Preparation,Structure And Properties Of Nano-/Ultrafine-structured Magnesium Alloys

Magnesium and its alloys have been widely applied in the field of aerospace,automotive industry and electronic products due to their advantages of large reserves,low density and high specific strength.However,the further applications of magnesium and its alloys is seriously restricted by their disadvantages of low absolute strength,poor plasticity and toughness,low thermal stability,and so on.Nano-/ultrafine-structured metallic materials show many outstanding physical and mechanical properties such as ultrahigh strength,hardness,wear resistance and low-temperature superplasticity.In this dissertation,magnesium and its alloys are thus selected to study.Based on the thermodynamic and kinetic stabilization mechanisms,nanocrystalline Mg-X binary alloys and Mg matrix nanocomposites are successfully fabricated,respectively,by mechanical alloyingandpowdermetallurgytechnologies.Inaddition,aseriesof nano/ultrafine-structured ternary Mg-Y-Zn multiple phase alloys are fabricated by adjusting the synthesis process of mechanical alloying and powder metallurgy.The corresponding relationships among microstructures,thermal stability and mechanical properties were studied systematically.According to the thermodynamic stability maps established by Darling et al.and Hume-Rothery rules,six possible stabilizing solutes,Ti,Zr,Ta,Co,Cr,La,are mechanically alloyed with Mg.Extended solid solubility of Ti in Mg is achieved,which is mainly attributed to three factors:the same crystallographic structure of Ti and Mg,the small atom size misfit and the difference in electronegativity value between Ti and Mg.The thermal stability is largely enhanced from¹00 ? for nanocrystalline Mg to⁴50? for Mg-1.5at%Ti alloy after 50 hours of milling.STEM observation and EELS analysis suggest that the enhanced thermal stability in Mg-Ti alloy be mainly attributed to the thermodynamic stabilization,i.e.,the segregation of Ti solute at grain boundaries during annealing.The excellent thermal stability of Mg-1.5at%Ti alloy enables us to consolidate the alloy powders at a high temperature and achieve a bulk nanocrystalline Mg-1.5at%Ti alloy?with an average grain size of¹12 nm?with both a high strength of 202 MPa and a large fracture strain of above 0.8.Mg-xMgO and Mg-10vol%MgF₂ nanocomposites are successfully fabricated by reactive milling.Dispersed MgO and MgF₂ nanoparticles with a fine particle size smaller than 10 nm are mainly situated at Mg grain boundaries and exhibit a strong atomic interfacial bonding with Mg matrix.The thermal stability is largely enhanced from¹00? for nanocrystalline Mg to 400 ? for Mg-10vol%MgO and⁴50 ? for Mg-10vol%MgF₂ due to the Zener pinning effect of the in-situ formed nanoparticles.The hardness of the Mg-MgO nanocomposites increases linearly with the content of MgO nanoparticles and reaches 3.65 GPa for Mg-40vol%MgO.The bulk Mg-10vol%MgO nanocomposite achieved by high-pressure consolidation has a high compressive yield strength of 562 MPa and a fracture strain of 16.3%.The bulk Mg-10vol%MgF₂nanocomposite has a high compressive yield strength of 582 MPa and a fracture strain of10.2%.Meanwhile,the MgF₂ nanoparticles can pin the grain-boundary and thus significantly improve the high-temperature hardness of nanocrystalline Mg.A ternary Mg-Y-Zn solid solution alloy is successfully fabricated by mechanical milling of prealloyed Mg₈₅Y₉Zn₆ powders for 30 hours.The Mg-Y-Zn solid solution alloy with an average grain size of⁸.3 nm shows a high hardness of 2.65 GPa.At 400 ?,the average grain size of the ternary Mg-Y-Zn alloy is still as small as⁶6.5nm.In addition,the hardness is still as high as².25 GPa.The enhanced thermal stability of the ternary Mg-Y-Zn solid solution alloy is mainly attributed to the Zener pinning effect of the in-situ precipitated nanoparticles.A LPSO-strengthened and ultrafine-structured Mg-Y-Zn multiple-phase alloy with a core-shell structure is successfully fabricated by high-pressure consolidating the mechanical alloyed Mg₈₅Y₉Zn₆ prealloyed powders and subsequent annealing treatments.The outer shell layer of the Mg-Y-Zn multiple-phase alloy consists of in-situ precipitated18R-LPSO,Mg₃Y₂Zn₃ and a large number of dispersed nanophases.The core domain is an in-situ precipitated elongated 18R-LPSO phase.In addition,a network structured Mg-rich phase appears at the original powder particle boundaries.The Mg-Y-Zn alloy fabricated by annealing at 400 ? shows a high compressive yield strength of 801 MPa,a fracture strength of 827 MPa and a fracture strain of 11.2%.The high strength is mainly attributed to the in-situ precipitated ultrafine 18R-LPSO phase and the strengthening effect of the dispersed nanoparticles in the outer shell layer.Meanwhile,the network structured Mg-rich phase improves the fracture resistance of the annealed alloy.The Mg-Y-Zn alloy after annealing at 300 ? for 50 hours maintains a high room-temperature hardness of 1.88GPa.In addition,The Mg-Y-Zn alloy shows a high-temperature hardness of 1.28 GPa and a high compressive yield strength of 382 MPa at 250 ?.The enhanced thermal stability and high-temperature mechanical properties of the Mg-Y-Zn alloy are mainly attributed to the high thermal stability of LPSO phase and the kinetic pinning effect of the in-situ precipitated nanoparticles.A LPSO-strengthened and ultrafine-structured Mg-Y-Zn multiple-phase alloy with a core-shellstructureissuccessfullyfabricatedbymechanicalmillingof prealloyed powders for 5 hours and subsequent spark plasma sintering?SPS?method.The outer shell layer of the Mg-Y-Zn multiple-phase alloy consists of in-situ precipitated18R-LPSO,a large number of dispersed nanophases and a small number of Mg₃Y₂Zn₃phase.The core domain is an in-situ precipitated elongated 18R-LPSO phase.The porosity-free Mg-Y-Zn alloy fabricated by SPS at 425 ? shows a high room-temperature compressive yield strength of 772 MPa,a fracture strength of 807 MPa and a fracture strain of 16.7%.The high strength is mainly attributed to the porosity-free microstructure,in-situ precipitated ultrafine 18R-LPSO phase and the strengthening effect of the dispersed nanoparticles in the outer shell layer.Meanwhile,the large fracture strain of the Mg-Y-Zn alloy fabricated by SPS is mainly attributed to the porosity-free microstructure and the decrease in the amount of the brittle Mg₃Y₂Zn₃ phase.In addition,The Mg-Y-Zn alloy fabricated by SPS at 425 ? shows excellent high-temperature mechanical properties with a high compressive yield strength of 432 MPa and a fracture strength of 503 MPa at 250?.