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Fully recrystallized hexagonal Zirconium was subjected to controlled hot compression tests. Deformation above 723 K showed both flow saturation, characteristic of dynamic recovery, and also the presence of fine equiaxed grains, indicating dynamic recrystallization (DRx). Though data from relative work hardening were inconclusive, a combination of transmission Kikuchi diffraction plus transmission electron microscopy clearly confirmed the presence of DRx grains. A separation between the deformed and the DRx grains was not possible with microtexture or nano-indentation data. Local internal friction measurements (or tanδ values), however, clearly distinguished the DRx grains from both statically recrystallized (SRex) and the hot deformed grains.

The choice of alloying method for ferrous powder metallurgy alloys is often dictated by the oxidation potential of the alloying elements used in powder and compact manufacture. Silicon is effective in improving properties of ferrous PM steels; however, if added to the melt prior to atomization, the likelihood of oxidation is high. Alternatively, Si-rich and more complex particulates can be combined with the base powder, i.e., iron or low-alloy steel, with alloying occurring by solid-state diffusion during sintering. The effectiveness of these additives to improve material properties is dependent on their distribution throughout the material volume, as determined by diffusion of each added element. In this study, the distribution of silicon was quantified using energy-dispersive spectroscopy on an iron-based alloy composition. These data were compared with the volume fraction of the various transformation products in cross sections of hardenability samples using both light and electron microscopy. Predictions on the effects of local chemical composition, cooling rate, and microstructure are made.

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Eutectoid wire rods were subjected to controlled thermo-mechanical processing (TMP). Both increased cooling rate and applied stress during the austenite-to-pearlite decomposition produced significant changes in the microstructure: major increases in the pearlite’s axial alignment and minor decreases in the interlamellar spacing. The pearlite alignment was correlated with changes in the ferrite crystallographic texture and the state of residual stress. Microstructural engineering, improved axial alignment of pearlite, through controlled TMP gave a fourfold increase in torsional ductility. TMP of eutectoid steel thus appears to have interesting technological possibilities.

Grain boundary engineering of Fe-based austenitic stainless steels and other materials has been successful in producing a large increase in twin and twin related grain boundaries from a wide range of thermomechanical treatments. However, the exact mechanisms and effective grain boundary network descriptors to create the heavily twinned microstructures are yet to be fully understood. In this study, we provide insight into the grain boundary engineering process by examining sequential progression of the same spatial location of a twin related microstructure through thermomechanical processing. The results show that clusters of twin related grain boundaries called twin related domains form during primary recrystallization. The size of the twin related domains increases as the level of strain falls toward the critical strain for recrystallization. Growth of twin related domains during recrystallization results in the formation of twin boundaries behind the migrating grain boundary front. Formation of higher order twin boundaries occurs when two separate grain boundary fronts of the same twin related domain impinge upon each other. We also present relevant microstructural descriptors with emphasis on twin related domain statistics to recrystallization phenomena in grain boundary engineering materials.

Near eutectoid fully pearlitic wire rod (5.5 mm diameter) was taken through six stages of wire drawing (drawing strains of 0 to 2.47). The as-drawn (AD) wires were further laboratory annealed (LA) to re-austenitize and reform the pearlite. AD and LA grades, for respective wire diameters, had similar pearlite microstructure: interlamellar spacing (lambda) and pearlite alignment with the wire axis. However, LA grade had lower hardness (for both phases) and slightly lower fiber texture and residual stresses in ferrite. Surprisingly, essentially identical tensile yield strengths in AD and LA wires, measured at equivalent spacing, were found. The work hardened AD had, as expected, higher torsional yield strengths and lower tensile and torsional ductilities than LA. In both wires, stronger pearlite alignment gave significantly increased torsional ductility.

This study involved fully pearlitic wires of seven different diameters (5.5-1.6 mm). All samples were laboratory annealed to re-austenitize and were then air-cooled to reform the pearlite structure. Morphological alignment of the pearlite, along the wire axis, improved significantly, 32% to 93%, as the wire diameter decreased. This improvement coincided with increases in the < 110 > ferrite fiber texture, and falls in the axial residual stresses. In all the wires, the majority of the pearlite lamellae appeared to align, in a 2-D analysis, with minimum elastic stiffness (E-Min, under simple compression) for the ferrite (alpha). This correlation increased from 80% to 98% with decrease in wire diameter and fall in axial residual stresses. 3-D microstructures by serial sectioning, 3-D rotations seeking E-min and observations on coarse pearlite, indicated that {011}(alpha) and < 001 > (alpha) were, respectively, the pearlite interface (or habit plane) and growth direction. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Microstructural evolution with superheating was studied in chromium carbide-nickel coatings deposited by laser cladding. At lower superheating, selective growth of aOE (c) 0001 > direction from the high density of Cr7C3 grains nucleated resulted in a columnar structure with (0001) texture. Increased superheating lead to the loss of columnar structure as well as the (0001) texture. The hexagonal Cr7C3 showed an unusual isotropic nanoindentation hardness evidently correlated with its low c/a ratio. However, the rod-like morphology of the carbide dendrites resulted in significant anisotropy in the hardness of the composite.

