A study of the dynamic impact behaviour of IN 718 and ATI 718Plus® superalloys

The dynamic impact response of IN 718 and ATI 718Plus®, in both the solution heat treated and age-hardened conditions, were investigated at different deformation temperatures and strain rates using a direct impact Hopkinson pressure bar. Analyses of the results provide a vital but previously not reported information that the ATI 718Plus® offers a higher resistance to damage during high strain rate ballistic impact deformation compared to the most widely used Iron-nickel based superalloy, Inconel 718. ATI 718Plus® showed higher strain hardening and strain rate sensitivity, in both heat treatment conditions, than IN 718. The difference in the deformation behaviour of both alloys, in the annealed condition, is attributable to the compositional modification in ATI 718Plus® which has been reported to lower its stacking fault energy and increases the tendency for deformation twinning. However, in the age-hardened condition, the difference is believed to be related to the disparity in the operative strengthening mechanism, of the precipitates present in both alloys. Furthermore, a higher susceptibility to strain location and the formation of adiabatic shear band, in aged IN 718, is attributable to the stronger temperature-softening characteristics observed in the alloy and to the limited strain hardening tendency under dynamic impact loading.     

高温高应变率下混凝土材料的损伤演化方程

采用?74 mm大口径分离式霍普金森压杆(SHPB)对不同温度(20、200、400℃)下的C45混凝土材料进行动态力学性能实验,得到了不同温度、不同应变率下混凝土材料的应力-应变曲线。实验结果表明:在20~400℃温度范围内,混凝土材料具有温度硬化和应变率硬化现象。基于上述实验数据给出了损伤变量关于塑性应变的关系式,并通过相关实验数据确定了不同温度、不同应变率下损伤演化方程的材料参数。将该损伤演化方程应用于混凝土材料的本构关系中,预测结果与实验数据具有较好的一致性,证明了所提出的高温、高应变率下混凝土材料损伤演化方程的合理性。     

Dislocation motion in high strain-rate deformation

We present a systematic investigation of dislocation motion, dislocation interactions, and the collective behaviour of dislocations in high strain-rate deformation. Based on results from three-dimensional dislocation dynamics simulations, we find that employing the accurate, full-dynamics, equation of motion (i.e. that includes inertial effects) significantly changes the predictions of microstructural evolution and the macroscopic response compared to the commonly used overdamped equation of motion (i.e. with no inertial effects), especially at high strain rates (10³–10⁶ s^?1). While we find that inertial effects cannot be neglected, the net velocities are not high enough that ‘relativistic’ effects are important. We also present results on the effects of high strain rates on single-crystal deformation, which show good agreement with experimental trends, including increased hardening with increasing strain rate.     

Effect of temperature on the anisotropy of AZ31 magnesium alloy rolling sheet under high strain rate deformation

In order to investigate the effect of temperature on the anisotropic behaviour of AZ31 magnesium alloy rolling sheet under high strain rate deformation, the Split Hopkinson Pressure Bar was used to analyse the dynamic mechanical properties of AZ31 magnesium alloy rolling sheet in three directions, rolling direction(RD), transverse direction (TD) and normal direction (ND). The texture of the rolling sheet was characterised by X-ray analysis and the microstructure prior and after high strain rate deformation was observed by optical microscope (OM). The results demonstrated that AZ31magnesium alloy rolling sheet has strong initial {0?0?0?2} texture, which resulted at the obvious anisotropy in high strain rate deformation at 20 °C. The anisotropy reflected in stress–strain curve, yield stress, peak stress and microstructure. The anisotropy became much weaker when the deformation temperature increased up to 250 °C. Continuing to increase the deformation temperature to 350 °C the anisotropy of AZ31 rolling sheet essentially disappeared. The decreasing tendency of anisotropy with increasing temperature was due to the fact that when the deformation temperature increased, the critical resolved shear stress (CRSS) for pyramidal 〈c + a〉 slip, which was the predominant slip mechanism for ND, decreased close to that of twinning, which was the predominant deformation mechanism for RD and TD. The deformation mechanism at different directions and temperatures and the Schmid factor (SF) at different directions were discussed in the present paper.     

