Grain growth in AZ31 alloy after uniaxial compression

The grain growth morphology,kinetics and texture change after uniaxial compression at 430 ℃ of an extruded AZ31 alloy were studied.The samples were loaded following two routes insuring two initial textures of the samples with compression direction parallel and normal to the extrusion direction.For both initial textures,a stable grain size is attained upon isothermal annealing and the grain growth kinetics can be described by:dn= dRn+kt with an n value of around 15.The annealing texture with grown grains is a retained hot deformation texture without emerging or strengthening other components.Abnormal grain growth is not observed for annealing time up to 10 000 h at 450℃.     

Size effects on the mechanical properties of thin polycrystalline metal films on substrates

An analysis of the effects of the thickness and grain size of polycrystalline thin films on substrates is presented with the objective of linking the film mismatch stress to the underlying characteristic size scales. The model is predicated on the notion that the relaxation of mismatch strain in the film is accommodated by the introduction of dislocation loops whose population, dimensions and interaction energies are controlled by the film thickness and microstructural dimensions. The model is capable of capturing the combined effects of these size scales by accounting for the interaction energies of the constrained dislocation structure, and provides quantitative predictions of the evolution of film stress during thermal excursions. The predictions of the analysis are compared with available experimental results for polycrystalline films of face-centered cubic materials on Si substrates. It is shown that the model correctly predicts the observed influence of film thickness and grain size on stress evolution during thermal excursions. Aspects of strain hardening in thin polycrystalline films with high dislocation densities are also discussed.     

Simulation of faceted film growth in two-dimensions: microstructure,morphology and texture

A series of simulations of the growth of polycrystalline, faceted films from randomly oriented nuclei in two spatial dimensions was performed. The simulations track the motion of all corners where facets from the same grain and different grains meet. Results are presented on the temporal evolution of the mean grain size, grain size distribution, surface roughness, crystallographic texture and growth zones as a function of α, a parameter describing the relative facet velocities. The mean grain size and r.m.s. surface roughness are shown to be a parabolic function of the film thickness, in agreement with experiment and theoretical results. The grain size distribution is temporally self-similar when scaled by the mean grain size and has the form of a gamma distribution. The crystallographic orientation distribution (i.e. texture) is Gaussian, peaks at an α-dependent orientation and the peak sharpens during film growth. The peak position is well described by the largest radius vector of the appropriate idiomorph in two and three dimensions. The grain size, roughness and texture evolution are intimately linked. This type of simulation may be used to examine the evolution of the microstructure of any film which exhibits faceted growth with prescribed facet velocities.     

The origin of stresses in magnetron-sputtered thin films with zone T structures

In order to understand the origin of the residual stress state in thin films and its thickness dependence, the structure–stress relation of magnetron-sputtered Cr and CrN layers with thicknesses ranging from 100 nm to 3 μm was investigated in detail. Based on correlations between the layer-thickness-dependent grain size, texture and morphology and the magnitude of the intrinsic and thermal components of the residual stress, a model is proposed that explains the origin of internal stresses of thin polycrystalline films with zone T structures. The model was further extended for the CrN/Cr dual-layer system, where the CrN top layer is epitaxially aligned with the underlying highly (2 0 0)-oriented Cr interlayer. It is shown for the first time that both the intrinsic and thermal stress components are thickness-dependent, which is associated with the layer microstructure.     

晶粒尺寸与保载载荷对Cu膜纳米压入蠕变性能的影响

利用纳米压入仪对Si片上的多晶Cu膜进行压入蠕变研究．实验结果显示晶粒尺寸大于200nm时，应力指数对晶粒尺寸不敏感．当晶粒尺寸小于200nm时，因压头底部更多的晶粒参与变形，应力指数随晶粒尺寸的降低而增大．认为薄膜材料存在一个对应力指数不敏感的最小If缶界晶粒尺寸ιc，Cu膜的应力指数随保载载荷增大而增大，其主要原因在于高载荷下位错强化机制使蠕变率降低。     

Microstructure and grain growth kinetics of nanocrystalline SnO2 thin films prepared by pulsed laser deposition

The isothermal grain growth of SnO₂ thin films prepared by pulsed laser deposition techniques was investigated at Si (100) substrate temperatures between 300 and 450 °C with 50 °C intervals for different annealing times. X-ray diffraction patterns proved that the average grain sizes are in the range of 2.4–27.8 nm. The grain growth data were analyzed using two different models. The first model, assuming normal grain growth as that in conventional polycrystalline materials, yields large grain growth exponent (n) and extremely low activation energy (Q). Although it can describe the evolution of grain sizes, it fails to give satisfactory physical interpretation of n and Q, both beyond the theoretical predictions. The second model is based on the structural relaxation of the interface component in nanocrystalline materials. In this case, the ordering of distorted interfaces by structural relaxation proceeds with grain growth. This structure relaxation model not only describes the evolutions of grain growth well, but also makes reasonable attribution of the low activation energy to the short-range rearrangement of atoms in the interface region as well.     

