B2 Solid Solution Research Articles

Self-diffusion of transition metals in (Ti,W,Cr)B2 solid solutions

The self-diffusivities of Ti and Cr were determined in (Ti x W y Cr z )B2 ceramics which are a model system for the development of in-situ reinforced boride ceramics by tailored precipitate formation. Homogeneous solid solutions of composition (Ti0.3W0.5Cr0.2)B2, (Ti0.5W0.3Cr0.2)B2, and (Ti0.4W0.5Cr0.1)B2 were studied in the temperature interval between 1100 and 1500°C, using ion implanted stable 49Ti and 54Cr isotopes and secondary ion mass spectrometry (SIMS). The diffusivities of each element obey an Arrhenius law. The Cr diffusivities of all three compounds can be fitted to a unique Arrhenius line with an activation enthalpy of 3.5 eV and a low pre-exponential factor of 2×10−7 m2/s. The Ti diffusivities are smaller by 1–2 orders of magnitude than the Cr diffusivities, showing, however, higher activation enthalpies of 3.9 eV and approximately the same pre-exponential factors. The consequences of the results for the formation kinetics of precipitates in supersaturated (Ti x W y Cr z )B2 solid solutions are discussed.

Kinetics of precipitate formation in (TixWyCrz)B2 solid solutions: influence of Cr concentration and Co impurities

Abstract Supersaturated (Ti0.5−xW0.5Crx)B2 (x = 0, 0.1, 0,2) solid solutions were produced by reaction sintering of transition metal diboride powders at a pressure of 60MPa at 1800°C. The formation kinetics of W2B5 precipitates were investigated at lower temperatures by X-ray diffractometry (XRD) and secondary electron microscopy (SEM). The precipitation rate constants as a function of reciprocal temperature show non-monotonic behaviour in the range between 1450 and 1700°C with a maximum at about 1650°C. At low temperatures, Arrhenius behaviour is approximately valid, where the precipitation kinetics are dominated by the diffusion of the transition metals. In contrast, at temperatures higher than 1650°C, the precipitation rates decrease rapidly, since supersaturation, and consequently nucleation, is strongly reduced. The rate constants of precipitation are identical within error limits for Cr concentrations of x=0.1 and x = 0.2, however, they are significantly lower for x=0. This means that the addition of small amounts of Cr accelerates the precipitate formation, while further addition has no further influence. Samples resulting from powders which were high-energy milled with WC/Co hard balls before reaction sintering show an increased density of precipitated crystallites but no increased rate constants of precipitation. This result can be explained under the assumption that for these samples a significant amount of additional pre-existing nuclei was present before precipitation starts, which were formed by heterogeneous nucleation at Co impurities, introduced by the milling process.

Precipitation kinetics of W2B5 in (Ti0.4W0.5Cr0.1)B2 solid solutions

The isothermal precipitation kinetics of W2B5 secondary phase from supersaturated polycrystalline (Ti0.4W0.5Cr0.1)B2 solid solutions were investigated with X-ray diffractometry and scanning electron microscopy in the temperature range between 1500 and 1700°C. The precipitate formation is described by a modified Johnson–Mehl–Avrami–Kolmogorov (JMAK) model, where W2B5 particles nucleate preferentially at grain boundaries and subsequently grow into the volume by a two-dimensional process controlled by volume diffusion of the transition metals. Numerical calculations are used to describe quantitatively the time dependence of the precipitated fraction and to determine a differential JMAK exponent n diff which gives information on the nucleation and growth modes. n diff decreases during the precipitation process from 2 to about 0.8 for all temperatures investigated. The first limit corresponds to the classical JMAK model (two-dimensional diffusional growth and constant nucleation rate) and the decrease in n diff is the consequence of an impingement of the nucleating and growing particles in the late stages of the process. Nucleation and growth rates are determined as functions of reciprocal temperature, where the first quantity shows a non-monotonic behaviour with a maximum at about 1650°C and the second quantity exhibits an Arrhenius behaviour with an activation enthalpy of 3.6 eV. From this it can be concluded that the overall precipitate formation is dominated by the kinetics of atomic motion at low temperatures and by the thermodynamics of nucleation at high temperatures.