Deformation of a nanocube with a single incoherent precipitate: role of precipitate size and dislocation looping

ABSTRACT

Precipitate hardening is a key strengthening mechanism in metallic alloys. Classical models for precipitate hardening are based on the average behaviour of an ensemble of precipitates, and fail to capture the complexity of dislocation-precipitate interactions that have recently been observed at individual precipitates in simulations and in-situ electron microscopy. In order to achieve tailored mechanical properties, detailed deformation mechanisms at specific precipitates that account for precipitate size, crystallography, and defect structure must be understood, but has been challenging to achieve experimentally. Here, in-situ scanning electron microscope mechanical testing is used to obtain the compressive stress–strain behaviour at an individual, incoherent Au precipitate within a Cu nanocube, and determine the influence of precipitate and cube size on yield strength and strain hardening. TEM imaging and strain mapping of the initial structure shows misfit dislocations at the Au precipitate, threading dislocations that traverse the Cu shell, and localised and anisotropic strain near the precipitate and threading dislocation. These nanocubes have yield strengths of 800–1000?MPa and strain hardening rate of 1–4?GPa. Yield strength is found to depend on the distance from the precipitate interface to the cube edge, while strain hardening depends on both cube size and precipitate size. An analytical model is developed to quantify the contribution of Orowan looping, Orowan stress, back stress and image stress to plasticity at the Au precipitate. Orowan stress is found to be the largest contributor, followed by back stress and image stress.     

Effect of thermomechanical processing on microstructural evolution and deformation behaviour of polycrystalline magnesium under quasi-static and high strain rate conditions

Abstract

The effect of thermomechanical processing on microstructure evolution and room temperature flow behaviour of polycrystalline magnesium in compression at strain rates of ~10^?2 and ~10³ s^?1 was investigated. Different initial microstructures were produced by optimising rolling and annealing cycles. Prior to annealing for 1 h at 350 °C, Mg samples were processed by two different treatments such as (i) hot rolling at 350 °C and (ii) hot rolling at 350 °C plus cold rolling at room temperature. Introduction of cold working step led to an increased fraction of hard oriented grains with a marginal grain size difference in post-annealed samples. A profound effect of thermomechanical processing on strain hardening rate as well as rate-sensitive flow behaviour of Mg was observed. The influence of prior processing history and strain rate on flow behaviour of Mg was clearly reflected in terms of texture strengthening/weakening phenomena and formation of microstructural deformation bands.     

A study of the hot and cold deformation of twin-roll cast magnesium alloy AZ31

Recent advances in twin-roll casting (TRC) technology of magnesium have demonstrated the feasibility of producing magnesium sheets in the range of widths needed for automotive applications. However, challenges in the areas of manufacturing, material processing and modelling need to be resolved in order to fully utilize magnesium alloys. Despite the limited formability of magnesium alloys at room temperature due to their hexagonal close-packed crystalline structure, studies have shown that the formability of magnesium alloys can be significantly improved by processing the material at elevated temperatures and by modifying their microstructure to increase ductility. Such improvements can potentially be achieved by processes such as superplastic forming along with manufacturing techniques such as TRC. In this work, we investigate the superplastic behaviour of twin-roll cast AZ31 through mechanical testing, microstructure characterization and computational modelling. Validated by the experimental results, a novel continuum dislocation dynamics-based constitutive model is developed and coupled with viscoplastic self-consistent model to simulate the deformation behaviour. The model integrates the main microstructural features such as dislocation densities, grain shape and grain orientations within a self-consistent viscoplasticity theory with internal variables. Simulations of the deformation process at room temperature show large activity of the basal and prismatic systems at the early stages of deformation and increasing activity of pyramidal systems due to twinning at the later stages. The predicted texture at room temperature is consistent with the experimental results. Using appropriate model parameters at high temperatures, the stress–strain relationship can be described accurately over the range of low strain rates.     

Atomistic simulation of the stacking fault energy and grain shape on strain hardening behaviours of FCC nanocrystalline metals

ABSTRACT

Ultra-fine grained copper with nanotwins is found to be both strong and ductile. It is expected that nanocrystalline metals with lamella grains will have strain hardening behaviour. The main unsolved issues on strain hardening behaviour of nanocrystalline metals include the effect of stacking fault energy, grain shape, temperature, strain rate, second phase particles, alloy elements, etc. Strain hardening makes strong nanocrystalline metals ductile. The stacking fault energy effects on the strain hardening behaviour are studied by molecular dynamics simulation to investigate the uniaxial tensile deformation of the layer-grained and equiaxed models for metallic materials at 300?K. The results show that the strain hardening is observed during the plastic deformation of the layer-grained models, while strain softening is found in the equiaxed models. The strain hardening index values of the layer-grained models decrease with the decrease of stacking fault energy, which is attributed to the distinct stacking fault width and dislocation density. Forest dislocations are observed in the layer-grained models due to the high dislocation density. The formation of sessile dislocations, such as Lomer–Cottrell dislocation locks and stair-rod dislocations, causes the strain hardening behaviour. The dislocation density in layer-grained models is higher than that in the equiaxed models. Grain morphology affects dislocation density by influencing the dislocation motion distance in grain interior.     

