Five-parameter grain boundary distribution of commercially grain boundary engineered nickel and copper

The five-parameter grain boundary distributions of grain boundary engineered nickel and copper specimens have been analyzed in detail. The relative areas of {1 1 1} planes in the entire population did not increase as a result of grain boundary engineering (GBE) and, in the Σ3-excluded population, decreased after GBE. This decrease occurred because the majority of the newly generated Σ3 grain boundaries were not coherent twins with {1 1 1} grain boundary plane orientations. GBE increased the proportion of Σ3 boundary length that was vicinal-to-{1 1 1} and the proportion of asymmetrical 〈1 1 0〉 tilt boundaries. There was a clear propensity for selection of particular planes or plane combinations which were associated with low energy. These plane types were analyzed in some detail, and it was shown that many of these boundaries were asymmetrical tilts comprising (or vicinal to) at least one low-index plane.     

Motion of a grain boundary facet in aluminum

The shape and migration of a capillary-driven 20.8°〈1 0 0〉 tilt grain boundary in aluminum bicrystals were investigated in situ in a scanning electron microscope at elevated temperatures. The moving boundary assumed a semi-faceted configuration composed of a singular facet connected to a curved boundary section joined at a distinct edge. At constant temperature the boundary with a facet moved steadily and its shape remained self-similar. Both the facet length and the connecting angle between the facet and the curved boundary section did not change over the investigated temperature range. The capillary-driven migration of the semi-faceted grain boundary in “quarter-loop” geometry was analyzed. The obtained results revealed that the major factor responsible for the formation of the mobile facet was the energy difference between the facet and the curved boundary. The temperature dependence of the facet mobility was determined. The migration activation enthalpy of the investigated capillary-driven planar boundary/facet was a factor of two higher than the activation enthalpy for migration of a geometrically similar boundary driven by an applied stress.     

Grain growth inhibition by connected porosity in sintered niobium

Pore–boundary interaction plays an important role in densification during solid-state sintering. This paper reports the evolution of porosity and grain size in niobium sintered at 2073 and 2273 K for different sintering times. The densification curves show a decrease in porosity up to 8 and 4 vol.% after 10,800 s for the samples sintered at 2073 and 2273 K, respectively. Grain growth is observed to take place together with this decrease in porosity. A new model for grain growth inhibition during sintering is proposed for connected porosity. This model considers that the moving grain boundaries and the outer surface of cylindrical pores remain in contact during grain growth and that energy dissipation takes place owing to the fact that the grain boundary is moving relative to the porosity. Our mechanism is akin to a friction between the grain boundary and the connected porosity at their contact region. In contrast to the traditional particle–grain boundary bypassing mechanisms, the present model is not a purely geometrical relationship but is material dependent. The model gives agrees well with experimental results obtained in this paper for sintered niobium as well as for other sintered materials reported in the literature. Our model is a novel approach to treating grain growth inhibition by pores during the sintering stage in which the porosity becomes interconnected.     

An insight into the role of the grain boundary in plastic deformation by means of a bicrystalline pillar compression test and atomistic simulation

A combination of experimental and molecular dynamics (MD) simulations is used to investigate the interaction of dislocations with a selected grain boundary (GB) in bicrystalline pillars (BCPs) with component crystals oriented for single slip and multiple slip. As a reference, single-crystalline pillars with the same orientations are also tested and compared with the BCPs. Orientations identical to the experiments are used to generate models in MD simulations. Further, electron backscatter diffraction (EBSD) measurements on the cross-section of the pillars are performed to investigate the crystal lattice rotation in correlation with the excess dislocation density. A clear change in mechanical behavior of the BCP was observed when the size of the component crystals reduced below 1 μm. The EBSD analyses of these small BCPs showed an increase in the misorientation in the vicinity of the GB. MD simulation provided atomistic insights into the dislocation nucleation process and the BCPs’ interaction with the GB. On the basis of these observations, it is concluded that in BCPs smaller than 1 μm the dislocation–GB interaction plays a more crucial role than the dislocation–dislocation interaction.     

