Serrated Plastic Flow Research Articles

Serrated plastic flow in deforming complex concentrated alloys: universal signatures of dislocation avalanches

Under plastic flow, multi-element high/medium-entropy alloys (HEAs/MEAs) commonly exhibit complex intermittent and collective dislocation dynamics owing to inherent lattice distortion and atomic-level chemical complexities. Using atomistic simulations, we report on an avalanche study of model face-centered cubic (fcc) NiCoCrFeMn and NiCoCr chemically complex alloys aiming for microstructural/topological characterization of associated dislocation avalanches. The results of our avalanche simulations reveal a close correspondence between the observed serration features in the stress response of the deforming HEA/MEA and the incurred slip patterns within the bulk crystal. We show that such correlations become quite pronounced within the rate-independent (quasi-static) regime exhibiting scale-free statistics and critical scaling features as universal signatures of dislocation avalanches.

Ultra-Large Compressive Plasticity of E-Ga2O3 Thin Films at the Submicron Scale.

Gallium oxide (Ga2O3) usually fractures in the brittle form, and achieving large plastic deformability to avoid catastrophic failure is in high demand. Here, ε-Ga2O3 thin films with columnar crystals and partial unoccupied Ga sites are synthesized, and it is demonstrated that the ε-Ga2O3 at the submicron scale can be compressed to an ultra-large plastic strain of 48.5% without cracking. The compressive behavior and related mechanisms are investigated by in situ transmission electron microscope nanomechanical testing combined with atomic-resolution characterizations. The serrated plastic flow and large strain burst are two major deformation forms of ε-Ga2O3 during compression, which are attributed to the dislocation nucleation and avalanches, formation of new grains, and amorphization. The ultra-large compressive plasticity of ε-Ga2O3 thin films at the submicron scale can inspire new applications of Ga2O3 in micro- or nano- electronic and optoelectronic devices, especially those that require impact resistance during processing or service.

Direct aging of additively manufactured A20X aluminum alloy

This study investigates the effect of direct aging on a newly developed high-performance aluminum alloy A20X for laser powder bed fusion. Three different temperatures (180 °C, 200 °C, and 220 °C) and nine different heat treatment durations (5 min to 30 h) were considered. The material exhibited significant softening (up to 23.7 % hardness drop) after direct aging. The extent of softening was proportional to the temperature and duration of heat treatment. During direct aging, incoherent Al2Cu (θ) precipitates ({110}θ ǁ {220}Al) coarsened and the volume fraction remained constant (4.52–4.63 %). The average precipitate size grew by up to ∼100 % through direct aging and no metastable precipitates (such as θ′ or Ω) formed, suggesting the absence of supersaturated matrix in as-printed A20X. Accordingly, the yield and tensile strengths of the alloy dropped by up to 27 % and 19.4 %, respectively. The continuous Mg/Ag solute wall observed at grain boundaries in the as-printed state caused serrated plastic flow due to the locking/unlocking of dislocations. Longer aging treatments generated discrete solute co-clusters along the grain boundaries and TiB2/Al interface promoting smoother plastic flow.

On microstructure and work hardening behavior of laser powder bed fused Al-Cu-Mg-Ag-TiB2 and AlSi10Mg alloys

In this paper, we investigate the correlation between microstructure and mechanical properties, including work hardening rates of two aluminum alloys, Al-Cu-Mg-Ag-TiB2 (A205) and AlSi10Mg, both additively manufactured using laser powder-bed fusion (LPBF) technique. Solutionizing followed by over-aging (T7) and artificial aging (T6) heat treatments were performed on as-built specimens of A205 and AlSi10Mg alloys, respectively, to investigate and compare the evolution of microstructure and tensile properties. Tensile tests based on ASTM E8/E8M standard were performed systematically on both materials in both as-built and heat-treated conditions. High-magnification microstructural characterizations including scanning electron and transmission electron microscopies and electron backscattered diffraction were performed to identify the microstructure-property correlations. The A205 alloy contains an ultrafine uniform equiaxed grain structure with no cell and/or epitaxial growth structure. However, the AlSi10Mg contains a cellular microstructure of primary α‐Al cells surrounded by a continuous cellular Si network. Tensile results on A205 (as-built) revealed discontinuous yielding, while the A205 (T7 HT) material serrated plastic flow instability. T7 heat treatment resulted in a 42.4% and 38% increase in yield stress (YS) and ultimate tensile strength (UTS), respectively, and elongation decreased from 12.5% to 9.5% consequently. However, considerable grain growth, coarsening of Si particles, and dissolution of some Mg2Si phases through solutionizing and T6 aging on AlSi10Mg resulted in increased elongation (5.49–14.20%) and decreased YS and UTS (14.5% and 19.4% respectively). The effect of distinct microstructural features of A205 and AlSi10Mg in as-built condition and their evolution in opposite directions upon aging (in terms of mechanical properties) is reflected in their work-hardening behavior. Considerably higher work hardening of AlSi10Mg alloy in both as-built and aged conditions compared to A205 is linked with the cellular structure containing hard eutectic-Si particles at the cell boundaries.

