The microstructure and room temperature fracture toughness of binary Cr/Cr 2 Nb alloys annealed at 1653 K for 30 h were investigated at both the hypo-and hypereutectic compositions. The experimental results indicate that the high temperature heat treatment has a beneficial effect on the room temperature fracture toughness of the Cr 2 Nb/Cr alloys. After the heat treatment, the room temperature fracture toughness of the hypo-and hypereutectic alloys are increased by about 212 and 203%, which are 15 and 8 times higher than that of as-cast Cr 2 Nb Laves phase (1?2 MPa m 1/2 ). The fractographic analysis indicates that in the annealed condition, the strengthen of lamellar eutectic cohesive strength can provide significant toughening of the matrix by crack deflection, crack blunting and crack bridging mechanisms.

The microstructures of binary Cr/Cr2Nb alloys doped with 0.1 at- rare earth element (REE) Y were characterised by fine interlamellar spacing, and the growth morphology of Cr2Nb primary phase was transformed from faceted to non-faceted. Moreover, the lamellar spacing was observed to decrease with the decreasing of solidification rate with the addition of REE Y. Furthermore, the mechanical tests showed that the addition of REE Y has a beneficial effect on the room temperature fracture toughness of Cr2Nb/Cr alloys. The room temperature fracture toughness of the Y doped Cr/Cr2Nb hypoeutectic and hypereutectic alloys increased by ∼130 and 140 over that of the undoped alloys respectively, which was caused by the refinement of lamellar eutectic and strengthening of lamellar eutectic bonding strength.

Abstract