From eutectic to peritectic: Effect of Ge on morphology, structure, and coarsening of Cr-Cr3Si alloys

The influence of germanium on microstructure, crystallography, and thermal stability of the eutectic Cr-Cr3Si alloy was investigated. Fine eutectic microstructures could be preserved but slightly degenerated when up to 2 at.% Si was substituted by Ge. A transition from eutectic to peritectic microstructure composed of primary dendrites and A15 Cr3Si matrix was observed when higher amounts of Ge were added. X-ray diffraction results showed no limit for substitution of Si and Ge in the studied composition range. The lattice parameter of both Cr solid-solution and A15 silicide increased with increasing the Ge/Si ratio. A macro was developed for the ImageJ program to automate the quantitative microstructural image analysis. Results showed that Ge addition increases the A15 phase fraction of the two-phase alloys. Microstructural investigations on annealed alloys at 1350 degrees C demonstrated that the coarsening of both binary Cr-Cr3Si and ternary Cr-Cr-3(Si, Ge) is interface-controlled. Although Ge addition to the eutectic alloy offered an isotropic microstructure, it decreased the thermal stability by degeneration of the lamellar structure.