Tensile Deformation Mechanisms Research Articles

Tensile properties and deformation mechanisms of two low-cost second-generation single crystal superalloys designed by optimization of Re and W compositions at various temperatures

In this work, two novel low-cost second-generation single crystal superalloys were designed by optimizing compositions of W and Re elements. The tensile properties and deformation mechanisms were systematically investigated from room temperature to 1070 ℃. It was found that yield strength of both alloys reached the maximum value (1099 MPa for 1.0Re alloy and 1086 MPa for 1.5Re alloy) at 760 ℃. At room temperature and 760 ℃, dislocations sheared into γ′ phase and dissociated into partial dislocations and stacking faults, which controlled the whole deformation process. At 760 ℃, the interactions between isolated stacking faults caused by the successive initiation of multiple slip systems led to the formation of Lomer-Cottrell locks, which effectively improved yield strength of 1.0Re alloy. 1.5Re alloy was strengthened effectively by isolated stacking faults, Kear-Wilsdorf locks and dislocation loops at 760 ℃. At high temperatures (980 ℃ and 1070 ℃), dislocation pairs with anti-phase boundary shearing into γ′ phase became the main deformation mechanism, and dislocations climbing and bypassing mechanisms could also be activated under higher thermal activation. Meanwhile, compared with that in 1.0Re alloy, interfacial dislocation networks in 1.5Re alloy is more regular and denser. Generally, the two novel self-designed single crystal superalloys characterized by outstanding tensile properties and cost reduction displayed great potentials for application of advanced aero-engines in the future.

Effect of temperature on tensile behavior, fracture morphology, and deformation mechanisms of Nickel-based additive manufacturing 939 superalloy

In the current work, uniaxial tensile tests, including interrupted tests at room temperature, intermediate temperatures (650 ℃ and 700 ℃), and high temperatures (850 ℃ and 950 ℃), were conducted on the additive manufacturing 939 superalloy to investigate the mechanical properties, fracture behaviors, and deformation mechanisms. According to the experimental results, the tensile mechanical properties exhibit a significant temperature sensitivity, as the yield, tensile strength, and elongation decrease with the increase in temperature. The fracture surfaces show that the material exhibits obvious plastic fracture characteristics at room temperature. Moreover, many multi-slip system steps can be observed when the temperature rises. Furthermore, according to the result of the transmission electron microscopy, the deformation mechanism at room temperature is primarily controlled by the single slip system. As temperature rises to the intermediate temperature region, more slip systems are activated by thermal energy, which promotes the movement of dislocations. The deformation mechanism is mainly dominated by dislocation shearing γ'. When the temperature reaches the high temperature region, the deformation mechanism firstly demonstrates large-scale stacking faults, and then transforms into the dislocation by-passing and climbing mechanism. Finally, the relationship between temperature and the deformation mechanism is discussed and deduced.

Twin thickness-dependent tensile deformation mechanism on strengthening-softening of Si nanowires

Twin thickness-dependent deformation and the transition from strengthening to softening in twinned silicon nanowires are investigated using molecular dynamics simulations with cylindrical and hexagonal cross sections. The results show that the transition from strengthening to softening occurs at critical twin thicknesses of 8.1 nm (11.0 TB s) with cylindrical cross section and 11.0 nm (8 TBs) with hexagonal cross section with decreasing twin thickness, and that the strongest twin thickness originates from a transition in the initial plasticity mechanism from full dislocation nucleation and interaction with the TBs to partial dislocation nucleation and gliding parallel to the TBs. Moreover, it is found that the relationship between peak stress and twin thickness can be divided into two regions. Several full and partial dislocations are formed in the regions with strengthening twin thickness range. The accumulation and pile-up of these dislocations and their interaction with the TBs at high density cause the Hall-Petch strengthening behavior. In contrast, few full and partial dislocations are formed with softening twin thickness range. These dislocations are nucleated and propagate parallel to the TBs, resulting in TB migration that causes inverse Hall-Petch softening behavior. Our simulation results provide sufficient insight into the mechanical behavior of twinned silicon nanowires with cylindrical and hexagonal cross sections. The study will be helpful to the further understanding of CTB-related mechanical behaviors of non-metallic materials and non-metallic system.

