Behavior Of Nanocrystalline Materials Research Articles

Deformation mechanism and inverse Hall – Petch behavior in nanocrystalline materials

Abstract It is widely accepted that for the very smallest grain sizes (typically below 20 – 30 nm), dislocations play no significant role in the deformation of nanocrystalline materials. However, the grain-boundary mechanisms responsible for the reported decrease in strength with decreasing grain size in this regime (the ‘inverse Hall–Petch effect’) remain unclear. Here, we demonstrate by molecular-dynamics simulation that, in the absence of both grain growth and any dislocations, nanocrystalline fcc metals deform via a mechanism involving an intricate interplay between grain-boundary sliding and grain-boundary diffusion. By quantitatively reproducing the well-known Coble-creep formula for coarse-grained materials, we show that the ‘inverse Hall–Petch effect’ arises from sliding-accommodated grain-boundary diffusion creep. Previous, apparently contradictory, suggestions that GB sliding, on the one hand, or GB-diffusion creep, on the other, are responsible for this behavior can thus be reconciled as originating from one and the same deformation mechanism. We discuss the reasons why we believe that these simulations also capture the room-temperature deformation behavior of nanocrystalline fcc metals in the absence of dislocation nucleation and microcracking.

Modelling and Experimental Validation of the Porosity Effect on the Behaviour of Nano-Crystalline Materials

Nano-crystalline metals have attracted considerable attention over the past two decades due to their increased mechanical properties as compared to their microcrystalline counterparts. However, the behaviour of nano-crystalline metals is influenced by imperfections introduced during synthesis or heat treatment. These imperfections include pores, which are mostly located in the area of grain boundaries. To study the behaviour of multiphase nano-crystalline materials, a novel fully parametric algorithm was developed. The data required for implementing the developed numerical model were the volume fraction of the alloying elements and their basic properties as well as the density and the size of randomly distributed pores. To validate the developed algorithm, the alloy composition 75 wt% tungsten and 25 wt% copper was examined experimentally under compression tests. For the investigation, two batches of specimens were used; a batch having a coarse-grained microstructure with an average grain diameter of 150 nm and a nanocrystalline batch having a grain diameter of 100 nm, respectively. The porosity of both batches was derived to range between 9% and 10% based on X-ray diffraction analyses. The results of quasi-static compression testing revealed that the nanocrystalline W-Cu material exhibited brittle behaviour which was characterised by an elastic deformation that led to fracture without remarkable plasticity. A compressive strength of about 1100 MPa was derived which was more than double compared to conventional W-Cu samples. Finite element simulations of the behaviour of porous nano-crystalline materials were performed and compared with the respective experimental compression tests. The numerical model and experimental observations were in good agreement.

Grain-Size-Dependent Grain Boundary Deformation during Yielding in Nanocrystalline Materials Using Atomistic Simulations

While the advantageous mechanical properties of nanocrystalline (NC) materials have stimulated numerous studies over the past decade, fewer studies have looked at the onset of yielding and deformation and the role of grain boundaries (GBs) prior to plastic flow. In this computational study, Ni microstructures with grain size between 6 nm and 20 nm were studied to elucidate the role of GBs during yielding. Residual strain was quantified by relaxing to zero stress during loading and calculating the resulting evolution of strain accommodation. The results of this work reveal the accumulation of plastic strain in GBs and how microplasticity changes with grain size, leading to the initiation of macroscopic yielding. Microplasticity accumulates homogeneously within large grain microstructures, then becomes localized with decreasing grain size. The resulting microstructural strain accommodation helps understand changes in the onset of dislocation plasticity and yielding behavior in NC materials and may have broader implications for their continual plastic deformation.

The crack nucleation in hierarchically nanotwinned metals

The nanocrystalline (NC) metals with hierarchical twin lamellae exhibit excellent mechanical properties. However, recent efforts are mainly focused on the effects of primary twin boundaries (TB) and grain boundaries (GB) on crack nucleation. Few investigations have been performed on secondary twin boundary (STB) effect on crack nucleation in hierarchically nanotwinned metal. A relevant model is established to describe the crack nucleation criteria quantitatively. Actually, the crack advance mainly depends on the energy of crack nucleation and the energy of pile-up dislocations. Furthermore, predictions about the site of crack nucleation are made for different hierarchically nanotwinned metals. This work will help us better understand the deformation mechanism and the failure behavior in NC materials with hierarchically nanostructures.

