The Forging Behavior Of High-strength Mg-Gd-Y-Zn-Zr Alloy

Light ultra high-strength Mg alloys are mainly used in aerospace, military andother high-tech areas, and be of great strategic significance for both the defense andcivilian science and technology progress. Especially the new high-strength andheat-resistant alloy of Mg-Gd-Y-Zn-Zr has broad application prospects. To furtherexpand its use, semi-continuous casting ingot of Mg-8.99Gd-2.85Y-1.89Zn-0.42Zr-0.2Ca alloy and extruded alloy bar of Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr alloy (wt.%) areused as raw materials in this article to study the behavior of thermal simulationcompression and actual forging deformation. Processing maps are used to discuss theparameter effects on microstructure evolution and mechanical properties ofMg-Gd-Y-Zn-Zr alloy of hot compression deformation with the help of microstructureanalysis. Real forging deformation is carried out to find out the microstructure evolutionand mechanical properties with changing conditons and reveal its mechanisms andstrengthening mechanism of high temperature deformation. Block material ofHigh-strength Mg-RE alloy is prepared and further researches can be carried out basedon the present study.As-cast Mg-8.99Gd-2.85Y-1.89Zn-0.42Zr-0.2Ca alloy consists of-Mg matrix,Mg5(Gd, Y) eutectic compound,14H-type LPSO phase and particle with compositionof Zr.500℃/12h is choosed as the homogenization temperature and time in the guide ofcalculated phase diagram and microstructural observation. Mg5(Gd, Y) phases dissolveduring heat treatment and the alloy after quenching water is mainly organized bysupersaturated-Mg matrix and residual LPSO phases located at grain boundaries. Alarge number of lamellar LPSO phases precipitate within the grains in the organizationby furnace cooling which lower the mechanical properties of the alloy.Single-hit isothermal compression tests(at a strain of0.9) were performed using aservo-controlled Gleeble-1500D system at constant strain rates of0.001s-1,0.01s-1,0.1s-1,1s-1, and10s-1and deformation temperatures ranging from300℃to500℃withthe as-soluted and as-extruded alloys. The true stress-strain curves belong to the CDRXtype which exhibits flow hardening followed by continuous flow softening. Therheological behavior shows that the peak stress increases with decreasing temperatureand increasing strain rate. The microstructure is characterized by dynamicrecrystallization which reveals the deformation mechanism. The efficiency of powerdissipation (η) and the flow instability domains under different conditions are analyzedwith the processing maps. the optimal thermal processing parameters are choosed as440℃/0.017s-1with a power dissipation efficiency of29%for as-soluted alloy and440℃/0.035s-1with a power dissipation efficiency of35%for as-extruded alloy. During the isothermal simulation compression test and the actual forgingdeformation，the LPSO phases localized at the grain boundaries and precipitated withinthe grains kink by shear stress to coordinate the organization. With the increasingdeformation degree, LPSO phases are broken by high stress concentration at the kinkband and interacted with the dislocation to promote the dynamic recrystallizationnucleation, as a deformation mechanism of dynamic recrystallization coordinated withLPSO phases. The deformation process and organizational performance, to a greatextent, are affected by LPSO phases. At lower temperatures, the dislocation slip ishindered by LPSO phases and the pileup of dislocation at the grain boundary is reducedwhich strengthen the alloys. Organization is characterized by the lower level ofrecrystallization at grain boundaries and highly kinked intracrystalline LPSO phase.Recrystallization mainly occurs within the grains and crosses the grains with increasingdeformation to refine the grain size. Represented by the annealing state alloy forged at425℃,the microstructure is characterized by intracrystalline and the yield strength,tensile strength and elongation to breake reached304Mpa and378Mpa and5.7%afterforging. And at high temperature, thermal activation promote the restructuring of highdensity dislocation obstacled by the LPSO phases to activate dynamic recrystallization.Represented by the annealing state alloy forged at475℃, recrystallization fullycompletes and grows up to a certain extent with lower mechanical properties.Due to a large number of bulky Mg5(Gd,Y,Zn) phase precipitated at the grainboundary during thermostatic multidirectional forging, the alloys after forging exhibitpoor mechanical performance. The alloys forged for1pass show better performanceand the yield strength、tensile strength and the elongation to break of the extruding statealloy after forging reach325Mpa、387Mpa and21.5%. The mechanical properties aftercooling multidirectional forging of three kinds of alloys are higher because of the highdegree of grain refinement, especially the yield strength increased significantly. Thehighest yield strength and tensile strength are399Mpa and419Mpa with elongation of14.8%obtained by extruding state alloy after forging. The precipitation of Mg5(Gd,Y,Zn) phase results in lower the matrix content of solute atoms which reduces theageing strengthening ability of alloys after forging. In contrast, the extruding statealloys after forging show better ageing strengthening ability with yield strength andtensile strength of358Mpa and440Mpa, elongation of11.5%. Extruding state alloy bycooling multidirectional forged is peak aged with the yield strength and tensile strengthup to417Mpa and434Mpa, the elongation of12.9%.