Size-dependent deformation mechanisms and strain-rate sensitivity in nanostructured Cu/X (X = Cr,Zr) multilayer films

Hardness, activation volume and strain-rate sensitivity of Cu/Cr (face-centered cubic (fcc)/body-centered cubic) and Cu/Zr (fcc/hexagonal close-packed) nanostructured multilayer films have been systematically measured as a function of modulation period (L) and modulation ratio (η), respectively. Significant size effects were found for all the three plastic deformation characteristics, i.e. enhanced hardness and activation volume but reduced strain-rate sensitivity with decreasing the dimension length L. Microstructure evolution was statistically examined to rationalize these size dependences. It was crucially observed that abundant nanotwins existed in the Cu grains, though nanotwin formation was depressed with smaller L. This inverse size-dependent nanotwin formation is responsible for the reduction in strain-rate sensitivity, because the negative effect induced by the decreased nanotwins predominates over the positive effect coming from the raised interfaces/boundaries. A mechanistic model is modified to account for the interface effect as well as the nanotwin effect, which yields calculations of strain-rate sensitivity in broad agreement with the experimental results when L is larger than about 20 nm. Below this critical length size, there are discrepancies between the calculations and the experimental results, due to the change in deformation mechanism from dislocation nucleation/slip in confined layers to dislocation crossing interfaces. A confined layer slip model is also modified by considering the nanotwin strengthening to quantitatively describe the L-dependent hardness. In addition, the effects of constituent phases and their relative content on the activation volume and strain-rate sensitivity of NMFs are discussed with regard to variation in η.     

Buckling behaviors and adhesion energy of nanostructured Cu/X (X = Nb,Zr) multilayer films on a compliant substrate

Two sets of Cu/Nb (face-centered cubic (fcc)/body-centered cubic) and Cu/Zr (fcc/hexagonal close-packed) nanostructured multilayer films (NMFs) have been prepared on a flexible polyimide substrate, with a wide range modulation period (λ) from 250 down to 5 nm. The mechanical properties of the two NMFs have been measured upon uniaxial tensile testing and the buckling behaviors have been systematically investigated as a function of λ. A significant difference in the bucking behaviors was found between the two NMFs, with the buckles in the Cu/Nb NMF being mostly cracked, while the buckles were nearly crack-free in the Cu/Zr NMF. The different buckling behaviors, dependent on the constituent phases, are rationalized in the light of the disparity in mechanical properties. The criteria to characterize buckle cracking have been discussed with respect to the mechanical properties (e.g. yield strength, ductility and fracture toughness) of the NMFs. A modified energy balance model has been employed to estimate the adhesion energy of the NMFs on the polyimide substrate. Within the λ regime below a critical size (λ_crit) of ～50 nm a λ-independent adhesion energy of about 1.1 and 1.2 J m^?2 has been determined for the Cu/Nb and Cu/Zr NMFs, respectively, which agrees well with previous reports on the metal film/polymer substrate systems. Within the λ regime greater than λ_crit, however, the measured adhesion energy exhibit a strong size effect, i.e. increasing with increasing λ. The λ dependence of the evaluated adhesion energy is discussed in terms of the size-dependent deformation mechanism in NMFs. A micromechanics model has been utilized to quantify the critical modulation period of ～50 nm, where the deformation mechanism changes from dislocation pile-up to confined layer slip.     

Magneto-optical properties of transition metal compounds XPt3 (X = V, Cr, Mn, Fe, Co) and X3Pt (X = Fe, Co)

We have studied theoretically the electronic structure and magneto-optical Kerr effect (MOKE) of XPt₃ (X = V, Cr, Mn, Fe, Co) and X₃Pt (X = Fe, Co) compounds from first principles using the full potential linearized augmented plane wave (FPLAPW) method within the local spin density approximation. The calculated magneto-optical spectra show the features of the experimental spectra which can be attributed to electronic transitions. For the Pt based compounds of 3d transition metals, we find the largest Kerr angle of −1.17° for MnPt₃ in the polar geometry which is in good agreement with the experimental data. Our calculations show an increase in Kerr rotation for XPt₃ series which can be correlated to an increase in the unpaired valence d-electrons of 3d transition metal (X) up to MnPt₃ and then a decrease for FePt₃ and CoPt₃. The Kerr spectrum is analyzed to ascertain the origin of the various peaks in terms of optical transitions. In addition, the effect of interchanging X and Pt atoms in XPt₃ is also studied in order to have greater insight for the dependence of Kerr spectra on configuration and local environment of these two atoms.     

