Grain Boundary Relaxation Research Articles

Stabilizing nanograins in metals with grain boundary relaxation

Abstracts Structural stability of nanograined (NG) metals at elevated temperatures or under mechanical loadings is much reduced relative to that of their coarse-grained counterparts, bottlenecking their processing and applications. In this paper, we present recent progress in stabilization of NG metals with grain boundary (GB) relaxation mediated by interactions of GBs and partial dislocations. Two strategies for inducing GB relaxation were identified in NG and submicro-grained pure metals: plastic deformation of nanograins below critical sizes and rapid heating for forming annealing twins. With the GB relaxation, both thermal stability and mechanical stability of nanograins in several pure metals are enhanced significantly.

Ultrafast Atomic Diffusion Paths in Fine-Grained Nickel Obtained by Spark Plasma Sintering

Short-circuit diffusion in fine-grained Ni samples processed by Spark Plasma sintering has been investigated by the radiotracer technique. Ni grain boundary self-diffusion is measured in samples sintered from commercial as-received powder and from a powder processed by mechanical milling (MM). Both samples displayed high penetration of the radiotracer and ultrafast diffusion rates, which exceed the diffusivity along general high-angle grain boundaries as they are present in pure polycrystalline Ni. A distinct profile was observed for each sample, dependent on the precursor powder. A stable penetration profile after pre-annealing at 773 K was observed when using commercial powder whereas a decrease in the grain boundary diffusion coefficient was depicted for the sample prepared from MM powders. The latter observation was interpreted in terms of partial relaxation of non-equilibrium grain boundaries generated by MM. Sample preparation by focused ion beam enabled the observation of interconnected porous paths in the sample prepared from commercial powders, which represent the main ultrafast diffusion path. A measurement of the surface diffusion coefficient through the pores was attempted considering a B–C-type kinetic regime. The isolated pores observed in the sample prepared from MM powder suggest a complex hierarchy of diffusion paths.

Effect of grain boundary relaxation on the corrosion behaviour of nanocrystalline Ni-P alloy

Grain boundary relaxation (GBR) in nanocrystalline materials is an approach to reduce the excess free energy of the material without any change in the grain size of the material and is carried out through controlled heat treatments at relatively low temperatures. In this work, we are investigating the effect of grain boundary relaxation on the corrosion behaviour of nanocrystalline Ni-P, a topic which has not been explored before. Nanocrystalline Ni-P foils were produced using the pulsed electrodeposition route using the modified Watts bath and were characterized by X-Ray Diffraction and Transmission Electron Microscopy. Differential scanning calorimetry (DSC) characterization technique was used to establish the GBR regime, and the same was validated by measuring the grain size post the grain boundary relaxation treatment. Once the relaxation regime was established, systematic heat treatments at a temperature of 473 K were carried out, which enhanced the hardness of the nanocrystalline material without any change in the initial grain size. Corrosion studies using potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) test were performed to understand the effect of GBR. The corrosion studies revealed an improvement in corrosion resistance of the relaxed structure vis-à-vis the as deposited nanocrystalline Ni-P. An increase in Ecorr and a decrease in icorr and ip were found for the relaxed structure. The EIS tests too revealed an improvement in the polarization resistance for the relaxed structure.

Probing defect relaxation in ultra-fine grained Ta using micromechanical spectroscopy

The study of grain boundaries (GBs) in polycrystalline materials is a field of major interest, as many fundamental properties are influenced by their actual structure. One of the main challenges in investigating GBs is that electron microscopy techniques capable to resolve their atomistic arrangement require very small sample volumes of a few tens of nanometers and might therefore change the natural state of the GB via removal of occurring material constraint. To counteract this influence one could apply indirect measurements such as internal friction to probe for changes of the GB structure. However, with the drive towards smaller volumes most of these techniques are at their limit. The current work proposes a new approach based on mechanical spectroscopy of micron sized specimens applied in-situ in a scanning electron microscope. Applying this novel concept to ultra-fine grained Ta we investigate changes in the defect structure between the as-deformed condition and after a heat treatment. In fact, we detect a decrease in internal friction of about 50% upon annealing. This pronounced change occurs in conjunction with an increase in flow stress and hardness and is related to thermally induced GB relaxation.

