Dislocation Slip Mechanism Research Articles

Creep behaviors of a Mg–Li based alloy at elevated temperatures

Improvement of the mechanical stability is a long challenge for the light and strong dual-phases MA21 (Mg-7.99Li-5.3Cd-4.57Al, wt.%) alloy with critical applications in aerospace and automotive industries. The present work experimentally investigated the relationship between the creep behaviors and microstructures of MA21 alloy at elevated temperatures. It was revealed that plate-like θ-phase precipitated in the α-phase and AlLi phase particles grew in the β-phase. These precipitate phase impeded the dislocation dynamics giving rise to the creep deformation. At low temperature and high stress, the creep behavior was dominated by dislocation climb accompanying Li atom diffusion, where the θ-phase was shear deformed. The AlLi particles were so hard to be penetrated by dislocation, so the dislocation slip can only propagate bypass it. In contrast, extensive atom diffusion of Al and Cd elements was revealed to support the dislocation slip mechanism with solute drag for the creep behaviors at high temperature and low stresses. The dislocation slip during creep resulted in grain reorientation with enhanced (0001) plane texture and transformation of large angle grain boundaries in the initial state into small angle grain boundaries. Our results shed new light on understanding of the time dependent deteriorated mechanical properties of Mg–Li commercial alloys and might give rise to a new strategy to stabilize the mechanical properties of MA21 alloy and relevant Mg–Li alloy system by engineering the precipitation phase in the α-Mg and β-Li phases.

Excellent plasticity of C and Mo alloyed TRIP high entropy alloy via rolling and heat treatment

The transformation-induced plasticity (TRIP) Fe50Mn30Co10Cr10 high entropy alloy (HEA) with 2 at% C and 1 at% Mo microalloying was prepared by non-consumable vacuum arc melting furnace. The as-casted alloy was then rolled at 70% and then annealed at 1000 °C for different times. The microstructure evolution and mechanical properties of the C and Mo alloyed TRIP-HEA were systematically investigated. The results showed that with the increase of annealing time, the strength of the C2Mo1 HEA increases, but the plasticity increases first and then decreases. Spuriously, when C2Mo1 HEA was annealed for 5 min at 1000 °C after 70% cold rolling, the total elongation was 121%, the tensile strength was 816 MPa, and its product of tensile strength and plasticity was 99.23 GPa, which is much better than the majority of HEAs. In the process of tensile deformation, <001> + <100>//TD double texture are formed, and dislocation slip and twin mechanism competed by each other, which affect the microstructure evolution in the process of plastic deformation, and finally as the great contribution to the product of tensile strength and plasticity.

Effect of Dislocation Slip Mechanism under the Control of Oxygen Concentration in Alpha-Case on Strength and Ductility of TC4 Alloy

The oxygen diffusion layer (alpha-case) is generally considered to have a negative impact on the mechanical properties and applications of titanium alloys. In this study, TC4 alloy specimens with four types of different oxygen concentrations in alpha-case were obtained by controlling the oxygen diffusion process parameters. Scanning electron microscopy and glow discharge spectrometry were employed to characterize the microstructure and oxygen concentration of alpha-case. The effect of alpha-case on strength and ductility of TC4 alloy was investigated via tensile test and new insights were provided. The results indicate that with the increase in the oxygen concentration in the alpha-case, the ductility of the TC4 alloy gradually decreased. Interestingly, the strength of TC4 alloy with the alpha-case first increased and then decreased, resulting in the existence of a peak corresponding to a lower oxygen concentration before the decline of strength. Furthermore, a relatively good ductility match was also observed at the peak. When the oxygen concentration was relatively high, both the strength and ductility decreased. This phenomenon is attributed to the fact that dislocations in the alpha-case controlled by the oxygen concentration were modified from wavy slip to planar slip. Finally, the dislocation’s slip morphology was characterized by transmission electron microscopy.

Elucidating the transition of cryogenic deformation mechanism of CrMnFeCoNi high entropy alloy

Here, the evolution of the deformation mechanism of the well-known CrMnFeCoNi high entropy alloy (HEA) under a cryogenic temperature was investigated via using transmission electron microscopy characterization after compressive experiments. It is found that, in contrast to the dominate mechanism of dislocation slip at 298 K, the deformation mode of the studied HEA at 123 K can be divided into two stages: (i) at the initial plastic stage, the plasticity is mainly induced by the planar dislocation glide; and (ii) at the late plastic stage, the combination of dislocation hardening and mechanical twinning facilitates high strength for the studied alloy. These findings are helpful to understand the temperature dependent deformation mechanism of high entropy alloys.