In-situ tracking the structural and chemical evolution of nanostructured CuCr alloys

We report the thermal stability of supersaturated CuCr nanocrystallines alloys at the atomic resolution using modern spherical aberration-corrected transmission electron microscopy (TEM) via performing in-situ structural and spectroscopy experiments. It is found that CuCr nanocrystallines are not only subjected to a structural change but also undergo a chemical evolution upon annealing. Chemical destabilization of supersaturated CuCr nanocrystallines occurs at a quite low temperature. Heating triggers a rapid separation of Cu and Cr grains at the forced intermixing zone, accompanied by an obvious decrease of average interface width whereas the grain growth is not significant. Elemental profiles and images recorded in real time reveal that the local compositions vary with heating, which in turn permits to derive the concentration of excess vacancy generated by deformation and observe its evolution with temperature, further to analyze the dynamic behavior in nanocrystalline materials. Electronic structure changes at the interface forced intermixing zone are revealed by the fine structure analysis. The study uncovers the interplay between the thermal stability and chemical decomposition process of bulk nanostructured materials in real-time.

Role of Localized Non-Equilibrium States in Nucleation of Plastic Deformation in Nanocrystalline Materials

A molecular dynamics simulation of the behavior of nanocrystalline materials in the fields of external influences was carried out. Crystallites of the fcc copper and bcc iron under different schemes of mechanical loading were investigated. Revealed specific localized non-equilibrium states served as the mechanism of formation and evolution of partial dislocations in fcc materials and twin growth in bcc materials. These non-equilibrium states were realized on the basis of local transformation of the martensitic type when the nearest surrounding of atoms – the centers of local rearrangements – changed according to the A-B(C) scheme, where A, B and C are types of crystal lattice. The bcc-fcc-bcc local rearrangements during twin growth were typical for bcc iron. The fcc-bcc-hcp and hcp-bcc-fcc local rearrangements during the partial dislocation movement were typical for fcc copper.

Temperature dependence of the radiation tolerance of nanocrystalline pyrochlores A2Ti2O7 (A = Gd, Ho and Lu)

A potentially enhanced radiation resistance of nanocrystalline materials, as a consequence of the high density of interfaces and surfaces, has attracted much attention both to understand the fundamental role of these defect sinks and to develop them for high-radiation environments. Here, irradiation response of nanocrystalline A2Ti2O7 (A = Gd, Ho and Lu) pyrochlore powders with grain sizes of 20–30 nm was investigated by 1-MeV Kr2+ ion bombardment. In situ transmission electron microscopy (TEM) revealed that the critical amorphization fluence for each nanocrystalline compound at room temperature was greater than that for their coarse-grained counterparts, indicating an enhanced amorphization resistance. The effect of temperature on the irradiation response of one of these compounds, nanocrystalline Lu2Ti2O7, was further examined by performing ion irradiation at an elevated temperature range of 480–600 K. The critical amorphization temperature (Tc) was found to be noticeably higher in nanocrystalline Lu2Ti2O7 (610 K) than its coarse-grained counterpart (480 K), revealing that nanocrystalline Lu2Ti2O7 is less resistant to amorphization compared to its coarse-grained phase under high temperatures. We interpret these results with the aid of atomistic simulations. Molecular statics calculations find that cation antisite defects are less energetically costly to form near surfaces than in the bulk, suggesting that the nanocrystalline form of these materials is generally less susceptible to amorphization than coarse-grained counterparts at low temperatures where defect kinetics are negligible. In contrast, at high temperatures, the annealing efficiency of antisite defects by cation interstitials is significantly reduced due to the sink properties of the surfaces in the nanocrystalline pyrochlore, which contributes to the observed higher amorphization temperature in the nano-grained phase than in coarse-grained counterpart. Together, these results provide new insight into the behavior of nanocrystalline materials under irradiation.

The effects of strain rate and grain size on nanocrystalline materials: A theoretical prediction

Abstract The article describes a dislocation-based model for the coupling effects of strain rate and grain size on the mechanical behavior of nanocrystalline materials. Two important experimental findings about nanocrystalline materials are explained by the model: 1. the difference of the dependencies of strain rate sensitivity (SRS) on grain size between FCC and BCC nanocrystalline materials and 2. the abnormal dependency of activation volume on thermally activated stress. Our analysis shows that the strain rate-dependent behavior of nanocrystalline materials is mainly determined by the ratio between the activation volumes in grain and grain boundary regions.

