Negative Strain Rate Sensitivity Research Articles

Experimental and numerical analysis of the Portevin–Le Chatelier effect in a nickel-base superalloy for turbine disks application

High-pressure turbine disks are subjected to extreme mechanical and thermal loadings throughout their lifetime. This is the reason why during the design of turboshaft engines, regulation rules impose on manufacturers to demonstrate the integrity of rotating parts, especially turbine disks, with over-speed experiments. To improve the prediction of rotation speed at burst, it is essential to study the viscoplastic behavior and the fracture of the material. The present work focuses on the approach used to calibrate accurately a viscoplastic behavior model at high temperature (500 °C) of nickel-based superalloy Inconel718. The experimental testing campaign includes standard tensile tests on axisymmetric and flat specimens with and without notches. The behavior is described using the McCormick model for dynamic strain aging. This model accounts for the viscoplastic behavior including the range of negative strain rate sensitivity and the Portevin–Le Chatelier (PLC) effect. In addition to global measurements, digital image correlation at high temperature using a speckle directly engraved on the specimen is used to detect and measure the strain carried by the PLC bands. This information is used in the parameter calibration. The consideration of the machine stiffness is shown to be essential in the analysis for a correct description of serrations and of the critical strain for PLC effects. 3D simulations on full specimen geometries samples are performed to validate the finite element model. Finally, a full 3D representative disk geometry simulation is performed to detect instabilities and their propagation due to geometrical singularities.

Microstructure and mechanical properties of a gas-tungsten-arc-welded Fe-24Mn-3.5Cr-0.4C high manganese steel pipe using a Fe-22Mn-2.34Ni-0.38C welding wire

In this study, Fe-24.34Mn-3.53Cr-0.42C high manganese steel sheets were joined using a gas‑tungsten arc welding (GTAW) and a plasma arc welding (PAW) using a newly-developed Fe-22.2Mn-2.34Cr-1.95Ni welding wire. Microstructure analysis revealed that Mn-depleted zone appeared in the weld zone due to Mn segregation during solidification. With plastic deformation, a small amount of ε-martensite was found in the Mn-depleted zone i.e., dendritic region, of the weld zone. Mechanical properties of the weld zone were found to be lower than in the base metal. On the other hand, due to recrystallization and grain growth induced by heat treatment at 1050 °C, the mechanical property differences between the base metal and the weld zone were reduced. Strain rate sensitivity test showed negative strain rate sensitivity in the base metal and heat-treated weld zone possibly due to the reduced work hardening caused by the reduction of twinning nucleation. On the contrary, the as-welded zone exhibited positive strain rate sensitivity due to the reduction in twinning stress caused by coarse grains. • The SFE of Mn-depleted zones decreased due to manganese segregation in weld zones, and low manganese content in welding wires. • Therefore, ε-martensite was formed in Mn-depleted zones due to transformation-induced plasticity effects during plastic deformation. • In weld zones, mechanical properties were improved, and their anisotropy also decreased largerly due to recrystallization effects through heat treatment. • Except for as-welded zone, twin formation decreased with increasing strain rate, thus showing negative strain rate sensitivity.

Two-temperature thermodynamics and a viscoplasticity model for FCC metals with grain boundary effect

A non-equilibrium thermodynamic route is pursued to model viscoplasticity, including the evolving grain boundary, in face-centered cubic (FCC) metals. The thermodynamic description includes two subsystems: the configurational subsystem describing the relatively slower motion of dislocations and grain boundaries, and a fast-evolving kinetic-vibrational (K-V) subsystem depicting atomic vibrations about equilibrium positions in the lattice. Grain boundary and dislocation densities as well as their interactions are incorporated within the model. The model is initially developed for homogeneous plastic deformation, which is then generalized to the inhomogeneous case via spatial variations in the thermodynamic states over a wide range of strain rate and temperature. The model is implemented as a user material subroutine (VUMAT) in ABAQUS to simulate a problem of industrial interest, viz. deep drawing on an AA 7075 aluminum alloy. Finally, the thermo-viscoplasticity model is used to predict negative strain rate sensitivity in the AA 2219 aluminum alloy at room temperature, for which an experiment is performed at a facility of the VSSC. The predictive quality of the model is demonstrated by a comparison of numerical simulations with the experimental evidence.

