Discontinuous Yielding Research Articles

Normalized characterization of the causes of discontinuous yield and crystal orientation deflection mechanism of β-Ti alloy during high-temperature deformation

To investigate the changes in grain orientation resulting from dislocation slip systems and propose a novel approach to characterizing discontinuous yield behavior during hot deformation, Ti–6V–5Al–5Mo–5Cr–3Nb–2Zr-0.2Si alloy was melted using vacuum arc furnace and subjected to hot compressed at 740, 790, 840 and 890 °C, along with strain rates (ε˙) of 0.001, 0.01, 0.1 and 0.5 s−1. The results indicate that discontinuous yield is observed at temperatures (T) of 740 or 790 °C and ε˙ of 0.1 and 0.5 s−1. Discontinuous yielding occurs at all strain rates when T is 840 and 890 °C. The presence of discontinuous yield is only observed when the power dissipation value exceeds the critical value, which in this study is 0.2657. Notably, this is the first instance in which the power dissipation value has been utilized to characterize the presence of discontinuous yield. The microstructure of the discontinuous yield reveals the activation of dislocation slip in the β grain. The dislocation slip is initially activated in the <001> oriented β grains during hot compression, followed by activation in the adjacent <101> oriented β grains. When the power dissipation exceeds the critical value, the dislocations more readily spread to the <101> oriented β grains due to higher activation energy, which is an important reason for discontinuous yield. The <101> oriented β grains gradually deflect to <2−3−1> oriented and finally rotate to <001> oriented during the subsequent compression process, as a means of reducing deformation resistance.

Effect of Grain Size on the Local Deformation Behavior Associated with the Discontinuous Yielding in a High Mn Austenitic Steel

Effect of Grain Size on the Local Deformation Behavior Associated with the Discontinuous Yielding in a High Mn Austenitic Steel

Phase transformation and deformation behavior under isothermal compression in β-quenched metastable Ti-10V-2Fe-3Al alloy

Comprehensive knowledge on the hot deformation behavior and microstructure evolution of metastable titanium alloys is critical for process optimization and microstructure regulation. The isothermal compression of β-quenched metastable titanium alloys (Ti-10 V-2Fe-3Al) was experimented at different temperatures (600 ℃−800 ℃) with the strain rate of 10−3 s−1 in present work. The stress-strain curves show that the flow stress reduces with increasing temperature, and the characteristics of the stress-strain curve at low temperature (600 ℃) are clearly distinct from those at other high temperatures. During the isothermal compression process at 600 °C, the deformation went through four stages: initial linear hardening (stage Ⅰ), Discontinuous yielding phenomenon (DYP, stage Ⅱ) followed by work hardening (stage Ⅲ) and final flow softening (stage Ⅳ). Further observations of the microstructure throughout each deformation stage revealed that the precipitation of αGB and the proliferation of dislocation lead to initial linear hardening behavior. The softening of deformation behavior during the discontinuous yield stage is originated from the fragmentation of αGB and the generation of a large amount of moving dislocations. Almost all β phase transformed into lamellar α phase as the deformation degree increased, which is the main cause of work hardening after discontinuous yield. The major causes of final flow softening are the formation of deformation bands and spheroidization of α phase. The present work analyses the correlation between the deformation behavior and microstructure features on the deformation mechanisms of metastable β titanium alloys and reveals the potential mechanism of metastable β alloy under thermal deformation and provides more comprehensive information for the optimization of the process parameters of hot deformation.

Discontinuous yielding behavior, strengthening effect and deformation mechanism of double wall copper-brazed steel tube under various continuous sectional cooling

Unlike traditional cooling, continuous sectional cooling (CSC) consists of various cooling modes, which is of decisive significance to the microstructure and performance of double wall copper-brazed steel tube (DWBT). In this work, three CSC (CSC-A/B/C) processes are chosen to reveal strengthening and deformation mechanisms of DWBT. The results show that there are significant differences in discontinuous yield behavior, natural aging phenomenon, and tensile properties of the DWBT among three CSC processes due to diverse grain size and precipitates, but a similar texture is obtained due to Kurdjumov-Sachs relationship. For discontinuous yielding, the propagation velocity of Lüders band is a key factor to determine the yield point elongation (YPE), which reduces with decreasing grain sizes due to the interaction between dislocation and grain boundary. Thus, the CSC-C (1120 °C→4s940 °C…360 °C→16s110 °C) with small grains has a longer YPE. Meanwhile, the precipitation strengthening dominates the highest strength and lowest elongation of the CSC-A (1120 °C…100 °C) due to abundant MC carbides. Compared with CSC-B (1120 °C…170 °C), the yield strength of CSC-C is higher while ultimate tensile strength is lower, which is attributed to different contributions of precipitation and grain boundary strengthening to both. The MC carbide causes the natural aging of DWBT. Moreover, the geometrically necessary dislocations (GNDs) and low angle grain boundaries (LAGBs) raise linearly with strain. The GNDs prefer to be distributed in small grains, and uniform grain can weaken the difference of GNDs distribution in different grain sizes. The LAGBs including dense dislocation walls or microbands are formed by the grain subdivision mechanism.

