High-temperature neutron diffraction and first-principles study of temperature-dependent crystal structures and atomic vibrations in Ti3AlC2, Ti2AlC, and Ti5Al2C3

Abstract

Herein we report on the thermal expansions and temperature-dependent crystal structures of select ternary carbide MAX phases in the Ti-Al-C phase diagram in the 100-1000• C temperature range. A bulk sample containing 38(±1) wt.% Ti5Al2C3 ("523"), 32(±1) wt.% Ti2AlC ("211"), 18(±1) wt.% Ti3AlC2 ("312"), and 12(±1) wt.% (Ti0.5Al0.5)Al is studied by Rietveld analysis of high-temperature neutron diffraction data. We also report on the same for a single-phase sample of Ti3AlC2 for comparison. The thermal expansions of all the MAX phases studied are higher in the c direction than in the a direction. The bulk expansion coefficients -9.3(±0

Thermally induced substitutional reaction of Fe into Mo2GaC thin films

“…This may be due to the weakened M-A interlayer bonding after the substitution of the A elements, allowing more movement of the new A elements around an equilibrium position. According to previous reports, the A element has a much higher atomic motion with respect to the M and the X elements in MAX phases due to weaker bonding of the A element [28,29]. In addition, both experimental and theoretical results indicate an anisotropic atomic displacement of the A element, which has a higher amplitude along the basal plane than along the [0001] direction [28].…”

A High‐Temperature Neutron Diffraction Study of Nb2AlC and TiNbAlC

“…The TEC's of the c and a lattice parameters, and their ratio, α c /α a , measured herein are listed in Table . Also listed are previous results for Ti 2 AlC, TiNbAlC and Nb 2 AlC . From these results, it is clear that the TECs for Nb 2 AlC along the a and c directions are almost equal.…”

Bonding and oxidation protection of Ti2AlC and Cr2AlC for a Ni-based superalloy

“…[44] We note in passing that both Ti2AlC and Cr2AlC discs were prepared in the same exact way and several HP bonding attempts were made with different Ti2AlC/NSA samples at lower stresses and temperatures, but none were successful. A more likely reason for the poor bonding could be the relatively large CTE differences, that can lead to thermal stress between Ti2AlC (~8•10 -6 °C -1 ) [36,37] and NSA (≥16•10 -6 °C -1 ) upon cooling. [39] Nevertheless, at the higher stress, the contact area between the Ti2AlC and the NSA surface is larger and hence the formation kinetic of initial DZ is enhanced.…”

“…Furthermore, studies based on growth models, documented that the grain boundary diffusion, in the Al2O3, layers scales are in accord with those obtained for one of the best oxidation resistant, alumina-forming FeCrAl alloys. [34,35] Another beneficial characteristic of some of the MAX phases is its coefficient of thermal expansion (CTE), [13] which in the case of Ti2AlC (8.2×10 −6 /°C) [36,37] is a better match to α-Al2O3 (9.3×10 −6 /°C), [38] as compared directly to NSA (15-16×10 −6 /°C), [39] ensuring better adherence. [30,31] In the case of Cr2AlC, the CTE value (12-13×10 −6 /°C) [36,37] lies between the CTE values of α-Al2O3 and NSAs that again prevents spallation, caused by thermal stresses.…”