Microanalytical Techniques Applied to Phase Identification and Measurement of Solute Redistribution at the Solid/Liquid Interface of Frozen Fe-4.3Ni Doublets

Abstract.

 A Fe-4.3Ni alloy has been solidified directionally by using the Bridgman system. The solidification conditions were chosen to obtain an oriented cellular structure of δ-ferrite. These are: a positive temperature gradient of about 60 K/cm and a growth rate of 6.6 µm/s. A change in these conditions can lead either to the formation of austenite or to the competitive growth of δ-ferrite/γ-austenite. The solid/liquid interface of δ-ferrite cells has been frozen and double instability has been revealed at the tip of the cells. The instability is described as the first harmonic wave of fundamental undulation, which appeared at the formerly planar solid/liquid interface. This means that a doublet structure is formed only with the imposed specific conditions of solidification. The Ni-solute redistribution after back-diffusion has been measured across the δ-ferrite doublet. Results of energy dispersive X-ray (EDX) measurements on the distribution of Ni and Fe correspond well to the theoretical prediction for redistribution developed especially for oriented structure formation (two dimensional solidification). Additionally, electron backscattered diffraction from the bulk Fe-4.3Ni alloy sample allowed us to determine the local structure, i.e. the distribution of single crystallite orientations in the microstructure.

A unique correlation between fluctuations of the Ni-solute redistribution and crystalline orientations in the δ-ferrite doublets has been demonstrated. Moreover, a relationship between geometrical asymmetry of the doublets and solute redistribution has also been found.