Strain-induced Transformation Research Articles

Correction: Influence of Plastic Anisotropy and Strain Path on strain-induced Phase Transformation of Cobalt

Correction: Influence of Plastic Anisotropy and Strain Path on strain-induced Phase Transformation of Cobalt

Controllable Synthesis of WSe2-WS2 Lateral Heterostructures via Atomic Substitution.

The atomic substitution in two-dimensional (2D) materials is propitious to achieving compositionally engineered semiconductor heterostructures. However, elucidating the mechanism and developing methods to synthesize 2D heterostructures with atomic-scale precision are crucial. Here, we demonstrate the synthesis of monolayer WSe2-WS2 heterostructures with a relatively sharp interface from monolayer WSe2 using a chalcogen atom-exchange synthesis route at high temperatures for short periods. The substitution was initiated at the edges of monolayer WSe2 and the lateral diffuse along the heterointerface, and the reaction can be controlled by the precise reaction time and temperature. The lateral heterostructure and substitution process are studied by Raman and photoluminescence (PL) spectroscopies, electron microscopy, and device characterization, revealing a possible mechanism of strain-induced transformation. Our findings demonstrate a highly controllable synthesis of 2D layered materials through atom substitutional chemistry and provide a simple route to control the atomic structure.

Effect of rare earth yttrium and the deformation process on the thermal deformation behavior and microstructure of pure titanium for cathode rolls

This study explores the challenges associated with microstructural refinement and homogenization control for large titanium cathode rolls utilized in electrolytic copper foil production. An innovative deformation control strategy was developed by incorporating yttrium microalloying within the β→α dual-phase region to refine the hot-deformed microstructure of pure titanium. Gleeble thermal simulation tests were performed to assess the impact of rare earth Y, deformation temperatures (700–900 °C), and interpass cooling rates (5 °C/s, 25 °C/s, and 50 °C/s) on the hot deformation behavior and microstructural evolution of pure titanium. The results reveal that Y2O3 particles generated by the addition of rare earth Y enhance dynamic recrystallization through the particle-stimulated nucleation (PSN) mechanism, effectively pinning grain boundaries and impeding recrystallized grain growth. Increased interpass cooling rates lead to finer grains and higher stored energy. The mechanism for grain refinement during dual-phase deformation is described as follows: β-phase nonrecrystallization temperature deformation → rapid cooling control → α-phase recrystallization deformation control, energy storage, strain-induced dynamic phase transformation (SIDT, α→β), and dynamic recrystallization (DRX). Under experimental conditions of β-phase region deformation at 980 °C → inter-pass cooling rate of 50 °C/s → α-phase region deformation at 750 °C, the addition of rare earth yttrium achieved the most effective grain refinement, reducing the average grain size to 2.56 μm.

Interaction between inclusions and the TRIP effect during tensile loading of an L-PBF 18Ni-300 maraging steel and effect on ductility

The influence of non-metallic inclusions on the tensile ductility of a direct aged 18Ni300 maraging steel produced by L-BBF was investigated on two materials having the same microstructure and a different content of non-metallic inclusions. The strain-induced transformation of austenite promotes the TRIP effect enhancing ductility. In the presence of a high amount of inclusions, the minimum strain needed to trigger strain-induced transformation is reached after necking, and the transformation cannot contribute to ductility. The effect of inclusions is to promote stress rising and strain accumulation, but mostly to make TRIP uneffective. To recover the effectiveness of TRIP and, in turn, increase ductility, the stability of reverted austenite should be reduced, in order to anticipate its transformation in martensite that is able to extend the uniform deformation step.

Study on mechanical behaviour and microstructure evolution of wire-arc directed energy deposition Co-free maraging steel under dynamic compression

ABSTRACT The dynamic compression property of maraging steel is an important basis for determining the impact resistance of the structure, and the related studies are few. To enrich the relevant data and elucidate the relevant mechanisms, in this work, a new Co-free maraging steel component is fabricated by wire-arc direct energy deposition (DED), and the quasi-static tensile properties and dynamic compression properties are tested. The results show that the deposition direction exhibits the highest quasi-static tensile properties. Strain rate strengthening effect, grain refinement and grain boundary strengthening are the main strengthening mechanisms during dynamic compression. With the increase of strain rate, dynamic recrystallization intensifies, causing the grain size and dislocation density to decrease. Under dynamic condition, strain-induced martensite phase transformation reduces the austenite content; temperature rise and dynamic recrystallization limit the degree of martensite phase transformation. Strain rate sensitivity of the strength is independent of the direction.