Computational Modeling of Dislocation Slip Mechanisms in Crystal Plasticity: A Short Review

The bridge between classical continuum plasticity and crystal plasticity is becoming narrower with continuously improved computational power and with engineers’ desire to obtain more information and better accuracy from their simulations, incorporating at the same time more effects about the microstructure of the material. This paper presents a short overview of the main current techniques employed in crystal plasticity formulations for finite element analysis, as to serve as a point of departure for researchers willing to incorporate microstructure effects in elastoplastic simulations. We include both classical and novel crystal plasticity formulations, as well as the different approaches to model dislocations in crystals.

Effect of heat treatment on the microstructure, microhardness and impact toughness of TC11 and TC17 linear friction welded joint

The service life of whole blisks for the aero-engine compressor is sharply decreased due to the dissimilar linear friction welded (LFWed) joint of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si (TC11) and Ti–4Mo–4Cr–5Al–2Sn–2Zr (TC17) alloys, which have the different microstructure and mechanical properties. To optimize the microstructure and performance of the joint, and improve service life of the whole blisks, the post-weld heat treatment (PWHT) with annealing, solution plus aging were adopted for the welded joint. The results show that the nucleation rate of primary αP phase increases obviously, which are spheres, rods and strips, and orthorhombic martensite αs phases transform into hexagonal martensite αs phases interlaced evenly on the β phase matrix on the WZ of TC17 side; meanwhile, acicular hexagonal martensite αs phases transform into lamellar αs phases on the WZ of TC11 side after solid solution and dual aging. The difference of the microhardness on both sides of the joint is narrowed significantly. The impact toughness of the joint is improved sharply from 9.3 J/cm2 after welding to 27.6 J/cm2 with an increase of 200%. In addition, the dissolution and growth model of primary αp phases were established, which can reasonably predict the nucleation and grow process of α phase. Furthermore, the narrowed discrepancy of the microhardness and increased impact toughness can be attributed to the decreased slice thickness of martensite αs phase, based on the dislocation slip mechanism and distorted discordant model.

Effect of initial forging temperature on mechanical properties and fatigue behavior of EA4T steel

An EA4T steel billet was forged at the different initial forging temperatures (1050, 1150, and 1250 °C). The microstructure and fatigue fracture of forged samples were observed by laser scanning confocal microscopy and scanning electron microscopy, respectively. The fatigue performance of samples forged at an initial forging temperature of 1250 °C was better owing to the presence of fine polygonal ferrite (PF). The difference in yield strength and ultimate tensile strength can be attributed to the average grain size of PF and acicular ferrite (AF) content, respectively. A relationship between the tensile strength and fatigue limit of EA4T steel is proposed. The mechanism of grain size and dislocation slip affecting the fatigue property was elucidated. Because the fine PF grains delayed the dislocations moving to the surface and inhibited the formation of intrusion and extrusion, premature cracks were not easily produced. This contributed to excellent fatigue behavior. Additionally, the pearlite colonies and fine PF delayed the crack growth rate and changed the crack propagation path. Studies on fatigue behavior provided a foundation for quality control and engineering application of EA4T steel.

The enhanced superplasticity of a 2024 matrix nanocomposite reinforced by TiC particles

Fine grains less than 10 μm are usually the prerequisite to superplasticity of alloys. Traditionally, multiple-step rolling and heat treatment are necessary to obtain fine grain microstructure of Al alloys. In this paper, a simple method to achieve superplasticity for 2024 alloys has been proposed. An inhomogeneous 2024–1.5TiC nanocomposite reinforced by TiC threadiness arrays was fabricated via casting and hot extrusion processes. By subsequent annealing, fine α-Al grains with average size of 4.1 μm were obtained. And then, elongation of 405% for the 2024–1.5TiC was achieved at 485 °C with initial strain rate of 3.33 × 10−4 s−1. According to experimental and theoretical analysis, the deformation is mainly controlled by grain boundary sliding and dislocation slip mechanisms. It was found that TiC nanoparticles accelerated nucleation of α-Al recrystallization during annealing, and the limited space between adjacent TiC threadiness arrays restricted α-Al grain growth. The high nucleation rate and low grain growth rate resulted in the formation of fine grains. Besides, TiC nanoparticles inhibited dynamic grain growth during tensile deformation, which also contributes to the superplasticity.

A molecular dynamics study on the mechanical properties of Fe-Ni alloy nanowires and their temperature dependence.