Discontinuous Yielding of Fe E420 under High Strain Rate Loading

The discontinuous yielding behaviour of Fe E420 steel was experimentally investigated through tensile tests conducted on laminated sheets. In order to assess the response under both quasi-static and dynamic loading conditions, uniaxial tensile tests were performed according to ASTM E8 standards, and dynamic tests were conducted on the Hopkinson bar using a specially designed flat specimen. Three nominal strain rates (2×10−4, 500, and 1000) s−1 were considered. The effect of heat treatment on the material was also considered. For this purpose, the proposed experimental campaign was carried out on both the as-is material and on one treated by water quenching. All dynamic tests were performed with the direct tensile split Hopkinson pressure bar and were recorded via a high frame rate camera. The post failure fracture surfaces were investigated using scanning electron microscopy technique.

Evolution of Poisson’s Ratio in the Tension Process of Low-Carbon Hot-Rolled Steel with Discontinuous Yielding

Low-carbon hot-rolled steel generally undergoes a deformation process composed of four phases, i.e., elastic deformation, discontinuous yielding, work hardening, and macroscopic plastic-strain localization in a tension test. The evolution of the Poisson’s ratio in terms of the average Poisson’s ratio and the local Poisson’s ratio in the deformation process from the non-load state to the onset point of specimen necking was investigated. The main results are as follows: (1) the average Poisson’s ratio cannot accurately represent the local Poisson’s ratio in the discontinuous-yielding phase; (2) the Poisson’s ratio varied significantly within a plastic band in the discontinuous-yielding phase, and the maximum Poisson’s ratio was reached within the plastic band; and (3) the strain rate greatly increased the Poisson’s ratio.

Discontinuous Yielding Mechanisms in Tempered Hierarchical Duplex Phase Steel with High-Density Dislocations

Discontinuous Yielding Mechanisms in Tempered Hierarchical Duplex Phase Steel with High-Density Dislocations

Toward Eliminating Discontinuous Yielding Behavior of the EA4T Steel.

Cold-rolled EA4T steel was heat-treated by inter-critical holding at 755 °C for 90, 120, 180, and 240 s, respectively, and then quenching in water. The tensile testing results of the EA4T specimens show an evident transition from the discontinuous yielding to the continuous yielding of the steel specimens by prolonging the holding time. A novel relationship between the discontinuous yielding behavior of tensile-deformed steel specimens and the carbide size was proposed based on experimental results and Cottrell’s theory. The model may provide a new clue for avoiding discontinuous yielding and improving mechanical properties of metals with static strain aging behaviors.

How the skin-pass rolling reduction ratio affects the strain aging behaviour of low-carbon steel sheets

ABSTRACT When producing cold-rolled steel sheets, the penultimate annealing process is followed by a final skin-pass rolling (or temper rolling) whose purpose is to eliminate any discontinuous yielding which creates stretcher–strain markings when the steel sheets are formed. To find out the critical reduction ratio eliminating the discontinuous yield phenomenon, the samples were annealed at 820°C for 3 min and then subjected to the temper rolling at various thickness reductions at room temperature. Results showed that the discontinuous yield phenomenon could not be eliminated in the case of a thickness reduction less than 0.5%, that a certain reduction ratio should be exceeded to eliminate the discontinuous yielding, and that the aging time varied depending on the reduction ratio. In addition, while the low-carbon steels did not undergo strain aging in three months with a 1% thickness reduction, they underwent strain aging in at least four months with higher thickness reductions.

