Mechanical Twinning Research Articles

Surface modification and its effect on the tensile and fatigue properties of 300M steel subjected to ultrasonic surface rolling process

Surface modification of 300M steel induced by ultrasonic surface rolling process (USRP) was studied regarding surface topography , microstructure, the distribution of shear strain, microhardness , and residual stress profiles. The effects of USRP on the tensile and fatigue properties were finally discussed. The results indicated that one pass of USRP can reduce the surface roughness ( R a ) by more than one order of magnitude (from 0.30 μm to 0.025 μm). Besides, a severe plastic deformation layer was noticed at the top surface, which led to an increase of surface microhardness by about 17% and an elevation of compressive residual stress by about 190%. In USRP, a high plastic shear strain of ~3 and shear strain rate of ~10 5 s −1 can be induced into the topmost surface, which motivates the dislocation activities in the form of dislocation tangles, dislocation walls, and dislocation cells. Dislocation slip supplemented by mechanical twinning dominates the microstructural evolution pattern of the USRP sample. The excellent surface finish, high microhardness, and high compressive residual stress result in an obvious increase in the resistance of high-cycle fatigue behavior. Besides, the tensile strength was improved by 3.8%, whereas the corresponding yield strength and elongation were reduced by 5.9% and 24.3%, respectively. • Only one-pass USRP can reduce the surface roughness by more than one order of magnitude. • A high plastic shear strain of ~3 and strain rate of ~10 5 s −1 appears on the topmost surface after USRP. • Dislocation slip assisted by the mechanical twinning dominates the microstructural evolution pattern. • An obvious increase in the resistance of high-cycle fatigue is obtained after USRP.

Effects of texture and stress sequence on twinning, detwinning and fatigue crack initiation in extruded magnesium alloy AZ31

Several types of cyclic stress were applied to specimens made of an extruded magnesium alloy, AZ31, to elucidate twinning, detwinning, and fatigue crack initiation mechanisms by electron backscattered diffraction analysis. In both the texture and the random orientation, twinning occurred under compressive stress exceeding the compressive yield strength. Detwinning occurred only in the texture upon the subsequent application of tensile stress less than the tensile yield strength of monotonic loading, whereas detwinning did not occur in the structure with random orientation. Under compression–compression fatigue test, successive twinning occurred, which changed the orientation of the basal plane. As a result, the random orientation of grains changed to the texture in which the normal of the basal plane was parallel to the loading direction. Cracks were formed along the boundary of grains with a high Schmid factor of the basal slip system and the misfit of grain on both sides was large. Near the crack initiation site, twin bands were observed; however, detwinning did not occur under the tensile stress after the cyclic compression loading.

Long-term (7 Ma) strain fluctuations within the Dead Sea transform system from high-resolution U-Pb dating of a calcite vein

Abstract The onset of the Dead Sea transform has recently been reevaluated by U-Pb age-strain analyses of fault-related calcite taken from several fault strands along its main 500-km-long sector. The results suggest that the relative motion between Africa and Arabia north of the Red Sea was transferred northward to the Dead Sea transform as early as 20 Ma and along a ~10-km-wide deformation zone that formed the central rift with contemporaneous bounding sinistral motion. The Gishron fault is the western bounding fault with normal and sinistral fault offsets that placed Proterozoic crystalline rocks and a cover of Cambrian sandstones in fault contact with Cretaceous-Eocene carbonates. Fault-related calcite veins are common in the Gishron fault zone, and we report the results of a detailed study of one sample with nine calcite fillings. Low fluid inclusion entrapment temperatures <50 °C, stable isotopes values of–3.3–0‰ (δ13C; PDB) and 15.3‰ to 17.6‰ (δ18O; SMOW), and low rare earth element (REE) concentrations within the nine calcite fault fillings indicate that a local, meteoric fluid fed the Gishron fault zone over ca. 7 Ma at depths of <2 km. Laser ablation U-Pb ages within the thin section range from 20.37 Ma to 12.89 Ma and allow a detailed fault-filling chronology with the oldest calcite filling in the middle, younging outward with shearing between the oldest eight zones, all of which are finally crosscut by a perpendicular (E-W) vein. All nine calcite fillings have unique mechanical twinning strain results (n = 303 grains). Shortening strain magnitudes (–0.28% to–2.8%) and differential stresses (–339 bars to–415 bars) vary across the sample, as do the orientations of the shortening (ε1) and extension (ε3) axes with no evidence of any twinning strain overprint (low negative expected values). Overall, the tectonic compression and shortening is sub-horizontal and sub-parallel to the Gishron fault (~N-S) and Dead Sea transform plate boundary. Most strikingly, the 7 m.y. period of vein growth correlates exactly with the timing of fault activity as evident within the 10-km-wide deformation zone in this evolving plate boundary (between 20 Ma and 13 Ma).

