Microstructure and Mechanical Properties of Cr2Nb-Based Laves Phase Alloys Prepared by Laser Surface Remelting

The microstructure and mechanical properties of Cr2Nb-based intermetallic matrix composites fabricated by laser surface remelting were investigated at hypoeutectic and hypereutectic compositions. In the hypoeutectic composition, the molten zones under different laser scanning velocities consist of fully α-Cr dendrites, whereas a series of morphology evolutions perpendicular to the scanning direction occurred in the hypereutectic composition: Cr2Nb primary phase + Cr/Cr2Nb eutectic → Cr/Cr2Nb eutectic → α-Cr primary phase + Cr/Cr2Nb eutectic → fully α-Cr dendrite. Based on the maximum growth rate criterion, the microstructure evolutions of the molten zones were analyzed by considering the competitive growth among the α-Cr phase, Cr2Nb Laves phase, and the Cr/Cr2Nb eutectic, which are in agreement with the experimental results. More importantly, the room-temperature fracture toughness values of the hypoeutectic and hypereutectic alloys attain 11.2 MPa m1/2 and 7.9 MPa m1/2, respectively, which increase by eight and six times over the as-cast Cr2Nb Laves phase (1.2 MPa m1/2), respectively. The remarkable enhancements in room-temperature fracture toughness were explained in terms of fine-grain toughening, second Cr phase toughening, and point defects toughening.