Intragranular Microcracking Research Articles

Semi-brittle deformation of antigorite serpentinite under forearc mantle wedge conditions

Serpentinization of the mantle wedge immediately above a subducting slab is a key process controlling the strength contrast between the slab and the overriding nominally anhydrous (i.e., olivine-rich) mantle. To assess the influence of serpentinite rheology on the degree of decoupling between the slab and mantle wedge, we conducted axial compression tests on partially foliated antigorite serpentinite using a modified Griggs-type apparatus at confining pressures (Pc) of 1.0–2.5 GPa, temperatures (T) of 300–600 °C, and a strain rate of 1.5 × 10−5 s−1. With increasing temperature, the dependence of maximum differential stress (i.e., strength) on Pc changes from positive to flat, suggesting an increasingly important contribution from plastic deformation. Microstructural observations show that increasing temperature promotes a transition in deformation mechanisms from distributed inter- and intra-granular microcracking to comminution and kinking localized within a through-going shear zone. The ductile-to-brittle transition can be attributed to the operation of thermally activated processes such as dislocation glide and grain boundary sliding in aligned antigorite grains. Comparison with strength data for dunite demonstrates that in the semi-brittle regime, the strength contrast between antigorite and olivine is < 2, which suggests that a thin antigorite layer will have only a weak decoupling effect at the slab–mantle wedge interface.

An experimental study on creep of partially molten granulite under high temperature and wet conditions

Samples of natural granulite were deformed in a gas medium apparatus to evaluate the flow strength of the lower crust. The sample consists of ∼52vol% plagioclase, ∼40vol% pyroxene, ∼3vol% quartz, ∼5vol% magnetite and ilmenite. Water content was ∼0.17±0.05wt% in the deformed samples. 40 creep tests were performed on 13 samples at 300MPa confining pressure, temperatures of 900–1200°C, and strain rates between 3.13×10−6 and 5×10−5/s, resulting in axial stresses of 12–764MPa and the total strain up to 7.8–20.5%.At low temperatures of 900–1000°C, the microstructural observations show that the granulite samples were deformed in semi-brittle deformation regime, mainly by dislocation glide and intragranular microcracking. At medium temperatures (MT) of 1050–1100°C, deformation was observed to be dominated by grain boundary migration recrystallization, corresponding to stress exponent nMT of 5.7±0.1, activation energies QMT of 525±34kJ/mol, logAMT of 1.3. At high temperatures (HT) of 1125–1150°C, the samples was deformed mainly by grain boundary migration recrystallization accommodated by partial melting and metamorphic reactions characterized by neo-crystallization of fine-grained olivine, with nHT of 4.8±0.1, QHT of 1392±63kJ/mol, and logAHT of 37.5. Partial melting at high temperatures of 1125–1200°C, which induces grain boundaries slip and enhances diffusion, has a significant weakening effect on the rheology of granulite, with an estimated strain rate enhancement by 5 times at melt fraction of ∼2vol%. Reaction from pyroxene to olivine may affect the flow law parameters and deformation mechanism. Based on our data, a wet and cool continental lower crust may still be in brittle deformation regime, whereas a hot lower crust may likely have a weak layer with plastic deformation.

Deformation and failure of alumina under high strain rate compressive loading

For development of structural ceramics e.g., alumina for high strain rate resistant applications, it is extremely important to understand the issues involved in compressive microfracture and the role of compressive microfracture in the global failure process at high strain rates. Thus the present work reports compressive strength of a dense (e.g., ρ ~97.2% ρth) 5μm grain size alumina exposed to high strain rate (e.g., 0.9×103s−1) loading in SHPB experiments. Concomitant utilization of high-speed videography has been exploited to study in-situ the details of the dynamic fragmentation process. The maximum compressive strength is measured to be ~3GPa. Post-mortem examination of the recovered alumina fragments has been performed by FESEM and TEM. Apart from conventional global brittle fracture, the results show the grain localized microcleavages, intragranular microcracking, plasticity, dislocations as well as formation of subgrain structure to occur in alumina. Based on these experimental results the reasons of compressive microfracture and its probable role in the global failure process of alumina ceramic are discussed.

