Microstructure characteristics and strengthening mechanism of semisolid CuSn10P1 alloys

Highlights: • High cooling rate and the semisolid slurry isothermal treatment induce the formation of metastable phase β′-Cu_13.7Sn. • The liquid-solid separation in rheo-squeeze casting can be significantly improved by holding the semisolid slurry for 20 s. • The strength and ductility of CuSn10P1 alloy can be improved simultaneously. • The fracture mechanisms has transformed from the brittle fracture to a combination of cleavage fracture and ductile fracture. Studies on the rheoforming of high melting point alloys are scarce due to the difficulties in preparing semisolid slurries. This work applied a novel enclosed cooling slope channel (ECSC) to prepare a semisolid slurry and squeeze cast a CuSn10P1 (weight percent) alloy. The microstructure and phase formation of the semisolid slurry, properties of liquid squeeze casting, rheo-squeeze casting with or without isothermal treatment of the semisolid slurry were investigated through X-ray diffraction, scanning electron microscopy, electron probe microanalysis, nanoindentation, and transmission electron microscopy. The results show that the microstructure is transformed from coarse dendrites of as-cast to equiaxed grains of semisolid. The high cooling rate of the ECSC process and the semisolid slurry isothermal treatment suppressed tin diffusion to induce the formation of the metastable phase β′-Cu_13.7Sn. The average values of the modulus and micro-hardness of the rheo-squeeze part after soaking for 20 s were 121.99 ± 14.03 GPa and 2.01 ± 0.94 GPa, which was lower than that without isothermal and liquid squeeze casting, indicating that it has good deformability. The tensile strength and elongation of the rheo-squeeze part after 20 s of soaking treatment are 417 MPa and 12.6%, which increase by 26.3% and three times, respectively, compared to those without isothermal and liquid squeezing, which is likely attributed to solution strengthening, fine grain strengthening, and microstructure homogenization. In addition, the fracture mechanism was transformed from the brittle fracture of liquid squeeze and rheo-squeeze casting without isothermal treatment to a combination of cleavage fracture and ductile fracture of rheo-squeeze casting with 20 s of soaking treatment.