Athermal ε-martensite transformation in a Co–20Cr alloy: Effect of rapid solidification on plate nucleation

• Published in: Acta materialia Vol. 111; pp. 138 - 147
• Main Authors: Ramirez-Ledesma, A.L., Lopez-Molina, E., Lopez, H.F., Juarez-Islas, J.A.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-06-2016
• Subjects: Athermal ε-martensite; Chill casting; Cobalt base alloys; Cobalt based alloys; Dendritic structure; Homogenizing; Martensite nucleation; Mathematical models; Nucleation; Plates (structural members); Rapid solidification; Stacking faults; Transformations; Chill casting; Rapid solidification; Athermal ε-martensite; Cobalt based alloys; Martensite nucleation
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2016.03.047

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.
[Display omitted]