Isostructural Decomposition Research Articles

Phase separation paths in metastable Zr1-xAlxN monolithic layers compared to multilayers with TiN: Growth versus annealing temperatures

Metastable super-saturated Zr1−xAlxN alloys tend to phase separate into the equilibrium cubic (c) ZrN and wurtzite (w) AlN due to a deep miscibility gap. Transformation is shown here to follow distinctly different paths depending on if Zr1−xAlxN (x = 0.3 and 0.6) is sputter deposited as a single layer or multi-layered with TiN, and further varied by post-deposition annealing. Using in situ high-energy synchrotron wide-angle X-ray scattering and analytical transmission electron microscopy, surface segregation effects are compared to secondary phase transformations occurring in as-deposited layers during thermal annealing up to 1000°C. For the primary phase transformation from the vapor phase, w-AlN nucleates and an AlN-ZrN labyrinthine structure evolves at elevated deposition temperature with semi-coherent interfaces over several nanometers, where the higher Al content narrows the structure in both single and multilayers. Transformation in thinner alloy layers is governed by epitaxial forces which stabilize single-phase c-Zr0.4Al0.6N, which enables c-Zr0.4Al0.6N/TiN superlattice growth at temperatures as low as 350°C. Regardless of the decomposition route, the formation of c-AlN is impeded and w-AlN instantaneously forms during transformation. In contrast, isostructural decomposition into w-AlN and w-Zr(Al)N occurs in w-Zr0.4Al0.6N alloys during annealing.

Theoretical investigation of mixing and clustering thermodynamics of Ti1−xAlxB2 alloys with potential for age-hardening

Metastable ternary ceramic alloys with clustering tendencies are candidates for hard coating applications. In this work, mixing thermodynamics and structural parameters of ceramic Ti1−xAlxB2 alloys are investigated with theoretical first-principles based techniques. Lattice dynamics and temperature dependent phase stability are explored. The effect of lattice vibrations on the total free energy is investigated and found to not significantly affect phase stability at temperatures below 1200 K. The isostructural phase diagram is derived using both cluster expansion-based Monte Carlo simulations and a mean field approach. The phase diagram shows a miscibility gap that does not close at temperatures below the melting or decomposition temperatures of the constituent binaries TiB2 and AlB2. The lattice mismatch between phases in the system is small regardless of their composition even at elevated temperatures. These findings support the prospect of age hardening due to coherent isostructural decomposition, such as spinodal decomposition, in coatings of Ti1−xAlxB2 as diffusion is activated at elevated temperature.

Self-structuring in Zr1\u2212xAlxN films as a function of composition and growth temperature

Nanostructure formation via surface-diffusion-mediated segregation of ZrN and AlN in Zr1−xAlxN films during high mobility growth conditions is investigated for 0 ≤ × ≤ 1. The large immiscibility combined with interfacial surface and strain energy balance resulted in a hard nanolabyrinthine lamellar structure with well-defined (semi) coherent c-ZrN and w-AlN domains of sub-nm to ~4 nm in 0.2 ≤ × ≤ 0.4 films, as controlled by atom mobility. For high AlN contents (x > 0.49) Al-rich ZrN domains attain wurtzite structure within fine equiaxed nanocomposite wurtzite lattice. Slow diffusion in wurtzite films points towards crystal structure dependent driving force for decomposition. The findings of unlikelihood of iso-structural decomposition in c-Zr1−xAlxN, and stability of w-Zr1−xAlxN (in large × films) is complemented with first principles calculations.

Structural and mechanical properties of nitrogen-deficient cubic Cr–Mo–N and Cr–W–N systems

Our computational and experimental results reveal the inherent driving force of cubic Cr–TM–N (TM=Mo, W) solid solutions for nitrogen deficiency, expressed chemically as Cr1-xTMxN1–0.5x. Their computationally predicted large positive mixing enthalpies indicate a high driving force for isostructural decomposition into B1-structured CrN and γ-TM2N. The calculations further predict an improved ductility with increasing TM-content and significant anisotropy of elastic constants and Young's modulus in the entire compositional range. The experimentally measured lattice parameters, nitrogen content in Cr-Mo-N, and elastic properties of Cr1-xTMxN1–0.5x corroborated our theoretical predictions. Our combined theoretical and experimental study provides new understanding and insights into these complex material systems.

A theoretical investigation of mixing thermodynamics, age-hardening potential and electronic structure of ternary M11–xM2xB2 alloys with AlB2 type structure

Transition metal diborides are ceramic materials with potential applications as hard protective thin films and electrical contact materials. We investigate the possibility to obtain age hardening through isostructural clustering, including spinodal decomposition, or ordering-induced precipitation in ternary diboride alloys. By means of first-principles mixing thermodynamics calculations, 45 ternary M11–xM2xB2 alloys comprising MiB2 (Mi = Mg, Al, Sc, Y, Ti, Zr, Hf, V, Nb, Ta) with AlB2 type structure are studied. In particular Al1–xTixB2 is found to be of interest for coherent isostructural decomposition with a strong driving force for phase separation, while having almost concentration independent a and c lattice parameters. The results are explained by revealing the nature of the electronic structure in these alloys, and in particular, the origin of the pseudogap at EF in TiB2, ZrB2, and HfB2.