Modeling the effects of initial grain size, martensitic transformation induced dynamic grain refinement, phases, and texture on strength of a high entropy alloy using crystal plasticity

Microstructurally flexible high entropy alloys (HEAs) can be tailored for strength via dual-phase strengthening mechanisms, which originate from the strain-induced phase transformation microstructural phenomena. One such alloy is a recently synthesized Fe42Mn28Co10Cr15Si5 (at%) HEA consisting of metastable gamma austenite (γ), stable epsilon martensite (ε), and stable sigma (σ) phases. In this work, a crystal plasticity homogenization incorporating a physically based phase transformation and hardening models is used to interpret and better understand the effects of strain induced phase transformation phenomena on the overall hardening response of the alloy. The transformation model is conceived based on the grain-scale stress sensitive motion of partial dislocations forming shear bands, while the hardening model is an extended Kocks-Mecking-type dislocation density-based formulation sensitive to grain size and shape. The deformation of constituent grains per phase in the alloy is modeled as a combination of anisotropic elasticity, crystallographic glide, and γ→ε phase transformation. Parameters of the hardening and transformation models within the homogenization are calibrated and validated on a suite of data including flow stress curves and phase fractions measured under compression, while the corresponding texture evolution data is used solely for verification. Good predictions of the model elucidate that the transformation induced dynamic Hall-Petch-type barrier effect is the primary origin of strain hardening along with the increase in the ε-phase fraction and dislocation density, while the individual strength of the phases gives rise to the overall strength. The modeling framework described in the present work is expected to be applicable to other HEA systems.

Multimodal assessment of mechanically induced transformation and damage in TRIP steels using X-ray nanotomography and pencil-beam diffraction tomography

A comprehensive multimodal measurement environment integrating X-ray computed tomography (CT) and special X-ray diffraction (XRD) using a collimated beam has been developed and was applied for the first time to investigate martensitic transformation and damage behaviour in transformation-induced plasticity (TRIP) steels. The transformation and damage behaviour of numerous γ grains were analysed individually to understand the behaviours and to identify the factors controlling these processes.A transition was observed from a stress-assisted transformation in the early loading phase, controlled by mechanical driving forces, to a strain-induced transformation controlled by dislocation density. A characteristic reversal in the transformation rate was observed whereby γ grains with a crystallographic orientation that initially undergo rapid transformation due to high mechanical driving force underwent a sudden deceleration of transformation. Conversely, γ grains with an orientation that initially underwent gradual transformation rapidly completed transformation. The preceding hard phase inhibited the transformation of neighbouring γ grains through the triple effects of reducing stress triaxiality, relaxing stresses and constraining plastic deformation.The growth of pre-existing voids was modest, and material damage was dominated by voids that were newly formed during deformation from α′ grains. The initiation and progress of damage were influenced by the combined effects of the γ grain morphology and their interaction with surrounding α′ grains. The complexity of the local morphology governed the martensitic transformation and damage behaviour, and geometrical parameters were identified as highly sensitive indicators for describing this phenomenon. Finally, industrial measures to design damage-resistant TRIP steels based on these research findings are discussed.

Unusual plastic strain-induced phase transformation phenomena in silicon

Pressure-induced phase transformations (PTs) in Si, the most important electronic material, have been broadly studied, whereas strain-induced PTs have never been studied in situ. Here, we reveal in situ various important plastic strain-induced PT phenomena. A correlation between the direct and inverse Hall-Petch effect of particle size on yield strength and pressure for strain-induced PT is predicted theoretically and confirmed experimentally for Si-I→Si-II PT. For 100 nm particles, the strain-induced PT Si-I→Si-II initiates at 0.3 GPa under both compression and shear while it starts at 16.2 GPa under hydrostatic conditions. The Si-I→Si-III PT starts at 0.6 GPa but does not occur under hydrostatic pressure. Pressure in small Si-II and Si-III regions of micron and 100 nm particles is ∼5–7 GPa higher than in Si-I. For 100 nm Si, a sequence of Si-I → I + II → I + II + III PT is observed, and the coexistence of four phases, Si-I, II, III, and XI, is found under torsion. Retaining Si-II and single-phase Si-III at ambient pressure and obtaining reverse Si-II→Si-I PT demonstrates the possibilities of manipulating different synthetic paths. The obtained results corroborate the elaborated dislocation pileup-based mechanism and have numerous applications for developing economic defect-induced synthesis of nanostructured materials, surface treatment (polishing, turning, etc.), and friction.

