Evidence Of Dynamic Recrystallization Research Articles

Quasi-in-situ observation and analysis of grain boundary evolution of GH4169 nickel-based superalloy during the micro-strain stage of thermal deformation

A systematic study was conducted to analyze the grain boundary microstructure evolution of GH4169 nickel-based superalloy during the micro-strain stage of thermal deformation. A quasi-in-situ EBSD (electron backscatter diffraction) approach was used for observation. Moreover, at a temperature of 1100 °C and a strain rate of 0.1 s−1, a continuous strain uniaxial compression experiment was carried out. We observed the development of the grain boundary structure and the grain boundary deformation. The experimental findings revealed that GH4169 underwent locally inhomogeneous thermal deformation during the micro-strain stage, and the simultaneous occurrences of grain boundary slip, grain rotation, and grain boundary migration, exhibiting microstructural diversity. The grain boundary structure changed continuously as a result of the random movement of high-angle grain boundaries. Twin grains displayed a stable structure during the migration of grain boundaries, and their content indicated a considerable decrease as the strain increased. This provides a basis for directing the design of grain boundary engineering since it suggests that twin boundaries are easier to migrate during thermal deformation. In addition, no evidence of dynamic recrystallization was found throughout the entire micro-strain stage and a <101>/<111> micro-texture was observed, which is compatible with common dislocation slips in face-centered cubic crystals. Then, on the basis of the transmission electron microscope images of the microstructure, the numerous forms of grain boundary deformation were rationally explained from the standpoint of the relation mechanisms among grain boundaries and dislocations. Finally, the synergistic mechanism of grain boundary slip and migration is established as the primary mechanism of thermal deformation in the micro-strain stage.

High burnup structure formation in U-Mo fuels

This study used electron microscopy techniques and phase field modeling to investigate the mechanisms responsible for developing the high burnup structure in U-Mo fuels. The results show that grain subdivision is initiated by polygonization; however, evidence of dynamic recrystallization is present with increasing fission densities. At 2.5 × 1021 fissions/cm3, LAGB misorientation of 4° was measured near the grain boundary of neighboring grains, which supports polygonization as the leading grain subdivision mechanism. However, with increasing fission densities, the formation of HAGB subgrain clusters are evident, marking the activation of another mechanism − dynamic recrystallization. Previously formed LAGBs transform into new HAGB grains, which is reflected in the increasing number frequency of HAGBs to LAGBs. At the same time, the HAGB subgrain clusters also undergo polygonization, suggesting that polygonization is a continuous mechanism. Based on the simulations performed in this study, polygonization may continue to occur until a mean grain diameter of ∼382 nm is achieved. Based on the results of this work, LAGB to HAGB transformation is associated with a strain-driven mechanism potentially caused by fission gas-induced plastic deformation and is consistent with recrystallization. Polygonization and dynamic recrystallization occurs in tandem with polygonization leading grain subdivision. Based on the fission densities evaluated in this study, authors postulate that the critical fission density at which dynamic recrystallization (DRX) becomes active is between 2.5 × 1021 fissions/cm3 and 3.5 × 1021 fissions/cm3.

Dynamic Tensile Extrusion Behavior of Fine-Grained Copper Fabricated by Powder Injection Molding

