Abstract
We combine theoretical and experimental tools to study elastic properties of Fe-Al-Ti superalloys. Focusing on samples with chemical composition Fe71Al22Ti7, we use transmission electron microscopy (TEM) to detect their two-phase superalloy nano-structure (consisting of cuboids embedded into a matrix). The chemical composition of both phases, Fe66.2Al23.3Ti10.5 for cuboids and Fe81Al19 (with about 1% or less of Ti) for the matrix, was determined from an Energy-Dispersive X-ray Spectroscopy (EDS) analysis. The phase of cuboids is found to be a rather strongly off-stoichiometric (Fe-rich and Ti-poor) variant of Heusler Fe2TiAl intermetallic compound with the L21 structure. The phase of the matrix is a solid solution of Al atoms in a ferromagnetic body-centered cubic (bcc) Fe. Quantum-mechanical calculations were employed to obtain an insight into elastic properties of the two phases. Three distributions of chemical species were simulated for the phase of cuboids (A2, B2 and L21) in order to determine a sublattice preference of the excess Fe atoms. The lowest formation energy was obtained when the excess Fe atoms form a solid solution with the Ti atoms at the Ti-sublattice within the Heusler L21 phase (L21 variant). Similarly, three configurations of Al atoms in the phase of the matrix with different level of order (A2, B2 and D03) were simulated. The computed formation energy is the lowest when all the 1st and 2nd nearest-neighbor Al-Al pairs are eliminated (the D03 variant). Next, the elastic tensors of all phases were calculated. The maximum Young’s modulus is found to increase with increasing chemical order. Further we simulated an anti-phase boundary (APB) in the L21 phase of cuboids and observed an elastic softening (as another effect of the APB, we also predict a significant increase of the total magnetic moment by 140% when compared with the APB-free material). Finally, to validate these predicted trends, a nano-scale dynamical mechanical analysis (nanoDMA) was used to probe elasticity of phases. Consistent with the prediction, the cuboids were found stiffer.
Highlights
Fe-Al-based alloys possess great potential for high-temperature applications [1,2,3,4,5,6] as well as for other functional and structural use [7,8,9,10,11,12,13,14,15]
The chemical composition agreed on average with the published thermodymanical assessment of the Fe-Al-Ti system [16]. In this assessment the composition Fe71 Al22 Ti7 is located in a two-phase region within the ternary Fe-Al-Ti phase diagram
It should be noted that the equilibrium higher-temperature Fe-Al-Ti phase diagrams do not contain any of the additional coherently-coexisting phases which we considered in our analysis
Summary
Fe-Al-based alloys possess great potential for high-temperature applications [1,2,3,4,5,6] as well as for other functional and structural use [7,8,9,10,11,12,13,14,15]. A subset of these materials is represented by two-phase Fe-Al-Ti superalloys in which sub-micron cuboids of an Fe-Al-Ti intermetallic phase are coherently embedded into an Fe-Al matrix The phase of cuboids is a variant of Fe2 TiAl which crystallizes in the Heusler L21 -structure. Material properties of stoichiometric Fe2 TiAl were intensively studied by quantum-mechanical calculations including thermodynamic and magnetic properties [22,23], electronic properties [24,25], and elastic properties under ambient conditions [26] or under hydrostatic pressures [23]. There is a long-lasting discrepancy related to the low-temperature magnetic moment of Fe2 TiAl which was theoretically predicted nearly an order of magnitude higher than the experimental value. Density functional theory (DFT) calculations resulted in 0.9 μB per
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