Serrated Grain Research Articles

A novel strategy creating serrated grain boundaries to improve ductility in a Fe–Cr–Al alloy

Grain boundaries serration has long been regarded as a way to improve mechanical properties at high temperatures in alloys. However, the traditional method of creating serrated grain boundaries (SGBs), solution treatment followed by slow cooling, could induce a significant coarsening behavior of the second phase, thereby adversely affecting the properties. In this work, a new strategy is proposed to obtain numerous irregular SGBs without producing coarser Laves precipitates (<400 nm) in Fe–Cr–Al alloys, in which these SGBs are formed through recrystallization annealing of a warm-rolled microstructure featuring kink bands (KBs). These SGBs can be divided into two types: low amplitude SGBs (LA-SGBs, in the range of 1∼10 μm), and high amplitude SGBs (HA-SGBs, over 20 μm). Through SEM and quasi in-situ EBSD observation, and combined with the Taylor factor model, the formation mechanism of irregular SGBs is revealed in detail. The pinning effect of Laves phase precipitated during annealing on the grain boundaries migration induces the formation of LA-SGBs. On the other hand, the discrepancy in deformation stored energy between the matrix and the KBs promotes the special nucleation and growth of new grains, thus resulting in the formation of HA-SGBs. Moreover, tensile tests are conducted at 298 K and 873 K. SGBs can significantly enhance ductility while maintaining strength, exhibiting an increase of 52.1% and 41.1% respectively. Compared to straight grain boundaries, at 298 K, SGBs can reduce the accumulation of the GNDs in the vicinity of themselves through the unique strain-hardening behavior mediated by high-density dislocation walls (HDDWs), alleviate the stress concentration, make the plastic strain distributions more uniform, and thus retard the crack initiation. As the deformation temperature rises to 873 K, the improved ductility is mainly ascribed to the deflection and passivation effects of SGBs on crack propagation during straining from necking to fracture while their effects on the strain-hardening stage are weakened.

Advancements in processing of Ni-based superalloys by microstructure engineering via discontinuous γ′ break-down

Wrought γ′-strengthened Ni-based superalloys are critical materials for gas turbine disks in aircraft and power-generation applications. However, their high inherent strength makes their hot formability challenging, and this is often related to intergranular cracking. We propose that this type of crack formation may be mitigated by grain boundary serrations which has not yet been demonstrated in superalloys. Here, we introduce engineered grain boundary serrations in a Ni-based superalloy via discontinuous γ′ precipitation during slow cooling after super-solvus solutionising. We compare the formability of this microstructure to a counterpart with globular γ′ and straight grain boundaries, during sub-solvus compression at 950–1130 °C. We demonstrate that the microstructure with discontinuous γ′ shows excellent cracking resistance, while the one with globular γ′ develops severe intergranular cracks during compression ≤1100 °C. The discontinuous γ′ microstructure exhibits up to 26% lower peak flow stresses and recrystallises at temperatures ≤1100 °C, compared to the globular γ′ microstructure where recrystallisation does not start until 1130 °C. Compression of the discontinuous γ′ microstructure at 1000–1050 °C yields recrystallised fractions of >75 vol.% and results in a duplex γ-γ′ microstructure with grain diameters <4 μm. We argue that its significantly improved formability results from these engineered serrated grain boundaries, the coarsened γ′ morphology, energy absorption by the break-down of discontinuous γ′, and the resulting development of a fine-grained duplex γ-γ′ microstructure. Integrating such discontinuous γ′ precipitation into advanced manufacturing routes will facilitate or, in some cases, even enable manufacturing of parts for aircraft turbines and gas engines at lower temperatures and forces.