On the plastic deformation of a CoCrFeNiW-C alloy at elevated temperatures: Part I. Serrated plastic flow and its latent dynamics

Multi-component Co-alloys are a class of promising metallic materials for high-temperature applications. The present work primarily focuses on the plastic deformation mechanisms of a CoCrFeNiW-C alloy at 650 °C, with emphases on dislocation slip intermittency and plastic flow features. To this end, by integrating in situ scanning electron microscopy-based tests, statistical analyses, and theoretical calculations, several mechanistic insights are revealed. In this material, the plastic flow is featured by evident serration events with a self-organized critical dynamic feature, where mixed Type A and B serrations operate at a moderate deformation level, followed by the onset of a higher magnitude Type B serration. As temperature increases up to 700 and 750 °C, although more evident Type C serration occurs, the self-organized criticality along with the spatial-temporal power-law scaling relation remains nearly unaffected. Microstructural relevance of the serration mechanisms and the deformation substructural characteristics are also explored in greater depth.

High temperature deformation behavior of Al0.2CoCrFeNiMo0.5 high entropy alloy: Dynamic strain ageing

High temperature deformation behavior of Al0.2CoCrFeNiMo0.5 high entropy alloy was investigated in fully annealed condition. Microstructural studies were performed using SEM and EBSD, prior to testing, clearly shows a fully annealed microstructure consists of secondary sigma phase in the FCC matrix. Uniaxial tensile tests were performed in the temperature range of 25–900 °C at an initial strain rate of 10−3 s−1. Serrated plastic flow behavior was observed in the intermediate temperature range of 400–600 °C. The influence of the strain rate on the critical strain corresponding to the onset of serrations were studied which indicated that the serration behavior was type A and type C. Strain rate sensitivity close to zero, decrease in the elongation and microstructural investigations suggesting that dynamic strain aging (DSA) is the responsible mechanism for the serrated flow and that the Al solute interactions with dislocations might be the possible reason for DSA. Additionally, the fractography analysis clearly indicates that there is transition in the failure mode from the transgranular fracture to the intergranular fracture.

Correlations between the plastic instabilities occurring in indentation and the Portevin Le-Chatelier effect in uniaxial tension

Instrumented indentation has demonstrated the capability of characterizing plastic instabilities at small scales. Nevertheless, due to the complex nature of stress and strain, it is challenging to use the results from indentation for quantitative evaluations of the Portevin–Le Chatelier (PLC) effect and the dynamic strain aging mechanisms. Here, we addressed this problem by correlating the plastic instabilities occurring in indentation to the PLC effect in uniaxial tension. A twinning-induced plasticity steel with notable PLC effect was used as the model material. Indentation tests were systematically carried out on single grains of a coarse-grained sample over a wide range of indentation strain rates from 5 × 10−4 to 100 s−1. For a direct comparison, uniaxial tensile tests coupled with digital image correlation were conducted on bulk polycrystalline samples at the equivalent uniaxial strain rates. It is demonstrated that indentation provides similar results for the respective ranges of strain rates for serrated plastic flow and negative strain rate sensitivity (nSRS), the value of nSRS, and the rate dependence of serration size, compared to those by uniaxial tension. In contrast to uniaxial tension, serrations in indentation may become stochastic at lower rates due to the limited sample volume. In addition, individual indentation serrations were also analyzed to reveal the physics behind their formation and rate dependent characteristics.