Dislocation-obstacle interaction evolution in rate dependent plasticity of AlSi10Mg as-built microstructure by laser powder bed fusion

Novel microstructural features obtained from laser powder bed fusion methods challenge our understanding of the rate controlling mechanisms for plasticity which are important for structural applications. The strain rate dependent plasticity of as-built AlSi10Mg laser powder bed fusion samples was systematically studied as a function of stress state (tension and compression) and strain rate at 293 and 78 K. Utilizing constant strain-rate and rate-change testing methodologies, a strong dependence of flow stress on strain rate was revealed for both stress states. The compressive yield stresses have a potent strain rate sensitivity in both quasi-static (10−4-10−1 s−1) and medium-dynamic (1-5 s−1) strain rate regimes. A positive strain rate sensitivity was observed in all strain rates except for a shift to negative values in the 10−2-10−1 s−1 range for both tensile and compression states. This negative strain rate sensitivity was correlated with the deformation-induced microstructural evolution, i.e., manifested as a gradual transition in the dominant dislocation-precipitate interaction mechanism from shearing to looping, along with the disappearance of dislocation-solute interaction in the form of dynamic strain aging (DSA). The precise measurement of activation volume evolution with flow stress further confirmed the transition from shearing to looping in the 10−2-10−1 s−1 range.

Compression temperature to activate kinking in pearlitic steel

The preferable condition for kinking in the compression of pearlitic steel is reported in this study. Spring steel (0.53 wt%C-1.48%Si-0.70%Mn-0.76%Cr–Fe) with a pearlitic structure was compressed at different temperatures ranging from ambient to 600 °C. The deformation microstructure clarified that the kinking occurred more oftenwhen the compression temperature was less than 400 °C. Three-dimensional reconstruction of the scanning electron microscopy images with focused-ion-beam serial sectioning revealed that the kinked structure comprised a band-like sheared area surrounded by the bent structures. The shear strain in the kinked structure is greater than the macroscopic strain applied by compression. A series of measurements of compressive flow stress (strain rate: 0.001/sec to 1/sec) elucidated negative strain rate sensitivity (m-value < 0) at ∼ 300 °C. Strain localization by kinking occurs preferentially at relatively lower temperature because a smaller strain rate sensitivity can promote local plastic instability.

Effects of strain rate on the tensile and creep-fatigue properties of 316H stainless steel

Since the unstable start-ups and shutdown processes of nuclear power plants, the effect of strain rate on tensile and creep-fatigue (CF) properties has attracted attention. This paper aims to study the strain rate dependent tensile and CF behavior of 316H stainless steel. The tensile and CF tests were independently performed with different strain rates of 0.0001, 0.0002, 0.0005, 0.001 and 0.005 s−1 at 600 °C. Due to the dynamic strain aging (DSA), the tensile stress-strain curve appears serrated flow phenomenon and shows negative strain rate sensitivity (SRS), which means the tensile strength of the material does not increase significantly with an increase of the strain rate, and even decreases. Meanwhile, it was found that peak tensile stress and inelastic strain increase in CF tests under low strain rates. The CF life of 316H steel was predicted using the hysteresis energy method. At the same time, based on microstructure and fracture observation, different dominant damage mechanisms were discussed.

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.

Strain rate sensitivity of a Cu60Zr40 metallic and nanoglass

The strain rate sensitivity, m, of a binary Cu60 Zr40 nanoglass (NG) and metallic glass (MG) are investigated using nanoindentation. Indentations were performed at different loading rates in the range of 0.26–8 mN/s, which gives equivalent indentation strain rates over three decades. The load vs. displacement curves of MG exhibited noticeable displacement bursts at low loading rates, which gradually decreased with increasing loading rate suggesting a transition from more to less severe heterogeneous plastic flow. While in the case of NG, no noticeable displacement bursts are present at any of the loading rates suggesting a near homogeneous plastic flow. In both NG and MG, the hardness decreases with increasing loading rate, resulting in negative strain rate sensitivity, m. The m for NG is higher than the MG, indicating a more homogeneous flow underneath the indentation. Interface indentation experiments and subsequent analysis of the deformation zone showed a larger number of fine secondary shear bands (SSBs) in NG as compared to the primary shear bands (PSBs), while the plastic flow in MG is accommodated mostly by the PSBs. The findings of the current study will help improve the understanding of the plastic deformation behavior of NGs and provide insights for designing the novel microstructural architecture of amorphous alloys with improved ductility.

