Transition In Deformation Mechanism Research Articles

Toughness behavior and deformation mechanisms in FCC-based Fe45Co30Cr10V10Ni5-xMnx high-entropy alloys: Insights from instrumented Charpy impact tests

This study investigated toughness properties of FCC-based Fe45Co30Cr10V10Ni5-xMnx high entropy alloys (HEAs). Ductile-dimpled rupture mode in all alloys and test temperatures complicated precise explanations on Charpy impact energy, but the instrumented Charpy test provided a breakdown of total energy (ET) into initiation energy (EI) and propagation energy (EP), offering critical insights into Mn-content- and temperature-related energy variations. Pmax and TP, reflecting maximum flow stress and time, respectively, until initiation of crack propagation from the notch tip, involved a transition of deformation mechanisms from slip to TWIP, then to BCC-TRIP, thereby inducing increased strain hardening and delaying plastic instability at the notch tip. At 25 °C, an increase in Mn content prompted a transition in deformation behavior from slip to TWIP and subsequently to BCC-TRIP, correlating well with increased Pmax and TP and consequently ET due to enhanced strain hardenability. At −196 °C, predominant BCC-TRIP activity in all alloys led to increased formation of BCC martensite, intensifying strain hardening and requiring higher Pmax for crack initiation than at 25 °C. Due to minimal slip line field formation associated with reduced plastic deformation, however, cracks initiated directly from the notch-tip center, representing fast initiation of crack propagation and consequently reduction in EI and ET. Thus, utilizing parameters from instrumented Charpy tests, including Pmax, TP, EI, EP, and ET, provided insights into fracture phenomena and their interrelations in the present HEAs.

Interface Amorphization Controls Maximum Wear Resistance of Multinanolayer DLC/WC Coatings.

Multilayer coatings offer significant advantages in protecting materials' surfaces by shielding the underlying materials hierarchically from damage and wear. The layering morphology and structure of multilayer coatings directly affect their wear resistance capacity. Using a systematic set of experiments and molecular dynamics (MD) simulations, we studied the effect of layering thickness on the macroscale wear response of DLC/WC multinanolayer coatings. Our study revealed the existence of a critical bilayer thickness where maximum scratch hardness and wear resistance can be achieved. Our large-scale MD simulations showed that reducing the WC layer thickness to a certain limit increases the scratch hardness due to the confinement of dislocation motion. However, when the thickness of the WC layers falls below 2 nm, the deformation mechanism transitions from the interface-induced dislocation confinement to the interface-mediated amorphization of WC layers, reducing the scratch hardness of the coating. This finding offers a procedure for optimizing the macroscale wear performance of multinanolayer coatings.

Dynamic Compression and Constitutive Model in Fe-27Mn-10Al-1C Duplex Lightweight Steel

Fe-Mn-Al-C lightweight steels have been of significant interest due to their excellent mechanical properties and unique microstructures. However, there has been limited focus on the dynamic deformation. Here, we systematically investigate the mechanical responses over various strain rates and corresponding microstructure evolution in quasi-static and dynamic compression to reveal the transition of deformation mechanisms. The present lightweight steel exhibits a significant strain rate effect, with the yield strength increasing from 735.8 to 1149.5 MPa when the strain rate increases from 10−3 to 3144 s−1. The deformation in ferrite under high-strain-rate loading is dominated by wave slip, forming a cellular structure (cell block). Meanwhile, the deformation in austenite is dominated by planar slip, forming dislocation substructures such as high-density dislocation walls and microbands. In addition, the deformation twinning (including secondary twinning)- and microband-induced plasticity effects are responsible for the excellent dynamic compression properties. This alloy delays damage location while maintaining high strength, making it ideal for shock loading and high-strain-rate applications. The Johnson–Cook (J–C) constitutive model is used to predict the deformation behavior of lightweight steel under dynamic conditions, and the J–C model agrees well with the experimental results.