Spatial orientations and structural irregularities associated with the formation of microbands in a cold deformed Goss oriented Ni single crystal

The crystallographic nature of microband boundaries was investigated in a Goss oriented nickel single crystal following cold deformation in channel die plane strain compression. Standard electron backscatter diffraction (EBSD), three-dimensional (3-D)-EBSD and transmission electron microscopy (TEM) were used in the investigation. When viewed in the three orthogonal sections microband boundary traces were classically aligned in the transverse direction section at an acute angle from the rolling direction (RD), but appeared wavy in the normal direction (ND) section. The latter observation may lead to the conclusion that microband boundaries are non-crystallographic. 3-D EBSD was used to reconstruct actual microbands in a deformed volume that revealed significant new information about their structure. Here microband surfaces are largely planar over large distances, but frequently interrupted by local distortions and undulations due to interactions between intersecting non-coplanar microbands. The combined EBSD/TEM investigation has revealed that microband boundaries are aligned close to an active {1 1 1} slip plane (i.e. they are crystallographic), but the undulations and distortions they contain are non-crystallographic in the sense that they deviate from an active slip plane. The non-crystallographic features of microbands (as revealed by their wavy structure in the ND section) may be explained by the crystallographic oscillations of up to ±7.5° towards RD that occur during plastic deformation. Such oscillations result in varying fractions of slip on a given {1 1 1} plane, resulting in varying degrees of interaction between the two sets of non-coplanar microbands. These local and intense microband interactions result in their deviation from their active slip planes.     

Grain boundary embrittlement by Mn and eutectoid reaction in binary Fe–12Mn steel

The grain boundary embrittlement in a binary Fe–12Mn is due to the grain boundary segregation of Mn. During tempering at 400 °C (higher than the equilibrium eutectoid reaction temperature 247 °C), reverted austenite particles were formed at lath and grain boundaries through the equilibrium reaction of lath martensite to ferrite + austenite. Surprisingly, hydrostatic pressure, which is induced by the transformation of epsilon martensite to austenite during heating at the tempering temperature, resulted in the nonequilibrium eutectoid reaction producing α-Mn precipitates at the interface between lath martensite and the transformed austenite during the tempering. The segregation concentration kinetics of Mn formed a convex profile due to the active grain boundary precipitation of the reverted austenite particles and the α-Mn particles, which act as a sink for the segregated Mn. Finally, the convex segregation profile of Mn corresponded to the concave profile of intergranular fracture strength.     

Linking recovery and recrystallization through triple junction motion in aluminum cold rolled to a large strain

Recovery mechanisms and kinetics have been studied in commercial purity aluminum (AA1050) cold rolled to a true strain of 5.5 (99.6% thickness reduction) and annealed at low temperatures from 140 to 220 °C. Transmission electron microscopy, electron backscatter diffraction (EBSD) and electron channeling contrast (ECC) are used to characterize the microstructural evolution during annealing. The microstructural characterization shows that a deformed lamellar structure coarsens uniformly during annealing by triple junction motion while maintaining the lamellar morphology, leading to a gradual transition into a more equiaxed structure, where recrystallization nuclei start to evolve. The apparent activation energy for the microstructural coarsening is estimated separately for different stages characterized by an increase in the lamellar boundary spacing measured by EBSD and ECC. The apparent activation energy increases during annealing, from 110 kJ mol^?1 at the beginning to 230–240 kJ mol^?1 at the end of uniform coarsening, linking the recovery stages to recrystallization. The increase in activation energy underpins operation of different diffusion mechanisms for migration of boundaries and their junctions during coarsening, and solute drag may become increasingly important as the structure coarsens. These findings form the basis for a discussion of the thermal behavior of a fine lamellar structure produced by cold rolling to a large strain of both scientific and applied interest.     

Structure and free volume of 〈1 1 0〉 symmetric tilt grain boundaries with the E structural unit

Atomistic simulations are used to investigate the structure and interfacial free volume of 〈1 1 0〉 symmetric tilt grain boundaries in copper containing the E structural unit from the Σ9(2 2 1)θ = 141.1° grain boundary. In this work, a stereologically-based methodology is used to calculate the grain boundary free volume along with the spacing and connectivity of free volume. After generating the minimum energy equilibrium grain boundary, we examine (i) the grain boundary structure, (ii) a measure of free volume associated with the grain boundary, (iii) spatial correlation functions of the distribution of free volume, and (iv) images of grain boundary free volume distribution. Using the results from these calculations, the influence of free volume spatial distribution and grain boundary structure on dislocation dissociation and nucleation is briefly discussed for boundaries with the E structural unit subjected to tensile loading normal to the interface along with the potential implications of free volume connectivity.     

Silicon Σ13(5 0 1) grain boundary interface structure determined by bicrystal Bragg rod X-ray scattering

The atomic structure of the silicon Σ13(5 0 1) symmetric tilt grain boundary interface has been determined using Bragg rod X-ray scattering. In contrast to conventional structural studies of grain boundary structure using transmission electron microscopy, this approach allows the non-destructive measurement of macroscopic samples. The interface was found to have a single structure that is fully fourfold coordinated. X-ray diffraction data were measured at Beamline I07 at the Diamond Light Source.     

