Heat Treatment And Strengthening Mechanism Of New-type High Strength Magnesium Alloys Containing Rare Earth Elements

Effects of added rare earth (RE) elements and heat treatment parameters on microstructure and mechanical properties of Mg-5%Zn-1%Y-0.6%Zr-0.8%(Ce/Nd)and Mg-15%Gd-3%Y alloys were investigated in this dissertation. The microhardness and tensile test combined with scanning transmission microscope (SEM) incorporating energy dispersive spectrum, X-ray diffraction analysis (XRD), high resolution electron microscope (HREM), revealed the relationship amongst chemical composition– microstructure– deformation behavior– mechanical properties.It has been pointed out that the refinement of magnesium alloy by adding rare earth was caused by high undercooling degree between the solid/liquid interface during solidification and high nucleation rate. A noticeable grain refinement by adding minor rare earth in the magnesium alloys was observed.It concluded that, for Mg-5%Zn-1%Y-0.6%Zr-0.8%(Ce/Nd)system, grain refinement and ageing strengthening are the main strengthening mechanisms. Besides the certain strengthening MgZn₂ phase in it, the quasi-crystal Mg-Y-Zn-Zr is also a possible strengthening phase.For Mg-15%Gd-3%Y system, besides the ageing precipitation as the main strengthening mechanism, work-hardening, dynamic recrystallization and twinning have also made contribution to deformation and mechanical properties. The strengthening precipitates in the alloy are meta-stable phase of Mg₅(G d , Y ),Mg₃Gd ,Mg₂₄Y₅ . MgGd₃ existing in the alloy is a possible strengthening phase. The precipitation sequence of this system is: SSSS→β'' ( DO₁₉)→β'( cbco )→β( fcc). The precipitation phases at peak aging state mainly consists of metastable phaseβ'. By analyzing mechanical properties combined with characteristic of precipitates for different ageing periods, it's concluded thatβ''andβ'are better precipitates thanβfor this alloy.