Fe–Ni alloy nanowires are widely used in high-density magnetic memories and catalysts due to their unique magnetic and electrochemical properties. Understanding the deformation mechanism and mechanical property of Fe–Ni alloy nanowires is of great importance for the development of devices. However, the detailed deformation mechanism of the alloy nanowires at different temperatures is unclear. Herein, the deformation mechanism of Fe–Ni alloy nanowires and their mechanical properties were investigated via the molecular dynamics simulation method. It was found that the local atomic pressure fluctuation of the Fe–Ni alloy nanowire surface became more prominent with an increase in the Ni content. At low temperature conditions (<50 K), the plastic deformation mechanism of the Fe–Ni alloy nanowires switched from the twinning mechanism to the dislocation slip mechanism with the increase in the Ni content from 0.5 at% to 8.0 at%. In the temperature range of 50–800 K, the dislocation slip mechanism dominated the deformation. Simulation results indicated that there was a significant linear relationship between the Ni content, temperature, and ultimate stress in the temperature range of 50–800 K. Our research revealed the association between the deformation mechanism and temperature in Fe–Ni alloy nanowires, which may facilitate new alloy nanowire designs.

Extraordinary ductility and strain hardening of Cr26Mn20Fe20Co20Ni14 TWIP high-entropy alloy by cooperative planar slipping and twinning

High-entropy alloys (HEAs), containing at least five major metal elements in equal or near equal atomic ratios, have drawn increasing attention because they open entirely new materials avenues for designing alloys with exceptional properties. In the literature, a well-studied equiatomic, face-centered cubic CrMnFeCoNi HEA reportedly exhibits a yield strength of 410 MPa and a ductility of 57% as well as a deformation mechanism of dislocation slip at room temperature [B. Gludovatz, et al., Science, 345 (2014) 1153–1158]. Some recent works also observed that twinning actually happens more or less in the equiatomic CrMnFeCoNi alloy at room temperature [Z.J. Zhang, et al., Nature Comm. 6 (2015) 10143]. In this study, we prepared a non-equiatomic, face-centered cubic Cr26Mn20Fe20Co20Ni14 HEA with a comparatively low stacking fault energy (SFE) by making an appropriate adjustment of the composition ratio. Our HEA has a yield strength of 180 MPa and a high strain hardening exponent, n, of 0.46 as well as a higher ductility (73%) than those of the CrMnFeCoNi alloy. Investigation of the deformation mechanisms at specific strain levels revealed a clear transition from planar slip dislocations in the initial deformation stage to twinning at high tensile strain. Cooperative planar slipping and twinning resulted from the comparatively low SFE and were responsible to the extraordinary ductility and strain hardening capability. Besides deformation twins, other hardening mechanisms including forest dislocations, sessile Lomer–Cottrell locks, dislocation-stacking fault interactions, sub-grain boundary and phase boundary were revealed.

Microstructure evolution of the TiB2/Al composites fabricated by powder metallurgy during hot extrusion

The microstructure evolution near the central axis during hot extrusion for the fine-grained TiB2/Al-Zn-Mg-Cu composites fabricated by powder metallurgy is studied by experimental observation at different locations in the remaining die. The significant increase of the fraction of ultrafine grains and texture evolution indicates the extensive dynamic recrystallization at the narrowest position of the die. The deformed grains are distinguished from the undeformed grains with the parameter of grain orientation spread (GOS). And the deformed grains are generally rotated around the extrusion direction. Additionally, the introduced submicron TiB2 particles have the potential to stimulate the dynamic recrystallization associated with progressively increased misorientation of subgrain boundaries, and even change the spatial distribution of the low angle grain boundaries. The preferential activation of {111} 〈1−10〉 system and non-octahedral (NOC) slip system {001} 〈110〉 contributes to the formation of strong deformation fiber texture according to the analysis of the Schmid factor. It is supposed that grain boundary sliding accommodated dislocation slip mechanism controls the microstructure evolution.

Structural Characterization of Low Carbon Foil by Asymmetrical Rolling

At room temperature, the low carbon steel were cold rolled from 0.5 mm to 1 μm without intermediate annealing by asymmetrical rolling (ASR) process. the microstructure were investigate by Electron Backscatter Diffraction (EBSD) and transmission electron microscopy (TEM). Lath structure (~300 nm) and local nanograins (~60nm) were obtained through ASR with extensibility up to 50,000%. The high angle boundaries were predominant. The results indicated that inhomogeneous deformation and combination deformation mechanism of dislocation slip and grain boundary slip in the foil ASR process.

Temperature Effect on Dislocation Slip Mechanism of Nanotwinned Mg with Void Defect at the Twin Boundary

In this paper, a molecular dynamics model is used to simulate the uniaxial tensile loading process at different temperatures of nano-twinned Mg with void defect at the twin boundaries. The dislocation slip mechanism during the stage of plastic deformation for different temperatures was studied. The results show that the initial edge dislocations formed from lattice misfit. It is found that the propagations of the slip bands emitted and the stacking faults also generated from Shockley partial dislocation.