The dynamic tensile extrusion (DTE) behavior and microstructural evolution of fine-grained (FG, ~1 μm &lt; d &lt; ~10 μm) Cu fabricated by powder injection molding (PIM) were investigated. The FGM Cu was fabricated by PIM with commercial micro-sized Cu powder sintering at 850 °C for 2 h, while the FGH Cu was developed by the hot isostatic pressing (HIP) of the FGM at 780 °C for 2 h under a pressure of 1000 bar. In order to compare the DTE behavior of the FG Cu manufactured using different methods, the ultrafine-grained-B (UFG, d &lt; ~1 μm) Cu was developed by performing 16 passes of equal-channel angular pressing with route Bc, and the FG-150 Cu was fabricated by annealing the UFG-B Cu bar at 150 °C for 1 h. The DTE tests were performed with identical flyer velocities using an all-vacuum gas gun. The fragments and remnants were carefully recovered after the DTE tests and examined by electron backscattered diffraction measurement and a micro-Vickers hardness test. A strong dual &lt;001&gt; + &lt;111&gt; texture was developed during the DTE for both FGM and FGH Cu. In contrast to the outcome of the UFG-B and FG-150, little evidence of dynamic recrystallization taking place during the DTE in the FGM and FGH was found during analysis of the grain morphology and grain orientation spread. Premature failure based on void coalescence was induced at the vertex region of the FGM fragments due to pre-existing pores. The HIP treatment on the FGM Cu increased the relative density by reducing the pre-existing pores and, as a result, increased the DTE ductility of the FGH Cu.

Microstructures and Fabric Transitions of Natural Ice from the Styx Glacier, Northern Victoria Land, Antarctica

We investigated the microstructures of five ice core samples from the Styx Glacier, northern Victoria Land, Antarctica. Evidence of dynamic recrystallization was found in all samples: those at 50 m mainly by polygonization, and those at 170 m, largely by grain boundary migration. Crystallographic preferred orientations of all analyzed samples (view from the surface) typically showed a single cluster of c-axes normal to the surface. A girdle intersecting the single cluster occurs at 140–170 m with a tight cluster of a-axes normal to the girdle. We interpret the change of crystallographic preferred orientations (CPOs) at &lt;140 m as relating to a combination of vertical compression, and shear on a horizontal plane, and the girdle CPOs at depths &gt;140 m, as the result of horizontal extension. Based on the data obtained from the ground penetrating radar, the underlying bedrock topography of a nunatak could have generated the extensional stress regime in the study area. The results imply changeable stress regimes that may occur during burial as a result of external kinematic controls, such as an appearance of a small peak in the bedrock.

Characteristics of grain refinement in oxygen-free copper processed by equal-channel angular pressing and dynamic testing

Oxygen-free copper was processed by equal-channel angular pressing (ECAP) at room temperature for 1, 4 and 8 passes and then the ECAP specimens were further deformed by dynamic testing at 298 K using a strain rate of 10 s−1. Experiments were conducted to investigate the influence of the initial microstructures induced by ECAP on the subsequent grain refinement and mechanical properties after dynamic testing. The results show the strength of copper increased with increasing numbers of ECAP passes and a significant additional grain refinement was produced in the ECAP specimens through the dynamic testing. Thus, the initial grain sizes after ECAP for 1, 4 and 8 passes were ~16, ~4.4 and ~2.9 μm, respectively, and these values were reduced to ~400, ~330 and ~300 nm by dynamic testing, The grains were refined by conventional dislocation processes in the 1-pass specimen but there was evidence for dynamic recrystallization in the specimen processed by 8 passes.

Dynamic Softening of Flow Stress during Dynamic Ferrite Transformation

This study using a 6Ni-0.1C steel confirmed the relationship between the change in a fraction of dynamically transformed ferrite and the dynamic softening in stress-strain curve during dynamic transformation above ortho-equilibrium austenite-to-ferrite transformation temperature. Dynamic softening in stress-strain curve was well-fitted with a form of Avrami equation as a function of strain, and it corresponded with the change in fraction of ferrite. The slope of work-hardening rate was increased due to the additional softening phenomenon, i.e. dynamic transformation, to dynamic recovery of austenite. Dynamic softening of austenite, which has been considered as a typical evidence of dynamic recrystallization, could be interpreted as a response of dynamic transformation to ferrite.