Grain boundary mobility and grain growth behavior in polycrystals with faceted wet and dry boundaries

The effect of an intergranular amorphous film on grain growth behavior has been studied in a faceted model system, BaTiO₃. We prepared two kinds of samples with and without intergranular amorphous films but with the same grain size and density. During annealing the samples at 1350 °C in air, abnormal grain growth occurred in samples with intergranular amorphous films while grain growth was inhibited in samples with dry boundaries, indicating the presence of a pinning force in samples with dry boundaries. To compare the mobilities of dry and wet boundaries, single crystal and polycrystal bilayer samples with or without amorphous films were prepared and annealed at 1340 °C. In contrast to the observed grain growth behavior in polycrystals, the growth of the single crystal into the polycrystal with dry boundaries was faster than that into the polycrystal with wet boundaries, demonstrating the higher mobility of a dry boundary, unlike the conventional understanding.     

Nucleation and variant selection of secondary α plates in a β Ti alloy

The microtexture of secondary α plates in Ti–4.5Fe–6.8Mo–1.5Al has been investigated by electron backscatter diffraction (EBSD) to obtain more insight in the nucleation and variant selection of these α plates. A statistical analysis of the EBSD data shows that for most β grain boundaries the variant selection of the α plates is in agreement with a commonly used variant selection criterion yielding that the α-{0 0 0 1} pole is nearly parallel to the closest β-{1 1 0} poles of the two adjacent β grains. For a small angle between the β-{1 1 0} poles nucleation is predominantly observed at both sides of the grain boundary, while with increasing angle some β grain boundaries exhibit nucleation of α plates at only one side. In the β grain interior many so-called Type 2 α–α grain boundaries are observed which are thought to originate from autocatalytic nucleation when a new α plate is formed at an existing α–β interface.     

The role of crystal misorientations during solid-state nucleation of ferrite in austenite

The role of grain and phase boundary misorientations during nucleation of ferrite in austenite has been investigated. Electron back-scatter diffraction (EBSD) was performed on a high-purity iron alloy with 20 wt.% Cr and 12 wt.% Ni with austenite and ferrite stable at room temperature in order to identify the crystallographic misorientation between austenite grains and between ferrite and austenite grains. It is observed that the specific orientation relationships between ferrite and austenite play a dominant role during solid-state nucleation of ferrite. Ferrite grains nucleate on grain faces independently of the misorientation between austenite grains, although random high-angle grain boundaries have a slightly higher efficiency. Different types of nucleation mechanisms are found to be active during ferrite formation at grain faces. A slight deformation of the austenite matrix was found to triple the number of ferrite nuclei during isothermal annealing.     

Formation and retention of low Σ interfaces in PTC thermistors

The distribution of grain boundary types in a barium titanate based positive temperature coefficient of resistance (PTC) thermistor was established using electron backscatter pattern analysis in a scanning electron microscope. A higher proportion of Σ = 3 interfaces than would be expected from a random grain distribution was observed, together with numerous Σ = 5 and Σ = 9 grain boundaries. All of these grain boundary types are reported in the literature to be PTC inactive, and hence should not contribute to the overall PTC response of the device. The Σ = 3 interfaces were further characterised on the basis of their microstructure as grain boundaries or annealing twins, from which it was established that the proportion of Σ = 3 grain boundaries in the thermistor increased only slightly during the sintering process, whereas the proportion of annealing twins increased significantly during the first 30 min of soak at the peak sintering temperature. Σ = 3 grain boundaries were found to be immobile during grain growth, in contrast with high angle grain boundaries, which showed considerable migration.     

Survey of computed grain boundary properties in face-centered cubic metals—II: Grain boundary mobility

The absolute grain boundary mobility of 388 nickel grain boundaries was calculated using a synthetic driving force molecular dynamics method; complete results appear in the Supplementary materials. Over 25% of the boundaries, including most of the non-Σ3 highest mobility boundaries, moved by a coupled shear mechanism. The range of non-shearing boundary mobilities is from 40 to 400 m/s GPa, except for Σ3 incoherent twins which have mobilities of 200–2000 m/s GPa. Some boundaries, including all the 〈1 1 1〉 twist boundaries, are immobile within the resolution of the simulation. Boundary mobility is not correlated with scalar parameters such as disorientation angle, Σ value, excess volume or boundary energy. Boundaries less than 15° from each other in five-dimensional crystallographic space tend to have similar mobilities. Some boundaries move via a non-activated motion mechanism, which greatly increases low-temperature mobility. Thermal roughening of grain boundaries is widely observed, with estimated roughening temperatures substantially among boundaries.     

