Rapid Dislocation Multiplication Research Articles

Nano hydride precipitation-induced disappearance of yield drop in zirconium alloy at elevated temperature

Hydrogen-induced variations in mechanical behavior of zirconium alloys impose detrimental influence on nuclear fuel cladding integrity. This work reports a disappearance of intrinsic yield drop in a recrystallized zirconium alloy following hydrogen-charging treatment. Microstructure characterizations reveal that the nano-hydrides precipitation, mediated by second phase particles Zr(Fe,Cr)2 acting as hydrogen trapping sites, leads to emission of substantial dislocations in α-matrix grains due to strong strain concentrations, as identified by high-angular resolution EBSD. These mobile dislocations preserved at elevated temperatures can maintain the applied plastic strain and impede rapid dislocation multiplication as well, a conclusion validated by comparative analysis of dislocation densities prior to and near yielding stage. These findings are expected to shed light on the underlying mechanisms governing the interaction between hydrogen and microstructural defects in Zr-based nuclear fuel cladding materials.

Microstructural evolution and mechanical behaviors of rock salt in energy storage: A molecular dynamics approach

The microstructure of rock salt significantly influences its macroscopic mechanical behaviors and deformation phenomena. Understanding the deformation and failure characteristics of rock salt at multiple scales is crucial for the secure and efficient functioning of energy storage in salt caverns. Although the macroscopic behaviors of rock salt are well understood, the microstructural changes occurring under uniaxial compression have not been thoroughly investigated. This study aims to investigate the evolving laws of rock salt microstructure and mechanical properties during the damage process, thereby providing a fundamental understanding of the underlying mechanism. To achieve this, a series of characterization tests on rock salt were conducted to study its microstructure and defects. Building on this, molecular dynamics simulations were utilized in rock mechanics to elucidate the mechanical behaviors, structural evolutions, and dislocation motions of rock salt during the damage process. The rock salt obtained from Qianjiang, China, is a polycrystalline material with an average subgrain size of 46 nm and an average dislocation density of 4.76 × 10−6 1/Å2. Uniaxial compression of single crystal NaCl exhibits three stages: elastic compression, rapid dislocation multiplication, and forest hardening. Scattered dislocations and isolated dislocation tangles trigger further plastic slippage, leading to decreased strength. Later, the formation of a dislocation cell network hinders further slip and reinforces the strength of rock salt. Plastic deformation is found to be a dominant factor in grain refinement and the formation of polycrystalline structures. Intergranular cracking results from cross-patterned dislocation lines on grain boundaries, which become vulnerable areas of the structure. This study describes the evolutionary process of rock salt microstructures and mechanical properties, identifies the micro-destruction mechanisms during the damage process at the ionic level, and provides insights into the micro-mechanical behavior of rock salt and for the design and stability assessment of underground energy storage facilities.

Synergistic work hardening by rapid multiplication of dislocations in a multi-principal-element alloy.

Synergistic work hardening by rapid multiplication of dislocations in a multi-principal-element alloy.

Harnessing instability for work hardening in multi-principal element alloys

The strength–ductility trade-off has long been a Gordian knot in conventional metallic structural materials and it is no exception in multi-principal element alloys. In particular, at ultrahigh yield strengths, plastic instability, that is, necking, happens prematurely, because of which ductility almost entirely disappears. This is due to the growing difficulty in the production and accumulation of dislocations from the very beginning of tensile deformation that renders the conventional dislocation hardening insufficient. Here we propose that premature necking can be harnessed for work hardening in a VCoNi multi-principal element alloy. Lüders banding as an initial tensile response induces the ongoing localized necking at the band front to produce both triaxial stress and strain gradient, which enables the rapid multiplication of dislocations. This leads to forest dislocation hardening, plus extra work hardening due to the interaction of dislocations with the local-chemical-order regions. The dual work hardening combines to restrain and stabilize the premature necking in reverse as well as to facilitate uniform deformation. Consequently, a superior strength-and-ductility synergy is achieved with a ductility of ~20% and yield strength of 2 GPa during room-temperature and cryogenic deformation. These findings offer an instability-control paradigm for synergistic work hardening to conquer the strength–ductility paradox at ultrahigh yield strengths.

