In situ TEM study of microplasticity and Bauschinger effect in nanocrystalline metals

In situ transmission electron microscopy straining experiments with concurrent macroscopic stress–strain measurements were performed to study the effect of microstructural heterogeneity on the deformation behavior of nanocrystalline metal films. In microstructurally heterogeneous gold films (mean grain size d_m = 70 nm) comprising randomly oriented grains, dislocation activity is confined to relatively larger grains, with smaller grains deforming elastically, even at applied strains approaching 1.2%. This extended microplasticity leads to build-up of internal stresses, inducing a large Bauschinger effect during unloading. Microstructurally heterogeneous aluminum films (d_m = 140 nm) also show similar behavior. In contrast, microstructurally homogeneous aluminum films comprising mainly two grain families, both favorably oriented for dislocation glide, show limited microplastic deformation and minimal Bauschinger effect despite having a comparable mean grain size (d_m = 120 nm). A simple model is proposed to describe these observations. Overall, our results emphasize the need to consider both microstructural size and heterogeneity in modeling the mechanical behavior of nanocrystalline metals.     

Peculiar elastic behavior of Ti–Nb–Ta–Zr single crystals

The cause of a low Young’s modulus was investigated in quaternary β-type Ti–Nb–Ta–Zr alloys, as the modulus is decreased to prevent bone absorption and degradation of bone quality when these alloys are implanted into human bones. This investigation was carried out using the alloys′ single crystals. Acoustic measurements and analysis by the Hill approximation revealed that a low Young’s modulus in a polycrystalline form is caused by the low shear modulus c′, related to the low β-phase stability, low c₄₄, and relatively low bulk modulus B compared with those of binary Ti-based alloys. Furthermore, it was found that the single crystals had strong orientation dependence on Young’s modulus, where that in the 〈1 0 0〉-direction E₁₀₀ is the lowest of all crystallographic orientations. For quaternary Ti–29Nb–13Ta–4.6Zr alloy (mass%), E₁₀₀ is only ∼35 GPa, which is similar to Young’s modulus of human cortical bones as a result of the low B and c′. These results indicate that decreases in c′, c₄₄ and B are essential for decreasing Young’s modulus of novel β-type Ti alloys which are expected to be developed in the near future.     

Hardening of pure metals by high-pressure torsion: A physically based model employing volume-averaged defect evolutions

A physically based model to predict the increment of hardness and grain refinement of pure metals due to severe plastic deformation by high-pressure torsion (HPT) is proposed. The model incorporates volume-averaged thermally activated dislocation annihilation and grain boundary formation. Strengthening is caused by dislocations in the grain and by grain boundaries. The model is tested against a database containing all available reliable data on HPT-processed pure metals. It is shown that the model accurately predicts hardening and grain size of the pure metals, irrespective of crystal structure (face-centred cubic, body-centred cubic and hexagonal close packed). Measured dislocation densities also show good correlation with predictions. The influence of stacking fault energy on hardening is very weak (of the order of ?0.03 GPa per 100 J mol^?1).     

A method for measuring single-crystal elastic moduli using high-energy X-ray diffraction and a crystal-based finite element model

This paper presents a method – based on high-energy synchrotron X-ray diffraction data and a crystal-based finite element simulation formulation – for understanding grain scale deformation behavior within a polycrystalline aggregate. We illustrate this method by using it to determine the single-crystal elastic moduli of β21s, a body-centered cubic titanium alloy. We employed a polycrystalline sample. Using in situ loading and high-energy X-rays at the Advanced Photon Source beamline 1-ID-C, we measured components of the lattice strain tensor from four individual grains embedded within a polycrystalline specimen. We implemented an optimization routine that minimized the difference between the experiment and simulation lattice strains. Sensitivity coefficients needed in the optimization routine are generated numerically using the finite element model. The elastic moduli that we computed for the β21s are C₁₁ = 110 GPa, C₁₂ = 74 GPa and C₄₄ = 89 GPa. The resulting Zener anisotropic ratio is A = 5.     