The effect of heat treatment on the microstructural changes in electrodeposited Ni-Mo coatings

Abstract Nanocrystalline Ni-Mo (16 wt% of Mo) coatings electrodeposited on steel substrates from citrate electrolyte solution were annealed in argon atmosphere, in the temperature range from 300 to 800 ˚C. The effect of the heat treatment on the coatings’ microstructure (surface morphology, phase composition and grain size), as well as their mechanical, tribological and corrosion properties was determined. It was found that, up to 500 °C, the average crystallite size remains almost unchanged, but relaxation of grain boundaries leads to significant increase of the coatings’ microhardness (up to 1190 HV). For Ni-Mo alloys annealed at 500 °C, the wear index and friction coefficient increase significantly from 33·10−6 mm3/Nm and 0.4 (for as-deposited samples) to 89·10−6 mm3/Nm and 0.68. Above 500 °C, thermally-induced gradual fluctuations of chemical composition in the Ni-Mo alloy, and interdiffusion of iron (from the substrate) and nickel (from the coating), were observed. The segregation of elements promotes an increase of grain size (from 10 nm to 33 nm for as-deposited and annealed at 800 °C Ni-Mo coatings, respectively) resulting in decreased microhardness to 487 HV. However, the coatings become less stiff (EIT up to 205 GPa) and are characterised by enhancement of wear (Wv about 2.2–7.8⋅10−6 mm3/Nm) and friction resistance (about 0.4–0.5). No significant influence of the heat treatment temperature on the corrosion parameters of the coatings in the sulphate environment was found. The corrosion resistance of the Ni-Mo coating annealed at 800 °C decreased slightly (Icorr=5.9 μA/cm2, Rp = 1530 Ω) in comparison to the as-deposited alloy (Icorr=4.8 μA/cm2, Rp = 2430 Ω) and that annealed at 500 °C (Icorr = 4.7 μA/cm2, Rp = 2220 Ω).

Annealing‐Induced Hardening in Ultrafine‐Grained and Nanocrystalline Materials

Annealing of deformed metals is considered as a process necessarily leading to softening due to the annihilation of lattice defects. However, in ultrafine‐grained (UFG) and nanocrystalline materials, annealing at moderate temperatures may induce hardening. This review summarizes those effects that can result in annealing‐induced hardening (AH) in fine‐grained materials. It is noted that only those hardening phenomena are considered as AH effects that are not accompanied by the change of the phase composition and/or the grain size. Therefore, herein, strengthening caused by precipitation is not discussed. It is shown that heat treatment of nanomaterials can cause hardening due to the relaxation of grain boundaries and segregation of alloying elements to the grain boundaries as these effects hinder the occurrence of grain boundary sliding. For UFG metallic materials processed by severe plastic deformation techniques, the annihilation of mobile dislocations and the clustering of the remaining dislocations into low‐angle grain boundaries during annealing can yield hardening. It is also shown that plastic deformation after annealing can cause a restoration of the yield strength and hardness to the same level as observed before annealing. The possible reasons of this deformation‐induced softening effect are discussed in detail.

Grain and grain boundaries contributions to dielectric relaxation of the clay-based ceramics

The current paper is a complementary dielectric study on the clay-based ceramics (CBC) to investigate the contribution of grain and grain boundaries to the dielectric relaxations. The dielectric measurements were carried out on the ceramic composed of 50 wt.% clay and 50 wt.% nepheline syenite) over wider ranges of frequency and temperature, using the advanced broadband dielectric spectrometer. Dielectric spectra suggested that the temperature has a significant effect on the electrical microstructure of CBC. They show the existence of two relaxation process associated to the grain boundaries (GBs) and grains in low- and high-temperature regions, respectively. The relaxation process associated with grain is more predominating over GBs relaxation over the temperature range studied. Interestingly, we reported an anomaly feature in dielectric behavior of CBC at − 40 °C at which all dielectric properties changed abruptly.

Achieving Ultrahigh Hardness in Electrodeposited Nanograined Ni-Based Binary Alloys.

