Grain-boundary Mechanism Research Articles

A molecular dynamics study of heterogeneous nucleation at grain boundaries during solid-state phase transformations

The study of solid-state nucleation through experiment is often limited by its tiny length range and short time scale. In this study, molecular dynamics (MD) simulations in a quasi 2D geometry are used to study the process of a BCC ferrite phase in pure Fe nucleating at a grain boundary (GB) in an FCC austenite polycrystalline system. In the MD simulations the critical nucleus can be identified, the bulk free energy difference between FCC and BCC is known for the Fe interatomic potential used and all relevant interface and GB energies are computed using a Gibbs–Cahn formulation. For nucleation events that exhibited low energy facets completely contained within the parent FCC phase, the results agreed well with predictions from classical nucleation theory (CNT) in terms of both the size and shape of the critical nucleus. For systems where the emerging nucleus contains facets that cross the GB plane the agreement with classical theory is less convincing and the observed nucleus does not exhibit parallel facets as predicted from the Winterbottom construction. The latter nucleation case involves a so-called pucker mechanism of the FCC grain boundary to accommodate the emerging nucleus and the effect of GB puckering on the incubation time is discussed.

Electrical properties and phase transition of [(CH3)3NH]CdCl3 compound

The [(CH3)3NH]CdCl3 compound was obtained by slow evaporation at room temperature and characterized by X-ray powder diffraction patterns, differential scanning calorimetry, and impedance spectroscopy. This compound was found to crystallize in the orthorhombic system with Pbnm space group and was characterized by four phase transitions (T 1 = 355 K, T 2 = 372 K, T 3 = 415 K, and T 4 = 446 K). The analysis of Nyquist plots has revealed the contribution of two electrically active regions corresponding to the bulk mechanism and distribution of grain boundaries. The modulus plots were characterized by the presence of two peaks associated with the grain and grain boundaries. Thermodynamic parameters such as the free energy for dipole relaxation ΔF, the enthalpy ΔH, and the change in entropy ΔS have been determined with the help of the Eyring theory. The temperature dependence of the electrical conductivity (σ g ), σ dc , and f p confirms the observed transitions in the calorimetric study.

A dual deformation mechanism of grain boundary at different stress stages

Molecular dynamics (MD) simulation with embedded-atom method (EAM) potential was carried out to study the structure and shear response of an asymmetric tilt grain boundary in copper bicrystal. A non-planar structure with dissociated intrinsic stacking faults was observed in the grain boundary. Simulation results show that this type of structure can significantly increase the ductility of the simulation sample under shear deformation. A dual deformation mechanism of the grain boundary was observed; the grain boundary can be a source of dislocation emission and migrate itself at different stress stages. The result of this study can provide further information to understand the grain boundary mediated plasticity in nanocrystalline materials.

Grain size-induced thermo-mechanical coupling in zirconium thin films

At the bulk scale, thermal conductivity of metals is not coupled with mechanical strain. However, this may not be the same for grain sizes below the electron mean free path, for which mechanical deformation mechanism and volume fraction of grain boundaries are drastically different from the bulk. To investigate this hypothesis of grain size-induced thermo-mechanical coupling, we developed an experimental setup to measure the thermal conductivity of 100-nm-thick freestanding nanocrystalline zirconium (Zr) films as a function of externally applied mechanical strain. Experimental results show strong mechanical strain–thermal conductivity coupling, thermal conductivity of Zr film dropped from 20 to 13 W m−1 K−1 with an increasing strain from 0 to 1.24 %. Thermal conductivity of Zr film was also measured as a function of average grain size ranging from as-deposited 10 nm to as-grown 250 nm. The results clearly show that for grain sizes above 10 nm, mechanical strain has no influence over thermal conductivity, which supports the proposed hypothesis.

