Large Subgrains Research Articles

Stress Relaxation Behavior of GH4169 Alloy

The relaxation properties of GH4169 alloy were studied contrastively at temperatures ranging from 600 oC to 700 °C and initial stress ranging from 550 MPa to 850 MPa. The relationship between the microstructure and relaxation behavior was evaluated using transmission electron microscopy techniques. It was found that the relaxation limit and relaxation stability of the alloy decreased obviously with the increase of temperature. Further investigations show that the relaxation behavior is mainly depend on both precipitate characteristics and its interaction with dislocations. The alloy with higher strength lever has more excellent stress relaxation stability, because of the inhibition of a large number subgrains on dislocations motion.

Measurement of creep deformation of ex-service 12% Cr steel using digital image correlation

The effective operation of ageing coal-fired power plants requires routine damage assessment of critical component materials throughout their service history. Traditional post-exposure mechanical testing typically requires large amounts of material for each testing condition. This study introduces an accelerated (high stress and temperature) creep testing technique that employs digital image correlation for strain measurement over a non-uniform temperature field generated by a thermomechanical tester. When applied to different ex-service (exposed to service conditions within a power plant) 12% Cr piping steels, multiple accelerated creep curves at temperatures ranging from 550°C to 600°C are measured from a single specimen for each material condition. Higher creep damage in the steel due to longer service exposure and higher void densities is revealed by shorter rupture times, faster creep rates and tertiary-dominated creep curves when compared to the lower creep damage state. Microstructural study via quantitative electron microscopy confirms the dislocation creep-driven growth of subgrains in the specimen gauge regions relative to the grips. Larger subgrains and a weakening of precipitate distributions for the high damage material were also noted. Digital image correlation–measured strains are in good agreement with traditional techniques of creep testing with standard extensometry. On a comparative basis, the technique is shown to be a plausible form of damage assessment of 12% Cr piping steels with varying levels of service exposure and serve as a suitable mechanical complement to the traditional void replica methodology.

High-energy x-ray diffraction microscopy study of deformation microstructures in neutron-irradiated polycrystalline Fe-9%Cr

In this study, we used far-field high-energy x-ray diffraction microscopy (FF-HEDM) to measure in 3D the spatial distributions of the sizes, orientations, and residual strains of grains and subgrains formed by room-temperature tensile deformation in a neutron-irradiated (450 °C, 0.01dpa) Fe-9Cr specimen and its unirradiated counterpart. We found that neutron irradiation under this condition alone had no effect on the grain size distribution. After deformation, grains fragmented into subgrains in both unirradiated and irradiated specimens: the irradiated specimen contained a few large subgrains which co-existed with many smaller subgrains, while the unirradiated specimen contained small subgrains with a relatively uniform size distribution. Prior to deformation, the irradiated specimen had higher residual strain spread compared to its unirradiated counterpart, while after deformation to the maximum uniform elongation, the strain distributions among subgrains were similar between the unirradiated and irradiated specimens. The FF-HEDM measurements provide new insight into the effects of neutron irradiation on the mechanical response of Fe-Cr ferritic alloys.

Mechanical properties and microstructure of ultrafine grained commercial purity aluminium prepared by cryo-hydrostatic extrusion

CP 99.5% aluminium was processed by cryo-hydrostatic extrusion with true strains up to 3.4 in one pass. The aim was to refine its microstructure and improve its mechanical properties. The properties of the thus processed material were compared with those obtained after the same process but run at room temperature. Cooling the billet with liquid nitrogen combined with water cooling of the extruded wire enabled suppressing partially the dynamic and static structural processes. The grain size was reduced to ~400nm in the cryo-extrusion, and to ~450nm in the room-extrusion. In the cryo-extrusion the increase of the yield strength to 168MPa and the hardness to 56HV0.2 with the respective reduction of the elongation to fracture to 13.6% were obtained. The cryo-cooling effectiveness and the influence of the adiabatic heat generated during the plastic processing on the structure, mechanical properties, hardness, and tensile impact toughness just after hydrostatic extrusion, an also after the post deformation annealing are discussed. In view of the intensive adiabatic heating amounting to 0.57Tm no special improvement of the mechanical properties after the post-deformation annealing was observed. The cryo-cooling became effective at the true strain ε>2, where the extrusion pressures clearly differed from the room-extrusion pressures and the defect density substantially increased. After the cryo-hydrostatic extrusion the mechanical properties were comparable to the highest values reported in the literature for cryo-rolling but, since they were obtained in a single deformation step and with twice as large subgrains, the ductility of the extruded aluminium was higher. During the cryo-hydrostatic extrusion conducted at high strains the reduction in ductility of the aluminium is hindered and thanks to the beneficial role of the hydrostatic stress active in the material the structural and mechanical effects which occur during severe plastic deformation are enhanced.

