Site Preference and Alloying Effect of Excess Ni in Ni-Mn-Ga Shape Memory Alloys

The formation energies and electronic structures of Ni-rich Ni-Mn-Ga alloys have been investigated by firstprinciples calculations using the pseudopotential plane wave method based on density functional theory. The results show that the alloying Ni prefers to occupy the Mn site directly in Ni9Mn3Ga4 and to occupy the Mn site and drive the displaced Mn atom to the Ga site in NigMn4Ga3, which is in accordance with the experimental result. According to the lattice constants and the density of states analyses, these site preference behaviours are closely related to the smaller lattice distortion and the lower-energy electronic structure when the excess Ni occupies the Mn site. The effect of Ni alloying on martensitic transformation is discussed and the enhancement of martensitic transformation temperature by Ni alloying is estimated by the calculated formation energy difference between austenite and martensite phases.     

Anomalous temperature dependence of elastic constant c44 in V, Nb, Ta, Pd, and Pt

The anomalous temperature dependence of elastic constant c₄₄ for elements V, Nb, Ta, Pd, and Pt, has been calculated using first-principles theory. It is shown that the variation of elastic constant for simple elements can be approximated as the sum of thermal expansion and electronic components. The thermal expansion contributes the normal linearly decreasing effect to the elastic constant with temperature, while electronic contribution is determined by the unique character of electronic structure of elements and leads to the anomalous effect to the elastic constant with temperature.     

Combined experimental and theoretical study on the effect of Nb content on martensitic transformation of NbRu shape memory alloys

The effect of Nb content on the martensitic transformation of NbRu high-temperature shape memory alloys is investigated by experiments and first-principles calculations. We calculate the lattice parameters, density of states, charge density, and heats of formation of Nb_50+xRu_50-x β phase. The results show that an increase in Nb content increases the stability of Nb_50+xRu_50-x β phase, leading to a significant decrease of the β to β' martensitic transformation temperature. In addition, the mechanism of the effects of Nb content on phase stability and martensitic transformation temperature is studied on the basis of electronic structure.     

Temperature and pressure derivatives of elastic constants in single crystal Au-47.5 at.% Cd near martensitic phase transformation

The behavior of the elastic properties and their pressure derivatives near the martensitic transformation β→β′ was determined from sound velocity measurements at a frequency of 10 MHz, on a single crystal of Au-47.5 at.% Cd.The transformation of the β phase to the martensitic β' phase was obtained from the anisotropic elastic constants c₁₁, c₄₄ and c′, and their pressure derivatives. The behaviour of c₁₁ and c₄₄ is normal as a function of temperature, while the variation of c′ is positive with temperature. Before the β→β′ phase change the value of c′ is very small and exhibits high elastic anisotropy. The small value of c′ and the anisotropy were explained according to the crystallographic mechanism of the martensitic transformation and the Zener theory of instability of the b.c.c. structure. The pressure derivatives of c′ are negative and the variation of $\text{d}\text{c}{}_{44}\text{d}\text{P}$ with temperature is anomalous. This behavior can be explained by the influence of pressure on the transformation temperature, and by the instability of the β phase. It was found that the elastic constants c′ determine the mechanism of the transformation. From the pressure derivatives of c₁₁, c₄₄ and c′ the Grüneisen parameters for different modes of vibration were calculated. For the c′ mode the values are negative, for the c₄₄ mode the variation of the parameter with temperature is negative. This anomalous behavior can be explained as due to the anisotropic softening of the atom bonding on the (110) planes of the cubic structure, as a consequence of the phase transformation mechanism.     