Creep anisotropy reduction and improvement via post-heat treatment in yttrium-added Hastelloy-X fabricated by laser powder bed fusion

The development of laser powder bed fusion (LPBF) has made it possible to produce complex three-dimensional components for high-temperature applications. The LPBF process needs to be refined to address several key factors, such as high-temperature elongation, microstructure heterogeneity, and mechanical anisotropy. Hastelloy-X Ni-based superalloy was used to illustrate these issues in this study. First, 0.046 wt.% of yttrium (Y) was added to Hastelloy-X (HX-y) to prevent grain boundary embrittlement. The second step involves two kinds of post-heat treatments; i) at 850 °C for 2 h (DA) for carbides and ii) solution heat treatment at 1240 °C for 8 h upon aging at 850 °C for 2 h (HTA). The creep properties of the samples were compared at 900 °C/ 80 MPa to understand the effect of Y addition and post heat treatments. The HX-y specimen was strengthened by solid solution and dispersion of Y-rich oxides, together with stabilization of oxygen-based contamination at the grain boundaries. The DA and HTA HX-y specimens had better creep properties than the HX specimens. The HX-y specimen showed superior creep properties to the HX specimen due to the presence of carbides M6C and Cr23C6 inside grains and at grain boundaries. However, carbides remained stable even at high temperatures within grains and at grain boundaries. Nevertheless, the HTA HX-y specimen exhibited superior isotropic creep properties. As a result of grain boundary pinning, serrated grain boundaries prevented grain boundary sliding. In contrast, HX specimens exhibited poor creep properties. This study confirmed that the optimal addition of Y together with the optimization of post-heat treatment drastically enhances the creep properties of materials fabricated by the LPBF process.

Investigation of the Heat Treatment Process and Formation Mechanism of Grain Boundary Serration for GH4795 Superalloy

Heat treatments, including solution treatment and isothermal heat treatment, were conducted to investigate the grain boundary serration of GH4975 superalloy. The two different heat treatment processes could both promote the formation of serrated grain boundaries within the present temperature and soaking time ranges, provided that the cooling rates were controlled to be quite slow. The samples subjected to furnace cooling exhibited a more obvious serrated grain boundary morphology by comparison with those subjected to air cooling. The interaction between precipitated phases and grain boundaries was focused to explore the formation mechanisms of serrated grain boundaries within GH4975 superalloy. Heat treatment temperature and soaking time strongly affected the morphology and size of precipitated phases, and consequently influenced the formation of serrated grain boundaries. The directional growth of grain boundary precipitates and its pinning effects on the migration of grain boundaries also affected the grain boundary morphology.

Dependence of microstructures on fatigue performance of polycrystals: A comparative study of conventional and additively manufactured 316L stainless steel

The fatigue properties and microstructural evolution of 316 L stainless steel (316LSS) manufactured by laser powder bed fusion (L-PBF) were systematically studied and compared with its wrought counterpart. The as-built L-PBF 316LSS shows a pronounced heterogeneity, not only structurally but also chemically, with a unique microstructure of highly serrated grain boundaries, bimodal grain structure, nano-precipitates, solidification cell structures, and chemical segregations. The microindentation test showed that the hardness of the as-built L-PBF 316LSS reached 2.589 GPa, which was about 1.6 times higher than that of the wrought solution annealed counterpart, and the sparser slip steps around indentations revealed its greater dislocation storage capability. The S-N curves indicated that the fatigue resistance of the as-built L-PBF 316LSS was significantly better than that of the wrought solution annealed samples, and this was ascribed to its unique microstructural characteristics, especially the pre-existing high-density dislocations and chemical microsegregation within cellular solidification features. Furthermore, the enhanced planar slip in L-PBF 316LSS by its unique microstructure, especially the formation of deformation twins, delays the strain localization and restrains slip band generation, thereby significantly inhibiting crack initiation, and contributing greatly to the fatigue performance. The unique cell structure appears to be more effective in improving the low-cycle fatigue performance of L-PBF 316LSS due to the enhanced ductility.