Nonequilibrium grain-boundary segregation mechanism of hot ductility loss for austenitic and ferritic stainless steels

Abstract

Features of Plastic Deformation and Fracture of a Dispersion-Strengthened V–4Ti–4Cr–(C, N, O) Alloy at Different Temperatures

4Using the methods of transmission and scanning electron microscopy, the defect substructure of a V–4Ti–4Cr– (C, N, O) alloy dispersion strengthened with nanosized oxicarbonitride particles and the features of its fracture after deformation via active tensile loading at the temperatures 20 and 800°C are investigated. It is shown that the most representative features of deformation at the elevated temperature is primarily activation of grainboundary mechanisms of deformation and fracture and secondly strain localization accompanied by crystallattice re-orientation. The reasons for these phenomena are discussed.

Microstructures, solid solution formation and high-temperature mechanical properties of ZrB2 ceramics doped with 5 vol.% WC

ZrB2 (ZB) and ZrB2–5vol.% WC (ZW) were prepared by hot-pressing at 2100°C. Combining experimental and theoretical methods, the tungsten (W) element in solid solution with ZrB2 and ZrC was investigated. Experimentally, solid solution formation in ZW samples was supported by XRD and EDS. Theoretically; atomic scale computer simulations based on density functional calculations were used to interpret the different amount of solid solution and the changes in lattice parameters. The flexural strength of the samples was measured from room temperature to 1600°C. The flexural strength of ZB and ZW changed differently as testing temperature increased. Then, the strength of ZB increased from 344±12MPa to 1281±39MPa, which is about 3.7 times of room temperature strength. Further temperature increasing led to strength decreasing. While for ZW, the strength increased continuously from room temperature to 1600°C. SEM and TEM techniques were employed to clarify the different mechanism of fracture surfaces and grain boundaries of ZB and ZW.

Ab Initio Modeling of Bulk and Intragranular Diffusion in Ni Alloys.

Knowledge of solid-state and interfacial species diffusion kinetics is of paramount importance for understanding mechanisms of grain boundary (GB) oxidation causing environmental degradation and cracking of Ni-base structural alloys. In this study, first-principles calculations of vacancy-mediated diffusion are performed across a wide series of alloying elements commonly used in Ni-based superalloys, as well as interstitial diffusion of atomic oxygen and sulfur in the bulk, at the (111) surface, ⟨110⟩ symmetric tilt GBs of Ni corresponding to model low- (Σ = 3/(111)) and high-energy (Σ = 9/(221)) GBs. A substantial enhancement of diffusion is found for all species at the high-energy GB as compared with the bulk and the low-energy GB, with Cr, Mn, and Ti exhibiting remarkably small activation barriers (<0.1 eV; ~10 times lower than in the bulk). Calculations also show that the bulk diffusion mechanism and kinetics differ for oxygen and sulfur, with oxygen having a faster mobility and preferentially diffusing through the tetrahedral interstitial sites in Ni matrix, where it can be trapped in a local minimum.

Compressive behavior of nanocrystalline nickel at various temperatures and strain rates

In this paper, compressive behavior of electrodeposited nano-crystalline (nc) Ni at various temperatures and strain rates is studied using a low temperature mechanic test system. Plastic deformation mechanisms of nc Ni caused by compression are characterized by the strain rate sensitivity index, the activation volume, and examined by scanning electron microscopy and high resolution transmission electron microscopic analysis. Results show that at low temperatures, the plastic deformation of nc Ni is mainly dominated by grain boundary accommodated dislocations. In other words, during plastic deformation of nc Ni at low temperatures, the intrinsic dislocation at the grain boundary bends up and expands without obstacles to the opposite grain boundary in the inner grain dislocation-free zone, until the occurrence of similar cutting forest-dislocation behavior appearing at opposite grain boundary. Moreover, the residual dislocations in the grain boundary bending out during plastic deformation could increase the strain compatibility and decrease the stress concentration. At room temperature, the plastic deformation mechanism of nc Ni is controlled by the deformation of grain boundary accommodated dislocations and grain slipping/rotating. Based on the above analyses, differences in compressive behavior of nc Ni at various temperatures and strain rates can be revealed by the correlation of deformation mechanisms of grain boundary accommodated dislocations and residual dislocation movement, temperature and defects in nc Ni.

