Peritectic coupled growth

Abstract

Even though peritectic coupled growth (PCG) has been often discussed in the literature, it remains poorly understood in comparison to eutectic coupled growth (ECG). We report the results of a combined experimental and numerical study that clearly establishes the existence of PCG in directional solidification of Fe–Ni alloys. Furthermore, the results shed light on the similarities, as well as the differences, between PCG and ECG. The main findings are that: (i) a necessary condition for PCG is a G/ v ratio close to or above the critical value for plane front growth of both solid phases, (ii) both lamellar and fibrous PCG is observed and the transition between the two morphologies correlates with the degree of asymmetry of solid volume fractions, as for ECG, (iii) lamellar PCG can be stable even though the slope of the undercooling-spacing relation is negative, in agreement with recent experimental and numerical findings for ECG, (iv) the stability of PCG is limited at both small and large spacings by short-wavelength oscillatory instabilities, whereas the stability of ECG is limited at small spacing by a known long-wavelength instability associated with lamellar elimination, and (v) PCG and ECG can be initiated by different mechanisms. In addition, cellular non-isothermal PCG is found when the G/ v ratio is below the limit for plane front growth of the primary phase and above that for plane front growth of the peritectic phase. The transition from isothermal PCG to cellular PCG with decreasing G/ v is discontinuous (sub-critical). Cellular PCG is characterised by diffusion-coupling between cells of one phase and nearly plane front of the other under the constraint of mechanical equilibrium at the triple junction.