Periodic formations of deformation twins and dislocation arrays via grain boundary sliding at serrated grain boundary in cold-shear-strained AZ31 alloy

It is well known that grain boundary sliding (GBS) provides stress concentration and initiates grain boundary fracture at the serrated grain boundary or grain triple junction during high-temperature creep. Recent reports on GBS occurring during deformation at room temperature suggest that stress concentration may occur due to GBS, which has a stress component parallel to the grain boundary plane, even in cold working. To identify if GBS can generate stress concentration and related defects at the grain boundary at room temperature, we experimentally investigated periodic defect formation along the serrated grain boundary during cold shear deformation. To obtain defect-free serrated grain boundary structure, optimal conditions of hot-torsion and annealing were determined. Cylindrical torsion specimens were then machined into notched tensile specimens, which provide the advantage of directly observing the GBS and lattice curvature. Extensive observations and orientation analysis on periodic defect formation along the serrated grain boundary suggest that GBS at room temperature can periodically nucleate twinning as well as dislocation arrays. Crystal plasticity simulation proved that serrated GB generates non-rotating region and split stress distribution with intensified value. Two stress concentration mechanisms during the accommodation of GBS at the serrated grain boundary were proposed: direct transfer from rotational stress and indirectly formed tensile component, namely “cleavage stress.” While direct energy transfer was likely to follow the local Schmid's law for direction of the grain rotation, cleavage accommodation represented geometrically necessary characteristics.

Quartz textural analysis from an anastomosing shear zone system: Implications for the tectonic evolution of the Ribeira belt, Brazil

Strain localization and influence of grain-size reduction processes were investigated from field and microstructural observations, and from quartz textural analysis using EBSD, in rocks deformed along an anastomosing network of shear zones in the Ribeira belt in Brazil. Rocks deformed along the Lancinha shear zone (LSZ) display high percentage of recrystallized grains, in which quartz recrystallized grain size from rocks in the southern LSZ reaches a maximum of 50 μm, and in the northern LSZ a maximum of 200 μm. The dominant serrated grain boundaries and the widespread (50–85%) presence of very small (>10–50 μm) recrystallized quartz grains, suggest that BLG was the dominant recrystallization mechanism responsible for grain-size reduction in the south portion of the LSZ. In the northern LSZ the presence of large recrystallized grains with sizes close to subgrains, suggests that grain-size reduction occurred by SGR, and the presence of lobate grain boundaries point to GBM recrystallization, which led to an increase in recrystallized grain size. This suggests grain-size reduction by dynamic recrystallization as a major factor leading to weakening and strain localization. Evidence of intracrystalline deformation and the well-developed quartz crystallographic fabrics suggest that strain localization in the LSZ occurred by dislocation creep deformation mechanisms. Dislocation creep is recorded by dominant activation of basal <a> slip and secondary contribution of rhomb <a> slip, in the southeast of the LSZ, and by activation of rhomb <a> + basal <a> and prism <a> in the northeast. These slip systems together with microstructural observations suggest that plastic deformation occurred at around 400 °C in the southern region, and at around at >400–500 °C in the northern region. S–C foliation, quartz <c> axis distribution and several kinematic indicators suggest sinistral sense of shear along the LSZ, and strain partitioning during progressive deformation. The dominance of sinistral indicators observed in the LSZ are also found in parallel shear zones to the east, such as the Putunã and Serra do Azeite. The sinistral kinematics in this overall dextral transpressional orogenic system may be explained by dual sense of shear on shear zones bounding extruding blocks, likely formed during progressive orogenic evolution instead of the polyphase evolution previously suggested for this southern part of the Ribeira belt.

Investigation on the formation of grain boundary serrations in additively manufactured superalloy Haynes 230

