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:?-Al₃Ni phase,?-Al₃CuNi phase,and?-Al₇Cu₄Ni phase.After only Ni is added to the alloy,the?-Al₃Ni phase is preferentially formed and tends to be block-like;as the Cu/Ni ratio increases,the Cu/Ni-rich phase transforms into the?-Al₃CuNi phase and the morphology changes into strip-like.When the Cu/Ni ratio is 5,the Cu/Ni-rich phase is transformed into the?-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.