Directional Solidification, Microstructures and Mechanical Properties of Cr-Cr3Si Eutectic Alloys

Alloys based on intermetallics have been considered for high temperature structural applications. However, many of these alloys suffer from intrinsic brittleness and low fracture toughness at ambient temperature. Therefore, ductile-phase-toughened intermetallic composites are being investigated as a means to improve the fracture toughness. A subset of this class of materials is in-situ composites produced by directional solidification of intermetallic eutectics. In this study, the Cr-Cr₃Si eutectic system is selected as a model system to investigate composites by directional solidification, where the strong, but brittle Cr₃Si is combined with a more ductile Cr-rich solid solution.

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 a fully lamellar structure is Cr-16.05 at.% Si, rather than the eutectic composition (Cr-15 at.% Si) indicated in the phase diagram. The eutectic microstructure consists of alternating lamellae of Cr (solid solution) and Cr₃Si (intermetallic). Uniform and well-aligned lamellar structures were obtained over a fairly wide range of solidification conditions, but not at very low or very high growth rates where degenerate and cellular structures, respectively, were obtained. The lamellar spacing was found to increase with decreasing solidification rate, in agreement with the Jackson-Hunt theory. In addition, for a fixed growth rate, the lamellar spacing was found to increase with increasing rotation rate. The growth directions in the lamellar eutectic were found to be <100> for the Cr₃Si phase and <111> for the Cr solid solution phase. Eutectic microstructures (rod or lamellar) could also be produced at off-eutectic compositions, but only for a limited range of growth conditions.

The mechanical properties of the individual lamellae and the Cr-Cr₃Si composites were examined by nanoindentation, Vicker’s hardness testing and three-point bend testing. It was found that the Vicker’s hardness of Cr-Cr₃Si composites is about HV847, independent of the lamellar spacing. The Young’s modulus of the Cr-Cr₃Si eutectic composites measured by ultrasonic techniques is 312 GPa, which is in reasonably good agreement with the nanoindentation results (within ~5%). The fracture toughness of single crystals of Cr₃Si is very low (~2.6 MPaÖm). Combination with a more ductile phase (Cr-rich solid solution) to make “ductile phase toughened” composites increases the fracture toughness to maximum 8.5 MPaÖm. The fracture toughness of lamellar microstructures with fine spacings is slightly higher than that of microstructures with coarse spacings.