Solid-solution and carbide-strengthened superalloys such as Haynes 230 are the materials of choice for the hot-section components of gas turbines, e.g., combustion cans and transition ducts. Under severe thermal conditions, to which those parts are exposed, creep strength is a crucial property of the related materials during their lifetime. Recently, the introduction of serrated grain boundaries in Haynes 230 has been intensively studied [J. G. Yoon, H. W. Jeong, Y. S. Yoo, and H. U. Hong, “Influence of initial microstructure on creep deformation behaviors and fracture characteristics of Haynes 230 superalloy at 900 °C,” Mater. Charact. 101, 49–57 (2015); L. Jiang, R. Hu, H. Kou, J. Li, G. Bai, and H. Fu, “The effect of M23C6 carbides on the formation of grain boundary serrations in a wrought Ni-based superalloy,” Mater. Sci. Eng. A 536, 37–44 (2012)], and nearly a triplication of the time to creep failure at high temperature and low stress conditions has been observed [J. G. Yoon, H. W. Jeong, Y. S. Yoo, and H. U. Hong, “Influence of initial microstructure on creep deformation behaviors and fracture characteristics of Haynes 230 superalloy at 900 °C,” Mater. Charact. 101, 49–57 (2015)]. The aim of this paper is to achieve serrated grain boundaries in Haynes 230 through an appropriate thermal process chain including the intrinsic heat treatments of the laser metal deposition (LMD) process, subsequent hot isostatic pressing and suitable heat treatments. The formation of serrations is a relatively new technique for Haynes 230 (i.e., first paper in 2012), and similar alloys and thus serrations have only been introduced in conventionally cast or wrought alloys so far. Optical and scanning electron microscopies are employed in this work to investigate the created microstructures, whose grain and carbide structure is finer compared to the recently studied conventionally processed alloys. Within the LMD samples, serrations were already found on almost all of the observed grain boundaries even in the as-build condition. This result was rather unexpected, as literature reports slow-cooling to be responsible for the formation of serrations, while fast-cooling is prevalent in LMD. Some authors associated the formation of serrations to the precipitation of M23C6-carbides at the grain boundaries during slow cooling conditions [L. Jiang, R. Hu, H. Kou, J. Li, G. Bai, and H. Fu, “The effect of M23C6 carbides on the formation of grain boundary serrations in a wrought Ni-based superalloy,” Mater. Sci. Eng. A 536, 37–44 (2012)]. The lower density of carbides along grain boundaries in the as-build state, however, makes this mechanism seem unlikely. Other authors attributed the emergence of serrations to a phenomenon similar to the faceting mechanism [J. G. Yoon, H. W. Jeong, Y. S. Yoo, and H. U. Hong, “Influence of initial microstructure on creep deformation behaviors and fracture characteristics of Haynes 230 superalloy at 900 °C,” Mater. Charact. 101, 49–57 (2015)]. It can be said that no uniform theory for the emergence of grain boundary serrations exists as of now. The electron backscatter diffraction (EBSD) investigations performed in this work indicated a correlation between serrated grain boundary segments, the {111}-directions of the crystal lattice, and possibly segregations along dendritic subgrain boundaries for a two-dimensional case. Serial sectioning in combination with EBSD analysis confirmed an agreement between the three-dimensional orientation of serrated grain boundary segments and the {111}-direction of adjacent grains. Hence, a mechanism different from the ones described in previous works is proposed for the formation of grain boundary serrations in the additively manufactured Haynes 230 alloy.

Mechanism of grain boundary serration during hot deformation of AZ31 alloy: Role of grain boundary dislocations and grain boundary sliding

Despite the industrial significance of grain boundary serration for improving creep resistance and retarding crack propagation, the mechanism of grain boundary serration during hot deformation is still unclear. The results of several studies indicated that increased inhomogeneity of the stored energy under high-stress deformation suppressed grain boundary serration. Thus, traditional strain-induced boundary migration, in which the stored energy difference between adjacent grains acts as the driving force for grain boundary serration, must be modified for hot deformation. The experimental results of this study suggested that dissociated grain boundary dislocations from lattice dislocations, rather than the elastic energy of the lattice dislocations, cause the nucleation of serrated grain boundaries. Grain boundary sliding on the serrated grain boundary asymmetrically generated an elastic energy field and an accommodated serrated grain boundary. The presence of an elastic energy field was predicted by dislocation climbing and statically distributed dislocations near the elastic energy field. Since the asymmetrically deformed serrated grain boundary exhibited sub-migration behavior in the elastic energy field, it was concluded that the applied shear stress (applied by grain boundary sliding) on the serrated grain boundary dominated the growth of serrated grain boundaries until dynamic recrystallization occurred.

