Search

Different grain coarsening behaviors (i.e. abnormal and homogeneous) are prevalently observed in gradient nanograined (GNG) Cu under stress controlled high-cycle and strain controlled low-cycle fatigue tests, respectively. In this paper, to comprehensively understand the intrinsic fatigue mechanism of gradient nanograined structures, both high and low cycle fatigue behaviors of GNG Cu are investigated under strain-controlled fatigue tests with a wide strain amplitude ranges. Cyclic behavior transition from abnormal grain coarsening at small strain amplitude to homogeneous grain coarsening at large strain amplitude is observd in GNG Cu. Microstrucural analysis reveals that the grain coarsening behavior in either abnormal or normal (homogeneous) mode is closely related to the spatial distribution of the cyclic plastic strain in the GNG layer (localized or delocalized) under cyclic loading. Such unique cyclic strain amplitude-dependent fatigue behavior is inherent to the gradient nanostructure, which fundamentally differs from the conventional strain localizing mechanism in metals with homogeneous structures under cyclic loading.

The mixed nanostructure mainly consisting of nanotwins and nanograins was obtained in a solid solution CuCrZr alloy by means of dynamic plastic deformation at cryogenic temperature. After subsequent aging treatments, the precipitation of Cr at nanometer scale provided further strengthening and brought substantial recovery of electrical conductivity. The aged nanostructured CuCrZr alloy exhibited a high tensile strength of 832 MPa and a high electrical conductivity of 71.2% IACS. The details of precipitation tuned by nanotwin boundaries were demonstrated in this work. The combined strengthening of nanostructures and nanoprecipitates was discussed.

A nanotwinned 316L austenitic stainless steel was prepared by means of surface mechanical grinding treatment. After recovery annealing, the density of dislocations decreases obviously while the average twin/matrix lamella thickness still keeps in the nanometer scale. The annealed nanotwinned sample exhibits a high tensile yield strength of ∼771 MPa and a considerate uniform elongation of ∼8%. TEM observations showed that accommodating more dislocations and secondary twinning inside the nanotwins contribute to the enhanced ductility and work hardening rate of the annealed nanotwinned sample.

Glioma is considered to be the most common brain malignancy in the central nervous system. At present, the aetiology of glioma is not clear. Due to its rapidly growth and easily recurrence, the prognosis of patients with glioma is very poor. N6-methyladenosine (m6A) methylation is an internal reversible modification in most RNAs, including messenger RNAs (mRNAs) and noncoding RNAs (ncRNAs). Recent studies have shown that the m6A regulators are abnormal expressed, and are extensively involved in the progression of glioma by targeting ncRNAs. Moreover, as the most important epigenetic regulators, ncRNAs can also affect the function of m6A regulators in glioma. This review summarized the expression and function of certain common m6A regulators in glioma. Also, the current review sum up the mutual interactions between m6A regulators and ncRNAs in glioma.

A gradient nanostructured (GNS) surface layer was induced on coarse-grained (CG) Cu and CuAl alloys by means of surface mechanical grinding treatment. The GNS/CG Cu-4.5Al sample subjected to tensile tests yields at a higher strength and fails at a higher uniform elongation (∼42%) in comparison with the GNS/CG Cu and Cu-2.2Al samples. The microstructures of the GNS/CG samples before and after tension at different strains were systematically investigated by transmission electron microscope. It is revealed that grain coarsening dominates the plastic deformation of nanograins in the GNS/CG Cu sample while the propensity of deformation twinning in nanograins increases in the GNS/CG CuAl samples. The experimental results suggested a transition of deformation mechanism of nanograins from grain coarsening to the partial dislocation associated deformation twinning in the GNS/CG Cu and CuAl alloys with increasing Al solute concentration. The obvious activation of deformation twinning accommodates the large tensile plasticity of the surface nanograins in the GNS/CG Cu-4.5Al sample. This work demonstrated that the partial dislocation associated deformation twinning is an effective deformation mechanism to retard the strain localization and to improve the tensile ductility of nanograins.