The AlxCoCrFeNi family of high entropy alloys (HEAs) has received considerable attention due to its promising thermal, mechanical, and corrosion-resistant properties which make it widely suited for aerospace and marine applications. While the formation of secondary phases has been studied at various annealing temperatures, the results have focused on the late stages of precipitation, highlighting the need for the analysis of the intermediate stage precipitation. Here we use in-situ heating in the transmission electron microscope (TEM) complemented by ex-situ characterization of bulk annealed specimens, thermodynamic calculations, and precipitation simulations to study the phase evolution of Al0.3CoCrFeNi. Due to the high density of nucleation sites in the thin film, in-situ TEM reveals the formation of an additional intermediate phase, Co-B2 at 550 °C, where hundreds of hours are predicted for this phase to be shown during ex-situ experiments. At higher annealing temperatures between 700 and 900 °C, in-situ TEM shows the formation of Cr-rich precipitates as the first intermediate phase, followed by NiAl precipitates that form co-precipitates. The formation of these precipitates occurs concurrently, contrary to the findings of previous studies. In conjunction with the in-situ and ex-situ TEM studies, thermodynamic calculations and precipitation simulations have been performed to predict the formation of these phases and are found to support the experimental results. The present work provides new insight into the microstructural evolution of HEAs and reveals the importance of intermediate stages of thermal evolution, enabling an enhanced predictive view of phase evaluation in this class of alloys.

In this work, we use novel protocols based on spherical nanoindentation and orientation imaging microscopy (OIM) to quantify the local changes in slip resistances in the grain boundary regions of deformed, polycrystalline aluminum. The new protocols involve the use of the recently developed methods for extracting indentation stress–strain (ISS) curves from raw nanoindentation data in conjunction with the measurement of local microstructure at the indentation site using OIM to study the changes in the local slip resistances as a function of distance from the grain boundaries. Eight grain boundaries were selected for this work such that they included a broad range of boundaries, including low and high (grain-to-grain misorientation) angle boundaries as well as low, moderate, and high deviations in the Taylor factors of the grains on either side of the boundary. It was concluded that there was additional hardening in the grain boundary region when a Taylor ‘soft’ grain was present next to a Taylor ‘hard’ grain. This hardening was consistently observed on the soft grain side with one exception where hardening was observed on both sides of the boundary. A positive correlation was observed between the difference in Taylor factor across the boundary and the amount of hardening on the ‘soft’ grain side. However, no correlation was observed between the grain-to-grain misorientation angle and the extent of hardening at the grain boundary.

Closed end burst tests, using room temperature water as pressurizing medium, were performed on a number of industrially produced zirconium (Zr) clads. A total of 31 samples were selected based on observed differences in burst ductility. The latter was represented as total circumferential elongation or TCE. The selected samples, with a range of TCE values (5 to 35 pct), did not show any correlation with mechanical properties along axial direction, microstructural parameters, crystallographic textures, and outer tube-surface normal (σ 11) and shear (τ 13) components of the residual stress matrix. TCEs, however, had a clear correlation with hydrostatic residual stress (P h), as estimated from tri-axial stress analysis on the outer tube surface. Estimated P h also scaled with measured normal stress (σ 33) at the tube cross section. An elastic–plastic finite element model with ductile damage failure criterion was developed to understand the burst mechanism of zirconium clads. Experimentally measured P h gradients were imposed on a solid element continuum finite element (FE) simulation to mimic the residual stresses present prior to pressurization. Trends in experimental TCEs were also brought out with computationally efficient shell element-based FE simulations imposing the outer tube-surface P h values. Suitable components of the residual stress matrix thus determined the burst performance of the Zr clads.

Microstructural development in laser clad layers of Chromium carbide (Cr x C y )-NiCrMoNb on SA 516 steel has been investigated. Although the starting powder contained both Cr 3 C 2 and Cr 7 C 3 , the clad layers showed only the presence of Cr 7 C 3 . Microtexture measurements by electron back scattered diffraction (EBSD) revealed primary dendritic Cr 7 C 3 with Ni rich FCC metallic phase being present in the interdendritic spaces. Further annealing of the laser clad layers and furnace melting of the starting powder confirmed that Cr 7 C 3 is the primary as well as stable carbide phase in this multi component system. Increase in laser power and scanning speed progressively reduced carbide content in the laser clad layers. Increased scanning speed, which enhances the cooling rate, also led to reduction in the secondary arm spacing ( λ 2 ) of the Cr 7 C 3 dendrites. The clad layer hardness increased with carbide content and with decreased dendrite arm spacing.

