Microstructure Evolution Of Fe-Ni Peritectic Alloys During Directional Solidification

Peritectic reaction can be found in many technologically important materials, such as steels, aluminum and copper alloys, rare earth permanent magnets, high Tc superconductors, and potential high temperature structure materials Ti-Al, Fe-Al, Ni-Al alloys. Recently, peritectic solidification behavior has drawn much attention for the following reasons: (1) many interesting microstructures have been found during directional solidification of peritectic alloys, which can better understand our knowledge on solidification behavior of alloys; (2) to enhance the properties of the above peritectic advanced materials, we must well control the solidification process to obtain suitable microstructures, which require us to know more about peritectic solidification behavior. However, our knowledge about peritectic solidification is very lack. It is clear that further experimental and theoretical studies remain needed to better understand peritectic solidification behavior.In this paper, we selected peritectic model alloys Fe-Ni system to investigate the peritectic solidification behavior by systematic directional solidification experiments. The main findings are: (1) the two phase growth evolution, especially peritectic coupled growth (PCG) and cellular peritectic coupled growth (CPCG) were high lighted; (2) peritectic reaction occurs near the trijunctions during the two-phase growth at different interface morphologies of the primary phase observed as planar, cellular or dendritic, and the influences of peritectic reaction on various microstructure evolution were discussed; (3) the applicability of maximum growth temperature criterion (MGTC) and nucleation and constitutional undercooling criterion (NCU) on the phase and microstructures selection during peritectic directional solidification was discussed.Systematic directional solidification experiments were carried out on Fe-4.0Ni, Fe-4.3Ni, and Fe-4.5Ni peritectic alloys at G of 6, 8 and 12 K/mm, respectively, to investigate the solidification behavior. Various microstructures were observed in the directionally solidified samples, such as bands, mixed bands, island banding, tree-like oscillatory pattern, coupled and cellular coupled growth structures. It is confirmed that PCG can be obtained in peritectic system, however, PCG is always morphological unstable and it need long initial transition stage and critical growth condition. It is difficult to grow well aligned in situ peritectic composites by PCG. However, cellular peritectic coupled growth (CPCG), coupled growth between cellular primary phaseδand near planar peritectic phaseγwas easy to be obtained and to reach a steady state to grow well aligned in situ peritectic composites. As a result, CPCG may be the candidate method to produce well aligned in situ peritectic composites.Peritectic reaction arises near the liquid/δ/γtrijunctions during the two-phase growth over a wide range of growth conditions. The dissolution ofδduring the reaction makes the liquid/δinterfaces concave near the trijunctions despiteδphase is planar, cellular, or dendritic. Peritectic reaction makes the liquid/δ/γtrijunctions region quite irregular and makes the trijunctions dynamics quite complicated, and thus plays an important role in the formation of various microstructures.Besides, the morphological instabilities of PCG and CPCG are mainly induced by peritectic reaction.The lamellar peritectic coupled growth in Fe-Ni peritectic system was investigated using the equilibrium Boettinger-Jackson-Hunt model. It was found that the slope of the undercooling vs lamellar spacing is very near zero around the minimum overheating, and the coupled growth can take place under this condition even the slope of the undercooling vs lamellar spacing curve is slightly smaller than zero. In addition, peritectic reaction can never reach completion during peritectic coupled growth. So the equilibrium peritectic coupled growth was modified by considering the incompletion of peritectic reaction. It was shown that when the fraction of peritectic reaction that reach completion in the range of 60%~80%, the calculated undercooling vs lamellar spacing curves agree well with the experimental observations in the directionally solidified Fe-Ni alloys.The morphology, distribution, and characteristic spacing of CPCG was investigated. It was found that the cell shapes of CPCG can be well described by the form of the Saffman-Taylor finger shape equation, and this presents a new evidence of the similarity between cellular coupled growth during directional solidification and Saffman-Taylor finger. The model predicting characteristic spacing of directional solidification microstructures based on single phase directional solidification can not predict CPCG, which is typical two phase growth. So we developed a new model to predict the characteristic spacing of CPCG. The prediction of this model agrees well with the results obtained in Fe-Ni alloys. Furthermore, a simplified model was developed to predict the growth region for stable CPCG. Good agreement was obtained between the theoretical predictions and the experimental observations. This model is very useful for producing well aligned in situ peritectic composites by directional solidification in peritectic system.