Effect of Discontinuous Yielding on the Strain Hardening Behavior of Fine-Grained Twinning-Induced Plasticity Steel

The yielding of a high Mn twinning-induced plasticity steel was examined in three fine-grained specimens recrystallized at 700°C for 5 min with different cooling conditions. While the stress-strain curves of furnace-cooled and air-cooled specimens exhibit a stress drop at yielding, the drop was not observed in the water-quenched specimen. A simple analysis of the displacement data indicates the occurrence of localized deformation at the beginning of the plastic deformation in the three tensile specimens with different cooling conditions. The localized deformation of all three specimens was confirmed as Lüders strain by digital image correlation (DIC) analysis. Based on this observation, the role of yielding behavior on the strain hardening rate evolution at an early stage of the tensile deformation was discussed.

Continuous and Discontinuous Yielding Behaviors in Ferrite-Cementite Steels

The continuous and discontinuous yielding behaviors in ferrite-cementite steels were complementarily investigated via nano- and macro-scale deformation examinations. The continuous yielding behavior was observed in heat-treated specimens with a lamellar or cementite-spheroidal structure even after strain-aging treatment. However, the discontinuous yielding behavior accompanied by the Luders elongation appeared in ferrite-cementite steel that was recrystallized after cold rolling. The results obtained by electron microscopy, synchrotron X-ray, and neutron diffractions indicate that the ferrite-cementite interface of the heat-treated specimen is semi-coherent with a high internal stress field, whereas that of the recrystallized one is incoherent with a low internal stress field. Moreover, coherency strain, which depends on the total area of the ferrite-cementite interface, and thermal strain, which is governed by temperature, are the two factors that influence peak broadening. The nanoindentation tests revealed that the critical loads are significantly lower near the semi-coherent interface than those near the incoherent interface and the ferrite grain boundary; this suggests that dislocations are easily emitted from the semi-coherent interface.

Discontinuous Yielding in High Temperature Deformation of Ti-5553 Alloy

The discontinuous yield behavior of Ti-5Al-5Mo-5V-3Cr-0.5Fe alloys, a new metastable β titanium alloy, was tested at 800∼860 °C and in strain rate range of 0.1∼10 s−1 on a Gleeble-1500 thermal Simulation testing machine. Results show that discontinuous yielding appears at the highest strain rate (10 s−1), and the magnitude of discontinuous yielding increases with increasing of the temperature. The above mentioned phenomenon is in agreement with Johnston and Gilman's dynamic yield theory, that is to say, the discontinuous yield behavior is related to the sudden multiplication of a great quantity of mobile dislocations in the alloy.

High Temperature Discontinuous Yielding in a New Near β Titanium Alloy Ti-7333

The high temperature discontinuous yielding behavior of Ti-7333 (Ti-7Mo-3Nb-3Cr-3Al), a new near β titanium alloy, has been examined between 770 and 845 °C over strain rates from 10−3 s−1 to 1 s−1. The results show that the discontinuous yielding of the alloy occurs when its strain rate is above 10−2 s−1 at all the experimental temperatures. The magnitude of the discontinuous yield drop for Ti-7333 alloy increases with decreasing of strain rate at a fixed temperature, and decreases with decreasing of temperature at a fixed strain rate. The magnitude is more sensitive to the strain rate than the temperature in this investigation. The discontinuous yield phenomena observed is associated with the mobile dislocation generated in grain boundaries during plastic deformation spreading to the grain interior with increasing of strain. The grain boundaries act as the sources of mobile dislocation generation. Further analysis suggests that the discontinuous yield drop is related to unlocking of dislocation sources through a localized dislocation climb, a boundary migration and/or a solute rearrangement.

1036 多結晶金属の不連続降伏の転位動力学解析(OS9,OS10 合同ポスターセッション,OS・一般セッション講演)

1036 多結晶金属の不連続降伏の転位動力学解析(OS9,OS10 合同ポスターセッション,OS・一般セッション講演)

The Petch–Hall Factor during Discontinuous Yielding

AbstractIt is shown that the dynamical properties of dislocations alone cannot usually explain the magnitude of the yield drop and the gtain-size-dependence of the Luders strain found experimentally for mild steels. On the basis of a great deal of experimental evidence a Petch–Hall term which decreases with increasing dislocation density is proposed. Explicit expressions of the lower yield stress and the Luders strain, containing the relevant dislocation-dynamical, structural, and experin1ental parameters, are presented.