This study involved edge regions of a friction stir processed (FSP) magnesium alloy (AZ31). Interleaved structures of ultra-fine, of few 100 nm, and coarser grains, several microns, were noted. Very short thermal anneals coarsened the former: generating typical micron-sized FSP grains. A model of microstructural development through grain boundary sliding of the ultra-fine grains was proposed. This model explains weaker texture in such grains and the fact that Mg-FSP shear texture did not change by successive passes. (C) The Minerals, Metals & Materials Society and ASM International 2015

Despite considerable research efforts on directionally solidified or freeze-cast materials in recent years, little fundamental knowledge has been gained that links model with experiment. In this contribution, the cryogenic characterization of directionally solidified polymer solutions illustrates, how powerful cryo-scanning electron microscopy combined with electron backscatter diffraction is for the structural characterization of ice–polymer composite materials. Under controlled sublimation, the freeze-cast polymer scaffold structure is revealed and imaged with secondary electrons. Electron backscatter diffraction fabric analysis shows that the ice crystals, which template the polymer scaffold and create the lamellar structure, have a-axes oriented parallel to the direction of solidification and the c-axes perpendicular to it. These results indicate the great potential of both cryo-scanning electron microscopy and cryo-electron backscatter diffraction in gaining fundamental knowledge of structure–property–processing correlations.

The microstructure evolution during isothermal static annealing of magnesium alloy AZ31 was investigated to critically evaluate the contribution of extension and contraction twins to the recrystallized microstructure after room temperature deformation, and to establish the potential of these twins to alter the recrystallized texture. Contraction twins were observed to be potent sites for recrystallized nuclei having many favorable new orientations, while extension twins were not effective nucleation sites for recrystallization. Although recrystallization nucleated preferentially inside the contraction twins, producing a distribution of new orientations (far from the undesirable starting c-axis fiber texture), these new grains did not significantly influence the final texture. This was attributed to four factors: (1) unusually extensive recovery before the onset of recrystallization after c-axis compression in most grains; (2) incomplete recrystallization within grains most closely aligned with the starting fiber texture; (3) sluggish growth of grains nucleated in the compression twins; (4) grain boundaries becoming an active site for recrystallized nuclei with a majority containing orientations from the starting texture.

Contrary to current understanding, an apparently stable single-phase quadrijunction, and one lacking any low-energy grain boundary member, has been experimentally observed during grain switching in large (∼mm) grained pure annealed aluminum. Electropolishing below the quadrijunction revealed a 3-D microstructure characterized by a rapidly shrinking grain with a faceted boundary indicating a significant anisotropy of energy and/or mobility. This unusual occurrence is described in detail in the expectation that a reasonable model for this observation might be found.

The effects of the superimposed oscillating load while using the continuous stiffness measurement (CSM) technique on the extracted indentation stress–strain (ISS) curves were systematically investigated using spherical nanoindentation tests performed on a variety of samples, using probes of 1 and 100 μm radii. Particular care was afforded to the corrections that were needed on the load, displacement and stiffness signals to account for the effect of the superimposed oscillations. A decrease in the oscillation frequency from the commonly used 45 Hz caused the measured stiffness values to decrease. This resulted in questionable high strain hardening values in the ISS curves. The noise levels in the recorded stiffness signal were significantly higher while using an oscillation amplitude of 1 nm. It was concluded that these observations are artifacts of the machine dynamic control systems and do not represent the material’s true behavior. The CMS signal with a 45 Hz oscillation frequency and 2 nm amplitude appears to produce most reasonable values of stiffness values and is therefore the best choice for reliably extracting ISS curves for the materials studied.

A commercial magnesium alloy was processed through multi-pass and multi-directional (unidirectional, reverse, and transverse tool movements) friction stir processing (FSP). Based on the FSP location, the dominant prior-deformation basal texture was shifted along the arc of a hypothetical ellipse. The patterns of deformation texture developments were captured by viscoplastic self-consistent modeling with appropriate velocity gradients. The simulated textures, however, had two clear deficiencies. The simulations involved shear strains of 0.8 to 1.0, significantly lower than those expected in the FSP. Even at such low shear, the simulated textures were significantly stronger. Microstructural observations also revealed the presence of ultra-fine grains with relatively weak crystallographic texture. Combinations of ultra-fine grain superplasticity followed by grain coarsening were proposed as the possible mechanism for the microstructural evolution during FSP. (C) The Minerals, Metals & Materials Society and ASM International 2016

The differences in the microstructure evolution during laser-roller hemming and conventional roller hemming (done at room temperature) of commercial-grade AZ31B sheet were studied using electron backscatter diffraction (EBSD). It was observed that the flanging operation, done as a precursor to roller hemming, produced a heterogeneous grain structure that remained throughout the subsequent hemming steps. Laser heating, applied during the roller passes, significantly reduced the amount of both extension and contraction twinning in the inner and outer band, respectively. More importantly, after two roller passes without laser heating, extension twinning in the inner band seemed to saturate. This forced the material in the inner band to accommodate further deformation by harder mechanisms, such as pyramidal slip and contraction twinning, during the third roller pass when failure occurred. The laser-hemmed samples exhibited much lower hardness values, especially in the inner band, which was deemed to be largely responsible for the success of the hemming operation with laser heating.