Fatigue behaviors of two Cu samples with different volume fractions of gradient nanograined (GNG) surface layer, prepared by surface mechanical grinding treatment, are investigated under stress-controlled high-cycle fatigue tests. With increasing the volume fraction of GNG layer from 4.3% to 13%, the fatigue endurance limit (at 107 cycles) of GNG Cu is enhanced from 88 to 98 MPa, much higher than that of coarse grained counterpart. Large volume fraction of GNG layer effectively postpones the extension of abnormal grain coarsening from a deeper subsurface layer to the topmost surface and thereby retards the initiation of surface fatigue crack.

Cracks often initiate at phase boundaries in conventional second phase reinforced alloys during cyclic loading, which limits their fatigue properties. Here, we prepared a nanotwin strengthened 316L stainless steel consisting of nanotwinned and recrystallized grains by using plastic deformation and subsequent partial recrystallization annealing. Fatigue tests revealed that interfaces separating hard nanotwinned grains from soft recrystallized ones exhibited excellent resistance to crack initiation. More than half of the cracks (57% in number fraction) are found in recrystallized grains while a small fraction (11%) is observed at the interfaces between nanotwinned and recrystallized grains. This is ascribed to the elastic homogeneity and cyclic deformation compatibility between nanotwinned and recrystallized grains. At small cumulative cyclic strains (below 4000 cycles at σa = 450 MPa), nanotwinned grains deform compatibly with the recrystallized grains without noticeable strain localization at their interfaces. Nanotwins can accommodate cyclic plastic strains by interaction of dislocations with twin boundaries, especially through the motion of the well-ordered threading dislocations inside the twin lamellae. At large cumulative strains, a moderate strain gradient is developed in recrystallized grains surrounding nanotwinned grains as a function of distance from the interfaces due to the occurrence of localized deformation in nanotwinned grains. The nanotwinned grains show high microstructural stability without notable de-twinnning, thus retarding crack initiation and propagation. Therefore, improved fatigue property with high fatigue limit of ∼350 MPa and high fatigue ratio of ∼0.45 is achieved in the nanotwin strengthened stainless steel, which is better than that of conventional second phase reinforced steels with comparable strength.

We investigated the low-cycle fatigue behavior of a novel nanotwinned 316L stainless steel consisting of nanotwinned austenitic grains embedded in the matrix with characteristic dislocation arrangements. We excitingly found that the α\prime-martensitic transformation is activated traversing twin lamellae to undertake the localized deformation of nanotwinned grains at high strain amplitudes. These α\prime-martensite nuclei are mainly originated from the intersections between “zigzag” slip bands and twin lamellae. Thanks to the effective release of stress/strain localization by the α\prime-martensitic transformation within nanotwins, the fatigue strength and fatigue life in the low-cycle regime are simultaneously enhanced.

Cu single crystals were subjected to dynamic compression plastic deformation to investigate orientation-dependent twinning. The experimental results showed that twinning is closely related to the ratio of the maximum Schmid factor for twinning partial (mT) to the maximum Schmid factor for perfect dislocation (mS), i.e., mT/mS, rather than mT. The twin volume fraction VT increases with the mT/mS value and the most favorable orientation for twinning has the maximum mT/mS value (1.15). The relationships of mT/mS with both effective stacking fault energy γeff and threshold stress for twinning τT were established for understanding orientation-dependent twinning. Further insights into the orientation-dependent twinning and guidance for developing bulk high density nanotwinned materials are provided.

The residual stress significantly influences the cyclic stress response of the hierarchical nanostructured materials. An obvious tension-compression asymmetry with minimum stress in compression larger than maximum stress in tension was observed in gradient nanograined (GNG) Cu under strain-controlled high-cycle fatigue tests, which gradually diminished with increasing cycles or after being annealed at a low temperature. The observed asymmetric response is primarily induced by the presence of the residual compressive stress in the GNG surface layer. The longer fatigue life can be achieved in GNG Cu with a higher residual stress, compared to that of annealed GNG Cu. Obvious tension-compression asymmetry was observed in cyclically deformed gradient nanograined Cu under strain control, caused by the residual compressive stress in the GNG surface layer.