Shear deformation of polycrystalline wadsleyite up to 2100 K at 14–17 GPa using a rotational Drickamer apparatus (RDA)

Shear deformation experiments on polycrystalline wadsleyite (water content, ∼200–2200 H/106 Si) have been conducted at 14.4–17.0 GPa, 1690–2100 K, and strain rates of 2.6–16 × 10−5 s−1 using a rotational Drickamer apparatus (RDA) at a synchrotron facility. The stress was measured from the orientation dependence of lattice spacing for the (013), (211), (141), (240) and (244) planes. On the basis of the mechanical and microstructural observations, we infer that deformation occurs by exponential creep through the Peierls mechanism at relatively low temperatures of 1690–2030 K. However, a sample deformed at the temperature of 2100 K showed significant grain‐size reduction, and most of small grains are dislocation‐free, although sub‐boundaries were observed in some grains in the sample. We interpret these observations as evidence for dynamic recrystallization and that diffusion creep (and grain boundary sliding) plays an important role after dynamic recrystallization caused by power law creep. Consequently, the strength observed in the high‐temperature conditions determined by the present study provides an important constraint on strength of diffusion creep and a lower limit for that of the power law dislocation creep. We conclude that the strength of wadsleyite in the power law dislocation creep is higher than or comparable to that of olivine and the strength of wadsleyite in the Peierls regime is similar to that of olivine.

Evidence of dynamic recrystallization in polar firn

Microstructural analyses have been performed on polar firn from the European Project for Ice Coring in Antarctica drilling site in Dronning Maud Land, Antarctica. The results derived from images of the firn structure in microscopic resolution indicate that dynamic recrystallization is active in firn at all depths, and it dominates the evolution of the microstructure when the firn density exceeds a critical value of 730 kg/m3 (overburden snow load ∼0.2 MPa). At the firn‐ice transition (density ∼820 kg/m3) the microstructure is characterized by many small grains and bulged or irregularly shaped grain boundaries. More than half of all grains show subgrain boundaries. Thus, strain‐induced boundary migration is an essential feature to describe the irregular grain structure. In agreement with previous studies, significant grain growth has been observed with depth for the largest grains in the samples. However, our microscopic analysis reveals that the grain growth with depth in fact vanishes if all grains larger than 65 μm in diameter are taken into account. This result reflects the fact that the growth of the largest grains is counteracted by grain size reduction by shrinking and subdivision of old grains, as well as production of new grains. Consequently, previous conclusions that grain growth in polar firn is essentially analogous to normal grain growth in metallic and ceramic sinters and that the stored strain energy is small in comparison with grain boundary energy can no longer be supported. Additionally, our observations show that the incipience of dynamic recrystallization in polar ice sheets is not as sensitive to temperature as supposed so far. A discussion of the change of the mean grain size due to the measuring technique is imperative.

Fluid inclusion characteristics of the Uti gold deposit, Hutti-Maski greenstone belt, southern India

The Uti gold deposit occurs within amphibolites of the late Archaean Hutti-Maski greenstone belt of southern India. Gold mineralization is associated with intense silicification, biotite-actinolite- and sulfide-rich alteration zones in between small anastomozing shear zones. Characteristic biotite-K-feldspar alteration and arsenopyrite thermometry constrain the mineralizing event to above 400 °C, which is concordant with evidence of dynamic recrystallization in mineralized quartz veins. Shearing, concomitant alteration and Au-mineralization took place at post-peak metamorphic conditions. A post-mineralization, sub-greenschist facies assemblage of prehnite/pumpellyite and hydrogarnet later imprinted the rocks. Fluid inclusion microthermometric studies on mineralized and barren quartz veins reveal similar fluid chemistry, P– T and, surprisingly, a complete absence of carbonic inclusions. Fluid inclusion textures and the shape of T h histograms point towards moderate reequilibration during a phase of isothermal decompression. The microthermometric data, coupled with stability relations of the hydrous Ca–Al silicates, indicate post-mineralization isobaric cooling followed by a near-isothermal decompression. The observed temperature–salinity variation is schematically explained by simple cooling of a low salinity heated meteoric fluid, followed by isothermal mixing with a high salinity granitic fluid. However, fluid mixing was later than, and unrelated to mineralization. On the other hand, formation of auriferous sulfide-bearing quartz veins at Uti was essentially due to remobilization and attendant fluid–rock interaction involving suitable host rocks that contained primary gold.