Microstructure-mechanical properties relationships and strain hardening mechanism for medium-Mn steel with low yield and ultra-high tensile strength

Considering the problems of forming ultra-high strength steel, a medium-Mn steel with low yield strength was prepared. The steel exhibited a yield strength of 537 MPa, a tensile strength of 1780 MPa, and an elongation of 17.8%. The study aimed to identify the key factors contributing to the low yield-to-tensile ratio and strain hardening behavior of the steel. To achieve this, interrupted tensile tests, loading-unloading-reloading tensile tests, and microstructure characterization were used, along with theoretical calculations. The results indicated that the low dislocation density in lath ferrite, initial martensite, and retained austenite, as well as the high-volume fraction of retained austenite, were the primary factors contributing to the low yield strength. During tensile deformation, strong solid solution strengthening in strain-induced martensite was the main factor driving strain hardening. Additionally, the rapid multiplication of dislocations in ferrite and martensite, along with the heterogeneous deformation-induced hardening caused by the heterogeneous structure composed of strain-induced martensite and other phases, were also significant factors contributing to strain hardening. When considering grain boundary strengthening, dislocation strengthening, solid solution strengthening, and heterogeneous deformation-induced hardening, the theoretical yield strength and flow stress calculated in this study closely matched the actual measured values.

Designing high ductility in magnesium alloys

The thermally activated pyramidal-to-basal (PB) transition of 〈c+a〉 dislocations, transforming glissile pyramidal dissociated core structures into sessile basal dissociated ones, lies at the origin of low ductility in pure magnesium (Mg). Solute-accelerated cross-slip and double cross-slip of pyramidal 〈c+a〉 dislocations have recently been proposed as a mechanism that can circumvent the deleterious effects of the PB transition by enabling rapid dislocation multiplication and isolating PB-transformed sessile segments. Here, the theory for solute-accelerated cross-slip is revisited with an explicit atomistic derivation, is extended to include multiple very dilute solute concentrations, and various aspects of the theory are demonstrated computationally. DFT inputs to the theory for a wide range of new alloying elements are presented. The theory is validated by comparing predicted ductility to literature experiments for a range of alloys. The theory is then applied to predict composition ranges for ductility in rare-earth free ternary and quaternary dilute alloys. The wide range of new alloys predicted to be ductile can serve as a guide to experimental development of new ductile Mg alloys.

Continuum simulation of the evolution of dislocation densities during nanoindentation

When nanoindenting dislocation-free regions of single crystals a so-called pop-in phenomenon is commonly observed. Molecular dynamics (MD) studies have revealed homogeneous nucleation of dislocations in a perfect crystal as a mechanism causing such pop-in behavior. In this work we transfer this knowledge gained on the atomic scale into a dislocation nucleation model that is applied within a dislocation density based crystal plasticity description. Furthermore, we develop a non-local formulation of a crystal plasticity model that is devised to yield a valid description of plasticity also in situations where the dislocation density is small or even vanishing and where conventional plasticity models fail. This is accomplished by studying the evolution of statistically stored and geometrically necessary dislocation densities separately. We apply this non-local crystal plasticity model to investigate the evolution of dislocation densities in the early stages of nanoindentation. The results of our continuum model show good agreement with MD simulations for cases where nanoindentation into an initially dislocation-free crystal is studied, i.e. where a pop-in occurs when the critical stress underneath the indenter reaches the critical value for homogeneous dislocation nucleation. After thus validating our model we study the influence of pre-existing homogeneous and local dislocation densities. Both cases show a good qualitative agreement with recent experimental findings and it is concluded that pre-existing local dislocations densities reduce the load at which a pop-in occurs and – more importantly – change the mechanism from homogeneous dislocation nucleation to rapid dislocation multiplication. In general, our results show that continuum plasticity formulations can be extended such that applications to nanoscale volumes become possible.

