Effects Of Alloying Elements Of Sn,Y And Gd On The Microstructure And Mechanical Properties Of Mg-5Al-5Ca Alloy

Magnesium alloys have become important lightweight materials for automotive,aircraft,aerospace,and electronics applications because of their advantages such as low density,high specific strength,high specific rigidity,and environmental friendliness in recent years.However,its low strength at high temperatures constrains the use of magnesium alloys as high temperature components.Therefore,the development of low-cost,high-performance heat-resistant magnesium alloy has become an important subject.Based on Mg-5Al-5Ca?AX55?alloy,AX55-x Sn,AX55-yY and AX55-z Gd?x,y,z=0.5%,1.0%,1.5%,2.0%?)alloy ingots were prepared by adding different contents of alloying elements Sn,Y and Gd.Two heat treatment processes,such as T61?solid solution 350?/32h+420?/16 h+aging 200?/24 h?and T62?solid solution 350?/32 h+450?/16 h+aging 200?/24 h?,were separately applied to the as-cast alloy.In the dissertation,the effects of three elements of Sn,Y,and Gd on the microstructure,room temperature mechanical properties,and tensile properties at high temperature?150?,175?,200?,225??of as-cast and heat-treated alloys were studied and obtained research results with certain value.At different temperatures,the alloys obtained the best tensile properties when the addition amounts of Sn,Y,and Gd were 1%,1.5%,and 1.5%,respectively.The addition of these alloying elements not only refined or homogenized the matrix structure of the as-cast alloy,but also the thick fish bone eutectic could be transformed into a fine reticulation morphology.In comparison,the effect of Sn on the alloy structure was most prominent,and the refinement of rare earth elements Y and Gd was not obvious.When the Sn content exceeded 1.5%,not only the matrix structure tended to be coarsened,but also a rod-like CaMgSn phase appeared in the alloy.When the addition amount of Y and Gd reached 2.0%,the coarse aggregate Al₂Y and Al₂Gd phases could be formed in the matrix which would reduce the tensile strength of the alloy.Overall,the decrease rate of the strength of each alloy decreased with the increase of the alloying element addition and increased with the increase of the tensile temperature.Compared with T61,T62 heat treatment process could effectively improve eutectic morphology and mechanical properties of the as-cast alloys.The major feature change of eutectic structure was spheroidization and granulation.When the addition of Sn,Y and Gd elements reached 1.5%,2.0%,and 2.0%in the alloys,the coarse CaMgSn,Al₂Y and Al₂Gd phases formed were refined under T62 heat treatment conditions and the distribution was improved.Among the three alloying elements,metal Gd had the most obvious effect on improving the alloy performance.Among them,the tensile strength and elongation at room temperature and high temperature of the T62-state AX55-1.5Gd alloy reached the maximum,and the increasing order from room temperature to 225?was 223.25 MPa,7.5%,206.07MPa,8.0%,190.84 MPa,and 7.9%,167.56 MPa,8.3%and 149.76 MPa,8.8%,respectively.