A bulk nanostructured solutionized iron-based superalloy consisting of nanotwins and dislocation structures was prepared by means of dynamic plastic deformation at a high strain rate. The nanotwins constitute ∼51% in volume with the average twin/matrix lamella thickness of ∼20 nm. The sample exhibits a yield strength as high as 1130 MPa but with nearly zero uniform elongation. Subsequent peak-aging treatment at 700 °C for 1 h results in the precipitation of nano-sized gamma prime phases with a random distribution in the nanotwins and deformed matrix. The nanotwins still remain and keep the corresponding strengthening after aging. Those dispersed nanoprecipitates further raise the strength of the aged sample to 1420 MPa. Meanwhile, the uniform elongation increases to ∼3% due to the intrinsic plasticity of nanotwins with a reduced dislocation density after aging annealing. This kind of combined strengthening from both nanotwins and precipitates provides a new strategy to further raise the strength of nanotwinned materials or precipitation-strengthened materials.

Both nanotwinned and nanograined samples were prepared in a CuAl alloy by means of plastic deformation. Tensile tests showed that the nanotwinned sample exhibits a high yield strength of ~357 MPa and a considerable uniform elongation of 18% while the nanograined sample has ultra-high strength of ~758 MPa with a very limited uniform elongation of 1.4%. The lower yield strength and higher ductility in the nanotwinned sample were discussed based on its deformation mechanism.

By means of plastic deformation and subsequent annealing of a Fe-Mn alloy, strain-induced ɛ martensite laths transform into austenite nano-scale twins in the form of bundles. This provides a potential approach to prepare bulk nano-twinned materials that are expected to exhibit high strength and good ductility. The transformation mechanism from ɛ martensite laths to austenite nano-scale twins is discussed.

Microstructural evolution of gradient nanograined (GNG) Cu was investigated under strain-controlled cyclic deformation. Mechanically driven abnormal grain coarsening initiated from the ultrafine grained subsurface layer and eventually extended to the nanograined top surface with increasing cycles. Abnormal grain coarsening and the formation of dislocation patterns sustain the cyclic plastic strain and postpone the formation of extrusions/intrusions at the top surface, contributing to the enhancement of overall cyclic properties of GNG Cu.

Nanotwin lamellae orientation effect on tensile properties of nanotwinned grains strengthening stainless steels was investigated. It is found that the nanotwinned grains with twin lamellae oriented roughly parallel to loading direction exhibit the much higher strengthening effect (at least a GPa higher) than the nanotwinned grains containing twin lamellae inclined to loading direction, which is related to the anisotropic strengthening mechanisms of nanotwins.

In this work, one group of solid solution CuCrZr alloy was pre-aged and followed by dynamic plastic deformation to prepare nanograins with randomly distributed precipitates and the other group of solid solution CuCrZr alloy was subjected to dynamic plastic deformation and followed by aging treatment to prepare nanograins with precipitates on grain boundaries. We investigated the effect of precipitate distributions on the nanograin growth in the strain-induced nanostructured CuCrZr alloy. The results showed that the onset temperature of nanograin growth is ~523 K when precipitates are distributed randomly in the nanograins. Grain growth occurs at 723 K for the nanograins with precipitates located on grain boundaries. The precipitates located on grain boundaries much enhance Zener pinning effect than those distributed randomly in the nanograins, thus raising the thermal stability of nanograins.

The fatigue damage behavior of a novel nanotwinned stainless steel consisting of nanotwinned austenitic grains, nanograins and dislocation structures was investigated. It is found that the nanotwinned grains can effectively suppress the generation of persistent-slip-bands-like shear bands and nucleation of fatigue cracks in comparison with the other two deformed regions. Such better fatigue damage resistance of nanotwinned grains is mainly ascribed to the higher microstructural stability of them without obviously structural coarsening.

A bulk solid solution Cu-5Ag alloy consisting of nanotwins and nanograins was prepared by means of dynamic plastic deformation at liquid nitrogen temperature. Continuous precipitation of Ag occurs in the nanotwins which exhibit higher thermal stability than the nano-grains upon annealing. A tensile strength of 870 MPa and an electrical conductivity of 78% International Annealed Copper Standard are achieved in the annealed nanostructure Cu-5Ag alloy which is strengthened with nanotwins and nanoscale precipitates.

Gradient nano-grained Cu subjected to tensile tests yields at a stress almost twice of that of the conventional coarse-grained Cu at or below the room temperature. Beyond the yield stress, a uniform plastic strain of larger than 30% can be achieved, accompanied by homogeneous grain coarsening in nano-grained surface layer. The observed grain coarsening may induce certain degree of “strain softening”. The measured grain coarsening rates strongly depend on temperature, stress and strain rate, suggesting that the grain coarsening is presumably limited by thermally activated dislocation activities at defective grain boundaries, rather than via diffusional process on atom-by-atom basis. A non-diffusional (source-limited) Mott-Turnbull rate equation has been proposed to interpret the observed grain coarsening phenomenon.

