Transition In Deformation Behavior 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.

Coupled chemo-mechanical modeling of point-defect diffusion in a crystal plasticity fast Fourier transform framework

Below the yield strength and at moderate-to-high homologous temperatures, the inelastic deformation of metals is mostly governed/rate-controlled by vacancy diffusion-mediated processes. As a function of grain size, stress, temperature and dislocation content, vacancies (or atoms) can adopt preferential diffusion paths across grain interiors, along grain boundaries, or towards and along dislocations, resulting in climb and self-climb. In the steady state and under constant load, grain boundary and grain bulk vacancy diffusion-mediated plasticity have been described in seminal works by Coble and by Nabarro and Herring, respectively. Yet, the interplay between all aforementioned potential diffusion pathways has not been comprehensively mapped. This work presents a thermodynamically-consistent full-field model integrated within a voxel-based elasto-viscoplastic fast Fourier transform framework, which considers the coupling between the diffusion-mediated plasticity mechanisms. In the proposed approach, the kinetics and kinematics of plastic deformation due to vacancy diffusion along grain boundaries and grain bulk, as well as the exchange between grain boundaries and bulk are described explicitly. A homogenization approach at the voxel level is further introduced to simultaneously consider bulk and grain boundary diffusion in a numerically efficient fashion. The new formulation predicts the expected strain rate dependencies and the scaling of the steady-state creep rate with respect to grain size, temperature, and stress. The model predicts the transition from grain bulk to grain boundary-dominated diffusion with reduction in grain size, a significant step towards capturing transitions in deformation behavior without any phenomenological or ad-hoc adjustments.

Unveiling the origins of work-hardening enhancement and mechanical instability in laser shock peened titanium

Laser shock-peening (LSP) produces complex gradients in microstructures and residual stresses, which enables fabrication of metallic engineering components with superior mechanical properties. Here we use non-destructive and spatially resolved high-energy X-ray diffraction (HE-XRD) to investigate the influences of LSP on microstructure, surface topography, and residual stress for high purity titanium plates. LSP is found to produce large compressive in-plane residual stresses near the peened surface which monotonically decay to zero about 2.5 mm below the surface. These properties were also tracked during in-situ tensile loading, allowing stress partitioning and work-hardening rates to be measured as a function of depth. The surface region is found to have the highest work-hardening rate and remain mechanically stable up to sample failure. In addition, a crystal rigid rotation along a transversal axis of about 27° was observed at large applied strain near the surface region, which is attributed to the formation of denser dispersed shear bands. Furthermore, a sudden drop in tensile stress was observed at a depth of ∼160 µm from the LSP surface, indicating the formation of a localized mechanical instability zone (LMIZ). The origin of this LMIZ could be attributed to the increase in shear stress induced by a load transfer from LSP surface due to the formation of denser dispersed shear bands. These observations indicate a localized transition in deformation behavior has occurred from dispersed shear banding to LMIZ and homogeneous deformation.

Size dependence of ductile to brittle transition of Zr-based metallic glasses under multiaxial loading

The size dependence of ductile to brittle transition of Zr-based metallic glasses was systematically investigated by small punch test. It was found that the metallic glasses with thickness ranging from 50 to 700 μm displayed an obvious transition from ductility to brittleness. Compared with thick samples, thin samples exhibit higher strength and better plasticity, which is attributed to the size-dependent transformation from soft state to hard state dominated by the deformation factor ( α = τ / σ ). Specifically, two significant transitions in deformation behavior at sample thickness of 200 and 500 μm are attributed to two critical deformation factors of α c 1 and α c 2 , which are responsible for the initiation of circumferential and radial shear bands, respectively. Specifically, only several micro-cracks can be observed on sample surface with α ≤ α c 2 . While α c 1 ≥ α ≥ α c 2 , multiple radial shear bands are activated and uniformly distributed on the sample surface. Further, when α ≥ α c 1 , multiple circumferential shear bands primarily initiate under high shear stress and large number of radial and circumferential shear bands can be observed on sample surface. The initiation and interaction of multiple radial and circumferential shear bands significantly enhance the ductility of the samples with α ≥ α c 1 .

