Electron Microscopy Study Of Precipitates In Mg-10Gd-3Y-0.4Zr Alloys

Mg-Gd-Y-Zr alloys are a type of alloys with strong age hardeningresponse，wich have potential to be studied among the high intensityMg-Rare Earth （RE） alloys. However, studies on the morphology,microstructure, and evolution of precipitates are still not sufficient forMg-Gd-Y-Zr alloys.By means of HRTEM, the evolution of the morphology andmicrostructure of precipitates during the aging process were studied forthe Mg-10Gd-3Y-0.4Zr alloy aged at225℃. Evolution of themicrostructure at early aging stage was studied and the nucleation of β″and β′phase was discussed. The morphology and microstructure changeof β′phase during the whole aging process was studied and the growthmechanism of β′phase was investigated.TEM results show that:Precipitates are mainly monolayers on the {1010}Mgplanescomposed of Gd and Y atoms with DO19structure for the specimen agedat225℃for0.5hour. The nucleation of β″and β′phases occurred in thisstage simultaneously. The nucleation of a number of β′phase wereoccurred when aged for2hours. The complete β’ phase with sawtoothmorphology was found to be Mg7Gd structure. The rapid growth of thehardness is due to the generation of β′phase.It is revealed that a large number of DO19monolayers lying onMgformed at first, then β′phase nucleated by periodic stacking ofthese monolayers in the <1（?）00>_Mg direction with the arrangement ofthree magnesium matrix layers between two monolayers, and finallyformed a complete Mg7Gd structure. β″phase also nucleated by periodicstacking of these monolayers with one magnesium matrix layer betweentwo monolayers. Though the nucleation of β" phase was observed in the beginning stages, but it was rarely observed throughout the aging process.Unlike previous studies, the present study demonstrat that there is norelationship between the nucleation of β′phase and β" phase.The growth of β’ phase after the nucleation was also observed. The β’phase distributed along the three directions of <11（?）0>_Mg. After aged at225℃for2hours, both the length of width of β′phase were under10nm.The β’ phase grew up with the length of20-30nm and width of about10nm after aged for16h. Due to the different growth speed ofMgand1100Mgdirection, the projection of β’ phases on the base surfacechanged from irregular shape to olive-like, then to the bamboo-like shape.At the same time the thickening in <0001>_Mgdirection resulted in thetransformation from disc-shaped morphology of β’ phase to rod-like.The rod-like β’ phase is the main strengthening phase for the alloy toobtain the peak hardness. The rare earth atoms occupied thecorresponding position by continuous transposition or filling, which leadto the growth of β’ phase. Although the hardness of the alloy after agingfor400hours has been significantly decreased, the precipitated phase inthis stage was still the β′phase. But the size of β’ phase can be up to300nm and the precipitate free zones are also wide.