Ultrasonic characterization of Cu-Al-Ni single crystals lattice stability in the vicinity of the phase transition

Measurements of elastic constants of the austenite phase when approaching the phase transformation either upon cooling or stressing is of the crucial interest for the shape memory alloy field. Acoustic properties (wave velocity and also attenuation changes) of the Cu-Al-Ni single crystal were investigated in situ during stress-induced martensitic transformation at constant (room) temperature. The parent austenite cubic lattice of the Cu-Al-Ni exhibits very high elastic anisotropy (anisotropy factor A approximately 12). The measurements were made using nine combinations of (i) applied uniaxial compression in a given crystal direction, (ii) the wave propagation and (iii) polarization vectors. The chosen configurations are sufficient for evaluation of all independent third order elastic constants (TOEC). The longitudinal modes were also measured by the immersion technique, using the transducer pair in a water tank installed on the testing machine. The device works as "a ultrasonic extensometer" measuring a transverse strain of the specimen. The dependencies of both natural and initial wave velocities on the applied stress may be evaluated. Three elastic constants of the stress-induced martensite were determined. The elastic properties were found to vary with the increasing stress above the Ms transformation temperature, which is interpreted as a precursor for the martensitic transformation. The onset of the transformation was additionally identified from the acoustic emission measurement.     

Temperature and volume dependence of the elastic constants of palladium

A separation between lattice and electronic contributions to the elastic constants of Pd is obtained. The temperature dependence at constant volume is measured by applying hydrostatic pressure. The apparatus and results for Pd are presented.     

Theoretical study of the structural, elastic, electronic and thermal properties of the MAX phase Nb2SiC

Structural, elastic, electronic and thermal properties of the MAX phase Nb₂SiC are studied by means of a pseudo-potential plane-wave method based on the density functional theory. The optimized zero pressure geometrical parameters are in good agreement with the available theoretical data. The effect of high pressure, up to 40 GPa, on the lattice constants shows that the contractions along the c-axis were higher than those along the a-axis. The elastic constants C_ij and elastic wave velocities are calculated for monocrystal Nb₂SiC. Numerical estimations of the bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, average sound velocity and Debye temperature for ideal polycrystalline Nb₂SiC aggregates are performed in the framework of the Voigt-Reuss-Hill approximation. The band structure shows that Nb₂SiC is an electrical conductor. The analysis of the atomic site projected densities and the charge density distribution shows that the bonding is of covalent-ionic nature with the presence of metallic character. The density of states at Fermi level is dictated by the niobium d states; Si element has a little effect. Thermal effects on some macroscopic properties of Nb₂SiC are predicted using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The variations of the primitive cell volume, volume expansion coefficient, bulk modulus, heat capacity and Debye temperature with pressure and temperature in the ranges of 0-40 GPa and 0-2000 K are obtained successfully.     

The effect of Fe on the martensitic transformation of TaRu high-temperature shape memory alloys:A first-principles study

The effect of Fe on the martensitic transformation of TaRu high-temperature shape memory alloys has been investigated using first-principles calculations. The site preference of Fe in TaRu alloys has been clarified for the first time, and the results show that Fe is predicted to occupy Ru sites. The addition of Fe increases the stability of the Ta₅₀Ru_50-xFe_x β phase, leading to a significant decrease in the β to β′ martensitic transformation temperature. In addition, the mechanism of the Fe alloying effect is explained on the basis of the electronic structure.     

Structural stability,electronic structure and mechanical properties of 4d transition metal nitrides TMN (TM=Ru,Rh, Pd)

Ab initio calculations are performed to investigate the structural stability, electronic, structural and mechanical properties of 4d transition metal nitrides TMN (TM=Ru, Rh, Pd) for five different crystal structures, namely NaCl, CsCl, zinc blende, NiAs and wurtzite. Among the considered structures, zinc blende structure is found to be the most stable one among all three nitrides at normal pressure. A structural phase transition from ZB to NiAs phase is predicted at a pressure of 104 GPa, 50.5 GPa and 56 GPa for RuN, RhN and PdN respectively. The electronic structure reveals that these nitrides are metallic. The calculated elastic constants indicate that these nitrides are mechanically stable at ambient condition.     

