Grain Size Of Polycrystalline Materials Research Articles

Quantitative analysis of laser-generated ultrasonic wave characteristics and their correlation with grain size in polycrystalline materials

Abstract The quantitative relationship between nanosecond pulsed laser parameters and the characteristics of laser-generated ultrasonic waves in polycrystalline materials was evaluated. The high energy of the pulsed laser with a large irradiation spot simultaneously generated ultrasonic longitudinal and shear waves at the epicenter under the slight ablation regime. An optimized denoising technique based on wavelet thresholding and variational mode decomposition was applied to reduce noise in shear waves with a low signal-to-noise ratio. An approach for characterizing grain size was proposed using spectral center frequency ratio (SCFR) based on time-frequency analysis. The results have demonstrated that the generation regime of ultrasonic waves is not solely determined by the laser power density; even at high power densities, a high energy with a large spot can generate an ultrasonic waveform dominated by the thermoelastic effect. It is ascribed to the intensification of the thermoelastic effect with the proportional increase in laser irradiation spot area for a given laser power density. Furthermore, both longitudinal and shear wave SCFRs are linearly related to grain size in polycrystalline materials; however, the shear wave SCFR is more sensitive to finer-grained materials. This study holds great significance for evaluating metal material properties using laser ultrasound.

Migration Pinning and Roughening Transition of a Ni Grain Boundary

To date, much research has been conducted into the effect of migration pinning on the grain size in polycrystalline materials. However, effects of migration pinning on the grain-boundary structure and its transition have not been illuminated. Here, using transmission electron microscopy (TEM) we have explored the pinning effects for the grain boundary in a Ni bicrystal. During TEM specimen preparation, a hole was intentionally drilled in the middle of the grain boundary as a pinning point against grain-boundary migration. The specimen was heated to 600 °C. The grain boundary is driven to migrate by both the surface energy anisotropy and the total strain energy reduction. Grain-boundary facets with a plane orientation of {0 3 2}//{1 1 1} appear near the hole. The facets undergo a structural transition from atomically flat to rough with increasing distance from the hole. A pinning force exerted by the hole suppresses the migration of the grain boundary near the hole, indicating that the grain-boundary region away from the hole is subjected to a higher driving force. It certainly appears that the phenomenon originates from a change in driving force with the distance from the hole, being a signature of kinetic roughening.

Signature of Kinetic Roughening Revealed by Migration Pinning in a Ni Grain Boundary

To date much research has been conducted into the effect of migration pinning on the grain size in polycrystalline materials. However, effects of migration pinning on the grain-boundary structure and its transition have not been illuminated. Here, using transmission electron microscopy (TEM) we have explored the pinning effects for the grain boundary in a Ni bicrystal. During TEM specimen preparation, a hole was intentionally drilled in the middle of the grain boundary as a pinning point against grain-boundary migration. The specimen was heated to 600oC. The grain boundary is driven to migrate by both the surface energy anisotropy and the total strain energy reduction. Grain-boundary facets with a plane orientation of {0 3 2}//{1 1 1} appear near the hole. The facets undergo a structural transition from atomically flat to rough with increasing distance from the hole. A pinning force exerted by the hole suppresses the migration of the grain boundary near the hole, indicating that the grain-boundary region away from the hole is subjected to a higher driving force. It certainly appears that the phenomenon originates from a change in driving force with the distance from the hole, being a signature of kinetic roughening. The same argument applies to the formation of short (0 1 0)//(1 -1 0) facets developing between {0 3 2}//{1 1 1} facets.

Grain size characterization by laser-based ultrasonics based on the centroid frequency shift method

To solve such problems as the sensitivity of ultrasonic attenuation method for grain size evaluation to geometric spreading, reflection and transmission effects, and the accuracy of ultrasonic echo amplitude measurements, this paper addresses the application of the centroid frequency shift method for characterization of mean grain size in polycrystalline materials. A general model was presented for the correlation of the mean grain size to the downshift in the spectral centroid frequency of a Gaussian pulse during propagation in a medium with a nonlinear frequency dependence of attenuation. The model was further extended to a particular case of laser-ultrasonic inspection, and an explicit formula for the mean grain size was derived in terms of the centroid frequency ratio of two successive echoes under a quadratic frequency assumption. Experimental inspection for mean grain size was then conducted in α-titanium alloys by a laser-based ultrasonic method. Microstructures with different mean grain sizes of 26–49 μm were obtained by annealing at 800°C for different holding times. The centroid frequency shift reveals a linear dependence on the mean grain size. Referring to the metallography analysis, the nondestructive evaluation errors over all the test samples are smaller than 5.4%. The provided results help support the application of the centroid frequency shift method for the grain size evaluation in polycrystalline materials, and this method is more insensitive to effects of wave diffraction, reflection, and transmission in comparison with some other nondestructive methods.

