Serrated Flow Research Articles

Benefits of curved serrations on broadband trailing-edge noise reduction

Far-field noise and flow field over a novel curved trailing-edge serration (named as iron-shaped serration) are investigated. Spectra of the far-field broadband noise, directivity plots and the flow-field over the iron-shaped serration are obtained from numerical computations performed using a compressible Lattice-Boltzmann solver. The new design is compared to a conventional trailing-edge serration with a triangular geometry. Both serration geometries were retrofitted to a NACA 0018 airfoil at zero degree angle of attack. The iron-shaped geometry is found to reduce far-field broadband noise of approximately 2dB more than the conventional sawtooth serration for chord-based Strouhal numbers Stc<15. At higher frequencies, the far-field broadband noise for the two serration geometries has comparable intensity. Near-wall velocity distribution and surface pressure fluctuations show that their intensity and spectra are independent on the serration geometry, but a function of the streamwise location. It is found that the larger noise reduction achieved by the iron-shaped trailing-edge serration is due to the mitigation of the scattered noise at the root. This effect is obtained by mitigating the interaction between the two sides of the serration, by delaying toward the tip both the outward (i.e., the tendency of the flow to deviate from the centerline to the edge of the serration) and the downward (i.e., the tendency of the flow to merge between the upper and bottom side of the serration) flow motions present at the root of the sawtooth.

Effect of SPD processing on mechanical behavior and dynamic strain aging of an Al-Mg alloy in various deformation modes and wide strain rate range

This work focuses on the effect of grain size and deformation mode on mechanical behavior and dynamic strain aging (DSA) process in an Al5083 alloy. The UFG material with average grain size of 0.4µm was produced by equal channel angular pressing with parallel channels (ECAP-PC) which resulted in dramatic improvement of mechanical strength while reducing tensile ductility of the alloy. Biaxial stretching formability of the UFG alloy is similar to that of its coarse-grained (CG) counterpart. Analysis of plastic flow in the strain rate range of 10−5–2.5·103s−1 showed that DSA process occurs in both CG and UFG alloys in the strain rate range of 10−5–10−3s−1, whereas it is suppressed at higher strain rates 10−1–2.5·103s−1 due to lack of time for solute atoms to ‘arrest’ gliding dislocations. The ECAP-PC processing also modified the DSA process, reducing the absolute value of negative strain rate sensitivity and activity of localized plastic flow, as well as changing the type of flow serrations on the stress-strain curves. These observations are rationalized based on grain refinement and reduction of concentration of solute atoms in the grain refinement due to their segregations at grain boundaries, which occurs during severe plastic deformation of Al alloys. An effect of deformation mode on the DSA process is also reported.

Improved Soft Abrasive Flow Finishing Method Based on Turbulent Kinetic Energy Enhancing

Soft abrasive flow(SAF) finishing can process the irregular geometric surfaces, but with the matter of low processing efficiency. To address the issue, an improved SAF finishing method based on turbulent kinetic energy enhancing is proposed. A constrained flow passage with serration cross-section is constructed to increase the turbulence intensity. Taking the constrained flow passage as the objective, a two-phase fluid dynamic model is set up by using particle trajectory model and standard k-e turbulence model, and the flow field characteristics of the flow passage are acquired. The numerical results show that the serration flow passage can enhance the turbulence intensity, uniform the particles distribution, and increase the particle concentration near the bottom wall. The observation results by particle image velocimetry(PIV) show that the internal vortex structures are formed in flow passage, and the abrasive flow takes on turbulence concentrating phenomenon in near-wall region. The finishing experiments prove that the proposed method can obtain better surface uniformity, and the processing efficiency can be improved more 35%. This research provides an abrasive flow modeling method to reveal the particle motion regulars, and can offer references to the technical optimization of fluid-based precision processing.