In-situ determination of strain during transient burst testing and the temperature dependence of Zircaloy-4 claddings

Understanding fuel system behavior during postulated loss-of-coolant accidents is pertinent for continued safe and efficient operation of light water reactors, particularly as higher burnups are being pursued and safety margins re-evaluated. Conventional mechanical models for the incumbent Zr alloys typically rely on the assumption that steady-state creep is the dominant fuel cladding response during transient accident conditions. To investigate this assumption, simulated accident burst testing was performed on Zircaloy-4 claddings with balloon behavior measured in-situ. Two distinct loading conditions were utilized during burst testing: (1) constant-gas-inventory where pressure was allowed to increase with temperature and (2) constant pressure. In-situ strains and strain rates were measured via 2-dimensional digital image correlation techniques and synchronized with temperature to determine deformation dependencies. The temperature dependence of strain rate was characterized by a two segment Arrhenius relationship, with a distinct transition between the high and low temperature/strain regimes. The average activation energy of the lower temperature/strain regime was 328 ± 25 kJ/mol, in agreement with the ∼320 kJ/mol used for conventional LOCA models. However, the higher temperature/strain segment, which encompassed most of ballooning, showed increased activation energies as well as a dependence on whether the burst region was in view. For tests that burst away from the camera view, the average high temperature/strain segment activation energy was 635 ± 150 kJ/mol. For samples where the rupture opening formed in view, the average activation energy was 1015 ± 179 kJ/mol. This observed shift in temperature dependence indicates a transition in deformation mechanism at the end of life, possibly to time independent failure mechanisms, which has not yet been visualized in the literature for Zr alloys. Parameters at the transition points were analyzed to determine thresholds for this change in behavior, which occurred at an average hoop strain of 6.9 ± 2.1 %.

Pressure-dependent deformation in brittle diamond

Distinguished from ductile metals, in brittle covalent materials like diamond, hydrostatic pressure is a prerequisite for triggering the plastic deformation. Consequently, the theoretical framework rooted in equilibrium lattice proves inadequate in describing the behavior of brittle covalent materials, making it essential to include extrinsic hydrostatic pressure effects. Here, we take diamond as a model and introduce hydrostatic pressure in the calculation of generalized stacking fault energies (GSFE). A deformation mechanism transition from perfect dislocation slipping on {200} plane to the partial dislocation slipping on {111} plane when the hydrostatic pressure exceeds 100 GPa is revealed by the pressure-coupled GSFE and further authenticated by nanocompression and diamond anvil cell experiments. Such pressure-dependent deformation mechanism is attributed to the different pressure sensitivities of lattice volume expansion during different slip systems operating. Our works provide insights into the activation of slip systems in diamond at the atomic scale and a route to study the plastic deformation behavior of brittle covalent materials.

Microstructure evolutions and deformation mechanisms of a Ni-based GH3536 Superalloy during loading at 293 K and 77 K

In this study, tensile tests and interrupted experiments at different true strain levels were performed at 293 K and 77 K to reveal the deformation mechanisms of GH3536 superalloy during loading. The microstructure evolutions of the alloy with strain at both temperature were studied via Transmission electron microscopy (TEM), and dislocation densities were calculated by X-ray diffraction (XRD) for quantifying the forest hardening contribution to the flow stress. Deformation twins were rarely observed in the GH3536 superalloy specimens deformed at 293 K, where deformation occurred solely by dislocation slip. While twinning was initially found at a strain of ∼7% at 77 K, and the corresponding stress at which twinning occurs is 808 ± 46 MPa. The twin volume fractions, their widths and spacings were determined by electron backscatter diffraction (EBSD), which is used to investigate the twin evolution of GH3536 alloy at 77 K. The deformation mechanisms of GH3536 alloy, as well as its twinning behaviors, depend on the competition between the maximum flow stress and critical stress for twinning. The maximum flow stress is large enough to activate twinning at cryogenic temperature, resulting in the transition of deformation mechanisms for GH3536 alloy as SLIP (dislocation slip) at 293 K and TWIP+SLIP (deformation twinning and dislocation slip) at 77 K, and the improved combination of ductility and strength at 77 K compared to 293 K is derived from additional deformation mode provided by twinning during the process of deformation.