Revaling the mechanism of microscopic kinking phenomena in Hf6Ta2O17 superstructures

As a promising candidate for a new-generation thermal barrier coating material, Hf6Ta2O17 is characterized by its unique long-period superstructures and excellent mechanical properties. In this study, the unique "kinking" phenomenon of the atomic arrangement in Hf6Ta2O17 crystals is directly visualized via high-resolution transmission electron microscopy for the first time. The evolution of the microstructure, and particularly the micro-mechanism of the "kinking" phenomenon, have been further explored by first-principles calculations. The study suggests that this phenomenon is primarily triggered by the rotation of the polyhedral structure, resulting from the partial elongation or shortening of the Ta-O bond length. These structural alterations are believed to directly influence the macroscopic mechanical properties and can improve the tensile properties of the Hf6Ta2O17 crystals. The findings of this study provide an important microscopic perspective for understanding the strain-induced crystal structure transformation. The quantitative analysis of the strengths of Ta-O and Hf-O bonds in different coordination polyhedra reveals the electronic structure changes behind the crystal structure transition, thus providing a new theoretical basis for an in-depth understanding of the behavior of crystals under the external stress.

Effects of Mean Normal Stress and Microstructural Properties on Deformation Properties of Ultrahigh-Strength TRIP-Aided Steels with Bainitic Ferrite and/or Martensite Matrix Structure.

The effects of mean normal stress on the deformation properties such as the strain-hardening, strain-induced martensite transformation, and micro-void initiation behaviors of low-carbon ultrahigh-strength TRIP-aided bainitic ferrite (TBF), bainitic ferrite/martensite (TBM), and martensite (TM) steels were investigated to evaluate the various cold formabilities. In addition, the deformation properties were related to the microstructural properties such as the matrix structure, retained austenite characteristics, and second-phase properties. Positive mean normal stress considerably promoted strain-induced martensite transformation and micro-void initiation, with an increased strain-hardening rate in an early strain range in all steels. In TM steel, the primary martensite matrix structure suppressed the micro-void initiation through high uniformity of a primary martensite matrix structure and a low strength ratio, although the strain-induced transformation was promoted, and a large amount of martensite/austenite constituent or phase was contained. A mixed matrix structure of bainitic ferrite/primary martensite in TBM steel also suppressed the micro-void initiation because of the refined microstructure and relatively stable retained austenite. Promoted micro-void initiation of TBF steel was mainly promoted by a high strength ratio.

Strain-induced giant enhancement and transformation of magnetocrystalline anisotropy in 1T-FeSe2 monolayer

The development of magnetic materials with large magnetic anisotropy energy (MAE) is crucial for magnetic storage and spintronics applications. The electronic structure and magnetic properties of 1T-FeSe2 monolayer have been systematically investigated by first-principles computational methods. The monolayer 1T-FeSe2 is confirmed to be a ferromagnetic semimetal with 100% spin polarization and exhibits pronounced magnetic anisotropy. It shows an easy magnetization plane in the two-dimensional plane and its MAE is as high as 8.134 meV. The MAE mainly originates from the contributions of transitions between Fe dyz and dz2, dxy and dx2−y2 orbitals, while the contributions of transitions between p x and p y, p x and p z orbitals of Se are also obvious. The biaxial strain significantly regulates the magnetic properties and electronic structures of the 1T-FeSe2 monolayer. Interestingly, the MAE increases significantly to 77.680 meV when the tensile strain increases to 3.5%, we can expect to achieve a reasonable value of MAE up to 98.951 meV by applying 5% tensile strain. In addition, the easy magnetization plane changes to perpendicular easy magnetization axis direction at −6% compressive strain or 7% tensile strain, which leads to a significant change in the band structure near the Fermi level. The 100% spin-polarized ferromagnetic semi-metallic behavior and the huge strain-induced magnetic anisotropy make the 1T-FeSe2 monolayer material promising for spintronics and magnetic data storage.