Recrystallization processes, microstructure and crystallographic preferred orientation evolution in polycrystalline ice during high-temperature simple shear

Abstract. Torsion experiments were performed in polycrystalline ice at high temperature (0.97 Tm) to reproduce the simple shear kinematics that are believed to dominate in ice streams and at the base of fast-flowing glaciers. As clearly documented more than 30 years ago, under simple shear ice develops a two-maxima c axis crystallographic preferred orientation (CPO), which evolves rapidly into a single cluster CPO with a c axis perpendicular to the shear plane. Dynamic recrystallization mechanisms that occur in both laboratory conditions and naturally deformed ice are likely candidates to explain the observed CPO evolution. In this study, we use electron backscatter diffraction (EBSD) and automatic ice texture analyzer (AITA) to characterize the mechanisms accommodating deformation, the stress and strain heterogeneities that form under torsion of an initially isotropic polycrystalline ice sample at high temperature, and the role of dynamic recrystallization in accommodating these heterogeneities. These analyses highlight an interlocking microstructure, which results from heterogeneity-driven serrated grain boundary migration, and sub-grain boundaries composed of dislocations with a [c]-component Burgers vector, indicating that strong local stress heterogeneity develops, in particular, close to grain boundaries, even at high temperature and high finite shear strain. Based on these observations, we propose that nucleation by bulging, assisted by sub-grain boundary formation and followed by grain growth, is a very likely candidate to explain the progressive disappearance of the c axis CPO cluster at low angle to the shear plane and the stability of the one normal to it. We therefore strongly support the development of new polycrystal plasticity models limiting dislocation slip on non-basal slip systems and allowing for efficient accommodation of strain incompatibilities by an association of bulging and formation of sub-grain boundaries with a significant [c] component.

Corrosion behavior of thermo-mechanically processed biomedical Ti-29Nb-13Ta-4.6Zr

The effects of thermomechanical processing (TMP) on the corrosion behavior of a titanium based bio-alloy are addressed in the present work. In this regard two distinct temperature regime, i.e. warm compression at two temperatures of 300 and 400 °C along with hot compression at 900 °C, which is above the corresponding β-transus temperature, were contemplated for TMP cycles. The results showed that the corrosion resistant of processed materials was extremely higher than that of as-received one. The electron backscatter diffraction (EBSD) studies characterized two different mechanisms which played important roles in the material corrosion behavior. In the warm worked specimens, the martensite plates were seen at the vicinity of different types of deformation bands. These martensite plates were thought to assist improving the corrosion rate of processed materials through creating a continuous passive surface film. On the contrary, the occurrence of extended dynamic recovery in addition to the partial dynamic recrystallization in the materials processed at 900 °C could raise the intrinsic susceptibility to local attack thereby increasing the corrosion rate. The latter was attributed to the higher dissociation rate of moving dislocations at non-equilibrium serrated grain boundaries (specially at lower strain rate) and higher fraction of high angle grain boundaries (specially at higher strain rate), which could act as preferred sites for corrosion attack initiation.

Hot corrosion behavior and mechanical properties degradation of a Ni–Cr–W-based superalloy

The effect of molten salts on the mechanical properties of a Ni–20Cr–18W (wt%)-based superalloy was investigated in this work. The hot corrosion characteristics and the microstructural evolution during the hot corrosion process were analyzed. The corrosion scale forms on the surface of the substrate, and its thickness increases gradually during the process. The element of S invades the substrate. The corrosion products mainly include NiO, Ni3S2 and Cr2O3. The hot corrosion in molten salt makes the specimens rough and flawed. The serration grain boundaries are observed in the hot corroded specimens. The tensile strength and elongation of the original alloy are ~1023 MPa and ~15 %, respectively. After corroded for 5 h, the tensile strength and elongation of the specimen decrease to ~771 MPa and ~7 % and diminish to ~814 MPa and ~9 % for the specimen corroded for 10 h. The hot corrosion process deteriorates the mechanical properties of the alloys. And the tensile strength and elongation of the specimen corroded for 20 h sharply decrease to ~539 MPa and ~3 %, respectively. The nonmonotonic variation of the mechanical properties of the specimen occurs for the specimens corroded for 10 h, which is related to the competition between the depredation of the molten salts and the strengthening process by carbides and the serrated grain boundaries. The corrosion characteristics and microstructural evolution of a Ni–20Cr–18W-based superalloy were investigated during the hot corrosion process. It is found that the attack of molten salts makes the tensile strength of the specimens deteriorate significantly. The nonmonotonic variation of mechanical properties is related to the competition between the destruction of molten salts and the strengthening process during the hot corrosion process.

