Order-disorder transformations in the heusler alloy Cd2AgAu

A sequence of order-disorder phase transformations was studied in the alloy Cd-Ag-Au at or near the Heusler alloy stoichiometry Cd₂AgAu. Total energy calculations based on an LMTO-ASA Hamiltonian were performed for a series of bcc compounds, from which equilibrium formation enthalpies were computed. A set of effective cluster interactions were fit to these results and then used as input to ternary cluster variation method calculations. The predictions for the B2 to Heusler alloy transition agreed well with experiment, as did the low-temperature portion of an isoplethal section of the ternary phase diagram. Predictions of sublattice occupations indicated a preference of Ag for the Cd sublattice in the phase with B2 symmetry.     

Combined ab initio and experimental study of A2 + L21 coherent equilibria in the Fe–Al–X (X = Ti,Nb, V) systems

A combined theoretical and experimental study of ordering and phase separation in α-Fe alloys in the Fe–Al–X (X = Ti, Nb, V) systems is presented. The theoretical part is dedicated to the assessment of the BCC-based phase equilibrium diagram in the iron-rich zone of the ternary systems via a truncated cluster expansion, through the combination of Full-Potential-Linear augmented Plane Wave (FP-LAPW) electronic structure calculations and of Cluster Variation Method (CVM) thermodynamic calculations in the irregular tetrahedron approximation. The stability and the solid solubility of Al in the Fe₂X Laves phases are also included in the discussion of the ternary BCC ground state diagrams. The approach was employed in order to explore particular vertical sections of the ternary systems where a coherent two-phase microstructure can be generated with an optimal combination of volume fraction of Fe₂AlX (L2₁) Heusler type precipitates and Al content in the α-Fe (A2) matrix. The results indicate that in the Fe–Al–Ti and Fe–Al–V systems there are two kinds of phase separations of the BCC phase, (A2+ L2₁) and (B2 (FeAl structure) + L2₁). A tie-line separates both two-phase fields that shrinks and moves toward the Fe-X binary system while its direction remains almost parallel as the temperature increases. Selected experiments were performed on three alloys of the Fe–Al–Ti system belonging to vertical sections that contain this tie-line. The microstructure and composition of the matrix and precipitate phases were characterized by transmission electron microscopy (TEM) and Energy Dispersive Spectroscopy (EDS), the theoretical predictions were borne out.     

Ab Initio Calculated Thermodynamic Properties of Mo5SiB2 Phase and Nb5SiB2 Phase

Due to their attractive high-temperature properties, multiphase Mo-Si-B alloys in the Mo-rich Mo-Si-B ternary system have been identified for high-temperature applications. The ternary intermetallic T₂ (Mo₅SiB₂) phase is a central feature of the phase equilibria within this ternary system. Experimental stability analyses of the T₂ phase shows its broad homogeneous composition ranges that can yield a constitutional defect structure such as vacancies for Mo-rich compositions and antisite defects for Mo-lean compositions. Previous thermodynamic model did not conform to the defect structures as reported in experiments, and thus subsequently a new sublattice thermodynamic model for the T₂ phase is initiated in this study. To support the new sublattice thermodynamic model, ab initio calculations were implemented to compute formation energy data. The calculated formation energy data explain a source for broad compositional homogeneity range of T₂ structure.     

Atomic site location by channelling enhanced microanalysis (ALCHEMI) in γ′-strengthened Ni- and Pt-base alloys

The additions of alloying elements to Ni- and Pt-base alloys influence the microstructure and thereby the creep properties, whereas the mechanism is uncertain. Therefore atomic site location by channelling enhanced microanalysis (ALCHEMI) was used to determine the site partitioning of ternary and quaternary alloying elements in the L1₂-ordered γ′-phase. Two ternary Ni–Al alloys with Cr and Ti additions were investigated. The measured site partitioning showed that Cr and Ti atoms prefer the Al-sublattice sites. For a ternary Pt–Al–Cr alloy, it was found that Cr atoms occupy Al sites. The influence of Ni as a fourth alloying element in a Pt–Al–Cr–Ni alloy on the site partitioning was also investigated. The detected results give evidence that in the quaternary alloy Cr and Ni atoms prefer the Pt sublattice. First principles calculations were used to support the experimental data.     

