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The mechanical properties of Al3X(X = Sc,Lu)were studied by density functional theory(DFT).The elastic constants and formation enthalpy indicate that the L12-Al3X(X = Sc and Lu)are mechanically and thermodynamically stable.The bulk moduli and shear moduli show that Al3Sc has better resistance to volume and shape changes than Al3Lu.However,the calculated results show that Al3Lu has better plasticity than Al3Sc.The properties of structural stability and elastic moduli of the crystal containing four major types of point defects in L12-A13X(X = Sc and Lu)were calculated.The mechanical properties of point defects show that point defects cause L12-Al3X lattice distortion and change the corresponding elastic constants.Point defects reduce the Young’s,shear and bulk moduli but have little effects on the crystal brittleness and toughness of Al3Sc and Al3Lu.Therefore,we have found that Lu addition into aluminum alloys is a very good replacement for expensive Sc addition when the L12 structures are desired for nucleation or strengthening precipitates in aluminum alloys.