P–253 Description of a rare spontaneous monozygotic blastocyst splitting into two discrete euploid blastocysts in vitro detected with time-lapse imaging and preimplantation genetic testing (PGT)

Abstract Study question Can spontaneous and complete blastocyst splitting into two, in vitro, be investigated using time-lapse imaging and biopsy of each trophectoderm, for inference of ploidy? Summary answer Time-lapse imaging combined with PGT-A gives insights into the incidence, dynamics and timing of rare blastocyst splitting and the ploidy status of each resulting blastocyst. What is known already It is well known that multiple births occur more often with Assisted Reproductive Technologies (ART) than spontaneous conception, even after single embryo transfer. The mechanism of Monozygotic Twinning (MZT) during ART is still unclear but cryopreservation, extended culture, PGT, maternal age and assisted hatching are reported risk factors. MZT is a rare phenomenon, with an incidence of 0.4% in natural conception compared with up to 4.9% in ART. The timing of embryo splitting dictates the type of twinning, in terms of chorionicity and amnionicty, and this is officially determined using ultrasound scanning. Study design, size, duration This is a case study describing the detection of the complete splitting of an IVF blastocyst at 140 hours post insemination (hpi), using time-lapse imaging. The 40-year-old patient previously experienced biochemical pregnancy and several miscarriages; an ectopic molar pregnancy and a probable cornual ectopic. The 39-year-old male partner was normozoospermic. Participants/materials, setting, methods Facilitative laser breaching was performed, according to standard operating procedure, of the morula at 96hpi of embryo development, prior to PGT. Images were collected every 10 minutes and developmental events and embryos morphology annotated using the EmbryoScope+™ time lapse incubator and software. Main results and the role of chance Over 50,000 hatching blastocysts have been time-lapse imaged, scrutinised and annotated within this group of fertility clinics. This is the first time that such a rare blastocyst splitting event has been recorded and studied. Following observation of two pronuclei following IVF and typical cleavage development to blastocyst, with facilitative zona breaching on, at 106.7hpi, the full blastocyst’s trophectoderm (TE) began to herniate and hatch. By 114.3hpi a second internal blastocoel cavity formed appearing to divide the inner cell mass (ICM) within the zona pellucida (ZP). This resulting blastocyst proceeded to hatch as its discrete ICM migrated out of the ZP, along with its TE. TE cells from the original blastocyst then began to hatch at 117.5hpi at the same breached site in the ZP with its ICM visibly evacuating the ZP. By 140hpi the blastocyst had split into two discrete blastocysts while hatching from the ZP. Both resulting blastocysts had clear and separate ICMs and TEs present. Biopsy of approximately 5 cells was performed for each TE, and the blastocysts were vitrified individually. Next Generation Sequencing (NGS) reported both blastocysts to be euploid. Limitations, reasons for caution This case may have been detectable without time-lapse imaging, as the splitting was completed prior to biopsy. More expert scrutiny of the images may result in earlier signs of twinning in progress being detected. Wider implications of the findings: The nature of this detectable in vitro blastocyst splitting, indicates these embryos (if they implanted) to be monozygotic, dichoriol-diamniotic ‘identical’ twins. However – as single embryo transfer is the preferred treatment plan; they may be born years apart. These observations could shed light on the debated models of monozygotic twinning. Trial registration number Not applicable

Unique transition of yielding mechanism and unexpected activation of deformation twinning in ultrafine grained Fe-31Mn-3Al-3Si alloy