Annealing hardening has recently been found in nanograined (ng) metals and alloys, which is ascribed to the promotion of grain boundary (GB) stability through GB relaxation and solute atom GB segregation. Annealing hardening is of great significance in extremely fine ng metals since it allows the hardness to keep increasing with a decreasing grain size which would otherwise be softened. Consequently, to synthesize extremely fine ng metals with a stable structure is crucial in achieving an ultrahigh hardness in ng metals. In the present work, direct current electrodeposition was employed to synthesize extremely fine ng Ni-Mo and Ni-P alloys with a grain size of down to a few nanometers. It is demonstrated that the grain size of the as-synthesized extremely fine ng Ni-Mo and Ni-P alloys can be as small as about 3 nm with a homogeneous structure and chemical composition. Grain size strongly depends upon the content of solute atoms (Mo and P). Most importantly, appropriate annealing induces significant hardening as high as 11 GPa in both ng Ni-Mo and Ni-P alloys, while the peak hardening temperature achieved in ng Ni-Mo is much higher than that in ng Ni-P. Electrodeposition is efficient in the synthesis of ultrahard bulk metals or coatings.

Size Dependence of Grain Boundary Migration in Metals under Mechanical Loading

The greatly increased grain boundary (GB) mobility in nanograined metals under mechanical loading is distinguished from that in their coarse-grained counterparts. The feature leads to softening of nanograined materials and deviation of strength from the classical Hall-Petch relationship. In this Letter, grain size dependences of GB migration in nanograined Ag, Cu, and Ni under tension were investigated quantitatively in a wide size range. As grain size decreases from submicron, GB migration intensifies and then diminishes below a critical grain size. The GB migration peaks at about 80, 75, and 38nm in Ag, Cu, and Ni, respectively. The suppression of GB migration below a critical size can be attributed to GB relaxation during sample processing or by postthermal annealing. With relaxed GBs the governing deformation mechanism of nanograins shifts from GB migration to formation of through-grain twins or stacking faults.GB relaxation, analogous to GB segregation, offers a novel approach to stabilizing nanograined materials under mechanical loading.

Enhancement of the corrosion resistance and mechanical properties of nanocrystalline titanium by low-temperature annealing

Annealing deformed metals usually leads to reduced strength. In the case of nanocrystalline materials, the opposite effect was observed after low-temperature annealing. The annihilation of mobile dislocations and a relaxation of grain boundaries are probable reasons for the observed phenomenon. Dislocation density and grain boundaries character affect also on the corrosion resistance. Hence, in this work the effect of low-temperature annealing on both the hardness and corrosion resistance of nanotitanium was investigated. The results indicated that low-temperature annealing is a fast, simple and inexpensive way to further enhance nanotitanium's mechanical and corrosion properties, which are crucial for implantable materials.

Hardening by Annealing and Implementation of High Ductility of Ultra-Fine Grained Aluminum: Experiment and Theory

Abstract The influence of low temperature annealing and subsequent deformation on microstructure, strength and ductility was investigated for the first time for high pressure torsion (HPT) processed commercially pure Al. Extremely high increases in the conventional yield stress (up to 50%) and ultimate tensile strength (up to 30%) were obtained by annealing of the ultrafine grained (UFG) samples in the range 90-200 °C for 1 h. Such increases were accompanied by a sharp drop in ductility up to 1%. Implementation of high ductility at the level of coarse-grained Al, while maintaining high strength of the HPT-processed sample was demonstrated for the first time and achieved by repeating the low temperature annealing followed by subsequent additional HPT deformation. The key role of relaxation of non-equilibrium high-angle grain boundaries (GBs) in the strengthening effect of UFG-Al by annealing is shown. Two theoretical models are suggested to explain the hardening by annealing and the implementation of high ductility in UFG structures. Within the models, plastic deformation occurs through emission of lattice dislocations from triple junctions of GBs containing pile-ups of grain-boundary dislocations, glide of the lattice dislocations across neighboring grains, their accumulation at and climb along the opposite GBs. The energy characteristics and the critical stresses of dislocation emission are determined in two different cases, for UFG Al subjected to annealing only and to annealing with subsequent additional HPT deformation. The calculated theoretical dependences of the flow stress on the plastic deformation value well fit qualitatively and quantitatively to our experimental data.

