Metallurgical Materials Design Laboratory

Department of Materials Science and Applied Chemistry
Faculty of Engineering | Graduate School of Science and Technology
Kumamoto University


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Researches

High performance Mg alloys with synchronized LPSO phase designed using a heterogeneity integration method


PROJECT BACKGROUND

Being considered assuredly superior in specific strength to all other lightweight structural metallic materials, Mg alloys are strong candidates for use in automotive, railway, and aerospace applications where weight reduction is of great importance. Unfortunately, it is well known that Mg metal possesses a HCP crystal structure with low symmetry, exhibits poor corrosion resistance, and has a low melting temperature. These properties impart undesirable qualities such as low ductility, low corrosion resistance, and low heat-resistance, thereby limiting the commercial use of Mg alloys. However, within the last one-and-half decades, Mg-Zn-rare earth (RE) alloys containing a long period stacking/order (LPSO) structure phase have received a large amount of attention, due to its improved mechanical performance. In the LPSO variants of a HCP structure of Mg crystal, either rhombohedral (R) or hexagonal (H) Bravais lattices appear, depending on the stacking period of the close-packed atomic layers. Four polytypes, 10H, 18R, 14H and 24R (after Ramsdell), have so far been reported for Mg?Zn?RE alloys. The Mg?Zn?RE LPSO structures exhibit a periodic chemical modulation as well as long-period stacking. Since the apparent chemical modulation occurs with confined Zn/RE distribution around stacking faults and synchronizes with the relevant stacking order, these structures are referred to as gsynchronized LPSO structuresh. The presence of the LPSO phase, with its unique microstructure, contributes to the strengthening of Mg alloys. For example, extruded Mg97Zn1RE2 alloys exhibit high yield strength (>350 MPa) and reasonable elongation (>5%) at room temperature, and maintains a high yield strength (>300 MPa) at 473 K. The LPSO phase acts as an alloy-strengthening component because of its unique plastic deformation behavior. During my research in LPSO phase-containing Mg alloys, I have developed a new heterogeneous multimodal microstructure alloy design method that can provide an ameliorable solution to the above-mentioned inveterate problems associated with conventional Mg alloys.


RESEARCH STRATEGY

We seek to design multifunctional Mg alloys with synchronized LPSO phase, based on metallurgical principles and a heterogeneity integration method that considers geometrical and electrochemical viewpoints.

Mg/LPSO two-phase alloy design based on metallurgical principles

What conditions results in synchronized LPSO phase formation in Mg alloys? A systematic investigation revealed the criteria of additive elements for formation of synchronized LPSO phase; important parameters are mixing enthalpy, solid solubility, and atomic size. I have developed more than twenty types of magnesium alloys with synchronized LPSO phase. A proposed time-temperature-transformation diagram for precipitation of synchronized LPSO phase and solute-segregated stacking faults in Mg-Zn-Gd alloys was also constructed.

Geometric heterogeneity control of Mg/LPSO two-phase alloys

The mechanical properties of alpha-Mg/LPSO two-phase alloys at room and elevated temperatures are considerably improved by thermo-mechanical processing such as extrusion, rolling, and forging. The microstructure of extruded Mg-Zn-RE alloys with LPSO phase is characterized by a heterogeneous multimodal microstructure that consists of three different regions. The alpha-Mg matrix is bimodally grained during extrusion; that is, it consists of fine DRXed grains with random orientation and coarse grains with a <10-10>fiber-texture normal to the transverse plane. The LPSO phase grains also develop this <10-10>fiber texture. While the fine DRXed -grain regions contribute to the improvement in ductility of the alloy, the coarse -Mg grains and the fiber-shaped LPSO phase contribute to its increased mechanical strength. In other words, <10-10>fiber-textured alpha-Mg and LPSO phase grains embedded in a random-textured fine-grained -Mg matrix can result in an improvement in both the strength and the ductility of the alloy simultaneously (Fig. 1, not printed here). The volume fraction of each region and the dispersion of LPSO phase in extruded Mg-Zn-RE alloys are controlled by changing the morphological features of the LPSO phase in the cast state, such as its secondary dendrite arm spacing and precipitation morphology. The effect of the LPSO phase on microstructure evolution during extrusion is classified into one of two types, depending on its grain shape and size. Large block-shaped LPSO phase grains promote dynamic recrystallization of alpha-Mg grains through particle-stimulated nucleation. Plate-shaped LPSO phase grains and/or solute-segregated stacking faults that precipitate coherently in the alpha-Mg matrix promote <10-10>//ED fiber texture development during extrusion, which is referred to as "LPSO phase-stimulated texture" evolution. In general, crystal anisotropy of the hcp structure is considered to be a disadvantage for deformation of Mg and Mg alloys. However, I have been able to make use of strong crystal anisotropy of LPSO phase as a means for heterogeneity control. Therefore, in this study, I am aiming to establish a multimodal microstructure design technique. Secondly, thermo-mechanical processing introduces profuse kink bands in the LPSO phase and alpha-Mg phase grains. Formation of these kink bands may strengthen the alloys, because kink bands formed perpendicular to the primary slip direction become an obstruction to basal slip. Intragranular misorientation axis (IGMA) analysis suggests that this kink deformation within the LPSO phase results in lattice rotation predominately about the <1-100> and <0-110>, and <0001> axes. (<1-100> and <0-110> in the 18R-LPSO phase are not equivalent because of its rhombohedral structure.) Assuming that kinking proceeds in the LPSO phase according to the dislocation-based model for kink deformation, these results suggest that kink deformation tends to proceed through these basal and prismatic slip systems and to some extent accommodates the plastic strain.

Electrochemical heterogeneity control of Mg/LPSO two-phase alloys

Unfortunately, from the viewpoint of corrosion science, the LPSO phase may be considered as a secondary phase causing a potential difference between it and the alpha-Mg matrix phase. The main reason for the low corrosion resistance of Mg alloys is galvanic attack due to impurities, alloying elements and secondary phases. Specifically, two-phase Mg alloy often shows low corrosion resistance. Therefore, the development of techniques that improve the corrosion resistance of -Mg/LPSO two-phase alloy is required. Application of rapid solidification processing to Mg alloys results in microstructure refinement, extended solid solubility and improved chemical homogeneity, which consequently results in improved mechanical properties and corrosion resistance. Rapidly solidified (RS) ribbon-consolidation is a newer technique that was developed to obtain large-scale highly corrosion-resistant alpha-Mg/LPSO two-phase alloys. Rapid solidification followed by the consolidation of RS ribbons produces nanocrystalline alloys containing a stacking fault-like LPSO phase that is dispersively precipitated within dynamically recrystallized alpha-Mg matrix grains. A morphological change from a massive block-shaped LPSO phase to a highly dispersed LPSO phase in the grain interior results in an improvement in corrosion resistance without compromising the excellent mechanical properties of the alloys, due to the increased electrochemical homogeneity. Additionally, controlling the Volta potential through chemical composition design of the LPSO phase is effective for reducing the potential difference between LPSO and alpha-Mg matrix phases. Recently, a novel LPSO phase was discovered in the Mg-Al-Gd alloy system. Substitution of Al for Zn in the LPSO phase present in the Mg-Zn-Gd alloy system causes a decrease of the potential of the LPSO phase, which results in improved corrosion resistance of the alloy due to the reduced potential difference between it and the alpha-Mg matrix phases, as shown in Fig. 2 (not printed here). The discovery of Mg-Al-Gd LPSO phase realizes the ability to reduce the electrochemical heterogeneity, while maintaining geometric heterogeneity.




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