Connectivity between electrical conduction and electrode structure in Mn–Co–Ni–O thick-film thermistors

In this paper, the electrical conduction of grain and grain boundary (GB) in Mn1.85Co0.3Ni0.85O4 negative-temperature-coefficient (NTC) thick-film thermistors with different finger electrode structures was studied carefully through the temperature-dependent ac impedance spectroscopy analyzing. It was found that the increase in finger length will drop the resistance of grain and GB sharply. The conduction mechanism of the grain was independent on the electrode structure and followed the small-polaron hopping model. However, the conduction mechanism of GB was dependent on the electrode structure. With the finger length increasing, the resistance–temperature (R–T) relation of GB was transformed from nonlinear to linear relation. These studies on the mechanisms concerning the dependence of conduction behaviors of grain and GB on electrode structure will provide a comprehensive understanding on the transformation behaviors of R–T relation of thick-film NTC thermistors.

The role of Co in the BaTiO3–Na0.5Bi0.5TiO3 based X9R ceramics

Ceramics in BaTiO3–Na0.5Bi0.5TiO3–Nb2O5–Co3O4 (BT–NBT–Nb–Co) system were fabricated to satisfy the EIA-X9R specification (−55~200°C, ΔC/C≤±15%) of multilayer ceramic capacitors. The role of Co in this system was investigated. The core–shell structure was found to exist in the system, as confirmed by transmission electron microscopy (TEM), accounting for the high dielectric temperature stability. The Co can regulate the reaction between Nb and BT–NBT to adjust the core–shell structure, where the optimum dielectric properties were achieved at Nb/Co=3, with dielectric constant being 930 at room temperature and capacitance variation being less than 15% in the temperature range from −55°C to 200°C. The impedance spectroscopy was fitted by 3RC equivalent circuit, corresponding to the grain, grain boundary and ceramic/electrode interface region for the Co modified BT–NBT system. The conduction activation energy of grain was found to decrease with increasing Co concentration, where the conduction mechanism of grain and grain boundary are both sensitive to the Co concentration.

Effects of the active layers deposition temperature on the electrical performance of p-type SnO thin-film Transistors

We investigated the effects of the active layers deposition temperature on the electrical performance of p-type tin-oxide thin-film transistors (TFTs). Negligible currents flowed between the source and the drain electrodes in the devices with tin-oxide thin films deposited at 60 °C and 220 °C, but devices with tin-oxide thin films deposited at 100, 140, and 180 °C exhibited typical p-type characteristics. For deposition temperatures ranging from 100 to 180 °C, the field-effect mobility decreased and the turn-on voltage moved in the negative direction as the deposition temperature was increased, which was mainly attributed to a decrease in the grain size based on the grainboundary charge-trapping mechanism. Our experimental results show that it is very important to optimize the active layers deposition temperature to improve the electrical performance in p-type tin-oxide TFTs.

Structure, deformation and fracture of arc evaporated Zr–Si–N hard films

Zr–Si–N films with varying Si contents were grown on WC–Co substrates by reactive cathodic arc deposition technique. The resulting microstructures of the films correlate to dominant variation in mechanical properties and deformation mechanisms. Si forms a substitutional solid solution in the cubic ZrN lattice up to 1.8at.% exhibiting a fine columnar microstructure. Further Si additions result in precipitation of an amorphous (a)-SiNx phase and evolution of a nanocomposite microstructure (nc ZrN/a-SiNx) which completely suppresses the columnar microstructure at 6.3at.% Si. The rotation-induced artificial layering during film growth is used as a marker to visualize the deformation of the film. A dislocation-based homogeneous plastic deformation mechanism dominates the columnar microstructure, while grain boundary sliding is the active mechanism mediating heterogeneous plastic deformation in the nanocomposite microstructure. Film hardness increases with increasing Si content in the columnar microstructure due to an effective solid solution strengthening. The deformation mechanism of localized grain boundary sliding in the nanocomposite microstructure results in a lower hardness. When cracking is induced by indentation, the fine columnar microstructure exhibits pronounced crack deflection that results in a higher fracture resistance compared to the nanocomposite films.