Radiation stability of nanocrystalline single-phase multicomponent alloys

Abstract

A Transfer Function for Relating Mean 2D Cross-Section Measurements to Mean 3D Particle Sizes

It is common practice to estimate mean 3D particle and grain size of polycrystalline materials by multiplying 2D cross-sectional measurements by a multiplication factor. However, the most frequently used multiplication factors apply only to uniform or specific dispersions of particles, and therefore can provide misleading results. In the present work, empirical equations are developed to more accurately predict the mean 3D grain size of a lognormal spherical particle dispersion, regardless of the dispersion’s width. The equations provide an improvement over scalar multiplier values by allowing the effects of particle size distribution to be accounted for using inputs that can be obtained by cross-sectional analysis.

Na incorporated improved properties of Cu2ZnSnS4 (CZTS) thin film by DC sputtering

Polycrystalline CZTS is an emerging candidate for photovoltaic, optoelectronic and gas sensing applications due to its availability and environment-friendly nature and also favorable light harvesting properties. The properties of polycrystalline materials depend upon the grain size. Grain size can be increased by increasing the annealing time, but this may lead to increased cost and also evaporation of certain volatile and low melting materials. Adding some growth enhancing elements is one of the novel methods to improve the grain size of polycrystalline materials. In this study, we studied Na induced CZTS film prepared by sputtering method. Before sulfurization, NaF (∼30 nm) thin layer was deposited on deposited CZT film using thermal evaporation method. UV-Visible, XRD and SEM/EDS analysis were used for studying optical, structural, elemental and morphological properties of CZTS films.

Size-Induced Structural Disorder Enables Ultrahard Oxides

It has been empirically observed that decreasing grain size in polycrystalline materials leads to an increase in hardness following a linear dependence with the inverse square root of the size. This so-called Hall–Petch relationship was originally observed in metals and was attributed to the blocking effect of grain boundaries on dislocation motion. However, the Hall–Petch theory is not tenable for explaining similar phenomena in oxide ceramics, where dislocations are virtually immobile at ambient temperature. In this study, multinuclear (27Al, 25 Mg) magnetic resonance spectroscopy is utilized to demonstrate that a reduction in the grain size of MgAl2O4 spinel nanoceramics induces structural disorder in the form of cation site inversion in the grain boundary region, which leads to an increase in the average bond density and bond strength. The rapid increase in the volume fraction of such structurally disordered and strong grain boundary regions with decreasing grain size of the nanoceramics is responsibl...

Shear-coupled grain boundary migration assisted by unusual atomic shuffling.

Shear-coupled grain boundary (GB) migration can be an efficacious mechanism to accommodate plastic deformation when the grain size of polycrystalline materials goes small. Nevertheless, how this kind of GB motion comes into play at the atomic level has not been fully revealed. Here, we have investigated the shear-coupled migration (SCM) of typical [100] group symmetrical tilt GBs in bcc W using atomistic simulations. Depending on GB character, the SCM is found to proceed via dislocation slipping in the 〈100〉 or 〈110〉 mode with striking shear strength difference between them. We demonstrate that there exists an unusual atomic shuffling along the tilt axis, which greatly assists SCM to operate in the easier 〈110〉 mode instead of the 〈100〉 one. The present results highlight the significant role of GB character in the atomistic SCM process and contribute to the future design and fabrication of high-performance materials in GB engineering.

The inverse hall–petch relation in nanocrystalline metals: A discrete dislocation dynamics analysis

When the grain size in polycrystalline materials is reduced to the nanometer length scale (nanocrystallinity), observations from experiments and atomistic simulations suggest that the yield strength decreases (softening) as the grain size is decreased. This is in contrast to the Hall–Petch relation observed in larger sized grains. We incorporated grain boundary (GB) sliding and dislocation emission from GB junctions into the classical DDD framework, and recovered the smaller is weaker relationship observed in nanocrystalline materials. This current model shows that the inverse Hall–Petch behavior can be obtained through a relief of stress buildup at GB junctions from GB sliding by emitting dislocations from the junctions. The yield stress is shown to vary with grain size, d, by a d1/2 relationship when grain sizes are very small. However, pure GB sliding alone without further plastic accomodation by dislocation emission is grain size independent.

Measurement of grain size of polycrystalline materials with confocal energy dispersive micro-X-ray diffraction technology based on polycapillary X-ray optics

The confocal energy dispersive micro-X-ray diffraction (EDMXRD) based on polycapillary X-ray optics was used to determine the grain size of polycrystalline materials. The grain size of a metallographic specimen of nickel base alloy was measured by using the confocal EDMXRD. The experimental results demonstrated that the confocal EDMXRD had potential applications in measuring large grain size.