Influence of strain rate on the work hardening, strain induced martensite formation, strain partitioning, and variant selection in a medium-Mn steel

In this study, we investigated the effect of strain rate on the work hardening, strain-induced martensite transformation, strain partitioning, and crystallographic variant selection in a medium-Mn steel. Uniaxial compression tests were performed under quasi-static and dynamic loading conditions at the strain rates of 10−3, 1 and 103s−1. Besides, to determine the contribution of temperature rise due to adiabatic heating at a higher strain rate, compression tests were carried out at 409 K (estimated temperature rise) under quasi-static conditions. The work hardening rate was higher at high strain rates and it rapidly decreased at all strain rates up to a true plastic strain of ∼0.05. Beyond the true plastic strain of ∼0.05, the work hardening rate increased at a low strain rate up to a true plastic strain of ∼0.15 while at a high strain rate, it remained relatively constant and gradually decreased beyond a true plastic strain of ∼0.15. From the analysis of microstructures obtained from electron backscatter diffraction, it was observed that the strain-induced transformation of retained austenite to α′-martensite is significantly impeded at higher strain rates and in the sample tested at 409 K at low strain rates. The higher stability of retained austenite is attributed to the adiabatic heating which decreases the chemical driving force for retained austenite to martensite phase transformation and increases the stacking fault energy. Analysis of the misorientation axis and angle evolution revealed that while the Kurdjumov–Sachs relationship was followed under quasi-static conditions, it deviated at higher strain rates. Analysis of crystallographic variants related to strain-induced martensitic transformation at a true plastic strain of ∼0.15 revealed that the samples deformed at a lower strain rate did not show any noticeable variant selection. However, in samples deformed at a higher strain rate, a significant variant selection was observed. It is reasoned that the martensitic transformation nucleates initially on favorably oriented habit planes of retained austenite grains at all strain rates and other variants get activated with an increase in strain. As the transformation is impeded beyond a critical strain at a higher strain rate, the variants activated at lower strains remains predominant.

Strain-Assisted Phase Transformation in Two-Dimensional Transition-Metal Dichalcogenides.

Two-dimensional polymorphic transition-metal dichalcogenides have drawn attention for their diverse applications. This work explores the complex interplay between strain-induced phase transformation and crack growth behavior in annealed nanocrystalline MoS2. Employing molecular dynamics (MD) simulations, this research focuses on the effect of grain size, misorientation, and annealing on phase evolution and their effects on the mechanical behavior of MoS2. First, examining phase transformation in monocrystalline MoS2 under various stress states reveals distinct behaviors depending on the initial phase (1T or 2H) and crystallographic orientation with respect to loading directions. Notably, transformation from a layered hexagonal to a body-centered tetragonal structure is more noticeable when strain in a zigzag direction is applied to the 1T sample. As such, single crystalline MoS2 with a 1T phase exhibits a 16% lower fracture stress in the armchair direction compared to that with a 2H phase. On the other hand, the 1T phase shows a 5% higher phonon lifetime compared to the 2H phase with similar phonon group velocities. Next, the influence of thermal energy and mechanical stress on the phase transformation of nanocrystalline MoS2 is investigated through annealing and quenching cycles, uncovering 60 and 44% irreversibility of phase transformation for an average grain size of 3 and 11 nm, respectively. Besides, the evolution of nanocrystalline samples with different initial phases and grain sizes is studied under uniaxial and biaxial stress. This study shows an inverse pseudo-Hall-Petch effect with exponents of 0.11 and 0.09 for 2H and 1T, respectively. The study reveals that phase transformation can occur concurrently with crack initiation and propagation with the 1T phase exhibiting a 19% lower grain size sensitivity of fracture stress compared to the 2H phase.