Microstructure and Deformation Mechanism of AZ31 Magnesium Alloy Under Dynamic Strain Rate

A hot-rolled AZ31Mg alloy sheet was subjected to dynamic plastic deformation parallel to the rolling direction, and microstructural evolutions and mechanical properties of the deformed samples were examined. It has been found that dynamic strain rate could facilitate {10-12} twin nucleation and growth and leads to a lower yield stress of about 20 MPa and an early end to twinning characteristic (happening at a strain point of about 6%) shown in the stress–strain curve. {10-12} twinning mechanism dominates the early plastic deformation; but when plastic strain exceeds ~ 9%, dislocation–slip mechanism instead of {10-12} twinning dominates the later plastic deformation. And this premature transformation of the dominant deformation mechanisms from {10-12} twinning to dislocation slip is caused by dynamic strain rate. The effect of dynamic strain rate on the number of twin nucleations remains unclear, and the more systematic researches are needed in the future.

Response of Ti microstructure in mechanical and laser forming processes

Microstructural deformation mechanisms present during three different forming processes in commercially pure Ti were analysed. Room temperature mechanical forming, laser beam forming and a combination of these two processes were applied to thick metal plates in order to achieve the same final shape. An electron backscatter diffraction technique was used to study the plate microstructure before and after applying the forming processes. Substantial differences among the main deformation mechanisms were clearly detected. In pure mechanical forming at room temperature, mechanical twinning predominates in both compression and tensile areas. A dislocation slip mechanism inside the compression and tensile area is characteristic of the pure laser forming process. Forming processes which subsequently combine the laser and mechanical approaches result in a combination of twinning and dislocation mechanisms. The Schmid factor at an individual grain level, the local temperature and the strain rate are factors that determine which deformation mechanism will prevail at the microscopic level. The final microstructures obtained after the different forming processes were applied are discussed from the point of view of their influence on the performance of the resulting formed product. The observations suggest that phase transformation in Ti is an additional microstructural factor that has to be considered during laser forming.

Influence of loading directions on dislocation slip mechanism of nanotwinned Ni with void defect at the twin boundary

In this study, a molecular dynamics model is used to simulate the compressive loading process along different crystal orientations of nano-twinned Ni with void defect at the twin boundaries. The loading angle is defined as the angle between the loading direction and the twin boundary, loading angles of 0, 15, 30, 45, 60, 75, and 90° were investigated in this study. The effects of different loading directions on the mechanical properties and the dislocation glide mechanisms were investigated. The dislocation glide process during the initial stage of plastic deformation for different loading directions was also studied. The results show that the dislocation glide mainly occurs along the {1 1 1} plane that is inclined to the twin boundaries when the loading direction is 0°. The dislocation glide process is constrained by the twin boundaries and, therefore, slips along the twin layers. As the loading angle increases from 0° to 45°, the dislocation gradually shifts and slips along the (1 1 1) slip plane that is parallel to the twin boundaries and twin migration and twinning occur. As the loading angle continues to increase to 90°, the dislocation slips along the {1 1 1} plane again during the loading process. In addition, the dislocation slips toward the adjacent twin layers because they are strongly hindered by the twin boundaries.

Strength and deformation of shocked diamond single crystals: Orientation dependence

Understanding and quantifying the strength or elastic limit of diamond single crystals is of considerable scientific and technological importance, and has been a subject of long standing theoretical and experimental interest. To examine the effect of crystalline anisotropy on strength and deformation of shocked diamond single crystals, plate impact experiments were conducted to measure wave profiles at various elastic impact stresses up to \ensuremath{\sim}120 GPa along [110] and [111] crystal orientations. Using laser interferometry, particle velocity histories and shock velocities in the diamond samples were measured and were compared with similar measurements published previously for shock compression along the [100] direction. Wave profiles for all three orientations showed large elastic wave amplitudes followed by time-dependent inelastic deformation. From the measured wave profiles, the elastic limits were determined under well characterized uniaxial strain loading conditions. The measured elastic wave amplitudes for the [110] and [111] orientations were lower for higher elastic impact stress (stress attained for an elastic diamond response), consistent with the result reported previously for [100] diamond. The maximum resolved shear stress (MRSS) on the {111}⟨110⟩ slip systems was determined for each orientation, revealing significant orientation dependence. The MRSS values for the [100] and [110] orientations (\ensuremath{\sim}33 GPa) are 25%--30% of theoretical estimates; the MRSS value for the [111] orientation is significantly lower (\ensuremath{\sim}23 GPa). Our results demonstrate that the MRSS depends strongly on the stress component normal to the {111} planes or the resolved normal stress (RNS), suggesting that the RNS plays a key role in inhibiting the onset of inelastic deformation. Lower elastic wave amplitudes at higher peak stress and the effect of the RNS are inconsistent with typical dislocation slip mechanisms of inelastic deformation, suggesting instead an inelastic response characteristic of shocked brittle solids. The present results show that the elastic limit (or material strength) of diamond single crystals cannot be described using traditional isotropic approaches, and typical plasticity models cannot be used to describe the inelastic deformation of diamond. Analysis of the measured wave profiles beyond the elastic limit, including characterization of the peak state, requires numerical simulations that incorporate a time-dependent, anisotropic, inelastic deformation response. Development of such a material description for diamond is an important need.

