Lattice stability and the effect of Co and Re on the ideal strength of Ni:First-principles study of uniaxial tensile deformation

Using first-principles density functional calculations, lattice stability of γ-Ni under [001], [110], and [111] uniaxial tensions and the effect of alloying elements Co and Re on the uniaxial tensile behavior of γ-Ni were studied in this paper.With elastic constants and phonon spectra calculations, we examined the mechanical stability and phonon stability of Ni during the uniaxial tensions along the three characteristic directions. The results show that the mechanical stability and phonon stability of a lattice occurs before the maximum stress–strain point under the [001] and [111] tension, respectively.The effects of Co and Re on the ideal tensile strength of γ-Ni show a significant directivity: Co and Re have little effect on the stresses in [001] and [111] directions, but increases the ideal strength of the system in the weakest uniaxial tensile direction. Moreover, the strengthening effect of Re is significantly better than that of Co. By further analyzing electronic structure, it is found that the effect of alloying elements on the uniaxial tensile behavior of γ-Ni comes from their interactions with host atoms.     

Chen’s lattice inversion embedded-atom method for Ni-Al alloy

The structural properties,the enthalpies of formation,and the mechanical properties of some Ni-Al intermetallic compounds(NiAl,Ni3Al,NiAl3,Ni5Al3,Ni3Al4) are studied by using Chen’s lattice inversion embedded-atom method(CLI-EAM).Our calculated lattice parameters and cohesive energies of Ni-Al compounds are consistent with the experimental and the other EAM results.The results of enthalpy of formation indicate a strong chemical interaction between Ni and Al in the intermetallic compounds.Through analyzing the alloy elastic constants,we find that all the Ni-Al intermetallic compounds discussed are mechanically stable.The bulk moduli of the compounds increase with the increasing Ni concentration.Our results also suggest that NiAl,Ni3Al,NiAl3,and Ni5Al3 are ductile materials with lower ratios of shear modulus to bulk modulus;while Ni3Al4 is brittle with a higher ratio.     

First-principles prediction of the hardness of fluorite TiO2

We present first-principles calculations on the elastic constants, ideal tensile and shear strengths of cubic TiO₂ with a fluorite structure (f-TiO₂). The results show that f-TiO₂ is mechanically stable at the ground-state structure. Both shear modulus and value of hardness predicted indicate that the hardness of f-TiO₂ is comparable with TiN but is lower than TiB₂. The ideal shear strength results suggest that the hardness of f-TiO₂ is reduced because of the lower stress on the shear (1 1 1) 〈1 1¯ 0〉 slip system.     

Exceptional ideal strength of carbon clathrates

We study by means of ab initio calculations the ideal tensile and shear strengths of the C-46 clathrate phase. While its bulk modulus and elastic constants are smaller than in diamond, its strength is found to be in all directions larger than the critical stresses associated with the diamond [111] planes of easy slip. This can be related to the frustration by the clathrate cage structure of the diamond to graphite instability under nonhydrostatic stress conditions [corrected] The criteria for designing strong materials are discussed.     

二元Ni-Al合金极端条件下的弹性和热力学性能：全电子准谐波近似 cited: 1)

通过在原子尺度上建模来研究Al、NiAl和Ni₃Al合金在极端高温和高压下的点阵常数、弹性常数、弹性模量、泊松比和弹性各向异性因子等性质．计算得到的弹性常数均满足相应的力学稳定条件．由于NiAl和Ni₃Al具有较高的B=G值,在0～30 GPa内都属于延展性材料．通过包含电子热运动对体系吉布斯自由能贡献的全电子准谐近似方法,得到了高温高压下Al、NiAl和Ni₃Al合金的热膨胀系数、体积模量、热容和熵等．计算值与已有的实验值和理论值符合较好     

Density functional theory study of 3R- and 2H-CuAlO2 in tensile stress

We report a direct computational result of a phase transformation from the 3R phase to the 2H phase in CuAlO₂ with the application of tensile stress using the first-principles density functional theory calculations. The calculations of enthalpy variation with tensile stress indicates the 3R-to-2H phase transformation is expected to occur around −26.0 GPa. As the applied tensile stress increases, the independent elastic constants of 3R- and 2H-CuAlO₂ show the presences of mechanical instability at −27.5 and −27.6 GPa, which are possibly related with the ideal tensile strength.     