Temperature dependence of low cycle fatigue for the Co-based single crystal superalloy

The low cycle fatigue behaviors of the Co-Al-W-Ti-Ta single crystal superalloy were systematically investigated from room temperature to 900 °C. With the increase of temperature, the main slip mode gradually changes from planar slip of stacking faults in both γ and γ′ to wavy slip of dislocations in γ, which has rarely been reported in previous work. The deformation mechanism transition can not only be attributed to the disappearance of stacking faults in γ but also the decrease of critical stress for stacking faults to shear γ′ phase, which induced the density and type of stacking faults to change. In consequence, the deformation homogeneity in γ/γ′ decreased with the increase of temperature and more strain was localized in γ. The finding provides us a new idea on high performance single-crystal superalloy design.

Dynamic deformation behaviors and mechanisms of CoCrFeNi high-entropy alloys

Due to their unique microstructures and chemical compositions, high-entropy alloys (HEAs) exhibit remarkable mechanical properties, such as high strength, excellent ductility and good thermal stability. However, to date, there have been limited studies on the dynamic behaviors of HEAs. Here, we systemically investigated the mechanical behaviors and deformation mechanisms of CoCrFeNi HEAs and their connections under dynamic loading by combining mechanical tests and molecular dynamics simulations. Compressive testing on polycrystalline CoCrFeNi HEA samples at different strain rates showed that both yield and flow stresses increase with strain rate and exhibit a significant strain rate sensitivity due to solid solution strengthening and lattice distortion of the HEA. The strain rate sensitivity of the CoCrFeNi HEA transitions from 0.010 at low strain rates (5.0×10−5–2.5×103 s−1) to 0.333 at high strain rates (2.5×103–6.5×103 s−1). Large-scale molecular dynamics simulations further revealed that this transition is related to the plastic deformation mechanism transition from dislocation nucleation and slip at low strain rates to massive dislocation nucleation and drag at high strain rates. Furthermore, the CoCrFeNi HEA exhibited a remarkable strain-hardening capability at high strain rates, which originated from the formation of primary and secondary nanoscale twins and twin-twin and twin-dislocation interactions. Our current study sheds light on the plastic deformation mechanisms of HEAs under dynamic loading, providing guidance for the design and fabrication of HEAs with excellent dynamic mechanical properties.

Tuning mechanical behavior and deformation mechanisms in high-manganese steels via carbon content modification

In this paper, two high-manganese steels with comparable grain sizes and manganese contents, namely 34Mn0.1C and 32Mn0.6C, were investigated to explore the effect of carbon content on the mechanical behavior and deformation mechanisms at both room temperature (RT) and liquid nitrogen temperature (LNT). The results indicate that increasing the carbon content promotes the transition of deformation mechanism from dislocation slip to deformation twinning, which is similar to the effect of reducing the deformation temperature. At RT, the strength and elongation are effectively improved by increasing the carbon content, which is attributed to the transformation of deformation mode. Specifically, the yield strength, the ultimate tensile strength and the total elongation increase from 262 MPa, 595 MPa and 50.4% for the 34Mn0.1C steel to 360 MPa, 846 MPa and 87.4% for the 32Mn0.6C steel, respectively. However, at LNT, the 32Mn0.6C steel exhibites higher strength but lower elongation than the 34Mn0.1C steel. The total elongation decreases from 70.6% to 52.3%, which is because the high carbon content and cryogenic temperature induce the rapid activation of deformation twins, leading to premature fracture.