First-Principles Investigation on Phase Stability,Elastic and Magnetic Properties of Boron Doping in Ni-Mn-Ti Alloy

The all-d-metal Ni-Mn-Ti Heusler alloy has giant elastocaloric effect and excellent mechanical properties, which is different from the conventional Ni-Mn-based Heusler alloys. In this work, the preferred site occupation, phase stability, martensitic transformation, magnetic properties, and electronic structure of the B-doped Ni₂Mn_1.5Ti_0.5 alloys are systematically investigated by the first-principles calculations. The results show that B atoms preferentially occupy the octahedral interstitial. The doped B atoms tend to exist in the (Ni₂Mn_1.5Ti_0.5)_1-xB_x (x = 0.03, 0.06, 0.09) alloy in the form of aggregation distribution, and the martensitic transformation temperature decreases with the increase in the B content. For octahedral interstitial doping, the toughness and plasticity of the (Ni₂Mn_1.5Ti_0.5)_1-xB_x alloys decrease, but the strength and rigidity are greatly enhanced. This is because a small part of the d-d hybridization in ternary Ni-Mn-Ti alloy is replaced by the p-d hybridization in Ni-Mn-Ti-B alloy.     

TEM study of structural and microstructural characteristics of a precipitate phase in Ni-rich Ni–Ti–Hf and Ni–Ti–Zr shape memory alloys

The precipitates formed after suitable thermal treatments in seven Ni-rich Ni–Ti–Hf and Ni–Ti–Zr high-temperature shape memory alloys have been investigated by conventional and high-resolution transmission electron microscopy. In both ternary systems, the precipitate coarsening kinetics become faster as the Ni and ternary element contents (Hf or Zr) of the bulk alloy are increased, in agreement with the precipitate composition measured by energy-dispersive X-ray microanalysis. The precipitate structure has been found to be the same in both Hf- and Zr-containing ternary alloys, and determined to be a superstructure of the B2 austenite phase, which arises from a recombination of the Hf/Zr and Ti atoms in their sublattice. Two different structural models for the precipitate phase were optimized using density functional theory methods. These calculations indicate that the energetics of the structure are not very sensitive to the atomic configuration of the Ti–Hf/Zr planes, thus significant configurational disorder due to entropic effects can be envisaged at high temperatures. The precipitates are fully coherent with the austenite B2 matrix; however, upon martensitic transformation, they lose some coherency with the B19′ matrix as a result of the transformation shear process in the surrounding matrix. The strain accommodation around the particles is much easier in the Ni–Ti–Zr-containing alloys than in the Ni–Ti–Hf system, which correlates well with the lower transformation strain and stiffness predicted for the Ni–Ti–Zr alloys. The B19′ martensite twinning modes observed in the studied Ni-rich ternary alloys are not changed by the new precipitated phase, being equivalent to those previously reported in Ni-poor ternary alloys.     

Thermodynamic optimization of the boron-cobalt-iron system

A thermodynamic optimization of the boron-cobalt-iron ternary system is performed based on thermodynamic models of the three constitutional binary systems and the experimental data on phase diagrams and thermodynamic properties of the ternary system. The liquid, fcc_A1, bcc_A2 and hcp_A3 solution phases are described by the substitutional solution model. The three intermediate line compounds, (Co,Fe)B, (Co,Fe)₂B and (Co,Fe)₃B, are described by the two sublattice model. A set of thermodynamic parameters are obtained. The calculated phase diagram and thermodynamic properties are in reasonable agreement with most of the experimental data.     

Solidification structures of Ti–Al–Cr alloys

Phase transformations in the ternary Ti–Al–Cr alloy system have been studied by combining preliminary phase equilibria calculations and microstructural studies of a number of model alloys. The results have contributed to a better understanding of phase equilibria in the Ti–Al–Cr alloy system above 1273 K. A liquid surface projection has been tentatively proposed. Micro-twins have been observed in the monolithic γ phase within a B2 matrix. This supports a previously proposed mechanism for the formation of such a structure in a B2 matrix. The results also suggest that there is no representative orientation relationship between γ and the Ti(Cr,Al)₂ Laves phase. The L1₂ τ phase can be in direct equilibrium with the liquid phase. The ω phase stability has also been studied. The stability of the ω phase is attributed to the electron density of the prior B2 phase. This leads to changes in the effective heat of formation of the ω structure, as concluded from total energy LMTO calculations.     

Structural analysis of a new precipitate phase in high-temperature TiNiPt shape memory alloys

Aging of the high-temperature shape memory alloy Ti₅₀Ni₃₀Pt₂₀ (at.%) results in precipitation of a previously unidentified phase, which plays a key role in achieving desirable shape memory properties. The precipitate phase has been analyzed with electron diffraction, high-resolution scanning transmission electron microscopy and three-dimensional atom probe tomography. The experimental observations show that the precipitates have unique crystallography due to their non-periodic character along one of the primary crystallographic directions. It will be shown that the structure can be explained in terms of crystal intergrowth of three variants of a monoclinic crystal. The monoclinic crystal structure is closely related to the high-temperature cubic B2 phase; the departure of the structure from the B2 phase can be attributed to ordering of Pt atoms on the Ni sublattice and relaxation of the atoms (shuffle displacements) from the B2 sites. The shuffle displacements and the overall structural refinement were deduced from ab initio calculations.     