Normal and abnormal grain coarsening, NGC and AGC, are briefly reviewed and discussed in the light of grain curvature and kinetic models - notably those of Mullins. The critical importance of the log-normal grain size distribution determined by Hull and the grain shapes measured by Rhines and Craig are reviewed. It is shown that in NGC, as in particle coarsening, the largest grains or particles in their respective distributions grow at the rates, larger by a factor of the ratio of the largest to average size, required to maintain the size distributions during coarsening. The particle curvature removal model for NGC anchorage, giving = 0.2 r/f, developed here, matches surprisingly well the data reviewed by Manohar et al. Finally the need is indicated for further experimental and modeling studies to test the current ideas and to answer the many remaining questions.

This paper describes the main results from an experimental investigation into the consequences of deformation twinning in AZ31 on various aspects of plastic deformation, including the anisotropic strain-hardening rates, the tension/compression yield asymmetry, and the evolution of crystallographic texture. It was seen that AZ31 exhibited unusually high normalized strain-hardening rates compared to α-Ti that occurred beyond the strain levels where extension twins have completely altered the underlying texture. This observation challenges the validity of the generally accepted notion in the current literature that the high strain-hardening rates in AZ31 are directly caused by extension twins. It is postulated here that the thin contraction twins are very effective in strain hardening of the alloy by restricting the slip length associated with pyramidal 〈c + a〉 slip. This new hypothesis is able to explain the major experimental observations made in this study and in the prior literature. We have also presented a new hypothesis for the physical origin of the observed differences in the thicknesses of the extension and contraction twins. The stress fields in selected matrix–twin configurations were modeled using crystal plasticity finite element models. The contraction twin ( 0 1 ¯ 1 1 ) [ 0 1 ¯ 1 2 ¯ ] was predicted to form an internal extension twin ( 0 1 1 ¯ 2 ) [ 0 1 ¯ 1 1 ] , resulting in the commonly observed “double twin” sequence. The extension twin is suggested to inhibit thickening of this double twin by loss of twin–matrix coherency. Extension twins were predicted to retain their coherency and thus thicken.

AA6022 samples were deformed at different temperatures and strain rates and to respective true strains of 1.0 and 2.0. Though the deformation was immediately followed by quenching, the as-deformed samples contained recrystallized grains. A convention was adopted, based on grain size and in-grain misorientation developments, to distinguish between the deformed and recrystallized grains/orientations. At the highest deformation temperature(s), a clear pattern of increase (with increase in strain) in deformed cube \( \left\{ {001} \right\}\left\langle {100} \right\rangle \) was established. Such a pattern could not be explained from Taylor type deformation texture simulations, even after allowing nonoctahedral as well as octahedral slip. Microstructural observations could link, statistically, the increased cube presence with the presence of thicker deformed cube bands. The process of thickening was hypothesized as strain-induced boundary migration (SIBM) of the deformed cube bands into the adjacent noncube.

In this work, we investigated experimentally the various factors influencing the extraction of indentation stress-strain curves from spherical nanoindentation on metal samples using two different tip radii. In particular, we focused on the effects of (i) the surface preparation techniques used, (ii) the presence of a surface oxide layer, and (iii) the occurrence of pop-ins at the elastic-plastic transition on our newly developed data analysis methods for extracting reliable indentation stress-strain curves. Rough mechanical polishing was shown to introduce a large scatter in the measured indentation yield strengths, whereas electropolishing or vibropolishing produced consistent results reflective of the pristine sample. The data analysis techniques used were able to discard the portions of the raw data affected by a thin oxide layer, present on most metal surfaces, and yield reasonable indentation stress-strain curves. Experiments with different indenter tip radii on annealed and cold-worked samples indicated that pop-ins are caused by delayed nucleation of dislocations in the sample under the indenter.

A processing route has been developed for recovering the desired λ fiber in iron-silicon electrical steel needed for superior magnetic properties in electric motor application. The λ fiber texture is available in directionally solidified iron-silicon steel with the \( {\left\langle {001} \right\rangle } \) columnar grains but was lost after heavy rolling and recrystallization required for motor laminations. Two steps of light rolling each followed by recrystallization were found to largely restore the desired fiber texture. This strengthening of the \( {\left\langle {001} \right\rangle } \) fiber texture had been predicted on the basis of the strain-induced boundary migration (SIBM) mechanism during recrystallization of lightly rolled steel from existing grains of near the ideal orientation, due to postulated low stored energies. Taylor and finite element models supported the idea of the low stored energy of the λ fiber grains. The models also showed that the λ fiber grains, though unstable during rolling, only rotated away from their initial orientations quite slowly.

The effect of heating rate on precipitation and recrystallization behavior in twin roll cast (TRC) AA3105 has been investigated by three different means: conventional air furnace, controlled infrared, and lead bath heating. Experimental results showed that as-recrystallized grain size decreased and became more equiaxed as the annealing heating rate increased. These results were explained via time-temperature-transformation (TTT) curves for both dispersoid precipitation and recrystallization. With the faster heating rate, recrystallization could occur before precipitation of Mn present in the unhomogenized TRC samples. At a heating rate of 50 °C/s, the material underwent grain growth after recrystallization at 500 °C. No sign of grain growth was observed in materials annealed with lower heating rates, 3 °C/s, 0.5 °C/s, and 0.01 °C/s, due to greater dispersoid precipitation.