Magnetic properties and electronic structure of R3T4X4 compounds (R = Pr,Nd; T = Cu,Ag; X = Ge,Sn)

Polycrystalline samples of R₃T₄X₄ (R = Pr, Nd; T = Cu, Ag; X = Ge, Sn) intermetallics were investigated by means of magnetometric and XPS methods. All these compounds crystallize in the orthorhombic Gd₃Cu₄Ge₄-type structure. They were found to be antiferromagnets, except Pr₃Ag₄Ge₄ which is a collinear ferromagnet. Analysis of the XPS valence band data indicates strong overlaps of the Pr 4f states with the Cu 3d states and the Nd 4f states with the Ag 4d states. Analysis of the R 3d states on the basis of the Gunnarsson–Schönhammer model gave the information on the hybridization of the 4f orbital with the conduction band. The results showed that the Pr-based compounds are characterized by larger values of the hybridization energy than the Nd-based compounds.     

Energy of formation, lattice parameter and bulk modulus of (Ni,X)Ti alloys with X = Fe, Pd, Pt, Au, Al, Cu, Zr, Hf

An analysis of the ternary ‘bridge’ Ni_50−yX_yTi₅₀ alloys with X = Fe, Pd, Pt, Au, Al, Cu, Zr, and Hf was performed using the BFS method for alloys. The lattice parameter, bulk modulus and energy of formation were determined for all the intermediate states in the (B2) transition NiTi to XTi.     

Experimental and theoretical study of Ti20Zr20Hf20Nb20X20 (X = V or Cr) refractory high-entropy alloys

We investigated the microstructure and mechanical properties of Ti₂₀Zr₂₀Hf₂₀Nb₂₀X₂₀ (X = V or Cr) high-entropy alloys (HEA), produced by induction melting and casting in inert atmosphere. The structures of these alloys were studied via X-ray diffractometry and scanning electron microscopy. Results show that Ti₂₀Zr₂₀Hf₂₀Nb₂₀V₂₀ has mainly the body centered cubic (BCC) structure, whereas the BCC matrix of Ti₂₀Zr₂₀Hf₂₀Nb₂₀Cr₂₀ contains small amount of Cr₂Nb and Cr₂Hf intermetallic compounds. Ti₂₀Zr₂₀Hf₂₀Nb₂₀V₂₀ alloy shows the high strength and the homogeneous deformation under compression at room temperature. The strength and hardness of Ti₂₀Zr₂₀Hf₂₀Nb₂₀Cr₂₀ alloy are further enhanced by the Cr-containing Laves phases segregated during casting. The structural and mechanical properties remained almost unchanged after a short time (10 min) heat treatment at 573, 773, 973 and 1173 K indicating the resistance to working temperature peaks for these two alloys. Ab initio calculations predict ductile behavior for these and similar refractory HEAs. The theoretically calculated Young's modulus E is in good agreement with the experimental ones.     

First-principles investigations of isotope effects in thermodynamic properties of TiX2 (X = H, D, and T) system

As hydrogen, deuterium and tritium storage materials, a series of investigations of mechanical and thermal properties of titanium hydrides, deuterides and tritides have been performed, however, very limited theoretical studies of thermodynamic properties for them can be found. Based on density-functional theory (DFT) and density-functional perturbation theory (DFPT) we have discussed systematically the hydrogen isotope effects on the thermodynamic properties of TiX₂ (X = H, D, and T) system. Our calculations indicate that for evaluating accurately their physical properties at absolute zero temperature, such as the equilibrium lattice constants, bulk modulus, and heat of formation, the zero-point energy correction must be taken into account. By performing the phonon calculation within quasiharmonic approximation (QHA), we obtain their vibrational free energies, vibrational entropies, and temperature dependence of specific heat, thermal expansion, and bulk modulus. Those results demonstrate that comparing with TiH₂, TiT₂ and TiD₂ are more stable and the zero-point effects play an important role in their thermal expansion. The increase in the force constant between Ti and H causes the higher value of specific heat of TiH₂ during the phase transition from FCC to FCT. In addition, comparing with available experimental values, we can conclude that QHA is feasible for describing the thermal properties of TiX₂.     