Microstructural Evolution and Tensile Properties of a Corrosion-Resistant Ni-Based Superalloys Used for Industrial Gas Turbines

As an important mechanical property, tensile behavior has been regarded as an indicator for the creep and thermal mechanical fatigue properties of Ni-based superalloys. The tensile property of Ni-based superalloys is closely related to the amount, size, and distribution of γ'-phase and carbides. To further clarify the tensile deformation mechanism of the CM247LC alloy, this study investigated its solidification characteristics and directionally-solidified and heat-treated microstructure. The dependence of tensile properties on the varied temperature ranging between 650 and 950 ℃ is discussed in detail. It was found that the deformation mechanism at 650 ℃ is dominated by the shearing of dislocations into the γ'-precipitates to form the superlattice stacking faults. At 800 ℃, the K-W lock leads to the anomalous yield effects. At 950 ℃, the deformation mechanism is dominated by the dislocations bypassing the γ'-precipiates. The results provide a comprehensive understanding of the CM247LC alloy and are beneficial for the development of corrosion-resistant Ni-based superalloys.

The Effect of Strain Rate on Hydrogen-Assisted Deformation Behavior and Microstructure in AISI 316L Austenitic Stainless Steel.

The influence of strain rate in the interval of (10-5-10-3) 1/s on room temperature tensile behavior, dislocation arrangement, deformation mechanisms, and fracture of austenitic stainless steel AISI 316L electrochemically charged with hydrogen was investigated. Independently on strain rate, hydrogen charging provides the increase in the yield strength of the specimens due to a solid solution hardening of austenite, but it slightly influences deformation behavior and strain hardening of the steel. Simultaneously, hydrogen charging assists surface embrittlement of the specimens during straining and reduces an elongation to failure, which both are strain rate-dependent parameters. Hydrogen embrittlement index decreases with increase in strain rate, which testifies the importance of hydrogen transport with dislocations during plastic deformation. The stress-relaxation tests directly confirm the hydrogen-enhanced increase in the dislocation dynamics at low strain rates. The interaction of the hydrogen atoms with dislocations and hydrogen-associated plastic flow are discussed.

In situ visualization of the tensile deformation mechanism of bacterial cellulose network

Bacterial cellulose (BC) pellicles are strong hydrogels composed of nanofibril networks. These hydrogels are considered attractive materials for synthetic biology, in which biological systems or modules are designed with user-defined functions. To develop BC-based materials with tailored mechanical properties, elucidation of the tensile deformation mechanism is essential. Therefore, in this study, BC hydrogels were fluorescently labeled, and the fiber network under tensile deformation was observed in situ using a device for simultaneous confocal laser scanning microscopy and uniaxial tensile deformation. As a result, strain-dependent deformation modes were identified and the generation of stress paths (stress-loaded fiber segments) during deformation was visualized. Furthermore, characteristic relaxation spectra of the nanofiber network were obtained from stress-relaxation measurements, revealing the existence of a first-order relaxation mode at approximately 1 s and higher-order relaxation modes over a long time period of 102–105 s. On this basis, we proposed a tensile deformation model of the BC hydrogel characterized by rearrangements of fiber segments accompanied by cleavage of cross-links. This model is expected to facilitate synthetic biology using BC hydrogels.

Crystal orientation-dependent tensile mechanical behavior and deformation mechanisms of zinc-blende ZnSe nanowires

Crystal deformation mechanisms and mechanical behaviors in semiconductor nanowires (NWs), in particular ZnSe NWs, exhibit a strong orientation dependence. However, very little is known about tensile deformation mechanisms for different crystal orientations. Here, the dependence of crystal orientations on mechanical properties and deformation mechanisms of zinc-blende ZnSe NWs are explored using molecular dynamics simulations. We find that the fracture strength of [111]-oriented ZnSe NWs shows a higher value than that of [110] and [100]-oriented ZnSe NWs. Square shape ZnSe NWs show greater value in terms of fracture strength and elastic modulus compared to a hexagonal shape at all considered diameters. With increasing temperature, the fracture stress and elastic modulus exhibit a sharp decrease. It is observed that the {111} planes are the deformation planes at lower temperatures for the [100] orientation; conversely, when the temperature is increased, the {100} plane is activated and contributes as the second principal cleavage plane. Most importantly, the [110]-directed ZnSe NWs show the highest strain rate sensitivity compared to the other orientations due to the formation of many different cleavage planes with increasing strain rates. The calculated radial distribution function and potential energy per atom further validates the obtained results. This study is very important for the future development of efficient and reliable ZnSe NWs-based nanodevices and nanomechanical systems.