Effect of sintering conditions on the electrical-transport properties of the SrZrO3-based protonic ceramic electrolyser membrane

Abstract The effects of sintering temperature and addition of 4 mol.% ZnO as sintering additive on the crystal structure, microstructure and electrical properties of SrZr 0.9 Y 0.1 O 3-δ are reported. The presence of ZnO as sintering aid brings about high densification at 1300 °C (relative density ∼97%); gas-tightness is not achieved for ZnO-free samples sintered below 1600 °C. Bulk conductivity (σ B ) is considerably higher in wet and dry O 2 on doping with ZnO, but only slight variations of σ B with sintering temperature are observed for the Zn-containing phases. Similarly, the apparent grain-boundary conductivities are much greater for the Zn-doped samples. The grain-boundary volume and accompanying resistances are much reduced on sintering at 1500 °C with ZnO addition in comparison to Zn-modified samples sintered below 1500 °C, with only minor changes in grain-boundary relaxation frequency observed. Conversely, in comparison to the undoped sample with sintering temperature of 1600 °C, there is an enormous improvement in the specific grain-boundary conductivity of two orders of magnitude for the ZnO-containing samples. Analysis on the basis of the core space-charge-layer model relates the enhancement of the grain-boundary transport to a higher concentration of charge carriers in the space-charge layer and associated lower potential barrier heights.

High-temperature dielectric properties of (Al, Nb) co-doped SrTiO3 ceramics

SrTi1-x(Al, Nb)xO3 (x=0, 0.005, 0.05, and 0.1) ceramic samples were prepared using the conventional solid-state reaction method. The dielectric properties of these samples were investigated in temperature range of 50–600°C and frequency range of 102–106Hz. The samples show two dielectric relaxations related to grain and grain boundary. The effects of (Al, Nb) co-doping were found to have influence on the grain-boundary relaxation by reducing its activation energy and enhancing the conductivity. Our results show that it is unsuitable to enhance the dielectric properties of SrTiO3 via (Al, Nb) co-doping.

Effect of annealing on mechanical properties of nickel electrodeposited using supercritical CO2 emulsion evaluated by micro-compression test

Nanocrystalline nickel film was electrodeposited with supercritical CO2 emulsion. The effects of annealing on nanocrystalline nickel were evaluated with scanning electron microscopy. The deposited films had a grain size of 16nm with 2at.% of interstitial carbon. A slight grain growth to 30nm grain size was observed after low temperature (<200°С) annealing. Higher temperature (>300°С) annealing caused Ni3C precipitation at grain boundaries and prominent grain growth above 300nm grain size. Micro-compression samples of each annealed films and as-deposited film were fabricated to investigate the mechanical properties. Low temperature annealed nickel was fractured by cracking with higher yield strength compared to the as-deposited nickel while high temperature annealed nickel simply had a decrease in strength. The brittle fracture after annealing is suggested to be related to the redistribution of carbon and relaxation of grain boundaries. High yield strength of 1GPa after 400°С annealing is achieved thanks to the fine precipitates of Ni3C.

Role of Relaxation on the Giant Permittivity and Electrical Properties of CaCu3Ti4O12 Ceramics

CaCu3Ti4O12 (CCTO) ceramics were synthesized under various sintering conditions to investigate the role of relaxation on permittivity and electrical properties. Two relaxation processes that respectively related to grain and to domain boundary at a temperature as low as 223 K were fitted according to the Cole–Cole theory. The results indicate that both relaxations largely account for the giant permittivity of CCTO ceramics. Moreover, the relaxation behaviors of grain and of the grain boundary can be processed via impedance plots that vary from 113 K to 473 K. It is shown that longer sintering duration leads to lower resistance of grain and of grain boundary: e.g., from 3200 Ω to 810 Ω and 1.76 MΩ to 0.48 MΩ, respectively. The activation energy related to grain-boundary relaxation drops from 1.14 eV to 0.80 eV, while the value of grain stays unchanged at about 0.11 eV. The Schottky barrier of the CCTO sample decreases from 0.65 eV to 0.57 eV. It is also proposed that the nonlinearity of current–voltage property for CCTO ceramics may be strongly related to the relaxation processes of grain boundaries.