Effect of temperature on inhomogeneous elastic deformation and negative stiffness of NiAl and FeAl alloy nanofilms

The uniaxial tension of NiAl and FeAl intermetallic alloy nanofilms at different temperatures has been investigated by the molecular dynamics method. It was previously shown that nanofilms at 0 K are elastically deformed by almost 40% and that, under strain-controlled tension, there is a region in the stress—strain curves, where an increase in the strain is accompanied by a decrease in the tensile stress, i.e., the stiffness of nanofilms is negative. Deformation of the films in the thermal instability region is associated with the appearance of domains with different elastic strains. The influence of the temperature on these effects is investigated. Particularly, it is shown that as the temperature increases, both the elastic strain and the negative stiffness of nanofilms decrease. The inhomogeneous elastic strain and negative stiffness for FeAl films are observed in a broader temperature range (to 1000 K) than for NiAl films (to 300 K), which constitutes 0.16 and 0.65 of the melting point of these materials, respectively.     

Insights into the fracture mechanisms and strength of amorphous and nanocomposite carbon

Tight-binding molecular dynamics simulations shed light into the fracture mechanisms and the ideal strength of tetrahedral amorphous carbon and of nanocomposite carbon containing diamond crystallites, two of the hardest materials. It is found that fracture in the nanocomposites, under tensile or shear load, occurs intergrain and so their ideal strength is similar to the pure amorphous phase. The onset of fracture takes place at weakly bonded sites in the amorphous matrix. On the other hand, the nanodiamond inclusions significantly enhance the elastic moduli, which approach those of diamond.     

Achievement of ultimate values of the bulk and shear strengths of iron irradiated by femtosecond laser pulses

Shock-wave phenomena generated by femtosecond laser pulses in submicron iron film samples have been studied by the interferometric method with the application of frequency-modulated diagnostics in the picosecond time range. The splitting of the shock wave into the elastic and plastic waves with a compression stress of up to 27.5 GPa behind the front of an elastic precursor has been detected. The corresponding maximum shear stress reaches 7.9 GPa, which is even somewhat higher than the calculated ideal shear strength. The measured spall strengths reach 20.3 GPa, which is also comparable to the calculated values of the ideal tensile strength.     

A new superhard carbon allotrope: Orthorhombic C20

A new superhard carbon orthorhombic allotrope oC20 is proposed, which exhibits distinct topologies including C4, C3 and two types of C6 carbon rings. The calculated elastic constants and phonon spectra reveal that oC20 is mechanically and dynamically stable at ambient pressure. The calculated electronic band structure of oC20 shows that it is an indirect band gap semiconductor with a band gap of 4.46 eV. The Vickers hardness of oC20 is 75 GPa. The calculated tensile and shear strength indicate that the weakest tensile strength is 64 GPa and the weakest shear strength is 48 GPa, which means oC20 is a potential superhard material.     

应力诱发NiAl单晶马氏体相变的分子动力学模拟

利用嵌入原子势（EAM）,对NiAl单晶在外应力作用下的动态拉伸过程进行了分子动力学模拟.应力-应变曲线分析以及原子构型分析表明外应力诱发NiAl合金发生了马氏体相变,原子结构由B2相转变为L1₀相.通过研究原子构型的演化过程,发现马氏体相变是通过多个{110}孪晶面的扩展和湮灭作用来完成的.同时探讨了马氏体相变的微观机理. 关键词： 马氏体相变 NiAl 分子动力学模拟     

Superdense tI12 carbon: Unexpectedly high elastic moduli but low ideal strength

The basic physical and chemical properties of new carbon allotropies are important to explore their further technique and industrial applications. Here, a systematic theoretical investigation on the electronic, dynamical, and elastic properties for the superdense carbon (tI12) are performed, especially the ideal tensile and shear strength and the corresponding bond-breaking modes are explored to uncover its intrinsic mechanical nature and the corresponding bond-breaking modes. Our results show that the bulk, shear and Young's modulus of tI12 carbon are ultrahigh, close to those of diamond, reflecting its excellent performance of the substance's resistance to be deformed elastically at small strains. However, the calculated tensile and pure shear strengthes are remarkably lower than that of diamond, which is attributed to its original structural anisotropy by analyzing the atomic structural deformation under different strains. The current results highlighted the need to carefully examine the stress response at large strains, which provide crucial insights for the bond-breaking modes and deformation mechanisms that may lead to conclusions different from those obtained from equilibrium structures.     

