Y. L. Xue

Phase stability of the laves phase Cr2Nb in a two-phase Cr-Cr2Nb alloy

The phase stability of the Laves phase Cr2Nb in a two-phase Cr-Cr2Nb alloy produced by arc-melting was investigated using X-ray diffraction, transmission electron microscopy and selected area electron diffraction. The experimental results indicated that the as-cast ingot was consisted of C15-Cr2Nb and Cr phases; the intermediate C36 and high-temperature C14 modifications of Cr2Nb, reported in literatures, were not detected in this study. These results, combined with a detailed analysis of the actual solidification conditions, revealed that the C14-Cr2Nb was a metastable rather than a stable high-temperature modification. Moreover, a kind of extremely fine lamellar structure was found to be randomly distributed in the eutectic cells, which may be formed by decomposition of the supersaturated C15-Cr2Nb via a discontinuous precipitation reaction.

Microstructure and fracture toughness of Cr/Cr2Nb alloys with trace Y addition

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.

Improvement of room temperature fracture toughness in Cr2Nb-based alloys by heat treatment

Abstract The microstructure and room temperature fracture toughness of binary Cr/Cr2Nb 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 Cr2Nb/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 Cr2Nb Laves phase (1·2 MPa m1/2). 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.