Change in the deformation mode resulting from beta-omega compositional partitioning in a Ti[sbnd]Mo alloy: Room versus elevated temperature

The effect of beta-omega compositional partitioning, introduced via isothermal annealing, on the deformation behavior of a TiMo alloy was investigated via tensile testing at room and elevated temperatures. While beta with compositionally congruent athermal omega precipitates primarily exhibits twinning, after beta-omega compositional partitioning the same alloy deforms via slip. This change in deformation mode can be rationalized based on the solute content within the beta matrix, with reference to the Bo-Md plot. Tensile testing at elevated temperatures revealed that both conditions deformed via slip while exhibiting serrated flow.

Bridging room-temperature and high-temperature plasticity in decagonal Al–Ni–Co quasicrystals by micro-thermomechanical testing

Ever since quasicrystals were first discovered, they have been found to possess many unusual and useful properties. A long-standing problem, however, significantly impedes their practical usage: steady-state plastic deformation has only been found at high temperatures or under confining hydrostatic pressures. At low and intermediate temperatures, they are very brittle, suffer from low ductility and formability and, consequently, their deformation mechanisms are still not clear. Here, we systematically study the deformation behaviour of decagonal Al–Ni–Co quasicrystals using a micro-thermomechanical technique over a range of temperatures (25–500 °C), strain rates and sample sizes accompanying microstructural analysis. We demonstrate three temperature regimes for the quasicrystal plasticity: at room temperature, cracking controls deformation; at 100–300 °C, dislocation activities control the plastic deformation exhibiting serrated flows and a constant flow stress; at 400–500 °C, diffusion enhances the plasticity showing homogenous deformation. The micrometer-sized quasicrystals exhibit both high strengths of ~2.5–3.5 GPa and enhanced ductility of over 15% strains between 100 and 500 °C. This study improves understanding of quasicrystal plasticity in their low- and intermediate-temperature regimes, which was poorly understood before, and sheds light on their applications as small-sized structural materials.

Aspect ratio effects on the serration dynamics of a Zr-based bulk metallic glass

Based on the constant strain rate compressive experiments to a Zr-based bulk metallic glass, the aspect ratio effects on the statistical properties of serration is systematically investigated. In the plastic deformation state, as the aspect ratio decreases, the stress drop magnitude increases dramatically and the reloading time in one serration event increases significantly. Moreover, distribution of the elastic energy density of each serration event is predicted, which obeys the squared exponential decay law. The size effect law is adopted to describe the serration dynamic-related parametric variation with different aspect ratios. It is noted that the scaling exponent reduces with the increase of the aspect ratio. In general, the effect of aspect ratio on the serration flow could be explained by the shear-band stability index.

Self-Similar Random Process and Chaotic Behavior In Serrated Flow of High Entropy Alloys.

The statistical and dynamic analyses of the serrated-flow behavior in the nanoindentation of a high-entropy alloy, Al0.5CoCrCuFeNi, at various holding times and temperatures, are performed to reveal the hidden order associated with the seemingly-irregular intermittent flow. Two distinct types of dynamics are identified in the high-entropy alloy, which are based on the chaotic time-series, approximate entropy, fractal dimension, and Hurst exponent. The dynamic plastic behavior at both room temperature and 200 °C exhibits a positive Lyapunov exponent, suggesting that the underlying dynamics is chaotic. The fractal dimension of the indentation depth increases with the increase of temperature, and there is an inflection at the holding time of 10 s at the same temperature. A large fractal dimension suggests the concurrent nucleation of a large number of slip bands. In particular, for the indentation with the holding time of 10 s at room temperature, the slip process evolves as a self-similar random process with a weak negative correlation similar to a random walk.

Time-resolved measurement of shear-band temperature during serrated flow in a Zr-based metallic glass

Severe localization of plastic flow into shear bands is typically accompanied by local heating. In metallic glasses, the small lateral extension of shear bands, the short duration of their intermittent plastic slips, and the unpredictable position of their initiation make in-situ investigations of local changes in temperature during shear banding difficult. An alternative is to estimate shear-band temperature using calculations for a given assumed deformation model, which generates predictions of temperature changes from 5 °C to more than 1000 °C. In this study we review four models of plastic slip accumulation and present first results on temporally resolved shear-band heating in metallic glasses via infrared thermography. We find that the heat evolution in a shear band during plastic slip is best modelled by shear that occurs simultaneously within serration time scales. This illustrates clearly that temperature changes within shear bands are less than 6 °C during stable plastic flow.