L12型Co3Ti基金属间化合物有序化行为的热力学研究

采用亚晶格模型,辅助以第一性原理总能计算,研究了L12结构的Co3Ti基金属间化合物中元素的占位有序化行为。结果表明,Co3Ti化学计量比合金呈现完全有序化;对于xCo/xTi为3:1的Co72Ti24M4合金,第3合金组元M为Si或Ta时,M与Ti共同占据1a位置,M为Cu、Ni、Pd、Rh、V或W时,M与Co占据3c位置,而当M为Al、Cr、Ge、Mn、Sc或Y时,M在1a和3c位置的占位分数相同,这些元素的占位行为均不受温度影响;而当M为Fe、Hf、Mo、Nb、Ru和Zr的占位情况随温度发生变化。随原子核外层电子的增加,原子占位逐渐倾向于从1a亚晶格转向3c位置。亦预测了xCo/xTi偏离3:1的部分合金的占位分数随合金成分和温度的变化细节,预测结果与文献进行了比较,并澄清了文献上的分歧     

Equilibrium segregation at coherent (1 0 0) interfaces in ternary fcc systems analyzed using the cluster variation method

In order to study equilibrium segregation, composition profiles around coherent (1 0 0) interfaces in ternary fcc systems were calculated using the cluster variation method within the tetrahedron approximation. A comparison with a previous regular-solution (RS) based model was made for interfaces between two disordered phases. This RS model overestimates the maximum atomic fraction of the segregant at the interface when its bulk value is >0.1. The oscillatory decay in segregant concentration from its maximum at the interface to its bulk value is much less damped in the case of the RS model. Analysis of the damping behavior leads to a very good agreement with results from free energy expansion approximations. Finally, the CVM calculations were extended to interfaces between ordered ternary fcc phases or anti-phase boundaries. Results for the interface between the L1₂-Cu₃Au(Ag) phase and a disordered Ag-rich phase in equilibrium at 583 K, and for a non-conservative APB in the same alloy, point to a strong suppression of order near the interfaces.     

Electron energy losses by channelling-enhanced microanalysis in a γ′-strengthened Ni-base alloy with Ti additions

The addition of Ti to a γ′-strengthened Ni-base model alloy shows a strong preference of Ti for the γ′-phase. Ti strongly influences the diffusion behaviour of constituents of the Ni₃Al (γ′) phase, although the mechanism is uncertain. Electron energy loss channelling experiments were therefore performed to determine the electronic structure in the vicinity of the Ti atom. The experimental results showed an increase in the Ti L₃/L₂ ratio when probing the Ni sublattice planes compared to the Al sublattice planes, indicating a strong charge transfer from the Ti to the Ni atoms. This finding was supported by first-principles calculations, indicating an increase in electron density in the vicinity of the Ti atom and a strengthening of the chemical bonds.     

全d族Ni-Mn-Ti基Heusler合金制冷性能研究进展

总结了近年来国内外对Ni-Mn-Ti基全d族Heusler合金制冷性能的研究进展。介绍了该系列合金的晶体结构和原子占位,晶体结构为价电子数最少的Ti原子取代原主族元素的位置上形成L2₁或B2有序结构。改变处理工艺和掺杂元素对马氏体相变温度与居里温度有一定影响,可以借此手段将相变温度调节到室温附近以达到实际应用的目的。着重分析目前Ni-Mn-Ti基全d族Heusler合金的制冷手段及其原理,对未来该系列合金在制冷性能方面的发展进行了展望。     

Effect of Ti (Macro-) Alloying on the High-Temperature Oxidation Behavior of Ternary Mo–Si–B Alloys at 820–1,300 °C

The aim of the present investigation was to gain an initial understanding of the effect of (macro-) alloying with Ti on the oxidation behavior of Mo–Si–B alloys in the ternary phase region of Mo_ss–Mo₃Si–Mo₅SiB₂ at 820–1,300 °C. Motivated by recent studies and thermodynamic calculations, the alloy compositions Mo–9Si–8B–29Ti (at.%) and Mo–12.5Si–8.5B–27.5Ti (at.%) were selected and synthesized by arc-melting. Compared to the reference alloy Mo–9Si–8B, superior initial oxidation rates at 1,100–1,300 °C as well as a significant density reduction by nearly 18 % were observed. Due to enhanced initial evaporation of MoO₃ and mainly inward diffusion of oxygen, a borosilicate-rutile duplex scale with a continuous TiO₂ phase had formed. Detailed investigations of the oxidation mechanism by SEM, EDX, XRD and confocal micro-Raman spectroscopy indicated that Ti alloying is promising with regard to further improvement of the oxidation resistance as well as the strength-to-weight ratio of Mo–Si–B alloys.     