Sub-micrometer structures generated during dry machining of copper

Deformation structures developed in the material ahead of the tool tip in commercial purity copper samples that were subjected to an orthogonal cutting process were investigated using transmission electron microscopy (TEM). The effect of the prior thermo-mechanical treatment has been considered by performing orthogonal cutting tests on hot extruded and fully annealed samples. Results showed that the microstructure of the primary deformation zone (PDZ) was characterized by the formation of elongated dislocation cell structures that were composed of heavily tangled dislocation walls. In the hot extruded samples, this structure was generated as a result of the activation of the pre-existing dislocations, but nucleation of fresh dislocations was needed in the annealed samples. Grain refinement occurred in the chip as a result of the subdivision of the elongated cells into smaller equiaxed sub-micron size (220 nm diameter) grains. Evidence for dynamic recrystallization has been found in the microstructure of the machined chips (MCs). However, calorimetric studies and the microhardness measurements have shown that the sub-micron structures of the machined chips were stable at temperatures up to 180 °C.

Inferring dynamic recrystallization in ferrite using the kinetics of static recrystallization

A general relationship between the kinetics of dynamic and static recrystallization is developed. It is predicted that conventional dynamic recrystallization will occur whenever the deformation time exceeds the adjusted start time for static recrystallization. This approach is verified using data for austenite and lead. It is then applied to current and previous work on ferrite. The model provides support for the contention that conventional dynamic recrystallization occurs in low carbon ferrite if deformation is carried out at high temperatures and low strain rates. In the present work, which was carried out at 700 °C, evidence for dynamic recrystallization was observed for strain rates less than around 0.01 s−1. At higher strain rates, the model predicts a critical strain for the onset of dynamic recrystallization that exceeds the critical strain for the beginning of the recovery steady-state region. While the model allows dynamic recrystallization to begin in this region, the critical strain for its onset is expected to increase rapidly with increasing strain rate and decreasing temperature once steady state has been reached.

High-temperature deformation in the Neoproterozoic transpressional Ribeira belt, southeast Brazil

The Neoproterozoic Ribeira belt is subdivided in two domains with contrasting tectonic characteristics. The northern domain is dominated by shallowly dipping foliations and orogen-normal thrust tectonics. The southern domain is characterized by a 1000-km-long network of anastomosing transcurrent shear zones parallel to the belt. This contrast is interpreted as reflecting continent–continent convergence that is almost orthogonal to the margins in the northern domain and significantly oblique in the southern domain. The central, transitional, domain of the Ribeira belt displays the northern termination of the transcurrent shear zone network: the Além Paraı́ba–Pádua shear zone system (APPSS). The 250-km-long Além Paraı́ba–Pádua system involves granulites facies mylonites deformed through transpression. A detailed study of the microstructure and lattice preferred orientation (LPO) of the rock-forming minerals in these granulite mylonites allow a better understanding of deformation mechanisms active at high temperature in the crust. Plagioclase crystals are plastically deformed; they display curved twins and cleavages, mechanical twins, and evidence of dynamic recrystallization. LPO of plagioclase is consistent with activation of the (010) [100] and (010) [001] slip systems. LPO of orthopyroxene and amphibole indicates that these minerals have been deformed through dislocation creep with the activation of the (100) [001] slip system. Quartz in granulite mylonite displays evidence of extensive growth through grain boundary migration. The LPO of quartz is therefore the result of a static transformation of an initial, syn-kinematic LPO, and cannot be straightforwardly interpreted in terms of deformation mechanisms active during mylonitization.