Quasi-static and dynamic plastic deformation behavior in titanium with vanadium addition

Compressive properties, microstructure features and deformation modes were investigated in binary Ti-(2,4,8) wt% V alloys during quasi-static (1 × 10−3 s−1) and dynamic (3 × 103 s−1) compressions. The compressive behavior shows a strong dependence on the loading strain rate and vanadium content contained in pure Ti, such that the flow stress increases with the increase in strain rate and vanadium content ranging from 2 wt% to 8 wt%. The microstructure features are clearly different from each other for alloys with different vanadium contents or under quasi-static and dynamic loading conditions. An examination of deformation microstructures by optical microscopy and electron backscattered diffraction indicates that twinning behavior occurs during quasi-static and dynamic compressions and the twinning density increases with strain rate increasing but decreases with vanadium addition. The existence of \(\left\{ {10\bar{1}2} \right\}\), \(\left\{ {11\bar{2}1} \right\}\) and \(\left\{ { 1 1 {\bar{{2}}{2}}} \right\}\) type twinning was further identified. With the help of the calculated Schmid factor map, the values of critical resolved shear stress of twinning types mentioned above have been obtained and verified to be rarely affected by the loading strain rate but sensitive to the vanadium content. In vanadium-rich alloys (Ti-8V), twins are rarely observed but dislocation slip mechanism is active by transmission electron microscopy investigations. With vanadium content increasing, both the critical resolved shear stress of twinning types and the content of β phase with abundant slip systems increase, reflecting a suppression of twinning but an active dislocation slip mechanism.

Orientation dependence of twinning in single crystalline CoCrFeMnNi high-entropy alloy

Tensile deformation behavior of the equiatomic CoCrFeMnNi high entropy alloy single crystals were studied along three different crystallographic orientations, i.e. [001], [1¯23], [1¯11], in order to reveal the orientation dependence of deformation twinning at room temperature. It was shown that initial yield behavior along these orientations is governed by dislocation slip mechanism and the critical resolved shear stresses for slip are independent of crystal orientation. Twinning starts at different strain levels after slip deformation, depending on the tensile testing orientation: in the [1¯11]-oriented crystals, twinning starts after 5% strain while in the [1¯23]-oriented crystals, it starts after about 25% strain. Deformation twinning was not detected in the [001]-oriented crystals under tensile loading. The critical resolved shear stresses for twinning are determined to be τcrtw = 110–140MPa. Onset of deformation twinning in the [1¯11]-oriented crystals at low strain levels, together with multiple active dislocation slip systems, leads to a significantly higher strain hardening coefficient and stress for fracture in comparison with the [001]-oriented crystals, where the plastic deformation occurs only by multiple slip systems, and the [1¯23]-oriented crystals, in which the deformation mainly occurs by single slip and then twinning in one system at later stages of the deformation.

Effect of twin boundaries on indentation behavior of magnesium alloys

The effect of twin boundaries on indentation behavior was investigated in Mg-0.3 at.%X (X = Al, Li, Y and Zn) binary alloys containing preexisting {101¯2} deformation twins. These alloys exhibited the highest hardness values reported among both fine- and coarse-grained alloys studied in the literature, irrespective of the alloying elements present. The activation volumes for plastic deformation were found to lie between 18b3 and 60b3, consistent with a rate-controlling mechanism of dislocation slip. These activation volumes were similar to previously reported results for alloys devoid of twins prior to testing, suggesting that although twin boundaries do not affect the dominant deformation mechanism, they lead to an increase in hardness. On the other hand, the activation volumes obtained from indentation tests were different from those obtained by uniaxial compression tests in samples without pre-existing twins, due to the occurrence of microstructural evolution, i.e., deformation twin formation, during plastic deformation.