Understanding the anisotropic tensile deformation behavior of commercially pure titanium by experiments and crystal plasticity simulations

Current work reveals the anisotropic tensile deformation mechanisms of commercially pure titanium (CP-Ti) along rolling direction (RD) and transverse direction (TD) by the statistical analysis of electron back scattered diffraction (EBSD) data and crystal plasticity (CP) simulations. The analysis of texture evolution, misorientation angle distribution, in-grain misorientation axis (IGMA) and Schmid factor indicates that the anisotropic tensile deformation behavior is originating from the anisotropic slip modes. Further CP simulation results confirm this conjecture and reveal that the higher fraction of basal slip accommodates the deformation and leads to the lower strain hardening behavior along TD. The lower uniform elongation along TD is caused by the heterogeneous strain distribution in the microstructure as deduced from digital image correlation (DIC) results and kernel average misorientation (KAM) maps.

Martensitic transformation dominated tensile plastic deformation of nanograins in a gradient nanostructured 316L stainless steel

We systematically investigated the tensile deformation mechanisms and mechanical properties of a gradient nanograined AISI 316 L stainless steel prepared by means of surface mechanical grinding treatment. The gradient nanograined samples exhibit a good synergy of high strength and remarkable tensile ductility at room temperature. Microstructural observations revealed that martensitic transformation co-operating with grain boundary migration accommodates the tensile plastic deformation of nanograins in the topmost layer of gradient nanograined 316 L SS, which is distinct from the traditional partial dislocation associated deformation twinning in homogeneous coarse-grained counterpart under tension loading. Accompanied by the decrease in grain boundary migration rate, the martensite content increases significantly with increasing tensile strain, reaching ∼50% at a true strain of 50%. The newly formed martensites as strengthening phase not only provide dynamic work hardening, but also effectively suppress the strain localization, resulting in the substantial tensile ductility of the gradient nanograins.

Dual-phase synergistic deformation characteristics and strengthening mechanism of AlCoCrFeNi2.1 eutectic high entropy alloy fabricated by laser powder bed fusion

Eutectic high entropy alloy (EHEA) possesses promising prospects for industrial application due to its controllable and near-equilibrium dual-phase structure. Due to the advantages of high material utilization, efficient production, and design freedom, the laser powder bed fusion (LPBF) technique provides a new path to prepare EHEA components with complex structure and excellent performance. In this study, near fully dense AlCoCrFeNi2.1 samples were obtained by adjusting the process parameters of LPBF. Considering the balling phenomenon and powder splashing during the LPBF process, laser remelting was selected as an optimized scanning strategy to further improve the forming quality of AlCoCrFeNi2.1. The microstructure of remelted AlCoCrFeNi2.1 sample exhibited regular eutectic lamellae consisting of nano-scale face-centered-cubic (FCC) and B2 phases, in which the FCC phase accounted for a higher proportion. By investigating the tensile behavior and deformation mechanism, it was revealed that the ultrafine eutectic lamellae could induce a strong dual-phase synergistic strengthening, thereby significantly improving the strength of the sample. Compared with the vacuum induction melted (VIM) sample, the remelted sample showed a 54% increase in ultimate tensile strength (UTS -1518 MPa) and a 130% increase in yield strength (YS -1235 MPa) with reasonable plasticity. This study indicates that by combining the design and manufacturing freedom of LPBF with the EHEA, it is expected to fabricate high-property 3D EHEA parts, expanding the application field of EHEA.