In this work, we report on the interdiffusion of Ge and Si in Ti3SiC2 and Ti3GeC2, as well as that of Nb and Ti in Ti2AlC and Nb2AlC. The interdiffusion coefficient, Dint, measured by analyzing the diffusion profiles of Si and Ge obtained when Ti3SiC2–Ti3GeC2 diffusion couples are annealed in the 1473–1773 K temperature range at the Matano interface composition (≈Ti3Ge0.5Si0.5C2), was found to be given by Dint increased with increasing Ge composition. At the highest temperatures, diffusion was halted after a short time, apparently by the formation of a diffusion barrier of TiC. Similarly, the interdiffusion of Ti and Nb in Ti2AlC–Nb2AlC couples was measured in the 1723–1873 K temperature range. The Dint for the Matano interface composition, viz. ≈(Ti0.5,Nb0.5)2AlC, was found to be given by At 1773 K, the diffusivity of the transition metal atoms was ≈7 times smaller than those of the Si and Ge atoms, suggesting that the former are better bound in the structure than the latter.

The ideal sheet texture for electric motor steels, the λ fiber, with 〈001〉 normal to the rolling plane, that can be produced by directional solidification, is largely lost by the required heavy rolling reduction and recrystallization needed to produce thin motor laminations. However by two successive light rolling reductions, followed in each case by recrystallization, the desired texture was largely recovered. Taylor and finite element modeling supported the hypothesis for this successful processing: the low Taylor factor of the λ fiber grains. Although starting from similar grain sizes, the second light deformation produced a much larger recrystallized grain size than did the first light deformation. This result suggests that a lower average Taylor factor in the structure before deformation, resulting in lower stored energy, can increase the recrystallized grain size.

The effect of alpha/beta solution temperature and cooling rate on the evolution of microstructure during the heat treatment of Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti6242Si) with a partially spheroidized starting microstructure of equiaxed + remnant lamellar alpha was established. Experiments comprising induction heating to a peak temperature of 971 °C or 982 °C followed by cooling at a rate of 11 °C/min or 42 °C/min revealed that the volume fraction of the equiaxed alpha grew much more rapidly than the lamellar constituent. These results were explained semiquantitatively using simple diffusion analyses of the growth of either spherical or elliptical particles, taking into account the soft impingement of the concentration fields. Despite the much lower diffusivity of molybdenum, which appears to control the growth of primary alpha in Ti6242Si, the similarity of the overall kinetics compared to those measured previously for Ti-6Al-4V was explained on the basis of the higher supersaturations developed during cooldown in the present alloy.

Novel experiments were conducted to elucidate the effect of deformation twinning on the mechanical response of high-purity α-titanium deformed at room temperature. Orientation-imaging microscopy (OIM), microhardness, and nanohardness evaluations were employed in conjunction with optical microscopy and quasi-static compression testing to obtain insight into the deformation mechanisms. Hardness measurements revealed that the newly formed deformation twins were harder than the matrix. This observation is perhaps the first experimental evidence for the Basinski mechanism for hardening associated with twinning, arising from the transition of glissile dislocations to a sessile configuration upon the lattice reorientation by twinning shear. This work also provided direct evidence for two competing effects of deformation twinning on the overall stress-strain response: (1) hardening via both a reduction of the effective slip length (Hall-Petch effect) and an increase in the hardness of twinned regions (Basinski mechanism) and (2) softening due to the lattice reorientation of the twinned regions.

The time for the static globularization of alpha lamellae in Ti-6Al-4V was predicted using analytical and finite-difference models based on the diffusion-controlled migration of the edges of remnant pancake-shaped particles. Similar to a previous two-dimensional analysis of the cylinderization of a semi-infinite, platelike second phase, the models quantified the influence of heat-treatment temperature, platelet thickness, and platelet diameter-to-thickness ratio on globularization kinetics. The present approaches were validated by comparison to previous observations of static globularization of Ti-6Al-4V at 955 °C.

International audience; An original model to simulate single grain boundary motion and its interaction with particles (Couturier G, Maurice C, Fortunier R. Phil Mag 2003;83:3387) is applied to model curvature driven grain growth. For single phase material, the single grain boundary model closely matches the grain coarsening kinetics of a 3D multi boundary vertex model. In the presence of spherical incoherent particles the growth rate slows down to give a growth exponent of 2.5. When the boundary is anchored there is a significantly higher density, four times higher, of particles on the boundary than the density predicted by the classic Zener analysis, but particles exert, on average, a drag force about half of the maximum value assumed by Zener. As a result the Zener drag is increased by a factor of 2.2. The limiting grain radius is compared with some experimental results.