Solidification Processing and Fracture Behavior of RuAl-Based Alloys

ABSTRACTAlloys of RuAl-Ru were processed using various solidification methods, and the fracture behavior was examined. The fracture toughness values for RuAl-hcp(Ru, Mo) and RuAl-hcp(Ru, Cr) alloys ranged from 23 to 38 MPa√m, while the volume fraction of RuAl ranged from 22 to 56 percent. Increasing the volume fraction of RuAl resulted in a decrease in fracture toughness. The hcp solid solution was shown to be the more ductile phase with a fracture toughness approaching 68 MPa?m, while the B2 solid solution (RuAl) was found to have a fracture toughness less than 13 MPa√m. An alloy of Ru-7Al-38Cr (at.%) that consisted of a hcp matrix with RuAl precipitates had the highest room temperature toughness and the greatest hardness.

Fabrication and evaluation of Zr0.5Ti0.5B2 solid solution

Fabrication and evaluation of Zr0.5Ti0.5B2 solid solution

Formation of periodic microstructures involving the L1 2 phase in eutectic Al–Ti–Cr alloys

The temperature–concentration parameters defining the existence of the eutectic transformation L⇄L1 2+β at the Al-rich corner of the Al–Ti–Cr phase diagram have been studied. The temperature of this transformation decreases from 1275 to 1250 °C with decreasing titanium content in the alloy, and its temperature interval is at most 10 °C. The univariant eutectic transformation line L⇄L1 2+β was constructed, whose coordinates on the concentration triangle are defined by the equation (in at.%): C Al=95.48–3.45 C Ti + 0.068 ( C Ti) 2. As a result of this transformation, periodic microstructures consisting of lamellae and fibres of the cubic L1 2 and β phases are produced. Transformations in the solid state which occur during cooling lead to the precipitation of intermetallic compounds from the β-solid solution: TiAlCr (structural type C14) or AlCr 2 (structural type C11 b). The position of the boundary at which the change of precipitating phases occurs was determined. The analysis of mechanical properties shows that the transition from single-phase L1 2 aloys to eutectic microstructures is accompanied by enhancement of both strength and plasticity, while retaining a high elastic modulus.

An assessment of Ir–Pt–Al alloys for high-temperature materials

To design high-temperature materials with suitable ductility and high strength at high temperatures, Ir–Pt–Al alloys were investigated. It is expected that precipitation hardening by the B2 phase and solid-solution hardening of Ir by Pt and Al occur in the Ir–Pt–Al alloys. The phase constitution and microstructure were investigated by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and electron probe microscopy analysis (EPMA). Compression creep tests were then carried out with some of these alloys at 1773 K under 30 MPa. The creep mechanism is discussed in terms of the microstructure and phase constituents. Finally, the potential of Ir–Pt–Al alloys as high-temperature materials is discussed.

Thermophysical Properties of 75Ni–15Mo–10Re Alloy

The true heat capacity, thermal expansion, thermal conductivity, electrical resistance, and density of NMR-75 alloy (75 wt % Ni, 15 wt % Mo, and 10 wt % Re) are investigated in a temperature range of 300–1300 K. The enthalpy, mean coefficient of thermal expansion, thermal diffusivity, and Lorentz number are calculated from the obtained experimental data. The measurement results show that a reversible structural transformation occurs in the alloy in a temperature range of 750–960 K. In accordance with the phase diagram, the ternary system of the alloy consists of an a-solid molybdenum–nickel solution, which is in equilibrium with a β-solid rhenium–nickel solution and a Ni4Mo intermetallic compound. On heating the alloy in a temperature range of 750–960 K, the intermetallic compound transforms into the α-solid molybdenum–nickel solution with the absorption of heat, while the ordered structure transforms into a disordered one. The thermal effect Q = 6 kJ/kg and the activation energy of alloy disordering E = 2.2 eV are estimated. The transformation proper is regarded as a second-order transition.

On mutual transformations of icosahedral phase and β-solid solution with participation of ordered ω-like displacements in quenched alloys of Al 61Cu 26Fe 13

In the rapidly-quenched alloys of Al 61Cu 26Fe 13 characteristic orientation relationships were found between the lattice of the icosahedral phase (ι-phase) and that of the β(CsCl)-solid solution (where β-solid solution contains fine regions of ω-like displacements): [111] β || A2, [110] β || A5. It was established that at the ι-phase (dendrite)/β-solid solution (interdendritic layers) boundary areas there was a state formed with a modulated structure of the ι-phase. It is assumed that the fine regions of ordered ω-like displacements are involved in the continuous transformations (direct and reverse) ι-phase→modulated ι-structure→heterogeneous β-solid solution, going on in the boundary areas due to the strained state of quenched alloys.