A reaction-layer mechanism for the delayed failure of micron-scale polycrystalline silicon structural films subjected to high-cycle fatigue loading

A study has been made to discern the mechanisms for the delayed failure of 2-μm thick structural films of n⁺-type, polycrystalline silicon under high-cycle fatigue loading conditions. Such polycrystalline silicon films are used in small-scale structural applications including microelectromechanical systems (MEMS) and are known to display ‘metal-like’ stress-life (S/N) fatigue behavior in room temperature air environments. Previously, fatigue lives in excess of 10¹¹ cycles have been observed at high frequency (~40 kHz), fully-reversed stress amplitudes as low as half the fracture strength using a surface micromachined, resonant-loaded, fatigue characterization structure. In this work the accumulation of fatigue-induced oxidation and cracking of the native SiO₂ of the polycrystalline silicon was established using transmission electron and infrared microscopy and correlated with experimentally observed changes in specimen compliance using numerical models. These results were used to establish that the mechanism of the apparent fatigue failure of thin-film silicon involves sequential oxidation and environmentally-assisted crack growth solely within the native SiO₂ layer. This ‘reaction-layer fatigue’ mechanism is only significant in thin films where the critical crack size for catastrophic failure can be reached by a crack growing within the oxide layer. It is shown that the susceptibility of thin-film silicon to such failures can be suppressed by the use of alkene-based monolayer coatings that prevent the formation of the native oxide.     

Solid solution strengthening in polycrystals of Mg-Sn binary alloys

Solid solution effects on the hardness and flow stress in Mg-Sn binary alloys with Sn content between 0.18% and 2.18% at temperatures ranging from ambient to 623 K were investigated in this study. At room temperature, the hardness increases with the Sn content as H_v0.5 = 28.3 + 6.88c, the 0.2% proof strength (corrected for grain size strengthening effect) and cⁿ follow a linear relationship, where c is the solute atom fraction and n = 1/2 or 2/3. The results suggest that the strengthening of basal planes controls the solid solution strengthening in polycrystals of Mg-Sn binary alloys. However, the cⁿ power-law is not applicable in the temperature range from 423 K to 623 K, which is proposed to be ascribed to the competition between solid solution strengthening and softening effect.     

Discrete and continuous deformation during nanoindentation of thin films

This paper describes nanoindentation experiments on thin films of polycrystalline Al of known texture and different thicknesses, and of single crystal Al of different crystallographic orientations. Both single-crystalline and polycrystalline films, 400–1000 nm in thickness, are found to exhibit multiple bursts of indenter penetration displacement, h, at approximately constant indentation loads, P. Recent results from the nanoindentation studies of Suresh et al. (Suresh, S., Nieh T.-G. and Choi, B.W., Scripta mater., 1999, 41, 951) along with new microscopy observations of thin films of polycrystalline Cu on Si substrates are also examined in an attempt to extract some general trends on the discrete and continuous deformation processes. The onset of the first displacement burst, which is essentially independent of film thickness, appears to occur when the computed maximum shear stress at the indenter tip approaches the theoretical shear strength of the metal films for all the cases examined. It is reasoned that these displacement bursts are triggered by the nucleation of dislocations in the thin films. A simple model to estimate the size of the prismatic dislocation loops is presented along with observations of deformation using transmission electron microscopy and atomic force microscopy. It is demonstrated that the response of the nanoindented film is composed of purely elastic behavior with intermittent microplasticity. The overall plastic response of the metal films, as determined from nanoindentation, is shown to scale with film thickness, in qualitative agreement with the trends seen in wafer curvature or X-ray diffraction measurements.     

Towards interrelationship of grain size,cell parameters and flow stress in type 316L stainless steel

In order to investigate the interrelationship amongst grain size, cell size, cell wall thickness and flow stress in type 316L stainless steel, tensile specimens of grain sizes 3.1–86.7 μm were deformed to a strain of 0.34 at the temperatures 297, 673 and 973 K and at a strain rate 1 × 10^–4 s^–1. The cell wall thickness is found to be larger than the cell interior size but the two are mutually related and only marginally decrease with the increase in grain size. The difference in the distributions of dislocations and cell structure in the vicinity of grain boundaries and in the grain interior leads to a variation in their relative contributions to strengthening. The smaller cell interior sizes and thicker cell walls result in a greater contribution of grain interior to strengthening of polycrystalline material than the grain boundaries, with the maximum effect observed at 673 K due to dynamic strain aging.     