Microstructure optimization for higher strength of a new Fe–Ni-based superalloy

Tensile properties and deformation mechanisms of a Fe–Ni-based superalloy with different size of γ′ precipitates obtained through varied aging time are investigated at 700 °C. The γ′ phase size increases with aging time, the yield strength of the alloy increases first and then decreases, which is consistent with the change trend of critical resolved stress of deformation mechanism. The dominant deformation mode of the alloy with fine γ′ phase is weakly-coupled dislocation pairs with slip bands, and then changes to Orowan looping at a γ′ size of approximately 30 nm, resulting in the occurrence of peak value of strength. As the γ′ size is above 40 nm, the deformation mechanism is Orowan bowing along with stacking fault shearing, decreasing the yield strength of the alloy. When γ′ size is between 30 nm and 40 nm, the dominant deformation mechanism is Orowan looping with strongly-coupled dislocation pairs and the fracture mode is intergranular. Intermediate temperature brittleness occurs in the secondary aging treated alloy with a γ′ phase size of approximately 30 nm, which is attributed to the increase in critical resolved stress and strain localization caused by the transformation of tensile deformation mechanism, and stress concentration due to dislocation entanglement at grain boundary. The sub-aging state of the alloy used for advanced ultra-supercritical plants may be a good choice for obtaining simultaneously favorable strength and ductility.

Atomistic tensile deformation mechanisms in a CrCoNi medium-entropy alloy with gradient nano-grained structure at cryogenic temperature

Large-scale molecular dynamics (MD) simulations have been utilized to reveal the atomistic deformation mechanisms of a CrCoNi medium-entropy alloy (MEA) with gradient nano-grained (GNG) structure in the present study. Strong strain hardening was observed in the gradient nano-grained structure at the elasto-plastic transition stage, which could be attributed to the Masing hardening. After yielding, obvious partitioning of tensile strain was detected in the gradient nano-grained structure, which indicates the existence of hetero-deformation induced (HDI) hardening effect and could account for the higher flow stress of the gradient nano-grained structure than that calculated by the rule of mixture from its homogenous nano-grained (NG) structured counterparts. Moreover, partitioning of stacking fault factor (corresponding to the partitioning of tensile strain), which demonstrates the intensity of dislocation behaviors, was also observed in the gradient nano-grained structure. The differences of factors for each grain size area were found to be smaller in the gradient nano-grained structure than those of its homogeneous nano-grained structured counterparts, which indicates the influence of forward stress and back stress on dislocation motion near the zone boundary between the hard zone with smaller grains and the soft zone with larger grains, further verifying the presence of hetero-deformation induced hardening in the gradient nano-grained structure.

Unraveling process-microstructure-property correlations in powder-bed fusion additive manufacturing through information-rich surface features with deep learning

A machine learning (ML)–based framework has been developed to optimize the process parameters and unravel the paramount process–microstructure–property (PMP) relationships rapidly and precisely, which is demonstrated using electron beam melting (EBM®)-processed Ti–6Al–4V alloy. The process maps are constructed using ML to discover the optimal process window based on the three criteria: fully dense (≥ 99.5% in relative build density), flat fusion surface and highest possible strength, without which the optimization outcome will be deficient. By using the ML-optimized process parameters, Ti–6Al–4V parts were fabricated with ultimate tensile strength ∼5% higher and ∼80% faster than the classical parameter setting developed by the original machine manufacturer. The α/β dual-phase microstructure and tensile deformation mechanism are further studied to understand the superior mechanical properties obtained in the ML-optimized sample. In addition, the information-rich top fusion surface is found to be an alternative route to implicitly materialize the microstructure-to-property relationships with a prediction accuracy of ∼98%. The full-scope surface and microstructure maps are then generated from the process parameters. Surface features that reflect superior and poor mechanical properties are particularly identified. Finally, the implications of the ML-based PMP framework with surface feature correlation are discussed in the context of improving resources utilization efficiency.

Tensile behavior, microstructural evolution, and deformation mechanisms of a high Nb-TiAl alloy additively manufactured by electron beam melting

To evaluate the tensile behavior, microstructural evolution, and deformation mechanisms of Ti-45Al-8Nb alloy additively manufactured by electron beam melting, uniaxial tensile experiments were performed at a constant strain rate of 2.5 × 10−4 s−1 at various temperatures. The experimental results indicated that the tensile behavior and flow stress are sensitive to temperature. The brittle to ductile transition temperature of Ti-45Al-8Nb alloy additively manufactured by electron beam melting is between 700 and 750 ℃. Below this temperature, the fracture was predominantly trans-granular, resulting in brittle failure. In addition, the brittle failure behavior is related to the localization of deformation within the grains and stress concentrations owing to plastic incompatibility at the interface of the α2+γ phases. However, above this temperature, the fracture transformed from the trans-granular type to the mixed mode of trans-granular and ductile dimples. Dynamic recovery and recrystallization are the primary processes leading to softening behavior. The flow behavior at elevated temperatures results from the competition between work hardening and softening. Furthermore, the Chaboche model was used to describe the tensile inelastic behavior at different temperatures.