Achieving high tensile strength-ductility synergy of a fully-lamellar structured near alpha titanium alloy at extra-low temperatures

The previous study has indicated that a near alpha titanium alloy of an equiaxed microstructure could hardly exhibit a desirable ductility at 20 K. This study aimed at achieving a promising balance of superhigh strength and excellent ductility of a Ti−3Al−3Mo−3Zr (wt%) alloy in cryogenic temperature field. Temperature dependence of tensile properties and deformation behavior was investigated systematically. It was found that the ultimate tensile strength (UTS) and elongation-to-fracture (EI) jumped from 730 MPa and 19.0% at 298 K to 1300 MPa and 23.0% at 77 K, respectively. Ultrahigh UTS up to 1600 MPa and an acceptable EI of 13.5% were achieved at 20 K. The ultrahigh UTS at 77 and 20 K was considered to be attributed to the sharp increase of CRSS of slip and the Hall-Petch effect of {101̅2}< 101̅1̅> twinning. The enhanced ductility at 77 K was owing to the improved strain hardening ability from large-scale twinning and prismatic and basal slips. Besides, a serrated plastic flow took place at 20 K, which was essentially because the intermittent start of twinning brought about alternate stress jump and cliff-like drop. A fully-lamellar microstructure is suggested to synchronously enhance the strength and ductility of a near-alpha titanium alloy at extra-low temperatures.

Micro-mechanical deformation behavior of CoCrFeMnNi high-entropy alloy

In the present study, the micro-mechanical behavior of CoCrFeMnNi high-entropy alloy was investigated using an in-house micro-tensile setup at room temperature and 550 °C at different strain rates. Micro-mechanical properties are compared with those obtained using a commercial macro-tensile setup to check a potential sample size effect. Results show that mechanical properties such as yield strength, ultimate tensile strength and uniform elongation are independent of the sample size. However, the total elongation-to-failure of micro-samples is found to be lower than those of macro-counterparts. Apart from this, the material exhibits serrated plastic flow, which is strain rate dependent in terms of the onset strain and shape of serrations at 550 °C. Furthermore, transmission electron microscopy investigations were performed to correlate the occurrence of serrations to the observed distinct dislocation structures. Microstructural results provide direct evidence that dislocations are curved and hence effectively pinned and unpinned at the lowest applied strain rate, which might be responsible for the occurrence of serrated plastic flow.

A critical dislocation velocity for serration mechanism transition in a nickel-chromium solid solution alloy

The influence of strain rate across three orders of magnitude (1.70 × 10−5/s to 1.43 × 10−2/s) along with the effect of the plastic strain accumulation (up to 10%) on the serrated plastic flow were investigated in the nickel-chromium (Ni-Cr) solid solution alloy Nimonic 75 by performing constant-strain-rate tension testing at 600 °C. As the strain rate decreased, the critical strain for the onset of serrations transitioned from normal behavior to inverse behavior. The serrated flow was characterized as Type A+B serration at high strain rate (1.43 × 10−2/s). In the intermediate strain-rate regime (1.43 × 10−3/s and 1.45 × 10−4/s), Type B serrations were observed and followed by a transformation to Type C+B serrations. At the low strain rate (1.70 × 10−5/s), the plastic flow immediately displayed Type C serrations, which later evolved into Type C+B serrations. Regardless of the strain rate, plastic strain, or dislocation density, a critical dislocation velocity falling in the range of 1.2 × 10−6 – 2.2 × 10−6 m/s was identified to signify the onset of Type C serration, whereby the mobile dislocations break free from the solute cloud for short bursts of deformation. Finally, a novel model by solute rearrangement across dislocation cores was used to understand how the critical dislocation velocity is quantitatively determined by the rate at which solute atoms are able to hop across the glide plane as a partial dislocation core moves through the lattice.