Microstructure and mechanical properties of continuous Al2O3 fibre reinforced Ni45Al45Cr7.5Ta2.5 alloy (IP75) matrix composites

Single crystalline Al₂O₃ fibres (sapphire), coated with the NiAl alloy IP75 by physical vapour deposition (PVD), were assembled to fabricate composites by means of diffusion bonding. The microstructure and chemistry of both as-coated fibre and as-diffusion bonded composites were investigated by electron microscopy and microanalysis. The interface shear stress for complete debonding was measured by fibre push-out tests at room temperature, and the composite tensile strength was measured at 900°C and 1100°C. An amorphous layer with a thickness of about 400?nm formed between the fibre and the matrix during the PVD process and was maintained during diffusion bonding. A Laves phase precipitated along NiAl grain boundaries in the IP75 matrix. This caused a lower tensile strength of the IP75/Al₂O₃ composite at high temperatures compared to as-cast monolithic IP75 and rendered the composite useless for structural applications.     

Deformation‐induced bonding evolution of iron tetraboride and its electronic origin

First‐principles density functional calculations are employed to provide a fundamental understanding of the structural features, mechanical properties, deformation behaviours and its electronic origin for the new synthesized FeB₄. The calculated elastic moduli suggest that FeB₄ has a low compressibility, but results of ideal shear strength and theoretical hardness indicate that FeB₄ is a hard material, not a superhard material. We find that the collapse of the unique corrugated B₆ units ring in FeB₄ under deformation is responsible for the failure under tensile and shear deformation based on the calculated charge density distribution and bonding evolution. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature-dependent ideal strength and stacking fault energy of fcc Ni: a first-principles study of shear deformation

Variations of energy, stress, and magnetic moment of fcc Ni as a response to shear deformation and the associated ideal shear strength (τ(IS)), intrinsic (γ(SF)) and unstable (γ(US)) stacking fault energies have been studied in terms of first-principles calculations under both the alias and affine shear regimes within the {111} slip plane along the <112> and <110> directions. It is found that (i) the intrinsic stacking fault energy γ(SF) is nearly independent of the shear deformation regimes used, albeit a slightly smaller value is predicted by pure shear (with relaxation) compared to the one from simple shear (without relaxation); (ii) the minimum ideal shear strength τ(IS) is obtained by pure alias shear of {111}<112>; and (iii) the dissociation of the 1/2[110] dislocation into two partial Shockley dislocations (1/6[211] + 1/6[121]) is observed under pure alias shear of {111}<110>. Based on the quasiharmonic approach from first-principles phonon calculations, the predicted γ(SF) has been extended to finite temperatures. In particular, using a proposed quasistatic approach on the basis of the predicted volume versus temperature relation, the temperature dependence of τ(IS) is also obtained. Both the γ(SF) and the τ(IS) of fcc Ni decrease with increasing temperature. The computed ideal shear strengths as well as the intrinsic and unstable stacking fault energies are in favorable accord with experiments and other predictions in the literature.     

Brittle and ductile fracture of semiconductor nanowires – molecular dynamics simulations

Fracture of silicon and germanium nanowires in tension at room temperature is studied by molecular dynamics simulations using several interatomic potential models. While some potentials predict brittle fracture initiated by crack nucleation from the surface, most potentials predict ductile fracture initiated by dislocation nucleation and slip. A simple parameter based on the ratio between the ideal tensile strength and the ideal shear strength is found to correlate very well with the observed brittle versus ductile behaviours for all the potentials used in this study. This parameter is then computed by ab initio methods, which predict brittle fracture at room temperature. A brittle-to-ductile transition (BDT) is observed in MD simulations at higher temperature. The BDT mechanism in semiconductor nanowires is different from that in the bulk, due to the lack of a pre-existing macrocrack that is always assumed in bulk BDT models.     

The temperature-dependent elastic properties of B2-MgRE intermetallic compounds from first principles

The temperature-dependent elastic modulus of MgRE (RE=Y, Dy, Pr, Sc, Tb) intermetallics with B2-type structure are presented from first-principles quasistatic approach, in which the static volume-dependent elastic constants are obtained by the first-principles total-energy method within density functional theory and the thermal expansion is obtained from the quasiharmonic approach based on density-functional perturbation theory. The comparison between the predicted results and the available experimental data for a benchmark material NiAl provides good agreements. At T=0 K, our calculated values of lattice parameter and elastic moduli for MgRE intermetallics show excellent agreement with previous theoretical results and experimental data. With temperature increasing, we find that the elastic constants satisfy the stability conditions for B2 structures and follow a normal behavior with temperature, i.e., decrease and approach linearity at higher temperature and zero slope around zero temperature. In addition, the sound velocities as a function of temperature for the NiAl and MgRE intermetallics are calculated and the relations to phonon spectrums have also been discussed.     