Strain rate sensitivity of binary Mg–Gd and Mg–Y solid solutions

The strain rate sensitivity (SRS) of binary Mg–Gd and Mg–Y alloys and the effect of solute on the rate-controlling mechanisms have been studied by analyzing rate changes during tension and compression tests at 78K and 298K. The steady-state SRS evaluated from the slope of Haasen plots at 78K increases with the solute concentration. The reverse behavior is observed at 298K, and the concentrated alloys exhibit the negative SRS. The activation work and the activation distance evaluated from strain rate jump tests at 78K discriminate regimes of plastic flow determined by solute–dislocation and dislocation–dislocation interactions. At the onset of plastic flow, dislocation–solute interactions control the plastic flow properties. This regime of low flow stress is represented by the activation work in the magnitude of 10−2eV and the activation distance in the range 10−2b - 10−1b. The deformation by mechanical twinning occurring early in the compression conforms to these parameters and suggest no fundamental difference in the thermally activated glide of ordinary and twinning dislocations through the field of solute obstacles. The results suggest that by-passing of Gd and Y atoms by basal dislocations occurs by sequential activation of partial dislocations. At higher flow stress, the dislocation–dislocation interactions determine the work-hardening. The thermally activated motion of jogs produced during interactions of basal and prismatic dislocations is thought to be the rate-controlling process. The activation work and the activation distance signatures of this process are W∗∼ 0.4 eV - 0.8 eV and d ∼ 0.5b - 1b, depending on the solute content.

Room temperature strain rate sensitivity of as-built 3D printed AlSi10Mg by laser powder bed fusion

Tensile and compression loading behavior of as-built AlSi10Mg alloy printed by laser powder bed fusion were studied at quasi-static to medium strain-rate regimes. A non-linear strain rate sensitivity behaviour was observed at all strain rates, a response linked to the extra-fine, non-equilibrium as-built microstructure. For both loading conditions, except for a negative strain rate sensitivity in 10−2–10−1 1/s range, a positive trend was observed. At the equivalent strain of 0.04, a strain rate sensitivity parameter of 0.010 and 0.019 (in 10−4–10−2 1/s range) was measured for tensile and compression loading, respectively, indicating a strong dependency especially in compression. For the medium regime (1–5 1/s), the strain rate sensitivity parameter in compression was further increased to 0.046.

Studies of High-Temperature Fatigue Behavior and Mechanism for Nickel-Based Superalloy Inconel 625

Strain-controlled continuous fatigue and creep–fatigue tests were carried out at 700 °C and 800 °C on Inconel 625 alloy. The effects of strain rate and tensile-hold time on cyclic stress response and fatigue life were investigated. Then, the microstructural analysis and the fractographic analysis of fatigue-fractured specimens were performed by scanning electron microscopy and transmission electron microscopy. The cyclic stress responses during high-temperature fatigue and the creep–fatigue–oxidation damage mechanism were discussed. The results showed that the strain rate and the tensile-hold time had little effect on the fatigue life at 700 °C, but there was a significant impact at 800 °C due to the creep–fatigue–oxidation interaction. The cyclic plastic deformation accelerated the precipitation of the γ″ phase, resulting in a continuous cyclic hardening and negative strain rate sensitivity. The fatigue failures at 700 °C under continuous fatigue conditions occurred with a transgranular fracture mode, while a transgranular-intergranular hybrid fracture manner was found at 800 °C. Furthermore, a frequency-modified total strain energy density model was proposed to consider the effects of creep and oxidation on fatigue life, and the predicted fatigue lives were located within the 1.5 times error band.

Portevin-Le Châtelier Effect in a Powder Metallurgy Co-Ni-Based Superalloy.

The Portevin-Le Châtelier (PLC) effect in a powder metallurgy (PM) Co-Ni-based superalloy was systematically investigated via the tensile tests at temperatures ranging from 200 to 600 °C and strain rates at 1.0 × 10−4 to 1.0 × 10−2. Both normal and inverse PLC effects were observed in the PLC regime, and the former appeared in the A and B types at a low temperature, whilst the latter appeared in the C type at an elevated temperature. Both positive and negative strain rate sensitivities (SRS) were shown in PLC regime, and SRS should be derived from same types of serrations. Based on the calculated activation energy, the substitutional atom Mo is considered to take primary responsibility for the PLC effect in present alloy.

Strain-rate dependent workability and plastic flow instability of a (Nb+V) stabilized microalloyed steel