A probabilistic crystal plasticity model for modeling grain shape effects based on slip geometry

A new statistical theory is introduced that takes into account the coupling between grain size, shape and crystallographic texture during deformation of polycrystalline microstructures. A “grain size orientation distribution function” (GSODF) is used to encode the probability density of finding a grain size D along a direction (given by unit vector n) in grains with orientation g. The GSODF is sampled from the input microstructure and is represented in a finite element mesh. During elastoplastic deformation, the evolution of grain size D (in direction θ) and the orientation g is tracked by directly updating the GSODF probabilities using a Lagrangian probability update scheme. The effect of grain shape (e.g. in high aspect ratio grains) is modeled by including the apparent grain size as seen by various different active slip systems in the grain within the constitutive law for the slip system resistance. The prediction of texture and strains achieved by the statistical approach is compared to Taylor aggregate and finite element deformation analysis of a planar polycrystalline microstructure. The role of grain shape and size in determining plastic response is investigated and a new adaptive GSODF model for modeling microstructures with multimodal grain shapes is proposed.     

Tensile testing of free-standing Cu,Ag and Al thin films and Ag/Cu multilayers

Free standing polycrystalline thin films with a strong 〈111〉 texture were tested in uniaxial tension. Studied were electron-beam deposited Ag, Cu and Al films, and Ag/Cu multilayers consisting of alternating Ag and Cu layers of equal thickness, between 1.5 nm and 1.5 μm (bilayer repeat length, λ, between 3 nm and 3 μm). The films had a total thickness of about 3 μm. A thin polymeric two-dimensional diffraction grid was deposited on the film surface by microlithographic techniques. Strains were measured in situ from the relative displacements of two laser spots diffracted from the grid. The average values of the Young’s moduli, determined from hundreds of measurements, are 63 GPa for Ag, 102 GPa for Cu, 57 GPa for Al and 87.5 GPa for Ag/Cu multilayers. In all cases, these values are about 20% lower than those calculated from the literature data and, for the Ag/Cu multilayers, are independent of λ. No “supermodulus” effect was observed. The 20% reduction in modulus is most likely the result of incomplete cohesion (“microcracking”) of the grain boundaries. The ductility of the Ag/Cu multilayers decreases when λ is reduced. For λ<80 nm, the films are brittle at room temperature: they break without macroscopic plastic flow. For λ>80 nm, the yield stress increases with decreasing λ according to a Hall–Petch-type relation. No softening with decreasing grain size was observed even at the lowest values of λ.     

The grain size dependence of flow stress in a Cu–26Ni–17Zn alloy

The effect of grain size in the range 15–120 μm on flow stress was studied at room temperature to investigate the Hall–Petch relationship in a Cu–26Ni–17Zn alloy. It was found that the Hall–Petch relation is valid for the alloy. The Hall–Petch constants, σ_oε and k_ε are related to true strain (ε) in such a way that σ_oε is proportional to ε and k_ε to ε^1/2. An equation for flow stress as a function of true strain and grain size has been derived from these results. Dislocation density model for grain size strengthening is found valid for this alloy. Solid solution strengthening in the Cu–26Ni–17Zn alloy is attributed to the interaction of nickel and zinc atoms with screw dislocations and the effective interaction is more due to modulus mismatch than size misfit.     

Size dependent phase transformation and mechanical behaviors in nanocrystalline Ta thin films

Nanocrystalline (NC) Ta thin films with various thicknesses (t = 600, 1200, and 2200 nm) were grown on Si (100) substrate by using magnetron sputtering system. A phase transformation from β-Ta to α-Ta was observed when the thickness reduced to 600 nm, which is rationalized by employing a thermodynamic model. It was interesting to find that the α-Ta phase exhibited tetrahedral pyramidal grain morphology, while the β-Ta had equiaxial grain. Hardness and strain rate sensitivity (SRS, m) were measured as a function of film thickness t. Compared with the reduction in SRS as the grain size decreased in the submicron bcc metals, the NC Ta thin films with average grain size smaller than ~72 nm showed an opposite trend, both in the α-Ta and in the β-Ta phase. Improved m was observed as the grain size reduced but the increasing trend was not continuous between different phases. This unusual variation trend of m in the NC Ta was explained by a model based on the traditional double-kink mechanism coupled with a GB-mediated dislocation process, which agrees well with the experiment results.     