AC impedance analysis, equivalent circuit, and modulus behavior of NaFeP2O7 ceramic

II-NaFeP2O7 was prepared by conventional ceramic fabrication technique. Rietveld refinement, impedance properties, conductivity dispersion, electric modulus, and their scaling were carried out as a function of frequency and temperature. Thus, X-ray diffraction analysis indicates that the sample exhibits a single-phase nature with a monoclinic structure. In addition, analysis of Nyquist plots as well as modulus analysis revealed the contribution of two electrically active regions corresponding to bulk mechanism and distribution of grain boundaries. The near values of activation energies, obtained from the impedance and modulus spectra, confirm that the transport happens through an ion hopping mechanism, dominated by the motion of the Na+ ions in the framework of the investigated material resulting from intersecting tunnels or voids where Na+ is located.

Simulation of transient-state recrystallization of Al–4.5Mg–1Zn alloy after hot deformation

In this paper, a modeling framework developed for the simulation of recrystallization after hot deformation is presented. Modeling work concerned the recrystallization of an Al–Zn–Mg alloy in the transient state during post-deformation annealing when the stored energy and subgrain size were changing. The initial stored energy as a result of deformation was calculated as a function of subgrain size related to the Zener Hollomon parameter and its evolution was correlated with subgrain growth. The as-deformed grain structure was mapped into the Monte Carlo simulation. The calculated stored energy was assigned to the mapped structure, taking the length scale of the simulation into consideration. The effects of the Zener drag pressure and the initial as-deformed grain structure of the material on recrystallization were also incorporated into the Monte Carlo growth model. Static recrystallization (SRX) through the mechanisms of grain boundary (GB) and particle-stimulated nucleation (PSN) were distinguished. The parameters used for correlating the Monte Carlo simulation time with real time were determined by fitting the simulation results to the experimental measurements. The predictions from the simulations were validated by comparing the predicated grain structures with those from experimental observations. It was found that by incorporating the evolution of the stored energy into the recrystallization model, the differences between the simulation and experimental results could be reduced from 15% to 3%.

High temperature tensile deformation behavior of AZ80 magnesium alloy

Tensile behaviors of an AZ80 alloy were investigated by elongation-to-failure tensile tests at 300, 350, 400 and 450 °C, and strain rates of 10−2 and 10−3 s−1. Strain-rate-change tests from 5×10−5 s−1 to 2×10−2 s−1 were applied to study deformation mechanisms. The experimental data show that the material exhibits enhanced tensile ductilities of over 100% at 400 and 450 °C with stress exponent of 4.29 and activation energy of 149.60 kJ/mol, and initial fine grains preserve in evenly deformed gauge based on microstructure studies. The enhanced tensile ductilities are rate controlled by a competitive mechanism of grain boundary sliding and dislocation climb creep, based on which a model can successfully simulate the deformation behavior.

Characteristics of Al-doped ZnO thin films prepared in Ar + H2 atmosphere and their vacuum annealing behavior