Understanding the serrated flow and Johari-Goldstein relaxation of metallic glasses

An increasing tendency of serrated flow with increasing loading rate of a La-based (La) metallic glass (MG) with pronounced Johari-Goldstein relaxation in nanoindentation is investigated, which is abnormal compared to precedent observations where the serrated flow normally decays with increasing loading rate as discovered in the referenced Zr-based (Zr) and Pd-based (Pd) MGs. Based on systematic analysis, a unified dimensionless number is proposed to elucidate the different origins of the abnormal serrated flow of the La MG and the normal serrated flow of the Zr and Pd MGs. The abnormal serrated flow behavior of the La MG provides critical implications in understanding the role of Johari-Goldstein relaxation in the flow of metallic glasses.

Competition between shear band nucleation and propagation across rate-dependent flow transitions in a model metallic glass

Shear transformation zone (STZ) dynamics is used to examine the transition between different regimes of flow serration in the strain rate dependent deformation of metallic glass. To capture the strain rate independent yield strength of Vitreloy 1 at low to moderate strain rates, the model is adapted to include STZ volume and activation energy that decrease with increasing strain rate. The different stages of shear banding are examined in a statistical fashion over six different strain rates ranging from 10−5 to 100 s−1, with twelve replicates at each strain rate. Examination of flow serration, shear band nucleation rates, propagation rates, and sliding rates in each simulation find support for the hypothesis that the flow transition is caused by high shear band propagation and sliding rates at low strain rates, and high shear band nucleation rates at high strain rates. The underlying cause for the flow transition is hypothesized to be a strain rate dependent critical shear band nucleus size that increases with increasing strain rate. This critical shear band nucleus size results from the strain rate dependent STZ volume and activation energy, in which very small variations can cause a large change in shear banding behavior.

Power-law scaling between mean stress drops and strain rates in bulk metallic glasses

Discrete, jerky stress-drop bursts (serrated flows) during plastic deformation of a Zr-based bulk metallic glass exhibit different magnitudes in response to variant strain rates. Through the classical statistical analysis of stress-strain curves during serrated flow, plastic dynamic behavior changes from Gauss to the power-law distribution with increasing the strain rates. In addition, a quantitative liner relation between mean stress drops and the logarithm of strain rates is obtained to describe the influence of strain rates on mean amplitudes of the stress drop. The power-law scaling behavior is illustrated in different views including the strain-rate dependence, energy dynamics, the Poisson's ratio of material, and shear-banding operation. These results are helpful for understanding the serration behavior at room temperature.

Experimental evidence for both progressive and simultaneous shear during quasistatic compression of a bulk metallic glass

Two distinct types of slip events occur during serrated plastic flow of bulk metallic glasses. These events are distinguished not only by their size but also by distinct stress drop rate profiles. Small stress drop serrations have fluctuating stress drop rates (with maximum stress drop rates ranging from 0.3–1 GPa/s), indicating progressive or intermittent propagation of a shear band. The large stress drop serrations are characterized by sharply peaked stress drop rate profiles (with maximum stress drop rates of 1–100 GPa/s). The propagation of a large slip is preceded by a slowly rising stress drop rate that is presumably due to the percolation of slipping weak spots prior to the initiation of shear over the entire shear plane. The onset of the rapid shear event is accompanied by a burst of acoustic emission. These large slips correspond to simultaneous shear with uniform sliding as confirmed by direct high-speed imaging and image correlation. Both small and large slip events occur throughout plastic deformation. The significant differences between these two types require that they be carefully distinguished in both modeling and experimental efforts.

