Miscible chemical ordering in Ti-Cr-Mo quinary system by solid solution of Mo2Ti2AlC3 and Cr2.5Ti1.5AlC3 o-MAXs

Abstract

Out-of-plane ordering is promising for separately adjusting the heterodesmic chemical bonding inside the MAX phase thus tuning their properties, while constructing the out-of-plane ordered-MAX (o-MAX) is still a challenge. In this work, a strategy towards o-MAX by solid solutions of two existing o-MAXs is verified, i.e., Cr_2.5Ti_1.5AlC₃ and Mo₂Ti₂AlC₃, with controllable stoichiometric ratios (1:2, 1:1, and 2:1). A miscible chemical ordering is observed in three Ti-Cr-Mo quinary MAXs, which inherits the out-of-plane ordering from both parental o-MAXs. Meanwhile, through density functional theory (DFT) calculations, the electronic structure and bonding states inside the quinary o-MAXs are analyzed. Based on the calculations, anisotropic and improved mechanical properties are predicted, which agree with the experimental observed high compressive strength and tunable capacity of energy dissipation. The present work proves a promising way for synthesizing multicomponent o-MAXs.

Fig. 1. Lattice structure and properties of o-MAXs compared to conventional MAXs.

a Schematic illustration of the out-of-plane ordering in M₃AX₂ (312) and M₄AX₃ (413) MAX with two different metal planes. The orange and black spheres represent A and X atoms in the MAXs’ lattice, respectively, whereas gray spheres represent homogeneous M atoms in the lattice. Green and pink spheres indicate M’ and M” atoms in the o-MAXs’ lattice, respectively. b Illustration of the solid solution of isomorphic fcc carbide lattice. c Illustration of the formation of an off-stoichiometric (Cr_0.9Mo_0.1)₂(Ti_0.8Mo_0.2)AlC₂ product from two 312 o-MAXs.

Fig. 2. Atomic resolution of out-of-plane ordering in the quinary o-MAXs.

Double Cs correction HAADF-STEM images, EDS mapping, corresponding line profile, and the crystal model showing the projection direction. a Mo₁Cr₂, b Mo₁Cr₁, and c Mo₂Cr₁. Line profiles are acquired along the respective yellow arrow and the HAADF signal of the lattice is shown as gray background. The orange, blue, pink, and red lines are corresponded to Cr, Ti, Mo, and Al, respectively. Source data are provided as a Source Data file.

Fig. 3. Phase structure of the quinary o-MAXs and schematic of the ordering inherits.

a Rietveld refinement of XRD pattern for annealed Mo₁Cr₂ MAX. The diffraction patterns of MAX and TiC phase are derived from the standard crystallographic data of α-V₄AlC₃ (ICSD 160754) and TiC (ICSD 1546). b The change in lattice parameters of the three quinary o-MAXs compared to Cr_2.5Ti_1.5AlC₃ and Mo₂Ti₂AlC₃. The red and blue plots correspond to the parameters of a-axis and c-axis, respectively, whereas the green plot shows the c/a ratio. c Schematic illustration of the stoichiometric solid solution of Cr_2.5Ti_1.5AlC₃ and Mo₂Ti₂AlC₃ to form the quinary o-MAX and the succession of the out-of-plane ordering. The red and blue knight phalanxes refer to the lattice of Cr_2.5Ti_1.5AlC₃ and Mo₂Ti₂AlC₃, respectively, whereas the equipped shield and spear, respectively, represent the 4e and 4f metal sites in each o-MAX. Source data are provided as a Source Data file.

Fig. 4. Electronic structure and bonding analysis.

a–c Total density of states (TDOS) and corresponding negative average crystal orbital Hamilton population (−COHP) of Mo₂Ti₂AlC₂ (a), Cr₂Ti₂AlC₃ (b), and Mo₁Cr₁ (c), respectively. d Schematic illustration of the coordination environments of Ti(14) atom at 4 f site with an adjacent Mo(1) atom in Mo₁Cr₁. e, f Average −COHP and integrated −COHP curves (−ICOHP) of (e) Ti-C(4 f) bonds in Mo₁Cr₁ and the two parental o-MAXs, (f) Ti-C(2a) bonds in Mo₁Cr₁ and the two parental o-MAXs. The −COHP curves of each bond are colored in red and blue, whereas the −ICOHP curves are colored in green and orange. Solid and dash lines represent the α and β spin, respectively. Source data are provided as a Source Data file.

Fig. 5. Bonding analysis of the 4e metal.

a Schematic illustration of the coordination environments of Cr(1) atom at 4e site with an adjacent Mo(4) atom. b–e Negative average crystal orbital Hamilton population (−COHP) and integrated −COHP curves (−ICOHP) of (b) Cr-C(4 f) bonds in Mo₁Cr₁ and Cr₂Ti₂AlC₃, (c) Mo-C(4 f) bonds in Mo₁Cr₁ and Mo₂Ti₂AlC₃, (d) Cr-Al bonds in Mo₁Cr₁ and Cr₂Ti₂AlC₃, (e) Mo-Al bonds in Mo₁Cr₁ and Mo₂Ti₂AlC₃. The −COHP curves of each bond are colored in red and blue, whereas the −ICOHP curves are colored in green and orange. Solid and dash lines represent the α and β spin, respectively. Source data are provided as a Source Data file.

Fig. 6. Mechanical properties of the quinary o-MAXs.

a Typical stress-strain curves of the three o-MAXs under a compressive rate of 10⁻³ s⁻¹. b The elastic modulus E (in green), nanoindentation hardness H (in blue), and the calculated E²/H (in red) of the three MAXs with a peak load of 5 mN. c Change on the compressive strength (in dark blue) and E (in green) of the three MAXs upon their chemical composition. d–f High-magnification SEM images show the typical surface morphology after compression tests: Mo₁Cr₂ (d), Mo₁Cr₁ (e), and Mo₂Cr₁ (f), respectively. The yellow arrows indicate the kinking within the grains. The error bars in (b) represent the standard deviations from ten parallel measurements, while the error bars of compressive strength in (c) represent the standard deviations from five parallel measurements. Source data are provided as a Source Data file.