Electronic origin of the isostructural decomposition in cubic M1−xAlxN (M=Ti, Cr, Sc, Hf): A first-principles study

Abstract

We have used first-principles calculations to investigate the mixing enthalpies, lattice parameters and electronic density of states of the ternary nitride systems Ti1−xAlxN, Cr1−xAlxN, Sc1−xAlxN and Hf1−xAlxN in the cubic B1 structure where the transition metals and aluminium form a solid solution on the metal sublattice. We discuss the electronic origins of the possible isostructural decomposition in these materials relevant for hard coatings applications. We find that in the systems Ti1−xAlxN and Hf1−xAlxN the electronic structure effects strongly influences the phase stability as d-states are localised at the Fermi level in AlN-rich samples. This leads to a strongly asymmetric contribution to the mixing enthalpy, an effect not present in Cr1−xAlxN and Sc1−xAlxN. The lattice mismatch is large in Sc1−xAlxN and Hf1−xAlxN, giving a symmetric contribution to the mixing enthalpies in those systems.

Cubic Sc1−xAlxN solid solution thin films deposited by reactive magnetron sputter epitaxy onto ScN(111)

“…16͒ can be seen. Such a small overestimation of the experimental values is common for calculations using the GGA functional and a similar shape of the calculated curve was also observed in lattice parameter calculations for different related systems, such as Ti 1−x Al x N, Cr 1−x Al x N, and Hf 1−x Al x N. 24 The experimental curve ͑with red dots͒ in Fig. 6 is a combination of the compositions achieved from RBS and the lattice parameters calculated from XRD data.…”

“…19 Calculations estimate the indirect band gap to be 0.9-1.6 eV ͑Refs. 20-23͒ and the lattice parameter to be 4.521 Å, 24 while experimentally the lattice parameter is measured to be 4.50 Å. 25 Its thermodynamically stable crystal structure is rocksalt, even though it is also predicted to form a metastable hexagonal phase.…”

“…17,21,23 This large difference in both volume and electronic structure gives the possibility to design materials with a wide range of both lattice parameters and band gaps. Recently Alling et al 24 reported of first-principles calculations on the mixing enthalpies, lattice parameters, and electronic density of states of c-Sc 1−x Al x N. There are, however, to the best of our knowledge, no reports on the synthesis of such solid solutions. This paper concentrates on thin film deposition and characterization of Sc 1−x Al x N starting from optimal conditions for pure ScN, followed by an increase in Al molar fraction reaching pure AlN.…”

Effect of Al substitution on Ti, Al, and N adatom dynamics on TiN(001), (011), and (111) surfaces

“…[12][13][14][15] This is believed to be responsible in part for the improved wear resistance of TiAlN coatings which has been extensively studied both experimentally [12,13,15] and theoretically. [12,[15][16][17][18] During growth, limited short-range clustering is likely to occur [19] and must be taken into account in order to obtain a complete atomistic understanding of these materials.…”

High temperature phase decomposition in TixZryAlzN

“…One such well studied example is (TiAl)N which is known to go from a solid solution phase of c-(TiAl)N to a phase separated state with a mixture of c-TiN and c-AlN upon heating, a process that drastically improves resultant material properties. [1][2][3][4][5][6][7] However, additional heating, around 900 • C, will deteriorate the material properties when the AlN changes its structure from cubic to wurtzite. 8 In our previous works [9][10][11] we demonstrated that the addition of Cr into the solution leads to a formation of intermediate metastable phases of (TiCr)N and (CrAl)N where Cr dissolved in the AlN delays the onset of a detrimental cubic-to-wurtzite phase transition to higher temperatures, up to above 1000 • C. Another alloy that have shown interesting high temperature properties is (ZrAl)N. [12][13][14][15][16][17][18] Also, alloying (TiAl)N with Zr have been shown to give improved high temperature properties.…”