Finite element analysis of deformation and failure mechanisms in nanoscale hexagonal cellular structures of metallic glasses

Cellular metallic glasses (MGs) have been found to be a potential candidate for structural and functional applications due to their attractive properties such as high strength to weight ratio, excellent energy absorption and enhanced plastic deformation. The experiments have shown transition in deformation mode from global failure caused by localization in a shear band to the local failure by damage confined to few cells with reduction in relative density of specimen from a large to moderate value. The mode of deformation again changes over to the collective buckling of ligaments through row by row collapse when the relative density is decreased to a sufficiently lower level. The atomistic simulations on nanoscale cellular MGs have also reported transition from localized but confined to few cells to almost homogeneous deformation with increasing cell size. They have also shown strain localization in a dominant shear band for cell spacing along diagonal direction above a threshold value which was correlated to shear bandwidth in monolithic MG. However, it is not clear as to why and how the shear band thickness in monolithic MG controls the threshold cell-spacing. Therefore, 2D plane strain finite element (FE) simulations of compressive loading are performed on nanoscale cellular MGs using thermodynamically consistent finite strain non-local plasticity model. The present FE simulations successfully predict the two transitions in deformation mode as observed in MD simulations and experiments. It is found that the interaction stress associated with the flow defects such as shear transformation zones (STZs) plays an important role in the deformation response of cellular MGs. Results show that the transition in deformation behavior is governed by the ratio of cell-wall thickness to the intrinsic material length associated with interaction stress. Also, the moderate change in sample size has marginal effect on the deformation response of MG cellular structure. The present work may provide guidelines for designing cellular MG structures capable of showing enhanced plastic deformation for practical engineering applications.

Probing the small-scale plasticity and phase stability of an icosahedral quasicrystal i-Al-Pd-Mn at elevated temperatures

Quasicrystalline materials possess a unique structure that is ordered yet not periodic. Despite their special configuration and many useful properties, they are typically very brittle at temperatures below \ensuremath{\sim}75% of their melting points, rendering them difficult to process and often unsuitable for practical implementations. Micro-compression offers an opportunity to unveil the fundamental mechanisms of quasicrystal plasticity. Here, we study the mechanical behavior of a typical icosahedral quasicrystal (i-Al-Pd-Mn) using microthermomechanical techniques over a temperature in the range 25--500 \ifmmode^\circ\else\textdegree\fi{}C. We observe a few interesting phenomena, including micropillar shrinkage, phase transformations, grain refinement, and thermally induced transitions in deformation behavior (from brittle fracture at room temperature to serrated plastic flows and then to homogeneous flows at elevated temperatures). Furthermore, we discuss the multiple underlying mechanisms on the mechanical behavior of the quasicrystal in this temperature regime, exploring surface evaporation/diffusion, diffusion-enhanced plasticity, dislocation activities, and grain boundary rotation/sliding. Our study bridges the gap between room-temperature and high-temperature plasticity in quasicrystals, demonstrating an opportunity to study complex intermetallic phases in broad size and temperature regimes.

Probing the Small-Scale Plasticity of an Icosahedral Quasicrystal I-Al-Pd-Mn at Elevated Temperatures

Quasicrystalline materials possess a unique structure that is ordered yet not periodic. Despite their special configuration and many useful properties, they are typically very brittle at temperatures below ~75% of their melting points, rendering them difficult to process and often unsuitable for practical implementation. Here, we study the mechanical behavior of a typical icosahedral quasicrystal (i-Al-Pd-Mn) using micro-thermomechanical techniques over the temperature range of 25-500 °C, which has never been explored before. A few interesting phenomena have been observed, including micro-pillar shrinkage, phase transformations, grain refinement, and thermally induced transitions in deformation behavior (from brittle fracture to serrated plastic flows, and then to homogeneous flows). Furthermore, we discuss the multiple underlying mechanisms on the mechanical behavior of the quasicrystal in this temperature regime, exploring surface evaporation/diffusion, diffusion-enhanced plasticity, dislocation activities, and grain boundary rotation/sliding. Our study bridges the gap between room-temperature and high-temperature plasticity in quasicrystals, demonstrating a new opportunity to study complex intermetallic phases in broad size and temperature regimes.

Microstructural evolution and deformation behavior of Al-Cu alloys: A Transmission X-ray Microscopy (TXM) and micropillar compression study

A unique approach to correlating an evolving 3D microstructure in an Al-Cu alloy and its micro-scale mechanical properties has been introduced. Using these nanoscale three-dimensional microstructures derived from Transmission X-ray Microscopy (TXM), individual contributions from different strengthening mechanisms were quantified. The spatial distribution and morphology of the individual θ′ and θ phases were seen to play an important role in influencing dislocation storage. Uniaxial micro-compression experiments were used to quantify the stress-strain response of the alloy at different aging times. Transmission electron microscopy (TEM) aided in discerning dislocation activity at these precipitates. A model is proposed to accurately predict the variation in yield stress by using appropriate morphological parameters from the 3D microstructure and its validity has been corroborated using experimental measurements. Distributions of 2D and 3D inter-precipitate spacing were seen to provide crucial insights on influencing deformation in such precipitation-strengthened alloys. Finally, the transition in deformation behavior and origin of numerous strain bursts were investigated using in situ micropillar compression testing.

