Nature of high hardness in the Co70Mo30 alloy

The Co70Mo30 alloy structure formed at different temperatures was studied by means of transmission electron microscopy and photoelectron spectroscopy. It has been found that at temperatures below the solidus temperature the alloy has a tendency toward phase separation which leads to formation of a quasi-cellular microstructure in the form of clusters strongly enriched in Mo and Co, respectively, but existing within a common fcc lattice of the solvent. At low temperatures, an allotropic phase transition α-Co → ε-Co occurs in the alloy and the strain contrast, arising at such transitions is superimposed on the quasi-cellular structure. The Co70Mo30 alloy acquires high hardness due to the fact that the crystal lattice in the clusters strongly enriched in Mo remains face-centered.

► Below the solidus temperature, a tendency toward phase separation leads to a fine-dispersed cellular structure. ► Due to high dispersity, a phase separation occurs only by chemical composition within a single Ni crystal lattice. ► The presence of Mo clusters within the Co crystal lattice determines the high-level strength of the alloy. ► Preservation of the phase separation structure right up to the solidus temperature determines the high heat resistance. ► Clusters strongly enriched in Co are responsible for α-Co. → ε-Co transition at 532 °C in the whole volume.