Fabrication of Nano/Ultra-Fine Grained IF Steel via SPD Processes: a Review

The grain size of polycrystalline materials plays a major role in dictating many critical properties including the strength and resistance to plastic flow. Nano/ultra-fine grained steels have the potential to exhibit outstanding physical, mechanical and chemical properties, which could, in principle, lead to new applications and novel technologies. Interstitial free (IF) steels in the coarse-grained condition possesses high ductility but low yield strength due to the decrease in the solid solution hardening effect of the interstitial atoms. Enhancing the strength with adequate ductility may increase the potential applications of IF steel sheets in new applications like those in aviation or defense industries. In fact, the overall trend in the development of IF steels are towards the high strength variety that will allow weight saving through down gauging. Considering the monophase microstructure of IF steel, strengthening methods to enhance its mechanical properties are limited and grain refinement seems to be the most feasible method. The most attractive method for the production of nano/ultra-fine grained IF steel is severe plastic deformation (SPD) processes. Therefore, this review is concerned with the production of nano/ultra-fine IF steel by using SPD methods such as accumulative roll bonding and equal channel angular pressing processes.

Positron annihilation study of defect distribution in 8YSZ nanostructure

The impact of the interfacial contribution on overall properties increases with decreasing grain size of polycrystalline materials. It is well known that distribution and size of cluster defects are rather different in bulk than grain boundaries. In light of “bottom-up” approaches, a study at the atomic level determining the distribution of crystallographic defects could clarify their contribution to the macroscopic properties, and then differentiate materials for outstanding or precise applications. In this work, Positron Annihilation Spectroscopy (PAS) is used to characterize the distribution of defects within 8 mol% Y 2O 3-stabilized zirconia (8YSZ) structures prepared by sintering through three different thermal treatments, i.e. a conventional thermal cycle in air and N 2/H 2 atmosphere, and a fast firing cycle in air, which lead to average grain sizes < 260 nm.

Predicting mesh-independent ballistic limits for heterogeneous targets by a nonlocal damage computational framework

During highly dynamic and ballistic loading processes, large inelastic deformation associated with high strain rates leads, for a broad class of heterogeneous materials, to degradation and failure by localized damage and fracture. However, as soon as material failure dominates a deformation process, the material increasingly displays strain softening and the finite element predictions of ballistic response are considerably affected by the mesh size. This gives rise to non-physical description of the ballistic behavior and mesh-dependent ballistic limit velocities that may mislead the design of ballistic resistant materials. This paper is concerned with the development and numerical implementation of a coupled thermo-hypoelasto-viscoplastic and thermo-viscodamage constitutive model within the laws of thermodynamics in which an intrinsic material length scale parameter is incorporated through the nonlocal gradient-dependent damage approach. This model is intended for impact and ballistic penetration and perforation problems of heterogeneous metallic targets such as metal matrix composites with dispersed particles at decreasing microstructural length scales. An evolution equation for the material length scale as a function of the material microstructural features (e.g. mean grain size in polycrystalline materials or particle size and inter-particle spacing in metal matrix composites), course of plastic deformation, strain hardening, strain-rate hardening, and temperature is presented. It is shown through simulating plugging failure in ballistic penetration of high-strength steel targets of different thicknesses by a hard blunt projectile that the length scale parameter plays the role of a localization limiter allowing one to obtain meaningful values for the ballistic limit velocity independent of the finite element mesh density. It is also shown that a local damage model incorporating viscosity and heat conduction as localization limiters, which are known to implicitly introduce length scale parameters, is insufficient in illuminating the mesh sensitivity at impact velocities close to the ballistic limit and that the mesh sensitivity increases as the target thickness increases. Therefore, the proposed nonlocal damage model leads to an improvement in the modeling and numerical simulation of high velocity impact related problems.

Micro–macro modelling of the effects of the grain size distribution on the plastic flow stress of heterogeneous materials

When the mean grain size of polycrystalline materials is larger than ∼100nm, it is commonly accepted for metals, intermetallics or ceramics that the plastic flow stress scales linearly with the inverse square root of the mean grain size (the so-called Hall–Petch behaviour). However, in this classic formalism, only the mean grain size is considered in a semi phenomenological way, and, the fact that the grains form a population of stochastic nature with different sizes and shapes is not stated. Here, a new self-consistent model making use of the “translated fields” technique for elastic–viscoplastic materials is developed as micro–macro scale transition scheme, and, the aggregate is composed of spherical randomly distributed grains with a grain size distribution following a log-normal statistical function. The constitutive behaviour of each grain is described by a partitioned strain rate into an elastic part and a viscoplastic part. The viscoplastic strain rate is described by an isotropic power law including the grain diameter through the reference stress. Numerical results firstly display that the plastic flow stress of the material depends on both the mean grain size and the grain size dispersion of the distribution. Besides, the role of the dispersion is more important when the mean grain size is on the order of the μm and the trend is a decrease of the flow stress with an increase of the dispersion. Secondly, predictions of second order internal stresses within the material indicate an increase in the internal stresses when grain size dispersion is increased, so that the plastic flow stress of the material depends on a competition between the respective distributions of internal stresses and individual flow stresses of the grains.

