Research On The Directionally Solidified Microstructure Evolution Of Ni-Ni₃Nb Hypereutectic Alloy At Constant And Changing Growth Rates

In this paper, directionally solidified microstructures of Ni-24.8%Nb hypereutectic were investigated at constant and changing growth rates. The solidification behaviors of Ni-24.8%Nb hypereutectic, including coupled growth zone and adjustment mechanisms of lamellar spacing of eutectic were researched by an abruptly changing growth rate in directional solidification. The influence of solidification process on the directionally solidified microstructures of Ni-24.8%Nb hypereutectic was presented. Finally, the mechanical property was studied through the compressive test.At the given temperature gradient of 200K/cm and the growth rate less than 1μm/s in directional solidification, coupled growth lamellar eutectic of Ni-24.8%Nb alloy was obtained, which was in agreement with the theoretical calculation result of 0.85μm/s. When the growth rate was larger than 5μm/s, primary Ni₃Nb phase grew ahead of the eutectic. With the increasing growth rate, the eutectic microstructure transformed from the regular lamellar to the irregular lamellar. When the growth rate was more than 10μm/s, α-Ni halos formed, which was closed to the calculated critical growth rate of 14μm/s for the formation of α-Ni halos.When the growth rate was increasing abruptly in directional solidification, the growth of β-Ni₃Nb primary phase was suppressed. The results showed that at higher growth rate, the coupled growth eutectic was obtained. It provided a new technical way for the production of in-situ composite effectively.Under the abruptly increase growth rate, eutectic lamellar would be branched. It took place both in Ni and Ni₃Nb phases. In the same time, the non-stability solidification microstructure was adjusted through three stages. Firstly, the regular lamellar spacing of eutectic reduced;secondly, the regular lamellar eutectic transformed to the irregular lamellar eutectic. Thirdly, the irregular lamellar spacing would be modified at various solidification conditions. With the higher acceleration growth rate, the length of non-stability transition zone reduced and the solidified microstructures were refined.Compressive tests showed that the compression strength and plasticity of samples, produced at the abruptly increase growth rate were improved than that of the samples manufactured at the constant growth rate in directional solidification. All the samples including primary Ni₃Nb phase appeared brittle fracture during the compression process. However;samples consisting of complete eutectic microstructure cannot break after compressive test, which showed the higher compression strength and toughness.