Dendrite arm climb by temperature gradient zone melting during solidification of a high-speed tool steel

Abstract

Allen and Hunt tl,21 have shown that dendrite side arms will climb up the stem (toward the dendrite tips) because concentration-driven solute diffusion through the liquid lowers the melting point at the trailing edge of each side arm, causing the material to remelt, while at the leading edge of the following arm, liquid solidifies because loss of solute has raised its melting point (Figure 1). Since the hotter arms climb somewhat faster, the process will contribute to the growth of the secondary dendrite arm spacings, and the solute concentration profile in the arms is smoothed down and skewed, as shown in Figure 1. Allen and Hunt 11,2] have proposed to adopt Pfann's phrase laj ~Temperature Gradient Zone Melting" (TGZM) for the phenomenon and have demonstrated it in experiments with transparent organic liquids. On the basis of Allen and Hunt's mathematical model, Lalli 14t has illustrated the effects of TGZM on solute profiles in dendrite arms, coarsening, and the amount of eutectic in an A14Cu alloy by numerical computations. A general discussion of the approach to the steady-state solute profile in TGZM has been given by Lograsso and Hellawell. ISl Experimental verification and quantification of TGZM of dendrite arms in metals has been missing so far. In alloys which form delta-eutectoid, like the highspeed tool steels, I61 the reaction front between the residual delta regions and the austenite is marked by a border of carbides (Figure 2). The border is concentric with the austenite skin which forms on the dendrites at the start of the peritectic reaction. Therefore, it can serve as a marker of the position of the dendrite cores at the moment when the peritectic isotherm passed the dendrite arm. After solidification, the side arms are outlined by thin layers of M6C eutectic (Figure 2), and by reference to the delta-eutectoid cores, they are seen to have moved in the direction of the dendrite tips. We interpret this shift as an effect of TGZM climb. This is supported by Figure 3, which pertains to an alloy similar to AISI M2. This alloy does not form delta-eutectoid under the present cooling conditions, but both the etching behavior and the microprobe profile of this specimen reflect a distortion of the originally symmetrical distribution of Cr (and other ferrite-partitioning elements), which is in accord with the expected effects of TGZM, as shown in Figure 1: dissolution of material causes the solute gradient to become steeper at the trailing edge, while the addition of lower solute material at the leading edge tends to reduce