Cyclic stresses generally lead to fatigue damage and failure with important implications for material and component design, safety, performance and lifetime costs in major structural applications. Here we present unique results for copper to demonstrate that a thin superficial layer of graded surface nanostructure over a coarse-grained core suppresses strain localization and surface roughening, thereby imparting unprecedented resistance to both low-cycle and high-cycle fatigue without compromising ductility. Progressive homogenization of the surface-graded copper is shown to be superior in fatigue properties compared to that of any of its homogeneous counterparts with micro-, submicro- or nano-grained structures. Since the findings here for enhancing resistance to fatigue are broadly applicable to a wide spectrum of engineering metals and alloys, the present results offer unique pathways to mitigate fatigue damage using a broad variety of processing routes, geometric design considerations, and structural parameters in many practical applications. Nanyang Technological University L.L acknowledges the financial support by the National Science Foundation of China (Grants Nos. 51420105001, 51471172 and U1608257) and the Key Research Program of Frontier Science, CAS. S.S. acknowledges support from the Distinguished University Professorship at Nanyang Technological University, Singapore.

Anisotropic tensile behaviors of a heterogeneous nanostructured Cu composed of isotropic nanograin matrix and anisotropic nanotwin bundles were investigated under loading directions parallel, normal, and 45° inclined to the twin boundaries (TBs). Distinct from the anisotropic strengthening effect in the homogeneous nanotwinned (NT) counterpart, the heterostructure exhibits the highest strength under parallel tension, and moderate strength under normal tension. High-resolution digital image correlation analysis indicated an orientation-dependent deformation compatibility between the nanotwin bundles and nanograin matrix, i.e., compatible deformation in the parallel orientation; but significant incompatibility in orientations both normal and 45° inclined to TBs. The results reveal that the strengthening effect of nanotwins in heterogeneous nanostructure is not only dependent on the strength of themselves, but also influenced by the deformation compatibility with surrounding components. The orientation-dependent deformation compatibility was attributed to the interactions between isotropic shear bands in nanograin matrix and anisotropic deformation in nanotwin bundles.

Introducing high density of nanotwins (NT) into metals is regarded an effective strategy to achieve superior mechanical properties. Our study shows that heterogeneous structured 304 austenitic stainless steel comprising of NT and micrometer-sized grains with a lower strength exhibits a comparable fatigue strength and larger fatigue limit/strength ratio, compared to its counterpart comprising of grains with NT and dislocation structures, distinct from the traditional fatigue view. Such superior high-cycle fatigue resistance arises from a weakened strain-localized fatigue mechanism where some NT grains co-deform plastically with a large volume fraction of surrounding microsized grains with enhanced dislocation slipping and martensitic transformation.

A randomly oriented copper embedded with nanotwin bundles was prepared by means of dynamic plastic deformation at cryogenic temperature. The subsequent cold rolling at room temperature induced shearing of nanotwin bundles, and resulted in a lamella structure after 90% thickness reduction. The volume fraction of copper-type rolling texture of the rolled nanotwinned copper is about 24.9±4.9 vol%, much less than the rolled coarse-grained (~76 vol%) and ultrafine grained ones. Detailed microstructure and texture characterizations show that the weaker texture is attributed to the evolution of nanotwin bundles retarding the formation of texture during cold rolling.

The plastic co-deformation behavior at the homophase interfaces between the hard nanotwinned grain inclusions and the soft recrystallized matrix grains in a duplex-microstructured AISI 316L austenitic stainless steel is examined through the analysis of long-range orientation gradients within the matrix grains by electron backscatter diffraction and transmission electron microcopy. Our analysis reveals that the mechanical accommodation of homophase interfaces until a macroscopic strain of 22% is realized within a small area of soft grains (about four grains) adjacent to the homophase interface. The activation of deformation twinning in the first two grain layers results in the occurrence of a ‘hump’ in the orientation gradient profile. We ascribe this effect to the role of deformation twinning on the generation of geometrically necessary dislocations. The smooth profile of the orientation gradient amplitude within the first 10 grain layers indicates a gradual plastic accommodation of the homophase interfaces upon straining. As a consequence, damage nucleation at such interfaces is impeded, resulting in an enhanced ductility of the single phase duplex-microstructured steel.