Tensile mechanical properties of fully recrystallized TWIP steel specimens having various grain sizes (d) ranging from 0.79 μm to 85.6 μm were investigated. It was confirmed that the UFG specimens having the mean grain sizes of 1.5 μm or smaller abnormally showed discontinuous yielding characterized by a clear yield-drop while the specimens having grain sizes larger than 2.4 μm showed normal continuous yielding. In-situ synchrotron radiation XRD showed dislocation density around yield-drop in the UFG specimen quickly increased. ECCI observations revealed the nucleation of deformation twins and stacking faults from grain boundaries in the UFG specimen around yielding. Although it had been conventionally reported that the grain refinement suppresses deformation twinning in FCC metals and alloys, the number density of deformation twins in the 0.79 μm grain-sized specimen was much higher than that in the specimens with grain sizes of 4.5 μm and 15.4 μm. The unusual change of yielding behavior from continuous to discontinuous manner by grain refinement could be understood on the basis of limited number of free dislocations in each ultrafine grain. The results indicated that the scarcity of free dislocations in the recrystallized UFG specimens changed the deformation and twinning mechanisms in the TWIP steel.

On the microstructural evolution pattern of 300 M steel subjected to surface cryogenic grinding treatment

In the present work, an ultrahigh strength 300 M steel was treated by the surface plastic deformation with surface cryogenic grinding treatment (SCGT) technique. The SCGT was carried out by rotational grinding wheel embedded with numerous zirconium corundum abrasive grits, which contributes to a decreasing distribution of strain, strain rate and strain gradient from the treated surface to the matrix. The depth-dependent microstructural evolution was systematically evaluated from micrometer to nanoscale and in particular, the deformation mechanisms and texture alternation were addressed. It was identified that the microstructural evolution starts from the formation of dislocation structures, that is subsequently followed by the distorted microstructures featured as the origination and evolution of ultrafine laminates and ultrafine grains in the form of dislocation activities. Nanocrystalline was generated in the topmost surface region and nanoparticle precipitates induced by the SCGT process occur within a few microns of the treated surface. In SCGT, a very high strain rate of over 10 6 s −1 and a strain gradient of over 0.2 μm −1 were induced on the topmost surface of 300 M steel sample, which impels the martensite-matrix lamella extended into the elongated structure and finally transformed into the nanosized grains during processing. A dislocation slip-dominated process that was supplemented by the mechanical twinning governs the microstructural evolution pattern during the SCGT process, the nucleus of dynamic recrystallization initiated from the pre-existing dislocation cells were observed, and the transformation from martensite to austenite was effectively suppressed. The increase of grain size from nanoscale to submicron and finally microscale contributes to a decrease of microhardness by 25.6% (from 7.8 GPa at the treated surface to 5.8 GPa at a depth of over 100 μm from the processed surface). • Gradient microstructures are observed after being subjected to the SCGT process • Nanoparticle precipitates induced by the SCGT process appear within a few microns of the treated surface • A dislocation slip-dominated process governs the microstructural evolution pattern • The relationship between the microstructural evolution and microhardness is analyzed

Hierarchically activated deformation mechanisms to form ultra-fine grain microstructure in carbon containing FeMnCoCr twinning induced plasticity high entropy alloy

Nanotructural evolution and grain refinement leading to heterogeneous bimodal structure were investigated during thermomechanical processing in carbon-containing FeMnCoCr twinning induced plasticity (TWIP) high-entropy alloy (HEA). The homogenized single-phase face-centered cubic alloy with the stacking fault energy of 19.4 ± 2 mJ m−2 was cold rolled up to thickness reduction of 84 % and annealed at 850 °C. Planar slip with profuse nanoscale deformation twinning was the dominant deformation feature at low rolling reduction (32 %). The heterogeneous structure could be obtained through subdivision of microstructure, continuous dynamic recrystallization and static recrystallization described as follows: (i) Hierarchical mechanical twinning, (ii) Interaction of twin-matrix (T/M) lamellae with shear-bands and (iii) Continuous dynamic recrystallization (CDRX) within the strain-induced boundaries (SIBs), (iv) Subsequent static recrystallization of the region with twin-matrix (T/M) lamellae and shear-bands. The strength and ductility enhancement during deformation was attributed to the hierarchy of microstructural evolution consisting of TWIP and microband-induced plasticity (MBIP). The heterogeneous bimodal structure composed of ultra-fine grains and larger grains with an average grain size of 0.5 μm and 3 μm respectively was achieved by post-rolling (84 %) annealing at 850 °C. Favorable strength-ductility combination with the ultimate tensile strength of 840 MPa and elongation of ~88 % was achieved by formation of heterogeneous bimodal microstructure through thermomechanical processing.

