Solidification thermal parameters and dendrite arm spacings have been measured in hypoeutectic Sn-Pb and Al-Cu alloys solidified under unsteady-state heat flow conditions. It was observed that both primary and secondary spacings decreased with increased solute content for Sn-Pb alloys. For Al-Cu alloys, the primary spacing was found to be independent of composition, and secondary spacings decrease as the solute content is increased. The predictive theoretical models for primary spacings existing in the literature did not generate the experimental observations concerning the Sn-Pb and Al-Cu alloys examined in the present study. The theoretical Bouchard-Kirkaldy’s (BK’s) equation relating secondary spacings with tip growth rate has generated adequately the experimental results for both metallic systems. The insertion of analytical expressions for tip growth rate and cooling rate into the predictive model, or into the resulting experimental equations in order to establish empirical formulas permitting primary and secondary dendritic spacings to be determined as functions of unsteady-state solidification parameters such as melt superheat, type of mold, and transient metal/mold heattransfer coefficient is proposed.

Heat flow parameters affecting dendrite spacings during unsteady-state solidification of Sn-Pb and Al-Cu alloys

The magnesium alloy AZ91 has been studied under different solidification conditions, to establish the details of the as-cast structure. The electron backscattered pattern (EBSP) technique has been used to determine the crystallographic directions of the dendrite stem and the secondary arms. Under unidirectional solidification conditions, two different stem directions were found: at a low temperature gradient and high growth velocity, the dendrites grew in a «11-20» direction, while at higher temperature gradients and lower growth velocities, the growth direction was (2245). Dendrites with «11-20» stems have arms in six directions around the stem, while dendrites with «22-45» stems have three secondary arm directions. The crystallographic directions of the secondary arms are the same as the two stem directions.

Dendritic solidification of magnesium alloy AZ91

The March 1987 Les Houches workshop on “Propagation in Systems far from Equilibrium” allowed us to compare the state of research in a number of different fields where instabilities occur. It appeared clearly that the field of solidification has been much less systematically explored than those, for example, of Rayleigh-Benard instabilities or convection in mixtures. Many questions have yet to receive an answer. The aim of the present text is to examine some of them in the light of the results of this Workshop. However, we shall begin by a brief review of the literature on the subject.

Lucien Capella

Papers

Unidirectional solidification of dilute PbBi alloys