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The high coefficient of thermal expansion (CTE) of magnesium alloys leads to poor thermal dimensional stability and limits its application in the electronic field. This work focused on understanding the effect of rare earth (RE) on the thermal expansion behavior of Mg and attempted to tailor the CTE with SiC particulates. Composites were manufactured by gravity casting with ultrasonic dispersion treatment. The effect of 3 wt.% RE (Sm, Nd, Gd, Y) on the microstructure and thermal expansion behavior of the Mg alloys was explored. Adding RE can lead to the formation of RE-containing second phases and marginally decreased the CTE of magnesium. The CTEs of Mg-3Sm, Mg-3Nd, Mg-3Gd and Mg-3Y alloys were 27.3 × 10−6/°C, 27.0 × 10−6/°C, 27.2 × 10−6/°C and 27.8 × 10−6/°C, respectively. The influence of SiCp on the thermal expansion behavior of Mg-3Y was also investigated. The SiCp promoted the precipitation of Mg24Y5 phase in the extruded Mg-3Y/SiCp composites and significantly reduced the CTE. The CTEs of Mg/3SiC, Mg-3Y/1SiC, Mg-3Y/3SiC and Mg-3Y/5SiC were 27.4 × 10−6/°C, 26.8 × 10−6/°C, 26.6 × 10−6/°C and 23.3 × 10−6/°C, respectively. Rule of mixture, the Turner model and the Kerner model were utilized to predict the CTE of the Mg-3Y/xSiCp (x = 1, 3, 5) composites. The predictions of the Kerner model show a better correlation with the experimental results. [ABSTRACT FROM AUTHOR]

The dynamic tensile properties and microstructural evolution of an extruded EW75 magnesium alloy deformed at ambient temperature and different high strain rates (from 1000 to 3000 s-1) along extrusion direction (ED) were investigated by Split Hopkinson Tension Bar (SHTB). The corresponding deformation mechanisms, texture evolution and microstructure changes were analyzed by optical microscope (OM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The results show that the extruded EW75 magnesium alloy along ED exhibits a conventional positive strain rate sensitivity that the dynamic flow stresses increase with increasing strain rate. Texture measurements show that after dynamic tension, the initial weak texture of extruded EW75 magnesium alloy tansforms to a relatively strong <10-10>//ED texture with increasing strain rates. The microstructural analysis demonstrates that dislocation motion are main deformatin mode to accommodate dynamic tensile deformation at high strain rates. In addition, the interactions of dislocation-dislocation and dislocation-second phase lead to the increase of flow stress and strain hardening with increasing strain rate. [ABSTRACT FROM AUTHOR]

[Display omitted] • A new progressive polycrystalline yielding criterion was successfully established based on four reasonable assumptions, particularly the assumption that elastic strains of comparable magnitudes during the yielding stage can coordinate with plastic strains. • The new yielding criterion can effectively replicate experimental results. • This new criterion better reflects the inherent mechanisms of yielding compared to phenomenological models and requires less computational effort than VPSC or EVPSC. When the equivalent plastic strain reaches 0.002, metals can be treated as yielded. In this study, at the yielding stage, using the assumptions such as: 1) elastic strain can assist in coordinating plastic strain; 2) ignoring the need to activate five independent slip systems simultaneously; 3) the strain hardening of slip systems to be linear; and 4) ignoring the intergranular strain coordination behavior, a new progressive polycrystalline yield criterion is established. This new criterion better reflects the inherent mechanisms of yielding compared to phenomenological models and requires less computational effort than VPSC or EVPSC. Experimental validation results demonstrated that the new criterion accurately reproduced experimental outcomes. [ABSTRACT FROM AUTHOR]