Phase field study on the microscopic mechanism of grain size dependent cyclic degradation of super-elasticity and shape memory effect in nano-polycrystalline NiTi alloys

A non-isothermal and crystal plasticity based phase field model was established by incorporating various inelastic deformation mechanisms, i.e., martensite transformation, martensite reorientation, the austenitic plasticity caused by dislocation slipping, and the martensitic plasticity caused by dislocation slipping and deformation twinning, in which the deformation twinning mechanism was newly considered. The grain size (GS) effect of martensite transformation and reorientation was characterized by introducing a grain boundary energy, and the GS dependent resistances of dislocation slipping and deformation twinning were considered in the phase field simulation for the first time to describe the GS dependence of plastic deformation. Through a series of phase field simulations on the inelastic cyclic deformation of two-dimensional nano-polycrystalline NiTi shape memory alloy (SMA) systems, the microscopic mechanism of the GS dependent cyclic degradation of the super-elasticity (SE), one-way shape memory effect (OWSME) and stress-assisted two-way shape memory effect (SATWSME) of such alloy systems was newly clarified. The simulated results show that with decreasing the GS, the cyclic degradations of the SE and SATWSME are both mitigated gradually, while that of OWSME is aggravated first and then mitigated; when reducing the GS to 10 nm, almost no functional degradation occurs. Further analysis shows that the GS dependence of the cyclic degradations of SE and SATWSME can be attributed to that: as reducing the GS, the plastic deformation resistances gradually increase due to the Hall-Patch effect, and the GS effect of martensite transformation and reorientation reduces the deformation mismatch introduced in the polycrystalline system and leads to the decrease of local internal stress level (i.e., the driving force of plastic deformation decreases) (Factor I) during the cyclic deformation, therefore, the generation and accumulation of plastic deformation are inhibited. However, for the cyclic degradation of OWSME, besides Factor I, since the reorientation-induced plasticity is mainly concentrated on the grain boundaries and their nearby grain interiors, the increase of grain boundary proportion results in the concentration and accumulation of plastic deformation at more locations during the cyclic deformation (Factor II). The competition between Factors I and II makes the cyclic degradation of OWSME show a non-monotonic variation with the decrease of the GS .

The predicted rate-dependent deformation behaviour and multistage strain hardening in a model heterostructured body-centered cubic high entropy alloy

Heterostructured high-entropy alloys (HEAs) exhibit a good combination of strength and ductility. However, the deformation mechanisms and related mechanical properties of the heterogeneous nanostructured body-centred cubic (BCC) HEAs remain largely unrevealed. Here, we report the effect of the strain rate on the mechanical properties and microstructural evolution in the heterostructured Al3CoCrCuFeNi HEA with the gradient grain size ranging from 9 nm at the surface to 45 nm in the center using the large-scale atomic simulations. Based on the characterization of microstructure evolution from the atomic simulation, a microstructure-based constitutive model that utilizes a single parameter set is established to study the effects of the multiple strengthening mechanisms at various deformation stages on the strength and strain hardening of the heterostructured HEA under different strain rates. The results show that the plastic deformation mechanisms are strongly influenced by the strain rate in the range of 1 × 106 to 1 × 1010 s−1. The dislocation and deformation twin are the main deformation mechanisms at the strain rate less than 1 × 108 s−1. The deformation twinning cooperated with multiple phase transformations plays a pivotal role at the strain rate of 1 × 109 s−1. The multiple phase transformations are the dominant plastic deformation model at the strain rate of 1 × 1010 s−1, which is different from the classic plastic-deformation mode, including the dislocation and deformation twin in traditional alloys. This phenomenon is ascribed to various types of atomic interactions and atomic-size mismatches, which produce strong lattice distortion. In particular, a large number of edge dislocations are observed, which is extremely different from screw dislocations invariably generated in traditional BCC alloys, and thus, could be responsible for high strength. Furthermore, the stress-strain response of the Al3CoCrCuFeNi HEA is derived from the individual contributions of the lattice distortion, dislocation, grain boundary, deformation twin, phase transformation, and back-stress strengthening mechanisms. The heterogeneous grain boundaries and lattice distortion contribute the most to the yield strength, and the dislocation, deformation twin, phase transformation, and back stress dominate the strain hardening. Therefore, these results highlight a new design strategy of HEAs to tailor their mechanical properties based on the partition of the multistage strengthening mechanisms by the heterostructure.