Microstructural analysis of wear micromechanisms of WC–6Co cutting tools during high speed dry machining

This original study investigates the damages of WC–6Co uncoated carbide tools during dry turning of AISI 1045 steel at mean and high speeds. The different wear micromechanisms are explained on the basis of different microstructural observations and analyses made by different techniques: (i) optical microscopy (OM) at macro-scale, (ii) scanning electron microscopy (SEM), with back-scattered electron imaging (BSE) at micro-scale, (iii) energy dispersive spectroscopy (EDS), X ray mapping with wavelength dispersive spectroscopy (WDS) for the chemical analyses and (iv) temperature evolution during machining. We noted that at conventional cutting speed Vc≤250m/min, normal cutting tool wear types (adhesion, abrasion and built up edge) are clearly observed. However, for cutting speed Vc>250m/min a severe wear is observed because the behavior of the WC–6Co grade completely changes due to a severe thermomechanical loading. Through all SEM micrographs, it is observed that this severe wear consists of several steps as: excessive deformation of WC–6Co bulk material and binder phase (Co), deformation and intragranular microcracking of WC, WC grain fragmentation and production of WC fragments in the tool/chip contact. Thus, the WC fragments accumulated at the tool/chip interface cause abrasion phenomena and pullout WC from tool surface. WC fragments contribute also to the microcutting and microploughing of chips, which lead to form a transferred layer at the tool rake face. Finally, based on the observations of the different wear micromechanisms, a scenario of WC–6Co damages is proposed through to a phenomenological model.

Optical properties of anisotropic polycrystalline Ce3+ activated LSO

Polycrystalline cerium activated lutetium oxyorthosilicate (LSO:Ce) is highly desirable technique to make cost effective and highly reproducible radiation detectors for medical imaging. In this article methods to improve transparency in polycrystalline LSO:Ce were explored. Two commercially available powders of different particulate sizes (average particle size 30 and 1500nm) were evaluated for producing dense LSO:Ce by pressure assisted densification routes, such as hot pressing and hot isostatic pressing. Consolidation of the powders at optimum conditions produced three polycrystalline ceramics with average grain sizes of 500nm, 700nm and 2000nm. Microstructural evolution studies showed that for grain sizes larger than 1μm, anisotropy in thermal expansion coefficient and elastic constants of LSO, resulted in residual stress at grain boundaries and triple points that led to intragranular microcracking. However, reducing the grain size below 1μm effectively avoids microcracking, leading to more favorable optical properties. The optical scattering profiles generated by a Stover scatterometer, measured by a He–Ne laser of wavelength 633nm, showed that by reducing the grain size from 2μm to 500nm, the in-line transmission increased by a factor of 103. Although these values were encouraging and showed that small changes in grain size could increase transmission by almost three orders of magnitude, even smaller grain sizes need to be achieved in order to get truly transparent material with high in-line transmission.

Micromechanics of shear rupture and the control of normal stress

Conjugate microfault zones and distributed tensile microcracks were observed in low porosity sandstone samples deformed by shear box rupture at different normal stresses. Statistical data collected from scanning electron microscopic mapping show that tensile microcracks and microfaults are more frequent in the higher normal stress cases. The average thickness of the cataclastic microfault zones increases proportionally to rupture normal stress. The density of tensile microcracks increases locally in microfault relay and intersection zones where their formation is an important mode of microfault zone cataclasis. Concentration of longer en échelon tensile microcracks occurs in process zones ahead of the microfault tips, connected by cross-cracks in an incipient breakdown zone to further propagate the fault. Fracture densities are much lower in calcite cement grains whose compliancy serves to arrest microfault growth temporarily, yet also aid intragranular microcracking in adjacent stiff grains and at grain boundaries. Tensile microcrack and microfault average dips from the sample rupture surface increase with rupture normal stress, with conjugate microfault orientation variability at higher normal stresses aiding their connectivity. Results of this investigation demonstrate how the fracture porosity and connectivity around rupture zones increase dramatically at increased rupture normal stress, impacting on hydrodynamic properties of rupture zones.