The work hardening behavior of nanotwinned austenitic grains in a novel type of nanotwinned austenitic stainless steels was investigated. Multiple Shockley partial dislocation activities in the nanotwinned grains could trigger occurrence of stacking faults and strain induced martensitic transformation before necking, resulting in an increase in work hardening rate. Consequently, the fully recovered nanotwinned austenitic stainless steel exhibits a uniform elongation of ~ 13% at the strength of 950 MPa.

Nano-twinned austenite grains were characterized in an austenitic stainless steel by using nanoindentation. The nano-twinned austenite grains exhibit identical elastic modulus with the micro-sized recrystallized austenitic grains. Nanoindentation hardness of the nano-twinned grains is about 4.5 GPa, ∼1.0 GPa higher than that of the recrystallized grains, but lower than that of the nano-sized grains with the comparable size owing to the presence of twinning partial dislocations.

Schematic illustrations of the residual stress measurement by X-ray diffraction

A novel type of nanotwinned austenitic stainless steel consisting of the nanotwinned grains embedded in coarsened nano-sized grains and recrystallized matrix was synthesized. Potentiodynamic polarization measurements indicate that the nanotwinned samples exhibit a higher passive ability in the acidic solution and a comparable pitting resistance in chloride solution in comparison with that of their coarse grained counterparts. The effects of the microstructure, specifically the presence of nanotwinned grains, on the corrosion resistance are discussed. The nanotwinned austenitic stainless steel possesses an excellent combination of strength and ductility as well as corrosion resistance.

Two types of austenitic stainless steel samples were prepared by means of dynamic plastic deformation followed by annealing: nanotwinned austenitic grains embedded in recrystallized austenite matrix and martensitic/austenitic duplex microstructures. Annealing at 923 K induced martensitic reversion while most nanotwinned grains are stable. An enhanced strength–ductility combination is observed in the annealed nanotwinned samples which exhibit a uniform elongation of ~21% and a yield strength of ~900 MPa, in contrast to a uniform elongation of ~12% with comparable strength in the martensitic/austenitic samples.

A bulk nanostructured CuCrZr alloy consisting of nanotwins and nanograins was prepared by dynamic plastic deformation at liquid nitrogen temperature. A tensile strength of 700 MPa and an electrical conductivity of 78.5% International Annealed Copper Standard are obtained in the nanostructured CuCrZr alloys processed by means of this one-step deformation without aging treatment. The reason for the increased strength without the sacrifice of its high electrical conductivity was discussed.

The microstructure, hardness and tensile properties of a modified 9Cr−1Mo steel processed by dynamic plastic deformation (DPD) to different strains (0.5 and 2.3) have been investigated in the as-deformed and annealed conditions. It is found that significant structural refinement and a high level of strength can be achieved by DPD to a strain of 2.3, and that the microstructure at this strain contains a large fraction of high angle boundaries. The ductility of the DPD processed steel is however low. Considerable structural coarsening of the deformed microstructure without pronounced recrystallization takes place during annealing of the low-strain and high-strain samples for 1 h at 650 °C and 600 °C, respectively. Both coarsening and partial recrystallization occur in the high-strain sample during annealing at 650 °C for 1 h. For this sample, it is found that whereas coarsening alone results in a loss of strength with only a small gain in ductility, coarsening combined with pronounced partial recrystallization enables a combination of appreciably increased ductility and comparatively high strength.