Nonlinear driving force–velocity relationship for the migration of faceted boundaries

A non-linear relationship between boundary migration and its driving force for faceted grain boundaries is demonstrated in a BaTiO₃ model system using the solid-state single-crystal growth technique. BaTiO₃ (0.4 mol.% TiO₂) samples with different grain sizes were prepared by sintering powder compacts in H₂ for various times. Single-crystal seeds with {1 0 0} and {2 1 0} orientations were joined to the sintered samples and annealed in air for various times up to 20 h. During the air annealing, the matrix grain size did not change, indicating that the driving force for the growth of the single-crystal seeds remained constant. Under driving forces that exceeded a critical value, the seed crystals grew into the polycrystals. The critical driving force was lower and the growth rate was ～10 times higher for the {1 0 0} crystal compared with the {2 1 0} crystal. These results demonstrate the presence of a critical driving force for the migration of a faceted boundary and the remarkable effect of the crystallographic orientation on the boundary migration. The observed nonlinear migration behavior of faceted boundaries is similar to that of faceted solid/liquid interfaces.     

Relative grain boundary area and energy distributions in nickel

The three-dimensional interfacial network of grain boundaries in polycrystalline nickel has been characterized using a combination of electron backscatter diffraction mapping and focused ion beam serial sectioning. These data have been used to determine the relative areas of different grain boundary types, categorized on the basis of lattice misorientation and grain boundary plane orientation. Using the geometries of the interfaces at triple lines, relative grain boundary energies have also been determined as a function of lattice misorientation and grain boundary plane orientation. Grain boundaries comprising (1 1 1) planes have, on average, lower energies than other boundaries. Asymmetric tilt grain boundaries with the Σ9 misorientation also have relatively low energies. The grain boundary energies and areas are inversely correlated.     

Microstructure evolution during dewetting in thin Au films

Thin metal films can degrade into particles in a process known as dewetting. Dewetting proceeds in several stages, including void initiation, void growth and void coalescence. Branched void growth in thin Au films was studied by means of electron backscatter diffraction (EBSD). The holes were found to protrude into the film predominantly at high angle grain boundaries and the branched shape of the holes can be explained by surface energy minimization of the grains at the void boundaries. (1 1 1) Texture sharpening during dewetting was observed and quantified by EBSD and in situ X-ray studies.     

Segregation engineering enables nanoscale martensite to austenite phase transformation at grain boundaries: A pathway to ductile martensite

In an Fe–9 at.% Mn maraging alloy annealed at 450 °C reversed allotriomorphic austenite nanolayers appear on former Mn decorated lath martensite boundaries. The austenite films are 5–15 nm thick and form soft layers among the hard martensite crystals. We document the nanoscale segregation and associated martensite to austenite transformation mechanism using transmission electron microscopy and atom probe tomography. The phenomena are discussed in terms of the adsorption isotherm (interface segregation) in conjunction with classical heterogeneous nucleation theory (phase transformation) and a phase field model that predicts the kinetics of phase transformation at segregation decorated grain boundaries. The analysis shows that strong interface segregation of austenite stabilizing elements (here Mn) and the release of elastic stresses from the host martensite can generally promote phase transformation at martensite grain boundaries. The phenomenon enables the design of ductile and tough martensite.     

Influences of chromium and hafnium additions on the microstructures of β-nial coatings on superalloy substrates

In this study, Hf-doped and Cr/Hf-modified NiAl coatings were deposited onto René N5^® substrates via direct current magnetron sputtering. Microstructural analysis using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron probe microanalysis (EPMA) showed the as-deposited coatings to be single phase with B2 crystal structures. Post-deposition annealing at 1000 °C in an Ar + 5%H₂ atmosphere resulted in the formation of nanometer-sized precipitates along grain boundaries and within grain interiors. TEM analyses showed most of the precipitates to be β′-Ni₂AlHf. Three-dimensional atom probe tomography (3D-APT) also revealed the presence of α-Cr precipitates within the NiAlCrHf coatings after annealing. The results have been analyzed and discussed relative to previous research on sputter deposited NiAl–Hf coatings.     

Inclination dependence of grain boundary energy and its impact on the faceting and kinetics of tilt grain boundaries in aluminum

The shape evolution and migration of <1 0 0> and <1 1 1> tilt grain boundaries with rotation angles θ in the range between 6° and 24° were investigated in situ in a scanning electron microscope at elevated temperatures. The results revealed that boundaries with misorientation θ < 15° did not attain a continuously curved shape in the entire temperature range up to the melting point and, thus, did not move under a capillary driving force. Instead, they remained straight or formed several facets which were inclined to the initial boundary orientation. Molecular statics simulations suggest that the observed behavior of low-angle boundaries is due to the anisotropy of grain boundary energy with respect to boundary inclination. This anisotropy diminishes with increasing misorientation angle, and high-angle boundaries assume a continuously curved shape and move steadily under the curvature driving force.