Constitutive modeling for strain rate-dependent behaviors of nanocrystalline materials based on dislocation density evolution and strain gradient

Abstract

Non-planar grain boundary structures in fcc metals and their role in nano-scale deformation mechanisms

This work presents the results of a comparative molecular dynamics study showing that relaxed random grain boundary structures can be significantly non-planar at the nano-scale in fcc metals characterized by low stacking fault values. We studied the relaxed structures of random [1 1 0] tilt boundaries in a polycrystal using interatomic potentials describing Cu and Pd. Grain boundaries presenting non-planar features were observed predominantly for the Cu potential but not for the Pd potential, and we relate these differences to the stacking fault values. We also show that these non-planar structures can have a strong influence on dislocation emission from the grain boundaries as well as on grain boundary strain accommodation processes, such as grain boundary sliding. We studied the loading response in polycrystals of 40 nm grain size to a level of 9% strain and found that the non-planar grain boundaries favour dislocation emission as a deformation mechanism and hinder grain boundary sliding. This has strong implications for the mechanical behaviour of nano-crystalline materials, which is determined by the competition between dislocation activity and grain boundary accommodation of the strain. Thus, the two interatomic potentials for Cu and Pd considered in this work resulted in the same overall stress–strain curve, but significantly different fractions of the strain accommodated by the intergranular versus intragranular deformation mechanisms. Strain localization patterns are also influenced by the non-planarity of the grain boundary structures.

Effect of Grain Rotation on the Strain-Softening Behavior in Nanocrystalline Materials

We postulated a softening model involving grain rotation that results in diffusion-accommodated grain-boundary sliding. This numerical model was used to compute the proportion evolution of grains within shear bands and was also employed to predict the softening of nanocrystalline materials considering non-homogeneous plastic deformation due to shear bands. The effect of softening mechanism for total stress-strain relation and the grain size and mean maximum Schmid factor effect was also considered in our model.

Constitutive Modeling of Nanocrystalline Materials with Shear Band

In hardening stage, a model was used to study the plastic deformation behaviors of nanocrystalline materials. The material was considered as a composite of grain interior phase and grain boundary (GB) phase. The constitutive equations of the two phases were determined in term of their main deformation mechanisms. In softening stage, a shear band deformation mechanism and the corresponding constitutive relation were presented. Calculation results have shown that the predications fit well with experimental data. The investigation using the finite-element method (FEM) provided a direct insight into quantifying shear localization effect in nanocrystalline materials.

Effect of Grain Size Distribution on the Local Mechanical Behavior of Nanocrystalline Materials

A theoretical model based on self-consistent approximation is proposed to explore the effect of grain size distribution on the local mechanical response of nanocrystalline (nc) materials. The representative volume element (RVE) is composed of grains randomly distributed with a grain size distribution following a log-normal statistical function. The grain interior and grain boundary are taken as an integral object to sustain deformation mechanisms of grain-boundary sliding, grain-boundary diffusion and grain-interior plasticity. Local plastic strains and internal stresses, developing within the RVE, have been recorded and discussed.

A new micromechanics-based scale transition model for the strain-rate sensitive behavior of nanocrystalline materials

An original two-step “three phase” elastic–viscoplastic scale transition model is developed based on the combined self-consistent and Mori–Tanaka schemes. A coated inclusion is embedded within a matrix, wherein the inclusion represents grain interiors and the coating of the inclusion mimics the effects of grain boundaries and triple junctions. The predominant behavior within the grain interiors is captured through dislocation glide, whereas grain boundary (GB) dislocation emission and absorption, as well as thermally assisted GB sliding, describe the deformation processes within the coating describing the GB affected zone. Furthermore, an imperfect interface is assumed between the inclusion and the coating to account for viscoplastic grain boundary sliding along a stick-slip mechanism. Results and discussion focus on the competitive roles of GB sliding, GB dislocation emission/absorption, dislocation sweep in grain cores and collective dislocation plasticity, and the origins of the pronounced strain rate sensitivity of fcc NC materials.

Understanding creep in nanocrystalline materials

Only a limited number of creep investigations have been carried out on nanocrystalline materials to-date. These studies have remained largely inconclusive in establishing the mechanisms of creep in nanocrystalline materials. The stress exponent and activation energy values obtained from nanocrystalline materials do not correlate well with conventional well established creep models. Furthermore discrepancies between experimentally determined deformation rates and theoretical predictions suggest that an entirely new mechanism of creep could be operational in these exotic materials. In this work we aim to develop an understanding of the creep behavior of nanocrystalline materials by considering a stress assisted grain growth mechanism that has been recently identified in these materials. In turn a model has been developed that provides a quantitative understanding of some of the observations made in creep literature.