Effects of aging temperature and grain size on the formation of serrated grain boundaries in an AISI 316 stainless steel

The effects of aging temperature and grain size on the formation of serrated grain boundaries have been investigated in an AISI 316 stainless steel. Grain size increased slightly over aging temperature ranges of 650–870 °C, resulting in predominantly serrated grain boundaries. However, when the temperature exceeded 880 °C, the grain size significantly increased, and grain boundary serration was not observed. The initial grain size also had an influence on the occurrence of grain boundary serration. For specimen having a large initial grain size of about 200 μm, no serrated grain boundary formed after aging treatment at 760 °C. Serrated grain boundaries were observed when “normal” initial grain sizes 55 μm were employed. It was found that the frequency of low angle boundaries markedly increased as the initial grain size increased from 55 to 200 μm. From the results obtained, it is possible to describe that the grain boundary serration could be considered as a spontaneous reaction that aims to reduce the total free energy of the system, and form a new interface of lower free energy. We proposed that the competition between grain growth and grain boundary serration during aging treatment reduces the total free energy of the alloy system: at temperatures exceeding 880 °C, the dominant process is the grain growth, while grain boundary serration predominates over the range of 650–870 °C.

On the formation of irregular-shaped gamma prime and serrated grain boundaries in a nickel-based superalloy during continuous cooling

Abstract Interrupted cooling tests have been carried out in order to study the microstructural evolution of a high γ′ fraction nickel-based superalloy during continuous cooling from above γ′ solvus. It was found that the formation of irregular-shaped γ′ during slow cooling is the result of coarsening and coalescing of γ′ precipitates which involved dissolution of adjacent even finer precipitates and solute flux from them. The formation of serrated grain boundaries was found to be associated with the presence of large γ′ precipitates at the grain boundaries which may have pinned grain boundaries during their migration.

Crack-seal microstructure evolution in bi-mineralic quartz–chlorite veins in shales and siltstones from the RWTH-1 well, Aachen, Germany

In core samples from the deep geothermal well RWTH-1 we studied Variscan quartz–chlorite veins formed by crack-seal processes in siliciclastics at the brittle to ductile transition. These sheared veins are common in sections of the well, which are interpreted as Variscan thrust zones based on image logs and seismic data. Microstructures interpreted to reflect different stages in the evolution of such crack-seal veins suggest the veins started in microcracks sealed by quartz and chlorite, to veinlets crossing multiple grains, and bundles of veinlets evolving by progressive localization into low-angle extensional shear veins and high-angle dilational jog veins. In the sheared veins, chlorite and quartz ribbons show evidence for crack-seal and simultaneous ductile shearing during vein evolution, forming peculiar fin-shaped microstructures in quartz ribbons. In high-angle dilational jogs fibrous crystals of quartz and chlorite point to multiple crack-seal events with simultaneous growth of two different mineral phases. This is interpreted to be the basic microstructural process in the veins. We extend earlier models of polycrystal growth in fractures and present a series of 2D simulations of the kinematics of crystal growth in these bi-mineralic veins for both localized and non-localized cracking. Results are compared with the observed microstructures. We show that when the relative growth rates of the two mineral phases are different, serrated grain boundaries evolve. The similarities between observation and model suggest that the assumption of our model is valid, although many second order processes require a more detailed study. We propose that the principles observed here can be applied to other bi-mineralic crack-seal veins.

