Micro-segregation on the Mono-variant Line of an Iron–Carbon–Chromium Ternary Alloy

Abstract

To examine an effect of diffusion in the solid on the micro-segregation on a mono-varinat line, a directional solidification experiment has been carried out in an iron–carbon–chromium ternary alloy. In an iron–carbon–metal ternary alloy, carbon is an interstitial solute in an iron-base solution (austenite or ferrite) and the diffusivity in the solution is not negligible, while the third metallic element in the iron-base solution is usually substitutional solute and its diffusivity is negligible compared with that of carbon. The second phase of the ternary alloy is usually a kind of carbide, and the diffusivity of carbon or the third metallic element can be neglected because of the strength of the bonding forces of the atoms. In such a situation with dendritic-eutectic mode, a micro-segregation model on the mono-variant line has been introduced, and it has been predicted that the progress of the solidification might terminate before reaching the three-phase eutectic point. If we assume a Scheil-type model of micro-segregation on a mono-variant line, the composition of the residual liquid will reach the terminal (for example, three-phase eutectic) point.
To confirm this prediction, an experiment has been carried out in an iron–carbin–chromium alloy. The initial composition is 3.17 mass% C and 25.6 mass% Cr and very near the eutectic valley of austenite–(Fe, Cr)₇C₃. With no diffusion of carbon in the austenite phase, the solidification will change to austenite–(Fe, Cr)₃C eutectic reaction through the peritecto-eutectic point. With the effect of carbon, using the model, the solidification will end before reaching the peritecto-eutectic point and (Fe, Cr)₃C formed during solidification will not be found. The metallographical observation revealed the exsistence of (Fe, Cr)₇C₃ phase but did not reveal the exsistence of (Fe, Cr)₃C phase. The EPMA measurement between the dendritic arms showed that the measured minimum chromium compositions inside the austenite agree with the predicted values within an experimental error and uncertainties of parameters of the system. The identificatin of the phases by using XRD indicated that austenite–(Fe, Cr)₃C eutectic did not form, as was predicted. These results suggest the validity of the model of micro-segregation on the mono-variant line in an iron–carbon–metal ternary system.