Phase Constitution and Oxidation Resistance of B2 (Ir, Co)Al

ABSTRACTB2 iridium aluminide (IrAl) is hopeful for use as an ultrahigh temperature oxidation resistant coating above 1600K. In this study, the effect of Co substitution for Ir on phase constitution, hardness and oxidation behavior was studied for IrAl alloys. Alloys of (Ir, Co)-50mol%Al with various Co contents were fabricated by Ar-arc melting followed by hot-forging at 1773K. Oxidation behavior was evaluated using thermogravimetry (TG) in Ar-67%O2 up to 1823K. XRD and SEM were also carried out for alloy characterization. It was found that a continuous B2 solid solution (Ir,Co)Al is formed between IrAl and CoAl. Depending on the Co concentration, the oxidation products identified after heating to 1873K in Ar-67%O2 were Ir, IrO2 and A2O3 and/or Co2AlO4. Thin and continuous Al2O3 layers were observed after isothermal oxidation at 1673K when Co content is more than 20mol%Co. In this case, the weight change by isothermal oxidation at 1673K becomes higher with decreasing Co content. The (Ir,Co)Al alloys containing 20–40mol%Co exhibit higher oxidation resistance than CoAl and IrAl, and thus oxidation resistance of CoAl is improved by Ir addition.

Structure parameters and degradation of the critical current of the alloy NT-50 after cryogenic drawing in an ultrasonic field

The parameters of the defect and phase structure of the alloy NT-50 are investigated after low-temperature (77 K) deformation by drawing in an ultrasonic field, and the influence of these parameters on the amount of degradation of the critical current under tensile loading at 4.2 K is studied. It is found that cryogenic ultrasonic deformation leads to intensified decomposition of the β-solid solution with precipitation of Ti-rich phases according to the kinetics of the spontaneous martensitic transformation and results in a reduction of the internal stress level in the alloys. The structural features found promote stabilization of the β-solid solution during a subsequent deep cooling, as is manifested in a lowering of the degree of completion of the low-temperature deformation-induced martensitic transformation, which in turn raises the threshold for degradation of the critical current and reduces the degree of degradation in a wide range of external mechanical loads.

Hydrogenation characteristics and observation of disintegration conditions of Ti–Mo and Ni-coated Ti–Mo thin films

The hydrogenation characteristics of the sputtered Ti–Mo (α–β alloy) films (∼900 nm) with and without Ni coating were investigated using high-energy non-Rutherford backscattering (non-RBS) and elastic recoil detection techniques. It was found that the Ti–Mo alloy had a good hydrogen absorption capability compared with pure Ti, and showed a high resistance to C contamination as well. It was also demonstrated that with a top layer of thin Ni film on the Ti–Mo, an improvement in hydrogenation capability was achieved. The non-RBS spectra revealed that the Ni layer reduced the surface contamination of C and O impurities effectively and, therefore, promoted hydrogen absorption activation. On the other hand, there was little change in their maximum hydrogen storage capability with increasing Mo concentration (Mo <20 at. %), and the phases in the samples hydrogenated at 473 K were composed of a δ hydride and a β-solid solution. Once the Mo content approached 20 at. %, the δ phase disappeared and only the β phase existed in the film. The durability of the sputtered films on the Mo substrate against the hydrogen absorption/desorption was also measured with the hydrogen absorption–desorption cycles method, and a significant improvement was obtained.

A study of transformations of β-solid solution in shape-memory alloys of the Cu-Zn-Al system

1. The β → α transformation in high-temperature decomposition of alloys of the Cu - Zn - Al system has features of diffusion and martensitic transformations. The shapes of the crystals, their merging, and the change in the chemical composition characterize the β → α transformation as a diffusion one. The spatial distribution of rods of the α-phase within the original grain and the regular structure in particles of the α-phase indicate the shear nature of the β → α transformation.

The Cu 2Se–HgSe–In 2Se 3 system at 670 K

The phase diagram of the quasi-ternary Cu 2Se–HgSe–In 2Se 3 system at 670 K was studied by X-ray diffraction and metallography. We defined regions of solid solubility of the components and the ternary phases. The existence of a new quaternary phase with the composition ∼Cu 1.4HgIn 16.6Se 26.6 has been discovered. A single crystal from the β-solid solution region of HgSe has been grown.