An original model, based on a variational formulation for boundary motion by viscous drag, is developed to simulate single grain boundary motion and its interaction with particles. The equations are solved by a 3D finite element method to obtain the instantaneous velocity at each triangular element on the boundary surface, before, during and after contact with one or more particles. After validation by comparison with some simple, analytical and numerical cases, it is adapted to model curvature driven grain growth. For single phase material, the single grain boundary model closely matches the grain coarsening kinetics of a 3D multi boundary vertex model. In the presence of spherical incoherent particles the growth rate slows down to give a growth exponent of 2.5. When the boundary is anchored there is a significantly higher density, by a factor of 4, of particles on the boundary than the density predicted by the classic Zener analysis, and many particles exert less than this Zener drag force. As a result the Zener drag is increased by a factor of about 2.2. The limiting grain radius is compared with some experimental results.

A method for in-situ studies of the dynamics of individual embedded subgrains during recovery is introduced. The method is an extension of 3DXRD principles for studies of grain dynamics in connection with recrystallisation. It is limited to studies of foils with a sample thickness of 10-100 subgrains due to diffraction spot overlap. The volume evolution during recovery (annealing at 300°C for 181 minutes) of nine individual subgrains in a deformed sample (38% cold rolled Aluminium) is presented.

The prior literature on abnormal grain coarsening (AGC) at low volume fractions (f) of stable second phase particles in high purity Al alloys is reviewed and reanalyzed in the light of developments in modeling particle inhibition of grain boundary migration. With the usual assumptions (i) of incoherent particles that retain their shape on contact with the grain boundaries and (ii) that all the grain boundaries are equally mobile, it appears impossible to account for process of AGC. Normal grain coarsening (NGC) is shown to be less inhibited by the particles than is AGC. This idea is explored using a new but simple model of particle inhibition by curvature removal. The curvature of the smallest grains is always larger than that of the larger grains. Two possible hypotheses to overcome this difficulty are proposed: First the possible change of shape of particles on slowly moving grain boundaries, of grains with near 14 neighbors should, after a small increment of NGC, promote AGC at low values of the volume fraction f. The second hypothesis involves the observed high density of immobile, low angle grain boundaries (LAGBs) found in recent experiments on high purity Al-Fe-Si alloys cast with very coarse grain sizes. These alloys undergo rapid AGC even at higher values of f (> 0.01). These LAGBs are expected to inhibit the shrinkage of many of the small grains, whose loss is the fundamental mechanism of NGC.

Deformation studies at grain level have been performed in order to model how individual crystals in a polycrystalline material deform. The experiment was carried out by plane–strain compression of a high–purity polycrystalline aluminium with columnar grain structure with near 〈100〉 fibre texture parallel to the constrained direction in the channel die. This structure was chosen to allow a fully three–dimensional characterization of the grain structure. The grain orientations were mapped by orientation image microscopy, as the directionally solidified material was deformed in steps of 10% to a total height reduction of 40%. The grains were found either to show nearly uniform rotations or to split into two types of deformation bands, either with repeating orientation fields or with non–repeating orientation fields. The Taylor model and the finite–element method (FEM) were, as usual, quite successful in predicting the average deformation texture, but the Taylor model failed totally to predict the rotation of individual grains. The FEM was more successful in predicting the individual grain rotations but did not, as in a previous study, predict the morphology of the deformation bands. The significant discovery, made here, was that it appeared possible to model the local deformation at a grain scale, from the observed individual deviations of the grain rotations from those predicted if each grain underwent just the plane–strain conditions imposed on the sample. Plastic work rates were computed allowing four shears (two shears in each of the two contact planes) that are compatible with the channel–die geometry. It was found that in all the ‘hard’ grains (those with high Taylor factors), the additional shears (in type and magnitude) that minimized the plastic energy dissipation rate were the same shears that were needed to match the observed grain rotations. Adjacent Taylor ‘soft’ grains were found to have been subjected to the additional shears imposed by their neighbouring hard grains. This was true even when these shears raised the plastic work of the soft grains. This effect was most marked when the soft grains were small in size. These additional shears found by this plastic work analysis were consistent with the observed additional shear seen in the overall shape change of the sample. The grains forming non–repeating orientation fields had low initial Taylor factors and were surrounded by high–Taylor–factor grains, usually of larger size, but which had adopted somewhat different extra shears. The grains showing repeating orientation fields were found to have an orientation, near ‘cube’, (001) 〈100〉, which was initially unstable, leading to a break–up into different orientation fields when deformed. These differing deformation bands in the cube grains followed different strain paths, which also minimized their plastic work.

An X-ray diffraction method is introduced for monitoring the change in volume and orientation of individual, embedded subgrains during static recovery. It is limited to plate-like specimens with a thickness of less than 100 subgrain sizes. Growth curves are presented for nine subgrains within a 38% cold-rolled aluminium specimen during annealing for 3 h at 300 °C.

The purpose of this study is to investigate the role of deformation twinning in the strain-hardening behavior of high purity, polycrystalline α-titanium in a number of different deformation modes. Constant strain rate tests were conducted on this material in simple compression, plane-strain compression and simple shear, and the true stress (σ)-true strain (e) responses were documented. From the measured data, the strain hardening rates were numerically computed, normalized by the shear modulus (G), and plotted against both normalized stress and e. These normalized strain hardening plots exhibited three distinct stages of strain hardening that were similar to those observed in previous studies on low stacking fault energy fcc metals (e.g. 70/30 brass) in which deformation twinning has been known to play an important role. Optical microscopy and Orientation imaging microscopy were conducted on samples deformed to different strain levels in the various deformation paths. It was found that the onset of deformation twinning correlated with a sudden increase in strain hardening rate in compression tests. The falling strain hardening rate correlated with saturation in the twin volume fraction. In shear testing a much lower rate of strain hardening was found, at all strains, and this correlated with a lower density of deformation twinning.