X線ラインプロファイル解析と透過電子顕微鏡観察による強圧下冷間圧延された純鉄，Fe-0.3 mass%Si合金，Fe-0.3 mass%Al合金の再結晶に伴う転位組織変化の評価

The change in dislocation structure with recovery and recrystallization of 99.8% cold-rolled pure iron, Fe-0.3 mass%Al and Fe-0.3 mass%Si alloys was investigated by means of X-ray line profile analysis (XLPA) and TEM observation. In as-rolled samples of Fe-Al and Fe-Si alloys, the fraction of edge dislocation density increased in comparison with that in pure iron. In the early stage of recovery, dislocation densities with both edge and screw components remarkably decreased in pure iron, whereas in the Fe-Al and Fe-Si alloys, only the edge dislocation density distinctively decreased. This suggests that edge dislocations with relatively high density in as-rolled Fe-Si and Fe-Al alloys are easily annihilated by the climbing up motion of edge dislocations. In the late stage of recovery, the dislocation density of Fe-Si alloy did not decrease. No structural changes were observed by TEM which suggests that the recovery of Fe-Si alloy is strongly retarded, since Si hinders dislocation movement. In the early stage of recrystallization of pure iron, the formation of large subgrains was clearly confirmed in the lamellar structure. The subgrains show a similar orientation to the neighboring unrecrystallized grains, which represents similar behavior regarded as continuous recrystallization. Fe-Al alloy is similar to pure iron. Whereas, Fe-Si alloy exhibits discontinuous recrystallization in the sense that grains with low dislocation density were confirmed in the unrecovered lamellar structure with high driving force for recrystallization. This is because the Si strongly hinders dislocation movement. This effect agrees with the behavior of dislocation in Fe-Si alloy confirmed by XLPA.

Microstructure characterization of 316L deformed at high strain rates using EBSD

Specimens from split Hopkinson pressure bar experiments, at strain rates between ~1000–9000s−1 at room temperature and 500°C, have been studied using electron backscatter diffraction. No significant differences in the microstructures were observed at different strain rates, but were observed for different strains and temperatures. Size distribution for subgrains with boundary misorientations >2° can be described as a bimodal lognormal area distribution. The distributions were found to change due to deformation. Part of the distribution describing the large subgrains decreased while the distribution for the small subgrains increased. This is in accordance with deformation being heterogeneous and successively spreading into the undeformed part of individual grains. The variation of the average size for the small subgrain distribution varies with strain but not with strain rate in the tested interval. The mean free distance for dislocation slip, interpreted here as the average size of the distribution of small subgrains, displays a variation with plastic strain which is in accordance with the different stages in the stress-strain curves. The rate of deformation hardening in the linear hardening range is accurately calculated using the variation of the small subgrain size with strain.

Quantitative characterization of the microstructure of heat-treated Zr-Excel by neutron line profile analysis

Neutron diffraction line profile analysis (DLPA) and transmission electron microscopy were used to characterize the components of the bimodal microstructure of Zr-Excel (Zr–3.5Sn–0.8Mo–0.8Nb), a nuclear structural material. The dual microstructure, consisting of equiaxed primary grains and martensitic domains both having hexagonal close-packed (h.c.p.) α crystal structure, forms when the as-received Zr-Excel alloy is heat treated at a high temperature and subsequently quenched,i.e. is solution treated. Because both microstructure components have the same crystal structure the reflections from the two components overlap significantly. The article presents how the multi-phase analysis capability of modern DLPA methods can be used to model the measured neutron diffraction patterns as the sum of two sub-patterns corresponding to the components of such a bimodal microstructure, which can be found in many hexagonal alloys relevant for industrial applications. The results show that the large equiaxed primary h.c.p. α grains have a highly correlated low-density dislocation structure and large sub-grains (∼300 nm), while the large martensitic domains have a randomly arranged very high density dislocation structure and sub-grains the size of ∼30 nm. The significantly different defect structures of the primary and martensitic phases manifest as large differences in the hardness and ductility of the individual components. As a result of this duality of the mechanical properties, solution-treated Zr-Excel materials can be considered as analogous to metal matrix composites where a softer ductile matrix contains a harder brittle reinforcing phase.