Spectral studies of new organic conductors: κ-(BDH-TTP)4Hg3X8, where X = Cl,Br

Raman spectra of single crystals of κ-(BDH-TTP)₄Hg₃X₈, where X = Cl, Br, were recorded in the 250–3000 cm⁻¹ frequency range for two polarizations of incident light. Additionally, the polarized reflectance spectra of the single crystals of the compounds were recorded from 600 to 7000 cm⁻¹ and fitted with the Lorentz model. The ab initio calculations of normal mode frequencies of the neutral BDH-TTP molecule, the BDH-TTP⁺ cation, and mercury-containing anions were performed. The calculations provided the assignment of vibrational modes observed in the experimental spectra. The analysis of the theoretical and the experimental Raman spectra suggests that both salts contain two kinds of donor molecules: differently charged or/and crystallographically independent ones.     

Microstructure and plastic deformation behavior of Ni3(Ti,X) (X = Nb,Al) single crystals with long-period geometrically closely packed crystal structures

The crystal structure, phase stability and plastic deformation behavior of Ni₃(Ti_1−xNb_x) [x = 0, 0.01, 0.03, 0.05 or 0.10] and Ni₃(Ti_1−yAl_y) [y = 0.16 or 0.28] single crystals were investigated. The substitution of Ti by Nb in Ni₃Ti induced the formation of various long-period stacking ordered (LPSO) structures with 18-fold, 10-fold or 9-fold stacking sequences of closely packed plane (CPP) depending on the Nb content. In compression tests, the yield stress anomaly (YSA) appeared in ternary LPSO crystals as well as in binary Ni₃Ti by slip on the CPP. The change in stacking sequence of CPP in the LPSO phases strongly affects the YSA behavior due to the change in APB energy on the non-closely packed plane.     

Lattice,magnetic and electronic transport behaviors of Ge-doped Mn3XC (X = Al,Zn, Ga)

The lattice, magnetic, and electronic transport properties of Mn₃X_1?yGe_yC (X = Al, Zn, and Ga; y = 0, 0.5) were investigated. Ge-doping does not change the antiperovskite structure, and only decreases the lattice constant in different degree. For all Mn₃X_1?yGe_yC, Ge-doping makes the magnetic transition temperature decrease. The total behavior of Mn₃Al_xGe_1?xC in temperature dependence of resistivity is metallic. For Mn₃Ga_xGe_1?xC, since the carrier densities in the ferromagnetic phase and antiferromagnetic one are different, with decreasing the temperature, the resistivity appears an abrupt increase.     

Oxygen incorporation in M2AlC (M = Ti,V, Cr)

Oxygen incorporation in the MAX phases Ti₂AlC, V₂AlC and Cr₂AlC was studied by ab initio calculations. Comparing the calculated energies for oxygen incorporation indicates that oxygen substitutes for carbon in Ti₂AlC and V₂AlC, but is incorporated interstitially in the Al layer of Cr₂AlC, even for carbon-deficient Cr₂AlC. To evaluate these predictions, combinatorial d.c. sputtering was used to deposit thin films with different oxygen concentrations. Two phase regions of Cr₂AlC and Cr₂Al were investigated in order to study oxygen incorporation in carbon-deficient Cr₂AlC. X-ray strain analysis data indicate that the a and c lattice parameters increase with oxygen content. These trends are in good agreement with the change in lattice parameters predicted by ab initio calculations and therefore corroborate the notion of interstitial oxygen incorporation in Cr₂AlC. A metastable solubility limit for oxygen of 3.5 at.% was determined experimentally. This is the first report on interstitial oxygen in a MAX phase and may be of relevance during the initial stages of oxidation.     

