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

Abstract

In this paper we report on the fabrication and characterization of Nb 2AlC (actual Nb:Al:C ratios: 52.5±0.5, 24.0±0.2 and 23.5±0.5 at.%, respectively) and (Ti,Nb) 2AlC (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 2AlC, (average grain size ≈14±2 μm) were fabricated by reactive hot isostatic pressing of Nb, graphite, and Al 4C 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) 2AlC. 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 2AlC samples are, respectively, 3.107±0.001 and 13.888±0.001 Å; the corresponding values for the solid solution are 3.077±0.001 and 13.790±0.001 Å. Since the hardness of the solid solution (5.8 GPa) is in between those of Nb 2AlC (6.1 GPa) and Ti 2AlC, and, at comparable grain sizes, Nb 2AlC 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 3SiC 2 and ice, and for the same reasons, the compressive stress of the Nb 2AlC sample decrease with decreasing strain rate. In contradistinction, and for reasons that are not understood, the strengths of the (Ti,Nb) 2AlC samples decrease with increasing strain rates. The grain growth kinetics of Nb 2AlC are quite sluggish; no appreciable grain growth was observed even after annealing at 1600 °C for 16 h.