NiTi形状记忆合金马氏体相变的第一性原理研究

为了理解NiTi形状记忆合金马氏体相变的机理，基于密度泛函的第一性原理研究了温度和应力对电子结构的稳定性的影响.通过对态密度的分析，发现随着温度的降低和变形的增加，B2相的态密度升高，能级兼并性提高，从而导致结构的稳定性降低，促进相变的发生. 关键词： NiTi 电子结构 第一性原理计算 相变     

Electronic structure calculations of lead chalcogenides PbS, PbSe, PbTe

In this work, we have extended our study of the mechanical properties and the electronic structure of PbTe to include other Pb chalcogenide compounds (PbSe, PbS). The calculations were performed self-consistently using the scalar-relativistic full-potential linearized augmented plane wave method. Both the local density approximation (LDA) and the generalized gradient approximation (GGA) to density-functional theory were applied.The equilibrium lattice constants and the bulk modulus of a number of structures (NaCl, CsCl, ZnS) were calculated as well as the elastic constants for the structures (NaCl, CsCl). The NaCl structure is found to be the most stable one among all the three phases considered. We have found that the GGA predicts the elastic constants in good agreement with experimental data.Both the LDA and GGA were successful in predicting the location of the band gap at the L point of the Brillouin zone but they are inconclusive regarding the value of the band-gap width. To resolve the issue of the gap, we performed Slater-Koster (SK) tight-binding calculations, including the spin-orbit coupling in the SK Hamiltonian. The SK results that are based on our GGA calculations give the best agreement with experiment.Results are reported for the pressure dependence of the energy gap of these compounds in the NaCl structure. The pressure variation of the energy gap indicates a transition to a metallic phase at high pressure. Band structure calculations in the CsCl structure show a metallic state for all compounds. The electronic band structure in the ZnS phase shows an indirect band gap at the W and X point of the Brillouin zone.     

Effect of alloying on the structure and phase transformations in Ni-Mn alloys alloyed by titanium

The crystal structure of NiMn alloy alloyed by titanium in a wide range of temperatures and compositions has been investigated using resistivity measurements, transmission electron microscopy, electron diffraction, and X ray diffraction. It is found that alloying by titanium not only decreases the martensitic transformation temperature but also changes the martensite crystal structure. The martensitic transformation temperatures are determined and the diagram of martensitic transformations for Ni₅₀Mn_{50 ? x} Ti _x alloys is constructed.     

First-principles studies of the electronic and elastic properties of Ti2GeC

We have performed first-principles studies on electronic structure and elastic properties of Ti₂GeC. The calculated band structure shows that this compound is electrical conductor. From the pressure dependence of elastic constants, we find that Ti₂GeC is most stable in the pressure range from 0 to 100 GPa. The strong Ti 3d, Ge 4p and C 2p hybridization may stabilize the structure of Ti₂GeC. By analyzing the ratio between the bulk and shear moduli, we conclude that Ti₂GeC is brittle in nature, and the brittleness of Ti₂GeC originated from the large value of Ti atom occupying the internal parameter z.     

First-principles study on the effect of Hf content onmartensitic transformation temperatureof TiNiHf alloy

In this paper a first-principles study of the electronic structure and stability of B2 TiDFT TiNiHf 电子结构 马氏体转化温度 平面波DFT, TiNiHf, electronic structure, martensitic transformation temperatureProject supported by the National Natural Science Foundation of China (Grant No 50471018).3/3/2006 12:00:00 AM6/7/2006 12:00:00 AMIn this paper a first-principles study of the electronic structure and stability of B2 Ti1-xNiHfx （x = 0.2, 0.4, 0.6） and B19′ Ti1-xNiHfx（x = 0, 0.5） alloys is presented. The calculations are performed by the plane-wave pseudopotential method in the framework of the density functional theory with the generalized gradient approximation. This paper calculates the lattice parameters, density of states, charge density, and heats of formation. The results show that the electronic structure and stability of B2 Ti1-xNiHfx change gradually with Hf content. However, Hf content has little effect on the electronic structure and stability of B19′ Ti1-xNiHfx. The mechanism of the effect of Hf content on martensitic transformation temperature of TiNiHf alloys is studied from the electronic structure.     

Changes in the electronic structure upon the B2–B19′ martensitic transformation in titanium-nickel

The self-consistent band structure of a TiNi intermetallic compound in two phases is calculated by the full-potential linearized augmented-plane-wave (FLAPW) method. The features of changes in the density of states upon B2–B19′ martensitic transformation are discussed. The influence of atomic positions on the electronic structure of the martensitic monoclinic phase is examined. The frequency dependence of the optical conductivity and the emission, absorption, and the characteristic electron-energy-loss spectra are calculated with due regard for the transition probability matrix element. The results of calculations are in reasonable agreement with the available experimental data.     