Grain boundary segregation in nickel base superalloys Astroloy: an atom-probe study

TEM and atom-probe techniques are used in a combined way for the study of straight and serrated grain boundaries in nickel base superalloys such as Astroloy. These two types of boundaries lead to very different mechanical properties. Experimental conditions are discussed. It is shown that the investigation of boundaries which are perpendicular to the tip axis may lead to biased composition data. Concerning serrated boundaries, microanalyses show that boron and a small amount of carbon segregate at grain boundaries, however, zirconium does not.

Serrated grain boundary formation potential of Ni-based superalloys and its implications

The serrated grain boundary formation potential of a large number of conventionally forged, powder processed, and investment cast Ni-based superalloys is reviewed. A mechanism of serrated grain boundary formation by which grain boundaryγ′ particles move and displace the local grain boundary segment is discussed and the prerequisite conditions for its occurrence are highlighted. The practical implications of the serrated grain boundary formation are also discussed. It is suggested that modifying the existing heat-treatment cycles in some investment cast and powder processed Ni-based superalloys would improve their properties. The possibility of minimizing weld cracking in superalloys by creating serrated grain boundaries in the base metal and the heat affected zone is also discussed.

On the mechanism of serrated grain boundary formation in Ni-based superalloys

Grain boundary serration is studied as a function of cooling rate in a number of Ni-based superalloys, e.g. IN 738, Nimonic 115 and Nimonic 105. The mechanism is related to γ′ precipitation in these alloys and a higher γ′ solvus temperature than the M 23C 6 solvus temperature appears to be a prerequisite for the development of serrations. A model based on the grain boundary primary γ′ particle movement causing the displacement of the local grain boundary segment and initiating serrated grain boundary formation has been proposed.

Differential stress determined from deformation‐induced microstructures of the Moine Thrust Zone

The dislocation structure, recrystallization, and elongation of quartz grains in tectonites from three localities along the Moine thrust fault have been analyzed by transmission electron and optical microscopy. The dislocation density, 5×108 cm−2, and the grain size after recrystallization, 15 μ, in the basal quartzite unit at the Stack of Glencoul are independent of distance from the fault, to the maximum sampling distance of ∼100 m. The differential stress level determined from the deformation‐induced microstructures is on the order of 100 MPa. The magnitude of the differential stress at the fault decreases by a factor of 2 from Knockan Crag to Loch Eriboll, a distance of 50 km. At the Stack of Glencoul, grains in the basal quartzite are progressively elongated and recrystallized approaching the fault, corresponding to a progressive increase in strain. At 100 m from the fault, the aspect ratio of relict quartz grains is ∼1:1; at 0.01 m it is 85:1. A quantitative estimate of the strain, based on the aspect ratios of the relict quartz grains, as a function of distance from the fault has been used to calculate strain rate and temperature profiles. For a creep activation energy of 0.19 MJ mol−1 the temperature decreases away from the fault with a gradient of 7.5×10−3 °C cm−1. For a differential stress of 100 MPa, a steady state thrust sheet velocity of 11 cm yr−1 would be required to produce this gradient. At 100 m from the fault, 5% of the quartzite is recrystallized from 1‐mm to 15‐μm grains; at 0.01 m it is 100% recrystallized to 15‐μm grains. This small grain size probably favored grain boundary deformation mechanisms over grain matrix mechanisms and thus helped to concentrate the strain at the fault. Systematic changes in the elongation and recrystallization of quartz grains in Lewisian Gneiss and Moine Schist are obscured by the constraints imposed by other mineral phases. The small size of the grains after recrystallization, highly serrated grain boundaries, and large densities of free dislocations suggest that very little static recovery followed the plastic deformation event.

Microstructure and Mechanical Properties of HIP-Consolidated René 95 Powders

The effects of heat-treatments on the microstructure of P/M Rene 95 (a nickel-based powder metal), consolidated by the hot-isostatic pressing (HIP), were examined. The microstructure of as-HIP'd specimen was characterized by highly serrated grain boundaries. Mechanical tests and microstructural observations reveal that the serrated grain boundaries improved ductility at both room and elevated temperatures by retarding crack propagation along grain boundaries.