Mechanical model for yield strength of nanocrystalline materials under high strain rate loading

To understand the high strain rate deformation mechanism and determine the grain size, strain rate and porosity dependent yield strength of nanocrystalline materials, a new mechanical model based on the deformation mechanism of nanocrystalline materials under high strain rate loading was developed. As a first step of the research, the yield behavior of the nanocrystalline materials under high strain rate loading was mainly concerned in the model and uniform deformation was assumed for simplification. Nanocrystalline materials were treated as composites consisting of grain interior phase and grain boundary phase, and grain interior and grain boundary deformation mechanisms under high strain rate loading were analyzed, then Voigt model was applied to coupling grain boundary constitutive relation with mechanical model for grain interior phase to describe the overall yield mechanical behavior of nanocrystalline materials. The predictions by the developed model on the yield strength of nanocrysatlline materials at high strain rates show good agreements with various experimental data. Further discussion was presented for calculation results and relative experimental observations.

Deformation Mechanisms of Nanocrystalline Materials

Nanocrystalline materials have been found to deform by mechanisms not accessible to their coarse-grained counterparts. These deformation mechanisms have rendered nanocrystalline materials unique mechanical behaviors such as high strength, low work hardening rate, high strain rate sensitivity, and usually low ductility In a few cases, both high strength and high ductility were observed simultaneously. To understand the mechanical behaviors of nanocrystalline materials and to design them for desired mechanical properties, it is critical to fully understand their deformation mechanisms. Significant progresses have been reported in the last few years on both the deformation mechanisms and mechanical behaviors of nanocrystalline materials. Here a brief overview on the deformation mechanisms of nanocrystalline materials is presented.

What is Behind the Inverse Hall-Petch Behavior in Nanocrystalline Materials?

AbstractAn inverse Hall-Petch effect has been observed for nanocrystalline materials by a large number of researchers. This result implies that nanocrystalline materials get softer as grain size is reduced below a critical value. Postulated explanations for this behavior include dislocation based mechanisms and diffusion based mechanisms. In this paper, we report an explanation for the inverse Hall-Petch effect based on the statistical absorption of dislocations by grain boundaries, showing that the yield strength is both dependent on strain rate and temperature, and that it deviates from the Hall-Petch relationship at a critical grain size.

Deformation mechanism and inverse Hall–Petch behavior in nanocrystalline materials

Abstract It is widely accepted that for the very smallest grain sizes (typically below 20–30 nm), dislocations play no significant role in the deformation of nanocrystalline materials. However, the grain-boundary mechanisms responsible for the reported decrease in strength with decreasing grain size in this regime (the ‘inverse Hall–Petch effect’) remain unclear. Here, we demonstrate by molecular-dynamics simulation that, in the absence of both grain growth and any dislocations, nanocrystalline fcc metals deform via a mechanism involving an intricate interplay between grain-boundary sliding and grain-boundary diffusion. By quantitatively reproducing the well-known Coble-creep formula for coarsegrained materials, we show that the ‘inverse Hall–Petch effect’ arises from sliding-accommodated grain-boundary diffusion creep. Previous, apparently contradictory, suggestions that GB sliding, on the one hand, or GB-diffusion creep, on the other, are responsible for this behavior can thus be reconciled as originati...

Model of high-temperature plastic deformation of nanocrystalline materials: Application to yttria tetragonal zirconia

The possibility of the influence of segregation-induced local electric fields in the bulk diffusion of the species controlling the plastic deformation of nanocrystalline materials has been pointed out. Until now, there is only a model applicable to the case of a monodimensional system. In spite of its simplicity, it predicts a significative influence of a local electric field in creep. Our work develops a different model applicable to three-dimensional systems. It takes as a starting point the diffusional model, and it can be generalized to those systems in which the grain-boundary sliding model accommodated by diffusional processes accurately describes plasticity in the submicron range of grain size. The range of validity, as well as the different behavior of nanocrystalline materials from the submicron ones is discussed. Preliminary results are in good agreement with the published data for yttria tetragonal zirconia (YTZP) nanocrystalline ceramics.