AlC. 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 (Ti 0.5 , V 0.5 ) 3 AlC 2 . 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.
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 MAX phases are a family of nanolayered, machinable, ternary carbides and nitrides, 2 having the general formula M n+1 AX nwhere n = 1, 2, 3; M is an early transition metal; A is an 3 A-group element (mostly groups 13 and 14); and X is C and/or N [1]. These phases combine 4 some of the best properties of metals and ceramics. Like metals they are machinable, electrically 5 and thermally conductive [2, 3], damage tolerant and not susceptible to thermal shock [4]. Like 6 ceramics, some of them are lightweight ( ≈ 4 Mg/m 3 ) and elastically rigid (Young's moduli > 7 300 GPa). These unique properties render the MAX phases desirable candidates for use as 8 reinforcement phases in metal matrix composites, MMCs. 9 Recently, magnesium, Mg, matrix composites reinforced with Ti 2 AlCa member of the 10 MAX phases -have been developed [5]. These Mg and Mg alloys-MAX composites were 11 manufactured by spontaneous melt infiltration, MI, of porous MAX preforms, at relatively low 12 processing temperatures (750 °C) yielding composites with attractive mechanical properties. For 13 example, the Young's modulus and ultimate compressive strengths, UCSs, of a Mg alloy (AZ61) 14 matrix composite reinforced with 50 vol. % Ti 2 AlC particles were found to be 136±6 GPa and 15 760±9 MPa, respectively. In addition to the excellent mechanical properties, these composites 16 dissipated almost 25 % of the applied mechanical energy at high stresses [5]. The resulting 17 composites were also most readily machinable, since both constituents are machinable. These 18 promising results spurred interest in developing Al-MAX composites with comparable or better 19 properties. 20 Fabricating Al-matrix MAX reinforced composites is complicated by the fact that Al is 21 not in equilibrium with most MAX phases [1]. For example, Wang et al. [6] attempted to 22 fabricate Ti 3 AlC 2 composites at elevated temperatures and found that above 950 °C, Ti 3 AlC 2 1 reacted with Al to form TiC and TiAl 3 . To circumvent this problem Wang et al. [6] hot pressed 2 Al and Ti 3 AlC 2 powders at a temperature (550 °C) at which the reaction kinetics were slow.3
a b s t r a c tHerein we report on the reactivity between silicon carbide, SiC, and pyrolytic carbon, PG, with the MAX phases, Ti 2 AlC, Ti 3 AlC 2 , Ti 3 SiC 2 and Cr 2 AlC. Diffusion couples were assembled and heated to 1300 • C under a load corresponding to a uniaxial stress of ∼30 MPa for 4, 10, and 30 h in a vacuum hot press, at a vacuum level of less than 1 Pa. 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 30 h at 1300 • C -it is concluded that neither Ti 3 SiC 2 nor Cr 2 AlC appear to react with SiC. The former also appears not to react with PG. When heated in the vacuum of the hot press, both Ti 2 AlC and Ti 3 AlC 2 dissociated to form TiC surface layers that were ≈15-20 m thick. After reaction of Ti 2 AlC with SiC and PG, the TiC layer was only ≈10 m thick, indirectly confirming that the dissociation of these phases in vacuum was due to the Al evaporation from the surfaces. The Ti 3 AlC 2 /SiC and Ti 3 AlC 2 /PG diffusion couples resulted in TiC layers that were ≈50 m and ≈100 m thick, respectively. The Cr 2 AlC/PG diffusion couple resulted in the formation of ≈10 m interfacial layer comprised of Cr 3 C 2 and Cr 7 C 3 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.
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