The Study of Dual-phase Steel after Cyclic Deformation at Various Strain Amplitudes

Low cycle fatigue tests under plastic straincontrol were carried out with a dual-phase steelcontaining 23 Vol.-％ martensite. Specimens hardenedrapidly at first few cycles followed by a slightsoftening to saturation stages when cycled athigher strain amplitudes, whereas at lower strainamplitudes the specimens presented continuallyhardening for a long time until saturation.TEM examination of the saturation dislocationstructures show that clusters, parallel wallsand cells were found at low, medium and highstrain amplitude, respectively. It also hasbeen found that the martensite/ferrite interfacesdid not affect the dislocation structures signi-ficantly when a specimen was fatigued at lowerstrain amplitude. However, the dislocation struc-ture adjacent to the two-phase boundary is dif-ferent to some extent from that in the remoteregions in the ferrite when a higher strainamplitude is applied.     

Modelling Residual Strains During Cycling of Ti–Ni and Ti–Ni–Cu Shape Memory Alloys in a Pseudoelastic Range of Behaviour Conditions

Abstract: Pseudoelastic behaviour of three types of Ti–Ni shape memory alloys in a pseudoelastic state has been studied under conditions of maximum strain‐ and maximum stress‐controlled cycling. Experimental results proved that residual deformation after unloading increases with the number of cycles; however, critical stress for the induction of martensite and the energy dissipated in one cycle decline during cycling. A higher critical stress for slip, and more intense cyclic dislocation hardening promoted by greater maximum deformation and greater maximum applied stresses, generally reduce the rate at which residual elongation grows with the number of cycles, and tend to stabilise the cyclic stress‐elongation diagrams. The small magnitude of critical stress for slip in low‐nickel alloys, and also cyclic strain hardening, induce greater internal stresses and a more marked decrease in critical stress for the induction of martensite as cycling progresses. Detailed analysis of plastic deformation propagation in cyclically loaded specimen helped develop a model of dependence of residual elongation on the number of cycles. This model enables identification of three main factors that govern the magnitude of residual elongation: one residual plastic elongation caused by dislocation hardening after the alloy is heat treated, and two cyclic strain hardening parameters describing how residual elongation grows with number of cycles, and how this residual elongation is reduced, as cycles increase, by the rising critical stress level for slip. The model has proved to yield very close agreement with experimental findings.     

Cyclic deformation of tungsten monofilament-reinforced multicrystalline copper composites

In an attempt to understand the cyclic deformation behavior of continuous fiber reinforced metal matrix composites, plastic strain controlled tests have been performed on tungsten monofilament-reinforced, multicrystalline, copper composites. The cyclic hardening response of the composites greatly depends on the fatigue dislocation structures corresponding to the strain amplitude. For example, at high strain amplitude, i.e. 1×10⁻³, secondary slip stimulated by the self-stresses of the primary dislocations becomes more active, and secondary hardening even occurs during saturation. At low strains, loop patches form and are associated with fine slip. At intermediate strains, persistent slip bands occur, but their distribution is altered by the presence of the fiber. The paper introduces a simple model to link the cyclic stress–strain response of the multicrystalline composites to those of monolithic single crystals and fibers. This model not only represents the fiber reinforcement by the rule of mixtures, but also adopts the Sachs model for the single crystal–polycrystal conversion factor. The results calculated by the model show very good agreement with the experimental data in all strain amplitudes at which the composites were fatigued. This encouraging outcome suggests that the new model could be applied to high-cycle fatigue of commercial continuous-fiber-reinforced polycrystalline metal matrix composites.     

Effects of grain size on cyclic plasticity and fatigue crack initiation in nickel

Fatigue experiments were conducted on polycrystalline nickel of two grain sizes, 24 and 290 μm, to evaluate the effects of grain size on cyclic plasticity and fatigue crack initiation. Specimens were cycled at room temperature at plastic strain amplitudes ranging from 2.5×10⁻⁵ to 2.5×10⁻³. Analyses of the cyclic stress–strain response and evolution of hysteresis loop shape indicate that the back stress component of the cyclic stress is significantly affected by grain size and plastic strain amplitude, whereas these parameters have little effect on friction stress. A nonlinear kinematic hardening framework was used to study the evolution of back stress parameters with cumulative plastic strain. These are related to substructural evolution features. In particular, long range back stress components are related to persistent slip bands. The difference in cyclic plasticity behavior between the two grain sizes is related to the effect of grain size on persistent slip band (PSB) morphology, and the effect this has on long range back stress. Fine grain specimens had a much longer fatigue life, especially at low plastic strain amplitude, as a result of the influence of grain size on fatigue crack initiation characteristics. At low plastic strain amplitude (2.5×10⁻⁴), coarse grain specimens initiated cracks where PSBs impinged on grain boundaries. Fine grain specimens formed cracks along PSBs. At high plastic strain amplitude (2.5×10⁻³), both grain sizes initiated cracks at grain boundaries.     