A joint study of experimental deformation and experimentally induced microstructures of pretextured peridotites

A series of deformation tests was performed on samples from natural xenoliths and dunites with well‐defined textures and microstructures. The rock samples were deformed perpendicular to the foliation, 45° to the foliation, and parallel to the foliation at temperatures from 1000°C to 1200°C, at a confining pressure of 300 MPa, and a constant strain rate of 10−5/s to obtain insight into the rheological behavior of anisotropic lithospheric mantle material. The experiments showed strong work hardening for all samples with a large increase of the flow stress at relatively low strain. In all samples, both brittle and plastic mechanisms were operative. We observed nonlocalized‐dilatant and localized‐dilatant deformation features. Nonlocalized and localized plastic deformation created subgrain boundaries, deformation lamellae, and an increase in dislocation density in the olivine crystals. Heterogeneous dislocation activities occurred on the [100] (0kl) system at 1000°C, and on the [100] (010) and [101] (010) systems at 1200°C. Strong crystallographic fabrics developed in samples with olivines that were initially oriented with low resolved shear stresses on their slip systems. The postdeformation fabric intensity was lower in specimens with strong initial petrofabrics. Interactions between dislocations of different type led to high work hardening rates during high‐temperature deformation, favoring intragranular microcracking. Fractures opened parallel to the [100] screw dislocations in the (010) plane, and in the {hk0} planes oblique to the (100) plane. The strength of the deformed samples was temperature and orientation dependent. Rocks deformed perpendicular to the foliation are strongest, and those deformed parallel to the foliation have intermediate rock strength similar to those deformed at 45° to the foliation. Based on the laboratory results, we find important implications for the rheology of the lithosphere: (1) Peridotites develop semibrittle rheology even at high temperature, and show characteristic microstructures and fabric changes. (2) The orientation of compressive stress and initial petrofabric are important parameters for defining a deformation regime if pressure, temperature, and strain rate are fixed. (3) Interaction of slip systems produces strong work hardening and intragranular microcracking.

The role of microcracking and grain-boundary dilation during retrograde reactions

Abstract Under conditions of sub-greenschist and low greenschist facies retrogression, both intragranular microcracking and grain-boundary dilatancy are important processes for facilitating fluid access into domains between fractures in metamorphic rocks. Both processes play a crucial role in promoting retrogression of high-temperature minerals. In a quartz-rich schist from Norway, garnet retrogression is clearly linked to microcracking in the vicinity of a minor fracture. Intense microcracking of quartz extends up to 12 mm from the fracture, and permits fluid infiltration. This causes extensive (70–100%) garnet retrogression to chlorite. At distances &gt;16 mm from the fracture there is no sign of fluid infiltration and garnets are entirely unaltered. The spatial distribution of grain-boundary reaction products in a scapolite-dominated rock from Scotland provides evidence for the significant role played by transient grain-boundary dilatancy in promoting grain-scale fluid access and resultant retrogression. By applying distinct element modelling (UDEC code) the geometrical differences in distribution of grain-boundary reaction products are directly related to subtle differences in the magnitude and orientation of applied stresses and patterns of transient grain-boundary dilatancy.

Micromechanics of brittle faulting and cataclastic flow in Berea sandstone

The micromechanics of failure in Berea sandstone were investigated by characterizing quantitatively the evolution of damage under the optical and scanning electron microscopes. Three series of triaxial compression experiments were conducted at the fixed pore pressure of 10 MPa and confining pressures of 20, 50 and 260 MPa, respectively, corresponding to three different failure modes: shear localization with positive dilatancy, shear localization with relatively little dilatancy and distributed cataclastic flow. To distinguish the effect of non-hydrostatic stress from that of hydrostatic pressure, a fourth suite of hydrostatically loaded samples was also studied. Using stereological procedures, we characterized quantitatively the following damage parameters: microcrack density and its anisotropy, pore-size distribution, comminuted volume fraction and mineral damage index. In the brittle regime, shear localization did not develop until the post-failure stage, after the peak stress had been attained. The microcrack density data show that very little intragranular cracking occurred before the peak stress was attained. We infer that dilatancy and acoustic emission activity in the prefailure stage are primarily due to intergranular cracking, probably related to the shear rupture of lithified and cemented grain contacts. Near the peak stress, intragranular cracking initiates from grain contacts and this type of Hertzian fracture first develops in isolated clusters, and their subsequent coalescence results in shear localization in the post-failure stage. The very high density of intragranular microcracking and pronounced stress-induced anisotropy in the post-failure samples are the consequence of shear localization and compactive processes operative inside the shear band. In contrast, Hertzian fracture was a primary cause for shear-enhanced compaction and strain hardening throughout the cataclastic flow regime. Grain crushing and pore collapse seem to be most intense in weakly cemented regions. Finite element simulations show that the presence of cement at grain contacts alleviates the tensile stress concentration, thus inhibiting the onset of Hertzian fracture and grain crushing.

