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 ela correlation with phase st ability. The substitution of Fe or Co for Cr in NbCr2 increases the average electron concentration of the alloy, thus leading to a systematic change o f the crystal structures from C15 to C14 and C14 to C15. The experimental c ritical ela values corresponding to the C15/C14/C15 phase transitions are c onsistent with those from the phase diagram survey: the C15 structure is st abilized at ela < 5.77; the C14 structure is stable at 5.85 < ela <7.50; th e C15 structure is stabilized again at e/a > 7.56. As the Fe or Co content increases in the Nb(Cr,X)2 phase, the volume of a 24-atom unit cell, decrea ses linearly, while the Vickers hardness increases over the whole C15 and C 14 ranges. The fracture toughness values are low for all. the alloys; never theless, the fracture toughness of the C15 structure is greater than that o f the C14 structure for the two pseudo-binary systems. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.