Herein we report on the reactivity between palladium, Pd, and the MAX phases, Ti2AlC, Ti3AlC2, Ti3SiC2 and Cr2AlC. Diffusion couples of Pd/MAX were heated to 900 degrees C under uniaxial stress of similar to 20 MPa for 2, 4, and 10 h in a vacuum (< 1 Pa) hot press. The diffusion couples were examined using X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. After heating to 900 degrees C for 10 h, the diffusion layer thicknesses in the Ti2AlC/Pd, Cr2AlC/Pd, Ti3AlC2/Pd and Ti3SiC2/Pd couples were found to be 35, 45, 105 and 410 mm, respectively. Thus, Ti2AlC is the most resistant to reaction, while Ti3SiC2 is least resistant, with Cr2AlC and Ti3AlC2, in between. In all cases, the reaction occurred by the diffusion of the A-group element into Pd, concomitant with Pd diffusion into the MAX phase. No diffusion of the M and X atoms was detected. (C) 2018 Published by Elsevier B.V.

Herein we report on the interaction of the M(n+1)AX(n) phases - Ti2AlC, Ti3AlC2, Ti3SiC2 and Cr2AlC - with static, pure molten sodium. The MAX phases were sealed in 316 stainless steel tubes filled with Na that, in turn, were heated to 550 degrees C or 750 degrees C for 168 h. Based on scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) of post-exposed samples we conclude that even after exposure for 168 hat 750 degrees C, Ti2AlC, Ti3SiC2 and Cr2AlC did not react with Na. In the case of Ti3AlC2, exposure to molten Na alters both the grains and the grain boundaries, despite the fact that no Na signal was detected in the post-exposed samples. These chemical alterations were observed in TEM and manifested themselves by a change in the hardness values of the exposed surfaces. (C) 2016 Elsevier Ltd. All rights reserved.

Herein, we report—for the first time—on the additive‐free bulk synthesis of Ti3SnC2. A detailed experimental study of the structure of the latter together with a secondary phase, Ti2SnC, is presented through the use of X‐ray diffraction (XRD), and high‐resolution transmission microscopy (HRTEM). A previous sample of Ti3SnC2, made using Fe as an additive and Ti2SnC as a secondary phase, was studied by high‐temperature neutron diffraction (HTND) and XRD. The room‐temperature crystallographic parameters of the two MAX phases in the two samples are quite similar. Based on Rietveld analysis of the HTND data, the average linear thermal expansion coefficients of Ti3SnC2 in the a and c directions were found to be 8.5 (2)·10−6 K−1 and 8.9 (1)·10−6 K−1, respectively. The respective values for the Ti2SnC phase are 10.1 (3)·10−6 K−1 and 10.8 (6)·10−6 K−1. Unlike other MAX phases, the atomic displacement parameters of the Sn atoms in Ti3SnC2 are comparable to those of the Ti and C atoms. When the predictions of the atomic displacement parameters obtained from density functional theory are compared to the experimental results, good quantitative agreement is found for the Sn atoms. In the case of the Ti and C atoms, the agreement is more qualitative. We also used first principles to calculate the elastic properties of both Ti2SnC and Ti3SnC2 and their Raman active modes. The latter are compared to experiment and the agreement was found to be good.

The unique properties of the MAX phases make them a desirable reinforcement material in metal matrix composites. The promising results of MAX reinforced Mg matrix composites have spurred interest in developing Al-MAX composites with comparable or better properties. Herein, two-phase Al-V2AlC composites were synthesized from elemental powders. The nominal compositions were chosen so that in situ reactions would produce either a 75/25 vol. % Al/V2AlC or a 50/50 vol. % Al/V2AlC composites. Elemental powders were heated to 1000 degrees C and reacted for 0.5, 2.5 or 10 h under flowing Ar. Water quenched samples from temperature >950 degrees C produced two-phase Al-V2AlC composites; furnace cooled samples did not. X-ray diffraction, scanning electron microscopy and differential scanning calorimetry were used to investigate the stability of V2AlC with Al in the 800-1000 degrees C range. The Al-V-C phase diagram was defined in the Al-rich corner. At 800 degrees C, the Al and V2AlC phases were found to be in equilibrium with both Al3V and Al4C3, but not with each other. This study is a requisite step for the development of advanced composites in the Al-V-C system. (C) 2016 Elsevier B.V. All rights reserved.

Herein we report on the reactivity between silicon carbide, SiC, and pyrolytic carbon, PG, with the MAX phases, Ti2AlC, Ti3AlC2, Ti3SiC2 and Cr2AlC. Diffusion couples were assembled and heated to 1300°C under a load corresponding to a uniaxial stress of ∼30MPa for 4, 10, and 30h in a vacuum hot press, at a vacuum level of less than 1Pa. The couples were then examined using optical and scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and orientation image microscopy. Based on the totality of the results – after 30h at 1300°C – it is concluded that neither Ti3SiC2 nor Cr2AlC appear to react with SiC. The former also appears not to react with PG. When heated in the vacuum of the hot press, both Ti2AlC and Ti3AlC2 dissociated to form TiC surface layers that were ≈15–20μm thick. After reaction of Ti2AlC with SiC and PG, the TiC layer was only ≈10μm thick, indirectly confirming that the dissociation of these phases in vacuum was due to the Al evaporation from the surfaces. The Ti3AlC2/SiC and Ti3AlC2/PG diffusion couples resulted in TiC layers that were ≈50μm and ≈100μm thick, respectively. The Cr2AlC/PG diffusion couple resulted in the formation of ≈10μm interfacial layer comprised of Cr3C2 and Cr7C3 at the interface between the two materials.

Herein, we report on the crystal structures of Nb2AlC and TiNbAlC—actual composition (Ti0.45,Nb0.55)2AlC—compounds determined from Rietveld analysis of neutron diffraction patterns in the 300–1173 K temperature range. The average linear thermal expansion coefficients of a Nb2AlC sample in the a and c directions are, respectively, 7.9(5) × 10−6 and 7.7(5) × 10−6 K−1 on one neutron diffractometer and 7.3(3) × 10−6 and 7.0(2) × 10−6 K−1 on a second diffractometer. The respective values for the (Ti0.45,Nb0.55)2AlC composition—only tested on one diffractometer—are 8.5(3) × 10−6 and 7.5(5) × 10−6 K−1. These values are relatively low compared to other MAX phases. Like other MAX phases, however, the atomic displacement parameters (APDs) show that the Al atoms vibrate with higher amplitudes than the Ti and C atoms, and more along the basal planes than normal to them. When the predictions of the APDs obtained from density functional theory are compared to the experimental results, good quantitative agreement is found for the Al atoms. In case of the Nb and C atoms, the agreement was more qualitative.

We synthesized the following previously unreported aluminum-containing solid solution Mn+1AXn phases: (Ti0.5, V0.5)3AlC2, (Nb0.5, V0.5)2AlC, (Nb0.5, V0.5)4AlC3 and (Nb0.8, Zr0.2)2AlC. Rietveld analysis of powder X-ray diffraction patterns was used to calculate the lattice parameters and phase fractions. Heating Ti, V, Al and C elemental powders—in the molar ratio of 1.5:1.5:1.3:2—to 1, 450°C for 2 h in flowing argon, resulted in a predominantly phase pure sample of (Ti0.5, V0.5)3AlC2. The other compositions were not as phase pure and further work on optimizing the processing parameters needs to be carried out if phase purity is desired.