In-situ synthesis of Mg-based bulk metallic glass matrix composites with primary α-Mg phases

Herein, we systematically investigate the in-situ synthesis and mechanical properties of Mg-based bulk metallic glass matrix composites (BMGMCs). From a well-known Mg65Cu25Gd10 bulk glass former, a series of Mg65+xCu20–2x/3Zn5Gd10−x/3 (x = 0–18 at%) alloys are systematically designed and Mg-rich (>77 at% Mg) glassy/crystalline matrix composites containing primary α-Mg phases are successfully developed. Mg77Cu12Zn5Gd6 BMGMC (x = 12 at%) exhibits over two times higher compressive fracture strength (773 ± 20.6 MPa) and specific strength (2.6 × 105 N m kg−1) than commercial Mg alloys such as AZ31 or AZ91. In particular, compared with monolithic Mg-based BMGs, the BMGMC displays obvious yielding, serrated plastic flow and plastic strain of 0.22 ± 0.02%. This result is attributed to the dispersed primary α-Mg phases which prevent the rapid propagation of shear bands and promote the formation of multiple shear bands. The Mg-rich crystalline matrix composites (x = 14-18 at%) containing primary α-Mg phases exhibit an enhancement in the plastic strain (from 0% to 1.51%) and pronounced work hardening at the expense of yield strength (<500 MPa) due to the absence of the amorphous phase. These results would give us a promising strategy to fabricate Mg-based BMGMCs with high specific strength and enhanced plasticity for lightweight structural applications.

Plastic instability in Ti–6Cr–5Mo–5V–4Al metastable β-Ti alloy containing the β-spinodal decomposition structures

Metastable β-titanium (Ti) alloys have been extensively used in aerospace industry owing to the outstanding mechanical properties. Temperature arising sometimes becomes unavoidable in service and thus it is necessary to evaluate plastic behavior of the alloys at elevated temperatures prior to applications. Here, plastic stability of metastable β-titanium alloys containing the β-spinodal decomposition structures at different temperatures is investigated for the first time using Ti–6Cr–5Mo–5V–4Al alloy as prototypical material. It is found that a serrated plastic flow with little frequency accompanied by overall strain softening appears in the tensile stress-strain curves at 298 K, while the serrated plastic flow occurs much more frequently and overall strain hardening capability is recovered appropriately when the tensile temperature is increased to 573 K. Microstructural characterizations reveal that ordinary dislocation slip dominates the alloy plasticity at all the testing temperatures. Dislocations strongly interact with the β-striations and dislocation debris are left around the striation interfaces. The resulting local stress accumulation destroys the β-striations in the form of shear bands so that sudden release of an avalanche of dislocations in the soft channels leads to stress drop. Subsequent dynamic reconstruction of the β-striations with the assistance of thermal activation especially at elevated temperature hardens the prior soft channels, which results in the following stress rise. The striking difference on deformation characteristics at 298 K and 573 K is that more homogeneous plasticity conferred by dense deformation bands at the latter, which is the direct reason on high serrated frequency and overall strain hardening. These findings offer valuable insights into plastic behavior of metastable β-Ti alloys, which will have important implications for the applications.

Dynamic strain aging behavior of accident tolerance fuel cladding FeCrAl-based alloy for advanced nuclear energy

The Fe-13Cr-4Al alloy has become a promising candidate material for accident tolerance fuel (ATF) cladding of light-water reactors (LWRs) due to its excellent oxidation resistance to high-temperature water vapor. However, the tensile deformation behavior of the Fe-13Cr-4Al alloy under different strain rates at different temperatures is still unclear. In the present study, the tensile behavior and deformed microstructure of the Fe-13Cr-4Al alloy were investigated at strain rates from 5 × 10–4 to 1 × 10–2 s−1 in the temperature range from RT to 600 °C. Serrations were observed in the tensile engineering stress–strain curves of the intermediate temperature ranging from 300 to 450 °C at all the three strain rates, indicating the occurrence of dynamic strain aging (DSA). As the strain rate increased, the temperature range where serrated plastic flow occurred shifted to the high temperature regime. Serrated plastic flow occurred after a certain critical plastic strain, and the critical plastic strain decreased with the increase in temperature and increased with the increase in strain rate. In the DSA regime of the Fe-13Cr-4Al alloy, the plateau in the yield strength, ductility minima, negative strain rate sensitivity, the peaks of strain hardening exponent and work hardening rate were observed, which were the typical manifestations of the DSA. The activation energies of serrated plastic flow evaluated by three different methods were 157 ± 35, 92 ± 10 and 93 ± 10 kJ/mol, respectively. According to the values of activation energy, the controlling mechanism responsible for the DSA of the Fe-13Cr-4Al alloy was found to be the interaction between substitutional aluminum atoms and dislocations. The fracture surfaces of the specimens tested at 400 °C under three strain rates showed the mixed fracture mode, which contained dimples and cleavage facets. The tensile samples at 600 °C showed a completely ductile fracture mode with large and deep dimples. A large number of dislocation tangles in the microstructure of the specimens tested at 400 °C under all the strain rates were observed, with a large amount of Fe2Nb Laves particles along the grain boundaries and in the matrix. Dislocation pile-up was also observed around the grain boundaries and second-phase particles. With the increase in the strain rate, the number of subgrains increased, and a clear dislocation cell structure was also observed.