Effect of Milling Time on the Structure Stability of FeMnNiCrAl Non-equiatomic High-Entropy Alloy

A non-equiatomic Fe34.9Ni30.2Mn18.6Cr9.3Al7 high-entropy alloy (HEA) was synthesized by mechanical alloying using different milling times. To study the effect of milling time on the structure stability, X-ray diffraction (XRD), scanning electron microscopy and energy-dispersive X-ray spectroscopy were conducted. For comparison, an Al-free alloy (Fe37.5Ni32.5Mn20Cr10) was produced at 25 h milling. XRD indicated a single FCC phase alloy after 5 h milling time and a dual FCC and BCC phase at 25 h milling time. Clearly, it is found that Al addition is not necessarily the main factor leading to BCC phase formation as reported for similar HEAs produced by casting route. Increasing the milling time, the lattice strain increased reaching an average maximum value of 0.8% with an increase in d-spacing while the crystallite size was reduced to about 5.7 nm. A dual-phase structure formation was related to a decrease in the accumulated strain (ca.32%) confirming a strain-induced transformation.

Uncovering the generic and alloy-specific governing parameters of deformation-induced martensitic transformation in austenitic steel

In this work, a hybrid modeling approach, combining machine learning (ML) and computational thermodynamics, has been applied to predict deformation-induced martensitic transformation (DIMT) and explore the generic and alloy-specific parameters governing DIMT in austenitic steels. The DIMT model was established based on the ensemble ML algorithms and a comprehensive set of physical variables. The developed model is highly generalizable as validated on unseen alloys. The generic governing parameters of DIMT are in good agreement with previous studies in the literature. However, the evaluated alloy-specific governing parameters reveal large differences between grades, e.g., 204 series of austenitic stainless steels has a quite balanced correlation between strain, stress, temperature, and DIMT, while the 301 series has much stronger correlation between stress and DIMT. The findings in the current study emphasize the importance that a general DIMT model for steels should include both stress and strain, as well as other governing parameters, since DIMT can be both stress-assisted and strain-induced transformation, and often the effect of applied mechanical driving force and the formation of new nucleation sites interact.Graphical abstract

Hexagonal-Ge Nanostructures with Direct-Bandgap Emissions in a Si-Based Light-Emitting Metasurface.

Si-based emitters have been of great interest as an ideal light source for monolithic optical-electronic integrated circuits (MOEICs) on Si substrates. However, the general Si-based material is a diamond structure of cubic lattice with an indirect band gap, which cannot emit light efficiently. Here, hexagonal-Ge (H-Ge) nanostructures within a light-emitting metasurface consisting of a cubic-SiGe nanodisk array are reported. The H-Ge nanostructure is naturally formed within the cubic-Ge epitaxially grown on Si (001) substrates due to the strain-induced nanoscale crystal structure transformation assisted by far-from-equilibrium growth conditions. The direct-bandgap features of H-Ge nanostructures are observed and discussed, including a rather strong and linearly power-dependent photoluminescence (PL) peak around 1562 nm at room temperature and temperature-insensitive PL spectrum near room temperature. Given the direct-bandgap nature, the heterostructure of H-Ge/C-Ge, and the compatibility with the sophisticated Si technology, the H-Ge nanostructure has great potential for innovative light sources and other functional devices, particularly in Si-based MOEICs.

Microstructure evolution and mechanical properties of Al/Mg–Li/Al laminate metallic composites via hot roll bonding and subsequent cryorolling

Hot roll (HR) bonded Al/Mg–Li laminated metallic composites (LMCs) were subsequent processed using room-temperature rolling (RTR) and cryorolling (CR), respectively. Afterward, the mechanical properties and the microstructure evolution of these LMCs under different rolling temperatures and reductions were studied. The results revealed that the Al/Mg–Li LMCs in the HR + CR1 group (a total reduction of 67 %) exhibited the best comprehensive mechanical properties, with the ultimate tensile strength and elongation ratio of 176.8 MPa and 10 %, respectively. The SEM and EBSD results revealed that the intermediate Mg–Li layer underwent an intense strain-induced phase transformation from the β-Li phase to the α-Mg phase during the RTR, while the strain-induced phase transformation during CR was inhibited due to the gradual atomic diffusion and insufficient driving energy. Therefore, the fraction of the β-Li phases in the Mg–Li layer was higher in HR + CR LMCs compared to HR + RTR LMCs, with the α-Mg phases in the former being evenly distributed in its β-Li matrix. Both β-Li and α-Mg phases in the Mg–Li layer held the stress and strain to ensure the ductility of the Mg–Li layer, which could consequently co-deform with the Al layers, thereby increasing the tensile strength and the elongation simultaneously in HR + CR LMCs. The XRD results revealed that during CR, multiple prismatic <a> slip systems and pyramidal <c+a> slip systems were activated in α-Mg phases, and the slip systems {110}<111>, {123}<111>, and {112}<111> were activated in β-Li phases simultaneously. The activation of a greater number of slip systems in both β-Li and α-Mg phases improved the overall ductility of the LMCs.