高温高应变率下纯锆的本构模型研究

 研究了常温至1 073 K、不同应变率下（4.1×10^－4 s^－1～2.8×10³ s^-1）锆的压缩力学性能。结果表明,锆的流动应力对温度和应变率的变化比较敏感,随应变率的提高而增大,随温度的升高而降低。基于位错动力学理论,建立了锆的本构模型,并考虑塑性变形过程中孪晶演化的影响,对模型进行了修正。修正后的本构模型预测结果与实验结果吻合较好,可描述很宽应变率和温度范围内锆的塑性变形行为。     

Strain softening and strain localization in copper based Cu-Al single crystals

The paper reports the results of the study of highly effective relaxation mechanisms of plastic deformation in Cu-Al single crystals and their effect on mechanical properties and tendency toward strain localization. The experiments concern a wide range of aluminium content (0–16 at. % Al) in copper based single crystals, tested in tension. These were oriented in the centre of stereographic triangle. Slip markings, changes of crystal shape, strain hardening rate and strain rate sensitivity provided independent information permitting to identify the mechanism of relaxation and to determine its kinetic features. It was found that selfinduced entry of critical slip system causes sudden decrease of hardening rate, changes the strain rate sensitivity and leads to strain localization. The effect occurs regardless the crystal composition but its extent is dependent on Al content. The model of the relaxation mechanism has been proposed.Presented at the 4^th International Symposium on Plasticity of Metals and Alloys, September 7–11, 1987, Srní, Czechoslovakia.     

Shear band formation and ductility in bulk metallic glass

The variations in the chemical compositions of the metallic glasses reported in the literature, as well as the overall lack of experimental data concerning the inhomogeneous deformation behaviour of metallic glass, make the evaluation of the effects of shear band/fracture behaviour on the mechanical properties of metallic glasses difficult. Isolating the effect of local shear band formation on bulk inhomogeneous flow would appear to be a first step in approaching this problem. The mechanical behaviour of Vitreloy metallic glass at room temperature and at various strain rates in tension and compression was investigated. The formation of multiple shear bands was observed at high strain rates. An increase in strain rate leads to enhanced ductility in tension and compression. Some aspects of the deformation processes in tension and compression are discussed.     

橡胶材料的动态压缩性能及其应变率相关的本构模型

 利用Instron万能试验机与LC4超硬铝合金分离式Hopkinson压杆设备,对3种不同波阻抗的橡胶材料——炭黑母胶（Carbon Black Rubber）、硅橡胶（Silicone Rubber）和泡沫橡胶（Foam Rubber）在较大应变率范围（0.002~15 000s^-1）内进行了单轴压缩实验,研究应变率对橡胶材料力学性能的影响。实验结果表明：3种橡胶的准静态与动态应力-应变曲线具有不同的应变硬化形式,且动态加载下随着应变率的增大,硬化效应逐渐增强;在准静态及高应变率（12 000~15 000 s^-1）压缩下,泡沫橡胶表现出多孔类材料压缩曲线的弹性、塑性崩塌及致密化3段特征。基于Rivilin应变能模型,构建了一个应变率相关的动态本构模型,模拟结果与实验结果吻合较好,可以用于描述较大应变率范围内3种橡胶的非线性应力-应变关系。     

Effect of initial orientation on the tensile properties of commercially pure titanium

