Intragranular Grain Research Articles

Measuring deformation processes in granular materials from intragranular to bulk scales

Granular materials (assemblies of contacting grains with inter-granular porosity) form a vast family of materials important in both natural and industrial contexts. Under mechanical loads such systems show complex behaviours relating to the inter- and intra-granular stresses, strains and force transmission as well as the individual grain kinematics and interactions leading to multi-scale, heterogeneous behaviour. To understand such systems requires detailed understanding of these different structures and processes at the pertinent scales. This paper provides a brief overview of some new opportunities for enhanced experimental description of structures and mechanisms relating to the deformation of granular materials exploiting the penetrating power of x-rays for both imaging, giving access to granular structures and their evolution, and diffraction-based measurements, giving access to intra-granular grain structure and changes as well as inter- and intra-granular force transmission.

Effect of Heat Treatment Temperature on Microstructure, Tensile Properties and δ-Precipitate Phase in Ni-based Superalloy

Here, we investigated the influence of δ-precipitate (orthorhombic D0a Ni<sub>3</sub>Nb-ordered phase) on the room- and high-temperature tensile properties in wrought nickel-based Inconel 625 superalloys subjected to solution and aging heat treatment. Typically, solution heat-treatment temperatures in these alloys affect the solid-state precipitation of δ-phase, which governs high-temperature tensile properties. While precipitation of fine D0<sub>a</sub> δ-phase is known to have beneficial effects on the mechanical properties owing to the retardation of grain coarsening, Widmanstätten δ precipitation plays a deleterious influence on the fracture toughness, tensile ductility, and fatigue resistance. Therefore, to enhance the mechanical properties of this alloy series, it is key to generate a high number density of fine D0<sub>a</sub> δ precipitate by adjusting solid solution treatment temperatures. In this study, solution heat treatments were conducted above and below δ-phase solvus temperatures. By applying solution heat treatment at 900°C and 970°C, this alloy was confirmed to have a Widmanstätten δ phase and is composed similarly to the annealed microstructure. This Widmanstätten δ precipitate was densely distributed at both intergranular and intragranular grains. On the other hand, when solution treatment was applied at 1040 and 1100°C, more coarse particles (approximately 30 μm) with a significant reduction of Widmanstätten type δ phase were obtained. We found that grain size and Widmanstätten δ-phases have an important role in the high-temperature tensile properties of Inconel 625 superalloy series.

Correlation of Structural, Microstructural, Dielectric, and Impedance Properties in Nd Doping on Bi4Ti2FeO12 Multiferroic Aurivillius Ceramic

Bi4−x Nd x FeTi2O12 (BNFTO, x = 0, 0.5, 1.0, and 1.5) Aurivillius three‐layered ceramics are prepared via a solid‐state reaction method. The crystal structure of the compounds reveals that the compounds exhibit a major orthorhombic structure with the B2cb space group. Slight distortions in the orthorhombic unit cell and an increase in cell volume with the doping of Nd are observed. Highly dense randomly oriented plate‐like nonuniform grains are observed in the micrographs of the ceramics. A decrease in energy bandgap values is noticed in Nd‐doped ceramics from 2.88 to 2.18 eV. An increase in polarization values is observed with Nd doping due to the improved grain boundaries. The appearance of a weak ferromagnetic nature at low temperature followed by a decline in magnetic interaction at room temperature (RT) is observed for all compounds. The frequency and temperature variations of the dielectric constant are investigated. The frequency and temperature variations of the impedance measurements indicate that compounds exhibit non‐Debye‐type electrical relaxations and a negative temperature coefficient of resistance. Furthermore, single‐peak relaxations are predominant for all measured samples, mainly the contribution of intragranular grain regions. This makes Nd‐dopant BNFTO Aurivillius ceramics have a great research value for RT multiferroic applications.

Influence of grain boundaries on the properties of polycrystalline germanium

High-speed thin film transistors based on plastic substrates are indispensable to realize next-generation flexible devices. Here, we synthesized a polycrystalline Ge layer, which had the highest quality ever, on GeO2-coated substrates using advanced solid-phase crystallization at 375 °C. X-ray diffraction and Raman spectroscopy revealed that Ge on plastic had a compressive strain, while conventional Ge with a glass substrate had a tensile strain. This behavior was explained quantitatively from the difference in the thermal expansion coefficients between Ge and the substrate. Electron backscatter diffraction analyses showed that the Ge had large grains up to 10 μm, while many intragranular grain boundaries were present. The potential barrier height of the grain boundary was lower for the plastic sample than that for the glass sample, which was discussed in terms of the strain direction. These features resulted in a hole mobility (500 cm2/V s) exceeding that of a single-crystal Si wafer. The findings and knowledge will contribute to the development of polycrystalline engineering and lead to advanced flexible electronics.

