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The effect of the precipitation of j,~ phase particles on the mechanical properties of single crystals of high strength MC2 superalloy was investigated. Plate-like and globular particles of a rhombohedral, W-rich, topologically close-packed (TCP) p phase precipitate within the dendrites during exposure in the temperature range 800-l lOO'C, together with cuboidal M6C carbides in a few interdendritic subgrain boundaries. The presence of p phase does not affect the fracture mechanism during the stress-rupture tests. Some tensile and high cycle fatigue tests were carried out on specimens previously exposed during 200 hours at 1050°C at a stress of 80 MPa and the results are compared to those obtained on unaged material. The decrease in yield strength observed at room temperature and 650°C is attributed to the coalescence of the strengthening y ' phase rather than to a softening effect due to the precipitation of p phase. Neither the tensile ductility nor the high cycle strength are affected by the presence of the TCP phase. A 200-hour exposure at 1050°C increases the impact strength at room temperature and 750°C of MC2 single crystals, due to the coarsening of the y ' phase.

Some fine-scale microstructure of nickel-based single crystal superalloys have been investigated by means of atom probe techniques.

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