On the properties of the eutectic alloy Al3(Nb,Cr) + Cr(Al,Nb)

The eutectic alloy Al₃(Nb,Cr) + Cr(Al,Nb) forms an in situ composite and the Al₃Nb presents high specific strength and low oxidation rate that may be improved by the combination with other phases. The purpose of this work is to investigate physical, mechanical and oxidation properties of the eutectic alloy. Therefore, Rietveld analysis was carried out for furnace cooled and water quenched samples and oxidation tests were performed on directional solidified samples. Compressive tests were performed for the eutectic alloy and also for the Nb–74.8% Cr–24.6% Al alloy in the as-cast condition. The alloy presents 12.9% Cr(Al,Nb) at room temperature, retained from the transformation Cr(Al,Nb) to Al(Nb)Cr₂. The combination of Al₃Nb with Cr(Al,Nb) and Al(Nb)Cr₂ considerably improves mechanical behaviour, leading the yield strength to 1525 MPa at 800 °C and 925 MPa at 900 °C. The oxidation tests showed the formation of several oxides at all temperatures studied and that from 900 °C on alfa Al₂O₃ is formed both in air and O₂ except under O₂ at 1000 °C. It is believed that the Cr(Al,Nb) phase acts as an Al reservoir for the formation of the various Al₂O₃ scales.     

On the crystal structure of new series of compounds, RPt2+xSb2−y (x = 0.125, y = 0.25; R = La, Ce, Pr)

A new compound CePt_2+xSb_2−y (x = 0.125, y = 0.25) was synthesized by arc-melting of the elements. The chemical and structural characterizations were carried out at room temperature on as-cast samples using X-ray diffractometry, metallographic analysis and EDS-microanalysis. According to the results of X-ray single crystal diffraction this antimonide crystallizes in I4cm space group (no. 108), Z = 32, ρ = 12.19 Mg/m³, μ = 89.05 mm⁻¹ (a = 12.5386(3) Å, c = 21.4692(6) Å (crystal I) and a = 12.5455(2) Å, c = 21.4791(5) Å (crystal II)). The structure and composition were confirmed by powder X-ray diffraction (a = 12.4901(2) Å, c = 21.3620(4) Å) and EDS-microanalysis respectively. Isotypic compounds were observed with La and Pr from X-ray powder diffraction of as-cast alloys at room temperature (a = 12.6266(4) Å, c = 21.4589(6) Å for LaPt_2+xSb_2−y and a = 12.5184(5) Å, c = 21.4178(7) Å for PrPt_2+xSb_2−y). The CePt_2+xSb_2−y structure is derived from CaBe₂Ge₂ (a = 2a₀ − 2b₀, b = 2a₀ + 2b₀, c = 2c₀) and comprises a new atomic arrangement with both vacancy on 4(b) pyramidal site and substitution of antimony atoms (X) by platinum (B) in the B–XX–B layers (referring to the subcell structure) forming two B––1/2B1/2XX–3/4B and two X–BB–X layers per cell. The structure of CePt_2+xSb_2−y is compared with those reported before for URh_1.6As_1.9 and CeNi_1.91As_1.94.     

Electronic structure of layer type tungsten metal dichalcogenides WX2 (X = S, Se) using Compton spectroscopy: Theory and experiment

In this paper, we report the first ever experimental Compton profile study of WS₂ and WSe₂ employing 20 Ci ¹³⁷Cs Compton spectrometer. To interpret our experimental data, the electronic properties of these compounds have been determined by linear combination of atomic orbitals, full potential linearised augmented plane-wave and spin polarised relativistic Korringa–Kohn–Rostoker (SPR-KKR) schemes. The band structure calculations show that both the WS₂ and WSe₂ are indirect-gap semiconductors. The SPR-KKR calculations are found to be relatively in poor agreement with the experimental electron momentum densities. The relative nature of bonding in both the dichalcogenides is explained in terms of equal-valence-electron-density profiles, Mulliken's population and valence band charge densities.     

