Oxidation of Cr2AlC (0001): Insights from Ab Initio Calculations

We performed an investigation of the initial stage of oxidation onto a relevant Cr₂AlC (0001) surface by ab initio calculations. For the most energetically stable Al-terminated Cr₂AlC (0001) surface, a detailed model describing the oxygen-surface interaction is developed by exploring the adsorption energetics. Based on the evaluation of the energetics and the structural properties of the atomistic models generated, the results point to an initial stage of the Cr₂AlC (0001) surface oxidation with some similarities with those observed in the Al (111) layer. Our findings on the bonding mechanism of single O adsorption atoms of the surface may lead to further alloying strategies to enhance oxidation resistance in a wide range of refractory-metal-based MAX phases.     

Crystallization kinetics of amorphous Cr2AlC thin films

Amorphous Cr₂AlC thin films were produced by room temperature magnetron sputtering on NaCl substrates with subsequent dissolution of the NaCl. The crystallization kinetics of Cr₂AlC was investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Two exothermal reactions are observed during DSC up to 1200 °C. Comparing lattice parameters obtained from XRD and ab initio calculations it is suggested that the first reaction is associated with the formation of hexagonal (Cr_,Al)₂C_x, while after the second reaction Cr₂AlC is formed. The activation energy for the phase transformations are 426 and 762 kJ/mol, respectively.     

Corrosion behavior of select MAX phases in NaOH, HCl and H2SO4

In this paper we report on the electrochemical corrosion of select MAX phases, namely Ti₂AlC, (Ti,Nb)₂AlC, V₂AlC, V₂GeC, Cr₂AlC, Ti₂AlN, Ti₄AlN₃, Ti₃SiC₂ and Ti₃GeC₂ in 1 M NaOH, 1 M HCl and 1 M H₂SO₄ solutions. Polarization characteristics recorded in 1 M NaOH show that V₂AlC, V₂GeC and Cr₂AlC undergo active dissolution at potentials more positive than the corrosion potential, while Ti₂AlC, (Ti,Nb)₂AlC, Ti₃SiC₂ and Ti₃GeC₂ passivate. In the 1 M HCl solutions, Ti₂AlC, V₂AlC and V₂GeC actively dissolve; Ti₃SiC₂ and Ti₃GeC₂ passivate. Depending on potential, (Ti,Nb)₂AlC and Cr₂AlC showed trans-passive behavior. In 1 M H₂SO₄ solutions, Ti₂AlC, (Ti,Nb)₂AlC, Ti₃SiC₂ and Ti₃GeC₂ passivate, V₂AlC and V₂GeC show active dissolution, while Cr₂AlC exhibits trans-passive behavior. Ti₂AlN and Ti₄AlN₃ were passive in all solutions except in 1 M HCl, where Ti₂AlN showed trans-passive behavior. Given that the corrosion behavior of (Ti,Nb)₂AlC is unlike either Ti or Nb, the behavior of the former cannot be predicted from that of the latter.     

Experimental and computational study on the effect of yttrium on the phase stability of sputtered Cr–Al–Y–N hard coatings

The effect of Y incorporation into cubic Cr–Al–N (B1) was studied using ab initio calculations, X-ray diffraction and energy-dispersive X-ray analysis of sputtered quaternary nitride films. The data obtained indicate that the Y incorporation shifts the critical Al content, where the hexagonal (B4) structure is stable, to lower values. The calculated critical Al contents of x ≈ 0.75 for Cr_1?xAl_xN and x ≈ 0.625 for Cr_1?x?yAl_xY_yN with y = 0.125 are consistent with experimentally obtained values of x = 0.69 for Cr_1?xAl_xN and x = 0.68 and 0.61 for Cr_1?x?yAl_xY_yN with y = 0.02 and 0.06, respectively. This may be understood based on the electronic structure. Both Cr and Al can randomly be substituted by Y. The substitution of Cr by Y increases the phase stability due to depletion of non-bonding (anti-bonding) states, while the substitution of Al by Y decreases the phase stability mainly due to lattice strain.     