Dependence of dynamic strain ageing on strain amplitudes during the low-cycle fatigue of TP347H austenitic stainless steel at 550 °C

Low-cycle fatigue (LCF) tests are carried out on TP347H stainless steel at a strain rate of 8 × 10⁻³ s⁻¹ with total strain amplitudes (Δε_t/2) of ±0.4% and ±1.0%, at room temperature (RT) and 550 °C. It is found that the stress responses and dislocation structures under cyclic loading strongly depend on the value of strain amplitude at 550 °C. Compared with those at the same strain amplitude at RT, the material shows a rapid strain softening, and finally attains a stabilized state at Δε_t/2 = ±0.4% and 550 °C, but the one presents an anomalous behavior, i.e., first a rapid hardening to the maximum stress, followed by a reducing softening at Δε_t/2 = ±1.0% and 550 °C. More cells resulting from dislocation cross-slip and planar structures due to dynamic strain ageing (DSA) restricting cross-slip develop at low strain amplitude of ±0.4% at the first cycle. However, there are more complicated dislocation structures, such as cells, elongated cells, walls/channels and planar structures at Δε_t/2 = ±1.0%. The observations of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) exclude the effects of martensitic transformation, creep, oxidation, and precipitations on these stress responses and microstructure evolutions, which result from DSA appearing at 550 °C.     

A high-cycle fatigue life model for variable amplitude multiaxial loading

A high‐cycle fatigue life model for structures subjected to variable amplitude multiaxial loading is presented in this paper. It treats any kind of repeated blocks of variable amplitude multiaxial loading without using a cycle counting method. This model based on a mesoscopic approach is characterized by the following features: (i) the choice of a damage factor related to the accumulated mesoscopic plastic strain per stabilised cycle; (ii) the use of a mesoscopic mechanical behaviour taking into account the fatigue mechanisms such as plasticity and void growth. This behaviour is a von Mises elastoplastic model with linear kinematic hardening and hydrostatic stress dependent yield stress. The fatigue life model has six parameters identified with one SN curve and two fatigue limits. In‐phase and out‐of‐phase experimental tests from the literature are simulated. The predicted fatigue lives are compared to experimental ones.     

AZ31B镁合金压-压循环载荷下变形行为及变形机制演化EI北大核心CSCD

为探讨AZ31B挤压态镁合金棒材沿径向取样的循环变形特征,开展了0.75%,1.0%,2.0%和4.0%应变幅下应变控制的非对称压-压循环变形实验。结果表明:在小应变幅(0.75%,1.0%)下,循环变形的滞回曲线表现出较好的对称性;在大应变幅(2.0%,4.0%)下,滞回曲线对称性差,且在滞回曲线上出现拐点;随着循环周次增加,塑性应变幅呈现下降趋势,材料均表现出循环硬化行为,在小应变幅下循环拉伸阶段对材料硬化率远大于压缩阶段的硬化率,而在大应变幅下这种区别并不明显。分析表明,沿径向取向的〈1120〉丝织构镁合金,小应变幅下位错滑移在整个寿命周期内作用更大;大应变幅下,随着塑性变形的增加,循环过程中变形机制发生演化,较低临界剪切应力(critical resolved shear stress,CRSS)的基面位错和拉伸孪生不能完全满足变形要求,较高CRSS滑移系启动及残余孪晶使得滞回曲线出现拐点;循环变形过程中不完全的孪生-去孪生过程使基体中存在大量残余孪晶,影响了循环变形过程的硬化率,同时降低了疲劳寿命。     

A new model for describing a stable cyclic stress–strain relationship under non-proportional loading based on activation state of slip systems

This paper proposes a new simple model for cyclic incremental plasticity based on activation states of slip systems describing stable cyclic stress–strain relationships under non‐proportional loading. In the model, the magnitude and the direction of incremental plastic strain are estimated by (1+αf_NP) and Q , respectively. Here, α is the constant related to the dependence of material on additional hardening and f_NP the intensity factor expressing the severity of non‐proportional loading. Q is the second‐order tensor describing the activation states of slip systems in polycrystalline metals and is given by the calculation using a virtual specimen. The model was examined by application to the prediction of the stable cyclic stress–strain relationship in extensive non‐proportional low cycle fatigue tests for type 304 stainless steel and 6061 aluminium alloy. The simulated results showed that the model gave a satisfactory prediction of the stable cyclic stress–strain relationship under complex non‐proportional multiaxial loadings for the two materials.     