Radial mosaic internal structure of rounded diamond crystals from alluvial placers of Siberian platform

The specific gray to almost black diamonds of rounded morphology are especially typical in alluvial placers of the northeastern part of the Siberian platform. The results of study of internal structure of these diamonds are presented. X-ray topography and birefringence patterns of polished plates of studied diamonds show their radial mosaic structure. Diamonds consists of slightly misorientated (up to 20′) subindividuals which are combined to mosaic wedge-shaped sectors. Electron back-scatter diffraction technique has demonstrated that subindividuals are often combined in the single large blocks (subgrains). The whole crystals commonly consist of several large subgrains misoriented up to 5° to one another. The total nitrogen content of these diamonds vary in the range 900–3300 ppm and nitrogen aggregation state (NB/(NB + NA)*100) from 25 to 64 %. Rounded diamond crystals of variety V are suggested to have been formed at the high growth rate caused by the high oversaturation of carbon in the crystallization medium. It may result in the splitting of growing crystal and their radial mosaic structure as a sequence. High content of structural nitrogen defects and the great number of mechanical impurities – various mineral and fluid inclusions may also favor to generation of this structure.

Effect of Sub-Grains and Crystal Defects on Monolike Si Solar Cell Performance

This paper investigates crystalline orientation in monolike silicon wafers and its effect on solar cell performance. Monolike silicon wafers from two different bricks cut from interior and corner region of an ingot were compared. The mono grain in the interior brick is nearly perfect, but there are some large oblong shaped sub-grains in the corner brick. The large sub-grains at corner brick wafers are oriented at (311), instead of (100) orientation. The (311) grains contain high density of dislocation and cannot be effectively textured by alkaline solution, therefore lowering the cell efficiency significantly. There is about 0.86% (abs) cell efficiency reduction on the monolike cells that contain large sub-grains.

Microstructure evolution in a 2618 aluminium alloy during creep-fatigue tests

Abstract Microstructure changes in the 2618 aluminium alloy during creep-fatigue tests were studied. These tests simulate the conditions of the application of this alloy in devices for the exhaustion of hot gasses generated during fire in closed or difficultly accessible areas. Creep-fatigue tests result in high dislocation density in subgrains and narrow subgrain boundaries, in contrast to creep tests reported in our previous work where large subgrains were observed with relatively wide subgrain boundaries and relatively low dislocation density in grains. Extensive precipitation occurred with denuded (precipitate-free) zones along grain boundaries. The coherent S-phase (Al2CuMg) transformed into partially coherent needle-shaped S′ precipitates. Superior stress amplitude caused reduced lifetime and wider denuded zones. A model of the formation of denuded zones along (sub)grain boundaries was proposed.

Structural Changes in a 9%Cr Creep Resistant Steel during Creep Test

Structural changes in a 9%Cr martensitic steel after 1%, 4% creep and creep rupture test at 650°C and stress of 118 MPa were examined. Heat treatment provided the formation of tempered martensite lath structure (TMLS) in the steel. The precipitations of second phase particles along block and lath boundaries provide effective stabilization of the TMSL under annealing/aging condition. This structure hardly changed under creep conditions in grip portion of crept sample. Significant coarsening of both the second phase particles and the martensite laths takes place in neck portion. In addition, the latter ones lose their original morphology and are replaced by large strain-induced subgrains. It should be noted that the increase of subgrain size is in almost direct proportion to the particle growth during the creep to 4% strain. The rapid growth of martesite laths followed by their evolution to deformation subgrains takes place within the tertiary creep regime.