Kinetic stages in the crystallization of deeply undercooled body-centered-cubic and face-centered-cubic metals

Crystallization velocities in several face-centered-cubic (fcc) and body-centered-cubic (bcc) metals are calculated using molecular dynamics computer simulations for the (1 0 0) and densely packed (1 1 1) or (1 1 0) planar interfaces. We show that the crystallization kinetics can be divided into high- and low-temperature regimes, separated at a crossover temperature, T_c, which is associated with kinetic arrest. In the high-temperature regime, the velocity in both fcc and bcc metals initially increases with the degree of undercooling before reaching a maximum somewhat above the glass temperature. The kinetics is characterized by a thermally activated process. In the low-temperature regime, stresses develop in the interface and reduce the apparent activation energies for interface mobility. For the fcc metals (Cu, Ni, Ag and Pt) the activation energies fall essentially to zero, indicating an athermal process. For bcc metals (Fe, Mo, V, Ta) the activation energies remain finite, varying from ≈0.013 eV (Ta) to ≈0.2 eV (Mo).     

Ductile tensile failure in metals through initiation and growth of nanosized voids

We here reveal the initiation of ductile failure in metals at the nanometer scale by molecular dynamics simulations coupled with a novel analytical model. This proceeds by the emission of a special type of dislocation shear loop, which can expand as a partial or perfect dislocation, evolve into a prismatic loop through reaction, or develop into twins. Molecular dynamics (MD) simulations predict a strong dependence of the stress required for the initiation of plastic flow at the surface of the void for both Cu (a model fcc metal) and Ta (a model bcc metal). The decrease in stress with increasing void size is also analyzed in terms of a new analytical approach based on the energetics of dislocation loop emission. For both fcc (copper) and bcc (tantalum) metals initiation of plastic flow in MD simulations takes place at voids as small as a tri-vacancy (radius R ≈ 0.1 nm). Extensive calculations for tantalum combined with the analytical model, which tracks the simulations, enable extrapolation to R ≈ 300 nm, in the realm of second phase particles and inclusions. Thus we conclude that this is a general mechanism of tensile failure in pure monocrystalline metals where other initiation sites are absent.     

Tensile and compressive behavior of tungsten,molybdenum, tantalum and niobium at the nanoscale

In situ nanomechanical tests are carried out to investigate the tensile and compressive behavior of 〈0 0 1〉-oriented body-centered cubic (bcc) metals W, Mo, Ta and Nb with nanometer dimensions. We find that the strength of these metals exhibits strong size dependence. The compressive size effect in Nb, as evaluated by the log–log slope of strength vs. nanopillar diameter, is ?0.93, a factor of 2.1 greater than that for the other three metals W, Mo and Ta (?0.44). In tension, however, Ta and Nb show higher size effect slopes (?0.80 and ?0.77) as compared with W and Mo (?0.58 and ?0.43). We also report that while the yield strength of these metals is a strong function of size, the strain-hardening behavior does not present any size-dependent trends. We further discuss the effects of strain-rate on deformation behavior and provide transmission electron microscopy analysis of microstructural evolution in the same Mo nanopillar before and after compression.     

Shape memory behavior of Ti–Ta and its potential as a high-temperature shape memory alloy

In this study, the effect of Ta content on shape memory behavior of Ti–Ta alloys was investigated. The shape memory effect was confirmed in Ti–(30–40)Ta alloys. The martensitic transformation start temperature (M_s) decreased by 30 K per 1 at.% Ta. The amount of ω phase formed during aging decreased with increasing Ta. A stable high-temperature shape memory effect was confirmed for Ti–32Ta (M_s = 440 K) during thermal cycling between 173 and 513 K. On the other hand, the high-temperature shape memory effect of Ti–22Nb, which has a similar M_s to Ti–32Ta, exhibited poor stability due to the large amount of ω phase formed during thermal cycling. It is suggested that Ti–Ta is an attractive candidate for the development of novel high-temperature shape memory alloys.     

Nanoquasicrystalline Al–Fe–Cr-based alloys. Part II. Mechanical properties

Nanoquasicrystalline Al-based alloys show considerable promise for elevated temperature applications compared with commercial Al-based alloys. In particular, a group of Al–Fe–Cr-based alloys-containing Ti, V, Nb or Ta have outstanding thermal stability. In the present work, the elevated temperature mechanical properties of these nanoquasicrystalline alloys were studied by tensile tests at a constant strain rate. Tests were designed in order to compare the mechanical behaviour at different test temperatures. Fractographic analysis was also carried out. The apparent activation energy for plastic deformation was found to be close to that for lattice self-diffusion for pure Al in the Al–Fe–Cr ternary alloy and in the Ti-containing alloy, and for grain boundaries diffusion for pure Al in the V-containing alloy, whereas the activation energy of the alloy with Ta additions was three times higher. All of the alloys showed similar sensitivity of plastic deformation to the strain rate in the range of 10^?3–5 × 10^?6 s^?1 at 350 °C. The apparent true stress exponent was n_app ～ 7, which can be associated with a deformation process controlled by dislocation mechanisms.     