Micromechanics of stress-induced permeability anisotropy and damage in sedimentary rock

A discrete element model for cemented granular material is described which combines simple mechanisms of granular deformation, intergranular and intragranular microcracking, and pore channel fluid flow. Although the microstructural mechanics are simulated with very simple and idealized models, the dominant physical processes appear to be captured with sufficient completeness that complex macroscopic behavior may be investigated, including non-linear inelastic deformation, creation and coalescence of microcracks into localized damage zones and shear bands, and stress-induced permeability alteration and anisotropy. Simulation results compare well with experimental observations, providing insight into the physical mechanisms which may control inelastic material behavior and stress-induced permeability anisotropy in weakly-cemented geological materials. The magnitude of stress-induced permeability reduction is related to the amount and strength of intergranular cementation. At low stress levels fluid permeability is reduced due to compression of intergranular flow channels. For near-hydrostatic loading permeability continues to decrease as the material compacts. At increasing deviatoric stress levels, however, compression-induced permeability reduction is counteracted by enlargement of additional flow channels due to shear and tensile damage to the intergranular bonds and compression-induced intragranular microcracking. The material yields in a dilatant manner. Because these stress-induced microcracks have preferred orientation parallel to the maximum load direction, permeability of the rock becomes anisotropic at the macroscopic level.

Modelling the swelling and microcracking of boron carbide under neutron irradiation

Abstract The internal production of helium gas and point defects in neutron-irradiated boron carbide causes a significant swelling, the size of which is roughly linear with the irradiation dose and weakly temperature dependent when the irradiation temperature is lower than 1500°C. Extensive intergranular and intragranular microcracking also causes destruction of this neutron-absorbing ceramic. Lenticular bubbles filled with helium gas (penny-shaped cracks) are responsible for both problems. Careful consideration of the equilibrium conditions of the bubbles and of their growth in the presence of point defects and gas atoms permits us to estimate the size of the swelling. Balancing the fraction of helium which accumulates in bubbles and the fraction which precipitates at the grain boundaries enables us to calculate the threshold dose for intergranular microcracking. The problem of gas release from the material is also examined qualitatively. The very unusual behaviour of boron carbide under neutron irradiat...

Behaviour of implanted helium in boron carbide in the temperature range 750 to 1720°C

Abstract Alpha implantations in boron carbide samples were carried out different controlled temperatures ranging from 750 to 1720°C and the irradiated samples were observed in an electron microscope. Below 1500°C one part of the implanted helium produces lenticular bubbles within the grains whereas another part accumulates at the grain boundaries where it produces strain centres. Above 1500°C, tridimensional bubbles are visible in the grains as well as on the grain boundaries. In boron carbide, the brittle to plastic transition takes place around 1500°C. Below this temperature, the helium accumulation at the grain boundaries and the organization of bubbles into strings within grains are responsible for inter-and intragranular microcracking respectively. Above this temperature, because of plasticity, the cracking disappears. But the swelling increases markedly. At 1720°C, for instance, while less than 1% of the implanted helium is retained by the sample, each helium atom is associated with 100 vacancies within tridimensional bubbles responsible for a swelling of 1% per percent of implanted helium.

The effect of austenite grain size on microcracking in martensite of an Fe-1.22C alloy

Austenitic grain sizes of ASTM No. 9 and coarser were produced in an Fe-1.22 pct C alloy austenitized by immersion in molten lead at 1640†F (893°C), a temperature just above theA cm for this alloy, for periods between 20 s and 1 h. Microcracking sensitivity,Sv, measured as crack area/unit volume martensite, was determined as a function of grain size in brine quenched specimens. Two locations of microcracks were observed in this investigation: 1) intragranular, resulting from the impingement of one martensite plate with another, and 2) grain boundary or intergranular resulting from the impingement of martensite plates at prior austenite grain boundaries. Intragranular microcracking sensitivity, the subject of previous investigations, increased and became the dominant type of cracking with increasing grain size, and reached a constant level for grain sizes of ASTM No. 4.5 and coarser. Total microcracking sensitivity, consisting of both intragranular and grain boundary microcracks, also increased with increasing grain size, then decreased to approach the intragranular value for grain sizes coarser than ASTM No. 3.5. On the other end of the scale, grain boundary microcracking made up a much larger proportion of the total microcracking in the fine grained specimens.