of LPSO-phase, were examined. Using several extruded alloys with different extrusion ratios, the influence of the extrusion ratio to the microstructure formation and the following mechanical properties were examined by compression tests. The tests were conducted in a temperature range between room temperature and 400 °C in a vacuum. Two loading orientations were selected for the compression test; one orientation is parallel to the extrusion direction (0° orientation) and the other is inclined at an angle of 45° from the extrusion direction (45° orientation) which clarifies the anisotropic mechanical properties of the extruded alloys.
As a result, the yield stress of the LPSO phase alloy was found to exhibit a strong orientation dependence varied with the extrusion ratio. Especially, the yield stress of the extruded alloy with the reduction ratio of 10 showed an extremely high value of ~460 MPa when loaded at 0° orientation while it was largely reduced when loading at 45° orientation. This strong anisotropy of the plastic deformation behavior was considered to be derived from the variation in the deformation mechanisms depending on the loading orientation because of the development of strong {10-10} fiber texture along the extrusion direction. The basal slip was found to govern the deformation behavior at 45° orientation while the predominate deformation mechanism varied from the basal slip to the formation of deformation of the kink band at 0° orientation as the extrusion ratio increased. In addition, it was found that the introduction the deformation kink band boundary during the extrusion process effectively acts as a strong obstacle against the motion of the basal slip. That is, "the kink band strengthening" was first quantitatively elucidated, which contributes to the drastic increase in the yield stress of the extruded LPSO-phase alloys in the wide temperature range below 400 ºC.

This paper will address the well-known, but practically ignored issue of unstable, thick and porous surface oxide layers of Mg alloys in terms of safety for manufacturing and applications and cleanliness for molten metal and manufactured parts. The unstable and porous oxide layers are solely responsible for the inherent obstacles of Mg alloys such as safety (burning), health (Beryllium and SO₂ gas), and environment (SF₆ gas). Mg alloys are also the main source for poor cleanliness (high inclusion level) which in turn greatly deteriorates fluidity and ductility. ECO-Mg alloys will be introduced as one practical possibility to produce the stable surface oxide layer through alloy design. Equal emphasis will also be placed on the importance of the stable oxide layer for controlling cleanliness, therefore improving fluidity (thinner and more complex consolidated die castings) and ductility (overcoming property or maintaining original property in other words).

Mg alloys have the lowest density among commercially available structural alloys which can provide significant weight savings in automobiles. For the widespread application of Mg alloys, however, Mg alloys should overcome a critical shortcoming: poor formability at room temperature mainly originating from strong basal texture developed during thermomechanical processing. Although several Mg alloys show random/weak texture and accordingly good room temperature formability, most of such alloys rely on the usage of expensive rare earth elements. It has been recently reported that the addition of Ca to Mg-Zn alloys weakens and randomizes the texture, similar to the effect of RE addition on modification of the texture. The texture of these Ca-containing Mg-Zn alloys can be described as the broadened angular distribution of basal poles along the transverse direction (TD) and split of basal poles along the rolling direction (RD) in as-rolled condition. A significant change in texture, however, occurs after the annealing process, splitting of basal poles toward the TD from the original RD in particular. Despite the weak texture intensity, their texture is less than ideal since one directional orthotropic texture developed during annealing would result in non-uniform deformation during stretch forming. The detailed mechanism of such texture evolution, however, has not been clearly revealed yet. In the present work, an attempt has been made for having a better understanding of the texture evolution during the annealing process of Ca containing Mg-Zn alloys. The details of their texture evolution have been analyzed by quasi-in-situ EBSD after various stages of annealing with particular emphasis on recrystallization and growth behavior.

Designing materials that contain complex microstructures and high performance is challenging using a reductionist approach to materials development. A powerful utility in this endeavor is the use of computational thermodynamic and kinetic tools. The integration of these tools into a systems-based materials design methodology that couples experimental research with theory and mechanistic modeling has been established to accelerate materials development. Microstructural properties can be expressed as thermodynamic parameters that are predictable by computational thermodynamic tools, while kinetic simulations can assist in elucidating processing-structure relationships to quantify microstructural evolution. This talk focuses on the high temperature magnesium alloy development as a model system and a new low-cost alloy is demonstrated with improved attributes to commercial high temperature magnesium alloys systems.

Designing materials that contain complex microstructures and high performance is challenging using a reductionist approach to materials development. A powerful utility in this endeavor is the use of computational thermodynamic and kinetic tools. The integration of these tools into a systems-based materials design methodology that couples experimental research with theory and mechanistic modeling has been established to accelerate materials development. Microstructural properties can be expressed as thermodynamic parameters that are predictable by computational thermodynamic tools, while kinetic simulations can assist in elucidating processing-structure relationships to quantify microstructural evolution. This talk focuses on the high temperature magnesium alloy development as a model system and a new low-cost alloy is demonstrated with improved attributes to commercial high temperature magnesium alloys systems.