On the existence of 5-line nodes on interdiffusion microstructure maps

Abstract

Department of Metallurgy and Materials Engineering, Institute of Materials Science,University of Connecticut, Storrs, CT 06269-3136, USA(Received August 5, 1999)(Accepted in revised form March 28, 2000)Keywords: Diffusion bonding; Coatings; Bulk diffusion; Kinetics theory and modeling; PhasediagramsIntroductionDiffusion couples are well known samples for studying atom mixing and reactions that take placebetween two alloys in contact, for example as in diffusion bonding or in coating/matrix interdiffusion.Interdiffusion Microstructure Maps (IMMs) are diagrams that predict the microstructures that arisewithin diffusion couples as a result of these interactions. IMMs are drawn for one alloy of the diffusioncouple, which is considered the “base alloy.” The IMM shows how the microstructure varies as afunction of the other alloy’s composition. This work concerns a two-dimensional IMM that contains a5-line node. Recognizing that such an unexpected topology can occur is important when constructingIMMs from microstructural data.An IMM Containing a 5-Line NodeIn recent work on the interdiffusion of Ni-Cr-Al alloys at 1200 °C [1–3], five different microstructureswere observed both experimentally and via computer simulation. In this work the base alloy was theg1g9 alloy indicated by the star in Fig. 1 and the variable alloys were a series of alloys in the g1bregion of the phase diagram as indicated by various symbols in Fig. 1. On Fig. 1 the g1b region issub-divided into five fields by dotted lines to form the IMM. The five fields are labeled A, B, C, D andE according to the microstructure that formed in the composition range of the field.The five microstructures are shown schematically in Fig. 2 where the dashed, vertical lines are theinitial interface of the diffusion couple and the solid vertical lines are boundaries between various singleand multiphase regions. Arrows indicate the direction of boundary motion, which are always away fromthe initial interface. Microstructures C and E were first found in computer simulations using DICTRAsoftware by Engstro¨m et al [4], while microstructures A, B, C and D were found both experimentallyand in simulations by Qiao [1]. Micrographs and simulations of all microstructures are in [2] and willbe reported in a future paper that describes the experimental procedure. However this work deals withkinetic theory and the results are independent of the alloy chemistry or precipitate morphology ofindividual samples.