Effects Of Sc And Zr Content On Microstructure And Mechanical Properties Of Al-Ni-Co Alloy Solidified Under Super-gravity Field

As a new type of heat-resistant aluminum alloy,the eutectic Al-Ni alloy provided excellent high temperature performance up to 400°C due to the Al₃Ni eutectic structure.In this paper,on the basis of eutectic Al-Ni alloy,the size of the eutectic Al₃Ni fibers was optimized by adding an appropriate amount of Co element and solidifying under super-gravity field to strengthen the Al-Ni matrix.The effect of adding Sc and Zr elements on the microstructure and mechanical properties of Al-Ni-Co alloy solidified under super-gravity field was studied.After direct aging,the effect of precipitation on the high temperature mechanical properties of the alloy was investigated.The refinement mechanism of super-gravity solidification on the microstructure,the microalloying mechanism of Sc and Zr,and the formation mechanism and strengthening mechanism of precipitates were discussed respectively.It was found that the super-gravity field solidification refined the eutectic Al₃Ni fiber size from about 138 nm to 116 nm.After the microalloying of Sc and Zr,the eutectic colony structure and eutectic Al₃Ni phase of Al-Ni-Co alloy were refined.The refining effect of Sc was better than that of Zr.The optimum Sc content was 0.3 at.%,and the average diameter of the Al₃Ni phase decreased from about 116 nm to about 94 nm.After direct aging,the Al₃Zr,Al₃Sc and Al₃（Sc,Zr）secondary phases were precipitated from the micro-alloyed alloy.It was found that the primary phase could be formed by the excessive addition of Zr.When the Zr content exceeded 0.3 at.%,the needle-like Zr-rich phase was formed in the Al-Ni-Co-Zr alloy,when the Sc+Zr content was 0.3 at.%,and the Sc/Zr ratio exceeded1:2,the cubic Al₃（Sc,Zr）phase was formed in the Al-Ni-Co-Sc-Zr alloy,and these primary phases were distributed in the edge of the macroscopic sample.The formation of the primary phase was inhibited by the super-gravity solidification.It was found that the hardness of Al-Ni-Co alloy was increased after microalloying Sc and Zr.The hardness of the alloy was highest at a Sc content of 0.3 at.%,rising from about 58 HV to about 82 HV.After direct aging,the hardness and high-temperature mechanical properties of the micro-alloyed alloy increased significantly,and the best high-temperature mechanical properties were achieved at a Sc content of 0.4 at.%,with high-temperature tensile strength,yield strength and elongation reaching about 138.7 MPa,105.0 MPa and 44.0%,respectively.The addition of Zr reduced the expensive cost of Sc elements,and the combined high-temperature mechanical properties of the Al-Ni-Co-Sc-Zr alloy with a Sc content of 0.15 at.%were comparable to those of the Al-Ni-Co-Sc alloy with a Sc content of 0.4 at.%.The nucleation rate of alloy was increased by super-gravity field solidification,thereby refining the microstructures,and the solid solubility of Sc was increased by super-gravity field solidification,thereby inhibiting the formation of primary phases.The Sc and Zr elements tended to dissolve in theα-Al matrix at the eutectic group boundary,thereby refining the eutectic colony structure.And the Sc elements were excluded from the front of the solid-liquid interface at the early stage of solidification,thereby refining the Al₃Ni phase.After direct aging,the hardness and high temperature mechanical properties of the alloy were improved due to the precipitation strengthening effect of the precipitated phase.