Mechanical Properties And Deformation Mechanisms Of Hard-Plate Rolled Rare-Earth Magnesium Alloys

Recently,Rare-earth（RE）magnesium（Mg）alloys,which exhibit high strength,have attracted extensive attention from researchers both domestically and internationally.However,high density of precipitates will form in the matrix of Mg-RE alloys during deformation,leading to an increased difficulty of deformation.The rolled Mg-RE alloy sheets generally exhibit high strength but low ductility and poor formability.Additionally,the various morphologies and sizes of precipitates in Mg-RE alloys introduce complexities in understanding their interaction of dislocation slip,which is necessary to make a detailed investigation.Furthermore,the eutectic phases with large size and high melting point are easily formed in RE-containing Mg alloys,which will greatly impair the ductility.Thus,it is important to explore methods for the improvement of the strength and ductility of alloys containing coarse eutectic phases.In this study,the Mg-4Y-3RE-Zr and Mg-8Al-0.5Zn-0.8Ce alloys are selected and studied around the themes of mechanical properties and deformation mechanisms as well as the microstructure evolution.The precipitation behaviors and deformation mechanisms of the bimodal-grained Mg-4Y-3Nd-0.5Zr with high strength-ductility synergy have been studied.The influences on the deformation mechanisms of the precipitates in the aged Mg-4Y-2Nd-1Gd-0.5Zr alloy have been revealed.The reasons for the improved strength and ductility by fabrication of high-density low-angle boundaries in the Mg-8Al-0.5Zn-0.8Ce alloy,which contains large Al₄Ce secondary phases,have been investigated.The main conclusions are as follows:（1）The precipitation behavior and deformation mechanisms of the bimodal-grained Mg-4Y-3Nd-0.5Zr alloy have been investigated.Based on the inhomogeneous deformation of the HPR process,the dynamic precipitation behavior of fine grains and coarse grains in the bimodal-grained Mg-4Y-3Nd-0.5Zr alloy is different,where equilibriumβphases are located at grain boundaries in fine grain area,while metastableβ₁ phases are located inherent coarse grains.The dominant deformation mechanism is basal<a>slips in fine grain area during room-temperature tension due to its weak basal texture,while a mass activation of non-basal slips have been observed in coarse grain area under the combined effects of grain orientation,solid solute elements,precipitates and the heterogeneous deformation of bimodal-grained alloy.（2）The reasons for the high strength and ductility of the bimodal-grained Mg-4Y-3Nd-0.5Zr have been revealed.The bimodal-grained Mg-4Y-3Nd-0.5Zr alloy exhibits high strength-ductility synergy,i.e.,yield stress of～311.7 MPa,ultimate tensile strength of～322.1 MPa and fracture elongation of～11.8%.The high yield is mainly attributed to the grain boundary strengthening from ultra-fine grains. The high ductility originates from the slip transfer between fine grains and the activation of non-basal slips in coarse grains.（3）The strongly impeding effect of prismatic plate-shaped precipitates on basal<a> dislocations in the aged Mg-4Y-2Nd-1Gd-0.5Zr alloy has been revealed.After peak aging at 200°C and 250°C,the prismatic plate-shapedβ’phases reduce the gap of the critical resolved shear stress（CRSS）between basal<a>and non-basal dislocation slips,which facilitates the activation of non-basal dislocation slips during tensile deformation.Additionally,the alloys peak aged 200°C and 250°C exhibit high work hardening ability,leading to strongly improved strength.The alloy aged at 200°C exhibits a yield strength of～221.3 MPa and an ultimate tensile strength of～334.4 MPa.（4）The dominant role of the effective planar inter-obstacle spacing（λ）of prismatic plate-shaped phases in the strongly impeding of basal<a>dislocation slips has been revealed.The proportion of the activated non-basal dislocation slips decreases with the increase of the aging temperature.Theλof the prismatic plate-shapedβprecipitates is largest in the sample aged at 300°C,resulting in a similar activation of dislocation slips to that of the as-solutionized sample without precipitation phases.（5）A high strength-ductility synergy has been achieved in the Mg-8Al-0.5Zn-0.8Ce alloy containing large Al₄Ce secondary phases.A rotated hard-plate rolling（RHPR）process has been developed based on the HPR process.The RHPRed Mg-8Al-0.5Zn-0.8Ce alloy exhibits high strength-ductility synergy,i.e.,a yield strength of～308.2 MPa,an ultimate tensile strength of～359.9 MPa and a fracture elongation of～13.8%,which shows strongly increase in yield strength of ～161 MPa and only a marginal decrease in elongation of～2.8%as compared to the initial extruded alloy.（6）The mechanisms for the formation of microstructure and the enhanced properties of the RHPRed Mg-8Al-0.5Zn-0.8Ce alloy have been revealed.A high density of low angle boundaries（LABs）has preserved in the RHPRed Mg-8Al-0.5Zn-0.8Ce alloy due to the incomplete continuous dynamic recrystallization caused by multi-passes rolling with reduced thickness reduction and the dislocation recovery during intermediate annealing.The high density of LABs not only leads to increased yield strength but also coordinates tensile strain,preventing the initiation of microcracks at Al₄Ce phases.