Effects of particulate metallic phase on microstructure and mechanical properties of carbide reinforced alumina ceramic tool materials

Alumina-based composite ceramic tool materials reinforced with carbide particles were fabricated by the hot-pressing technology. Choice of metallic phase added into the present composite ceramic was based on the distribution of residual stress in the composite. The effects of metallic phase on microstructure and mechanical properties of composites were investigated. The metallic phase could dramatically improve room temperature mechanical properties by refining microstructure, filling pores and enhancing interfacial bonding strength. However, it also led to sharp strength degradation at high temperature because the metallic phase was easier to be oxidized and get soft at high temperature in air. The effects of metallic phase on strengthening and toughening were discussed. The improved fracture toughness of composite with metallic phase was attributed to the lower residual tensile stress in the matrix and the interaction of more effective energy consuming mechanisms, such as crack bridged by particle, crack deflection and intragranular grain failure.

Field, experimental, and numerical studies of deformation localization bands

Characterization and Formation Mechanisms of Deformation Localization Bands(Shear, Compaction, and Dilatancy): Field and Experimental Data,Theoretical Analysis and Numerical Models;Montpellier, France, 9–10 May 2011. The origin of fractures and discontinuities in rocks has been for decades a subject of common interest in both the academic world and industry, as these features influence hydrocarbon reservoir production. Fractures (notably joints) and deformation bands are usually analyzed on a separate basis. Joints have commonly been interpreted as mode I fractures susing fracture mechanics, whereas the bifurcation theory has been used for deformation bands. This diversity was called into question by the research carried on by the Geo‐FracNet consortium sponsored by Total and Shell International. New findings (A. I. Chemenda et al., J. Geophys. Res., 116, B04401, doi:10.1029/2010JB008104, 2011, and references therein) suggest that at least a part of joints could have been formed as dilatancy (dilation) bands, i.e., due not to the strong effective tensile stress concentration at the opening fracture tip but to the porosity increase and decohesion (perigranular and/ or intragranular grain breakage) of the material within a band several grains thick. The band (material damage and dilatancy within it) can propagate along the strike without opening and can eventually be opened.

Intragranular nucleation sites of massive γ grains in a TiAl-based alloy

Massive γ grains were generated in a TiAl-based alloy through ice-water quenching from the α domain. Apart from those located along α 2 /α 2 grain boundaries, a few massive γ grains were detected inside the α 2 grains. Some of these intragranular grains were revealed to be attached to particular α 2 twins ( K 1 : “ { 2 ¯ 12 10 ¯ 3 } ” η 1 : 〈 5 1 ¯ 4 ¯ 6 ¯ 〉 ), in a Blackburn orientation relationship with them. Others were identified as developing in feathery colonies of similar crystallographic orientation. Whatever the nucleation site, further growth of all massive γ grains involves successive twinning on {1 1 1} γ planes.

Ceramic nanocomposites obtained by sol–gel coating of submicron powders

MgAl 2O 4–ZrO 2 nanocomposites were fabricated by conventional sintering of composite powders obtained by sol–gel coating of a submicron spinel powder. In the composite powder the zirconia grains remain narrow sized and completely tetragonal even after being heat treated at temperatures where a free xerogel is completely monoclinic. The sintered material exhibits a dense, fine and highly homogeneous microstructure. The zirconia nanoparticles are located at both inter- and intragranular positions and exhibit heteroepitaxial relationships with the surrounding crystals. Tetragonal zirconia seems to be stabilised by an interface effect. Both the scale of the microstructure and the fraction of intragranular grains were controlled by adjusting the mean grain size of spinel grains before coating and sintering conditions.

Effect of testing environment on intergranular microsuperplasticity in an aluminum MMC

A correlation between the testing environment and the observed high temperature deformation characteristics of a Al 6090 (Al Mg Si Fe Cu alloy) + 25 vol.% SiCp metal matrix composite (MMC) has been conducted. Post deformation observations were performed on similar material specimens tested in uniaxial tension to failure within either air or argon high temperature environments. Intragranular grain boundary whiskers that exhibited exceptionally high local superplastic-like elongations were observed when deformed in air at a temperature which was approximately 80% of, or greater than, the incipient melting point of the matrix and at a strain-rate of 4 × 10 −4 s −1; whereas in a relatively pure argon environment under similar deformation conditions, the whiskering phenomenon (i.e. microsuperplastic whiskering, MSPW) at failure was not observed. Differential thermal analysis of the MMC microstructure verified that the observed MSPW behavior was a solid-state deformation process rather than being related to the presence of a liquid or semi-solid. The chemical constitution of the observed whiskers is thermodynamically most likely to be of a spinel compound, MgAl 2O 4.