Secondary orientation effects on the low cycle fatigue behaviors of rectangular‐sectional Ni‐based single crystal superalloys at medium and high temperatures

AbstractTurbine blades made of Ni‐based single crystal superalloys (NBSXs) have long‐strip shaped cross sections and rectangular‐sectional structures, where the secondary orientation produces potential effects even the primary orientation is fixed at [001]. Low cycle fatigue behaviors between [010] and [110] transversely oriented rectangular‐sectional NBSX specimens were compared. Obvious differences existed under 600°C but disappeared under 850°C, with the deformation mechanism and fracture mode transitions. Secondary orientation effects on stress asymmetry and fatigue life cannot be described by the conventional LCP model and critical plane method but were well explained by dislocation path length‐dependent back‐stress model and A.N. May's random slip model.

Size dependence for mechanical properties of Ti/Cu multilayered nanowire with a semi-coherent interface

Metallic multilayered nanowires have a wide application prospect in micro-nano devices because of their superior physical and chemical properties and microstructure designability. Size effects on the tensile behaviors of Ti/Cu multilayered nanowires are investigated by molecular dynamic simulations. Aspect ratios of 1:4, 1:3, 1:2, 1:1, 1:0.75, and 1:0.67 and sectional dimensions of 3, 4, 5, 6, and 7 nm are adopted to construct nanowires with different sizes. Simulation results indicate that the strength of Ti/Cu nanowires decreases with the decrease of aspect ratio in the large aspect ratio range (>1:2) and all simulated sectional dimension ranges, showing a reverse Hall-Petch effect. The Hall-Petch law can only be satisfied in a small aspect ratio range (<1:2). Deformation mechanism transition is found in the critical aspect ratio of 1:2. When the aspect ratio is larger than 1:2, crystalline phases of Ti and Cu layers dominate the plastic deformation of Ti/Cu nanowires. Crystal phases and interface both bear plastic deformation when the aspect ratio is smaller than 1:2. Interface is an important factor in the strength and deformation of Ti/Cu nanowires. The variation of interface fraction and interaction between interface and dislocation motion determine the tendency of strength variation for Ti/Cu nanowires.

A great enhancement of both strength and plasticity in a dual-phase high-entropy alloy by multi-effects of high carbon addition

High-entropy alloys (HEAs) have become an interesting research area for the novel alloying design paradigm. Interstitial carbon has usually been introduced to enhance mechanical properties of HEAs. To achieve addition of high carbon, this work designed a dual-phase Al10(FeNiCoMn)90 HEA without strong carbide-forming elements. The results showed that adding 3 at.% carbon did not cause the precipitation of carbides in this HEA, and its yield strength and plasticity increased by 237 MPa and 26.2 %, respectively, which was due to the increase of fcc phase fraction, the distribution change of bcc phase and interstitial strengthening. Addition of 4 at.% carbon only caused the precipitation of nano-scale carbides in the matrix and the yield strength and plasticity increased by 455 MPa and 5.5 %, respectively, resulted from phase modulation, interstitial strengthening and precipitation strengthening. Simultaneously, the transition from wavy slip to planar slip and the formation of microbands after carbon addition, which also led to an increase of plasticity. This work provides a promising method that if coarse carbides can be avoided, high carbon addition enhances both strength and plasticity in a dual-phase HEA by multi-effects including phase modulation, interstitial strengthening, precipitation strengthening and deformation mechanism transition.

Dynamic Behavior of Additively Manufactured FeCoCrNi High Entropy Alloy

Additively manufactured face-centered-cubic high entropy alloys have a combination of high strength and good ductility, and are promising impact-resistant structural materials. However, the dynamic behavior of additively manufactured face-centered-cubic high entropy alloys is seldomly reported. In this study, FeCoCrNi high entropy alloy was fabricated, using the laser beam powder bed fusion technique, and dynamic tests were performed by means of a Split Hopkinson Pressure Bar. The high entropy alloy showed a more excellent combination of yield stress and toughness at high strain rates, than previously reported alloys. This was attributed to the dislocation cell structure of the additively manufactured FeCoCrNi HEA, which provided high local stress concentration, leading to the formation of microbands and deformation twins. The high entropy alloy showed higher strain rate sensitivity than the cast counterpart, at both quasi-static and strain rates over 3000 s−1. Interestingly, the yield stress kept stable at a strain rate from 1000 s−1 to 3000 s−1, showed a steep decrease of strain rate sensitivity and a four-fold increase in activation volume, implying a transition in deformation mechanism to collective dislocation nucleation.