Microstructure and some properties of TiAl-Ti2AlC composites produced by reactive processing

The TiAl–Ti₂AlC composites with and without impurities, Ni, Cl and P, were prepared by combustion reaction from the elemental powders and cast after arc melting. The resulting composites had about 18 vol% Ti₂AlC in the lamellar matrix consisting of γ-TiAl and Ti₃Al (α₂). In the homogenized specimens, the α₂ phase decomposed to γ-TiAl and Ti₂AlC. The composite material had a high strength both at ambient and elevated (1173 K) temperatures; about 800 and 400 MPa, respectively, with an ambient temperature ductility of 0.7% at bending test. The fracture toughness test also proved that the homogenized composite has higher toughness than the as cast one. The toughness value reached to 17.8 MPa m^1/2. The zigzag cracks propagated in the homogenized composite and the reinforcement Ti₂AlC particles and the finely precipitated Ti₂AlC particles were main obstacles to the crack propagation. The composite with impurities showed a marginal improvement in the oxidation resistance over the composites without impurities.     

Formation of TiAl–Ti2AlC in situ composites by combustion synthesis

Preparation of TiAl–Ti₂AlC in situ composites with a broad range of composition was conducted by self-propagating high-temperature synthesis (SHS) with compressed samples from the mixture of elemental powders. When compared with SHS formation of monolithic TiAl, the addition of carbon particles to the Ti–Al powder mixture enhances the sustainability of the reaction. It was found that no prior heating was required for the samples prepared to produce the composites containing more than 20 mol% Ti₂AlC, in contrast to the need of preheating at 200 °C for single-phase TiAl formation. This is attributed to the fact that formation of Ti₂AlC is more exothermic than that of TiAl. As a result, the combustion temperature and combustion wave velocity increase with the content of Ti₂AlC formed in the TiAl–Ti₂AlC composite, and approach the values associated with formation of single-phase Ti₂AlC when considerable amounts of Ti₂AlC are yielded. The XRD analysis of the end products confirms formation of TiAl–Ti₂AlC in situ composites. Moreover, simultaneous formation of Ti₂AlC promotes the phase evolution of the aluminide compounds. That is, the secondary aluminide phase, Ti₃Al, was no longer detected in the TiAl–matrix composites containing Ti₂AlC of 30 mol% or above.     

Stability and elastic properties of L12-(Al,Cu)3(Ti,Zr) phases: Ab initio calculations and experiments

The total energies and equilibrium cohesive properties of L1₂, DO₂₂ and DO₂₃ structures along Al₃Ti–Al₃Zr and Al₃X–Cu₃X (X = Ti, Zr) sections are calculated from first principles employing electronic density-functional theory (DFT), ultrasoft pseudopotentials and the generalized gradient approximation. Calculated heats of formation are consistent with a narrow field of stability of the L1₂ structure at 12.5 at.% Cu for ternary (Al,Cu)_0.75Zr_0.25 and (Al,Cu)_0.75Ti_0.25 intermetallics at low temperatures. Experimentally, samples homogenized at 1000 °C establish a more extensive stability field for the L1₂ phase in quaternary alloys with Cu concentrations ranging from 6.7 to 12.6 at.% Cu. Two L1₂ phases were observed in as-cast alloys with near equal amounts of Ti and Zr, as well as alloys homogenized at 1000 °C. Good agreement is obtained between calculated and measured values of lattice parameters and elastic moduli. These results demonstrate high accuracy of ab initio calculations for phase stability, lattice parameters and elastic constants in multicomponent trialumide intermetallics.     

