The Influence of Solid State Diffusion on Microstructural Development During Solidification

The primary factors that effect solid state diffusion during solidification are described and binary solute redistribution equations that permit estimation of the significance of solid state diffusion are discussed. Model calculations suggest that solid state diffusion of substitutional alloying elements in FCC alloys is insignificant under most processing conditions, while that of interstitial alloying elements is likely to be complete. Experimental data that supports these results are presented. Several cases that highlight the practical importance of microsegregation on performance of engineering alloys are described as well as methods for avoiding or minimizing microsegregation for improved properties. A solute redistribution model for handling the limiting cases of solute diffusion in ternary alloys is presented and model calculations are reviewed to reveal the strong influence diffusion can have on the solidification path and resultant microstructure. Introduction Solidification processing is used in a variety of fabrication methods such as casting, welding, laser surface treatment, and crystal growth. In many cases, products prepared from these processes are utilized in the as-solidified condition. In such applications, the mechanical properties and corrosion performance of the component are strongly influenced by the distribution of alloying elements and relative fraction of secondary constituents which form during the solidification process. It is often useful to make quantitative estimates of the resultant solidification microstructure as a means for controlling the ultimate component performance. The degree of solid state diffusion that occurs during solidification plays an important role in microstructural development. The objective of this article is to review factors that influence the extent of solid state diffusion that occurs during solidification, describe methods for estimating the expected degree of solid state diffusion, and discuss various models that can be used to account for the influence of solid state diffusion on the solidification path and resultant phase formation. Pertinent solute redistribution models for binary and ternary alloys will be described, and practical examples that highlight the importance of solid state diffusion will be provided. Limiting Cases of Solute Redistribution in Binary Alloys Many models have been developed in the solidification literature on solute redistribution and microstructural development in binary alloys that are capable of accounting for factors such as solid state diffusion, dendrite tip undercooling, and coarsening, and several review articles have been published on the subject [1-4]. While these models are quite useful, it is worth noting that most all conditions of solute redistribution in binary alloys must fall within two very simple cases: the lever law and so-called non-equilibrium Scheil equation [5]. Each of these conditions assumes that equilibrium is maintained at the solid/liquid interface, there is complete diffusion in the liquid, and there is no undercooling during nucleation or growth. The only difference between the two conditions is the assumption made about solid state diffusion – the lever law assumes solute diffusion is infinitely fast in the solid while the Scheil equation assumes diffusion is negligible. The relation between liquid composition and fraction liquid for equilibrium solidification is given simply by