Application of Lamb wave scattering for grain size analysis in thin copper sheets: Results from laboratory and industrial trials

It is well established that attenuation of ultrasonic bulk waves may be used as a measure of grain size in polycrystalline materials. However, thinner samples usually do not allow for attenuation measurements with through-thickness ultrasonic bulk waves, and in order to overcome this obstacle, the present study was made to see whether attenuation of guided Lamb waves may be used instead. The analysis method, which employs Fourier analysis to reveal the frequency components in the ultrasonic Lamb traces, is tested on thin copper sheets of varying grain sizes. In addition, some of the tests have been carried out on-line at a continuously annealing line during production of copper sheets. The experimental data indicate that the estimation of attenuation from these measurements could be used as a measure of grain size. In addition, the experimental data have been compared to attenuation calculated from the grain scattering theory of Lamb waves and show reasonably good agreement.

Heterogeneous Deformation and Mechanical Strength of Materials

Grain size in polycrystalline materials was changed from larger than 10 w m to smaller than 10 nm, and the effects of both grain size and strain rate on the strength has been investigated from a view point of heterogeneous deformation. Grains of 10 nm or less in size were obtained by crystallization of amorphous alloys. The experimental results are summarized as follows: (a) Heterogeneous deformation is effectively suppressed when grain size decreases smaller than about 0.1 w m. As a result, the strength remarkably increases in this grain size range, and takes the maximum value when grains of 10 nm in size are homogeneously formed. (b) When grain size becomes smaller than 10 nm, those ultrafine grains are embedded into the amorphous matrix, and the strength decreases with increasing volume fraction of amorphous phase. (c) Heterogeneous deformation is also effectively suppressed by increasing strain rate in general as well as decreasing grain size. The maximum strength obtained experimentally is compared with the theoretical strengths estimated under various conditions.

Grain Size Analysis Using Automated-EBSP

This work discusses using the automated Electron BackScatter Pattern (EBSP) technique of electron diffraction in the Scanning Electron Microscope (SEM) for measuring grain size in polycrystalline materials. A quantitative analysis of 2-D grain size from a standard metallographic section can be made in less than one hour using an automated-EBSP system on a traditional SEM and dedicated analysis software. Thus, the automated EBSP technique provides a useful and cost-effective way to determine grain size. A common measure used to describe grain size is mean intercept length (MIL) and the present discussion will be limited to determining MIL using EBSP.Grain size analysis using optical microscopy is strongly influenced by sample preparation, which involves etching of the polished section to reveal boundaries. Boundaries that are to be avoided in determining MIL, such as sub-grain boundaries and twin boundaries, may be revealed and mistaken for random high angle boundaries.

Grain Refinement of Aluminum using Equal-Channel Angular Pressing

AbstractUsing the technique of equal-channel angular (ECA) pressing, it is possible to reduce the grain size of polycrystalline materials to the submicrometer level. Thus, this processing technique has the potential for producing materials which may exhibit superplasticity. This paper describes various factors affecting the development and evolution of the microstructure produced by ECA pressing. Optimization of such factors is then presented for the advent of superplasticity.

A study of the relation between the grain size and the dimensions of spots on x-ray diffraction photographs of polycrystalline materials

Clark and Zimmer reported a method of determining the average grain size of polycrystalline materials from a knowledge of the spot dimension on the X-ray diffraction rings in the range of 10 −2–10 −3 cm. All the factors influencing the spot dimension do not seem to have been taken into consideration by them for deducing the average grain size. Here an attempt is made to study systematically the relation between the grain size of a polycrystalline sample and the X-ray spot dimension taking into consideration the factors influencing it, viz. the divergence of the X-ray beam, the Bragg angle and the specimen-to-film distance. A method is devised to determine the average grain size of a polycrystalline material from the measurement of X-ray spot dimension under any experimental conditions. A method of calculating theoretically the spot dimension due to a known grain size is also developed. The average grain size results of the spot-dimension method are compared with those of the spot-cointing method and are found to agree with each other.