A novel type of duplex microstructure is generated in a single-phase austenitic steel (AISI 316L; X2CrNiMo19-12), consisting of plastically compliant recrystallized austenitic grains as the matrix containing coarse non-recrystallized grains with a nanotwinned austenitic ( nt-γ ) structure as strengthening inclusions. This novel type of single-phase yet duplex microstructured steel exhibits an excellent combination of strength and ductility. We study the plastic co-deformation mechanisms between the nanotwinned and the recrystallized grains under tension using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). At tensile strains below 5%, the nt-γ grains nearly deform homogeneously in conjunction with the surrounding statically recrystallized (SRX) grains without generating notable strain localization near their interfaces. The anisotropic plastic deformation of the nt-γ grains with predominant shear parallel to the twin boundaries results in a higher dislocation density in the neighboring SRX grains. As the strain exceeds 12%, localized deformation occurs within the nt-γ grains in the form of shear banding. A strain gradient is developed in the surrounding SRX grains as a function of distance from the nt-γ /SRX interface. Deformation twinning is observed in the SRX grains near the nt-γ grains, while away from nt-γ grains dislocation slip dominates the deformation. The strengthening effect of the strong and ductile nt-γ grains may offer a novel approach to strengthen austenitic steels and related alloys by generating a nanotwinned/recrystallized duplex microstructure.

A cylindrical hexagonal-close-packed Ti sample was pre-deformed by dynamic compression to produce coarse-grained and ultrafine-grained structures in different parts of the sample followed by further dynamic compression to failure, making it possible to explore the effect of stored strain and grain boundary energy on shear banding in the material. A long shear band that formed during the final compression process passed through a complete diagonal of the sample. Electron backscattered diffraction was used to systematically investigate the shear-banding-induced structural evolution. Results show that the original stored energy in the matrix plays a significant role in the competition between deformation-induced grain refinement and grain growth, which determines the final average grain size in a shear band. Shear banding leads to grain reorientation such that one close-packed 〈 1 1 2 ¯ 0 〉 direction and one 〈 1 0 1 ¯ 0 〉 direction in most grains are parallel to the local shear direction and the normal direction to the local shear plane, respectively. The grain orientation in the shear band favours prismatic 〈a〉 slip, while the texture in the matrix, which is a stable compression texture, benefits the basal 〈a〉 slip. The results advance our understanding of the shear banding behaviour in heterogeneous deformation conditions and also the overall mechanical behaviour of materials under dynamic compression.

Lamellar nanostructures were induced in a plain martensitic low-carbon steel by using dynamic plastic deformation at room temperature. The nanostructured steel was hardened after annealing at 673 K for 20 min, with a tensile strength increased from 1.2 GPa to 1.6 GPa. Both the remained nanostructures and annealing-induced precipitates in nano-scale play key roles in the hardening.

The passive behavior of a bulk nanostructured 316L austenitic stainless steel consisting of nanometer-sized grains (NG) and nano-twin bundles (NT) are investigated. The electrochemical results indicate that the spontaneous passivation ability and growth rate of passive film are improved. The X-ray photoelectron spectroscopy (XPS) shows that a Cr 0 -enriched layer forms at the passive film/metal interface. More nucleation sites afforded by the nanostructures and the enhanced diffusion rate of charged species across the passive film are believed to be responsible for the improved passive ability. The PDM model is introduced to elaborate the microscopic process of passivation.

With surface mechanical grinding treatment, a gradient nanograined (GNG) surface layer is produced on a bulk coarse-grained (CG) pure Cu, where the grain size increases gradually from 20 nm (topmost surface) to micrometer scale. Microhardness measurements of the GNG/CG sample after tension revealed that tension induces softening for grains smaller than 165 nm and induces hardening above this size. This critical size agrees with the strain-induced saturation grain size of Cu subjected to severe plastic deformation.

The deformation behaviour of ultrafine-grained hexagonal close-packed Ti processed by dynamic plastic deformation was investigated using electron microscopy techniques. Results show that the deformation twinning propensity in ultrafine-grained Ti decreases monotonously while the number of dislocations having the component increases with decreasing grain size, indicating that the transformation of the deformation mechanism from twinning to slip occurs and that the twinning mechanism does not change within the observed grain size range. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Cyclic deformation was studied in Cu samples with a gradient nanograined (GNG) surface layer. Compared with the coarse-grained sample, the Cu samples with a GNG surface layer exhibit a greatly enhanced fatigue limit under stress-controlled cyclic deformation. The cyclic deformation induced an abnormal grain coarsening that initiated from the subsurface layer and grew along 45° to the stress axis toward the top surface layer, where the fatigue cracks were formed.

