Synthesis and mechanical properties of Nb2AlC and (Ti,Nb)2AlC

In this paper we report on the fabrication and characterization of Nb 2 AlC (actual Nb:Al:C ratios: 52.5±0.5, 24.0±0.2 and 23.5±0.5 at.%, respectively) and (Ti,Nb) 2 AlC (actual Ti:Nb:Al:C atomic ratios: 24.4±0.5, 27.3±0.5, 24.0±0.3 and 24.4±0.5 at.%, respectively). Polycrystalline, fully dense, predominantly single-phase samples of Nb 2 AlC, (average grain size ≈14±2 μm) were fabricated by reactive hot isostatic pressing of Nb, graphite, and Al 4 C 3 at 1600 °C for 8 h and 100 MPa. The identical procedure resulted in predominantly single-phase samples—with an average grain size of 45 μm—of (Ti,Nb) 2 AlC. To obtain finer-grained (≈15±3 μm) samples of the solid solution the powder mixtures were hot pressed at 1450 °C for 24 h. The a and c lattice parameters of Al-poor Nb 2 AlC samples are, respectively, 3.107±0.001 and 13.888±0.001 A; the corresponding values for the solid solution are 3.077±0.001 and 13.790±0.001 A. Since the hardness of the solid solution (5.8 GPa) is in between those of Nb 2 AlC (6.1 GPa) and Ti 2 AlC, and, at comparable grain sizes, Nb 2 AlC is stronger, we conclude that no solid solution strengthening occurs in this system. All samples explored in this work are quite damage tolerant and thermal shock resistant. A 300 N Vickers indentation in a 1.5 mm thick, four-point bend bar decreases the strengths by anywhere from 25 to 50% depending on grain size. Quenching in water from 1200 °C reduces the four-point flexural strength by 40 to 70%; i.e., it is not catastrophic. Both compositions are strain rate sensitive. Like Ti 3 SiC 2 and ice, and for the same reasons, the compressive stress of the Nb 2 AlC sample decrease with decreasing strain rate. In contradistinction, and for reasons that are not understood, the strengths of the (Ti,Nb) 2 AlC samples decrease with increasing strain rates. The grain growth kinetics of Nb 2 AlC are quite sluggish; no appreciable grain growth was observed even after annealing at 1600 °C for 16 h.