Influence of stresses on structure and properties of Ti and Zr- based alloys from first-principles simulations

Computer simulations in the framework of the Density Functional Theory have become an established tool for computer simulations of materials properties. In most cases, however, information is obtained at ambient conditions, preventing design of materials for applications at extreme conditions. In this work we employ ab initio calculations to investigate the influence of stresses on structure and stability of Ti-Mo and Zr-Nb alloys, an important class of construction materials. Calculations reproduce known phase stability trends in these systems, and we resolve the controversy regarding the stability of body-centered cubic solid solution in Mo-rich Ti-Mo alloys against the isostructural decomposition. Calculated results are explained in terms of the electronic structure effects, as well as in terms of physically transparent thermodynamic arguments that relate phase stability to deviations of concentration dependence of atomic volume from the linear behavior.

Crystallography of phase transitions in metastable titanium aluminium nitride nanocomposites

The isostructural decomposition of the titanium aluminium nitride supersaturated solid solution crystallising in the face centred cubic (fcc) crystal structure into the titanium-rich fcc-(Ti,Al)N and almost titanium-free fcc-(Al,Ti)N and the transformation of metastable fcc-(Ti,Al)N into the wurtzitic aluminium nitride are discussed from the crystallographic point of view by taking the observed orientation relationships between the adjacent phases into account. It is shown that the isostructural decomposition of fcc-(Ti,Al)N into Ti-rich fcc-(Ti,Al)N and fcc-(Al,Ti)N and the transformation of the metastable fcc-(Ti,Al)N into the thermodynamically stable wurtzitic phase are concurrent processes, which are controlled not only by the thermodynamic stability of the respective compound and by the diffusivity of Al and Ti, but also by the local lattice strains. A part of the local lattice strains is regarded to result from the lattice misfit at the interfaces between the titanium aluminium nitrides having different [Al]/[Ti] concentration ratios and, in the case of the fcc/wurtzite-type interface, also having different crystal structures. The phase transition of the fcc-(Ti,Al)N to the wurtzitic one was predicted to be facilitated by stacking faults. The results of crystallographic considerations were verified experimentally by using in situ high-temperature synchrotron diffraction experiments that were performed on cathodic arc evaporated (Ti,Al)N thin films containing titanium and aluminium in different amounts.

Microstructure evolution during the isostructural decomposition of TiAlN—A combined in-situ small angle x-ray scattering and phase field study

This paper describes details of the spinodal decomposition and coarsening in metastable cubic Ti0.33Al0.67N and Ti0.50Al0.50N coatings during isothermal annealing, studied by in-situ small angle x-ray scattering, in combination with phase field simulations. We show that the isostructural decomposition occurs in two stages. During the initial stage, spinodal decomposition, of the Ti0.50Al0.50N alloy, the phase separation proceeds with a constant compositional wavelength of ∼2.8 nm of the AlN- and TiN-rich domains. The time for spinodal decomposition depends on annealing temperature as well as alloy composition. After the spinodal decomposition, the coherent cubic AlN- and TiN-rich domains coarsen. The coarsening rate is kinetically limited by diffusion, which allowed us to estimate the diffusivity and activation energy of the metals to 1.4 × 10−6 m2 s−1 and 3.14 eV at−1, respectively.

Surface directed spinodal decomposition at TiAlN/TiN interfaces

In contrast to the monolithic c-Ti1−xAlxN, the isostructural spinodal decomposition to c-AlN and c-TiN in c-Ti1−xAlxN/TiN multilayers has almost the same onset temperature for the compositions x = 0.50 and 0.66. Differential scanning calorimetry also shows that the decomposition initiates at a lower temperature compared to the monoliths with the same Al-content. Z-contrast scanning transmission electron microscopy imaging reveals a decomposed structure of the multilayers at temperatures where the monoliths remain in solid solution. In the multilayers, the decomposition is initiated at the internal interfaces. The formation of an AlN-rich layer followed by a TiN-rich area parallel to the interface in the decomposed Ti0.34Al0.66N/TiN coating, as observed in atom probe tomography, is consistent with surface directed spinodal decomposition. Phase field simulations predict this behavior both in terms of microstructure evolution and kinetics. Here, we note that surface directed spinodal decomposition is affected by the as-deposited elemental fluctuations, coherency stresses, and alloy composition.

