Dislocation Pile Research Articles

Hot deformation behavior and microstructural evolution of titanium‐aluminum based alloy during hot compression

AbstractIn this paper, titanium‐aluminum based alloy was successfully prepared by introducing titanium powders using powder metallurgy. The experimental results indicated that the microstructures of alloys were composed of the new trititanium‐aluminium layers skeleton and the γ+α2 phases filler, which exhibited excellent compression properties. The compressive strength of the titanium‐aluminum based alloy (10 wt.% titanium) were 509.9 MPa, higher than monolithic Ti‐48Al‐2Cr‐2Nb alloy at 800 °C and 1×10−4 s−1. The deformation mechanism is mainly referred to the motion and rotation of γ+α2 areas and dynamic recrystallization. The γ+α2 areas were surrounded by complete new trititanium‐aluminium layers, which is beneficial to dislocation pile‐up, cross and tangle at grain boundaries, resulting in high strength. Besides, the dislocation pile of γ, α2 phase, and twins in γ phases, are the deformation mechanism in alloys.

Dislocation motion induced by pulse current in Ti/Al bimetal composite observed by in-situ TEM

The in-situ transmission electron microscopy (TEM) was applied to investigate the effect of pulse current on the dislocation evolution of Ti/Al bimetal composite sheet. By comparison of electrical in-situ and thermal in-situ experiment, the dislocation moving along the electron wind direction were observed in the Al matrix and interface region. The results revealed that the dislocation behavior of the Al matrix and interface were affected not only by Joule heating effect, but also by non-thermal effect. In contrast, the dislocation motion of the Ti matrix was more significantly affected by Joule heating effect. The effect of the pulse current on dislocation evolution can be divided into three stages. When the current was <0.8 mA, the current has little effect on the dislocation of Ti/Al bimetal composite sheet. When the current was between 0.8 mA and 2.2 mA, the dislocation motion appeared to be enhanced by current. But the effect was confined, and the dislocation pile up did not improve. When the current is >2.4 mA, the dislocation motion was promoted significantly due to the pulse current, and the dislocation entanglements were facilitated to open. The dislocation density of Ti/Al bimetal composite sheet decreased.

Dependence of tensile deformation of twinning-induced plasticity steel on temperature

• Investigating the mechanical behaviors and underlying micromechanics in TWIP steels. • Profuse deformation twins are introduced in TWIP steel at cryogenic temperature. • The cross GBs and TBs provide an easy mean for cracking in TWIP steel. We study the tensile deformation behaviour and microstructure evolution of twinning-induced plasticity (TWIP) steel at room and cryogenic temperatures. The TWIP steel shows the higher strain hardening capability, excellent strength and obvious ductility loss at cryogenic temperature. The TWIP steel has excellent strength at cryogenic temperature due to the synergistic effect of dislocation slip and deformation twinning. High-density nanotwin structure can cause strong strain hardening capability. In general, strong strain hardening leads to uniform plastic deformation, which provides high ductility. According to the microstructure characterization, the intersecting position of high-density twin-grain and twin-twin boundaries act as the nucleation sites for nanovoids, which were formed by dislocation pile up. Therefore, its ductility reduced at cryogenic temperature even with continuous steady strain hardening.

Revealing the Role of Cross Slip for Serrated Plastic Deformation in Concentrated Solid Solutions at Cryogenic Temperatures

Serrated plastic deformation is an intense phenomenon in CoCrFeMnNi at and below 35 K with stress amplitudes in excess of 100 MPa. While previous publications have linked serrated deformation to dislocation pile ups at Lomer–Cottrell (LC) locks, there exist two alternate models on how the transition from continuous to serrated deformation occurs. One model correlates the transition to an exponential LC lock density–temperature variation. The second model attributes the transition to a decrease in cross-slip propensity based on temperature and dislocation density. In order to evaluate the validity of the models, a unique tensile deformation procedure with multiple temperature changes was carried out, analyzing stress amplitudes subsequent to temperature changes. The analysis provides evidence that the apparent density of LC locks does not massively change with temperature. Instead, the serrated plastic deformation is likely related to cross-slip propensity.