Notch sensitivity in nanoscale metallic glass specimens: Insights from continuum simulations

Recent experiments have shown that nano-sized metallic glass (MG) specimens subjected to tensile loading exhibit increased ductility and work hardening. Failure occurs by necking as opposed to shear banding which is seen in bulk samples. Also, the necking is generally observed at shallow notches present on the specimen surface. In this work, continuum finite element analysis of tensile loading of nano-sized notched MG specimens is conducted using a thermodynamically consistent non-local plasticity model to clearly understand the deformation behavior from a mechanics perspective. It is found that plastic zone size in front of the notch attains a saturation level at the stage when a dominant shear band forms extending across the specimen. This size scales with an intrinsic material length associated with the interaction stress between flow defects. A transition in deformation behavior from quasi-brittle to ductile becomes possible when this critical plastic zone size is larger than the uncracked ligament length. These observations corroborate with atomistic simulations and experimental results.

Transition of deformation behavior and its related microstructure evolution in Nimonic 80A under hot-to-warm working

Transitions of deformation behavior and microstructure evolution were observed in Nimonic 80A during hot-to-warm compression from 800°C to 1180°C (0.67Tm–0.91Tm) at a constant strain rate of 1s−1. The relationship between the peak stress (σp) and the deformation temperature was found to be bilinear with the slope of −4.0 in warm working range (800°C–950°C) and −1.0 in hot working range (1100°C–1180°C). The peak stress is much more temperature-dependent in warm working range than in hot working range, which is attributed to the increase of deformation activation energy (Q) from 175kJ/mol to 932kJ/mol. After deformation to a large strain of 1.1, homogeneous grain structures obtained above 1000°C gradually changed into inhomogeneous ones consisting of deformed initial grains as the temperature decreased into warm working range. The dynamic recrystallized grain size (DR) can be related to the peak stress by an empirical power–law function of σpGDRn=K. The grain size exponents (n) are −0.39 and −0.71 for warm compression (800°C–950°C) and hot compression (950°C–1180°C), respectively. These transitions of deformation behavior and microstructure evolution with the temperature variation are associated with the change of the operating dynamic recrystallization (DRX) mechanism. In hot working, new grains form via the bulging of initial grain boundaries (i.e. discontinuous DRX), while in warm working, new grains develop from subgrains via the gradual increase of sub-boundary misorientations into high angle type upon straining (i.e. continuous DRX). There is a temperature range for the transition in Nimonic 80A, which is approximately from 950°C to 1100°C where both DDRX and CDRX take place during the deformation.

On Temperature and Strain Rate Dependent Strain Localization Behavior in Ti–6.5Al–3.5Mo–1.5Zr–0.3Si Alloy

Deformation behaviors of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloys with α/β lamellar structure were investigated systematically at different temperatures from room temperature to 950 °C and different strain rates. Results reveal that when the deformation temperature is higher than a critical temperature of 600 °C, an evident transition of deformation behavior from localized shear banding to α/β lamella kinking, flow softening and temperature/strain rate-dependent peak flow stress occurred in the alloy. The critical conditions for the occurrence of internal cracking and strain localization behavior associated with temperature and strain rate were determined.

Transition of deformation behavior in an ultrafine grained magnesium alloy

Abstract The deformation behavior of friction stir processed magnesium alloy with average grain size ranging from 25 μm to 240 nm was studied. An evaluation of critical grain size for transition of deformation mode from twinning to slip was carried out by employing tensile and compressive tests. The role of critical grain size in transition of deformation behavior from strain hardening to softening was established. Inhomogeneous yielding was observed during tensile test of ultrafine-grained magnesium alloy below the critical grain size.