Chaotic state to self-organized critical state transition of serrated flow dynamics during brittle-to-ductile transition in metallic glass

We study serrated flow dynamics during brittle-to-ductile transition induced by tuning the sample aspect ratio in a Zr-based metallic glass. The statistical analysis reveals that the serrated flow dynamics transforms from a chaotic state characterized by Gaussian-distribution serrations corresponding to stick-slip motion of randomly generated and uncorrelated single shear band and brittle behavior, into a self-organized critical state featured by intermittent scale-free distribution of shear avalanches corresponding to a collective motion of multiple shear bands and ductile behavior. The correlation found between serrated flow dynamics and plastic deformation might shed light on the plastic deformation dynamic and mechanism in metallic glasses.

A Brief Review of High Entropy Alloys and Serration Behavior and Flow Units

Multicomponent alloys with high entropy of mixing, e.g., high entropy alloys (HEAs) and/or multiprincipal-element alloys (MEAs), are attracting increasing attentions, because the materials with novel properties are being developed, based on the design strategy of the equiatomic ratio, multicomponent, and high entropy of mixing in their liquid or random solution state. Recently, HEAs with the ultrahigh strength and fracture toughness, excellent magnetic properties, high fatigue, wear and corrosion resistance, great phase stability/high resistance to heat-softening behavior, sluggish diffusion effects, and potential superconductivity, etc., were developed. The HEAs can even have very high irradiation resistance and may have some self-healing effects, and can potentially be used as the first wall and nuclear fuel cladding materials. Serration behaviors and flow units are powerful methods to understand the plastic deformation or fracture of materials. The methods have been successfully used to study the plasticity of amorphous alloys (also bulk metallic glasses, BMGs). The flow units are proposed as: free volumes, shear transition zones (STZs), tension-transition zones (TTZs), liquid-like regions, soft regions or soft spots, etc. The flow units in the crystalline alloys are usually dislocations, which may interact with the solute atoms, interstitial types, or substitution types. Moreover, the flow units often change with the testing temperatures and loading strain rates, e.g., at the low temperature and high strain rate, plastic deformation will be carried out by the flow unit of twinning, and at high temperatures, the grain boundary will be the weak area, and play as the flow unit. The serration shapes are related to the types of flow units, and the serration behavior can be analyzed using the power law and modified power law.

Enhanced plasticity of a Fe-based bulk amorphous alloy by thin Ni coating

A thin Ni coating well adhering on a Fe81Mo1P7.5C5.5B2Si3 bulk amorphous alloy (BAA) has been successfully prepared by electroless plating. It has been found that the plastic strain of the BAA is significantly increased to 8% (28 times enhanced) by the thin Ni coating. It shows that the coating with nanograin could promote the nucleation of shear bands, constrain the propagation of them and change the flow serration into a self-organized critical state, which drastically increase the plasticity of the BAA.

Spatio-temporal characteristics of plastic instability in AA5182-O during biaxial deformation

Spatio-temporal characteristics of plastic instability denoted by locally extreme values in strain rate contours computed with digital image correlation are investigated in biaxial deformation of AA5182-O sheet. Temporal characteristics are similar to those of Portevin–Le Châtelier (PLC) bands in uniaxial tension. Spatial characteristics, however, dramatically differ from PLC band morphologies in uniaxial tension. Initiation occurs in a localized deformation ring (LDR) that does not include the pole, followed by a rapid transition to a circular region that includes pole, i.e. a localized deformation circle (LDC). A critical strain is required to trigger serrated flow at the pole, and a negative strain rate sensitivity of the flow stress at the pole suggests that the underlying microstructural mechanism is dynamic strain aging. Plastic instability hops between the LDR and the LDC until a critical strain is reached at which it appears in a localized deformation band, with no circular symmetry, that hops around the pole. Future model development will require modifications to existing theoretical frameworks to account for the unique spatial characteristics of plastic instability during biaxial deformation of AA5XXX alloys.