A comparative first-principles study on electronic structures and mechanical properties of ternary intermetallic compounds Al8Cr4Y and Al8Cu4Y: Pressure and tension effects

An investigation into the bulk properties, elastic properties and Debye temperature under pressure, and deformation mode under tension of Al₈Cu₄Y and Al₈Cr₄Y compounds was investigated by using first principles calculations based on density functional theory. The calculated lattice constants for the ternary compounds (Al₈Cu₄Y and Al₈Cr₄Y) are in good agreement with the experimental data. It can be seen from interatomic distances that the bonding between Al1 atom and Cr, Y, and Al2 atoms in Al₈Cr₄Y are stronger than Al₈Cu₄Y. The results of cohesive energy show that Al₈Cr₄Y should be easier to be formed and much stronger chemical bonds than Al₈Cu₄Y. The bulk modulus B, shear modulus G, Young's modulus E and Poisson's ratio ν can be obtained by using the Voigt–Reuss–Hill averaging scheme. From the results of elastic properties, Al₈Cr₄Y has the stronger mechanical behavior than Al₈Cu₄Y. Our calculations also show that pressure has a greater effect on mechanical behavior for both compounds. The ideal tensile strength are obtained by stress-strain relationships under [001](001) uniaxial tensile deformation, which are 15.4 and 23.4 GPa for Al₈Cu₄Y and Al₈Cr₄Y, respectively. The total and partial density of states and electron charge density under uniaxial tensile deformations for Al₈Cu₄Y and Al₈Cr₄Y compounds are also calculated and discussed in this work.     

Response of fcc metals and L12 and D022 type trialuminides to uniaxial loading along [100] and [001]: ab initio DFT calculations

Ab initio density-functional calculations have been used to investigate the response of the face-centred cubic (fcc) metals Al and Cu, and of the L1₂- and D0₂₂-type trialuminides Al₃(Sc,Ti,V) to uniaxial loading along the [100] and [001] directions. The results obtained under uniaxial strains are compared to the response to biaxial (epitaxial) strains. The ideal tensile and compressive strengths and their limitation by shear instabilities along these deformation paths have been calculated. Although the response of both pure fcc metals could be expected to be very similar, our results show a fundamental difference: whereas for Cu a special invariant state with C ₂₂?=?C ₂₃, leading to a bifurcation from the tetragonal to an orthorhombic deformation path, is reached at a strain of 10%, for Al this state is reached only at a strain of 33% close to the critical strain defining the ideal tensile strength. The reaction of the L1₂-type trialuminides is comparable to the response of Al; no bifurcation to an orthorhombic deformation path is predicted. The response of the D0₂₂-type trialuminides is different from that of the L1₂-type phases because of the difference in the stacking of the atomic planes along the [001] direction. For D0₂₂-type trialuminides, the uniaxial compression along this direction or epitaxial tension in the (001) plane leads to the formation of a stress-free D0₃ structure, in complete analogy to the fcc???bcc transformations observed for the pure metals. Under uniaxial [100] loading the guiding symmetry along the deformation path is orthorhombic and leads to the formation of special structures under both tension and compression parts, which are related to the D0₃ structure in the same way as the parent D0₂₂-lattice is related to the L1₂ structure.     

Phonon instabilities and the ideal strength of aluminum

We have calculated the phonon spectra of aluminum as a function of strain using density functional perturbation theory for <110>, <100>, and <111> uniaxial tension, as well as relaxed <112>[111] shear. In all four cases, phonon instabilities occur at points away from the center of the Brillouin zone and intrude before the material becomes unstable according to elastic stability criteria. This is the first time the ideal strength of a metal has been shown to be dictated by instabilities in the acoustic phonon spectra. We go on to describe the crystallography of the unstable modes, all of which are shear in character. This work further suggests that shear failure is an inherent property of aluminum even in an initially dislocation-free perfect crystal.