This paper addresses the fracture properties of commercially cold-drawn (53%) MP35N (35% Co, 35% Ni, 20% Cr, and 10% Mo) in both the cold-worked and the cold-worked and aged conditions. MP35N is a superalloy that exhibits a remarkable combination of high strength (about 2 GPa) and high fracture toughness (about 100 MPa m ). The fracture toughness of MP35N was investigated systematically, with particular attention to the influence of the post deformation aging heat treatment, the sample thickness, the orientation of crack plane and the direction of crack propagation with respect to the drawing direction in the sample. These measurements yielded the following important results: (1) The plane strain fracture toughness of the commercially cold-drawn MP35N is about 120 MPa m , and is about 110 MPa m in the cold-drawn and aged condition. (2) The plane stress fracture toughness values for the cold-drawn MP35N were significantly lower than the corresponding values for the cold-drawn and aged material, in spite of the fact that the later had a higher strength value. (3) The direction of the crack propagation did influence the fracture toughness, reducing its value significantly in T–L direction, as compared with values in T–T and L–T directions.

The multiphase alloy MP35N (35% Ni, 35% Co, 10% Mo, 20% Cr) is a high strength, high toughness alloy of choice for several safety-critical applications in aerospace, oil drilling, and biomedical industries. Several previous attempts in literature to measure the plane strain fracture toughness of commercially drawn MP35N did not produce reliable values since they violated one or more of the criteria stipulated by ASTM standards for a valid measurement of K Ic . In most cases, the requirements for plane strain and small scale yielding conditions were not met, since the commercially drawn material was available only with limited cross-sectional dimensions. In this investigation specially designed specimens (modified compact tension (CT) specimens) have been used to measure the plane strain fracture toughness of MP35N in both the unaged and the aged conditions. The K Ic of the commercially drawn (53% reduction level) MP35N was measured to be 126 MPa√m, while that of the commercially drawn and aged MP35N was measured to be 98 MPa√m. Both these measurements satisfied all the required criteria stipulated by ASTM standards for valid measurements of K Ic . The new procedure used in this study has been verified by the measuring of fracture toughness of Al alloy, using both the modified specimen, as designed here, and the standard one. The results for plane strain fracture toughness of MP35N alloy have been verified by the standard measurement of J Ic values for both the aged and the unaged alloys. Finally, on a suitably normalized plot, introduced in this paper, the toughness-strength envelop for MP35 is higher than most of structural alloys, but significantly lower than that of the TRIP steels.

The sudden increase of strain hardening rates seen after small strains in titanium, was shown to correlate with the onset of deformation twinning. This result appears to match quantitatively with Hall–Petch grain size strengthening. The new twin boundaries appear to reduce the effective grain size.

An algorithm is used to estimate mesoscopic strains in a deformed polycrystalline material. This requires comparison of microstructures before and after imposed macroscopic plastic deformations, in order to estimate the local/mesoscopic strains from the displacements of identifiable grain boundary segments. The algorithm was applied to lightly plane strain compressed (PSC) polycrystalline zirconium. Very large (up to 1.2) near-boundary mesoscopic shear strains were estimated. These were well above the estimated measurement uncertainties and remarkably larger than the extremely small (0.01-0.04) PSC strains imposed. Opposing local shears, on both sides of a grain boundary, appeared to compensate each other. Direct correlations were noted, in the same grain, between mesoscopic shear strains and (i) in-grain misorientations and (ii) subsequent grain fragmentation. 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Following our recent studies of the influence of mechanical twinning on the strain hardening of low SFE FCC metals deformed by simple compression, the investigation was extended to two different deformation modes. These were plane strain compression and simple shear carried out on 70/30 brass, which exhibits only strain hardening, and on MP35N, a Co–Ni based alloy that also shows secondary hardening by deformation promoted precipitation. It was found that the magnitude of the primary strain hardening in both alloys, and the secondary hardening in MP35N, was dramatically reduced under simple shear compared to the other deformation paths. This reduced hardening in simple shear appears to be a consequence of the bulk of the deformation twins, and also the secondary hardening precipitates, forming on planes that were parallel to the primary {111} slip planes in this deformation path. These hypotheses are supported by deformation path change tests in which the shear samples that show low flow stress under continued shear, when subjected to simple compression showed a significant increase (jump) in the flow stress, reaching values that are similar to those of the alloy continuously compressed to the same equivalent strain. That is, the reduced strain hardening in shear deformation is due not to reduced twinning, but to the twins produced by shear providing only limited barriers to continued strain by simple shear. Shear banding was found to be more marked in plane strain compression than in simple compression after cold working, and particularly after the additional secondary hardening in MP35N.

