Mechanical Behaviors And Fracture Micromechanisms Of High Strength And Heat Resistant Cast Mg-Gd-Y-Zr Alloy

Cast Mg-Gd-Y-Zr alloys are promising for application in aerospace and defense industries for their excellent strength and creep resistance.In the past two decades,extensive research has been conducted to investigate the microstructures and mechanical properties of Mg-Gd-Y-Zr alloys.However,so far little attention has been paid on the safety-concerning mechanical properties related to fracture processes,including notch strength and fracture toughness,in these alloys.Moreover,knowledge of the fracture behaviors,the operating fracture micromechanisms,and the impacts of fracture on the mechanical properties in these alloys is lacking.In this study,a series of mechanical tests were performed on cast Mg-Gd-Y-Zr alloys;the deformation and fracture behaviors were investigated by means of electron microscopy,electron backscattered diffraction analysis,and finite element analysis.First,the mechanical behaviors of a cast Mg-Gd-Y-Zr alloy tested in tension at ambient to elevated temperatures were investigated;the transition in fracture modes with increasing temperature and its impact on tensile ductility were revealed.Premature fracture occurs with the Considere criterion unsatisfied,resulting in a low uniform elongation of～4%.At 200℃,ductile fracture occurs after slight necking,with an enhanced uniform elongation of～9%.At 250℃,however,grain boundary sliding occurs and induces intergranular cracks,resulting in damage and accelerated ductile fracture,reducing the uniform elongation to～7%.Cleavage microcracks formed along the basal plane are observed in samples deformed at room temperature,and are considered as the source of premature fracture.Deformation twinning does not induce fracture at room or elevated temperatures.The deformation and fracture behaviors under the effect of stress concentrations induced by notches were investigated.The plastic deformation at the notch tip is dominated by basal slip,with trivial twinning activity.Notch fracture is controlled by the critical von Mises stress at the notch tip,and occurs at a rather low plastic strain level;the result is serious notch weakening.The low-strain notch fracture is initiated by the multiplication and coalescence of basal cleavage microcracks,which are nucleated by the stress concentration due to basal<a>dislocations piled up against grain boundaries,in the plastically deformed region adjacent to the notch tip.The relationship between the formation of cleavage microcracks and the microstructure was investigated.During the tensile deformation of a cast Mg-Gd-Y-Zr alloy at room temperature,basal cleavage microcracks show clear tendencies to form in grains orientated with high Schmid factors for basal slip,and in those with large sizes.The criterion for cleavage fracture of a single grain adopts the form of τ-τ0=(1/k)d-1/2,where τ and d represent the resolved shear stress acting on the basal slip systems,and the grain diameter,respectively.A quantitative model was proposed based on the criterion to depict the multiplication of basal cleavage microcracks during roomtemperature tension,and showed good consistency with the experimental results.The fracture toughness of cast Mg-Gd-Y-Zr alloy was investigated,and the deformation and fracture micromechanisms operating at the crack tip were examined.The initial extension is induced by the formation of basal cleavage microcracks in the crack-tip plastic zone.Afterwards,stable extension occurs by generation of discontinuous basal cleavage microcracks and twin cracks.The KЛC values of the solution-treated and peak-aged conditions are 13.0 MPa·m1/2 and 11.7 MPa·m1/2,respectively.The low fracture toughness is attributed to the early crack extension due to basal cleavage microcracks.This study reveals the operating fracture micromechanism of basal slip-induced cleavage microcracking in cast Mg-Gd-Y-Zr alloys.This mechanism can be activated at low plastic strain levels where basal slip dominates the plastic flow,leading to premature fracture,and is considered as an immediate cause of the poor roomtemperature ductility and toughness exhibited by cast Mg-Gd-Y-Zr alloys.