Catastrophic vs Gradual Collapse of Thin-Walled Nanocrystalline Ni Hollow Cylinders As Building Blocks of Microlattice Structures

Lightweight yet stiff and strong lattice structures are attractive for various engineering applications, such as cores of sandwich shells and components designed for impact mitigation. Recent breakthroughs in manufacturing enable efficient fabrication of hierarchically architected microlattices, with dimensional control spanning seven orders of magnitude in length scale. These materials have the potential to exploit desirable nanoscale-size effects in a macroscopic structure, as long as their mechanical behavior at each appropriate scale - nano, micro, and macro levels - is properly understood. In this letter, we report the nanomechanical response of individual microlattice members. We show that hollow nanocrystalline Ni cylinders differing only in wall thicknesses, 500 and 150 nm, exhibit strikingly different collapse modes: the 500 nm sample collapses in a brittle manner, via a single strain burst, while the 150 nm sample shows a gradual collapse, via a series of small and discrete strain bursts. Further, compressive strength in 150 nm sample is 99.2% lower than predicted by shell buckling theory, likely due to localized buckling and fracture events observed during in situ compression experiments. We attribute this difference to the size-induced transition in deformation behavior, unique to nanoscale, and discuss it in the framework of "size effects" in crystalline strength.

Transition in deformation behavior of martensitic steel during large deformation under uniaxial tensile loading

More than 50% elongation of as-quenched martensite was achieved under uniaxial tensile loading using a multilayered structure combining martensitic and austenitic steel, and the deformation behavior of as-quenched martensite was observed in situ. It was demonstrated that work-hardening of as-quenched martensite is enhanced as applied strain exceeds a certain level, 15–20%, and that the transition from poor work-hardening to improved hardening is associated with a shift in the deformation mode of martensite.

Strengthening and softening of nanocrystalline nickel during multistep nanoindentation

Multistep load-unload nanoindentation was employed to address the effect of deformation-induced microstructural evolution on mechanical behavior of nanocrystalline Ni. Deformation discontinuity was deliberately introduced by unloading-reloading during nanoindentation testing, which allows us to examine the influence of microstructural evolution on the successive deformation. Strain strengthening/softening of nanocrystalline nickel, associated with the transition of deformation behavior from dislocation activity at high loading rates to a grain-boundary-mediated process at low loading rates, was uncovered by means of this experimental methodology.

X-ray line-broadening analysis of deformation mechanisms during rolling of commercial-purity titanium

X-ray line-broadening techniques were used to establish deformation mechanisms in textured commercial purity (CP) titanium specimens that were rolled at various temperatures. The results indicated that conventional line-broadening techniques developed specifically for polycrystalline powders could be used on textured materials to deduce the slip-system activity and dislocation density that develop during thermomechanical processing if appropriate averaging procedures are followed. The deduced slip-system activity and densities revealed a transition in deformation behavior from a heavily-twinned to an untwinned mode with an increase in deformation temperature for CP titanium.

The effect of grain morphology on longitudinal creep properties of INCONEL MA 754 at elevated temperatures

The longitudinal creep behavior of two heats of coarse grained INCONEL* MA 754 have been examined at temperatures of 1000 °C and above. Both heats exhibit a pronounced transition in deformation behavior. At high stresses, dislocation creep is observed and high stress exponents (n ∼40) are measured. Fracture in this regime is transgranular with high creep ductilities. At lower stresses, the stress exponents are low and fracture is intergranular. In this regime, the stress exponent depends strongly on the grain morphology. Heat 1, with a uniform fiber grain morphology, exhibits significantly higher stress exponents than Heat 2, which has a duplex grain morphology consisting of coarse grains along with pockets of fine, equiaxed grains. Microstructural examination of specimens deformed at the lower stresses provides evidence that cavitation of the transverse grain boundaries occurs by means of diffusive cavity growth. In the heat with the uniform fiber morphology, cavity growth is constrained by creep of adjacent grains. Cavity growth for the heat with the duplex grain morphology is apparently limited by the sliding of pockets of fine grains. The implications of these results for optimizing creep resistance of MA 754 are discussed.

Instabilities of the deformation process in cold drawing of poly(ethylene terephthalate) and other polymers

AbstractThe relationship between drawing rate and drawing stress was studied for amorphous poly(ethylene terephthalate) (PET) under various experimental conditions. Three types of expriments were performed: simple drawing with necking at constant rate, drawing through a conical die, and drawing at constant stress. Under constant stress conditions a transition between two stable regimes of drawing can be observed. The transition occurs at a critical stress σc at which the rate of neck propagation changes by some orders of magnitude. Such a transition was found both below and above the glass transition of PET. With constant drawing rates instabilities of neck propagation were observed under certain experimental conditions. Such self‐oscillations, described by other authors, are not due to heat effects as has been proposed, but are related to the existence of the critical stres σc.Stress‐induced transitions in deformation behavior as in PET were observed for polypropylene and nylon 6 but not for polycarbonate. The results obtained by various methods including morphological studies do not support the assumption that the instabilities are caused by thermal effects due to the dissipation of deformation energy. Rather, a model is proposed which is based on the existence of a “spinodal transition” from the isotropic into the highly oriented state.