Phase Selection In Undercooled Melts Of Ni-Ge And Fe-Ge Peritectic Alloys

Peritectic alloy is a kind of widely applied engineering alloy.Phase selection behavior during non-equilibrium solidification has a very significant influence on the final microstructure and properties of the alloy.The studies on the phase selection Nd-Fe-B permanent magnet alloys have shown that high undercooling can suppress the formation of primary phase in favor of the formation of a peritectic phase with excellent magnetic properties.Therefore,the research of the phase selection during the solidification process of peritectic alloy can not only provide a new method to improve the microstructure of the alloys,but also help scholars to understand the behavior and mechanism of phase selection.In the thesis,Ni-Ge and Fe-Ge alloys were chosen as model peritectic systems to study phase selection in undercooled melts.In the present experiments,the compositions were selected as Ni100-xGex(36?x?50)and Fe100-yGey(38.5?y?50).During the experiments,the alloy samples were undercooled by the glass fluxing technique,and the recalescence behavior of the samples was studied by the infrared temperature-measuring technique.After the end of the experiments,the microstructure of the alloy sample was analyzed by scanning electron microscopy and energy dispersive spectrometer.The phase selection laws of the undercooled melts of the peritectic alloy was determined by examining the recalescence characteristic in cooling curves and solidification microstructures.The phase selection mechanisms were discussed.The conclusions are shown as follows:(1)The maximum undercooling attained in the cases of Ni64Ge36 and Ni50Ge50 alloy samples were 203 K and 192 K,respectively.The primary phases of the two alloys did not change within the undercooling range obtained in the experiment,and the primary phases are Ni5Ge3 and Ni3Ge2,respectively.The maximum undercooling obtained in the case of Ni61Ge39 alloy sample was 199 K,and the primary phase transforms from the Ni5Ge3 to the Ni19Ge12 with the increase of undercooling.The maximum undercooling attained in the cases of Ni58Ge42 and Ni55Ge45 alloy samples were 162 K and 198 K,respectively.With the increase of undercooling,the primary phases of Ni5gGe42 and Ni55Ge45 alloy samples are Ni5Ge3,Ni19Ge12 and Ni3Ge2.The maximum undercooling attained in the case of Ni53Ge47 was 199 K,and the primary phase of Ni53Ge47 transforms from the Ni19Ge12 to the Ni3Ge2 with the increase of undercooling.(2)The maximum undercooling attained in the cases of Fe61.5Ge38.5 and Fe54.5Ge4.5 alloy samples was 222 K and 199K,respectively.The primary phase of the two alloys did not change in the undercooling range obtained in the experiments,and was always the Fe5Ge3 phase.The maximum undercooling attained in the cases of Fe57.5Ge42.5 and Fe50Ge50 alloy samples was 204 K and 194 K,respectively.The primary phase transforms from the Fe3.2Ge2 to the Fe18gGe13 with the increase of undercooling.(3)Analysis of the experimental results indicates that the phase selection mechanism is closely related to the structure of the phases which compete with each other.If the symmetry of crystal structure of the peritectic phase is lower than that of primary phase,the peritectic phase can solidify preferentially by nucleation competition mechanism at the low undercooling condition.At the high undercooling condition,the gap in nucleation rate is narrowed by the decreases of the critical nucleus radius and nucleation energy,and the phase with faster growth rate preferentially solidifies from the undercooled melts.When the crystal structures of the two competitive phases are the same,the nucleation rate of the primary phase is always higher than that of the peritectic phase at the low undercooling condition.The nucleation rate of the two phases is reduced only at the high undercooling condition,and the preferentially solidification of peritectic phase is controlled by competitive growth mechanism.