Microstructure Evolution and Pinning of Boundaries by Precipitates in a 9 pct Cr Heat Resistant Steel During Creep

Structural changes in a 9 pct Cr martensitic steel during a creep test at 923 K (720 °C) under the applied stress of 118 MPa were examined. The tempered martensite lath structure (TMLS) was characterized by M23C6-type carbide particles with an average size of about 110 nm and MX-type carbonitrides with a size of 40 nm. The M23C6 particles were located on the packet/block/lath boundaries, whereas the MX precipitates were distributed homogeneously throughout TMLS. TMLS in the grip portion of the crept specimen changed scarcely during the tests. In contrast, the structural changes in the gauge section of the samples were characterized by the evolution of relatively large subgrains with remarkably lowered density of interior dislocations within former martensite laths. The formation of a well-defined subgrain structure in the gauge section was accompanied by the coarsening of M23C6 carbides and precipitations of Laves phase during creep. The most pronounced structural changes occurred just at the beginning of the tertiary creep regime, which was interpreted as a result of the change in the mechanism of grain boundary pinning by precipitates.

50%冷間圧延を施したAl-Mg-Si合金の焼鈍に伴う組織変化の同一視野観察 —その(2)回復により形成される組織—

Changes in microstructures at recovery stage after cold-rolling at 50% reduction rate were observed in an Al-Mg-Si alloy by means of SEM-EBSD system. The same area in the specimen was observed before and after cold-rolling. Intermittent annealing at 673 K, Ar ion-polishing, and EBSD observation were repeated up to the annealing time of 3.6 ks. Dislocation cells formed by cold-rolling changed into subgrains with annealing. Low and high angle boundaries were formed with surrounding the subgrains depending on misorientations between the neighboring subgrains. The high angle boundaries migrated to form recovered microstructures. Original orientation of the dislocation cell was maintained in the recovered microstructure. Some of the recovered microstructures grew with invading others. Therefore, new discrete poles did not appear in a discrete pole figure during annealing but those in the invaded subgrains disappear, which made the pole figure of the annealed specimen different from that of the cold-rolled specimen. Secondary recovered microstructures were observed to be formed with invading primary formed recovered microstructures. The recovered microstructures were finally invaded by recrystallized grains to disappear. In a region where growth of recrystallized grains was delayed, coarse recovered microstructures composed of large subgrains were formed.

NanoSIMS analysis and Auger electron spectroscopy of silicate and oxide stardust from the carbonaceous chondrite Acfer 094

We have detected 138 presolar silicate, 20 presolar oxide and three presolar complex grains within the carbonaceous chondrite Acfer 094 by NanoSIMS oxygen isotope mapping. These grains were further investigated by scanning electron microscopy (SEM) and Auger electron spectroscopy for morphological and chemical details and their distribution within the meteorite matrix. The three complex grains consist of Al-rich oxides (grossite and hibonite) attached to non-stoichiometric Si-rich silicates. Refractory Al-rich oxides therefore serve as seed nuclei for silicates to condense onto, which is proposed by condensation theory and astronomical observations. However, in the majority of presolar silicates we did not find any indications for large subgrains. Most of the grains (80%) belong to O isotope Group I ( 17O-enriched) and come from 1 to 2.5 M ⊙ asymptotic giant branch (AGB) stars of close-to-solar or slightly lower-than-solar metallicity. About 60% of these grains are irregular in shape; ∼40% display elliptical morphologies together with smooth, platy surfaces. Three grains with large 17O enrichments ( 17O/ 16O > 3 × 10 −3) have highly irregular shapes and are very small (<250 nm); these grains may have formed in binary star systems or around higher mass ( ∼ 3 M ⊙ ) AGB stars. About 10% of the presolar silicates in this study can be assigned to the O isotope Group IV, which most likely originate from type II supernovae (SNeII). These grains are also generally smaller than 300 nm and are often irregular in shape (88%), consistent with the SNII origin scenario. The presolar grains are generally evenly distributed within the matrix on an mm scale, although in one case a statistically significant clustering of five grains in one 10 × 10 μm 2 sized field is observed. This could be an important hint that the distribution of presolar material in the parental molecular cloud was heterogeneous on a very fine scale. The matrix-normalized abundance of silicate stardust in Acfer 094 is 163 ± 14 ppm, which is among the highest abundance of O-rich stardust in primitive meteorites. Oxide stardust comprises 26 ± 6 ppm of the matrix. Auger Nanoprobe measurements of 69 presolar silicates and oxides (30 on a quantitative, 39 on a qualitative basis) indicate that most of the grains are Fe-rich (Mg/(Mg + Fe) of 0.82 and lower), which is either due to non-equilibrium condensation, secondary alteration, or both. (Mg + Fe)/Si ratios of the silicates are mostly non-stoichiometric and scatter around pyroxene-like rather than olivine-like compositions, which is consistent with recent Auger and transmission electron microscopy observations and astrophysical predictions. Mg-rich grains (Mg/(Mg + Fe) > 0.5) more likely exhibit elliptical, smooth surfaces (14 out of 18 grains), which is an indication that these grains have not been strongly altered since their circumstellar condensation. We identified only one grain similar to the “glass with embedded metal and sulfides” (GEMS) with a statistically significant sulfur content (>2–3 at.%). It remains unclear why the typical high-sulfur GEMS grains are only found in interplanetary dust particles, but have not yet been unequivocally identified in primitive meteorites.