Electric Relaxation Studies on La2−xNdxMo2O9 Oxide-Ion-Conductor Ceramics

The La2−xNdxMo2O9 (x = 0.4, 0.6, 0.8, 1.0) ceramics were successfully synthesized via the sol–gel method. Structural and microstructural analyses revealed that all the samples crystallize in the cubic phase and exhibit dense microstructures, although their particle sizes vary. Dielectric, impedance, and electric modulus spectroscopic analyses revealed two relaxation processes in both the temperature and the frequency spectra. Further investigation clarified that the two relaxation processes are associated with grain and grain-boundary relaxation. At temperatures less than 683 K, the bulk response is a localized relaxation process, whereas long-range conductivity dominates when the temperature is increased to 713 K.

Combined volumetric, energetic and microstructural defect analysis of ECAP-processed nickel

Difference dilatometry and differential scanning calorimetry (DSC) are used to investigate defect annealing in ultrafine grained nickel processed by equal channel angular pressing (ECAP) at various temperatures. Different defect types and processes such as vacancies, dislocations, grain boundaries and grain-boundary relaxation can be detected. They can be distinguished due to their distinct kinetics as revealed by the release of excess volume and excess heat during linear heating. The data are quantified in combination with a detailed characterization of the microstructure. Values for the absolute vacancy concentration, the dislocation density, the grain boundary expansion and the excess of grain boundary expansion in ECAP-processed nickel are derived.

The interplay between grain boundary structure and defect sink/annealing behavior

We present a series of results from atomistic simulations in three different materials (3 crystal structures) that demonstrate that the multiplicity of grain boundary (GB) structures at fixed macroscopic GB degrees of freedom is both extremely large and ubiquitous. The GB energy vs. misorientation curve that is commonly discussed is in fact a wide band, with many GB states very close in energy. The existence of so many GB states suggests that GB configurational entropy Sc is important for GB properties. We demonstrate that the GB Sc consists of two major contributions, one of which is geometric in nature and one that depends on bonding. We then show how this concept can be employed to predict GB relaxation dynamics by analogy with Adam-Gibbs theory, originally derived to predict the properties of glass forming liquids. Finally, we apply these predictions to understand GB denuded zone size during irradiation.

Dielectric, ac-impedance and modulus spectroscopic studies of nano-crystalline Bi0.5Na0.5TiO3 synthesized by using one pot glycine assisted solution combustion from inexpensive TiO2

Nano-crystalline Bi0.5Na0.5TiO3 (BNT) ceramic has been successfully synthesized for the first time by solution combustion synthesis using glycine as the fuel, inexpensive solid TiO2 powder as the raw material, and metal (Bi and Na) as nitrates. Phase formation, the crystalline nature, morphology, and chemical purity of the fabricated BNT were investigated with TGA, XRD, SAED patterns, SEM, TEM, and EDX analyses. TG/DTA analysis of the dry powder gives pre information about the formation of final product around 900 °C, which is a relatively lower temperature than other conventional ceramic methods. XRD patterns confirmed the formation of a single phase of all the sintered ceramics. The bright-field TEM image revealed that the particle size was in the range of 20–35 nm, which was in near agreement with the average crystallite size obtained from XRD. SEM images of the sintered BNT ceramics showed the average grain sizes were in the range of 150 nm–0.5 µm. EDX studies showed the presence of bismuth, sodium, titanium, and oxygen, which confirmed the stoichiometry and purity of the ceramics. The AC conductivity spectrum obeyed the Jonscher power law. The natures of relaxation behavior of the ceramics were rationalized by using impedance and modulus spectroscopy. The dielectric behavior of the ceramics exhibited Debye-like relaxation, and could be explained based on a Maxwell–Wagner model. The activation energies calculated from the grain-boundary relaxation time constant were found to be in the range of 1.5–1.7 eV.

Influence of Microalloying with Yttrium on the Relaxation Properties of Copper

The influence of microalloying with yttrium on the relaxation properties of copper is investigated. The grain-boundary and “impurity” grain-boundary relaxation peaks of the internal friction are discovered. The energy of binding of yttrium atoms with grain boundaries of copper is determined by measuring the “impurity” grain-boundary internal friction.