An examination of strain ageing in a Mg–Y–Zn alloy containing Gd

Dynamic strain ageing has been observed on straining a Mg–Y–Zn–Gd alloy in both the quenched and the annealed states, where long-period-structural-order phase and solution or precipitation of Gd is found. Deformation mechanisms are analysed in terms of solute diffusion to mobile dislocations, the role of excess vacancies and the presence of a Taylor dislocation network. In quenched material, the operation of multiple deformation modes leads to the creation of a three-dimensional (Taylor) dislocation network such that pipe diffusion along forest dislocations can readily lead to pinning of a moving dislocation. The many planar precipitates present in annealed material inhibit non-basal flow such that the Taylor dislocation network does not form and bulk diffusion of solute is required for flow serrations to appear.

The mechanism of power-law scaling behavior by controlling shear bands in bulk metallic glass

Bulk metallic glasses deform irreversibly under a stress through shear-banding courses that manifest as the serrated flow behavior. The compressive deformation and dynamic serrated flow behavior of Zr52.5Cu17.9Ni14.6Al10Ti5 bulk metallic glass samples with different aspect ratios have been investigated. The yield strength nearly remains a constant value of approximately 2GPa, while the compressive plasticity increases obviously with decreasing aspect ratio. It is found that the serrated flows display a power-law scaling behavior at different aspect ratios. The power-law scaling behavior is discussed by controlling shear bands in BMG. In addition, a new method was proposed to study the power-law-scaling behavior. When the aspect ratio is small, the friction between the sample and the platen will play a significant role that attributes to a lateral constraint. The uniaxial stress and the lateral constraint will cause a hydrostatic pressure on the sample close to the platen. The shear bands are controlled by the different stress states, which leads to a power-law-scaling behavior in serrated flows. The investigations have a contribution to understanding the plastic-deformation mechanism of BMGs.

Investigation using digital image correlation of Portevin-Le Chatelier Effect in Hastelloy X under thermo-mechanical loading

The plastic behavior of Hastelloy X subjected to thermo-mechanical loading was investigated in this work, and the phenomenon of dynamic strain aging was studied under isothermal conditions, using digital image correlation (DIC). The material of interest, Hastelloy X a Ni-based superalloy, is widely used in high temperature applications. It was seen that Hastelloy X exhibits serrated plastic flow behavior between temperatures of 300°C and 700°C, as a consequence of the dynamic strain aging effect, which, depending on temperature, manifests as type A/B or A and B oscillations on the uniaxial tension stress–strain curve, and is characterized by strain concentration bands propagating along the length of the specimen. The characteristics of Portevin-Le Chatelier (PLC) band internal strain distribution in uniaxial tension experiments were also investigated. The bands themselves were seen to have internal (sub)bands of two or three smaller strain jumps. An analysis of the strain increments induced by these bands was performed and clearly illustrates the inhomogeneous nature of the deformation – both spatially and temporally. In addition to the uniaxial behavior, biaxial stress states were investigated by studying PLC band formation near a notch in a double notched tensile geometry. Similar macroscopic results to the uniaxial case were seen while the strain bands themselves were seen to initiate in the notch region and travel away from the notches.

Serration Dynamics in a Zr-Based Bulk Metallic Glass

Intermittent or serrated plastic flows have been widely observed in irreversible deformation through shear banding in bulk metallic glasses (BMGs). The strain-rate-dependent plasticity under uniaxial compression at 2 × 10−3, 2 × 10−4, and 2 × 10−5 s−1 in a Zr-based BMG is investigated. Serration events have a typical time scale at a relatively higher strain rate (2 × 10−3 s−1), while at lower strain rates, there is a lack of typical time scale. During serrations, the stress is falling rapidly, and the amplitude of the stress drop between the neighboring serrations is approximately equal. The stress drop vs time satisfies the exponential decay rule during jerk flows. Due to the serrated flow corresponding to the internal shear process, the free-volume model and stick–slip model are introduced to explain how the shear bands form and propagate and the cooperation of multiple shear bands. The mechanism is explained by relating the atomic-scale deformation with the macroscopic shear-band behavior, offering key ingredients to fundamentally cognize serrations in jerk flows.