Electronic structure and physical properties of hcp Ti3Al type alloys

According to the basic information of sequences of Ti and Al characteristic atoms in hcp Ti-Al system, the compositional variations of the electronic structure, atomic potential energies, atomic volumes, lattice constants and cohesive energies of the ordered hcp Ti3Al type alloys were calculated by the framework of systematic science of alloys（SSA）. The electronic structure of the hcp Ti3Al compound consisted of ψ4h^Ti and ψ0h^Al atoms is 0.75[Ar] （3dn）^0.573（3dc）^2.1685（4sc）^0.972（4sf）^0.3093＋0.25[Ne]（3sc）^1.32（3pc）^1.19（3sf）^0.49. The factors of controlling lattice stability are electronic structure, atomic energies and atomic concentration. The ψ4h^Ti atoms play a determinative role in forming D019 structure with a=0.287 2 nm, c=0.456 4 nm, atomic cohesive energy e=4.810 8 eV/atom and heat of formation △H=-0.332 8 eV/atom. These calculated values are in good agreement with experimental values （a=0.287 5 nm, c=0.46 0 nm, △H=-0.27, -0.29 eV/atom）. The calculated cohesive energy of the hcp Ti3Al compound is slightly bigger than that of the fcc Ti3Al.This is a good sign that makes it feasible to stabilized L 12 structure of the hcp Ti3Al compound by ternary element, The new element should have more dc-electrons than Ti-metal and occupy at the Ti-lattice points.     

Properties of plasma sprayed NiCrAlY ＋ ZrO2 coating on TiAl alloy surface

The NiCrAIY＋ZrO2 thermal barrier coating was prepared on the surface of TiAI alloy by plasma spraying technique. The microstructure and phase structure were analyzed using scanning electron microscopy （SEM） and X-ray diffractometry （XRD）. The high temperature oxidation resistance of the plasma sprayed samples at 850℃ was investigated. The results show that the bonding between thermal barrier coating and substrate is very good. Surface hardness of TiAI alloy is improved too, The microhardness of the coating surface is about HV 900 after the oxidation test at 850 ℃. The oxidation resistance of the samples is improved remarkably.     

The influence of alloying on the α2/(α2+γ)/γ phase boundaries in TiAl based systems

By employing both experimental and theoretical approaches, a comparative study of the α₂/(α₂+γ)/γ phase boundaries in some Ti–Al–X (X=Nb, Ta, V, Cr, Mn, Fe or Ga) systems has been carried out. The phase constitution of Ti–(33–51 at.%)Al–(1–3 at.%)X alloys with a composition step of 1 at.% Al was determined experimentally by using the X-ray diffraction method on bulk samples equilibrated at 1173 K. The α₂/(α₂+γ) and (α₂+γ)/γ phase boundaries were calculated using a model that describes the phase boundaries in terms of sublattice site occupancies of alloying species in the two ordered phases. The differences between the predicted and experimentally determined phase boundaries were found to be less than about 1 at.% in most cases. The predicted volume fractions of constituent phases were also compared with experimental measurements by a combination of metallography and transmission electron microscopy. Agreement between the two is good for 14 ternary and two quaternary alloys containing 46 at.% Al. The present work suggests that, in the order of increasing strength of stabilization at 1173 K, V, Nb and Ta stabilize the α₂ phase, whereas Cr, Mn, Fe and Ga stabilize the γ phase.     

Microstrucmre and mechanical properties of Al2O3/TiAl composite

Al2O3/TiAl composites were fabricated by PAXD （pressure-assisted exothermic dispersion） method. The effects of Nb205 content on the microstructure and mechanical properties of the composites were investigated. The results show that the ultimate phases of the composite consist of TiAl, Ti3Al, Al2O3 and a small amount of NbA13. SEM reveals that a submicron γ＋（α2/γ） dual phases structure can be presented after sintered at 1 200 ℃, Furthermore, with the increase of Nb205 content, the ratio of TiAl to Ti3Al phase decreases correspondingly, the grains of the corflposites are remarkably refined, and the produced Al2O3 particles are uniformly dispersed. When 6% Nb205 is added, the composite has the best comprehensive properties. It exhibits a Vickers hardness of 4.77 GPa and a bending strength of 642 MPa. Grain-refinement and dispersion-strengthening are the main strengthening mechanisms.     

Boundary trend of α1／（α1＋α2 ） phase region at lower Cu side in Al-Zn-Cu system

The range of miscibility gap above 300℃ at low Cu side in Al-Cu-Zn ternary system was obtained by EPMA of the designed alloys and diffusion-couples treated for equilibrium. The results about the boundary trend of the α1/(α1 α2) phase region was obtained. The α1/(α1 α2) boundary moves towards the lower Zn side with the in-crease of Cu content. The results are opposite to traditional phase diagrams obtained by experiments, but consistent with recent thermodynamic calculations.     