Cyclic Saturation Dislocation Pattern Observation in the Vicinity of Grain Boundaries by Electron Channeling Contrast Technique

State Key Laboratory for Fatigue and Fracture of Materials, Institute of Metal Research, Chinese Academy of Sciences,Shenyang, 110015, China)Abstract:The cyclic saturation dislocation patterns within grains and in the vicinity of low-angle grain boundaries in fatigued copper crystal were successfully observed by electron channeling contrast technique in SEM. The results show that the dislocation patterns within grains consisted of typical two-phase structure, i.e. persistent slip bands (PSB) and veins. With increasing plastic strain amplitude (γp1 ≥1.7×10-3), large amount of PSBs and regufar dislocation walls were observed.The dislocation walls and PSBs could cross through the low-angle grain boundaries continuously except that the dislocation-free zone (DFZs) appeared at some local regions. Combining with the cyclic stress-strain response and dislocation patterns, the effect of low-angle grain boundaries on cyclic deformation behavior was discussed.     

Modellierung des Einflusses von Temperatur und Verformungsgeschwindigkeit auf die Fließspannung von Ck 45 bei Temperaturen T ≲ 0,3 Ts

Modelling of the temperature and strain-rate dependance of the flow stress of Ck 45 at temperatures T ≲ 0.3 T_s After presenting the theoretical background of thermally activated dislocation slip an evaluation method is discussed, which allows to describe the influence of temperature and strain-rate on the flow stress. For this purpose it is neccessary to measure the flow stresses in tensile tests as well as the thermal flow stress jumps in strain-rate jump tests at different temperatures and strain-rates. The quality of this method is shown using experimental results obtained at specimens of normalized Ck 45 (SAE 1045). The constitutive law allows a reliable extrapolation of flow stress values to strain-rates up to 10⁺⁴ s⁻¹. The influence of plastic strain on the constitutive constants of the modelling law is discussed.     

The influence of pressurization-induced dislocations on the plastic deformation of LiF and NaCl monocrystals

Single crystals of LiF containing voids and of NaCl containing Na₂SO₄ precipitates were pressurized to introduce dislocations in the vicinities of the discontinuities and subsequently compressed along 100 at room temperature. The yield stress was raised in both materials; additionally, in LiF discontinuous yielding and easy glide were suppressed and work hardening rate increased by the pressurization-induced dislocations. Following pressurization at 0.85 GN m^–2, for example, the 0.1 % shear flow stress of LiF was doubled to 4 MN m^–2 and stage II work hardening rate quadrupled to 180 MN m^–2. Pressurization of NaCl above 0.6 GN m^–2 resulted in an increase in the 0.1 % flow stress from 1.2 to 2.0 MN m^–2. If the slip bands in LiF were initiated by a precompression, pressurization prevented the broadening of these fresh slip bands during subsequent plastic flow. Deformation now took place at a higher stress both in LiF and NaCl. These effects resemble in some ways latent hardening in that oblique as well as conjugate dislocation intersections must take place to continue the deformation. In contrast to latent hardening data, the strain hardening rate was increased in LiF and was approximately proportional to the pressurization-induced dislocation density. This ratio, 5 to 6 dyne per dislocation, is in fair agreement with two sets of independent calculations reported by Gilman and Johnston. The results suggest, therefore, that in the present case also hardening may be due to defects left in the wakes of pressurization-induced moving dislocations.     