Uncovering the Mystery of Harper–Dorn Creep in Metals

In the present investigation, the occurrence of Harper–Dorn creep in Pb was studied under the condition of large strains. In performing the study, two Pb grades, commercial-purity Pb (99.95 pct) and high-purity Pb (99.999 pct), were creep tested at 573 K. The mechanical data showed that 99.95 Pb, unlike 99.999 Pb, did not exhibit accelerated creep rates at stresses <0.1 MPa (the region of Harper–Dorn creep). Also, the data on 99.999 Pb revealed that the creep curves associated with Harper–Dorn creep exhibited periodic accelerations and that the stress exponent was approximately 3, not unity as previously reported for Pb and other materials. An examination of the substructure developed during creep revealed that the substructural details in 99.95 Pb were markedly different from those characterizing Harper–Dorn creep in 99.999 Pb. In 99.95 Pb, the substructural observations included the presence of very large subgrains and triple junctions of subgrains. By contrast, in 99.999 Pb, the substructural features included the nucleation of new grains and the presence of twins. Combining the present data on Pb and those reported recently for Al has led to three important conclusions. First, the accelerated creep rates noted in the region of Harper–Dorn creep are produced by a creep process for which the dominant restoration mechanism is dynamic recrystallization. Second, the linear-stress dependence of creep rate previously reported for Harper–Dorn creep is most probably a direct consequence of short-term measurements involving small strains (total creep strains ≤0.01). Finally, Harper–Dorn creep will be observed in a large-grained metal at very low stresses if its purity level is high. This condition favors dynamic recrystallization as a restoration mechanism.

Quantifying Heterogeneous Microstructures: Core and Mantle Subgrains in Deformed Calcite

Calcite deformed by high temperature creep develops a heterogeneous microstructure consisting of deformed and recrystallised grains. The deformed grains either contain homogeneously distributed subgrains of similar size, or heterogeneously distributed small subgrains at grain boundaries (mantle subgrains) and relatively large subgrains in the core of grains (core subgrains). This paper demonstrates a method using electron backscattered diffraction (EBSD) to distinguish between the different types of grains and subgrains and to measure their sizes separately. In geological materials the average subgrain size, regardless of the subgrain type, is often used to estimate the deformation stress. However, this paper shows that mantle and core subgrain types only show a weak or no stress dependence.