The microstructure and electrical–optical properties of Al-doped ZnO (AZO) films have been studied as a function of H2 flux in the magnetron sputtering process at 150 °C and postannealing temperature in vacuum. As H2 flux increases in the sputtering gas, the AZO films deposited have a (002) preferred orientation rather than the mixed (100) and (002) orientations, the grain size shows a tendency to first increase then decrease, and (002) diffraction peak position is inclined to shift to higher angles first then to lower angles. The resistivity of the films first decreases then increases with H2 flux, and the lowest resistivity of 4.02 × 10−4 Ω cm is obtained at a H2 flux of 10 sccm. The average transmittance in the visible region shows little dependence on H2 flux. As a whole, the AZO films with higher values of figure of merit are obtained when the H2 flux is in the range of 6–12 sccm. The AZO films deposited in Ar and Ar + H2 exhibit different annealing behaviors. For the AZO film deposited in Ar, the grain size gradually increases, the stresses are relaxed, the resistivity first decreases then increases, and the average transmittance in the visible region is unchanged initially then somewhat decreased as annealing temperature is increased. The optimum annealing temperature for improving properties of AZO films deposited in Ar is 300 °C. For the AZO films deposited in Ar + H2, annealing does not significantly change the microstructure but increases the resistivity of the films; the average transmittance in the visible region remains unchanged initially but greatly reduced with further increase in annealing temperature. The carrier transport in the as-deposited and annealed films appears to be controlled by a mechanism of grain boundary scattering, and the value of Eg increases with the increase in carrier concentration due to Burstein–Moss effect.

High temperature plasticity in yttria stabilised tetragonal zirconia polycrystals (Y-TZP)

The literature data on the superplastic deformation of high purity yttria stabilised tetragonal zirconia polycrystals is reviewed in detail. It is shown that, based on the existence of a threshold stress, the single mechanism of grain boundary sliding (GBS) accommodated by diffusional processes can explain the superplasticity of these materials over all the ranges of temperature, stress, grain size, and surrounding atmosphere that have been studied. The origin of the threshold stress and its quantitative dependence on temperature and grain size is explained in terms of the segregation of yttrium atoms at the grain boundaries. A new model for GBS accommodated by lattice or grain-boundary diffusion is presented which can explain the transition of the stress exponent from 2 to 1.

On the conditions of strain localization and microstructure fragmentation under high-rate loading

The paper reports on research in the deformation and fragmentation mechanisms of coarse- and fine-grained materials under high-rate loading. The study was performed by an experimental procedure based on collapse of thick-walled hollow cylinders and by molecular dynamics simulation. The key issue was to study the formation of plastic strain localization bands. It is found that the pattern of plastic deformation is governed by loading conditions and characteristic grain sizes. For a coarse-grained material, the governing mechanism is dislocation deformation resulting in localization bands. For a fine-grained material, the governing mechanism is grain boundary sliding with attendant fragmentation of the material. A dependence of the strain rate and degree on the critical grain size was disclosed. The computer simulation revealed mechanisms of grain boundary sliding on the scales studied.

The origin and geophysical implications of a weak C-type olivine fabric in the Xugou ultrahigh pressure garnet peridotite

We report here a comprehensive investigation of mineral chemistry, water content and deformation features (including the crystallographic preferred orientation and dislocation substructures of olivine) of the Xugou garnet peridotite from the southern Sulu ultrahigh pressure (UHP) terrane. Our results suggest: (1) the Xugou garnet peridotite was equilibrated and deformed at high pressure (4.8–6.6 GPa) and relatively low temperature (810–930 °C) conditions. (2) In contrast to the pronounced shape-preferred orientations of olivine, pyroxene and garnet, electron backscattered diffraction (EBSD) measurements show a weak C-type fabric in olivine, a strong (100)[001] fabric in clinopyroxene and orthopyroxene and a nearly random fabric in garnet, respectively. The deformation mechanism map suggests a dominant deformation mechanism of dislocation-accommodated grain boundary sliding (DisGBS) in olivine. (3) The trivial amount of water in olivine (15±10 ppm), orthopyroxene (21±17 ppm), clinopyroxene (64±48 ppm) and garnet (dry) correspond to a water-poor deformation condition. We proposed that the C-type olivine fabric was generated by a dominant DisGBS with some contribution from diffusion creep at elevated pressures. Calculated seismic properties of the peridotite show the fastest P-wave velocity and the fast polarization direction of S-waves are oriented normal to the foliation plane and perpendicular to the lineation in the foliation plane, indicating that the C-type olivine fabric could also be a candidate to explain the observed strike-parallel faster S-wave polarization near trenches and in collisional orogenic belts.