Evidence for dynamic recrystallization during Harper–Dorn creep

Harper–Dorn creep refers to the anomalous creep behavior first reported in 1957 by Harper and Dorn in their study on the creep behavior of large-grained aluminum. The creep rates associated with Harper–Dorn creep are two orders of magnitude faster than those predicted by Nabarro–Herring creep (diffusional creep). The present investigation provides new evidence that dynamic recrystallization occurs during Harper–Dorn creep. Such evidence is manifested by the occurrence of periodic accelerations of the creep rate. In addition, it is shown that the creep substructure along with other considerations is consistent with the characteristics of dynamic recrystallization.

Microstructure of high-strain, high-strain-rate deformed tantalum

Hat-shaped specimens of polycrystalline tantalum are subjected to high plastic shear strains (γ = 170–910%) at strain rates exceeding 5 × 104/s in a compression split Hopkinson bar. The dynamic shear tests are performed at room and 600 K initial temperatures, under adiabatic and quasi-isothermal conditions, using UCSD's recovery Hopkinson technique [1]. The microstructure of the post-test specimens is examined with transmission electron microscopy (TEM). The plastic deformation is highly concentrated, producing a narrow shear-localization region of approximately 200 μm in width. Slip of perfect screw dislocations, on the {110} primary planes along the 〈111〉 directions, is found to be the dominant deformation mechanism. Dynamic recovery takes place in the shear-localization regions of all adiabatically tested specimens, and evidence of dynamic recrystallization is observed in the specimen deformed to a shear strain of 910% at a 600 K initial temperature. The substructures of the adiabatically tested specimens include well-defined dislocation arrays, grouped dislocations, elongated dislocation cells, subgrains, and recrystallized micron-sized grains. The microstructure of isothermally tested specimens, on the other hand, features high dislocation density and inhomogeneous dislocation distribution. In light of the TEM observations, the relation between the microstructure and shear stress, the causes of strain inhomogeneity, the estimated adiabatic temperature within the shear-localization zone, the rapid quenching of the shearband at the end of the dynamic testing, the slip characteristics of dislocations in tantalum, and the formation mechanisms of dislocation loops, are discussed.

Hot working and micro structure in 409 ferritic steel

After continuous casting, homogenization and hot working, 409 ferritic stainless steel was deformed by torsion to different strains up to fracture. At 800–1000 °C flow curves exhibited clearly defined peaks followed by gradual softening which was greater at higher stresses. At 600 °C with ge = 0.01 s −1, deformation curves show only strengthening to failure but a peak is present at 700 °C. At 900–1000 °C, flow curves approach a steady state. The optical microstructure at 600 °C consists of deformed grains with little recovery. At 700–1000 °C, elongated grains contain substructure which is more developed at higher T and ε and lower ge with no evidence of dynamic recrystallization.

Grain structure and microtexture evolution during superplastic forming of a high strength AlZnMgCu alloy

Grain structure and microstructure evolution during superplastic forming were studied on an unrecrystallized sheet of a modified 7050 superplastic alloy. A SEM-based local orientation technique was used to cover a large number of (sub)grain boundaries in combination with other metallographic techniques. The gradual boundary misorientation and microtexture evolution during superplastic forming (SPF) confirmed that a continuous evolutionary process was occurring. There was no evidence of dynamic recrystallization at the stress maximum. The fraction of high angle boundaries increased rapidly once the mean misorientation reached a critical value. These and other results suggest that both grain boundary sliding (GBS) and dislocation slip were operative initially until the stress maximum was approached, beyond which GBS was predominant. The results of quantitative orientation distribution function (ODF) analyses suggest that grain rotation, which resulted in texture randomization, became important from slightly beyond the stress maximum through most of the stress-strain curve.