Polycrystalline deformation and its modeling by currently used crystal plasticity models has been investigated by means of an experiment involving direct measurement of deformation induced orientation changes. The experiment used a polycrystalline aluminum sample with quasi-columnar grains, whose initial lattice orientations were mapped using the Orientation Imaging Microscopy (OIM) technique. The sample was then compressed 40% (along the axis of the columnar grains), and the lattice orientations after deformation were studied by OIM. It was found that most of the grains had significant in-grain misorientations in the form of deformation bands with two morphologies — either elongated on the grain scale or nearly equiaxed. In many, but not all cases, more than one similarly oriented deformation band was found in an individual grain. The deformation was then simulated using (i) a classical Taylor-type model, and (ii) a finite element model of the polycrystalline aggregate imposing equilibrium and compatibility between and within the constituent grains (in the weak numerical sense). A comparison of the predictions with the experimental results indicated that the Taylor-type model captured well the tendency to move towards a fiber texture but failed to predict correctly which pole was rotating towards the compression axis in the individual grains, and also by its implicit assumptions could not predict any in-grain misorientation. The finite element model predicted, reasonably well, grain rotations as well as the magnitude of the in-grain misorientations in most, but not all, of the individual grains, but failed completely to predict the morphology of the deformation bands that developed within the grains.

The transition in deformation textures between low stacking fault energy f.c.c. metals (e.g. brass textures) and the medium to high stacking fault energy f.c.c. metals (e.g. copper textures) is addressed. A detailed microscopy investigation was conducted in parallel with texture measurements on deformed samples of copper and 70/30 brass to different strain levels in three different deformation paths, namely, plane strain compression, simple compression, and simple shear. The objective of the study was to identify the specific trends in the transition between the brass textures and the copper textures that correlated with the onset of deformation twinning and those that correlated with the onset of micro-scale shear banding. It was found that several important transitions in the evolution of the deformation textures, especially in the rolled samples, correlated not with the onset of deformation twinning but with the onset of micro-scale shear banding. These results strongly suggest that the critical feature in texture transition is not twinning directly, but the shear banding promoted by the high strain hardening rates of low stacking fault energy f.c.c. metal.

MP35N (35% Co, 35% Ni, 20% Cr, and 10% Mo) is a superalloy that exhibits a remarkable combination of high strength and high fracture toughness. This alloy derives its strength mainly from two processes — cold work and a subsequent aging heat treatment. This study examines the influence of the amount of cold-work, imposed in the drawing and the rolling processes, and the subsequent aging heat treatment on the strength and ductility (which is also an indicator of toughness) of this alloy. It was found that high levels of cold-work followed by the aging heat treatments resulted in a drastic drop in the ductility of the alloy. This drastic loss of ductility was correlated with appearance of macro-scale shear bands in the samples. The physical origin of these localized shear bands is investigated in this paper. Furthermore, this study established a processing window for MP35N (in cold-drawing and cold-rolling processes) where the aging heat treatment provides the desired increase in the strength without adversely influencing the ductility of the alloy.

This article investigates the microstructural variables influencing the stress required to produce deformation twins in polycrystalline fcc metals. Classical studies on fcc single crystals have concluded that the deformation-twinning stress has a parabolic dependence on the stacking-fault energy (SFE) of the metal. In this article, new data are presented, indicating that the SFE has only an indirect effect on the twinning stress. The results show that the dislocation density and the homogeneous slip length are the most relevant microstructural variables that directly influence the twinning stress in the polycrystal. A new criterion for the initiation of deformation twinning in polycrystalline fcc metals at low homologous temperatures has been proposed as (σ tw −σ 0)/G=C(d/b)A, where σ tw is the deformation twinning stress, σ 0 is the initial yield strength, G is the shear modulus, d is the average homogeneous slip length, b is the magnitude of the Burger’s vector, and C and A are constants determined to have values of 0.0004 and −0.89, respectively. The role of the SFE was observed to be critical in building the necessary dislocation density while maintaining relatively large homogeneous slip lengths.

Two recent ideas for understanding and modeling the development of cube recrystallization texture in rolled and plane strain deformed aluminum and copper have been tested by experimental studies. The ideas are: (i) the concept called either ‘orientation pinning’ or ‘variant inhibition’—the inhibition of the growth of recrystallized grains by contact with regions in the deformed material with similar orientation, and (ii) the simple model, αC = NCdR/λC, for predicting the frequency of recrystallized cube grains based on nucleation from closely spaced deformed cube bands inherited from pre-existing cube grains. λC is the mean spacing between deformed cube bands,dR is the thickness of the recrystallized grains, both measured in the normal direction, ND, and NC is the number of cube grains per cube band in a linear traverse in ND. Detailed experimental results by orientation imaging microscopy have confirmed the importance of orientation pinning in preventing the development of the majority deformation texture components in warm deformed aluminum since here dR > λV, the inter-variant spacing of the rolling texture variants. In the same material cold rolled to an equivalent strain, retained rolling texture developed since, after cold rolling,dR