Determination of Dislocation Velocities and Densities from the Deformation Waves of Discontinuous Yielding

The deformation waves that accompany the repeated discontinuous yielding of metals are analyzed with respect to utilizing their characteristics to determine dislocation velocities and densities as functions of stress and strain. Repeated yielding experiments were performed on an annealed 2024 aluminum alloy and an α-brass using a ``dead-weight'' loading apparatus. Measurements were taken of deformation wave velocities and widths and the associated plastic strain increments and strain rates. The calculated dislocation velocities and densities appear to be reasonable and consistent.

Internal friction and electron microscope studies of strain-ageing in zirconium

Internal friction experiments and transmission electron microscopy have been used to throw further light on the mechanism of strain-ageing which occurs at about 300 °C in zirconium and zirconium-oxygen alloys. Internal friction specimens which have been heat-treated and deformed in a manner which leads to a discontinuous yield point in a tensile test show only logarithmic damping and the implication is that under these conditions the dislocations are strongly pinned. On the other hand in internal friction specimens treated in such a way that discontinuous yielding is not observed a transition from logarithmic to non-logarithmic damping takes place. Electron microscope studios of dislocation arrangements in sheet specimens given the same treatment as the internal friction wires have shown that the specimens which display a discontinuous yield point contain a well defined cellular structure of tangled dislocations. It is believed that with such a structure the dislocations are strongly pinned as a result of both solute atom locking and dislocation interactions. In specimens deformed under such conditions that no discontinuous yield point occurs a more uniform array of dislocations is formed. Here the pinning of the dislocations is much weaker because the effects of both solute locking and interaction locking are reduced and, under these conditions, the stresses imposed on the specimen during the internal friction experiments are sufficient to cause unpinning to take place. Some observations on the variation of logarithmic damping with temperature are also reported. These have been interpreted as arising from sub-grain boundary relaxation effects.

Allen–Wallace Zone of the San Andreas Fault and Discontinuous Yielding of a Slip Band in a Crystal

Weertman has suggested that a model derived for the discontinuous yielding of a slip band in a crystal could be applied to occurrence. In this article the application of the model to a region of the San Andreas fault is discussed and some predictions are made.

Grain-size dependence of discontinuous yielding in strain-aged steels

Recent studies of discontinuous yielding in polycrystalline metals support interpretations of the Hall-Petch relationship which relate k y to the local stress required to spread yielding across grain-boundaries at a yield front. Changes in yield-point behaviour due to strain-ageing can be rationalised on the assumption that the growth of k y provides a measure of the increase in the average value of this local stress. In low-carbon steels, the first phase of ordinary strain-ageing is concerned with Cottrell-atmosphere formation. In test-pieces aged within this first stage, mobile dislocations are almost certainly released by unpinning, but after later stages of ageing the magnitude of the discontinuous yield becomes insensitive to continued solute segregation to dislocations: yield-nucleation by unpinning is then unlikely. After the first stage of ageing it is probable that mobile dislocations are nucleated from grain-boundaries. Thus a reduction in solute segregation at boundaries, by quenching from ~700°C before prestraining, is accompanied by a decrease in the second-stage (“plateau”) values of Luders strain and k y . It is suggested that prestraining causes a reduction in the stress required to generate dislocations from boundaries and that the final slow increase in k y is due to a transfer of interstitial solute from grain interiors to unsaturated grain-boundaries.

Role of Grain Boundaries in the Discontinuous Yielding of Low-Carbon Steels

The grain-size-dependence of the lower yield stress in a low-carbon steel, as defined by k y in the Petch relationship, can be reduced by rapid cooling from temperatures above ∼ 400° C. The rate of return of k y towards its saturation value, during subsequent ageing, has been measured in a partially decarburized steel quenched from 700° C, and in a low-carbon steel quenched from 450° C. The time taken to restore k y during ageing at 90° C ( > 103 min) showed that the process depends on interstitial-solute segregation to grain-boundary regions, rather than on relocking of dislocations in the body of the grains. Plastic deformation took place during quenching from 700° C and many of the new dislocations were stopped at grain boundaries; possibly such boundary dislocations play a part in nucleating slip at a yield front in samples quenched from 700° C. No significant plastic deformation occurred in quenching from 450° C; in this case the reduction in k y on quenching, and its subsequent restoration in ageing, provide evidence that k y can depend on solute segregation to the boundaries themselves. Metallographic observations of dislocation distributions in the early stages of plastic deformation support the idea that dislocations are generated from some high-angle boundaries without the help of intense stress concentrations. It is concluded that, in slowly cooled and in fully strain-aged steels, k y is probably related directly to the local stresses required to nucleate dislocations from grain boundaries at the yield front.