Evolution Of Phase In Al-Mg₂Si-Cu-Ni Alloy And Its Effect On Mechanical Properties

The morphology evolution of Cu/Ni-rich phase in Al-15%Mg₂Si alloy was studied by means of x-ray diffractometer（XRD）,field emission scanning electron microscope（FSEM）equipped with an energy spectrometer（EDS）and automated serial slicer（UES Inc.,Robo-Met.3D）with three-dimensional analysis software（FEI,Avizo Fire 7）,etc.The volume fraction of Cu/Ni-rich phase was measured by Image Pro software,and the variation of the volume fraction of Cu/Ni-rich phase in the alloy was researched.The influence of volume fraction of Cu/Ni-rich phase on the hardness of the alloy was analyzed.The promotion rate of the elevated-temperature strengthening phase to the tensile strength of the alloy was calculated.The strengthening mode and fracture mechanism of the alloy at 25℃,350℃,450℃ were analyzed by combining the fracture morphology of the alloy and the internal three-dimensional topography of the Mg₂Si phase and the Cu/Ni-rich phase.Experimental and analytical results show that:（1）After adding different proportions of Cu and Ni elements in Al-15%Mg₂Si alloy,three strengthening phases were produced:ε-Al₃Ni phase,δ-Al₃CuNi phase,andγ-Al₇Cu₄Ni phase.After only Ni is added to the alloy,theε-Al₃Ni phase is preferentially formed and tends to be block-like;as the Cu/Ni ratio increases,the Cu/Ni-rich phase transforms into theδ-Al₃CuNi phase and the morphology changes into strip-like.When the Cu/Ni ratio is 5,the Cu/Ni-rich phase is transformed into theγ-Al₇Cu₄Ni phase,and the morphology is more likely to skeleton-like.（2）The volume fraction of the rigid Cu/Ni-rich phase increases with the Cu/Ni ratio rises.Moreover,Cu/Ni-rich phase is uniformly distributed inside the alloy to form a three-dimensional network structure,which surrounds the Al-matrix and hinders the plastic deformation of the matrix phase,thereby improving the hardness of the alloy.（3）At room-temperature,the Mg₂Si phase plays a major strengthening role,and the promotion of the Cu/Ni-rich phase to tensile strength is not obvious.At elevated-temperature,the Cu/Ni-rich phase with higher hardness and more stable performance at high temperature bears the load,and can effectively hinder dislocation motion,and increase the resistance of the alloy to deformation.The Cu/Ni-rich phase contributes more to the increase in tensile strength of the alloy at high temperature（132%）.（4）At 25℃,microcracks are preferentially generated at the interface of the Mg₂Si phase and the Al-matrix or at the sharp corners of the Mg₂Si,and propagate,expand and interconnect through the three-dimensional network structure of the Mg₂Si.At 350℃,microcracks are generated and preferentially accumulate on the stable Cu/Ni-rich phase,and propagate along the surface of the three-dimensional network structure of Cu/Ni-rich phase or through the Cu/Ni-rich phase.At 450℃,during continuous deformation,microvoids are formed at the interface between the primary Mg₂Si phase and Cu/Ni-rich phase with the Al-matrix along the loading direction,and expand with the load increasing,eventually forming a tear-like fracture morphology.