Bauschinger and size effects in thin-film plasticity

We present an experimental investigation of the effects of surface passivation, film thickness and grain size on the plastic behavior of freestanding Cu thin films. The stress–strain curves of the films are measured using the plane–strain bulge test. Films with a passivation layer on one or both surfaces have an offset yield stress that increases significantly with decreasing film thickness; the yield stress of unpassivated films, by contrast, is relatively independent of film thickness and increases mainly as a result of grain-size strengthening. The stress–strain curves of passivated films show an unusual Bauschinger effect on unloading. This effect is not observed for unpassivated films. Our experimental results suggest that passivation layers prevent dislocations from exiting the films and that they block slip bands at the film–passivation interface. The back stresses associated with these blocked slip bands increase the resistance to forward plastic flow on loading and cause reverse plastic flow on unloading. The effect of the back stresses increases with decreasing film thickness and leads to the observed strengthening of the passivated films. The constraint of a passivating layer on dislocation motion and hence on plastic flow cannot be described by classical plasticity theories, but can be modeled with some strain–gradient plasticity theories. We evaluate the suitability of the strain–gradient plasticity theory developed by Fleck and Hutchinson to describe our experimental results in a continuum framework. Comparison between experimental results and calculations yields very good agreement for the effect of film thickness, but the strain–gradient plasticity model fails to describe the Bauschinger effect observed in passivated films.     

Effects of oxygen partial pressure on film growth and electrical properties of undoped ZnO films with thickness below 100 nm

The dependences of electrical and structural properties on film thickness below 100 nm have been studied on polycrystalline undoped zinc oxide (ZnO) thin films on glass substrates at 200 °C prepared by plasma-assisted electron-beam deposition. From Hall effect measurements, we find that resistivity decreases from 0.47 to 0.02 Ω cm with increasing film thickness, whereas carrier concentration remains almost constant, 1.65-2.0 × 10¹⁹ cm^− 3, Hall mobility increases from 1.7 to 16.7 cm²/Vs with increasing film thickness. From both high-resolution out-of-plane and in-plane X-ray diffraction (XRD) data, we find substantial changes in the lattice parameters with increasing film thickness below 40 nm; a reduction in the lattice parameter of the a-axis and an increase in the lattice parameter of the c-axis. Williamson-Hall analysis reveals an increase in in-plane grain size with increasing film thickness. This indicates that the dominant scattering mechanism that determines electrical properties is a boundary scattering mechanism.     

Study of the size effects in the electrical resistivity of ultrathin (

Saci Messaadi  Hadria Medouer  Mosbah Daamouche 《Journal of Alloys and Compounds》2010,489(2):609-613

Stress fields and geometrically necessary dislocation density distributions near the head of a blocked slip band

We have examined the interaction of a blocked slip band and a grain boundary in deformed titanium using high-resolution electron backscatter diffraction and atomic force microscopy. From these observations, we have deduced the active dislocation types and assessed the dislocation reactions involved within a selected grain. Dislocation sources have been activated on a prism slip plane, producing a planar slip band and a pile-up of dislocations in a near screw alignment at the grain boundary. This pile-up has resulted in activation of plasticity in the neighbouring grain and left the boundary with a number of dislocations in a pile-up. Examination of the elastic stress state ahead of the pile-up reveals a characteristic “one over the square root of distance” dependence for the shear stress resolved on the active slip plane. This observation validates a dislocation mechanics model given by Eshelby, Frank and Nabarro in 1951 and not previously directly tested, despite its importance in underpinning our understanding of grain size strengthening, fracture initiation, short fatigue crack propagation, fatigue crack initiation and many more phenomena. The analysis also provides a method to measure the resistance to slip transfer of an individual grain boundary in a polycrystalline material. For the boundary and slip systems analysed here a Hall–Petch coefficient of K = 0.41 MPa m^½ was determined.     

镍基高温合金低能孪晶界密度与热塑性变形参数的响应关系

为深入理解甚至描述热塑性变形过程中低能孪晶界密度(BLD∑3n)的演化,建立以平均晶粒尺寸和储能为变量的改进孪晶密度模型.对于Nimonic 80A高温合金,在温度范围1273～1423 K、应变速率范围0.001～10 s-1下进行等温压缩和EBSD实验,基于EBSD数据统计的晶粒尺寸和BLD∑3n结果对其孪晶密度模...     

Grain growth in dilute alloys of copper

An experimental study of grain growth in dilute binary alloys of copper was performed. It was found that the grain growth law D = Kt ⁿ fit most of the data. The grain growth exponent n decreased with solute content until a saturation value was obtained at slightly under 0.2 for both silicon and aluminum solutes. It is concluded that the grain growth exponent n depends upon solute adsorption at the grain boundaries. An estimate of the activation energy was made and it was found that the activation energy decreased with temperature. The activation energies for grain boundary migration compare well with values obtained from internal friction studies on identical alloys. This contributes strong experimental support for the two-step mechanism for complete grain boundary stress relaxation.