Enhancing strength and ductility in a near medium Mn austenitic steel via multiple deformation mechanisms through nanoprecipitation

High-Mn austenitic steels, usually deformed via the TWIP (twinning induced plasticity) mechanism, suffer high cost and other technical issues (e.g., hot-dip galvanizing, welding, etc.) from the high Mn content. Nevertheless, decreasing the Mn content would result in a shift of deformation mechanisms from TWIP to TRIP (transformation induced plasticity), usually causing quasi-cleavage brittle fracture. Herein, we report that by massive nanoprecipitation of coherent disordered particles, the grain sizes of a near medium Mn austenitic steel are successfully refined to 0.9 ± 0.4 μm, leading to a significant stacking fault energy increment of 6.4–7.9 mJ m–2 and accordingly, the transition of deformation mechanism from TRIP to multiple deformation mechanisms, namely, stacking faults, dislocation slip, nanotwinning and ԑ-martensite transformation. Moreover, the high-density of coherent nanoprecipitates effectively refines ԑ martensite and nanotwins from 20–500 nm and 10–50 nm to a few atomic columns and 1–15 nm, respectively. More importantly, these deformation mechanisms were sequentially activated at different deformation stages, resulting in a consistently high work hardening rate. Based on the synergistic refinement effects of grains, twins and martensite and the transition of deformation mechanism, a near medium Mn ultrafine-grained (UFG) austenitic steel with 15 wt.% Mn is developed, which shows a unique combination of high tensile strength (1210 ± 19 MPa) and large elongation (72 ± 6 %). These findings provide a new route to addressing the trade-off between the Mn content and mechanical performance of high-Mn austenitic steels which could facilitate their widespread applications.

Misorientation-dependent transition between grain boundary migration and sliding in FCC metals

Grain boundaries (GBs) play a crucial role in the mechanical behaviors of nanocrystalline materials. The dynamics of GBs depend on a variety of intrinsic and extrinsic factors, including GB misorientation, inclination, curvature, metal type and loading history. Controlling GB geometry and deformation behavior thus provides effective means to tailor the mechanical properties of nanostructured materials. However, the relationship between deformation mechanism and GB geometry is still lacking, largely due to the vast amount of metastable GB structures and deformation paths. Here, the relation between GB misorientation (θ, a critical degree of freedom for GB structure) and GB deformation mechanisms was studied by large-scale molecular dynamic simulation in face-centered cubic (FCC) metals. With increasing GB misorientation, the GB deformation mechanism transitions from migration to sliding and further to migration again. The critical transition thresholds (critical misorientations) vary with metal types. An energetic model that links deformation mechanism with GB structure was further developed with a focus on predicting the critical GB misorientation at which GB sliding supersedes GB migration. The influence of metal type on the transition threshold is well captured. We finally extend this model to general GBs with complex atomic structures and compare the theoretical predictions with data reported in the literatures and obtained from the present work. Our findings shed lights on the misorientation-dependent GB deformation mechanisms and can be utilized in GB engineering that pursues high-performance nanocrystalline metals.

Transition of deformation mechanisms from twinning to dislocation slip in nanograined pure cobalt

• Gradient nanograined structures were prepared by plastic deformation in a pure cobalt sample. • Atomic-scale observation for microstructure characteristics was conducted in nanograined cobalt. • Comparing deformation mechanisms between nanograins and submicron-grains indicates a transition of governing deformation mechanism as reducing the gain size to nanoscale. Deformation mechanisms of nanograined and submicron-grained pure cobalt processed by means of high strain rate shear deformation at cryogenic temperatures were studied. Microstructural analysis revealed a transition of governing deformation mechanism from deformation twinning and < a > dislocation slip in submicron-grains to < c + a > and < a > dislocations slip in nanograins. Microhardness tests illustrated that the Hall-Petch relation slope changes consequently with the transition of deformation mechanism.