Моделювання взаємодії дислокацій пластичної зони з границею зерна в області температур крихко-пластичного переходу в молібдені

The (DD) method was used to model the formation of the plastic zone of the top of the cracks in polycrystalline molybdenum. Special attention was paid to take into account the interaction of dislocations in the plastic zone with grain boundaries. Structural sensitivity of fracture toughness was analyzed under brittle-ductile condition. Simulations were performed for a range of grain sizes from 400 to 100 μm, at which a sudden increase in fracture toughness with a decrease of grain size was experimentally shown. We calculated the value of K1c taking into account the shielding action of dislocations. The position of all dislocations in the plastic zone at fracture moment was calculated. Based on these data, we obtained the dependences of dislocation density on the distance from the crack tip thereby confirming significant influence of the grain boundaries on plastic zone formation. At large grain sizes, when the plastic zone does not touch the boundary, the distribution of dislocations remained unchanged. As grains reduce their size to size of the plastic zone, they start formating a dislocation pile – up near the boundaries. Dislocations on plastic zone move slightly toward the crack tip, but the density of dislocations in the middle of the grain remains unchanged, and fracture toughness remains almost unchanged. Further reduction of the grain size leads to the Frank-Reed source activation on the grain boundary Forming dislocation pile-up of the neighbor grains. Its stress concentration acts on dislocations of the first grain and causes redistribution of plastic zone dislocations. If the reduction in grain size is not enough to form a strong pile-up, density of dislocations on plastic zone increases slightly and crack resistance increases a few percent. Further reduction of grains promotes strong pile-up, dislocations move to crack tip, and its density on plastic zone increases. Crack is shielded and fracture toughness increases sharply. The calculation showed that the fracture toughness jump is observed at grain sizes of 100—150 μm, in good agreement with the experiment. Keywords: dislocation dynamics simulation, molybdenum, fracture toughness, grain size, plastic zone, brittle-ductile transition.

A Comparison Study on the Strengthening and Toughening Mechanism between Cu-Bearing Age-Hardening Steel and NiCrMoV Steel.

Cu-bearing age-hardening steel has significant potential in shipbuilding applications due to its excellent weldability as compared to conventional NiCrMoV steel. Not much research has been carried out to analyze the differences in the mechanisms of strength and toughness between Cu-bearing age-hardening and NiCrMoV steel. Both steels were heat treated under the same conditions: they were austenized at 900 °C and then quenched to room temperature, followed by tempering at 630 °C for 2 h. The uniaxial tensile test reveals that the Cu-bearing age-hardening steel exhibits relatively lower strength but larger plasticity than NiCrMoV steel. The lower contents of Carbon and other alloying elements is one of possible reasons for these differences in mechanical properties. Transmission Electron Microscope observations show that two types of precipitates, Cr carbides and Cu-rich particles, exist in tempered Cu-bearing age-hardening steel. Cu-rich particles with sizes of 20–40 nm can inhibit the dislocation motion during deformation, which then results in dislocation pile ups and multiplication; this makes up the strength loss of Cu-bearing age-hardening steel and simultaneously improves its plasticity.

Size-dependent plasticity of hetero-structured laminates: A constitutive model considering deformation heterogeneities

Deformation mechanism-based constitutive modeling can serve as a flexible tool to optimize the microstructure of hetero-structured metals for achieving better mechanical performance. In this work, a dislocation density-based constitutive model, which considers geometrically necessary dislocations (GNDs) and the back stress induced by internal heterogeneous deformation, is developed for describing the deformation of hetero-structured laminate materials. The heterogeneous laminates are considered to consist of alternating coarse-grained and nano-grained layers separated by extended interfaces (interface affected zones, IAZs). The respective deformation mechanisms of each phase are established based on experimental information. The evolution laws of back stress and GNDs for the coarse-grained layers, nano-grained layers and IAZs are formulated within a unified framework based on dislocation pile up theory. The simulated tensile responses are in quantitative agreement with experimental data for heterogeneous copper/bronze laminates with different interface spacings and different volume fractions of the nanostructured layers. The model also allows to represent the back stress and strain-hardening rate accurately. It is found that smaller interface spacing, implying a higher volume fraction of the IAZ, leads to a higher density of GNDs and more significant back stress. The IAZ is demonstrated to be the key to understanding the strength-ductility combination of laminates. The developed model can be readily applied to other kinds of metallic materials with architectured microstructures.