Experimental and theoretical study of Ti20Zr20Hf20Nb20X20 (X = V or Cr) refractory high-entropy alloys

We investigated the microstructure and mechanical properties of Ti₂₀Zr₂₀Hf₂₀Nb₂₀X₂₀ (X = V or Cr) high-entropy alloys (HEA), produced by induction melting and casting in inert atmosphere. The structures of these alloys were studied via X-ray diffractometry and scanning electron microscopy. Results show that Ti₂₀Zr₂₀Hf₂₀Nb₂₀V₂₀ has mainly the body centered cubic (BCC) structure, whereas the BCC matrix of Ti₂₀Zr₂₀Hf₂₀Nb₂₀Cr₂₀ contains small amount of Cr₂Nb and Cr₂Hf intermetallic compounds. Ti₂₀Zr₂₀Hf₂₀Nb₂₀V₂₀ alloy shows the high strength and the homogeneous deformation under compression at room temperature. The strength and hardness of Ti₂₀Zr₂₀Hf₂₀Nb₂₀Cr₂₀ alloy are further enhanced by the Cr-containing Laves phases segregated during casting. The structural and mechanical properties remained almost unchanged after a short time (10 min) heat treatment at 573, 773, 973 and 1173 K indicating the resistance to working temperature peaks for these two alloys. Ab initio calculations predict ductile behavior for these and similar refractory HEAs. The theoretically calculated Young's modulus E is in good agreement with the experimental ones.     

Microstructure and spin polarization of quaternary Co2Cr1−xVxAl,Co2V1−xFexAl and Co2Cr1−xFexAl Heusler alloys

The microstructures, magnetic properties and spin polarization of quaternary Co₂Cr_1−xV_xAl, Co₂V_1−xFe_xAl and Co₂Cr_1−xFe_xAl alloys were investigated. Phase separation into A2 and B2/L2₁ structure occurs in Co₂CrAl and Co₂Cr_0.6Fe_0.4Al, whereas Co₂FeAl exhibits a single-phase B2 structure. The ordered L2₁ structure becomes more stable with increasing vanadium concentration (x ⩾ 0.35). The saturation magnetization measured at 5 K for Co₂Cr_1−xV_xAl alloy changes from 1.4 to 2.0 μ_B when x increases from 0.0 to 0.5 and then becomes 1.4 μ_B for x = 1.0. This behavior can be attributed to the variation in the local magnetic moment of Co atoms. The saturation magnetization of Co₂V_1−xFe_xAl and Co₂Cr_1−xFe_xAl alloys increases with increasing Fe concentration. The spin polarization decreases from 0.62 to 0.56 with increasing x for Co₂Cr_1−xFe_xAl alloy. Also, the spin polarization decreases with increasing x for Co₂Fe_1−xV_xAl and Co₂Cr_1−xV_xAl alloys. Possible reasons for the reduced spin polarization in these alloys are discussed.     

Structure,phase stability and elastic properties in the Ti1–xZrxN thin-film system: Experimental and computational studies

The composition-dependence of the structure and elastic properties of ternary Ti_1–xZr_xN alloys is systematically investigated by combining thin film growth and ab initio calculations. Single-phase Ti_1–xZr_xN thin films (0 ? x ? 1) with a rocksalt structure have been deposited using dc reactive magnetron sputtering at T_s = 300 °C in Ar/N₂ plasma discharges. The structure, stress state and phase stability upon thermal annealing were studied by X-ray diffraction (XRD), while the acoustic and elastic properties were measured using Brillouin light spectroscopy, picosecond ultrasonics and nanoindentation. First-principles pseudopotential calculations of the total energy, lattice constants, bulk modulus, and single-crystal elastic constants C_ij for several cubic ordered structures of Ti_1–xZr_xN alloys were also carried out. The positive values of the computed formation energies indicate that the homogeneous Ti_1–xZr_xN alloys can be only stabilized at high temperatures. However, the magnetron-sputtered thin films were found to retain their as-grown single-phase cubic structure during post-deposition annealing at 850 °C for 3 h. The calculated equilibrium lattice parameters are in good agreement with the stress-free lattice parameters a₀ determined experimentally from XRD using the sin²ψ method: they both exhibit a positive deviation from Vegard-like linear interpolation. The calculated bulk modulus, elastic constants and Poisson’s ratio gradually decrease from TiN to ZrN. These computed values were used to interpret the experimentally derived elastic constants and Young’s modulus as functions of composition.     