Grain size effects on contact resistance of top-contact pentacene TFTs

Multiple top-contact OTFTs with various channel lengths (L_c) were successfully scaled-down to the L_c of 1.8 μm by using the membrane shadow mask and the interface between the evaporated Au and pentacene was analyzed based on the channel resistance method. For large grain pentacene (S-80) deposited at 80 °C, the parasitic resistance (R_p) at V_GS = −20 V has 1.8 ± 0.2 kΩ cm, whereas for small grain pentacene (S-20) deposited at 20 °C has 4.2 ± 0.2 kΩ cm, which means that R_p depends on the grain size of pentacene. The grain size and grain boundary trap density for pentacene can be possibly origins to determine R_p, which is critically correlated with bulk transport in pentacene. The grain boundary trap density (N_t) for S-80 and S-20 was extracted as (5.6 ± 0.5) × 10¹¹ and (1.2 ± 0.3) × 10¹² cm⁻² from the Levinson plots, respectively. In addition, activation energy of R_p for S-80 is in the range from 42 to 48 meV, whereas for S-20 is from 72 to 108 meV.     

On-chip laboratory suite for testing of free-standing metal film mechanical properties,Part II – Experiments

In this paper, we demonstrate the fabrication of electrostatically loaded, free-standing Al–0.5 wt.%Cu thin-film samples, realizing a near-zero compliance support post. We measure Young’s modulus E = 74 GPa using cantilevers, in good agreement with grain texture measurements. We measure residual stress σ_R ranging from 30 to 60 MPa using fixed–fixed beams and find that processing induces significant plastic straining, which leads to residual stress values significantly less than the as-deposited value. Strength of this alloy is at least 172 MPa if the film is not severely strained, and the material exhibits no room-temperature fatigue up to 1 billion cycles at this stress level. Notched devices that have been subjected to process-induced plastic straining of ∼4% are weaker and fatigue logarithmically with the number of cycles. We compare deformation processes on the samples using ex situ TEM. The mechanism for the high strength value is attributed to the grain size and the thin surface oxide which constrain dislocation glide, while fatigue of the highly strained material is associated with the appearance of persistent slip bands.     

Growth behavior of superconducting DyBa2Cu3O7–x thin films deposited by inclined substrate deposition for coated conductors

Superconducting DyBa₂Cu₃O_7–x (DyBCO) films were grown on biaxially textured MgO buffer layers deposited by inclined substrate deposition (ISD) on Hastelloy substrates. Despite the large lattice mismatch (8.5%) between DyBCO and MgO, the DyBCO grew epitaxially on the MgO buffer layer and the biaxial texture of the MgO was well transferred to the DyBCO. Typical critical current densities, j_c, of the DyBCO film were 2.1 MA cm^?2 at 77 K in a self-field. Biaxial texturing is the key for reaching the high critical current densities and was investigated by transmission electron microscopy. DyBCO grains were found to be ～130–500 nm in size, with faceted grain boundaries. The c-axis of the DyBCO grains was tilted away from the substrate normal by 29° such that it was perpendicular to the MgO (0 0 2) facets. A high dislocation density of ～7.4 × 10¹¹ cm^?2 and stacking faults along the ab-planes were observed in the DyBCO film. Interface, grain boundary and volume energies of the DyBCO film were calculated and a growth model for the DyBCO film is discussed. ISD offers the potential for high-quality, biaxially textured MgO buffer layers suitable for long-length superconducting coated conductors.     