Strong electron correlations stabilize paramagnetic cubic Cr1−xAlxN solid solutions

The stability of rock salt structure cubic Cr1−xAlxN solid solutions at high Al content and high temperature has made it one of the most important materials systems for protective coating applications. We show that the strong electron correlations in a material with dynamic magnetic disorder is the underlying reason for the observed stability against isostructural decomposition. This is done by using the first-principles disordered local moments molecular dynamics technique, which allows us to simultaneously consider electronic, magnetic, and vibrational degrees of freedom.

Pressure and temperature effects on the decomposition of arc evaporated Ti0.6Al0.4N coatings in continuous turning

The isostructural decomposition of arc evaporated Ti0.6Al0.4N coatings at elevated temperatures and high stresses occurring during metal cutting has been studied. Comparisons are made with short time (t=10min) anneals at temperatures typical for steel turning operations. The evolution of the spinodal wavelengths is studied by analytical transmission electron microscopy from samples originating from the rake face. Temperature and force measurements during turning allowed for separation of the effects of the temperature and stresses on wavelength evolution. The results show a peak temperature of around 900°C and a peak normal stress of around 2GPa during cutting. The overall wavelength is longer after cutting compared to the annealed sample at the same temperature. The results suggest that pressures generated during cutting promote coherent isostructural decomposition which is in line with theoretical studies but for considerably higher pressures.

Effects of pressure and vibration on the thermal decomposition of cubic Ti1-x Al x N, Ti1-x Zr x N, and Zr1-x Al x N coatings: a first-principles study

Thermodynamic properties as well as the miscibility gap (binodal) and spinodal decompositions of the cubic Ti1-x Al x N, Ti1-x Zr x N, and Zr1-x Al x N coating alloys have been computed using first-principles calculations. Herein, the cluster expansion method and especially the special quasirandom structure are employed to describe the disordered alloys. The effects of pressure and lattice vibration on the miscibility gaps and spinodal decompositions of the above alloys have been investigated by means of Helmholtz free energy with the vibrational contribution depicted with the Debye-Gruneisen model. It is found that the application of hydrostatic pressure promotes the isostructural decomposition of Ti1-x Al x N, Ti1-x Zr x N, and Zr1-x Al x N alloys, whereas the vibrational contribution decreases the consolute temperature of the phase separation. Our results indicate that the improved age-hardening behavior of cubic Ti1-x Al x N coatings with the addition of Zr arises from the enlarged composition range of binodal and spinodal curves at specified temperatures. Our results are in good agreement with the available experimental data and provide a useful insight into the investigation of age-hardening and characterization of Ti–Al–Zr–N-based coatings for high-temperature applications.

Effect of Hf on structure and age hardening of Ti–Al-N thin films

Protective coatings for high temperature applications, as present e.g. during cutting and milling operations, require excellent mechanical and thermal properties during work load. The Ti1−xAlxN system is industrially well acknowledged as it covers some of these requirements, and even exhibits increasing hardness with increasing temperature in its cubic modification, known as age hardening. The thermally activated diffusion at high temperatures however enables for the formation of wurtzite AlN, which causes a rapid reduction of mechanical properties in Ti1−xAlxN coatings. The present work investigates the possibility to increase the formation temperature of w-AlN due to Hf alloying up to 10at.% at the metal sublattice of Ti1−xAlxN films. Ab initio predictions on the phase stability and decomposition products of quaternary Ti1−x−yAlxHfyN alloys, as well as the ternary Ti1−xAlxN, Hf1−xAlxN and Ti1−zHfzN systems, facilitate the interpretation of the experimental findings. Vacuum annealing treatments from 600 to 1100°C indicate that the isostructural decomposition, which is responsible for age hardening, of the Ti1−x−yAlxHfyN films starts at lower temperatures than the ternary Ti1−xAlxN coating. However, the formation of a dual phase structure of c-Ti1−zHfzN (with z=y/(1−x)) and w-AlN is shifted to ~200°C higher temperatures, thus retaining a film hardness of ~40GPa up to ~1100°C, while the Hf free films reach the respective hardness maximum of ~38GPa already at ~900°C. Additional annealing experiments at 850 and 950°C for 20h indicate a substantial improvement of the oxidation resistance with increasing amount of Hf in Ti1−x−yAlxHfyN.

Thermally enhanced mechanical properties of arc evaporated Ti0.34Al0.66N/TiN multilayer coatings

Cubic metastable Ti0.34Al0.66N/TiN multilayer coatings of three different periods, 25+50, 12+25, and 6+12 nm, and monoliths of Ti0.34Al0.66N and TiN where grown by reactive arc evaporation. Differential scanning calorimetry reveals that the isostructural spinodal decomposition to AlN and TiN in the multilayers starts at a lower temperature compared to the monolithic TiAlN, while the subsequent transformation from c-AlN to h-AlN is delayed to higher temperatures. Mechanical testing by nanoindentation reveals that, despite the 60 vol % TiN, the as-deposited multilayers show similar or slightly higher hardness than the monolithic Ti0.34Al0.66N. In addition, the multilayers show a more pronounced age hardening compared to the monolith. The enhanced hardening phenomena and improved thermal stability of the multilayer coatings are discussed in terms of particle confinement and coherency stresses from the neighboring TiN-layers.