First principles study of site substitution of ternary elements in NiAl

Site substitution of ternary elements in ordered compounds influences the electronic structure and hence the properties of compounds at the continuous level. The electronic structure and binding energy of a number of NiAl-X alloy systems (X=Ti, V, Cr, Mn, Fe, Co, Zr, Nb, Mo, Hf, Ta, W, Si, Ga, or Ge) were calculated using the discrete variational cluster method based on the local density approximation of the density functional theory. The site preference of the ternary additions to NiAl was investigated by employing the Bragg–Williams model to analyse the calculated binding energy. The results show that all the considered ternary elements possess stronger preference to the Al sublattice sites than a Ni atom does. A new method of identifying sublattice substitution of ternary additions in NiAl was proposed by comparison of the binding energies per atom of the ternary and the binary clusters involving the fourth nearest neighbours. The analysis suggests that Fe and Co atoms occupy the Ni sublattice sites, whereas Si, Ga and Ti atoms occupy the Al sublattice sites. The remaining elements may substitute for both sublattices: Mn is most likely to go for the Ni sublattice; V, Cr, Zr, Nb, Mo, Hf, Ta, W and Ge have a larger preference for the Al sublattice, but Cr and W do not show significant preference to any sublattice. The densities of states involving alloying additions of Co, Si and Cr were further investigated to clarify the site preference of the alloying additions.     

Thermodynamic study of phase equilibria in the Ni-Si-B system

A thermodynamic analysis of the phase equilibria in the Ni-Si-B ternary system was conducted. A regular solution approximation based on a sublattice model was adopted to describe the Gibbs energies for the individual phases in the binary and ternary systems. A set of thermodynamic parameters for the individual phases was evaluated from literature data on phase boundaries and thermochemical properties. The optimized parameters reproduced the experimental data, for the most part, satisfactorily. However, in the calculated isothemal section at 850 °C, phase equilibria between the fcc phase and Ni₆Si₂B or Ni₃Si(β ₁) and Ni₆Si₂B were found instead of the experimentally observed equilibria between Ni₃Si(β ₁) and Ni₃B or Ni₅Si₂(γ) and Ni₃B. Further, in the primary crystal surface for the fcc phase, the calculated liquidus temperatures were higher than the reported values by approximately 80 °C. Therefore, it is considered that the fcc phase evaluated in the Ni-Si system by Lindhólm and Sundman is too stable.     

Thermodynamic study of phase equilibria in the Ni-Si-B system

A thermodynamic analysis of the phase equilibria in the Ni-Si-B ternary system was conducted. A regular solution approximation based on a sublattice model was adopted to describe the Gibbs energies for the individual phases in the binary and ternary systems. A set of thermodynamic parameters for the individual phases was evaluated from literature data on phase boundaries and thermochemical properties. The optimized parameters reproduced the experimental data, for the most part, satisfactorily. However, in the calculated isothemal section at 850 °C, phase equilibria between the fcc phase and Ni₆Si₂B or Ni₃Si(β ₁) and Ni₆Si₂B were found instead of the experimentally observed equilibria between Ni₃Si(β ₁) and Ni₃B or Ni₅Si₂(γ) and Ni₃B. Further, in the primary crystal surface for the fcc phase, the calculated liquidus temperatures were higher than the reported values by approximately 80 °C. Therefore, it is considered that the fcc phase evaluated in the Ni-Si system by Lindhólm and Sundman is too stable.     

Microstructure and room temperature mechanical properties of NiAl–Cr(Mo)–(Hf,Dy) hypoeutectic alloy prepared by injection casting

The NiAl–Cr(Mo)–(Hf,Dy) hypoeutectic alloys were prepared by conventional casting and injection casting techniques respectively, and their microstructure and room temperature mechanical properties were investigated. The results reveal that with the addition of Hf and Dy, the Ni₂AlHf Heusler phase and Ni₅Dy phase form along the NiAl/Cr(Mo) phase boundaries in intercellular region. By the injection casting method, some Ni₂AlHf Heusler phase and Ni₅Dy phase transform into Hf and Dy solid solutions, respectively. Moreover, the microstructure of the alloy gets good optimization, which can be characterized by the fine interlamellar spacing, high proportion of eutectic cell area and homogeneously distributed fine Ni₂AlHf, Ni₅Dy, Hf solid solution and Dy solid solutions. Compared with conventional-cast alloy, the room temperature mechanical properties of injection-cast alloy are improved obviously.