Tensile Behavior, Constitutive Model, and Deformation Mechanisms of MarBN Steel at Various Temperatures and Strain Rates.

To reduce harmful gas emission and improve the operational efficiency, advanced ultra-supercritical power plants put forward higher requirements on the high temperature mechanical properties of applied materials. In this paper, the tensile behavior and deformation mechanisms of MarBN steel are discussed at different strain rates (5 × 10-3 s-1, 5 × 10-4 s-1, and 5 × 10-5 s-1) under room temperature and 630 °C. The results show that the tensile behavior of the alloy is dependent on temperature and strain rate, which derived from the balance between the average dislocation velocity and dislocation density. Furthermore, observed dynamic recrystallized grains under severe deformation reveal the existence of dynamic recovery at 630 °C, which increases the elongation compared to room temperature. Finally, three typical constitutive equations are used to quantitatively describe the tensile deformation behavior of MarBN steel under different strain rates and temperatures. Meanwhile, the constitutive model of flow stress for MarBN steel is developed based on the hyperbolic sine law.

Effects of stacking fault energy and temperature on grain boundary strengthening, intrinsic lattice strength and deformation mechanisms in CrMnFeCoNi high-entropy alloys with different Cr/Ni ratios

Tensile properties and deformation mechanisms at 293 and 77 K of single-phase face-centered-cubic (FCC) CrxMn20Fe20Co20Ni40-x (0 ≤ x ≤ 26 at.%) high-entropy alloys (HEAs) with grain sizes (d) in the range [6–283 µm] were studied. A specificity of these alloys is that their stacking fault energy (SFE) strongly decreases with increasing the Cr/Ni ratio while all the other characteristics affecting strength (elastic moduli and solute misfit volumes) remain approximately constant. Therefore, these alloys allow to investigate how the SFE affects the tensile behavior and deformation mechanisms of FCC HEAs. Grain boundary strengthening is found to be nearly independent of SFE, composition, and temperature. While the intrinsic lattice strength (σ0 ≈ 136 MPa) remains approximately constant at 293 K, it strongly increases with increasing Cr/Ni ratio and thus decreasing SFE at 77 K, presumably due to a reduction of dislocation line tension. Deformation twinning occurs at 293 K when x ≥ 20 at.% (SFE ≤ 35 mJ/m2) and in all alloys at 77 K. The strength-ductility combination is found to increase with decreasing SFE only when 14 ≤ x ≤ 24 at.% at 77 K because twinning must be triggered at relatively low plastic strains to yield a significant dynamic Hall-Petch effect, allowing to sustain a high work hardening rate and postpone necking instability. In addition to twinning, an FCC-to-HCP (hexagonal close-packed) martensitic transformation was observed at 77 K in the Cr26Mn20Fe20Co20Ni14 HEA, which induced early rupture when d = 212 µm.

Atomistic modeling of tensile deformation and fracture of carbon fibers: Nanoscale stress redistribution, effect of local structural characteristics and nanovoids

The mechanisms of tensile deformation and fracture of carbon fibers are investigated in large-scale molecular dynamics simulations. The simulations performed for computational samples with different structural characteristics enable analysis of the structural sensitivity of the mechanical properties. It is found that the carbon fiber nanostructure can be represented by a fine (nanoscale) mixture of stiff and soft regions. Despite the nanoscale structural heterogeneity of the computational fibers, their fracture strain and tensile strength exceed the experimental values by almost an order of magnitude. This observation is explained by the ability of the carbon fiber nanostructure, largely consisting of curved and folded layers of turbostratic carbon, to exhibit continuous nanoscale load redistribution during the deformation. The transient, fluid nature of a percolating cluster of stress-holding structural elements prevents stress localization at the early stage of the deformation and enables the fiber nanostructure to adapt to the increasing load. Overall, the results of the simulations suggest that the design of advanced carbon fiber manufacturing and post-processing methods focused on the elimination of critical mesoscopic structural defects (internal voids, foreign inclusions, and surface flaws) can yield a substantial increase in the strength and fracture strain of carbon fibers.