Effect of crystallographic texture on uniaxial tensile deformation of commercially pure titanium was studied using in situ as well as post-mortem electron backscatter diffraction and elastoplastic self-consistent simulations. Correlation of mechanical properties and strain hardening response with deformation micromechanisms like different modes of slip and twinning was established. Tensile specimens were machined along rolling direction in the plane perpendicular to normal and transverse direction (sample A and C, respectively) as well as along transverse direction in the plane normal to rolling direction (sample B) to obtain different initial texture from cold rolled and annealed plate of commercially pure titanium. Sample B showed higher strength but lower strain hardening rate and ductility than the orientations A and C. It showed extension twinning with lateral thickening while the other samples showed coexistence of extension and contraction twinning. Schmid factor accounted for most of the observed twinning although some contraction twinning in sample A is attributed to the effect of internal stresses. A combination of in situ tensile test in a field emission gun scanning electron microscope with electron backscatter diffraction facility and elastoplastic self-consistent simulations aid in obtaining high-fidelity Voce hardening parameters for different slip and twinning systems in commercially pure titanium. The variation in tensile properties can be explained on the basis of propensity of twinning which tends to provide strain hardening at lower strain but contributes to failure at higher strain.     

Deformation behaviour of ultrafine and nanosize-grained Mg alloy synthesized via mechanical alloying

Bulk Mg–5Al alloys were consolidated from powders that had been mechanically alloyed over different milling durations. The microstructural evolution, and physical and mechanical properties of the alloys were investigated. Mechanical measurements revealed a change in deformation behaviour after certain milling durations. At short milling duration, high yield strength was obtained through dislocation strengthening mechanisms predominantly by grain refinement and to a lesser extent by solid solution strengthening and particle dispersion strengthening. However, at longer milling durations, low yield strength was observed and the strengthening mechanisms at work in short milling durations appeared to be no longer effective. Enhanced ductility with no work hardening behaviour was observed in samples with a mean grain size of 45?nm. It appeared that the significantly large increase in the grain boundary regions played an important role in the room temperature deformation of the alloys. The possibility of a diminishing effect of the dislocation strengthening mechanisms and the onset of grain boundary deformation modes for the softening phenomenon and the absence of work hardening at some nanoscale grain sizes are discussed.     

Hardening and softening during plastic deformation of hexagonal metals

The influence of temperature on the deformation behaviour of single crystals and polycrystals is investigated. The temperature dependences of the critical resolved shear stress, work hardening rates, the stress at the onset of stage C., the yield stress and maximum stress are reported. Possible deformation mechanisms concerning hardening and softening during plastic deformation of hexagonal metals are discussed.     

Numerical simulation of flat-tip micro-indentation of glassy polymers: Influence of loading speed and thermodynamic state

Flat-tip micro-indentation tests were performed on quenched and annealed polymer glasses at various loading speeds. The results were analyzed using an elasto-viscoplastic constitutive model that captures the intrinsic deformation characteristics of a polymer glass: a strain-rate dependent yield stress, strain softening and strain hardening. The advantage of this model is that changes in yield stress due to physical aging are captured in a single parameter. The two materials studied (polycarbonate (PC) and poly(methyl methacrylate) (PMMA)) were both selected for the specific rate-dependence of the yield stress that they display at room temperature. Within the range of strain rates experimentally covered, the yield stress of PC increases linearly with the logarithm of strain rate, whereas, for PMMA, a characteristic change in slope can be observed at higher strain rates. We demonstrate that, given the proper definition of the viscosity function, the flat-tip indentation response at different indentation speeds can be described accurately for both materials. Moreover, it is shown that the model captures the mechanical response on the microscopic scale (indentation) as well as on the macroscopic scale with the same parameter set. This offers promising possibilities of extracting mechanical properties of polymer glasses directly from indentation experiments.     

On the homogeneous nucleation and propagation of dislocations under shock compression

The dynamic response of crystalline materials subjected to extreme shock compression is not well understood. The interaction between the propagating shock wave and the material’s defect occurs at the sub-nanosecond timescale which makes in situ experimental measurements very challenging. Therefore, computer simulation coupled with theoretical modelling and available experimental data is useful to determine the underlying physics behind shock-induced plasticity. In this work, multiscale dislocation dynamics plasticity (MDDP) calculations are carried out to simulate the mechanical response of copper reported at ultra-high strain rates shock loading. We compare the value of threshold stress for homogeneous nucleation obtained from elastodynamic solution and standard nucleation theory with MDDP predictions for copper single crystals oriented in the [0 0 1]. MDDP homogeneous nucleation simulations are then carried out to investigate several aspects of shock-induced deformation such as; stress profile characteristics, plastic relaxation, dislocation microstructure evolution and temperature rise behind the wave front. The computation results show that the stresses exhibit an elastic overshoot followed by rapid relaxation such that the 1D state of strain is transformed into a 3D state of strain due to plastic flow. We demonstrate that MDDP computations of the dislocation density, peak pressure, dynamics yielding and flow stress are in good agreement with recent experimental findings and compare well with the predictions of several dislocation-based continuum models. MDDP-based models for dislocation density evolution, saturation dislocation density, temperature rise due to plastic work and strain rate hardening are proposed. Additionally, we demonstrated using MDDP computations along with recent experimental reports the breakdown of the fourth power law of Swegle and Grady in the homogeneous nucleation regime.     