Studying the oxidation of Ti2AlC MAX phase in atmosphere: A review

The oxidation of Ti₂AlC MAX phase has been studied in this paper. MAX phases are a class of materials with nano-layered structure. These materials have been formed from a transition metal (M), an element from the IIIA or IVA groups (A) and carbon or nitrogen (X). Ti₂AlC MAX phase is a ternary carbide with layered structure and hexagonal crystalline lattice. Physical and chemical properties are the most significant characteristics of the Ti₂AlC which have introduced this material as the most practical MAX phase known so far. This material is currently considered as the most used MAX phase at elevated temperatures, the application of which requires correct recognition of the oxidation mechanism. Many of researchers investigated on the oxidation of Ti₂AlC. Therefore, it has been tried in this paper to introduce this MAX phase and also report all the investigations of its oxidation characteristics at elevated temperatures.     

Processing and characterization of porous Ti2AlC with controlled porosity and pore size

In this work, we demonstrate a simple and inexpensive way to fabricate porous Ti₂AlC, one of the best studied materials from the MAX phase family, with controlled porosity and pore size. This was achieved by using NaCl as the pore former, which was dissolved after cold pressing but before pressureless sintering at 1400 °C. Porous Ti₂AlC with samples a volume fraction of porosity ranging from ～10 to ～71 vol.% and different pore size ranges, i.e. 42–83, 77–276 and 167–545 μm, were successfully fabricated. Fabricated samples were systematically characterized to determine their phase composition, morphology and porosity. Room temperature elastic moduli, compressive strength and thermal conductivity were determined as a function of porosity and/or pore size. For comparison, several samples pressureless-sintered without NaCl pore former, or fabricated by spark plasma sintering, were also characterized. The effects of porosity and/or pore size on the room temperature elastic moduli, compressive strength and thermal conductivity of porous Ti₂AlC are reported and discussed in this work. It follows that porosity can be a useful microstructural parameter to tune mechanical and thermal properties of Ti₂AlC.     

Structure,elastic properties and phase stability of Cr1–xAlxN

The effect of composition and metal sublattice population on the phase stability, structure and elastic properties of cubic (c), hexagonal (h) and orthorhombic spin-polarized Cr_1–xAl_xN was studied using ab initio calculations. Excellent correlation between ab initio and experimentally obtained lattice parameters and elastic constants was obtained. The energy of formation suggests that the cubic phase can be stabilized for x in the range 0.48–0.75, depending on the metal sublattice population. The broad range of x, which is also observed in experiments, can be understood by considering the Al distribution induced changes in the configurational contribution to the total energy.     

Effect of lattice defects on thermal conductivity of Ti-doped,Y2O3-stabilized ZrO2

The evolution of lattice structure and thermal conductivity has been studied systematically for a range of Ti-doped, Y₂O₃-stabilized ZrO₂ (YSZ) solid solutions. The mechanism of reducing the thermal conductivity by Ti doping has been determined. Ti⁴⁺ mainly substitutes for Zr⁴⁺ below a critical composition factor (x ? 0.08), above which the interstitial Ti⁴⁺ need to be considered separately. The effect of lattice defects caused by mass and radius differences between Ti⁴⁺ and Zr⁴⁺ ions on the phonon scattering coefficient was discussed quantitatively. And the reduction of oxygen vacancy by interstitial Ti⁴⁺ ions which increases the thermal conductivity at high Ti doping content was also determined. Concerning the integrated phase stability and thermo-mechanical properties, Ti-doped YSZ is believed to be a promising candidate for thermal barrier coatings at higher temperature.     