Subgrain Rotation Recrystallization in Minerals

Subgrain rotation is a common mechanism of continuous dynamic recrystallization in minerals and some metals. The mechanism involves new grain boundary formation by progressive rotation of subgrains or subgrain boundary migration in regions with an orientation gradient. This paper reviews the status of our current knowledge of rotation recrystallization in minerals. In minerals a misorientation angle (θ) of 10˚ is often taken as the transition from subgrain boundary to grain boundary but recent studies on olivine indicate a much higher transition angle between 15-25˚. In contrast to a high transition angle, the onset of subgrain boundary mobility may occur at much lower angles between 3-10˚. In consequence, rotation recrystallization in minerals often involves an initial stage of subgrain rotation followed by subgrain growth once medium angle boundaries have formed. Current models assume that all subgrain boundaries increase in misorientation with strain. However, recent studies show that many different types of subgrain boundary develop in minerals. The formation of new high angle grain boundaries is only likely along some types of geometrically necessary boundary (GNB). The mineral halite (NaCl) is often quoted as the classic example of rotation recrystallization yet recent electron backscattered diffraction (EBSD) studies show that only limited grain sub-division occurs in NaCl polycrystals. This grain sub-division occurs on the scale of large subgrains that divide the old grain into a few domains and not by the rotation of the smaller equiaxed subgrains, as envisaged in current models. The small scale, equiaxed, mainly low angle network of subgrain boundaries that develop in many minerals may be incidental boundaries, as found in metals, or could be smaller length-scale GNBs. As minerals have high plastic anisotropy and a limited number of slip systems GNBs may dominate over incidental subgrain boundaries formed by trapping of statistically stored dislocations. New and extended models for rotation recrystallization are needed that consider i) incidental subgrain boundaries as well as different types of GNB, ii) the potential high mobility of medium angle (3-15˚) subgrain boundaries and iii) a link between the development of subgrain misorientation and texture development.

Analysis of X‐ray extinction in crystals with inhomogeniously distributed dislocations

AbstractThe secondary extinction theory of Zachariasen for mosaic crystals as well as the formalism of Becker&amp;Coppens have been used for substructure analysis (lattice disorientations, block size, density of excess dislocations, etc.) in crystals with inhomogeniously distributed dislocations in the Bragg case of diffraction geometry. In the case of large crystals DS ≫ Λhkl the mean total density ρD of randomly distributed dislocations was also estimated taking into account additionally the primary X‐ ray extinction treatment. In this connection two cases are considered: I) pure secondary extinction related to arrangement of dislocation walls and II) mixed extinction in crystals with large subgrains related to randomly distributed dislocations and arrangement of dislocation walls. In order to check the considerations, the experimental and calculated data were compared for Be (model I) and Cu (model II) single crystals. The weakly distorted single crystals of Be and Cu were experimentally investigated in the Bragg case of diffraction geometry using Cu Kα1 radiation by means of double crystal diffractometer. A new experimental procedure was proposed. Using alternative technique for substructure analysis (for instance EBSD) the reliability of analysis based on extinction phenomenon in weakly distorted single crystals has been checked. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

On the extrusion microstructural evolution of Al–Al 3Ni in situ composite

A directionally solidified Al–Al 3Ni in situ composite with fine Al 3Ni eutectic fibers was extruded at 200 and 400°C. Evolution of the microstructures was then investigated by examining the partially and fully extruded portions of the billets. According to the results, extrusion at 200°C will generate numerous tangled dislocations initially. When fiber breakage occurs, subgrains which are formed preferentially near the cracks will grow to eliminate the tangled dislocations and separate the pieces of broken fibers further apart, resulting in a fine subgrain structure with rod-like Al 3Ni particles distributed intergranularly. On the other hand, the billet extruded at 400°C shows formation of large subgrains with many intragranular Al 3Ni fibers in the early stage. Upon further extrusion, the fibers are broken while the large subgrains are refined by forming dislocation bands and changing the bands to sub-boundaries. Consequently, the 400°C-extruded billet shows fine subgrains with intergranular and intragranular rod-like Al 3Ni particles.

Deformation of cube-oriented grains and formation of recrystallized cube grains in a hot deformed commercial AlMgMn aluminium alloy

The cube orientation is usually found to be the strongest recrystallization texture component in annealed aluminium. The purpose of the present work was to obtain a better understanding of the cube orientation, both with respect to the behaviour of cube-oriented grains during deformation and the formation of recrystallized cube grains. Extensive use was made of the EBSP technique to characterize samples of a hot deformed AlMgMn alloy and plane strain compression tests were used to simulate hot rolling under a wide range of conditions of strain, strain rate and temperature. The investigations included: (i) the stability of cube-oriented grains during deformation, (ii) characterization of cube-oriented regions in the as-deformed state and (iii) nucleation of recrystallized cube grains. The work has demonstrated that cube-oriented grains present in the material prior to deformation remain orientation metastable during deformation and are deformed to bands. These cube bands have a unique subgrain size distribution with a long tail of large subgrains, making them very potent as nucleation sites for recrystallized grains. Nucleation of recrystallized cube grains takes place preferentially from those bands which are surrounded by the S deformation texture component.