Abstract
In the present work, a drastic increase in the formation of athermal ε-martensite was experimentally found after casting a Co–20Cr alloy in a water cooled Cu-mold. Under these conditions, rapid solidification was achieved with an alloy cooling rate of 278 K/s (approx.). The amount of precipitated athermal ε-martensite measured using X-ray diffraction was 97.26 vol. %. An inspection of the exhibited microstructure indicates that rapid solidification promotes appreciable dendrite grain refinement and alloy homogenization. In addition, the dendritic structure contained numerous visible striations associated with the athermal γ ↔ ε transformation. Transmission electron microscopy showed the development of an extremely large amount of stacking faults and stacking fault intersections, including ε-martensite plates. The microstructural characterization included an estimation of average ε-martensite plate dimensions by assuming an ellipsoidal plate geometry. These results where compared with predictions of critical ε-martensite embryo dimensions by using Eshelby's inclusion theory. In addition, a determination of the chemical volumetric free energy change, ΔGV using a regular solution model indicated that this term is rather small. Consequently, the energy barrier for spontaneous nucleation in the Co–20Cr is relatively large when compared with other alloy systems.
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