Elastic anisotropy of γ′-Fe4N and elastic grain interaction in γ′-Fe4N1−y layers on α-Fe: First-principles calculations and diffraction stress measurements

The three independent single-crystal elastic-stiffness constants C_ij of cubic γ′-Fe₄N (face-centred cubic (fcc)-type iron substructure) have been calculated by first-principles methods using the density functional theory: C₁₁ = 307.2 GPa, C₁₂ = 134.1 GPa and C₄₄ = 46.0 GPa. The Zener elastic-anisotropy ratio, A = 2C₄₄/(C₁₁ − C₁₂) = 0.53, is strikingly less than 1, implying 〈1 0 0〉 as stiffest directions, whereas all fcc metals show A > 1. This elastic anisotropy is ascribed to the ordered distribution of N on the octahedral interstitial sites. X-ray diffraction lattice-strain measurements for a set of different h k l reflections recorded from γ′-Fe₄N_1−y layers on top of α-Fe confirmed the “abnormal” elastic anisotropy of γ′-Fe₄N_1−y. Stress evaluation, yielding a compressive stress of about −670 MPa parallel to the surface, was performed on the basis of effective X-ray elastic constants determined from the calculated single-crystal elastic constants C_ij and allowing a grain interaction intermediate between the Reuss and the Voigt models.     

Microstructural evolution in copper subjected to severe plastic deformation: Experiments and analysis

The evolution of microstructure and the mechanical response of copper subjected to severe plastic deformation using equal channel angular pressing (ECAP) was investigated. Samples were subjected to ECAP under three different processing routes: B_C, A and C. The microstructural refinement was dependent on processing with route B_C being the most effective. The mechanical response is modeled by an equation containing two dislocation evolution terms: one for the cells/subgrain interiors and one for the cells/subgrain walls. The deformation structure evolves from elongated dislocation cells to subgrains to equiaxed grains with diameters of ∼200–500 nm. The misorientation between adjacent regions, measured by electron backscatter diffraction, gradually increases. The mechanical response is well represented by a Voce equation with a saturation stress of 450 MPa. Interestingly, the microstructures produced through adiabatic shear localization during high strain rate deformation and ECAP are very similar, leading to the same grain size. It is shown that both processes have very close Zener–Hollomon parameters (ln Z ∼ 25). Calculations show that grain boundaries with size of 200 nm can rotate by ∼30° during ECAP, thereby generating and retaining a steady-state equiaxed structure. This is confirmed by a grain-boundary mobility calculation which shows that their velocity is 40 nm/s for a 200 nm grain size at 350 K, which is typical of an ECAP process. This can lead to the grain-boundary movement necessary to retain an equiaxed structure.     

Increase of Co solubility with decreasing grain size in ZnO

Nanograined (grain size 10 nm) ZnO films with various Co contents (0–52 at.% Co) were synthesized by a novel liquid ceramics method. The solubility limit for Co was determined at 550 °C. The lattice parameter c of the ZnO-based solid solution with würzite structure ceases to grow at 33 at.% Co. The peaks of the second phase (Co₂O₃ or with cubic lattice) become visible in the X-ray diffraction spectra at 40 at.% Co. The same second phase appears in the bulk ZnO already at 2 at.% Co [Bates CH, White WB, Roy R. J Inorg Nucl Chem 1966;28:397]. A few years ago it was predicted theoretically that ZnO could become ferromagnetic at room temperature and above by doping with Co and other transition metals. Recently published papers on the structure and magnetic behaviour of Co-doped ZnO allowed us to obtain the size dependence of Co solubility in ZnO for the polycrystals and small single-crystalline particles. The overall Co solubility drastically increases with decreasing grain size. The quantitative estimation leads to the conclusion that, close to the bulk solubility limit, the thickness of a Co-enriched layer is several monolayers in grain boundaries and at least two monolayers in the free surfaces.     

Nanoplastic deformation of nanoindentation: Crystallographic dependence of displacement bursts

The present paper summarizes the crystallographic dependence of the displacement bursts in nanoindentation using single crystalline aluminum and copper with three kinds of surface indices, namely (0 0 1), (1 1 0) and (1 1 1). It is found that the experimental linear relation between the critical indent load and the burst width of the indent depth at the first displacement burst has significant crystallographic dependence. From the critical indent loads, the critical resolved shear stresses of the dislocation nucleation were estimated to be 3.3 GPa for Al and 3.6 GPa for Cu, using Hertz contact mechanics, which are both close to the ideal values. We explain the nanoplastic mechanics by a comprehensive energy balance model to describe the linear relation between the indent load and the burst width, and by the collective dislocation nucleation model consisting of three-dimensional dislocation loops to evaluate the number of dislocations nucleating. The former model can derive the linear relation qualitatively and link the burst width to the collective dislocation slipping. The latter relates the theoretical critical load and burst width with the function of the punch radius and the distance between dislocations nucleating d_D, in which setting d_D as about 10b (b is the Burgers vector) can fairly represent the Al experimental data.     