Lattice and electronic contributions to the elastic constants of palladium

We measured second order elastic constants of Pd from 295 to 850 °K and under hydrostatic pressure up to $\text{7000 kg}\text{cm}{}^2$. A study of the temperature behavior and the volume dependence of these constants allows to separate the electronic contributions from those of the lattice. Grüneisen constants are calculated. With our model a certain relation between elastic shear constants and the magnetic susceptibility is successfully applied and the deformation potential is determined.     

The mechanical,thermodynamic and electronic properties of Al3Nb with DO22 structure: A first-principles study

The lattice constants, elastic properties, electronic structure and thermodynamic properties of Al₃Nb with DO₂₂ structure have been investigated by the first-principles calculation. The calculated lattice constants were consistent with the experimental values, and the structural stability was also studied from the energetic point of view. The single-crystal elastic constants (C_ij) as well as polycrystalline elastic parameters (bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio υ and anisotropy value A) were calculated, and brittleness of Al₃Nb was discussed in detail. Besides, the electronic structure of tetragonal Al₃Nb was studied, which indicates a mixture of metallic bond and covalent bond in Al₃Nb and reveals the underlying mechanism of the stability and elastic properties of Al₃Nb. Finally, the thermodynamic properties of Al₃Nb were calculated and the physical properties such as heat capacity and Debye temperature were predicted within the quasi-harmonic approximation.     

Band parameters for cadmium and zinc chalcogenide compounds

Theoretical investigations of the electronic, optical and elastic properties of cadmium and zinc chalcogenides in the zinc-blende structure are performed using a pseudopotential formalism. Our results are in reasonable agreement with the available experimental data. Polynomial expressions are obtained for the electron effective mass and the static dielectric constant as a function of the fundamental energy band-gap. Relations of elastic constants ratio to the ionicity are also examined and discussed.     

Modelling of dislocation-induced martensitic transformation in anisotropic crystals

The structural transformation caused by dislocation-induced heterogeneous nucleation in the fcc?→?bcc martensitic transformation in elastically anisotropic crystals is investigated by using the phase field microelasticity model. The three-dimensional microstructure of the dislocation-induced martensitic embryos is obtained. It is found that the embryos are not single-domain particles as is usually assumed but rather a complex self-organized assemblage of stress-accommodating twin-related microdomains. Sessile metastable martensitic embryos around the dislocation loops form in the prototype Fe–Ni alloy system above the temperatures of the martensitic transformation. A possibility that the presence of these pre-existing embryos could be responsible, at least, for a part of the elastic modulus softening with the temperature decrease observed in many martensitic systems is discussed. The effects of elastic anisotropy, the “chemical” energy barrier and structural anisotropy of the Landau free energy on the formation and growth of martensitic embryos are investigated. The assumptions of elastic isotropy and a choice of the anisotropic term in Landau polynomial do not significantly affect the microstructure of martensitic embryos but may appreciably change the undercooling that is necessary to eliminate the total nucleation barrier and start the athermal martensitic transformation.     

Theoretical investigations of NiTiSn and CoVSn compounds

The structural, elastic and electronic properties of NiTiSn and CoVSn half-Heusler compounds have been calculated using the full-potential linear muffin-tin orbital (FP-LMTO) method. The computed equilibrium lattice constants are in excellent agreement with the available experimental and theoretical data. The elastic constants C_ij are calculated using the total energy variation with strain technique. The polycrystalline elastic moduli (namely: the shear modulus, Young's modulus, Poisson's ratio, Lamé's coefficients, sound velocities and the Debye temperature) were derived from the obtained single-crystal elastic constants. The ductility mechanism for the studied compounds is discussed via the elastic constants C_ij and their related parameters. The electronic band structure calculations show that the conduction band minimum (CBM) is located at the X point for both compounds, whereas the valence band maximum (VBM) is located at the Г point for NiTiSn and at the L point for CoVSn, resulting in indirect energy band gaps of 0.46 and 0.75 eV for NiTiSn and CoVSn, respectively. The pressure and volume dependences of the energy band gaps have been calculated.