The scanning electron microscopy transmission Kikuchi diffraction technique was used to characterize the microstructure of ultrafine-grained Ti processed by dynamic plastic deformation at room temperature. Contrary to the general conception that twinning plays little role in the plastic deformation of ultrafine-grained hexagonal close-packed Ti, nanocrystalline { 1 1 2 ¯ 2 } and { 1 0 1 ¯ 2 } deformation twins were widely observed in the material. The observation provides insight into the deformation mechanisms and mechanical behaviour of nanostructured metals with hexagonal close-packed structures.

A surface layer with a gradient decrease in twin density has been produced in a Fe–Mn austenitic steel, which corresponds to a gradient drop in hardness from 5.3 GPa in the top surface to 2.2 GPa in the coarse grained core. The dependence of hardness on the twin thickness was determined, showing a weaker strengthening effect of twin boundaries than that of conventional grain boundaries in this alloy. Superior strength–ductility synergy was observed in tensile tests of the gradient nanotwinned layer.

A novel strategy for strengthening austenite steels is introduced by using the unique strengthening effect of nanoscale twins. Austenite grains containing multiple nanoscale twins can be strengthened to a yield strength of a few GPa with very high work-hardening rates. Compared with the conventional dual-phase steels, several austenite steels strengthened by using ultra-strong nanotwinned austenite grains exhibit a superior strength–ductility synergy. Perspectives on future development of the nanotwinned austenite steels and the nanotwin strengthening strategy are presented.

Bulk dynamic plastic deformation (DPD) materials comprise a composite structure of nanoscale twin bundles and nanoscale grains. The tribological properties of DPD-processed pure nano-Cu have been investigated in this study and compared with conventional coarse-grained (CG) Cu under both monotonic and repeated frictional sliding. We demonstrate that DPD nano-Cu and CG Cu exhibit steady-state mechanical characteristics after repeated frictional sliding that are similar to those seen in nanotwinned (NT) Cu produced by pulsed electrodeposition. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

An austenitic Fe-25Mn steel with a low stacking fault energy was subjected to dynamic plastic deformation (DPD) followed by thermal annealing. The as-DPD sample is structurally characterized by a mixed nanostructure consisting of nanosized grains with an average size of 43 nm and bundles of nanoscale twins (with an average twin/matrix lamella thickness of 5 nm), as well as some dislocation structures, which exhibits a high yield strength of about 1470 MPa but a limited tensile ductility. Thermal annealing leads to static recrystallization (SRX) of the nanostructures forming a hierarchical structure of nanotwinned grains embedded in microsized SRX grains, owing to the higher thermal stability of the nanotwinned bundles than that of nanosized grains. With an increasing number of SRX grains the yield strength and ultimate tensile strength drop while the tensile ductility increases. The calculated yield strength of the nanotwinned grains is about 1.5 GPa, much lower than that determined from Hall-Petch strengthening extrapolated to the nanoscale. Work hardening rates of the nanotwin grains, comparable with that of the microsized grains, are higher than that of the original coarse grained sample. The micrograined austenitic Fe-Mn samples strengthened by nanotwinned grains exhibit enhanced strength ductility synergy in comparison with the deformed samples. A combination of a 977 MPa yield strength with a uniform elongation of 21% is achieved in the annealed samples, well above that of the deformed samples. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

By means of dynamic plastic deformation (DPD) with high strain rates, a bulk nanostructured 316L austenitic stainless steel consisting of nano-sized grains embedded with bundles of nanometer-thick deformation twins was synthesized. The average transverse grain size is similar to 33 nm and the twin/matrix lamellar thickness is similar to 20 nm. The nano-twin bundles constitute similar to 24% in volume. The nanostructured samples exhibit a high tensile strength of similar to 1400 MPa but a limited ductility with a uniform elongation of similar to 2%. Subsequent thermal annealing of the as-DPD samples in a temperature range of 730-800 degrees C led to a single-phased austenite structure consisting of static recrystallized (SRX) micro-sized grains embedded with remaining nano-twin bundles and nano-grains. The annealed DPD samples exhibit an enhanced strength-ductility synergy and much more enhanced work-hardening rates than the as-deformed samples. Work-hardening rates of the annealed DPD samples can be even higher than that of the original CG sample. Tensile ductility was found to increase almost linearly with the volume fraction of SRX grains. A combination of 1.0 GPa tensile strength with an elongation-to-failure of similar to 27% is achieved in the annealed DPD 316L stainless steel samples. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.