Dynamic strain aging and serrated flow behavior of Cr-Ti-B low carbon steel during warm deformation

The compression behavior of Cr-Ti-B low carbon steel was studied over the temperature range from 250 to 650 °C at three strain rates: 0.01 s−1, 0.1 s−1, and 1 s−1. Moreover, the serrated plastic flow was analyzed during compression, which revealed that type A serrations occurred at high strain rates, while types B or B + C serrations formed at low strain rates. The temperature range (250–350 °C) of negative strain rate sensitivity (NSRS) was slightly lower than that (300–400 °C) of dynamic strain aging (DSA). A pronounced increase of peak stress was observed at 350 °C deformation temperature and a 1 s−1 strain rate, which was attributed to the interactions between the interstitial atoms or atmospheres and movable dislocations. The average diffusion activation energy (73 kJ/mol) at low temperature (250–350 °C) also indicated that the serration mechanism was associated with the diffusion of interstitial atoms. The serration behavior at high temperatures (550–650 °C) was attributed to the pinning of dislocations by precipitates. The steel tended to exhibit planar slip of dislocations in the DSA region, and dynamic recovery was dominant during deformation at 550 °C and above.

Size Polydispersity Tunes Slip Avalanches of Granular Gouge

AbstractGranular materials have frequently been used as representations of natural fault gouges. Although they can reproduce proper avalanche behaviors, the universality of the scaling exponent of avalanche size remains debatable. As a core issue in both amorphous plasticity and geophysics, avalanche universality may help reconcile the avalanche behaviors of earthquake and granular materials into the same universality class. We examine numerically the signatures of stress avalanches emerging from quasi‐static shear of granular materials with different size polydispersity. A persistent serrated plastic flow phenomenon is observed in our models with varying polydispersity. The stress drop is well defined by a truncated power law distribution P(s) ~ s−τexp(−s/smax). The exponent τ and cutoff stress drop smax show a clear dependence on polydispersity, which reflects a tuned criticality. We further calculate the effective temperature from the statistics of energy fluctuations. The effective temperature volatility can be used to explain the tuned critical behaviors of granular gouge.

Spatial correlation and temporal evolution of plastic heterogeneity in sheared granular materials

Granular materials will yield and undergo serrated plastic flow when subjected to loading. The serrated plastic flow is related to the spatial concentration and temporal evolution of the microscopic plastic events. Limited by the temporal resolution of experimental techniques available, the initialization and localization of the plastic events within timescale much less than the structural relaxation time is still poorly understood. We use combined finite and discrete element method to model the simple shearing of an assembly of irregularly shaped particles. At small temporal scales, nonaffine motions originate at particles experiencing large stress fluctuations. The cooperative dynamics and long-ranged dynamic heterogeneity is responsible for the annihilation and merging of plastic zones at the timescale close to the structural relaxation time, and the plasticity is manifested as irreversible rearrangement at large enough strain scales. The spatiotemporal variability of plastic activity is confirmed by the transient nature of the high mobile clusters.

Spatio-temporal dynamics of jerky flow in high-entropy alloy at extremely low temperature

ABSTRACT Despite a large body of literature, mechanisms contributing to low temperature jerky flow remain controversial. Here, we report a cross-over from a smooth at room and liquid nitrogen temperatures to serrated plastic flow at 4.2 K in high-entropy CrMnFeCoNi alloy. Several complimentary investigations have been carried out to get a coherent physical picture of low temperature jerky flow in these alloys. Microstructural characterisations at 77 K and 4.2 K show that the number of Lomer-Cottrell (L-C) locks at 4.2 K is much higher than that at 77 K, inducing stronger barriers for dislocation glide at 4.2 K. A stability analysis shows that the jerky flow results from an interaction between dislocation inertial motion with L-C locks. The instability results from a competition between inertial and viscous time scales characterised by a Deborah number. A detailed nonlinear time series analysis of experimental serrated stress signals shows that jerky flow is chaotic characterised by the existence of a finite correlation dimension and a positive Lyapunov exponent. Further, the minimum degree of freedom required for the chaotic dynamics turns out to be four, consistent with four collective modes degrees of freedom used in our model equations. These results highlight the crucial ingredients for jerky flow at liquid helium temperatures.

