The structure of directionally solidified two-phase Sn-Cd peritectic alloys

Abstract

The structure of Sn-Cd two-phase peritectic alloys directionally solidified at various values ofG/υ (temperature gradient in the liquid divided by growth rate) is reported. The minimum value of G/υ as a function of composition required for the solidification of two-phase peritectic alloys with a planar liquid-solid interface is estimated using a simple constitutional supercooling stability criterion. At a value ofG/υ just below this minimum value, these alloys solidify with a nonplanar interface consisting of cells of α (the high temperature phase) and intercellularβ (the low temperature phase). This produces a coarse rod-like microstructure consisting of rods of α phase imbedded in aβ matrix. At a value ofG/υ above this minimum value, these alloys solidify with a planar interface which alternately deposits bands of α andβ transverse to the growth direction. No coupled growth of α andβ at a planar interface is observed in Sn-Cd two-phase peritectic alloys as was expected. To understand this, an analysis of coupled (eutectic-like) growth of two-phase peritectic alloys is presented and contrasted with the results of the Jackson-Hunt theory of lamellar eutectic growth. This calculation indicates that the coupled growth of two-phase peritectic alloys is unlikely on theoretical grounds.