Magnetic non-destructive monitoring of a ship's propeller blade after long-term operation

This study examines the potential of magnetic non-destructive techniques for monitoring surface damage to ship propeller blades made of stainless steel. The propeller considered in this investigation operated for 35 years in the Danube river. Sheet and vortex cavitations, as well as random mechanical impacts on the trailing and leading edges, were detected as the major mechanisms of damage to the blade. Non-destructive monitoring of this damage was based on the Barkhausen noise and Helmholtz feritscope techniques. The blades investigated here had a composite structure in which the ferrite stainless body was mixed with the deposition of austenite on the trailing and especially the leading edges. Damage to the ferritic phase was mostly associated with sheet cavitation and a decreasing magnitude of Barkhausen noise as a result of repetitive plastic deformation. Conversely, vortex cavitation and random impacts to the edges in the austenite regions were associated with increasing Barkhausen noise due to strain-induced martensite transformation, a result that was also confirmed by data obtained from a Helmholtz feritscope. Information about plastic deformation and the fraction of strain-induced transformation was also confirmed by X-ray and electron backscatter diffraction techniques.

Effects of mechanical deformation on dislocation density and phase partitioning in 4130 steel

Interrupted tensile tests were performed on an AISI 4130 pressure vessel steel and investigated by neutron diffraction and scanning microscopy techniques. Analysis of the neutron diffraction patterns reveal a partitioning of ferrite and martensite phases resulting from deformation. A modified Williamson-Hall approach was used to model the broadening of Bragg peaks associated with the two phases as a function of applied strain, revealing an order of magnitude increase in their dislocation densities when the material was strained beyond the ultimate tensile strength (UTS). Lattice strains measured in the ferrite phase were consistently larger than those measured in the martensite phase for all the applied strain levels investigated. Moreover, a strain-induced phase transformation from a predominately martensitic steel to a ferritic steel was observed, with the average martensite phase fraction of an as-received specimen going from 78% to 22% when pulled to failure. Electron Backscatter Diffraction (EBSD) and Scanning Kelvin Probe Force Microscopy (SKPFM) were used to characterize the microstructure and phase fractions of ferrite and martensite associated with the various strain levels. These results agree well with those obtained from neutron diffraction and demonstrate the utility of SKPFM to distinguish between metallic phases with similar crystal structures that may be difficult to detect using conventional methods such as EBSD.

Hydrogen-induced phase with martensitic-like characteristics in Ti-Ni shape memory alloys

It has been reported that functional and mechanical properties of Ti-Ni shape memory alloys could be degraded by hydrogen absorption. Many studies have indicated the formation of a hardened layer on the surface in hydrogen-absorbed Ti-Ni alloys. However, the underlying microstructural features and their impact on the functional and mechanical properties remain unclear. In the present study, ex situ and in situ electron microscopy characterizations of the high-hardness layer in a hydrogen-charged Ti-Ni alloy were performed for the first time. Electron diffraction and atomic resolution imaging revealed that the hardened layers have an orthorhombic structure with a four-layer stacking sequence and a martensitic-like crystallographic nature such as lattice correspondence including 12 variants of the orthorhombic phase against each orientation of the parent cubic (B2) phase. In situ and ex situ observations in an electron microscope confirmed that the orthorhombic phase undergoes athermal transformation upon cooling and stress- and/or strain-induced transformation upon deformation, which further characterizes the martensitic-like nature of this phase. The effect of hydrogen on the original phase transformation from the cubic (B2) to the monoclinic (B19′) structure is also discussed.