放電プラズマ焼結によるＴｉ‐Ｗ‐Ｃｒ‐Ｂ混合粉末の反応合成

The reactive sintered compacts of Ti-W-Cr-B mixed powders were manufactured by a pulse electric current technique. Identification and characterization of the resulting boride phase were done using EPMA, XRD and other methods.The density of the sintered compacts rose rapidly with sintering temperature up to 1773 K, at which temperature the relative density was 94%. Above this temperature, the density rose only slightly with increasing sintering temperature. The borides of Ti and W were synthesized from mixed metal powders by this method. The type of boride formed and its composition depended on sintering temperature. Compacts sintered at lower temperatures consisted of WB2 and TiB2 phases, but at the highest sintering temperature, 2173 K, the main phase was (Ti, W, Cr)B2 solid solution, in which W and Cr were dissolved in TiB2. There was also a very small amount of β-(W, Ti, Cr)B phase. By annealing compact sintered at high temperature, the (Ti, W, Cr)B2 solid solution phase decomposed and the amount decreased.

Effect of high-temperature treatment on the physical and mechanical properties of titanium alloys

1. It has been established that the curves describing the dependence of the electric conductivity and the mechanical properties on the temperature have extrema. 2. Thermodynamic calculations show that a binodal decomposition of the β-solid solution can occur in titanium alloys with β-isomorphic elements, which intensifies the effect of high-temperature hardening on the mechanical and physical properties. 3. Measurement of the electric conductivity of some titanium compositions has shown that this method can be used successfully for studying phase transformations.

Peculiarities of Thermoelastic Martensitic Transformation in the Cu-Zn-Al and Cu-Zn-Al-V after Decomposition of the β-Solid Solution

Dilatometric, metallographic, and electron-microscopic methods were used to study martensitic transformations in Cu-Zn-Al alloys that were subjected to a heat treatment to the decomposition of β solid solution at temperatures of α-phase precipitation (635-530°C) and bainite formation (300-125 °C). The partial decomposition and the accompanying redistribution of the components were shown to alter the type of martensitic transformation from nonthermoelastic to thermoelastic with a corresponding change in the martensite morphology from plate type to needle type (spear-shaped). A decrease in the temperature of isothermal anneal in the α+β range from 600 to 530°C substantially decrease the M. temperature (by 140 °C). The temperature hysteresis of the transformation changes from 0 to 70 K.

Damage of first wall materials of fusion reactor during plasma disruption

The calculations of running-off under the action of gravity of the melted surface layer of the first wall subjected to a high heat flux have shown that the effect (especially on the free surface of the melt) is considerable even for a single pulse heat event. Experiments have been performed on pulse laser irradiation (PLI) of VTi alloys and aluminium alloys. The PLI is followed by the destruction of β-solid solution and precipitation of oxides, nitrides and oxidenitrides of Ti. The phase and structure transformations and the formation of surface cracks in aluminium alloys have been observed.

Structural conversions in VT22 titanium alloy during aging

For two-phase high-strength alloys of the VT22 type, as for β-alloys of titanium, the presence of an incubation period for the formation of α-phase during aging is characteristic. The signs for this process are the following: enchanced etchability of the surface of thin sections; formation of zones which do not have an interface with the matrix but differ from it in etchability (zones of presegregation of α-phase); a change in the intensities of the β-phase lines on the x-ray diffraction patterns. Decomposition of the β-solid solution in titanium alloys of the VT22 type is a multistep process, including formation of segregates (onset of stratification of the β-solid solution), regions of the Guinier—Preston zone type, various intermediate states, coherent states of the formed α-phase, and finally the appearance of isolated α-phase, having an interface with the matrix β-solid solution.

ALSV investigation of the phase composition of electrolytic Cu + Sn alloys

Anodic linear sweep voltammograms (ALSVs) have been recorded for thin layers of Cu + Sn alloys electrochemically deposited on graphite from a pyrophosphate bath. Three characteristic peaks were found. The first peak, appearing in the ALSVs of all samples, could be ascribed both to pure Cu (in the deposit obtained at low cathodic polarization) and to the η-Cu 6Sn 5 phase, while one of the other two peaks appearing only in the samples obtained at low cathodic polarization should reflect the presence of the ϵ-Cu 3Sn phase. The other peak, appearing only in the samples obtained at high cathodic polarization, is likely to reflect the presence of the tin-rich β-solid solution. The phase composition, in terms of the content of different phases, was determined as a function of the thickness of the alloy as well as of the deposition potential.