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Interface segregation of solute atoms has a profound effect on properties of engineering alloys. The occurrence of solute segregation in coherent twin boundaries (CTBs) in Mg alloys is commonly considered to be induced by atomic size effect where solute atoms larger than Mg take extension sites and those smaller ones take compression sites in CTBs. Here we report an unusual solute segregation phenomenon in a group of Mg alloys—solute atoms larger than Mg unexpectedly segregate to compression sites of {10 1 ¯ 1} fully coherent twin boundary and do not segregate to the extension or compression site of {10 1 ¯ 2} fully coherent twin boundary. We propose that such segregation is dominated by chemical bonding (coordination and solute electronic configuration) rather than elastic strain minimization. We further demonstrate that the chemical bonding factor can also predict the solute segregation phenomena reported previously. Our findings advance the atomic-level understanding of the role of electronic structure in solute segregation in fully coherent twin boundaries, and more broadly grain boundaries, in Mg alloys. They are likely to provide insights into interface boundaries in other metals and alloys of different structures. Segregation of solute atoms at interfaces affects the properties of alloys and needs to be understood to allow their rational design. Here the authors report an unusual solute segregation phenomenon in a group of Mg alloys, driven by chemical bonding, where solute atoms larger than Mg segregate to compression sites of specific fully coherent twin boundary. [ABSTRACT FROM AUTHOR]

The equilibrium intermetallic phase in Mg-Nd-Ag alloys has not been well understood. In this work, intermetallic particles in the solution-treated microstructure of commercial magnesium alloy QE22 (Mg-2.5Ag-2.0Nd-0.7Zr, wt pct) have been investigated using scanning electron microscopy, electron diffraction, atomic-resolution imaging and mapping techniques of scanning transmission electron microscopy (STEM) and thermodynamic modelling. The intermetallic particles are distributed in the inter-dendritic regions. They have coarse irregular shape and share the same crystal structure. The intermetallic phase (designated δ) is determined to have an orthorhombic structure (space group Cmcm, a = 1.02 nm, b = 1.18 nm, c = 1.00 nm) and a composition of NdAgMg11, which are different from those reported previously. An atomic model is proposed for the δ phase based on atomic-resolution STEM images and atomic-scale energy-dispersive X-ray spectroscopy maps. The δ lattice is structurally related to that of Mg12Nd phase in binary Mg-Nd alloys. The Gibbs energy of formation of NdAgMg11 is determined from the equilibrium study at 793 K (520 °C), including the entropy of formation using the present experimental phase analysis data obtained at lower temperature. Implications to the formation temperature range and thermal stability of this phase and alloy solidification are discussed based on the calculated Mg-Nd-Ag phase diagram and Scheil solidification paths of alloys. [ABSTRACT FROM AUTHOR]

Stacking faults in magnesium alloys have received considerable attention in recent years, but only intrinsic and extrinsic faults are studied at experimental and computational levels. Twin-like fault, designated as T 2 fault, has received little attention. In this study, we report for the first time the occurrence of T 2 fault in Mg–9.8 wt%Sn alloy. The T 2 fault and solute segregation in regions surrounding the fault were characterized using aberration-corrected scanning transmission electron microscopy and first-principles calculations. Sn atoms orderly distribute in the fault plane and its neighbouring planes, which drastically decreases the T 2 fault energy. [ABSTRACT FROM AUTHOR]

Additions of Zn can significantly improve the age-hardening response of binary Mg-Sn alloys by refining the distribution of Mg 2 Sn precipitates, but its role in the precipitation is still unclear. In this study, the Mg 2 Sn precipitates with different morphologies and orientations in a Mg-9.8Sn-1.2Zn (wt.%) alloy are characterized by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and STEM X-ray mapping. It is found that Zn atoms invariably segregate to the Mg 2 Sn-Mg interface, irrespective of the interfacial structures and orientation relationships of Mg 2 Sn. This finding provides insight to the understanding of the enhanced nucleation of Mg 2 Sn precipitates in Mg-Sn-Zn alloys. [ABSTRACT FROM AUTHOR]