Effect of γ′ size on room temperature low cycle fatigue behaviour of a nickel base superalloy

Abstract

The results of a study on the effect of γ′ particle size on the room temperature (23°C) low cycle fatigue (LCF) behaviour of a Ni base superalloy, Nimonic 90, is reported. The γ′ particle sizes were estimated from transmission electron micrographs. Ranges of particle sizes corresponding to underaged, peak aged, and overaged conditions were identified by examining the age hardening response curve. The solutionised samples had longer LCF lives compared with the aged alloys. Coffin-Manson and cyclic stress–strain plots showed bilinearity at a plastic strain amplitude of around 0.4% in the solutionised, underaged, and peak aged conditions. The observed bilinearity could be attributed to a change in the deformation mode from single slip to multiple slip. The cyclic stress response showed relatively stable behaviour for the peak aged and the overaged specimens compared with the underaged and the solution treated specimens.     

The study of grain boundary density effect on multi-grain thin film under tension

The grain-size effect on the yield strength and strain hardening of thin film at sub-micron and nanometer scale closely relates to the interactions between grain boundary and dislocation. Based on higher-order gradient plasticity theory, we have systematically investigated the size effect of multi-grain thin film arising from the grain boundary density under tensile stress. The developed formulations employing dislocation density and slip resistance have been implemented into the finite element program, in which grain boundary is treated as impenetrable interface for dislocations. The numerical simulation results reasonably show that plastic hardening rate and yield strength are linear to the grain boundary density of multi-grain thin film. The aspect ratio of grain size and orientation of slip system have distinct influence on the grain plastic properties. The research of slip system including homogeneous and nonhomogeneous distribution patterns reveals that the hardening effect of low-angle slip system is greater than that of high-angle slip system. The results agree well with the experimentally measured data and the solutions by discrete dislocation dynamics simulation.     

Internal friction,acoustic emission and fatigue in metals for high amplitude ultrasonic frequencies

High amplitudes (strains up to 3 × 10^?3) have been used in studying internal friction, acoustic emission and fatigue in metals. The internal friction in the high amplitude range is due to breakaway of dislocations from pinning points and by the generation of Frank-Read loops which results in plastic strain. There are two plastic regions, one when the added loops are held up by the grain boundaries, and the second when the loops break through the grain boundaries. In the second region, slip bands are produced in the metal. As the amplitude increases slip bands can join up and fatigue in the metal occurs. Ultrasonic frequencies are useful for studying fatigue since a large number of cycles can occur in a reasonable time.Acoustic emission—that is, a noise in the sample associated with the dislocation motion—can be studied by putting transducers on the sample. It is shown that the emission is closely associated with the internal friction since it goes through two regions in the plastic range.     

Strain hardening in nanolayered thin films

Experimental results indicate that metal–ceramic multilayered thin films have unusual properties such as high strength, measurable plasticity and high strain hardening rate when both layers are nanoscale. Furthermore, the strength and strain hardening rate show a pronounced size effect, depending not only on the layer thickness but also on the layer thickness ratio. We analyze the strain hardening behavior of nanoscale multilayers using a three-dimensional crystal elastic–plastic model (3DCEPM) that describes plastic deformation based on the evolution of dislocation density in metal and ceramic layers according to confined layer slip mechanism. These glide dislocations nucleate at interfaces, glide inside layers and are deposited at interfaces that impede slip transmission. The high strain hardening rate is ascribed to the closely spaced dislocation arrays deposited at interfaces and the load transfer that is related to the layer thickness ratio of metal and ceramic layers. The measurable plasticity implies the plastically deformable ceramic layer in which the dislocation activity is facilitated by the interaction force among the deposited dislocations within interface and in turn is strongly related to the ceramic layer thickness.     

Microstructure and texture of high manganese steel subjected to dynamic impact loading

ABSTRACT

Dynamic impact response of high Mn-steel at a strain rate of 3000?s^?1 was investigated using the Split Hopkinson Pressure bar. The investigated steel depicted continuous yielding at high strain rates. Additionally, the yield stress displayed a positive strain-rate sensitivity with an increasing strain rate. Microstructural evaluations displayed that strain-induced martensitic transformation and dislocation multiplication during slip were dominant plastic deformation mechanisms in the absence of deformation twinning which contributes to the strain hardening. Adiabatic shear band and martensite to austenite reversion or dynamic recrystallisation were also attributed to strain softening during impact deformation. The {001}<110> R-cube, {011}<110> R-Goss, and ({111}<110>) E texture components were strengthened after impact loading compared with as-received condition, while the intensities of Cube, Cupper, Brass, and S texture components were decreased.     