Epitaxial Ti2AlN(0 0 0 1) thin film deposition by dual-target reactive magnetron sputtering

Ultrahigh-vacuum dual-target reactive magnetron sputtering, in a mixed Ar/N₂ discharge was used to deposit epitaxial single-crystal MAX phase Ti₂AlN(0 0 0 1) thin films, without seed layers, onto Al₂O₃(0 0 0 1) substrates kept at 1050 °C. By varying the N₂ partial pressure a narrow process window was identified for the growth of single-crystal Ti₂AlN. The film microstructure was characterized by a combination of X-ray diffraction, spherical aberration (C_s) corrected transmission electron microscopy (TEM), high-resolution image simulation and high-resolution scanning TEM. Nitrogen-depleted deposition conditions resulted in the concurrent formation of N-free Ti–Al intermetallics at the film/substrate interface and a steady-state growth of Ti₂AlN together with N-free intermetallic phases. At higher N₂ partial pressures the growth assumes a columnar epitaxial nature. 1 Å resolution of the lattice enabling location of all elements in the Ti₂AlN unit cell is demonstrated.     

HiPIMS复合热处理制备高纯Ti2AlC MAX相涂层

Ti₂AlC MAX 相涂层是一类兼具金属和陶瓷特性的具有密排六方结构的高性能陶瓷涂层,在电接触、高温防护、宽温域摩擦等领域具有广阔的应用前景。然而 MAX 相涂层的成相成分窗口窄,性能受杂质相影响大,实现高纯、致密 Ti₂AlC MAX 相涂层的制备目前仍存在挑战。考虑沉积气压与溅射等离子体能量密切相关,采用高功率脉冲复合直流磁控溅射技术在钛合金基体上制备了 TiAl / Ti-Al-C 涂层,经后续热处理退火得到高纯 Ti₂AlC MAX 相涂层,重点研究不同沉积气压对涂层退火前后的成分、微观结构以及力学性能的影响和作用机制。结果表明,随着气压不断增大,沉积态涂层厚度先增加后减少。其中低沉积气压下沉积态涂层退火后,结构中除了 Ti₂AlC MAX 相外,还含有一定量杂质相;而在高气压下沉积态涂层退火后几乎全部转变为 Ti₂AlC MAX 相,呈现高纯、表面光滑致密的 MAX 相涂层特征。相较于沉积态涂层,退火后的涂层硬度变化不大,但由于生成了 Ti₂AlC MAX 相,涂层弹性模量有所提高。     

High-temperature oxidation and hot corrosion of Cr2AlC

High-temperature oxidation and hot corrosion behaviors of Cr₂AlC were investigated at 800–1300 °C in air. Thermogravimetric–differential scanning calorimetric test revealed that the starting oxidation temperature for Cr₂AlC is about 800 °C, which is 400 °C higher than other ternary transition metal aluminum carbides. Thermogravimetric analyses demonstrated that Cr₂AlC displayed excellent high-temperature oxidation resistance with parabolic rate constants of 1.08 × 10⁻¹² and 2.96 × 10⁻⁹ kg² m⁻⁴ s⁻¹ at 800 and 1300 °C, respectively. Moreover, Cr₂AlC exhibited exceptionally good hot corrosion resistance against molten Na₂SO₄ salt. The mechanism of the excellent high-temperature corrosion resistance for Cr₂AlC can be attributed to the formation of a protective Al₂O₃-rich scale during both the high-temperature oxidation and hot corrosion processes.     

