Influence of hexagonal to orthorhombic phase transformation on diffusion-controlled dendrite evolution in directionally solidified Sn-Ni peritectic alloy
(1. School of Materials and Energy, Lanzhou University, Lanzhou 730000, China;
2. School of Physics and Electronic Information Engineering, Qinghai Normal University, Xining 753000, China;
3. Gansu Provincial Hospital of Traditional Chinese Medicine, Lanzhou 730000, China)
2. School of Physics and Electronic Information Engineering, Qinghai Normal University, Xining 753000, China;
3. Gansu Provincial Hospital of Traditional Chinese Medicine, Lanzhou 730000, China)
Abstract: The hexagonal to orthorhombic (HO) transformation from β-Ni3Sn2 (hexagonal) phase to α''''-Ni3Sn2 (orthorhombic) phase was confirmed in directionally solidified Sn-Ni peritectic alloys. It is shown that the remelting/resolidification process which is caused by both the temperature gradient zone melting (TGZM) and Gibbs-Thomson (G-T) effects can take place on secondary dendrites. Besides, the intersection angle between the primary dendrite stem and secondary branch (θ) is found to increase from π/3 to π/2 as the solidification proceeds. This is the morphological feature of the HO transformation, which can change the diffusion distance of the remelting/ resolidification process. Thus, a diffusion-based analytical model is established to describe this process through the specific surface area (SV) of dendrites. The theoretical prediction demonstrates that the remelting/resolidification process is restricted when the HO transformation occurs during peritectic solidification. In addition, the slope of the prediction curves is changed, indicating the variation of the local remelting/resolidification rates.
Key words: directional solidification; dendritic solidification; peritectic microstructures; Gibbs-Thomson effect; temperature gradient zone melting; HO transformation