Effects of composition on lamellar microstructures of near-eutectic Cr–Cr 3Si alloys

Abstract

A series of binary Cr–Si alloys with silicon concentrations ranging from 13 to 24 at.% were produced by arc melting and drop casting. These compositions span the composition (15 at.% Si) at which a eutectic reaction is reported in the phase diagram. The microstructure of the Cr-16.05 at.% Si alloy is fully lamellar and devoid of any pro-eutectic phases suggesting that the best composition for obtaining fully lamellar structure is Cr-16.05 at.% Si, rather than the eutectic composition (Cr-15 at.% Si) indicated in the phase diagram. The alternating lamellae in the eutectic alloy consist of Cr (solid solution) and Cr 3Si (intermetallic). Alloys that are chromium-rich relative to the 16.05 at.% Si contain pro-eutectic chromium whereas those that are silicon-rich contain proeutectic Cr 3Si. Directional solidification of drop-cast Cr–Cr 3Si alloys with nominal compositions ranging from 15 to 16.05 at.% Si was carried out using a high-temperature optical floating zone furnace. Uniform and well-aligned lamellar microstructures of Cr–Cr 3Si were observed when the growth conditions and alloy composition were optimized. Fully lamellar microstructures were obtained after directional solidification only when the starting composition was slightly enriched in chromium (Cr-15.95 at.% Si) relative to the composition of the fully lamellar drop cast alloy (Cr-16.05 at.% Si). A possible reason for this discrepancy is chromium evaporation during directional solidification.