Prediction of high-temperature point defect formation in TiO2 from combined ab initio and thermodynamic calculations

A computational approach that integrates ab initio electronic structure and thermodynamic calculations is used to determine point defect stability in rutile TiO₂ over a range of temperatures, oxygen partial pressures and stoichiometries. Both donors (titanium interstitials and oxygen vacancies) and acceptors (titanium vacancies) are predicted to have shallow defect transition levels in the electronic-structure calculations. The resulting defect formation energies for all possible charge states are then used in thermodynamic calculations to predict the influence of temperature and oxygen partial pressure on the relative stabilities of the point defects. Their ordering is found to be the same as temperature increases and oxygen partial pressure decreases: titanium vacancy → oxygen vacancy → titanium interstitial. The charges on these defects, however, are quite sensitive to the Fermi level. Finally, the combined formation energies of point defect complexes, including Schottky, Frenkel and anti-Frenkel defects, are predicted to limit the further formation of point defects.     

Microstructural evolution during high-temperature oxidation of Ti2AlC ceramics

Microstructural development during high-temperature oxidation of Ti₂AlC below 1300 °C involves gradual formation of an outer discontinuous TiO₂ layer and an inner dense and continuous α-Al₂O₃ layer. After heating at 1400 °C, an outer layer of mixed TiO₂ and Al₂TiO₅ phases and a cracked α-Al₂O₃ inner layer were formed. After heating to 1200 °C and cooling to room temperature, two types of planar defect were identified in surface TiO₂ grains: twins with (2 0 0) twin planes, and stacking faults bounded by partial dislocations. Formation of planar defects released the thermal stresses that had generated in TiO₂ grains due to thermal expansion mismatch of the phases (TiO₂, α-Al₂O₃ and Al₂TiO₅) in the oxide scale. After heating to 1400 °C and cooling to room temperature, crack propagation in TiO₂ grains resulted from the thermal expansion mismatch of the phases in the oxide scale, the high anisotropy of thermal expansion in Al₂TiO₅ and the volume changes associated with the reactions during Ti₂AlC oxidation. An atomistic oxidation mechanism is proposed, in which the growth of oxide scale is caused by inward diffusion of O^2? and outward diffusion of Al³⁺ and Ti⁴⁺. The weakly bound Al leaves the Al atom plane in the layered structure of Ti₂AlC, and diffuses outward to form a protective inner α-Al₂O₃ layer between 1100 and 1300 °C. However, the α-Al₂O₃ layer becomes cracked at 1400 °C, providing channels for rapid ingress of oxygen to the body, leading to severe oxidation.     

Integrated design of Nb-based superalloys: Ab initio calculations,computational thermodynamics and kinetics,and experimental results

An optimal integration of modern computational tools and efficient experimentation is presented for the accelerated design of Nb-based superalloys. Integrated within a systems engineering framework, we have used ab initio methods along with alloy theory tools to predict phase stability of solid solutions and intermetallics to accelerate assessment of thermodynamic and kinetic databases enabling comprehensive predictive design of multicomponent multiphase microstructures as dynamic systems. Such an approach is also applicable for the accelerated design and development of other high performance materials. Based on established principles underlying Ni-based superalloys, the central microstructural concept is a precipitation strengthened system in which coherent cubic aluminide phase(s) provide both creep strengthening and a source of Al for Al₂O₃ passivation enabled by a Nb-based alloy matrix with required ductile-to-brittle transition temperature, atomic transport kinetics and oxygen solubility behaviors. Ultrasoft and PAW pseudopotentials, as implemented in VASP, are used to calculate total energy, density of states and bonding charge densities of aluminides with B2 and L2₁ structures relevant to this research. Characterization of prototype alloys by transmission and analytical electron microscopy demonstrates the precipitation of B2 or L2₁ aluminide in a (Nb) matrix. Employing Thermo-Calc and DICTRA software systems, thermodynamic and kinetic databases are developed for substitutional alloying elements and interstitial oxygen to enhance the diffusivity ratio of Al to O for promotion of Al₂O₃ passivation. However, the oxidation study of a Nb–Hf–Al alloy, with enhanced solubility of Al in (Nb) than in binary Nb–Al alloys, at 1300 °C shows the presence of a mixed oxide layer of NbAlO₄ and HfO₂ exhibiting parabolic growth.