In this paper, in situ electron back scattered diffraction combined with scanning electron microscopy and digital image processing techniques were used to study the microscopic deformation compatibility of AZ31 Mg alloy rolled sheets with strong basal texture during biaxial tension at room temperature. The results firstly quantitatively showed that the distribution of microscopic strain in AZ31 rolled sheet was inhomogeneous during formation. Strain concentrations happened in some regions even at early stage of deformation. Short distorted bands also appeared in these regions and extended to connect each other with increasing macroscopic strain. The appearance of distorted bands play an important role in strain compatibility since the materials lack easy deformation modes at room temperature. Besides slips and twins, the grain distortion should also be an indispensable mechanism in accommodating plastic deformation in some parts in distorted bands. According to the analysis from the Schmid factor, the geometric compatibility factors and slip trace identification, the comprehensive effect of strong basal texture and biaxial tensile stress state not only led to the quite low SFs for prismatic , pyramidal slips and tensile twinning, but limited the compatibility between slip systems and tensile twins. This mechanism was the main reason for the lack of easy deformation modes and the appearance of distorted bands, which further led to the inhomogeneous strain distribution at room temperature. Therefore, the formability of Mg alloys rolled sheets with strong basal texture was weakened at room temperature. [ABSTRACT FROM AUTHOR]

In this paper, two kinds of micro-tensile test systems for in-situ Electron Back-Scattered Diffraction (EBSD) are developed to explore the difference in microstructure evolution of AZ31 alloy under biaxial tensile stress state (BTSS) and uniaxial tensile stress state (UTSS). The in-situ EBSD results show that there is an apparent difference of texture evolutions during deformation under BTSS and UTSS and the different responses of tensile twins (TTs) in various stress states are proved to be the main reason. A modified calculation of Schmid factor (SF) for varied stress states further reveals that the distribution of TT SF on {0001} pole figure (PF) determined by BTSS significantly differs from that under UTSS. In addition, the change of stress states also affects the deformation compatibility of TT to slip systems. The poor formability of Mg alloy rolled sheets partly results from the incompatibility between TT and slip systems or tensile twinning under BTSS. [ABSTRACT FROM AUTHOR]

• The Mg-1Gd/Mg-13Gd (wt.%) laminate with dual-heterostructure grain size and texture between alternating layers was designed and fabricated via accumulated extrusion bonding, exhibiting an excellent strength-ductility synergy. • Strain delocalization in the fine-grained (FG) layers was realized by generating dispersed stable strain bands, which promoted the activation of ductile mechanisms. • The lower level of geometric compatibility factor at the interface, with its heterogeneous texture, aggravated deformation incompatibility and boosted the back stress. The higher shear stress field at the interface generated by back stress led to the activation of pyramidal slip and enhanced the strength of the coarse-grained layers. • The elevated forward stress induced by the high back stress triggered the activation of non-basal slips at the interface in the FG layers, which acted together with the abundant basal slips caused by the random texture to improve the ductility of the FG layers. Heterostructures can effectively break the traditional strength-ductility trade-off dilemma. However, how the texture of materials with heterostructure under hetero-deformation induced stress (including back stress and forward stress) influences the activation of the deformation mechanism is still not clear. In this paper, a Mg-1Gd/Mg-13Gd (wt.%) laminate with an alternating distribution of dual-heterostructure grain size and texture, where the coarse-grained (CG) layers presented a bimodal texture along the extrusion direction and the fine-grained (FG) layers showed random texture, exhibited an excellent strength-ductility synergy. Strain delocalization in the Mg-13Gd layer was realized by generating dispersed stable strain bands, which promoted the activation of ductile mechanisms. In the CG layers, the bimodal texture facilitated the activation of pyramidal slip to maintain the continuity of strain at the interface. The lower level of geometric compatibility factor at the interface, with its heterogeneous texture, aggravated deformation incompatibility and boosted the back stress. The higher shear stress field at the interface generated by back stress led to the activation of pyramidal slip and enhanced the strength of the CG layers. In the FG layers, the forward stress promoted the activation of prismatic and pyramidal slips at the interface; they acted together with the abundant basal slips caused by the random texture to improve the ductility of the FG layers. The results of this work will promote the development of heterogeneous theory in textured Mg alloys. [ABSTRACT FROM AUTHOR]

