Investigation On Microstructure Evolution And Mechanical Properties Of Two Gd Contained Magnesium Alloys

The application of present magnesium alloys are strictly limited by their relatively lower strength and poor deformability, especially when magnesium alloys are used in very important field. The latest investigation shows that when magnesium and its alloys add proper amount of heavy Rear Earth elements (Gd and Y), the magnesium alloys will obtain appropriate heat treatment strength effect, as a result the conventional and high temperature mechanical properties will increase significantly.Two magnesium alloys AZ81-xGd and Mg-Gd-Y chosen as basic alloy and the effect of Rear Earth elements on the microstructure of the two basic alloys are investigated in the present dissertation, the effect of heat treatment on the microstructure and mechanical properties of the alloys are studied in the mean while. The strengthening mechanism of Rear Earth elements in the two magnesium alloys is discussed.The experimental results of effects of Gd on the microstructure and mechanical properties of AZ81 magnesium alloys demonstrate that Gd change the microstructure constitute of the alloy, a new irregular shaped phase of Al2Gd formed in the alloy. With increasing of Gd content the a-Mg grain size decreases and the net-and block shapedβ-Mg17A12 phase became discontinue and dispersed small block. The mechanical properties of the alloy change dramatically with Gd content. When the Gd content is 5wt% the mechanical properties of the alloy are reach the maximum value:the tensile strength is 262Mpa, while the elongation is 10.04%. The mechanical properties of the alloy are further improved through heat treatment:the tensile strength is 278MPa, while the elongation is 13.47% after solid solution; the tensile strength is 280MPa, while the elongation is 12.1% after aging. The high temperature mechanical properties of the alloy is also increased with Gd addition:at 170℃the tensile strength increase 25.8% and the elongation increases two times of that of the ambient value, while at 220℃the tensile strength increases 17% and the elongation increases 40%. The microstructure of AZ81-4%Gd magnesium alloy after solid solution is observed by SEM and EDS, the results show that with increasing the solid solute time the (3-Mg17Al12 phase increases gradually then vanishes completely after 24 hours. The Al2Gd phase persists within the a grain during the heat treatment. The grain size of the matrix increases with increasing the solid solution time and after 8 hours the grain size have a slight change. In the later stage of solid solution some twins are formed in the microstructure and volume fraction of twins increase with increasing heat treatment time. After 24 hours of solid solution the alloy have highest hardness value due to the annealing twins.The microstructure of AZ81-4%Gd magnesium alloy after aging at 150℃,200℃250℃,300℃are investigated, the results show that theβ-Mg17Al12 phase precipitated in two modes which dependence on the aging time:in the early stage of aging theβ-Mg17Al12 phase precipitated in the grain boundary and in the discontinue mode, after a curtain time of aging theβ-Mg17Al12 phase precipitated within the grain and in the continue mode.When the aging temperature is relatively low theβ-Mg17Al12 phase precipitated in the above tow modes, while in the relatively high temperature theβ-Mg17Al12 phase only precipitated in the continue mode.The SEM and TEM observation results demonstrate that the morphology of discontinuesβ-Mg17A112 phase changes from lamina to grain then to long rod and sphericity. The orientation relationship of discontinued long rod precipitates and the matrix are[0110]α//[220]β, (0001)α//(110)β; the morphology of continueβ-Mg17Al12 phase are block, long rod and diamond-shaped lamina. The separation angle of some long rod and diamond-shaped lamina precipitates are 120°.The orientation relationship of continue precipitates and the matrix are [1010]α//[211]β, (0001)α//(110)β.The mechanism of discontinue and continue precipitateβphase are as follows:for the discontinue precipitationβphase nucleated at grain boundary then grown to one of the grain; while for the continue precipitation when the elements concentration is over a curtain value theβphase should precipitate continuously. The growing mode of theβphase depends on the atom arrangement of theβphase and matrix. In the (110) plane ofβphase and (0001) plane of matrix the atom misfit of the interface is relatively small and interface energy is relatively low due to the (110) and (0001) plane are close packed plane, as a result theβphase is prefer to grow along random direction in the (0001) plane than to grow along (0001) direction, result in theβphase of lamina.The as-extruded Mg-13Gd-4Y-0.4Zr wrought magnesium is solid solution at 500℃for 10 hours then quenching in water, then aging in the range of 150-300℃, the hardness of the above heat treated alloy testing results show that the higher the aging temperature the higher the hardness and when the alloy aging at 300℃the hardness value is the highest.The closed inspect of SET and TEM shown that there are two kinds of precipitates in the microstructure after the alloy aging at 300℃, one is the white block phase and the phase rich of alloy elements and the other is small lamina phase. The former phase is precipitated in the early stage of aging while the later is precipitated in the later stage of aging. The lamina phase aligns along three directions and the separate angles among the three directions are 120°and the content of solute elements in the phase was slightly high than the matrix. The orientation relationship of the lamina phase and the matrix are [0001]α//[111]β, (0110)α//(220)β。...