Geochronology and geochemistry of multiple generations of monazite from the Wepawaug Schist, Connecticut, USA: implications for monazite stability in metamorphic rocks

U-Pb isotope analyses, rare earth and trace element analyses, and petrographic observations are presented for monazites from the Wepawaug Schist in southern Connecticut, USA. Two samples of kyanite zone schist were collected less than a meter apart. Each sample contains a different variety of monazite with distinct morphology, chemistry, and Pb isotopic composition. One sample preserves a largely amphibolite facies mineralogy, including kyanite, staurolite, garnet, biotite, and chlorite, with little textural evidence of later shearing. Monazites from this sample are xenoblastic with about 1 wt% ThO2, 0.3 wt% CaO, and a more LREE enriched pattern than monazites from the second sample. These xenoblastic monazites preserve textural evidence of a retrograde reaction to apatite which involves chlorite, indicating that these monazites became unstable during retrograde chloritization of biotite. These monazites give strongly discordant U-Pb ages which fit a chord with an upper intercept age of 411±18 Ma, interpreted as the minimum growth age of these xenoblastic monazites, perhaps during amphibolite facies metamorphism. The second sample contains S-C banding, evidence of dynamic recrystallization, and abundant retrograde chlorite. This sample contains idioblastic monazites with about 3 wt% ThO2, 0.8 wt% CaO, and with less fractionated REE patterns. These monazites give close to concordant U-Pb ages with a mean 207Pb*/206Pb* age of 388 ± 2 Ma. This age is interpreted as probably representing the time of monazite growth during retrogression of the sample from an amphibolite to greenschist facies mineralogy.

Mechanical response and structural development during the hot extrusion of a rapidly solidified Al-20Si-7.5Ni-3Cu-1Mg alloy powder

The consolidation of an air-atomized Al-20Si-7.5Ni-3Cu-1 Mg alloy powder was performed utilizing hot extrusion, to determine its extrudability and understand its structural development in relation to process parameters. One of the main features exhibited by the material in this process was a high degree of softening over a peak extrusion pressure, which has been explained by the simultaneous onset of dynamic recovery and recrystallization during deformation. The peak extrusion pressure was shown to be strongly dependent upon the temperature applied, and this dependence has been described with temperature compensated strain rate. It was also observed that the process parameters had a fairly narrow range applicable to the extrusion of the powdered alloy and a significant influence on the deformation behaviour of the powder particles. The combination of heating and deformation, primarily used to convert the loose powder particles into an engineering material, resulted in the decomposition of the meta-stable aluminium matrix and transformations of constituent phases, initially formed in the rapidly solidified powder. Additionally, it was found that the extrusion temperature had an effect on the lattice size and perfection of the as-extruded matrix in the material. Three intermetallic dispersoids containing nickel were detected in the consolidated material, independent of extrusion temperature, and their formation was promoted by hot deformation. The silicon crystal phase in the extruded material was reshaped, and its size was insensitive to the extrusion temperature, which is thought to be caused by a high volume fraction of the coexistent dispersoids. The dispersions of the silicon crystals and intermetallic compounds with various sizes in the matrix substantially modified the deformation mode of the alloy. Evidence of dynamic recrystallization was found, which co-operated with dynamic recovery during deformation, giving rise to a duplex microstructure in the extruded material.

Superplastic Deformation of Zinc Sulfide Near Its Transformation Temperature [1020°

Polycrystalline ZnS was deformed in compression to large strains (up to 100%), near its α⇄β transformation temperature (1020°), at strain rates that ranged from 10−5 to 3 × 10−3 s−1. The flow stress showed a minimum near 1020°. The strain rate sensitivity and the grain size dependence of the flow stress suggest a superplastic mechanism of deformation. There was some evidence of dynamic recrystallization and stress‐induced β (fcc) to α (hcp) phase transformation. The polycrystals had a tendency to form in tergranular cavities at the higher temperatures and at faster strain rates.

Microstructural evidence for dynamic recrystallization during superplastic deformation

Microstructural evidence for dynamic recrystallization during superplastic deformation