Acoustic emission, microstructure, and damage model of dry and wet sandstone stressed to failure

Twenty‐three uniaxial compression tests were performed on dry and wet Flechtingen sandstone from Germany. Compressive strength of wet core is 60% of the strength of dry core. Before fracture, the transverse P wave speed drops by 13% and the pulse amplitude by 22% for wet and 37% for dry cores. Accumulated strain energy doubles for dry core. Acoustic emissions (AE) are detected with 10 sensors for 19 cores. AE activity starts at 84% of the fracture strength of wet cores (55 MPa) and at 91% of the strength of dry cores (87 MPa). The ratio of located to recorded AE is 0.37 for dry and 0.13 for fully wet cores. AE hypocenter patterns document the development of two opposite fracture cones. The negative slope of cumulative AE‐amplitude frequency distribution drops by 50% before failure in dry cores. The slope of the wet core drops and recovers. Energy discrimination of AE detected by a broadband sensor resolves different stages of damage and captures the onset of the dilatant throughgoing macrofracture. Using the analogy to visible light microfracturing events are separated into high‐energy short pulses (blue AE) and low‐energy pulses with long duration times (red AE). Blue AE are explained by intragranular grain breakage, red AE by multiple stick slip on crack planes or grain boundaries. Deformed cores show highly fractured calcite cement and mostly intact quartz grains. The stochastic damage model for brittle composites developed highlights that microfracturing of the sandstone is controlled by the amount and distribution of the weak mineral (calcite).

Control of Microstructure of Alumina Ceramics by Dispersion of Nano SiC Particulates

Al2O3/SiC composites were prepared by hot-pressing in order to investigate the microstructural characteristics and nano-disperse behavior of alumina matrix composites. The grain size of the monolithic alumina ceramics increased with increasing hot-pressing temperature, and the exaggerated grain growth occurred above 1800°C. However, the addition of SiC particulates effectively suppressed the growth of the alumina matrix in the composites. The average matrix grain size was smaller and the grain size distribution was narrower than those of the monolithic alumina ceramics. Very fine SiC particulates were well dispersed in composites as nano-sized intragranular grains in the alumina matrix, on the other hand, coarse SiC particulates were found to exist at grain boundaries of the alumina matrix. The intragranular SiC seems to be trapped like a pores in the matrix grains during the densification by hot-pressing. It is concluded that nano-sized SiC particulates are grain growth inhibitors and microstructural stabilizers thus giving a uniform microstructures with fine grains.

Temperature‐induced, low‐frequency acoustic emission of quartz related to formation of microcracks, fluid inclusion decrepitation, and phase transitions in the crystal structure

Quartz, as the most ubiquitously occurring mineral, was chosen for investigations of the acoustic emission upon heating. Various samples from a wide range of conditions of formation were analyzed in the frequency range of 400–10 000 Hz with a high‐resolution decrepitometer. The acoustic emission patterns showed a variation of 5000–80 000 total signals over the entire temperature range of 90 °C‐610 °C (1‐cm3 sample, grainsize fraction 250–500 μm). Microthermometric investigations of the sound‐emitting processes and correlation with microcrack patterns determined by scanning electron microscopy (SEM) showed that various sources contribute to the sometimes vehement noise production. Stress built up because of thermal anisotropy causes the formation of microcracks along inter‐ and intragranular grain boundaries over most of the observed temperature range. Accumulation of signals from the decrepitation of fluid inclusions reveals information about the conditions of formation and subsequent alterations of the investigated sample. Acoustic emission caused by phase transitions in the crystal structure (α〈 − 〉β‐transition at 573 °C) is related to changes in the twinning state of the sample. Measurement of the complex patterns of acoustic emission produces a “fingerprint” of the conditions of formation and subsequent alterations the sample underwent in its geological history. They can be used to determine the extension of mineralizations and alterations or detection of geological structures even when covered by soil. Real‐time frequency analysis of the signals shall reveal further information about the origin of single acoustic signals. [Work supported by the DFG.]

Dynamic recrystallization and chemical evolution of clinoamphibole from Senja, Norway

Clinoamphibole from a mylonitic amphibolite exhibits microstructures characteristic of dynamic recrystallization, including porphyroclasts in a finer grained matrix of needle-shaped amphibole. The matrix amphibole defines an LS fabric and porphyroclasts have core and mantle structures with a core containing undulose to patchy extinction and (100) deformation twinning surrounded by a mantle of recrystallized grains. In addition intragranular grains also occur within the cores. TEM analyses of the porphyroclasts reveal that they contain a wide variety of lattice defects including high densities (5 × 108cm−2) of free dislocations and dislocation arrays, dissociated dislocations, stacking faults, and (100) micro-twins. TEM also shows that matrix grains and intragranular grains have relatively low defect densities, and that the intragranular new grains occur at localities in the porphyroclasts characterized by high densities of dislocations. These observations along with the chemical and orientation relationships between the recrystallized grains and porphyroclasts indicate that the new grains may have formed by heterogeneous nucleation and that further growth probably occurred by both strain assisted and chemically induced grain boundary migration or liquid film migration. This recrystallization event is interpreted to be synkinematic based on the fact that no recrystallization textures are present in the matrix grains and that the matrix grains define an LS fabric. However, the low defect densities in the matrix grains and the lack of intracrystalline strain in other phases indicate that post-kinematic recovery processes were active.