Mg alloys usually deform under complex biaxial compressive stress states (e.g. during drawing, forging and extrusion). In order to better understand how the biaxial compressive stress state influences the evolution of the microstructure and the deformation compatibility, the uniaxial and biaxial compression tests of AZ31 rolled sheets were performed at room temperature. The new self-developed biaxial compression devices offered a possibility to obtain the evolution of the microstructures by in situ measurements via electron backscatter diffraction. The analysis of geometrically necessary dislocation and the simulation based on the visco-plastic self-consistent method were used to examine the microscopic behaviors in the sheets during the uniaxial and biaxial compression tests. The results indicated that the reorientation of the texture during the biaxial compression was different from that during the uniaxial compression. The simulated results based on the visco-plastic self-consistent model suggested that the equivalent yield strength under a biaxial compressive stress state was lower than that under a uniaxial stress state. This was due to the higher relative activity of extension twins under biaxial compressive stress state. This also led to a lower geometrically necessary dislocation density in the biaxial compressed sample. The geometrically necessary dislocations were also more homogeneously distributed with a biaxial compressive stress state. The analysis of the activation of all the deformation mechanisms proved that the appearance of extension twinning variants limited the activation of prismatic slip during the uniaxial compression while promoting prismatic slip during the biaxial compression. This changeable interaction between extension twinning and prismatic slip combining with the lower geometrically necessary dislocations, the lower flow stress inferred that the application of the biaxial compressive stress state during processing at room temperature was a way to improve the deformability of Mg alloys. [ABSTRACT FROM AUTHOR]

In this paper, the Zn additions combined with a novel severe plastic deformation (SPD) technology, screw twist extrusion (STE), is used to prepare Mg-xZn-1Mn alloys (x=0, 2, 4, 6) with fine grains and high ductility. By observing the microstructures, it is found that a gradient microstructure is obtained perpendicular to the extrusion direction (ED) in all four samples after STE. The STE process introduces twist extrusion strain into the samples, which causes the grain to twist and break, resulting in the weakening of the texture and the refinement of the grain. In addition, MgZn 2 phase is formed by the addition of high Zn, and the combination of MgZn 2 with twist shear strain further promoted dynamic recrystallization (DRX) to refine the grain and obtain the preferred texture, resulting in the increase of fracture elongation (FE). The FE of Mg-6Zn-1Mn sample improved from 18.4% to 27.8%, an 51.1% increase. This is mainly associated with the DRX mechanisms of continuous dynamic recrystallization (CDRX), discontinuous dynamic recrystallization (DDRX) and particle simulated nucleation (PSN). Meanwhile, the Schmid factor (SF) of basal slip increased with the addition of Zn, the basal slips in various samples are promoted obviously and FE is further enhanced. Thus, the novel STE technology combining with Zn additions provides an effective approach to improve the ductility of Mg-Zn-Mn alloys. • The ductility of Mg-Zn-Mn alloy is enhanced by Zn addition combined with STE process. • A variety of DRX behaviors have been studied in STEed Mg-χZn-Mn alloys. • The method proposed in this paper to enhance the ductility of Mg-Zn-Mn alloys is a good choice for the application of degradable biomedical devices. [ABSTRACT FROM AUTHOR]

Mg-RE (magnesium-rare earth) binary alloys, such as Mg–Y alloys, usually exhibit weakened texture and excellent plasticity compared with traditional RE-free Mg alloys. However, the weakened texture could lead to a significant decrease in the strength of Mg-RE binary alloys. In this work, the weak-textured Mg–3Y (wt. %) extruded sheet displayed good strength-ductility synergy with a 41% increase in the yield strength and a good ductility similar to the initial sheet via simple pre-twinning and annealing treatment (PT). After PT, the average grain size of the sheet decreased from 11.2 μm to 6.0 μm and 34.2% initial soft texture component (<11–21> //normal direction (ND)) was transformed into the hard texture component (<0001> //transverse direction (TD)). The effect of pre-twinning on deformation mechanisms was systematically investigated by slip trace analysis and activation stress ( τ ) analysis. Basal slip was the dominant deformation mechanism during the tension of the as-extruded (AE) specimen while pre-twinning promoted the activation of non-basal slip, especially prismatic slip in the PT specimen. The texture transformation and reduced critical resolved shear stress (CRSS) ratio between non-basal slip and basal slip induced by pre-twinning were primarily responsible for the high activity of non-basal slip. The underlying strengthening mechanism and the reason for good ductility were also rationally discussed. The strengthening effect of single grain refinement was relatively lower in the weak-textured Mg–3Y alloy. Texture hardening could effectively improve the effect of fine-grain strengthening. The excellent deformation compatibility between basal slip of parent grains and prismatic slip of twins as well as relatively higher activity of the second-order pyramidal slip was the origin of the good ductility of the PT specimen. [ABSTRACT FROM AUTHOR]