First-principles calculations of structural energetics of Cu–TM (TM = Ti,Zr, Hf) intermetallics

A systematic and comprehensive study of cohesive properties of Cu–TM (TM = Ti, Zr, Hf) intermetallics is carried out using a first-principles method. Specifically, the total energies and equilibrium cohesive properties of 95 intermetallics in the Cu–TM (TM = Ti, Zr, Hf) systems are calculated employing electronic density-functional theory (DFT), ultrasoft pseudopotentials and the generalized gradient approximation for the exchange-correlation energy. The intermetallic phases considered in our first-principles investigation are classified as stable, metastable and virtual types. The concentration dependence of the heat of formation (ΔE_f) in the Cu–Ti system is only slightly asymmetric, while in the Cu–Zr and Cu–Hf systems they are distinctly asymmetric, being skewed towards the Cu-rich side with a minimum in ΔE_f at Cu₁₀TM₇. Based on the observed differences between ab initio and calorimetric heat of formation, we conclude that additional careful experiments are needed to validate ab initio alloy energetics. We also note that the calphad model parameters representing alloy energetics vary significantly from one assessment to another in these systems. For the stable intermetallics, the calculated zero-temperature lattice parameters agree to within ±1% of experimental data at ambient temperature. For the stable phases with unit cell-internal degree(s) of freedom, the results of ab initio calculations show a good agreement when such data are available from X-ray and other diffraction results. For intermetallic compounds where no such experimental data are available, we provide optimized unit cell geometries which may be verified in future experiments. For most structures we also provide zero-temperature bulk moduli and their pressure derivatives, as defined by the equation of state. The bonding between Cu and Zr is discussed based on the analyses of density of states and bonding charge densities in Cu₅Zr and CuZr.     

Cu/X(X=Cr,Nb)纳米多层膜力/电学性能的尺度依赖性

利用纳米压痕实验以及四探针法,系统研究了相同层厚Cu/X(X=Cr,Nb)纳米金属多层膜的力学性能(强/硬度)和电学性能(电阻率)的尺度依赖性.微观分析表明:Cu/X多层膜调制结构清晰,Cu层沿{111}面择优生长,X层沿{110}面择优生长.纳米压入结果表明,Cu/X多层膜的强度依赖于调制周期,并随调制周期的减小而增加.多层膜变形机制在临界调制周期(λ~c≈25 nm)由Cu层内单根位错滑移转变为位错切割界面.多层膜的电阻率不仅与表面/界面以及晶界散射相关,而且在小尺度下受界面条件显著影响.通过修正的FS-MS模型可以量化界面效应对多层膜电阻率的影响.Cu/X纳米多层膜可以通过调控微观结构实现强度-电导率的合理匹配.     

Syntheses and single-crystal structures of La3AgSnS7, Ln3MxMS7 (Ln = La, Ho, Er; M = Ge, Sn; 1/4 ≤ x ≤ 1/2)

In our investigation of non-centrosymmetric rare earth sulfides in the La₃AgSnS₇/KBr, LaAlGeS₅/NaBr, HoAlGeS₅/KBr, ErAlGeS₅/NaBr, Er₃AgGeS₇/KBr and La₃NaSnS₇/NaBr systems, five compounds belonging to the R₆B₂C₂Q₁₄ family have been obtained. These compounds crystallize in the P6₃ space group, and the crystal data are as follows—La₃AgSnS₇: a = 10.3780(15) Å, c = 5.9900(12) Å, Z = 2; La₃Ge_0.25GeS₇: a = 10.2970(15) Å, c = 5.8120(12) Å, Z = 2; Ho₃Ge_0.272(10)GeS₇: a = 9.6480(14) Å, c = 5.7920(12) Å, Z = 2; Er₃Ge_0.330(10)GeS₇: a = 9.5930(14) Å, c = 5.8490(12) Å, Z = 2; La₃Sn_0.25SnS₇: a = 10.2770(15) Å, c = 6.0030(12) Å, Z = 2. Single-crystal analysis indicated that the crystal structures consist of three types of building block: LnS_n, MS₄, and AgS₃ (for La₃AgSnS₇) or MS₆ units (for Ln₃M_xMS₇, Ln = La, Ho, Er; M = Ge, Sn; 1/4 ≤ x ≤ 1/2), as any other compounds belonging to the R₆B₂C₂Q₁₄ family. Ln₃M_xMS₇ (Ln = La, Ho, Er; M = Ge, Sn; 1/4 ≤ x ≤ 1/2) are deficient compounds with the B sites occupied partly by M(II), and/or M(IV).