Study On Microstructures And Mechanical Behaviors Of High Strength And Heat Resistant Cast Mg-Gd-Y-Zr Alloys

Mg-Gd-Y-Zr?GWK?alloy is one of the new high-strength and heat-resistant magnesium alloys that have the greatest potential for aerospace applications.The current research is devoted to further improving strength or comprehensive mechanical properties through alloying,improving processing technology,etc.However,to achieve successful and mature applications of the alloy system in aerospace parts that may undergo complex service conditions,mechanical behaviors of the alloy under various internal and external conditions must be thoroughly investigated.However,in general,there is little research on this aspect,especially on the plastic deformation mechanism of casting alloys.In this paper,the effects of Gd/Y content,strain rate and loading mode on the mechanical behavior of room temperature are systematically studied.The results show that for the Mg-Gd-Y-Zr alloy with high rare earth content,when the total amount of Gd and Y is equal?13wt.%?,with the decrease of Gd/Y ratio,the room temperature tensile ductility of the as-solutionized alloy decreased significantly.Under different aging conditions,the volume fraction and the spacing of precipitates are the key parameters determining the yield strength and plasticity of the alloy,respectively.When the volume fraction of dispersed distributed precipitates is the same,the smaller the spacing of precipitates,the worse the tensile ductility at room temperature.The effect of strain rate on the microstructure and mechanical behavior of GW103K alloy was studied.The results show an anomalous phenomenon that the tensile ductility of Mg-Gd-Y-Zr alloys in both as-solutionized and as-aged states is positively correlated with strain rate at room temperature within quasi-static strain rate range(10^-5 to 10^-1 s^-1).The reason is that RE elements can reduce the critical shear stress of non-basal slip and inhibit {1012} twinning.Combined with high local stress at grain boundaries caused by fast dislocation pile-up at high strain rate,non-basal slips are significantly activated,especially the pyramidal<c+a>,which promotes the non-Schmid factor behavior of the basal slip,as well as multi-slip and cross-slip,ensuring that the alloy has good work hardening and strain coordination.By comparing the tensile and compressive microstructures and mechanical behaviors of a GW103K alloy,it was found that cast GWK alloys don't have tensile-compressive yield asymmetry.Compared with tension,compression exhibit higher maximum shear stress with higher softness coefficient a=2,which directly facilitated the occurrence of twinning.Since twinning was the coordinating deformation mechanism,the room temperature compression ductility of this cast alloy is independent of the strain rate.The medium-high temperature mechanical behaviors of GWK alloy were also studied.It was found that this alloy has ductility drop behavior at intermediate-temperature?of?350??,which is independent of strain rate.Partial precipitation at grain boundaries,which is associated with the segregation of rare earth elements,is the cause of the grain boundary embrittlement at intermediate-temperature?350 ??that responsible for the loss of ductility.The introduction of a large number of twin lamellas in the initial microstructure can effectively eliminate this intermediate-temperature ductility drop phenomenon,and was also found facilitate to the ductility.The twinned GWK alloy was even superplastic at high temperatures.The perfect static recrystallization of the twinned structure during the short-term heat preservation process before high temperature stretching is the decisive factor of subsequent dynamic recrystallization and superplasticity.The superplastic deformation mechanism of the twin is dominated by the slide-controlled dislocation creep mechanism assisted by grain boundary sliding.In summary,a comprehensive and systematic study on the microstructures and mechanical behaviors of new high-strength heat-resistant GWK alloys was conducted in this work,the results of which supplements the mechanical properties database of the alloy system.With revealing the deformation mechanism of the alloy under different deformation conditions,this work provides a theoretical basis for the regulation of mechanical properties of this GWK alloy via adjusting microstructures.