In the current work, the strain-rate dependent workability and flow instability has been investigated in a (Nb+V) stabilized microalloyed steel. Uniaxial compression tests were conducted in intercritical and single phase austenitic temperature domain (700–1100 °C) at several strain-rates (0.01–10 s−1), using a thermo-mechanical simulator (Gleeble®−3800). The results show that the flow stress increases at higher strain-rates. A good sample processing window arises at medium strain-rates (0.1–1 s−1); whereas, flow instability (serrations) at high (10 s−1) and low (0.01 s−1) strain-rate plastic deformations. After a detailed sample characterization, it appears that the reasons for flow instability during the hot and warm deformations are the formation of micro-cracks or void nucleation at low (0.01 s−1) strain-rate; whereas, flow localization and shear banding by adiabatic heating at a higher strain-rate (10 s−1). In both cases (0.01 and 10 s−1), the serration arises at periodic intervals with negative strain-rate sensitivity. At a higher strain-rate (10 s−1), the flow instability dominates till 1100 °C, because of relieving the deformation-induced stored energy mainly by dynamic recovery. At lower deformation temperatures (i.e., 700–800 °C), fine ferrite grains nucleate around shear bands by the diffusional transformation of austenite. The dynamic recrystallization is either absent or incomplete in agreement with the experimental determination of Tnr (non-recrystallization temperature) and texture analysis. The overall flow instability characteristics in terms of temperature, strain and strain-rate substantiate well with the dynamic materials model, by the superposition of flow instability and power dissipation efficiency, on revisiting the processing map.

Accurate finite element modeling of the mechanical behavior of SS304 across the peak dynamic strain aging temperatures

ABSTRACT Dynamic strain ageing or DSA is exhibited by ferrous and aluminium alloys typically at elevated temperatures and low strain rates. This effect, in association with viscoplasticity, produces a complex material behaviour which is manifested through positive strain rate sensitivity of the flow stress in certain pockets of temperatures and strain rates and negative strain rate sensitivity in others. The conventional material models of DSA either lack the ability to accurately capture the DSA effect or to incorporate the effect of the temperature variation. The objective of this work was to propose a constitutive model which can do both. The desired material model was proposed by performing necessary augmentations to a conventional material model. The suitable conventional model for this purpose was identified through rigorous comparison of the existing models. The proposed constitutive model could accurately capture the DSA behaviour of austenitic stainless steel 304 for different strain rates over a range of temperatures across the peak DSA temperatures.

Negative strain rate sensitivity of yield strength of Al-Si alloy additive-manufactured using laser powder bed fusion

This article reports an unexpected finding of the mechanical behavior of Al-12%Si alloy manufactured using the laser powder bed fusion (L-PBF) process. The as-manufactured specimen has a negative strain rate sensitivity of flow stress at an early stage of tensile deformation (below 3% in strain). The post heat-treated specimens exhibited a positive strain-rate sensitivity and none of these specimens exhibited a serrated stress-strain curve, indicating a mechanism different from the Portevin–LeChatelier effect. Transmission electron microscopy revealed the dynamic precipitation of the nano-sized Si-phase inside the supersaturated solid solution of the α-Al matrix in the as-manufactured specimen deformed at ambient temperatures. The dynamically precipitated Si phase interacted with the introduced dislocations, enhancing the flow stress during the early stages of plastic deformation. The dynamic precipitation could contribute to the negative strain-rate sensitivity of the flow stress at an early stage of tensile deformation (including the yield strength).

Plastic instability and texture modification in extruded Mg-Mn-Nd alloy

Even though Mg alloys containing Mn and rare earth elements lead to higher ductility and lower yield asymmetry due to the weak texture after extrusion, plastic instability, commonly known as the Portevin–Le Chatelier (PLC) effect, causes unexpected fragility in the service environment. In the present study, the PLC phenomenon and texture development during the deformation of Mg-Mn and Mg-Mn-Nd extruded alloys were investigated under various temperatures and strain rates. The addition of Nd causes not only texture weakening but also severe PLC phenomenon. The PLC phenomenon was significantly affected by the temperatures and the strain rates, which causes a difference in mechanical properties and development of texture. In the conditions of high temperature and low strain rate, the strength increased while the elongation decreased significantly, and obvious PLC phenomenon with severe serration and negative strain rate sensitivity. The initial texture was maintained even after deformation only under severe PLC conditions, and this is due to the restriction of basal slip and suppression of lattice rotation in PLC conditions. The series of results indicate that the PLC phenomenon causes a reduction of formability even at high temperature.

Dynamic strain aging in DP1000: Effect of temperature and strain rate

The present study reports on an extensive experimental campaign performed to identify the occurrence of dynamic strain aging (DSA) in a fine-grained DP1000 steel. Tensile tests are performed covering 7 orders of strain rate, i.e., 0.0001 s-1 to 1080 s-1, and temperatures ranging from −40 °C to 400 °C. The test results are thoroughly analysed with particular attention to typical features of DSA, such as serrated flow, thermal hardening, negative strain rate sensitivity and blue brittleness. In addition to the plastic strain, DSA is sensitive to both temperature and strain rate. DSA effects are most dominant at higher temperatures and lower strain rates. However, even at the highest strain rate, traces of DSA are found in DP1000. At temperatures below 100 °C, no DSA is observed. The way and extent to which DSA manifests itself depend on the specific combination of temperature and strain rate. Analysis of the serrations stimulate the hypothesis that quasi-periodic dislocation cells might lie at their origin. In addition, scanning electron microscopy images of the fracture surfaces reveal finer and shallower dimples with flat features in the dynamic strain aging regime.

