RELATION BETWEEN GRAIN-SIZE AND COHERENCY PARAMETERS IN ALUMINUM-ALLOYS

During equiaxed dendritic solidification, dendritic grains form at the point where primary dendritic crystals become coherent (or impinge on each other). An experimental and theoretical approach to evaluate the gain size in the alloys is proposed, using thermal and constitutional parameters, and the parameters at the dendrite coherency point. For a n alloy with a solute concentration below the solid solubility limit, the grain size is inversely proportional to (dT/dt)(1/2) Sigma m(i)C(o i)(k(i) - 1) and the coherency parameters (T is the temperature, t is the time, m is the liquid line slope, C-o is the original solute conce ntration, k is the solute distribution, and subscript i is the ith all oying element). For an alloy with a solute concentration above the sol id solubility limit, the grain size can be calculated using the dendri tic growth interval Sigma k(i)(T-Li - T-eui), where T-L and T-eu are t he liquidus and eutectic temperatures, respectively, instead of the de ndritic restriction factor Sigma m(i) C-oi(k(i) - 1). For commercial a lloys, the grain size cart be calculated using the equation of thermal balance in a dendritic grain and the parameters at the dendrite coher ency point. The calculations are compared with experimental measuremen ts. (C) 1995 The Institute of Materials.