On the role of pre-existing defects in influencing hardness in nanoscale indentations — Insights from atomistic simulations

Using in-situ nanoindentation experiments it is possible to study the dislocation mechanisms which unfold under an indenter.Large-scale atomistic simulations of the same are possible due to similarities in length scale, provided that defects can be included in the simulation. Yet, nanoindentation simulations have so far been mostly undertaken on defect free samples, while studies with pre-existing defects are few. The latter show that the average hardness is not affected by the presence of pre-existing defects, which justifies the use of ideal crystals in such simulations. However, this observation is counter-intuitive, as indenter-defect interactions should lead to work hardening and manifest themselves in hardness calculations. Our simulations along with a new look at the evolution of dislocations under the indenter, show for the first time, that hardness in atomistic simulations is influenced by pre-existing defects in the sample. Utilizing a face-centred tetragonal TiAl bicrystal with misfit dislocations at the interface, to populate the sample with defects, we correlate the contact-pressure variations to defect-indenter interactions. We show that the measured contact-pressure is affected by the presence and nature of defects under the indenter. Dislocation pile ups lead to intermittent rise in contact pressure, while seamless growth leads to steady convergence. The sensitivity to detect such defect interactions depends upon indenter size while convergence to average hardness is a result of curvature accommodation near the surface. Our findings prove that pre-existing defects have a profound influence on calculated hardness in indentation simulations which also corroborates with experimental observations in the literature.

Resistance spot-welding of AISI-1008 steel joints with MWCNT coating interlayer

ABSTRACT Inclusion of multi-walled carbon nanotubes (MWCNTs) as an inter-layer sandwiched between the mating sides of AISI 1008 steel joints in resistance spot-welding and its impact on microstructure/mechanical behavior of the subsequent lap joint is discussed here. A thin layer of MWCNTs approximately 50 μm is incorporated as a coating at one of the mating surfaces of the lap joint. It was found that the MWCNTs get entrapped in weld-nugget during the spot-welding procedure, which subsequently leads to weld strengthening through various mechanisms. While the weld strength is reliant mainly on the welding current, time, and also on the nugget diameter, an improvement of ~45% in weld strength is attained in lap-shear tensile tests due to the MWCNT interlayer. A detailed microstructural and XRD studies corroborate the experimental observations. Peak shifts were observed in Raman spectroscopy which confirms the compressive stress associated with the weld nugget-entrapped MWCNTs. Transmission electron microscopic studies of the nugget region elucidate the prevailing strengthening mechanisms including dislocation pile up, interfacial stress transfer, and mechanical interlocking.

Origin of strong solid solution strengthening in the CrCoNi-W medium entropy alloy

Solid solution strengthening is one of the most conventional strategies for optimizing alloys strength, while the corresponding mechanisms can be more complicated than we traditionally thought specifically as heterogeneity of microstructure is involved. In this work, by comparing the change of chemical distribution, dislocation behaviors and mechanical properties after doping equivalent amount of tungsten (W) atoms in CrCoNi alloy and pure Ni, respectively, it is found that the alloying element W in CrCoNi alloy resulted in much stronger strengthening effect due to the significant increase of heterogeneity in chemical distribution after doping trace amount of W. The large atomic scale concentration fluctuation of all elements in CrCoNi-3W causes dislocation motion via strong nanoscale segment detrapping and severe dislocation pile up which is not the case in Ni-3W. The results revealed the high sensitivity of elements distribution in multi-principle element alloys to composition and the significant consequent influence in tuning the mechanical properties, giving insight for complex alloy design.

Abrupt elastic-to-plastic transition in pentagonal nanowires under bending.

In this study, pentagonal Ag and Au nanowires (NWs) were bent in cantilever beam configuration inside a scanning electron microscope. We demonstrated an unusual, abrupt elastic-to-plastic transition, observed as a sudden change of the NW profile from smooth arc-shaped to angled knee-like during the bending in the narrow range of bending angles. In contrast to the behavior of NWs in the tensile and three-point bending tests, where extensive elastic deformation was followed by brittle fracture, in our case, after the abrupt plastic event, the NW was still far from fracture and enabled further bending without breaking. A possible explanation is that the five-fold twinned structure prevents propagation of critical defects, leading to dislocation pile up that may lead to sudden stress release, which is observed as an abrupt plastic event. Moreover, we found that if the NWs are coated with alumina, the abrupt plastic event is not observed and the NWs can withstand severe deformation in the elastic regime without fracture. The coating may possibly prevent formation of dislocations. Mechanical durability under high and inhomogeneous strain fields is an important aspect of exploiting Ag and Au NWs in applications like waveguiding or conductive networks in flexible polymer composite materials.

