Influence of a Partial Substitution of Co by Fe on the Phase Stability and Fatigue Behavior of a CoCrWC Hard Alloy at Room Temperature

Abstract

The deformation-induced phase transition from fcc to hcp causes local embrittlement of the metal matrix in Cobalt-base alloys, facilitating subcritical crack growth under cyclic loading and reducing fatigue resistance. Our approach to increasing the fatigue life of Co-based hard alloys is to suppress the phase transition from fcc to hcp by an alloy modification that increases the stacking fault energy (SFE) of the metal matrix. Therefore, we substitute various contents (15, 25, and 35 mass pct) of Co by Fe and analyze the effect on the fatigue life and resistance against subcritical crack growth. Subcritical crack growth in the specimens takes place in a cyclic load test. The proceeding crack growth and the occurrence of phase transformations are monitored by scanning electron microscope (SEM) investigations and electron backscatter diffraction (EBSD). We determined an SFE of 35 mJ/m2 at an iron content of 35 mass pct, which leads to a change of the main deformation mechanism from deformation-induced martensitic transformation to deformation twinning. Analysis of cyclically loaded specimens revealed that the resistance against subcritical crack growth in the metal matrix is facilitated with increasing Fe content, leading to a significant increase in fatigue life.

Laser Powder Bed Fusion of Hard‐Phase Containing Co‐Base Alloy

“…With this lower deformation capacity, a stabilizing effect on the hard phases is maintained, which counteracts hard phase fragmentation and the formation of a material mixed layer (cf. the work of Varga et al [ 8 ] Krell et al [ 9 ] and Brackman et al [ 10 ] ). In addition, Wood et al described in their work that Co‐alloys can form stable oxide layers in an oxygen‐containing atmosphere, leading to a wear‐resistant, hard, and low‐friction glaze layer due to tribo‐mechanical stress.…”

Diseño de aleación de alta entropía de alto desempeño mecánico en condiciones de bajas y altas temperaturas para aplicaciones energéticas y aeroespaciales.

“…Para diseñar aleaciones que contengan estos refuerzos es necesario establecer criterios de selección de la composición química de la aleación. La utilización del Co como elemento aleante produce un estado de compatibilidad de energía con elementos que producen un aumento significativo de la SFE como lo es el Ni [128,129]. Desde lo anterior, si no se contempla Co, en un sistema que contenga Ni, el contenido de Ni no debería superar un 6%.…”