Microstructure And Mechanical Properties Of Mg-Gd-Y-Zn-Zr Alloy Processed By High Pressure Torsion

Magnesium alloy,as one of the lightest and green engineering structural materials,has many advantages such as low density,high specific strength and specific stiffness.It has been widely applied in many fields such as aerospace,automobile,electronic communication and so on.In recent years,Mg-RE-Zn alloys have received widely attetion because of their special long periodic stacking ordered?LPSO?structure and excellent mechanical properties.However,as compared with aluminum alloys,magnesium alloys still show low strength and poor plasticity at room temperature,whichrestricttheirfurtherpracticality.Inpresentwork,the Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr?wt.%?alloy with different initial microstructures is subjected to high pressure torsion?HPT?processing at room temperature in order to obtain the nanoscaled grains,and both the microstructure evolution and mechanical properties during HPT processing are analyzed;The ageing response and thermal stability of Mg-Gd-Y-Zn-Zr nanostucture are systematically investigated,in which the ageing precipitation behaviour,hardening mechanism and thermal stability mechanism are revealed.The as-cast Mg-Gd-Y-Zn-Zr alloy is deformed by HPT processing at room temperature.When the equivalent strain increased up to 6.0,the mean grain size decreases to 55 nm,and the hardness reaches a saturated value of¹15 HV.The coarse netlike Mg₃RE second phase is gradually broken into fine dispersed particles and the dislocation density increases.Such grain refinement is is much stronger than that for HPT-processed conventional Mg alloys.The hardness is higher than that achieved by conventional extrusion/rolling of this alloy.The main hardening mechanism is the grain refinement.The homogeneously distributed fine second phase particles and the high dislocation density also play a role in hardening.The solution-treated Mg-Gd-Y-Zn-Zr alloy is subjected to HPT processing for 10 turns at room temperature to produce a nanostructure with a mean grain size of 48 nm.The corresponding hardness is increased to 126 HV.Under the similar HPT processing conditions,the solution-treated alloy exhibits finer grains and higher dislocation densities as compared to as-cast of this alloy.The increased solute contents in the matrix enhance the solute-dislocation and/or dislocation-dislocation interactions,resulting in a higher dislocation density.This causes a larger saturated hardness than that of the as-cast alloy deformed by HPT.During HPT processing of the annealed Mg-Gd-Y-Zn-Zr alloy containing numerous LPSO phase at room temperature,as the strain increases,the LPSO phase experiences kink bending,fragmentation and dissolution,and eventually a supersaturated solid solution forms.The decomposition of LPSO phase can be attributed to the introduction of amounts of defects in the LPSO phase and the Mg matrix.The grain size is refined to 52 nm as the equivalent strain increases to 6.6,and the corresponding hardness reaches 128 HV.The combined effects of nanosized grains,high dislocation density and dissolved alloying elements are accounting for the hardness enhancement.HPT processing is carried out at room temperature on the peak-aged Mg-Gd-Y-Zn-Zr alloy.At the early stages of deformation,dislocation generation and pile-ups are promoted by the nanosized??precipitates.As the strain increases,the precipitates are cut by the moving dislocations and gradually dissolved into the?-Mg matrix aided by dislocations,which work as a diffusion channel for the solute atoms.After 2 turns the hardness reaches a peak,and with further deformation,the microstructure refinemenrt continues and the dislocation density is increased,but the softening effect deriving from dissolution of precipitates causes a reduction in hardness.After 16 turns,the dissolution of precipitates is complete,and the mean grain size is³3nm.It reveals that the effect of nanosized??phase in grain refinement during HPT is more significant.As compared to the conventional thermomechanical processing of Mg-Gd-Y-Zn-Zr alloy,HPT-processed nanostructure shows a different ageing precipitation behaviour and ageing strengthening mechanism.The temperature of optimal peak ageing is decreased remarkably and the corresponding ageing time is shortened.The peak-aged hardenss?120 ?/12 h?reaches 145 HV.For the HPT-processed nanostructure,the main structural factor of ageing hardening is the solute segregation formed at grain boundaries?GBs?,rather than any other precipitates.The stable?phase precipitates directly in the nanostructure during overageing,and the metastable phase precipitated in the conventional micronsized alloys cannot be observed.The solute segregation at GBs is the main ageing strengthening mechanism of the Mg-Gd-Y-Zn-Zr nanostructure,and a strengthening model for solute segregation is proposed.The nanostructured supersaturated solid solution,which is prepared by the peak-aged Mg-Gd-Y-Zn-Zr alloy processed by HPT for 16 turns,is subjected to peak ageing at 120 ?.The hardness increases to 156 HV,which far exceeds those of other reported Mg alloys obtained by thermal mechanical processing and ageing treatment,and also is higher than that of the T4+HPT+T5 peak-aged sample.In addition to the hardening effects of solute segregation and high dislocation desity,the significant grain refinement?33 nm?leads to an ultrahigh hardness of the alloy.The Mg-Gd-Y-Zn-Zr nanostructure shows a good thermal stability.The grain size remains 55 nm after annealing at 300 ? for 0.5 h.As the temperature rises to 400 ?,the grain size is controlled within 500 nm.The formation of solute segregation at200-300 ? and the precipitation of the?equilibrium phase at 350-400 ? can inhibit the grain growth by the solute drag effect and Zener pinning effect,respectively.Due to the formation of solute segregation at GBs,a maximum hardness can be achieved by annealing of the HPT-processed Mg-Gd-Y-Zn-Zr nanostructure at 300 ?.