Microstructure and strength of TiAl/40Cr joint diffusion bonded with vanadium-copper filler metal

1　INTRODUCTIONInrecentyears,considerableinteresthasfocusedonTiAlintermetallicsbecauseofuniquepropertiessuchaslowdensity,goodstiffness,highelevatedtemperaturestrength,andexcellentoxidationresistance[1～4].TiAlintermetallicshasbeenconsideredasidealnewhight…     

双相TiAl合金α2/γ界面电子结构计算与增韧机制分析

基于余氏固体与分子经验电子理论(EET)和程氏改进的TFD理论提出了计算双相TiAl合金层片状结构α2/γ界面电子结构的计算模型与方法,计算了含常用合金元素的单相TiAl合金与双相TiAl合金中的异相界面的电子结构,利用界面电子结构给出的信息-界面结合因子ρ, △ρ,σ,以合金元素Mn为例初步分析讨论了双相TiAl合金层片状结构增加韧性的微观机制.     

Discontinuous coarsening of primary α2/γ lamellae at colony boundaries in γ-TiAl-based alloys

The discontinuous coarsening of primary α₂/γ lamellae at lamellar colony boundaries was studied by TEM and optical microscopy for the Ti–40Al, Ti–42Al and Ti–45Al (in at%) alloys which were isothermally aged at 1000°C for various time after being solution-treated in the α single-phase field, and then water quenched to room temperature. Discontinuous coarsening nuclei form at lamellar colony boundaries and grow into the neighboring colony. With increasing Al concentrations in the alloys, the degree of the discontinuous coarsening becomes greater. The size of discontinuous coarsening cells increases linearly with time in the first hour for the Ti–42Al alloy and the growth velocity is about 1.17×10⁻⁸ m s⁻¹. However, at larger times, the microstructure stabilizes and further discontinuous coarsening occurs at much slower rates.     

Directional solidification of TiAl–Re–Si alloys with aligned γ/α2 lamellar microstructures

The effect of Re additions on the solidification behavior of Ti–Al–Re and Ti–Al–Re–Si alloys was investigated to determine if such additions would be compatible with a seeding technique developed previously to align the γ/α₂ lamellar microstructure. Rhenium additions to TiAl were found to enlarge the primary β composition range. Rhenium also tends to segregate to the β-dendrite cores leading to the formation of the B2 phase. After the L+β→α peritectic reaction, Re tends to segregate to the interdendritic liquid. For the Ti–Al–Re alloys, the maximum Re content found in the lamellar microstructure was near 0.5 at% and the lamellar orientation in master alloys containing this amount of Re could successfully be grown from the seed material. However, such levels of Re had little effect on the measured yield stress either at room temperature or at 1000°C and tensile failure usually occurred by the coalescence of microcracks that nucleated from large silicide particles. Finally, it was found that the γ/α₂ lamellar microstructure of Ti–43.5Al–3Si–0.5Re can successfully be aligned by using an appropriately oriented seed even though β is the primary solidification phase indicating that the α-phase from both the L+β→α peritectic and the L→α+Ti₅Si₃ eutectic reactions must be continuous with that of the seed material.     

Thermodynamic analysis of β → α(α_2) phase transformation temperature in a Ti-Al-H alloy system

The T0 face equation of a Ti-Al-H alloy system was set up by the regular solution model,and the relationship between the β phase stabilizing parameter of hydrogen and the equilibrium phase compositions was attained.According to the T0 face equation and the thermodynamic parameters from literature,the effect of hydrogen on the β→α(α2) transformation temperature was evaluated.The calculated results were in a better consistence with the measured ones.     

Magnetic properties and structure of Ni80Fe20/Ni48Fe12Cr40 bilayer films deposited on SiO2/Si（100） by electron beam evaporation

Ni80Fe20/Ni48Fe12Cr40 bilayer films and Ni80Fe20 monolayer films were deposited at room temperature on SiO2/Si（100） substrates by electron beam evaporation. The influence of the thickness of the Ni48Fe12Cr40 underlayer on the structure, magnetization, and magnetoresistance of the Ni80Fe20/Ni48Fe12Cr40 bilayer film was investigated. The thickness of the Ni48Fe12Cr40 layer varied from about 1 nm to 18 nm while the Ni80Fe20 layer thickness was fixed at 45 nm. For the as-deposited bilayer films the introducing of the Ni48Fe12Cr40 underlayer promotes both the （111） texture and grain growth in the Ni80Fe20 layer. The Ni48Fe12Cr40 underlayer has no significant influence on the magnetic moment of the Ni80Fe20/Ni48Fe12Cr40 bilayer film. However, the coercivity of the bilayer film changes with the thickness of the Ni48Fe12Cr40 undedayer. The optimum thickness of the Ni48Fe12Cr40 underlayer for improving the anisotropic magnetoresistance effect of the Ni80Fe20/Ni48Fe12Cr40 bilayer film is about 5 nm. With a decrease in temperature from 300 K to 81 K, the anisotropic magnetoresistance ratio of the Ni80Fe20 （45 nm）/Ni48Fe12Cr40 （5 nm） bilayer film increases linearly from 2.1% to 4.8% compared with that of the Ni80Fe20 monolayer film from 1.7% to 4.0%.