The effects of Zn additions on the age hardening response and microstructure of Mg–9Al–6Sn (wt.%) alloy have been investigated. It is found that the Zn additions can remarkably increase the alloy hardness in the as-quenched condition. The peak hardness value of the Mg–9Al–6Sn alloy increases from 90 ± 0.87 HV to 103 ± 3.83 HV, when the Zn addition is 3 wt.%, which is the highest among the magnesium casting alloys that are free of rare-earth elements. The enhanced hardness in the Mg–9Al–6Sn–3Zn alloy is attributable to an increased volume fraction of smaller Mg 2 Sn particles in the solution-treated condition and the formation of [0001] Mg Mg 17 Al 12 rods and Mg 21 (Zn, Al) 17 precipitates in the peak-aged condition. [ABSTRACT FROM AUTHOR]

Abstract: The present work reports the effect of annealing treatment on detwinning deformation in Mg alloy AZ31 and pure Mg that have pre-strained twins. It shows that appropriate annealing enhances, rather than reduces, the yield strength of the pre-strained AZ31, but it does not cause any strengthening of the pre-strained pure Mg. STEM–EDS mapping shows that both Al and Zn segregate to twin boundaries in the pre-strained AZ31 after the annealing process. It is proposed that it is the pinning of twin boundary by segregated solute atoms that results in an increased activation stress for detwinning deformation and, hence, annealing hardening. [Copyright &y& Elsevier]

Developing ultra-high strength in rare-earth-free Mg alloys using conventional extrusion process is a great challenge. What is even more difficult is to achieve such a goal at a lower processing cost. In this work, we report a novel low-alloyed Mg-2Sn-2Ca alloy (in wt. %) that exhibits tunable ultra-high tensile yield strength (360–440 MPa) depending on extrusion parameters. More importantly, there is little drop in mechanical properties of this alloy even when it is extruded at a speed several times higher than those used in the reported high strength Mg alloys. Examination of as-extruded microstructures of this Ca-containing Mg alloy reveals that the ultra-high strength is mainly associated with the presence of surprisingly submicron matrix grains (down to ∼0.32 μm). The results suggest that the Ca addition promotes accumulations of the pyramidal dislocations, which eventually transform into the low angular grain boundaries (LAGBs). The high number density of LAGBs separate the α-Mg matrix via either discontinuous dynamic recrystallization (DDRX) mechanism in the early stage or the continuous dynamic recrystallization (CDRX) mechanism in the later stage of extrusion, which effectively enhances the nucleation rates of the DRXed grains. More importantly, large amount of Ca segregation along LAGBs, accompanied with dynamically precipitated Mg 2 Ca nano-phases, are detected in the present non-severely deformed samples. It is the combination of solute segregations and numerous Mg 2 Ca nano-precipitates that contributes to the formation of the ultra-fine grains via pinning mechanism. The ultrafine grains size, Ca enrichment in most LAGBs, and residual Mg 2 Ca nano-precipitates would in turn contribute significantly to the enhancement of the yield strength of the as-extruded Mg-2Sn-2Ca (wt.%) alloy. The low content of alloying elements and the fast one-step extrusion process render the present alloys low-cost and thus have great potential in large-scale industry applications. [ABSTRACT FROM AUTHOR]

In the present study, a comparative study about the hardening effect of solute segregation at { 10 1 ¯ 2 } twin boundaries (TBs) on a { 10 1 ¯ 2 } twinning predominant deformation and a detwinning predominant one was carried out. The influence of the pre-straining and subsequent annealing conditions on mechanical behavior was systematically addressed. Our results show that solute segregation at TBs can occur even at 100 °C. The annealing at 100 °C for 20 min induces a partial segregation at TBs, while that at 150 °C or higher temperature for 20 min can induce a complete solute segregation. The annealing conditions and pre-strain levels generate quite different effects on deformation by twinning and that by detwinning. Both annealing hardening and annealing softening might happen during the twinning predominant recompression. Annealing hardening occurs only with pre-strains of 3.0% and 5.5% after annealing at 100 °C for 6 h. A higher pre-strain or a higher annealing temperature or a longer annealing time generate a higher annealing softening effect. However, during the detwinning predominant recompression, all used annealing treatments generate hardening effect in all the pre-strained samples. With a complete solute segregation at TBs, a hardening of about 11–25 MPa is generally achieved. It is also found that solute segregation at TBs reduces the strain hardening rate of deformation by TBs migration. [ABSTRACT FROM AUTHOR]