In this work, a novel texture control strategy, shear strain-induced twin orientation regulation (SITOR), was employed to fabricate the Mg–3Al–1Zn (AZ31) alloy with significantly improved ductility. The SITOR-1P sample processed at 250 °C exhibited a homogeneous microstructure and an almost ideal shear texture (c-axes of grains tilted 45° from the extrusion direction to the transverse direction), which resulted in an enhanced fracture elongation from 14.3% to 30.6% owing to the high activity of basal slip when compared with as-extruded Mg alloys. With increasing shear strain, the concentration of shear texture components in the SITOR-4P sample increased, leading to further improvement in its mechanical performance. While specimens produced by the conventional shear strain-induced orientation regulation (SIOR) method also underwent the same 4 passes of shear deformation, both their texture deflection and performance enhancement remained limited. The current investigation proposes a viable way to largely activate basal slip via the SITOR scheme, therefore offering a fresh perspective for addressing the poor plasticity of Mg. • Excellent ductility was achieved via the newly proposed shear strain-induced twin orientation regulation (SITOR) scheme. • The ductility improvement of SITORed samples can be ascribed to the rapid formation of shear texture and the promoted activation of basal slip. • The present SITOR scheme can act as a new promising way for plasticity enhancement in Mg. [ABSTRACT FROM AUTHOR]

In this study, accumulative extrusion bonding (AEB) was used to process Mg–1Gd/Mg–13Gd laminated composites, in which the Mg–13Gd layers had much finer grains than the Mg–1Gd layers. This grain size difference was maintained after 2-pass, but diminished after the 3-pass. The 2-pass composite samples exhibited an excellent combination of ultimate tensile strength (291 MPa), elongation (20.6%) and yield strength (144 MPa), which are much superior than the individual component materials. Moreover, the 2-pass samples also exhibited maximum extra strength and ductility that is higher than what is predicted by the rule-of-mixture. The observed high strength and ductility are mainly attributed to back-stress strengthening and work hardening, respectively. The 2-pass samples exhibited the highest back stress, indicating an optimum laminate thickness for producing the highest back stress hardening. This work provide a new understanding on the design of heterogeneous laminated Mg alloys for improving mechanical properties. [ABSTRACT FROM AUTHOR]

The special microstructure distribution of the friction stir welded (FSW) Mg alloys joints is usually responsible for the non-uniform deformation and poor mechanical properties. Hence, it was possible to improve the joint mechanical properties by modifying the microstructure. In the present study, two kinds of AM60/AZ31 Mg alloys dissimilar FSW joints with different microstructure were obtained using different rotation rates (800 rpm and 1600 rpm). The results of mechanical tests showed that the tensile strength and elongation of the joint at 1600 rpm were 13.5% and 115% higher than those at 800 rpm, respectively. The microstructure observation indicated that the grain size of various zones in the joint at 1600 rpm was obviously larger than that at 800 rpm, especially at the AM60 side. However, there was no obvious difference in the texture distribution of the two welds. In addition, microstructure observation after fracture showed that there were more twins in AM60 side of the joint with coarse grains. These coarse grains promoted the activation of basal slip and twinning in the regions far away from the nugget zone (NZ) interface. This weakened the strain localization of the joint to some extent. Meanwhile, the coarse grain joint had lower concentration degree of the strain and dislocation near the NZ boundary compared with the fine grain joint. These factors led to better mechanical properties of the joint with coarse grains. This work could provide some guidelines for improving the inhomogeneous deformation behavior of dissimilar Mg alloys FSW joints. [ABSTRACT FROM AUTHOR]