Hot deformation behavior of Co-base ODS alloys

Co-base ODS alloys, strengthened by nanosized oxide dispersion and γ′ precipitates, were prepared by mechanical alloying. Hot compression tests were performed in the temperature range of 25–970°C with the strain rate varying from 0.0001 to 0.01s−1. The influence of deformation parameters and Y2O3 content on the flow behavior and the microstructure of compressive specimens were investigated. It was observed that Co-base ODS alloys had exhibited the homogeneous distribution of ultrafine γ′ precipitates (0.26μm), nanosized oxide particle (12.5nm), and small grains (1.1μm). The enhanced flow stress at low temperatures and high strain rates was attributed to the rapid multiplication of dislocations due to the pining effect of oxide dispersion, γ′ precipitates, and fine grains. The peak stress had decreased with increasing the deformation temperature as well as by decreasing the strain rate. The obvious improvement in flow stress was acquired by increasing the Y2O3 contents. Grain boundary sliding and porosity growth had been considered to be the main softening mechanisms during the hot deformation at elevated temperature. Moreover, It was found that the flow softening could be accelerated by decreasing the strain rate. The activation energy had increased with increasing temperature, but decreased with decreasing strain rate.

A Study of High Temperature Viscoplastic Deformation of Beta Titanium Alloy Considering Yield-Point Phenomena

In this paper, the high temperature, deformation behaviour of beta titanium alloy Ti-20V-4Al-1Sn sheet is studied by performing uniaxial tension experiments at three different strain rates at high temperatures of 700°C, 750°C and 800°C. The stress-strain curves for these temperatures show strain rate sensitivity, yield point phenomena and continuous flow, softening patterns. Microstructures of deformed specimens at several representative deformation stages and different strain rates are studied using an optical microscope. Dynamic recovery does not occur at the early stage of deformation including the yield-point and the subsequent yield drop regime, but it is activated at a large deformation stage, where it is affected by both strain rate and strain. A viscoplastic, constitutive model, based on the assumption of rapid dislocation multiplication, is proposed to describe such high temperature, yield-point phenomena. In this modelling, the softening effect due to dynamic recovery is also considered. The stress-strain responses, predicted by the constitutive model, well capture the yield-point phenomena, strain rate sensitivity and subsequent continuous flow, softening behaviour of the beta titanium alloy.

Yield-Point Phenomena of Ti-20V-4Al-1Sn at 1073 K and Its Constitutive Modelling

The deformation behaviour of β titanium alloy Ti-20V-4A1-1Sn sheet at 1073K was investigated by performing uniaxial tension experiments. The stress-strain curves show a sharp yield point and the subsequent abrupt yield drop followed by the strain softening. From the EBSD analysis, a crystallographic misorientation was found in grains even at an early stage of yielding, even though the dynamic recovery had not yet taken place. To describe such a characteristic stress-strain response, a viscoplastic constitutive model is proposed that is built on the premise that the yield point phenomena are associated with the rapid dislocation multiplication at an early stage of yielding.

A plasticity model describing yield-point phenomena of steels and its application to FE simulation of temper rolling

To describe the yield-point phenomena of steels, an extended version of the first author’s model (Yoshida, F., 2000. A constitutive model of cyclic plasticity. International Journal of Plasticity 16, 359–380) is proposed on the premise that the material behavior of sharp yield point and the subsequent abrupt yield drop result from a rapid dislocation multiplication and the stress-dependence of dislocation velocity. A specific feature of this model is that it describes well a high upper yield point, the rate-dependent Lüders strain at the yield plateau and the subsequent workhardening, as well as cyclic plasticity characteristics, such as the Bauschinger effect and rate-dependent ratcheting. Using this model, an FE simulation of temper rolling process is conducted in order to clarify its role for the elimination of the yield point of steel sheets. Particularly, the effect of upper yield point on the deformation characteristics in the process is discussed.

