The Research On Microstructure And Properties Of Mg-10Gd-2Y Magnesium Alloys

Since magnesium-rare earth alloys not only possess all superior properties of ordinary magnesium alloys, but also exhibit incomparable mechanical properties both at room and elevated temperature, increasing attention has been attracted in academic and industrial fields. Compared with casting magnesium alloys, wrought magnesium alloys show demonstrate many superior characteristics such as high strength and plasticity, dense microstructure, high performance stability, diverse products. In addition, rare earth containing magnesium alloys also have high creep resistance and thermal stability, and thus become an important stream in the development of magnesium industry. However, strengthening mechanisms and the impacts of microstructural evolution and crystal orientation variation after plastic deformation are not sufficient understood. Consequently, the application of magnesium alloys in the fields of aviation, aerospace, ordnance and automobile are constrained. Therefore, investigation on the above issues is important both from the aspects of theoretical and engineering applications.In the present work, the phase diagrams of Mg-Gd-Y ternary system were calculated by phase diagram calculation software (PANDAT6.0) and a high-performance magnesium alloy was designed on the basis of calculation. The microstructure of the as-cast and extruded, crystal orientation of extruded of Mg-Gd-Y alloys were characterized by optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) assisted with an attached Link INCA EDX system. Thermal behavior and mechanical properties of this alloy was tested by differential scanning calorimetry (DSC) and tensile test. Fracture mechanisms, strengthening mechanisms and heat treatment processing were discussed and following conclusions could be withdrawn.Chemical composition of the studied alloy was Mg-9.8Gd-1.6Y-0.02Zn-0.5Zr(wt.%)(designated as GW102alloy). The as-cast alloy was composed of a(Mg) solid solution, divorced eutectic compound Mg5(Gd0.6Y0.2Zn0.2), particle Mg5(Gdo.4Yo.6) or Mg24(Y,Gd)5, non-equilibrium crystallized phase Mg3(Gdo.5,Yo.5). According to the Mg-Gd-Y ternary phase diagram, the equilibrium phases of the alloy was composed of a(Mg)+Mg24Y5+Mg5Gd+a(Zr). The tensile strength and elongation of as-extruded GW102alloy were about309MPa,243MPa,215MPa and7.0%,6.0%,2.4%in three directions (extrusion direction,45°direction, vertical direction). After aging treated at220℃for60h, the tensile strength can reach377MPa,345MPa,268MPa and elongation were7.5%,6.5%,3.5%.Strengthening mechanisms of GW102alloy included grain refinement strengthening, dislocation strengthening and aging reinforcement. The results showed that crack propagation process of GW102alloy was, at the early stage, micro-crack initiated from the interfaces between particles and matrix or interfaces between agglomerated particles because the cohesive force of these areas was weak, these places would debond firstly under applied loading, and then the micro cracks grew up and forward. With the increase of stress, the crack tip widen continuously. When the crack tips were dulled, grains would rotate by themselves to produce secondary crack for stress releasing. When the value of applied loading exceed value of dulled, the crack just forward extension and lead to material fracture at last.The results of TEM analysis showed that yield strength of the studied alloy was reduced by the precipitates-free zone (PFZ) and plastic deformation took place readily in these areas, which would result in intergranular damage and strength weakening.There were two peaks and a trough in the hardness curve of GW102alloy as the solution time increases. The formation of the trough is due to the precipitation of14H(LPSO) from solid solution. The LPSO and its interaction with dislocations improved the elongation of the alloy. The hardness and tensile properties of the alloy were first increased and then decreased at long aging time. After aging treated at200℃for63h, the hardness and tensile strength of the alloy was reached128HV and432MPa. The precipitation sequence of GW102alloy was Mg(SSSS)â†'β"(140℃)â†'β’(240℃)â†'β(276℃)â†'β(510℃). The precipitation phases at peak aging state consisted of β" and β’, which hindered the movement of the dislocation and improved the strength of the alloy.