Phase stability and mechanical behavior of NbCr2-based Laves phases

The phase relations in pseudo-binary Nb(Cr,X)₂ systems, where X = Fe or Co, were determined experimentally to verify the e/a correlation with phase stability. The substitution of Fe or Co for Cr in NbCr₂ increases the average electron concentration of the alloy, thus leading to a systematic change of the crystal structures from C15 to C14 and C14 to C15. The experimental critical e/a values corresponding to the C15/C14/C15 phase transitions are consistent with those from the phase diagram survey; the C15 structure is stabilized at e/a7.56. As the Fe or Co content increases in the Nb(Cr,X)₂ phase, the volume of a 24-atom unit cell decreases linearly, while the Vickers hardness increases over the whole C15 and C14 ranges. The fracture toughness values are low for all the alloys; nevertheless, the fracture toughness of the C15 structure is greater than that of the C14 structure for the two pseudo-binary systems.