Modelling and simulation of dynamic recrystallisation based on multi-phase-field and dislocation-based crystal plasticity models

ABSTRACT

The mechanical properties of metals used as structural materials are significantly affected by hot (or warm) plastic working. Therefore, it is industrially important to predict the microscopic behaviour of materials in the deformation process during heat treatment. In this process, a number of nuclei are generated in the vicinity of grain boundaries owing to thermal fluctuation or the coalescence of subgrains, and dynamic recrystallisation (DRX) occurs along with the deformation. In this paper, we develop a DRX model by coupling a dislocation-based crystal plasticity model and a multi-phase-field (MPF) model through the dislocation density. Then, the temperature dependence of the hardening tendency in the recrystallisation process is introduced into the DRX model. A multiphysics simulation for pure Ni is conducted, and then the validity of the DRX model is investigated by comparing the numerical results of microstructure formation and the nominal stress–strain curve during DRX with experimental results. The obtained results indicate that in the process of DRX, nucleation and grain growth occur mainly around grain boundaries with high dislocation density. As deformation progresses, new dislocations pile up and subsequent nucleation occurs in the recrystallised grains. The influence of such microstructural evolution appears as oscillation in the stress–strain curve. From the stress–strain curves, the temperature dependence in DRX is observed mainly in terms of the yield stress, the hardening ratio, and the change in the hardening tendency after nucleation occurs.     

A multiscale gradient-dependent plasticity model for size effects

The mechanical behaviour of polycrystalline material is closely correlated to grain size. In this study, we investigate the size-dependent phenomenon in multi-phase steels using a continuum dislocation dynamic model coupled with viscoplastic self-consistent model. We developed a dislocation-based strain gradient plasticity model and a stress gradient plasticity model, as well as a combined model, resulting in a theory that can predict size effect over a wide range of length scales. Results show that strain gradient plasticity and stress gradient plasticity are complementary rather than competing theories. The stress gradient model is dominant at the initial strain stage, and is much more effective for predicting yield strength than the strain gradient model. For larger deformations, the strain gradient model is dominant and more effective for predicting size-dependent hardening. The numerical results are compared with experimental data and it is found that they have the same trend for the yield stress. Furthermore, the effect of dislocation density at different strain stages is investigated, and the findings show that the Hall–Petch relation holds for the initial strain stage and breaks down for higher strain levels. Finally, a power law to describe the size effect and the transition zone between the strain gradient and stress gradient dominated regions is developed.     

Stimulation and Suppression of Grain Boundary Sliding by Intragranular Slip in Zinc Bicrystals

The influence of intragranular slip on grain boundary sliding is studied in originally compatible zinc bicrystals with symmetric tilt boundary. The experiment is designed to separate different effects of intragranular slip on the boundary sliding and establish their mechanisms. Grain boundary sliding with and without development of intragranular slip is observed. The rate of sliding accompanied by slip is more than five times of that without slip. A good correlation between the boundary sliding and intragranular slip prior to slide hardening is established. Slide hardening followed by the negative sliding near one end of the boundary and strain hardening in the boundary vicinity, are observed at the last stages of deformation. For the case of formation of slip induced glissile grain boundary dislocations of opposite signs the possibility of their contribution to total grain boundary sliding, is analyzed. The effect of the increase in the rate of sliding is explained in terms of the accommodation of sliding by slip and appearance of additional glissile grain boundary dislocations of one sign due to strain incompatibility. Contribution of these different dislocation mechanisms to the increase in the sliding rate is determined for the stage of deformation preceding slide hardening. It is supposed that the effect of slide hardening and negative sliding as well as boundary curving is created by non-smooth boundary and small degree of incompatibility caused by straining.