The Effects Of Trace Elements Sc And Zr On The Microstructure And Properties Of ZA27 Alloy

In this paper, by adding the trace elements of Sc and Zr into the ZA27 alloy, the effects of Sc addition, Zr addition and combination of them on the microstructure and mechanical properties of the ZA27 alloy were studied, and the strengthening mechanism were discussed.As far as the Sc addition is individually concerned, the refinement result of the ZA27 alloy with 0.5% Sc addition is comparatively best. The coarse dendrite microstructure of the ZA27 alloy is refined and transformed to the uniform and small floccus microstructure, the eutectics among the grain boundaries also become finer. The tensile strength of the alloy is up to 495MPa, and the hardness of the alloy is up to 120.2HB, while the elongation of the alloy is 7.6%. In the ZA27 alloy, the trace element of Sc combines with Al to form the Al₃Sc particles, the lattice of which (LI₂-type) and the cell size of which are extremely similar to the Al matrix. So these particles play the role of heterogeneous nucleation, refining the microstructure of the alloy and improving the properties of the alloy.When the trace element of Zr is individually added, the refinement result of the ZA27 alloy with 0.2% Zr addition is relatively the best. The coarse dendrites of the ZA27 alloy are refined to become the uniform and small floccus microstructure. The tensile strength of the alloy is 435MPa, while the elongation is 4.3%. However, the hardness of the ZA27 alloy with 0.3% Zr addition is relatively better. In the ZA27 alloy, the trace element of Zr combines with Al to form the Al₃Zr particles, the lattice of which is LI₂-type and similar to the Al matrix. Appropriate amount of these particles can play the role of heterogeneous nucleation, refining the microstructure of the alloy and improving the properties of the alloy.The combination of minor Sc and Zr has much stronger refinement effect on the as-cast microstructure of the ZA27 alloy. In the microstructure of the ZA27 alloy with 0.1%Sc and 0.1% Zr additions, the thick dendrites are almost disappeared. As for the ZA27 alloy with 0.4%Sc and 0.1% Zr additions, the refinement effect of its microstructure is the best, the coarse dendrite microstructure of the ZA27 alloy is refined and transformed to the uniform and small floccus microstructure, and the tensile strength of the alloy is up to 513 MPa, the elongation is up to 8.6%, and the hardness is up to 125.6HB. After adding appropriate amount of minor Sc and Zr into the ZA27 alloy, the Al₃(Sc,Zr) particles can be formed, which has the lattice type of LI₂ and is completely coherent with the matrix a(Al), playing the part of heterogeneous nucleation and promoting the fine crystalline strengthening of the alloy.For the ZA27 alloy with minor Sc and Zr additions, solution treatment at 360℃for 2 hours and then being air-cooled can make the phase of CuZn₄ and the zinc-richβ-phase solving together, thus eliminating the eutectics among the grain boundaries and improving the plasticity of the alloy.