A Constitutive Model for the Mechanical Behavior of Single Crystal Silicon at Elevated Temperature

AbstractSilicon in single crystal form has been the material of choice for the first demonstration of the MIT microengine project. However, because it has a relatively low melting temperature, silicon is not an ideal material for the intended operational environment of high temperature and stress. In addition, preliminary work indicates that single crystal silicon has a tendency to undergo localized deformation by slip band formation. Thus it is critical to obtain a better understanding of the mechanical behavior of this material at elevated temperatures in order to properly exploit its capabilities as a structural material. Creep tests in simple compression with n-type single crystal silicon, with low initial dislocation density, were conducted over a temperature range of 900 K to 1200 K and a stress range of 10 MPa to 120 MPa. The compression specimens were machined such that the multi-slip <100> or <111> orientations were coincident with the compression axis. The creep tests reveal that response can be delineated into two broad regimes: (a) in the first regime rapid dislocation multiplication is responsible for accelerating creep rates, and (b) in the second regime an increasing resistance to dislocation motion is responsible for the decelerating creep rates, as is typically observed for creep in metals. An isotropic elasto-viscoplastic constitutive model that accounts for these two mechanisms has been developed in support of the design of the high temperature turbine structure of the MIT microengine.

Cyclic Plasticity Behavior of Mild Steel with Special Emphasis on Yield-Point Phenomena.

Annealed steels exhibit a sharp yield point and the subsequent abrupt yield drop followed by the yield plateau (Luders deformation) in their stress-strain curve of uniaxial tension. This paper provides some typical examples of experimental data of cyclic plasticity with special emphasis on the yield-point phenomena for an annealed mild steel, i.e., uniaxial tension at several crosshead speeds;cyclic straining;stress- and strain-controlled ratchetting. In order to describe the stress-strain response characteristics such as the yield-point phenomena, cyclic hardening/softening, the Baus-chinger effect and rate-dependent ratchetting, constitutive modelling of cyclic elasto-viscoplasticity is discussed on the premise that the sharp yield point and the subsequent abrupt yield drop result from the rapid dislocation multiplication and the stress-dependence of dislocation velocity.

A constitutive model of cyclic plasticity

Abstract This paper addresses a constitutive model of cyclic plasticity with special emphasis on the yield-point phenomena. In order to point out the deformation characteristics of a mild steel, four types of experiments were conducted, i.e. uniaxial tension at several crosshead speeds, cyclic straining, and stress- and strain-controlled ratchetting. A viscoplastic constitutive model of cyclic plasticity is proposed on the premise that the phenomena of sharp yield point and the subsequent abrupt yield drop result from rapid dislocation multiplication and the stress-dependence of dislocation velocity. Besides, cyclic plasticity behavior, such as the Bauschinger effect, cyclic hardening/softening characteristics and ratchet-strain accumulation, is described by some kinematic and isotropic hardening rules. The cyclic stress–strain responses predicted by this model agree well with the corresponding experimental results.

Plastic deformation of single glide oriented Cu-2 to 15at.%Al crystals at elevated temperatures

Single glide oriented Cu-2 to 15at.%Al single crystals were deformed in tension at temperatures between 300 and 1000 K with repeated strain rate changes and stress relaxations. On several crystals the development of slip bands was observed during deformation through an optical microscope and recorded by video. The gradual transition from smooth to serrated (Portevin-Le Châtelier, PLC) slip was examined by determining the load fluctuations in correlation with the number of active slip and bundles and their evolution. Rapid data recording permitted the evaluation of load drops in the PLC regime similar to stress relaxations with respect to the apparent activation volumes. The results are discussed in terms of breakaway of a source dislocation from its solute atmosphere, rapid multiplication of dislocations during the load drop, and their recapture by diffusing solutes. The transition from PLC serrations to smooth deformation at higher temperatures, where beyond the maximum of the critical resolved shear stress and the strain rate sensitivity the slip lines disappear completely, was investigated by comparison of the measured strain rate sensitivity with the concepts of the formation of solute atmospheres at gliding dislocations and their viscous drag.

