The Effect Of Sc And Cr On The Microstructure And Mechanical Properties Of MP Aluminum Alloy

This paper is based on the alloy composition of the heat treated MP aluminum alloy produced by Rheinland Company in Germany.Through copper mold casting technology and the addition of Sc and Cr alloy elements,a similar heat treated aluminum alloy is designed.Using thermodynamic calculations to study the effect of alloying elements on the phase composition of MP aluminum alloy,observing the microstructure through optical microscopy and scanning electron microscopy,and comparing and analyzing the microstructure of MP alloys with different compositions;XRD and DTA were used for phase analysis and differential thermal analysis of MP alloys with different compositions;The room temperature tensile properties of 9 types of MP aluminum alloys were tested using a tensile testing machine.Based on the above results,analyze the influence of Sc and Cr on the microstructure and mechanical properties of MP aluminum alloy,and provide data support for China’s independent development and design of heat treatment free strengthened aluminum alloy.The main research findings are as follows:The MP alloy without adding other elements is abbreviated as S1 alloy.The addition amounts of Sc elements in the three types of MP alloys containing Sc are 0.1%,0.2%,and 0.3%,respectively,which are abbreviated as Sc1,Sc2,and Sc3.The thermodynamic calculation results indicate that the main differences among the three MP alloys are the precipitation temperature and molar percentage of Al₃Sc phase.The precipitation temperatures of Al₃Sc phase are 550℃,630℃,660℃,and the molar percentages are 0.24mol%,0.48mol%,and 0.72mol%,respectively.From the photos of the optical microscope,it can be seen that there is a large amount of strengthening phase Mg₂Si at the grain boundaries of the MP alloy.With the addition of Sc element,the fibrous Mg₂Si phase at the grain boundary first transforms into granular and short rod-shaped,then changes into coarse fibrous phase.From the scanning electron microscope photos,it can be seen that with the addition of Sc element,the white precipitates transform from irregular to granular and short rod-shaped.The tensile tests were conducted on four alloy samples,and the comparison showed that Sc2 samples had the best mechanical properties,with a tensile strength of 169MPa and an elongation of 1%.The addition amounts of Cr element in three types of MP alloys containing Cr are 0.1%,0.2%,and 0.3%,respectively,which are abbreviated as Cr1,Cr2,and Cr3.According to thermodynamic calculations,a new phase Al₇Cr appeared in the Cr2 and Cr3 samples,which precipitated at the liquidus temperature and dissolved in the matrix below the solidus temperature;All three samples showed the appearance of a new phase Al（FeCr）Si,with a precipitation temperature of480℃.From the photos of the optical microscope,it can be seen that with the addition of Cr element,the Mg₂Si phase at the grain boundary gradually transforms from fibrous to granular or short rod-shaped,and is then covered by a large amount of agglomerated iron phase.From the scanning electron microscope photos,it can be seen that with the addition of Cr element,the white precipitates transform from irregular shapes to granular and short rod-shaped,and the volume fraction increases significantly,while also gathering at the grain boundaries.Tensile tests were conducted on four alloy samples,and the comparison showed that the mechanical properties of Cr2 samples were the best,with a tensile strength of 205.5MPa and an elongation of 1.25%.Among MP alloys with Sc and Cr elements added simultaneously,ScCr1alloy with Sc addition of 0.1%and Cr addition of 0.2%,and ScCr2 alloy with Sc addition of 0.2%and Cr addition of 0.1%,respectively.According to the thermodynamic calculations of the two alloys,Al₇Cr phase appeared in ScCr1alloy,which precipitated at the liquidus temperature and dissolved in the matrix below the solidus temperature;Al₃Sc phase appeared in ScCr2 alloy,which precipitated at 630℃（liquidus temperature is 642℃）with a content of0.48mol%at room temperature.From the photos of the optical microscope,it can be seen that the grain size of ScCr1 alloy is significantly larger than that of ScCr2 alloy.From the scanning electron microscope photos,it can be seen that the white precipitates of ScCr1 alloy are mostly granular and have a size of about 3μm.The white precipitates of ScCr2 alloy are mainly rod-shaped,with a length of 10-15μm and a width of about 7μm.Comparing the tensile test results of 9 types of MP alloys,ScCr1 alloy has the highest tensile strength of232.5MPa and an elongation of 1.5%;ScCr2 alloy has the highest elongation of2.5%and a tensile strength of 199.5MPa.