Reactions between Si melt and various ceramics

Reactions between different kinds of ceramics and silicon were studied to evaluate ceramics as candidates for their use in the process of silicon-crystal growth. Three types of ceramic plates, Al2O3, ZrO2 and quartz (SiO2), were put into contact with a silicon wafer via annealing at 1450 ?C for 30min under an Ar atmosphere. Defects appeared at the Si-ceramics interface. Among these, a crack and a dislocation pile up were found at the Si-SiO2 dissolution couple. In addition, two intermetallic compounds, Y2Si2O7 and Zr-Si, produced by the diffusion of Y, O and Zr from the ZrO2 into the Si, were found at the Si-ZrO2 dissolution couple. At the interface of the Si-Al2O3 dissolution couple, no intermetallic compounds and few defects were found. The oxygen concentration and electrical resistivity near the interface were high and gradually decreased away from the interface for all Si-ceramics dissolution couples.

EBSD Analysis of Orientation Gradients Developed near Grain Boundaries

The local misorientation and orientation gradient development near grain boundaries (GBs) are analyzed in a deep drawing quality steel sheet (AKDQ) subjected to interrupted tensile tests in a notched sample. The microstructure is studied using electron backscatter diffraction (EBSD) with subgrain-level spatial resolution. The evolution of misorientation accumulation for particular GBs was traced in grains located in different zones inside the notch, identifying the effective area of influence of GBs inside the neighboring grains. A local study was performed, and the evolution in misorientation development near GB was investigated. The results show a low correlation between GB width and sharpness of the orientation gradient with the mesoscopic strain, but instead orientation gradients between GB zones and the interior of the grains were observed with increasing strain. The increase in severity observed in some GBs can be related to dislocation pile up development, which reduces the permeability of a boundary to dislocation transmission.

A review of EBSD: from rudimentary on line orientation measurements to high resolution elastic strain measurements over the past 30 years.

Considering that the Electron Back Scatter Diffraction technique, EBSD, features in more than 60% of the papers published in the current conference proceedings, this review concentrates on the most recent development in the last ten years, that is, High Resolution EBSD. An outline of the theory is presented and four-point bend test results are shown proving the sensitivity of the technique for measuring elastic strain is 1 part in 10000. Other examples of its use included here are strains surrounding indents in silicon, mapping the stress concentration at grain boundaries ahead of dislocation pile ups and dislocation generation at grain boundaries due to strain ahead of nano indentations. An application is presented to distinguish the c axis direction in slightly tetragonal PZT crystals and developments in obtaining absolute strain measurement as opposed to the relative strain measurement currently the norm are discussed.

Phase morphology, variants and crystallography of alloy microstructures in cold dwell fatigue

This paper examines microscale crystal slip accumulation, cold creep, and stress redistribution (load shedding) related to dwell fatigue in a range of α–β Ti alloy microstructures. The role of basal slip and prism slip is evaluated in load shedding in a rogue grain combination. The results enrich the Stroh dislocation pile up interpretation of dwell by accounting for the anisotropic rate dependence of differing slip systems together with morphology.Microstructural morphology has been found to play an essential role in cold creep and load shedding in dwell fatigue. Basketweave structures with multiple α variants have been shown to give the lowest load shedding for which the mechanistic explanation is that the β lath structures provide multiple, small-scale α variants which inhibit creep and hence stress relaxation, thus producing more uniform, diffuse stress distributions across the microstructure through microscale kinematic confinement, imposed by multi (α)-to-single (β) BOR relations (i.e. multiple α variants sharing the same parent β grain). The critical consequence of this is that alloys typically having multi-variant basketweave structure (e.g. Ti-6246), remain free of dwell fatigue debit whereas those alloys associated with globular colony structures (e.g. Ti-6242) suffer significant dwell debit. This understanding is important in microstructural design of titanium alloys for resisting cold dwell fatigue.