Pressure enhancement of the isostructural cubic decomposition in Ti1−xAlxN

The influence of pressure on the phase stabilities of Ti1−xAlxN solid solutions has been studied using first principles calculations. We find that the application of hydrostatic pressure enhances the tendency for isostructural decomposition, including spinodal decomposition. The effect originates in the gradual pressure stabilization of cubic AlN with respect to the wurtzite structure and an increased isostructural cubic mixing enthalpy with increased pressure. The influence is sufficiently strong in the composition-temperature interval corresponding to a shoulder of the spinodal line that it could impact the stability of the material at pressures achievable in the tool-work piece contact during cutting operations.

Electronic origin of the isostructural decomposition in cubic M 1− xAl xN (M=Ti, Cr, Sc, Hf): A first-principles study

We have used first-principles calculations to investigate the mixing enthalpies, lattice parameters and electronic density of states of the ternary nitride systems Ti 1− x Al x N, Cr 1− x Al x N, Sc 1− x Al x N and Hf 1− x Al x N 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 Ti 1− x Al x N and Hf 1− x Al x N 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 Cr 1− x Al x N and Sc 1− x Al x N. The lattice mismatch is large in Sc 1− x Al x N and Hf 1− x Al x N, giving a symmetric contribution to the mixing enthalpies in those systems.

First-principles study of the effect of nitrogen vacancies on the decomposition pattern in cubic Ti1−xAlxN1−y

The effect of nitrogen substoichiometry on the isostructural phase stabilities of the cubic Ti1−xAlxN1−y system has been investigated using first-principles calculations. The preferred isostructural decomposition pattern in these metastable solid solutions was predicted from the total energy calculations on a dense concentration grid. Close to the stoichiometric Ti1−xAlxN1 limit, N vacancies increase the tendency for phase separation as N sticks to Al while the vacancies prefers Ti neighbors. For nitrogen depleated conditions, N sticks to Ti forming TiNδ (0<δ<1) while Al tends to form nitrogen-free fcc-Al or Al–Ti alloys.

Comparison of thermodynamic properties of cubic Cr1−xAlxN and Ti1−xAlxN from first-principles calculations

In order to investigate the stability of the cubic phase of Cr1−xAlxN at high AlN content, first principles calculations of magnetic properties, lattice parameters, electronic structure, and mixing enthalpies of the system were performed. The mixing enthalpy was calculated on a fine concentration mesh to make possible the accurate determination of its second concentration derivative. The results are compared to calculations performed for the related compound Ti1−xAlxN and with experiments. The mixing enthalpy is discussed in the context of isostructural spinodal decomposition. It is shown that the magnetism is the key to understand the difference between the Cr- and Ti-containing systems. Cr1−xAlxN turns out to be more stable against spinodal decomposition than Ti1−xAlxN, especially for AlN-rich samples which are of interest in cutting tools applications.

The transformation sequences in the cubic → tetragonal decomposition

The decomposition of a generic supersaturated binary cubic solid solution into a mixture of cubic and tetragonal phases is investigated by phase field microelasticity modeling and simulations. It is shown that the decomposition in such a system is not necessarily developed by conventional nucleation and growth of the tetragonal phase. There are three temperature and composition ranges where the sequences of transient structures formed are different. The transformation pathways are predicted and the corresponding thermodynamic parameters are identified. In particular, the simulations reveal unusual transformation sequences occurring in the process of isostructural decomposition followed by cubic → tetragonal MT confined within one of the decomposed cubic phases. Mechanisms for the formation of the stress-accommodating multi-domain aggregates of the tetragonal phase and the checkerboard-like structures comprised of parallel rods of cubic and tetragonal phases are discussed.

β1→γ′ transformation in Cu-Mn-Al alloys after low temperature aging

Investigation of martensitic transformation in inhomogeneous solid solutions is a difficult problem. Convenient objects for study of this question are Cu-Mn-Al alloys, properties of which were described in cited references. Accordingly the eutectoid transformation temperature drops to 400 C by addition of 6.5 wt. % manganese. An isostructural decomposition of the {beta}{sub 1}-phase takes place within a relatively low temperature region. In this article the authors considered the influence of concentration inhomogeneities in the {beta}{sub 1}-solid solution on the martensitic transformation. The change of the martensitic points in the quenched Cu-Mn-Al alloys as a result of concentration variations is investigated.