Excellent tensile properties and deformation mechanisms in a FeCoNi-based medium entropy alloy with dual-heterogeneous structures

A dual-heterogeneous structure with both heterogeneous grain structure and coherent L12 nano-precipitate was obtained in a (FeCoNi)86Al7Ti7 medium-entropy alloy (MEA) after cold rolling, critical annealing, and aging treatments. The volume fraction of the L12 phase is found to increase from 0% to 34.5% after aging treatment, resulting in severer heterogeneity. The unaged samples of (FeCoNi)86Al7Ti7 MEA are found to have a much better synergy of strength and ductility as compared to the FeCoNi MEA. Moreover, the tensile properties for the aged samples of (FeCoNi)86Al7Ti7 MEA with dual-heterogeneous structures are even better than those for the corresponding unaged samples. The hetero-deformation-induced hardening plays a more important role in the aged samples than in the unaged samples, producing a higher density of geometrically necessary dislocations. High density dislocations are discovered in grain interiors for the aged samples and the deformation is dominated by the planar slip in the unaged samples. Meanwhile, a high density of dislocations are observed inside L12 nano-precipitates; thus, L12 nano-precipitates can be sheared by dislocations, indicating that L12 nano-precipitates are strong but deformable. The strong interactions between dislocations and L12 nano-precipitates should provide additional strain hardening for better tensile properties in the aged samples.

Enhancing the strength–ductility synergy of medium-Mn steel by introducing multiple gradient structures

In this study, novel multiple gradient structures with various distributions of austenite/martensite phase, grain sizes, and dislocation densities were fabricated in medium-Mn steel by using torsion treatment, and the corresponding tensile properties and deformation mechanisms were investigated. The results showed that the yield strength and the total elongation of the multiple-gradient-structured samples increased by 27% and 25%, respectively, compared with their homogeneous counterparts. A distinct phase transformation behavior was observed in the present gradient medium-Mn steel. That is, the strain-induced martensitic transformation was promoted in the center during the entire tensile deformation process; however, it was suppressed at the surface during the initial deformation stage and then significantly triggered during the remaining deformation process. Moreover, active strain partitioning at the macroscale and microscale occurred during plastic deformation, which led to a higher hetero-deformation induced (HDI) stress in the multiple-gradient samples. A combination of a strong and persistent transformation-induced plasticity effect, active HDI strengthening and HDI hardening, and dislocation strengthening contributed to the superior strength–ductility synergy of the gradient-structured medium-Mn steel.

Enhanced dynamic deformability and strengthening effect via twinning and microbanding in high density NiCoFeCrMoW high-entropy alloys

High density alloys with enhanced deformability and strength are urgently required in energy, military and nuclear industries, etc. In this work, we present a new kind of NiCoFeCrMoW high entropy alloys (HEAs) which possess higher densities and sound velocities than copper. We systematically investigate the phase structure, quasi-static tensile, dynamic compression and related deformation mechanism of these HEAs. It is shown that single FCC or FCC + μ dual phases were formed in the HEAs depending on Mo and W content and annealing temperature. Excellent quasi-static tensile and dynamic compression properties have been achieved for these HEAs, e. g. Ni30Co30Fe21Cr10W9 HEA annealed at 1573 K exhibited a yield and ultimate tensile strength and elongation of ∼364 MPa, ∼866 MPa and ∼32%, respectively, in quasi-static test; a yield strength of ∼710 MPa and no fracture under the dynamic strain rate of 4100 s−1. Superior strain rate sensitivity (SRS) of yield strength than that of previously reported FCC HEAs have been evidenced. The dynamic stress-strain constitutive relation can be described by the modified Johnson-Cook model. As for the dynamic deformation mechanism, it is envisaged that the regulation of stacking fault energy and Peierls barrier in current HEAs resulted in occurrences of abundant nanoscale deformation twins and microbands during high strain rate compression. The synergistic microbanding and twinning effectively contributes to the enhanced dynamic deformability and strengthening effect. Besides, the interactions of dislocations with precipitates, stacking faults (SFs) with twins, and between SFs also contribute to extraordinary work-hardening capacity.