Modeling cyclic deformation of austenitic stainless steels at elevated temperatures using a physically-based mesoscale crystal plasticity framework

Austenitic stainless steels demonstrate significant changes in key macroscale deformation responses at elevated temperatures; these changes are relatively well-understood at micro- and macroscales in the framework of dynamic strain aging (DSA). At the microscale, temperature-dependent behavior is attributed to enhanced solute mobility and subsequent pinning of dislocations by solute atmospheres. At the macroscale, phenomenological equations are used to approximate microscale material behavior. However, the connection between unit dislocation processes at the microscale and macroscale deformation behaviors is unclear. In the current study, a dislocation density-based crystal plasticity model is used to relate micro- and macroscale responses for cyclic loading of austenitic stainless steels at elevated temperatures. Specifically, the variation in deformation behavior at elevated temperatures is attributed to modifications in the heterogenous distribution of dislocations on the mesoscale. The proposed framework demonstrates good agreement with experiments for strain- and stress-controlled cyclic loading scenarios. The model is also shown to be able to address key phenomena associated with DSA, i.e., negative strain rate sensitivity and stress-strain curve serrations.

Twinning-Induced Abnormal Strain Rate Sensitivity and Indentation Creep Behavior in Nanocrystalline Mg Alloy.

Nanocrystalline materials exhibit many unique physical and chemical properties with respect to their coarse-grained counterparts due to the high volume fraction of grain boundaries. Research interests on nanocrystalline materials around the world have been lasting over the past decades. In this study, we explored the room temperature strain rate sensitivity and creep behavior of the nanocrystalline Mg–Gd–Y–Zr alloy by using a nanoindentation technique. Results showed that the hardness and creep displacements of the nanocrystalline Mg–Gd–Y–Zr alloy decreased with increasing loading strain rate. That is, the nanocrystalline Mg–Gd–Y–Zr alloy showed negative strain rate sensitivity and its creep behavior also exhibited negative rate dependence. It was revealed that the enhanced twinning activities at higher loading strain rates resulted in reduced hardness and creep displacements. The dominant creep mechanism of the nanocrystalline Mg–Gd–Y–Zr alloy is discussed based on a work-of-indentation theory in this paper.

The Symmpact: A Direct-Impact Hopkinson Bar Setup Suitable for Investigating Dynamic Equilibrium in Low-Impedance Materials

BackgroundMeasuring the dynamic behavior of low-impedance materials such as foams is challenging. Their low acoustic impedance means that sensitive force measurement is required. The porous structure of foams also gives rise to dynamic compaction waves, which can result in unusual behavior, in particular if the foam material is so thick, that dynamic force equilibrium is not reached.ObjectiveThis work investigates comparatively large polyurethane foam specimens with densities in the range of 80 – 240 kg/m3 to deliberately achieve a state away from force equilibrium during high-rate compaction. The aim is to understand how an increase in strain rate leads to a reduction in strength for such materials.MethodsA specialized direct-impact Hopkinson bar is employed. It uses polycarbonate bars to achieve the required long pulse duration of 2.6 ms to compress the large specimens into the densification regime. In contrast to existing setups, both striker and output bar are instrumented with strain gauges to monitor force equilibrium. The absence of an input bar allows monitoring force equilibrium more accurately. Special attention is paid to the calibration of strain gauges, taking non-linear effects, wave dispersion and attenuation into account. Digital Image Correlation is employed to analyze elastic and plastic compaction waves by means of Lagrange diagrams.ResultsDepending on density, the specimens show saturation of dynamic strength increase at high rates of strain approx 500 /s, or even negative strain rate sensitivity in case of the lowest density. The occurrence of apparent negative strain rate sensitivity is accompanied by a localized structural collapse front, moving at a low velocity of approx 10 m/s through the material. This apparent strain rate sensitivity is a structural effect which is related to the thickness of the specimen.ConclusionsThe primary aim of material characterization using Hopkinson bars is to achieve a state of force equilibrium. For this reason, very thin specimens are usually employed. However, data gathered in this way is not representative for thick foam layers. Here, an increase of strain rate can lead to a decrease of strength if homogeneous deformation is replaced by a dynamic compaction wave. This behavior can occur at strain rates encountered under conditions such as automotive crash.