Atomic-scale investigation on fretting wear mechanism of γ phase in a cast Ti-45Al alloy

The atomic-scale investigation on fretting wear mechanism of γ phase in a cast Ti-45Al alloy was conducted. The fretting wear of the alloy at 25 °C was a mixture of abrasive and adhesive wear. The results showed that serrated waves were observed in the force-time curves, which was associated with the local welding between the abrasive particles and substrates when fretting wear occurred. The main plastic deformation mechanism was mechanical twinning in the substrates. The introduction of order-twin (RT) interfaces restricted the downward movement of abrasive particles. The twinning across lamellae was dominant in the substrates with a RT lamella. Dislocation pile-ups in RT interfaces induced the formation of twin embryo. The successive dislocation dissociation 1/6[-12-1]→1/6[011]+1/6[-11-2] was the nucleation mechanism of twinning across lamellae.

Gradient nanotwinned CrCoNi medium-entropy alloy with strength-ductility synergy

In this study, a high-strain rate ultra-precision machining technology named single point cubic boron nitride turning is developed to fabricate a gradient nanotwinned CrCoNi medium entropy alloy layer. The grain size of the ~ 150 µm-thick gradient layer is gradually refined from the original ~ 17 µm to ~ 25 nm in the topmost surface, exhibiting a significantly enhanced yield strength (from ~ 450 MPa to ~ 1100 MPa) and well-retained ductility of ~ 27%. High-resolution transmission electron microscope and atomistic simulations were mainly performed to unveil the size-dependent twinning mechanisms governing the gradient refinement process from the core to the topmost surface, i.e. transiting from the parallel twins segmenting ultrafine grains, twin-twin intersections refining rhombic blocks and rotating the intersected nanograins, and finally to the zero-macrostrain deformation nanotwinning in the refined nanograins. The machining process provides sufficient equivalent stress to activate the twinning partials for forming the gradient nanotwinned structure.

Atomic-scale mechanism of rhombohedral twinning in sapphire

Deformation twinning is a fundamental plastic deformation mode in crystals. Upon twinning, individual atoms must move in different directions to satisfy the twin symmetry, which is called atomic shuffling. However, actual atomic motions during shuffling are still unknown, especially for ionic compounds. Here, we report the dynamic twinning behavior dominated by the atomic shuffling in sapphire (α-Al2O3). The propagation and annihilation of twins are revealed to be mediated by migration of step structures on the matrix/twin interfaces. The step migration is driven by cooperative motions of a group of five atoms with relatively few recombination of Al–O bonds. Our findings imply that the atomic shuffling associated with twinning is determined by a collective property of a group of several atoms.

Annealing twin mechanism for 304 stainless steel solution treatment process

Annealing twin mechanism for 304 stainless steel solution treatment process

A Method for Classification of Surface Defect on Metal Workpieces Based on Twin Attention Mechanism Generative Adversarial Network

Aiming at the insufficient sample size and unbalanced category of the metal workpiece surface defect dataset in industrial production. But traditional image augmentation methods based on generative adversarial network (GAN) do not effectively correlate the dependency between local feature and global feature, and the loss function is not rational, which causes the problem of insufficient image diversity and poor quality. A relative mean generative adversarial network (TARGAN), driven by a twin attention mechanism, is proposed to generate high-quality defect images. This method employs the twin-attention mechanism that integrates the spatial attention module and the channel attention module. Moreover, it has validated the ability of the twin attention mechanism to improve the refinement of defective features by ablation study. Meanwhile, the loss function during training is improved from the absolute relationship judgment to the relative mean value of the different relationships to improve the diversity of the generated images. Compared with current GANs, this model is superior in generating high-quality and diverse metal workpiece surface defect images thus effectively improving the performance of the MobileNetV2-based classifier. By extended MGSD and NEU-UB based on this method, the accuracy of the defect classification model reached 99% and 98.25% respectively, which is 9.5% and 13.25% higher than the model without TARGAN expansion.