Mechanical properties, failure mechanisms, and scaling laws of bicontinuous nanoporous metallic glasses

Molecular dynamics simulations are employed to study the mechanical properties of nanoporous CuxZr1-x metallic glasses (MGs) with five different compositions, x = 0.28, 0.36, 0.50, 0.64, and 0.72, and porosity in the range 0.1 < ϕ < 0.7. Results from tensile loading simulations indicate a strong dependence of Young's modulus, E, and Ultimate Tensile Strength (UTS) on porosity and composition. By increasing the porosity from ϕ = 0.1 to ϕ = 0.7, the topology of the nanoporous MG shifts from closed cell to open-cell bicontinuous. The change in nanoporous topology enables a brittle-to-ductile transition in deformation and failure mechanisms from a single critical shear band to necking and rupture of ligaments. Genetic Programming (GP) is employed to find scaling laws for E and UTS as a function of porosity and composition. A comparison of the GP-derived scaling laws against existing relationships shows that the GP method is able to uncover expressions that can predict accurately both the values of E and UTS in the whole range of porosity and compositions considered.

Deformation Mechanism Transition in Additively Manufactured Compositionally Graded Fe-Base Alloys

Deformation Mechanism Transition in Additively Manufactured Compositionally Graded Fe-Base Alloys

Grain boundary evolution in nanograined metals: from relaxation to Schwarzation

High density of grain boundaries (GBs) in nanograined metals may become unstable and evolve in various ways such as migration, sliding, or relaxation under thermal or mechanical stimuli. In this report, we will present our recent studies on structural evolution of GBs in nanograined Cu during intensive plastic deformation. As the grain size decreased below 200 nm, GB migration become obvious assisted by dislocation activities. Below 70 nm, accompanied by a transition in dominant deformation mechanism from full dislocations to partials, GB migration decays gradually, while the GB structures relax into lower energy states through their interactions with partials. The population of low-energy GBs increases with a decreasing grain size. Below 10 nm, further refinement of grains results in a violent structural evolution of the GB network into a 3-dimensional periodic minimal surface structure, Schwarz-D, through a phase-transformation-like process, which is termed as Schwarzation.

The Strain Rate Sensitivity of Heterogeneous Thin Film Metallic Glasses: Interplay Between Nanoscale Heterogeneity and Dynamic Plasticity

The dynamic mechanical properties of metallic glasses (MGs) are crucial to capturing the deformation signatures as well as for structural and functional applications. In this work, we investigate the influence of nanoscale structural heterogeneity of MGs on dynamic plasticity, focusing on the variation in strain rate sensitivity and the transition of deformation mechanisms, using a combined experimental and simulation approach. The Cu50Zr50 thin-film MGs with different nanoscale heterogeneities are synthesized using magnetron sputtering and further characterized using dynamic force microscopy and nanoindentation. All the films exhibit a strain rate hardening effect, but a transition in strain rate sensitivity as the indentation rate increases has been found in the MG with a higher heterogeneity. To understand the underlying mechanisms, mesoscale shear transformation zone dynamics simulations are performed on model Cu50Zr50 MGs. The simulation results are able to capture the experimental trend. Notably, the transition in strain rate sensitivity for a heterogenous MG stems from a change in deformation mechanisms: from structure-dictated strain localization at a lower strain rate to stress-dictated strain percolation into a shear band at a higher strain rate. The observed strain rate sensitivity and the corresponding mechanisms are summarized in a deformation mechanism map where nanoscale structural heterogeneity and strain rate are varied. We envision our study not only providing insights into the structure and property relationship of MGs on the nanoscale but also will facilitate the design of heterogeneous MGs for dynamic applications.