Dislocation-based serrated plastic flow of high entropy alloys at cryogenic temperatures

Serrated plastic deformation at temperatures close to 0 K has been previously reported in some metals and alloys, and is associated with two possible origins: (i) thermomechanical instability or (ii) mechanical instability. While some recent results indicate that serrations are a mechanical dislocation-based phenomenon, a comprehensive model does not exist. CoCrFeMnNi, an expectedly ideal candidate, exhibits severe serrated plastic deformation with large stress drops in excess of 100 MPa. Furthermore, it also shows cryogenic serrated plastic deformation at a higher temperature (35 K) than previously reported for any other alloy. The exacerbated nature of serrated plastic deformation in CoCrFeMnNi led to the following inferences: (i) temperature and dislocation density are indisputable controlling parameters for cryogenic serrated plastic deformation and they cannot supersede each another; (ii) a phenomenological model is elucidated based on the increasing difficulty for cross-slip with decreasing temperature, leading to sudden massive dislocation proliferation event; (iii) the model establishes a gradual transition from completely non-serrated to completely serrated deformation, mediated by cross-slip, as opposed to the conventional model which proposed a discrete transition; (iv) solute dislocation interaction and associated Stacking Fault Energy (SFE) during deformation plays a key role in controlling dislocation constriction and cross-slip and correspondingly serrated plastic deformation; (v) the need/direct influence of deformation twinning, transformation induced plasticity and especially thermomechanical factors on serrated plastic deformation is invalidated. Some of these points were further clarified through comparisons with CoCrNi and CoNi, also presented in the present article.

Beyond Serrated Flow in Bulk Metallic Glasses: What Comes Next?

This manuscript is based on an oral contribution to the TMS 2020 annual meeting and is dedicated to Prof. Peter Liaw, who for decades has shown great interest in serrated plastic flow. Here we will focus on the case of bulk metallic glasses, and begin with briefly summarizing some aspects of serrated and non-serrated inhomogeneous flow—a phenomenon that has perplexed materials scientists for decades. Four directions of research are identified that emerged out of the desire to fundamentally understand the intermittent inhomogeneous flow response. These research directions gear away from the phenomenological stress–strain behavior but put the underlying shear defect into focus. Unsolved problems and future research topics are discussed.

Nanoscale serration characteristics of additively manufactured superalloys

Structural elements made of nickel-based superalloys usually operate at high temperatures. Many surface failure mechanisms such as creep, fatigue, fretting fatigue, corrosion, and stress corrosion cracking start from the surface. Thus, studying the surface mechanical properties and deformation behavior of materials is vital in order to draw a correlation between the surface properties and failures. Herein, a nanoindentation technique was implemented to characterize surface properties of a widely used additively manufactured (AM) superalloy, i.e., Inconel 625. The specimens were tested in a range of temperatures in a vacuum chamber. Serrated plastic flow characterized by pop-in events during nanoindentation, known as Portevin-Le Chatelier (PLC) effect, was observed in all three tested elevated temperatures. This phenomenon depicts itself as bursts of plasticity in the loading section of the load-displacement curves. These bursts were studied comprehensively to explore incipient plasticity. Hertzian contact mechanics were implemented to extract the maximum shear stress beneath the indenter from the pop-in loads. The results show that the initial pop-in load increases as the temperature increases. The average initial pop-in load increases by almost four times from 300 °C to 650 °C. This was attributed to the formation of strengthening precipitates. The average size of the serrations and the magnitude of dislocation nucleation increase from 300 °C to 500 °C and decrease from 500 °C to 650 °C. The serrations are attributed to the dislocation generation and movement as well as their interaction with the solute atoms and precipitates.