Влияние насыщения водородом на структуру и механические свойства аустенитной стали 01Х17Н13М3, формируемые в процессе прокатки при разных температурах

Introduction. The development of hydrogen energy implies a decrease in the dependence of various human activities on fossil energy sources and a significant reduction in carbon dioxide emission into the atmosphere. Therefore, the requirements for the quality of structural materials, which have the prospect of being used for storage and transportation of hydrogen, as well as for the creation of infrastructure facilities for hydrogen energy, are increasing. Therefore, the scientific researches on the hydrogen-assisted microstructure and mechanical behavior of structural materials in various loading schemes are of great importance. The aim of this work is to establish the effect of chemical-deformation treatment, including rolling combined with hydrogen saturation, on the microstructure, phase composition, and mechanical properties of 316L-type austenitic stainless steel. Methods. Transmission electron microscopy and backscattered electron diffraction, X-ray diffraction, X-ray phase and magnetic phase analysis, microindentation and uniaxial static tension are utilized. Results and Discussion. It is shown experimentally that after rolling with 25 and 50 % upset, the morphology of the defect structure and the phase composition of 316L steel substantially depends on the deformation temperature (at room temperature or with the cooling of the samples in the liquid nitrogen) and on hydrogen saturation rate (for 5 hours at a current density of 200 mA/cm2). The main deformation mechanisms of the steel in rolling are slip, twinning, and microlocalization of plastic flow, which all provide the formation of ultrafine grain-subgrain structure in the samples. In addition, deformation-induced ε and α' martensitic phases are formed in the structure of the rolled samples. Regardless of the regime of chemical-deformation processing, grain-subgrain structures with a high density of deformation defects are formed in steel, but its morphologies are dependent on the processing regime. The experimental data indicate that both preliminary hydrogen saturation and a decrease in the deformation temperature contribute to the more active development of mechanical twinning and deformation-induced phase transformations during rolling. Despite the discovered effects on the influence of hydrogen saturation on the deformation mechanisms and the morphology of a defective microstructure formed during rolling, preliminary hydrogenation has little effect on the mechanical properties of steel at a fixed degree and temperature of deformation. These data indicate that irrespective of the morphology of the defective grain-subgrain structure, grain refinement, accumulation of deformation defects and an increase in internal stresses lead to an increase in the strength characteristics of the steel.

Manipulating deformation mechanisms with Y alloying of Mg

The effect of Y concentration on the slip and twinning mechanisms in binary Mg–Y alloys are investigated using transmission electron microscopy, electron backscattered diffraction, and visco-plastic self-consistent polycrystal constitutive modeling. Four concentrations of Y are studied in hot-rolled and recrystallized sheet material. The materials were deformed in tension and compression in the rolling direction and compression in the normal direction in order to invoke distinct proportions of slip and twin mechanisms with each test. Within the single crystal hardening model used in polycrystal modeling, a slip-twin interaction law is introduced to account for dislocation density reductions due to dislocation absorption during twin boundary migration. We show that increasing Y concentration reduces the intensities of both the initial and deformation textures. During deformation, the plastic anisotropy in yield stress, the tension-compression asymmetry, and amount of 101‾2⟨1‾011⟩ twinning is shown to decrease with increasing Y. For each alloy, the model identifies a single set of material parameters that successfully reproduced all measured stress-strain curves and achieved agreement with measured deformation textures and twin area fractions. Transcending texture effects, the model interpretation of the flow responses suggests that increased concentrations of Y increase the critical resolved shear stress for basal slip but have negligible effects on the other slip modes. The reduced plastic anisotropy with increases in Y is explained by a concomitant decrease in the prismatic-to-pyramidal slip critical resolved shear stress ratio. The model suggests that their nearly equivalent critical resolved shear stress values lead to the enhanced non-basal activity of Mg–Y alloys, which was confirmed by transmission electron microscopy. The calculations suggest that beyond any texture differences, this reduction in twinning can be attributed to a slightly increased resistance for 101‾2⟨1‾011⟩ twin propagation, particularly in the binary with the highest Y content.

High-temperature compressive behavior and kinetics analysis of Al0.4MnCrCoFeNi high entropy alloy

In the present study, AlxMnCrCoFeNi alloys (x = 0, 0.4) were produced by vacuum melting and casting. For this aim, ingots were subjected to the homogenization, cold roll, and annealing. X-ray diffraction analysis and SEM images indicated that the crystal structure is FCC solid solution for MnCrCoFeNi. Moreover, in addition to FCC solid solution, the alloy with aluminum (x = 0.4) ordered B2 precipitates. Besides, hot compressive tests were conducted at different temperatures and strain rates of 0.01, 0.05 and 0.1/s. Investigating the stress-strain diagrams and work hardening rate, it was revealed that twins played a role in the deformation mechanism. In the constitutive equation, the values of the stress exponent and activation energy were measured and they were 6.86 and 434 kJ mol−1, respectively. The exponential equations of peak stress and peak strain, as well as steady-state stress, were obtained due to the Zener-Hollomon parameter. Regarding the linear relationship between the work hardening rate and the dislocation annihilation coefficients, the activation energy of deformation changes at 600 °C. Finally, another effect of the Al addition was shown to be increase in initiation temperature of necklace structure about 200 °C in hot compression test. In addition, B2 nanoprecipitate and nanotwins indicated that there was twin deformation mechanism in alloy containing Al.