Nanoscaled grain boundaries and pores,microstructure and mechanical properties of translucent Yb:[LuxY(1−x)O3] ceramics

Translucent ceramics of Yb:[Lu_xY_(1?x)O₃] system doped by ZrO₂ was sintered from nanopowder synthesized by laser evaporation. The relative density of the ceramics was 99.97%, residual pores had sizes from 8 nm to 20 nm, Young modulus was 200 GPa at the applied load of 2000 mN, the microhardness was 12.8 GPa. The grains of ceramics had sizes 1–10 μm, but the thickness of grain boundaries was about 1 nm. The transcrystalline type of the crack propagation was detected in the specially broken ceramics. The results indicated high strength of grain bonds and good perfection of grain boundaries in the studied ceramics but an increased content of pores (higher than 10^?3 vol.%) and stoichiometry deviation (Lu:Y:O = 0.21:0.79:3) from the required one (Lu:Y:O = 0.25:0.75:3).     

Mechanical properties of micro-sized copper bending beams machined by the focused ion beam technique

Micro-sized bending beams with thicknesses, t, from 7.5 down to 1.0 μm were fabricated with the focused ion beam technique from a copper single crystal with an {1 1 1}〈0 1 1〉 orientation. The beams were loaded with a nano-indenter and the force vs. displacement curves were recorded. A strong size effect was found where the flow stress reaches almost 1 GPa for the thinnest beams. A common strain gradient plasticity approach was used to explain the size effect. However, the strong t^−1.14 dependence of the flow stress could not be explained by this model. Additionally, the combination of two other dislocation mechanisms is discussed: the limitation of available dislocation sources and a dislocation pile-up at the beam centre. The contribution of the pile-up stress to the flow stress gives a t⁻¹ dependence, which is in good agreement with the experimental results.     

The effect of vacuum annealing on the microstructure,mechanical and electrical properties of tantalum films

The microstructure, mechanical and electrical properties of vacuum annealed tantalum films were studied. X-ray diffraction spectra confirmed the presence of mixed (α and β) phases in the as-deposited Ta films. After vacuum annealing (at 750 °C for 1 h), the metastable β-phase was completely transformed to stable α-phase. The grain size increased (from 35 ± 3 nm to 92 ± 3 nm) with the increase in annealing temperature. The mixed (α and β) phases resulted in higher hardness and higher Young's modulus. The film annealed at 750 °C for 1 h exhibited lower resistivity (52 ± 4 μΩ-cm), lower hardness (H = 10.4 ± 1.3 GPa) and lower Young's modulus (Y = 185 ± 5 GPa) as compared to the as-deposited and annealed (at temperature < 750 °C) films. This is attributed to the phase transformation from β to α at an annealing temperature of 750 °C.     

Preparation of WC nanoparticles by twice ball milling

In order to improve the ball milling efficiency of WC powders and thus to fabricate nano-grained WC–Co cemented carbides with high mechanical properties, WC nanoparticles were prepared by twice ball milling in nylon vessels. The best technology to disperse WC powders in alcohol was investigated at first. Based on the dispersion results, 2 wt.% PEG was used with La₂O₃ as additive to improve ball milling efficiency. The particle size, crystal structure, surface morphology and surface properties were tested by a laser particle sizer, XRD, FE-SEM and FT-IR, respectively. During the first ball milling, sample d achieved the best milling performance, including average particle size (168 nm) and grain size (27.2 nm) among samples a (pure WC), b (with PEG), c (with La₂O₃) and d (with PEG and La₂O₃). La₂O₃ could greatly decrease particle size and grain size while PEG could narrow particle size distribution. During the second milling, the particle size and grain size of sample d reached 89 nm and 13.2 nm at 96 h, respectively. The results indicated that twice ball milling can greatly improve particle size and grain size compared with the first ball milling, and further narrow the size distribution. In conclusion, multiple ball milling can reduce the particle size of certain powders with suitable milling technology.