High temperature tensile deformation behavior of β-Ti alloys

The high temperature tensile deformation behavior of β-Ti alloys has been investigated as function of strain rate, temperature and type of alloying elements. It was found that the magnitude of the flow stress drop increases with strain rate and solute concentration and decreases with temperature. For a given temperature and strain rate the extent of the drop decreased with a decrease in the relative atomic size between the solvent (titanium) and solute (manganese and vanadium) and a decrease in the diffusivity of the alloying element in the β phase. In addition, it was found that prestraining to a stress below the peak stress significantly reduces the peak stress and the magnitude of the flow stress drop. On the basis of these results it is suggested that the initial flow stress drop is due to the rapid multiplication of dislocations. The steady state behavior after the initial flow stress drop was attributed to dynamic recovery leading to the formation of subgrains.

Elevated temperature yield effect in iron-aluminum

The pronounced stress drop or yield-point phenomenon that is observed in partially ordered Fe-Al alloys which contain approximately 25 at.% Al has been evaluated by means of elevated temperature, constant strain-rate tests. The magnitude of the effect is found to depend on the particular combination of strain rate and test temperature employed. No stress drop is observed when complete Fe 3Al order is present. However, a stress drop is observed whenever FeAl or partial Fe 3Al order is present; this corresponds to a temperature region where the flow stress is unusually sensitive to the strain rate. Transmission electron microscopy revealed that the stress drop at yield is associated with an initially low dislocation density and rapid dislocation multiplication. Consequently, the yield phenomenon is believed to be of the type first discussed by Gilman and Johnston wherein a stress drop is caused by rapid dislocation multiplication in a structure in which the dislocation velocity is relatively insensitive to the stress.

Plastic deformation of anthracene single crystals

Tensile tests have been carried out on single crystals of anthracene over a range of temperatures and strain rates. A yield point is observed and plastic deformation occurs by slip in either the [010] or [110] directions in the basal plane. The yield point appears to be due to an initial difficulty in dislocation generation followed by a rapid multiplication of dislocations on yielding, rather than an impurity-locking effect. The critical shear stress for yielding and the appearance or non-appearance of the yield point are dependent on the surface finish of the crystals, presumably because the active dislocation sources are mainly at the surface. At temperatures less than T/ T M = 0.67 for slip on (001) [010] the Peierls force appears to be a major factor in determining the critical shear stress for plastic deformation to begin, while for slip on (001) [110] the stress field due to defects probably makes a major contribution.

Deformation mechanisms of metals hardened by a dispersed, incoherent, second phase

Hardening of fcc metals by incoherent particles is analysed. The presence of the particles indirectly determines the rapid multiplication of dislocations in the matrix. The hardening is related to the dense tangles so formed. Recovery of these tangles occurs by cross-slip, climb, and probably by some other complex, thermally activated mechanisms.

The relationship between flow stress and dislocation density in deformed vanadium

The dislocation configuration and density produced by plastic deformation of vanadium in the temperature range 77°K to 673°K have been investigated. In material of constant grain size (2 d = 0.0013 cm) the dislocation density N is proportional to the strain ε and √ N is proportional to the flow stress τ over the whole temperature range investigated. Plastic deformation in and just above the jerky flow region results in a more rapid increase of N with increasing ε compared with plastic deformation below this temperature range where N is independent of temperature for equivalent strains. This rapid rate of increase of N with ε in the jerky flow region is thought to be a result of rapid multiplication of dislocations unpinned from impurity atmospheres. Above the jerky flow region the high rate of increase of N with increasing ε can be explained if diffusing solute atoms decrease the dislocation velocity. In material of different grain sizes deformed to the same strain at 20°C, √ N is proportional to d − 1 2 and the slope of the line is related to the K-parameter for flow. The value of κ f obtained in this way is in good agreement with the result of a grain-size analysis of the flow stress for the same strain.