Effect Of Alloying On Microstructure And Hardness Of Mg-Bi Alloy

The precipitation strengthening effect of Mg-Bi alloy was optimized by adding a small amount of Cu,Y and Sr elements,which provided theoretical basis and experimental support for the subsequent study of Mg-Bi alloy.In this paper,Mg-5Bi-xCu（x=0.2、0.5、1.0）、Mg-5Bi-x Y（x=0.2、0.5、1.0）and Mg-5Bi-x Sr（x=0.02、0.05、0.1、0.5）ingots were prepared.The as-cast alloy was held at 525±2 for 72h and water quenching.The solid solution samples were divided into 15 groups and subjected to isothermal aging at 175±1 in a constant temperature oil bath.The microstructure was observed by optical microscope and scanning electron microscope,the morphology of precipitated phase was observed by transmission electron microscope and the orientation relationship was analyzed,and used vickers microhardness tester to test hardness.The results show that,the as-cast Mg-5Bi-xCu alloy is mainly composed ofα-Mg dendrite and interdendritic eutectic,and the eutectic second phases are mainly the symbiotic structure of Mg-Cu phase and Mg₃Bi₂ phase.The orientation relationship（OR）between Mg₃Bi₂ phase andα-Mg matrix is（^<sub>2110）_Mg3Bi2∥（0^<sub>111）_Mg、[01^<sub>12]_Mg3Bi2∥[01^<sub>12]_Mg;The OR between Mg₂Cu phase and Mg₃Bi₂ phase is（11^<sub>1）_Mg2Cu∥（01^<sub>10）_Mg3Bi2、[^<sub>110]_Mg2Cu∥[0001]_Mg3Bi2.The microstructure and of as-cast alloy with Cu addition are obviously refined,and the hardness increased significantly.The as-cast Mg-5Bi-xYalloy is mainly composed ofα-Mg matrix and Mg3Bi2 phase.With the increase of Y content,the grain size of the matrix is refined,but a small number of large mesh structures are formed.In as-cast Mg-5Bi-x Sr alloy,the microstructure is mainly composed ofα-Mg matrix and Mg₃Bi₂ phase when the Sr content is less than 0.1 wt.%.When the Sr content is increased to 0.5 wt.%,the rod-shaped second phase increases,and Mg₁₇Sr₂ phase may also be formed in the alloy.After solid solution treatment,0.5 wt.%Cu alloy shows a good solid solution strengthening effect,except for a small amount of undissolved microstructure at the grain boundary,the second phase is almost completely dissolved,and the hardness is slightly improved.Rare earth Y has a higher strengthening ability in Mg alloy in terms of solid solution strengthening,and the hardness of solid solution increases with the increase of Y content.The alloy with 1.0 wt.%Y has the highest hardness after solid solution,which is 63.1 HV.Among the alloys added with Sr element,the hardness of Mg-5Bi-0.5Sr treated by solid solution is the highest,which is 54.4 HV,but the solution effect is poor.After aging treatment,high density granular and short rod precipitates are precipitated along a specific direction in the alloy with Cu element added,and the hardness is higher than that of the solid solution alloy.The aged alloy with element Y precipitate point precipitated phases,but the hardness is slightly lower than that of the solid solution alloy.The reason for this phenomenon is that the effect of Y on solid solution strengthening in Mg alloy is greater than that of aging strengthening.Rod-shaped precipitated phases are precipitated after aging treatment of the alloy with Sr element.However,due to the existence of a large number of large grain structures in the alloy,the properties of the alloy are not favorable,and the hardness of samples before and after aging is not obvious.