The dependences of deformation temperature on the strain-hardening characteristics and fracture behavior of Mn–N bearing lean duplex stainless steel

The tensile properties and fracture behavior of lean duplex stainless steel (LDSS) containing Mn–N was investigated through tensile deformation by varying the deformation temperature from 353 K to 213 K. The ultimate tensile strength (UTS) and yield strength (YS) of the test LDSS increased continuously as the temperature was lowered from 353 K to 213 K. The total elongation (TE) did not decline smoothly as the temperature decreased, instead, the TE peaked at the room temperature (293 K). This temperature-dependent mechanical response was mainly associated with the kinetics of strain-induced martensite transformation (SIMT) in the metastable austenite (γ). A quantitative kinetic model of temperature-dependent SIMT was established, which indicates a positive relationship that the lower the deformation temperature, the faster the SIMT rate. With respect to the nucleation of strain-induced martensite, three modes were clearly manifested by the nucleation mechanism during tensile deformation at different temperature ranges. The first case corresponded to the direct transformation of γ→α′ at 213 K. The second mode of γ→ε→α′ dominated the SIMT at the 213 K–293 K range. The α′-martensite nucleated at the mechanical twinning interactions at 293–333 K, i.e., γ→Twins→α’, which corresponded to the third mode. Notably, as the test temperature was lowered, a continuous fracture mechanism transition gradually emerges from fully ductile to brittle. Microstructural observations from in-situ tensile tests and EBSD showed that damages at the ferrite/martensite (α/α′) interface dominated at 293 K. At 213 K, besides the interfacial fractures that occurred at 293 K, cleavage-type fractures were also found inside ferrite and martensite. Void coalescence was mainly responsible for the ductile fractures over 353 K where SIMT was absent.

Tensile deformation behavior of nickel-free high-manganese austenitic cryogenic-temperature steel

Nickel-free high-manganese austenitic Fe–24.4Mn–4.04Al–0.057C steel was produced by smelting, and the homogenized forged billet was hot-rolled. The plastic deformation mechanism was investigated through tensile testing of the hot-rolled sample. Different characterization techniques such as scanning electron microscopy, transmission electron microscopy, electron backscattered diffraction, and X-ray diffraction were used to analyze the microstructural evolution of steel under different strain levels. The steel had a single austenite phase, which was stable during deformation. After hot rolling, annealing twins were observed in the microstructure of the steel. The steel showed an excellent combination of mechanical properties, like a tensile strength of 527 MPa, impact energy of 203 J at − 196 °C, and an elongation of 67% till fracture. At the initial deformation stage, the dislocations were generated within the austenite grains, entangled and accumulated at the grain boundaries and annealing twin boundaries. Annealing twins participated in plastic deformation and hindered the dislocation movement. As the deformation progressed, the dislocation slip was hindered and produced stress concentration, and the stacking faults evolved into mechanical twins, which released the stress concentration and delayed the necking.

Ceramic buckling for determining the residual stress in thin films

A new technique for determination of residual stress in thin films and coatings has been presented. The method consists of focused ion beam milling to create a lamella of thin film, followed by analysis of stress driven buckling profile of undercut lamella (beam) to extract residual stress. The residual stresses in crystalline TiN and Al2O3 films, produced by reactive magnetron sputtering and thermal oxidation, respectively, have been successfully determined by this new technique and validated by conventional X-ray diffraction and photoluminescence piezospectroscopy techniques, respectively. This new technique successfully measures and tracks the evolution of residual stress in as-deposited and different thermally cycled amorphous SiAlN films induced by thermal mismatch or relieved via mechanical twinning in interlayer, where diffraction methods are not applicable, thereby evaluating the thermal cycling performance of amorphous SiAlN coatings for protection of Ti at high temperature.