Superdense microbands strengthening of textured low alloy ferritic steel

Microstructure evolution of Ti-Mo microalloyed ferritic steel was studied via warm rolling process. Results revealed that rolling at 500 °C (R3) mainly facilitated the formation of superdense microbands (∼200 nm in spacing), especially in the γ textured region. Average spacing of microbands (MBs) increased with temperature and it turned to be about 850 nm when rolling at 650 °C (R2). As rolling temperature further increased to 700 °C (R1), average ferritic grain size was refined to near 1.9 μm whereas no MBs appeared. Additionally, due to the preexisting superdense MBs, ultimate tensile stress of R3 increased to 1041 MPa and the elongation kept high at 22.9% simultaneously. MBs are characterized by high density dislocation walls and formed through the activation of {112}<111> slip systems, which is induced by the pile up of dislocations during grain boundary sliding. In that process steps appear in the grain boundary and gradually heighten with the extension of MBs. Large dynamic recovery rate could slack the dislocation pile up rate and increase the spacing of MBs. Moreover, preexisting of superdense MBs sheds new insights on the development of high strength ferritic steels with excellent combination of strength and ductility.

Fractographic correlations with mechanical properties in ferritic martensitic steels

The ultimate continuum of a material is nothing but the process called fracture. Fracture surface retains the imprint of the entire deformation history undergone in a material. Hence, it is possible to derive the approximate deformation and fracture properties of a material from a systematic fracture feature analysis. There has been large volume of literature available in the open domain correlating different mechanical and fracture responses of reduced activation ferritic martensitic grade steels under various testing conditions/circumstances with corresponding microstructural interpretation. There has been no such literature available to establish the relationship between the two-dimensional fracture geometry/topography with its corresponding deformation and mechanical properties of the material as a function of testing temperature, which has been the primary aim in the current investigation. A comprehensive literature survey has been carried out to realize this fact. In order to establish the above hypothesis, many tensile experiments were carried out at constant strain rate by systematic variation of the test temperature. The initial void volume fraction or the inclusion content of material was kept unaltered and the test temperature has been varied orderly on different multiple specimens to vary the deformation-induced nucleation sites of micro voids (i.e. different carbides, phase interfaces, dislocation pile up etc), which results in a change of fracture topography under uniaxial tensile deformation. A conventional metallographic technique followed by optical microscopy has been employed to understand the basic morphologies and characteristics of the alloy exposed at different temperatures. Fractographic investigation of the broken tensile specimens at various temperatures is carried out to measure the fracture features by using quantitative fractography on representative scanning electron fractographs through image processing.

Estimation of Dislocation Concentration in Plastically Deformed FeCrNi Alloy by Positron Annihilation Lifetime

Dislocations would be induced after plastic deformation, which might change the mechanical properties of solids. FeCrNi austenitic model alloy and its Mo-diluted alloy were cold rolled with different degree of thickness reduction. Positrons are sensitive to point defects, which are easily trapped and annihilated around the trapping sites. The mean positron lifetimes have been used to estimate the average dislocation concentration in solids. Meanwhile, the trapping efficiency μ was calculated from the lifetime results. The trapping efficiency value is estimated about 3.31×10-7 cm3s-1 for FeCrNi alloy and 3.31×10-7 cm3s-1 for Mo-diluted alloy, respectively. The increment of the hardness value during plastic deformation is related to the increase of the dislocation density and dislocation pile up in solids.

Microstructural and crystallographic response of shock-loaded pure copper

Abstract

The intergranular corrosion susceptibility of 2024 Al alloy during re–ageing after solution treating and cold–rolling

The intergranular corrosion (IGC) susceptibility of 2024 Al alloy during re–ageing after solution treating and cold–rolling was investigated by accelerated corrosion testing, open circuit potential testing, transmission electron microscopy and scanning electron microscopy. The absence of IGC in both the peak–re–aged and over–re–aged samples is related to the dislocation pile–ups which prevent the supersaturated solutes from diffusing into the grain boundaries and precipitating the continuous S–Al2CuMg phase. The aggregated pitting corrosion in the over–re–aged samples arises from the S–phase precipitates on the dislocation cell walls which accelerate the anodic dissolution of the cell interiors.