Cyclic stress–strain characteristics of two microalloyed steels

Abstract

The cyclic stress–strain behaviour of two microalloyed steels with different microstructures has been characterised at room temperature under strain controlled low cycle fatigue. The cyclic stress–strain curve in the double logarithmic plot shows a linear relation for both steels. A transition of the cyclic stress–strain curve from softening to hardening with increasing strain amplitude has been observed with respect to the corresponding tensile curve. The strain amplitude for the onset of cyclic softening to hardening transition has been found to be dependent on grain size. The strain lifetime behaviour, estimated from modified universal slopes equation, shows similar trends as Nb or V bearing microalloyed steels. The cyclic characteristics of the two microalloyed steels have been compared with corresponding predeformed state carried out under stress controlled conditions. While, cyclic saturation was observed in case where the extent of predeformation was within the Lüders strain, cyclic softening occurred when it exceeded the Lüders strain. It has been attempted to provide a mechanistic understanding of the differences in the cyclic behaviour of the two steels owing to the microstructure and predeformation.     

Constitutive modeling for flow stress of 55SiMnMo bainite steel at hot working conditions

The hot deformation behavior of 55SiMnMo bainite steel was studied through isothermal hot compression tests conducted using a Gleeble 3500 at 950–1100 °C, with strain rates of 0.01 s⁻¹ to 10 s⁻¹. A constitutive equation was established using the experimental results to describe the stress–strain relationship based on the dislocation density variation, considering the influence of the dynamic softening mechanism. When dynamic recovery is the only softening mechanism, a constitutive equation for flow stress was obtained from the variation of the dislocation density during hot deformation based on work hardening and dynamic recovery. When dynamic recrystallization occurs, the relationship between the dislocation density and the volume fraction of dynamic recrystallization was used to predict the flow stress after the peak. The reliability of the model was verified through a comparison between the predicted flow stress curves from the model and the experimental data.     

Low Cycle Fatigue Life Threshold for Titanium Aluminides

Although there exists a good understanding of dislocation glide mechanisms during fatigue of titanium aluminides, 1 fatigue life prediction remains a crucial issue because of the low amount of macroscopic plastic strain and the existence of microstructural inhomogeneities. The concept proposed here aims at a threshold value, allowing a reasonable low cycle fatigue life for isothermal and thermomechanical fatigue conditions. It can be stated that under isothermal testing conditions a maximum strain amplitude of Δε/2 = 0.35% (corresponding to a stress amplitude of about Δσ/2 = 500 MPa) for temperatures up to 750 °C is feasible for TNB‐V2 in order to reliably reach a sufficient LCF life. Concerning satisfactory thermomechanical fatigue life the proposed concept suggests that the acting combination of mean stress and strain amplitude needs to be located below the Goodman line. It can be concluded that with increase in maximum cyclic temperature and temperature range, the tolerable strain amplitude decreases. Because of the well‐documented unfavorable combination of tensile mean stresses and environmental attack during OP‐TMF, an acceptable strain amplitude for TMF conditions is significantly lower than under isothermal conditions.     

Strain rate effects on the mechanical behavior of nanocrystalline Au films

The effect of fabrication, film thickness, and strain rate on the mechanical behavior of Au films with 100 nm (evaporated gold) and 200 nm (electroplated gold) average grain sizes was investigated. Uniaxial tension was imposed at 10^− 3-10^− 6 s^− 1 strain rates on evaporated 0.5 μm and 0.65 μm thick Au specimens, and at 10^− 2-10^− 5 s^− 1 on electroplated 2.8 μm thick Au specimens. Strain rates between 10^− 3 and 10^− 5 s^− 1 had a marked impact on the ultimate strain of evaporated films and less significant effect on their yield and saturation stress. The ductility increased with decreasing strain rate and it varied between 2-4.5% for 500-650 nm thick films and 3.4-10.6% for 2.8 μm thick films. When compared at the same strain rate, the thick electroplated films were more ductile than the thin evaporated films, but their yield and saturation stresses were lower, possibly due to their larger grain size. Qualitatively, the stress-strain behavior was consistent at all rates except at the slowest that resulted in significantly different trends. A marked decrease of the maximum strength, effective Young's modulus, and yield strength occurred at 10^− 6 s^− 1 for thin, and at 10^− 5 s^− 1 for thick films, while for 500 nm thin films multiple stress localizations per stress-strain curve were recorded. Because of temperature, applied stress, and grain size considerations this behavior was attributed to dislocation creep taking place at a strain rate comparable to the applied strain rate.