Initial Dislocation Research Articles

A Theoretical Model of the Flow Properties of Postprocessed Direct Metal Laser Sintering Ti6Al4V (ELI)

Heat treatment of direct metal laser sintering (DMLS) Ti6Al4V (ELI) generates different mechanical properties of the alloy depending on the heat treatment cycle adopted. This is due to the different aspects of the microstructure, such as phase fraction, grain size, texture, and dislocation density, which vary with heat treatment. Other external factors, such as the prevailing level of strain, strain rate, and temperature, also affect the mechanical properties of the material. This paper presents the development of a theoretical model that couples the effects of strain rate, temperature, strain, grain size, and initial dislocation density to describe the flow properties of DMLS Ti6Al4V (ELI). According to the model, higher initial dislocation density results in higher yield stress, low strain hardening, and earlier saturation of flow stress. The model shows that the parabolic shape of the stress-strain curve of the alloy is dictated by the initial dislocation density, which is generally a factor of grain size.

MTS Model Application to Materials Not Starting in the Annealed Condition.

Application of the Mechanical Threshold Stress constitutive model becomes challenging when the material of interest is not supplied in the annealed condition with a low initial dislocation density. When the material has some existing warm or cold work, the evaluation of the internal state variables, specification of the activation energies, and analysis of strain hardening can be affected. This paper gives an example of this in molybdenum and presents options for proceeding with the model development. A hypothetical Body-Centered Cubic (BCC) alloy with known model variables is used to demonstrate the issues and solution options.

Low cycle fatigue behavior of near-alpha titanium alloys used in deep-driving submersible: Ti-6Al-3 Nb-2Zr-1Mo vs. Ti-6Al-4 V ELI

The low cycle fatigue (LCF) of Ti-6Al-3 Nb-2Zr-1Mo and Ti-6Al-4 V ELI with a bimodal structure were compared at different total strain amplitudes (∆εt/2) from 0.5 % to 1.0 %. Because the Ti-6Al-3 Nb-2Zr-1Mo exhibited a higher tensile elongation, but a lower yield strength than Ti-6Al-4 V ELI, it displayed a slightly longer fatigue life than Ti-6Al-4 V ELI when ∆εt/2 was larger than 0.7 % (plastic deformation predominated), while its fatigue life became shorter when ∆εt/2 was less than 0.7 % (elastic deformation predominated). Both Ti-6Al-3 Nb-2Zr-1Mo and Ti-6Al-4 V ELI showed a continuous cyclic softening behavior due to the rearrangement or mutual annihilation of initial dislocations during cyclic deformation, and the variation of cyclic softening rate depended on ∆εt/2 was consistent with the variation of fatigue life depended on ∆εt/2. In cyclic deformation, the prismatic and basal slips were easier to activate, and the dislocation bands were denser and more homogeneous in the Ti-6Al-3 Nb-2Zr-1Mo than that in the Ti-6Al-4 V ELI due to the actual Al content in former was smaller than that in latter, which was beneficial to reduce local stress concentration and prolong fatigue life.

Influence of free-end torsion on compressive behavior of extruded AZ31 rod at various temperatures

The influence of free-end torsion on compressive behavior of an extruded AZ31 rod at various temperatures was studied. Pre-torsion generates a high density of dislocations and a large number of twins in the matrix, which can largely enhance the compressive yield strength at RT and 100 °C. However, with increasing temperature, hardening effect via pre-torsion gradually decreases. When the compressive temperature reaches 300 °C, pre-torsion reduces the compressive yield strength. Moreover, initial dislocations and twins via torsion help to refine the sub-structure and accelerate the continuous dynamic recrystallization during compression at 200 °C. Thus, twisted sample exhibits more rapid flow softening behavior than the as-extruded sample at 200 °C. When compressed at 300 °C, the twins and dislocations via torsion were largely eliminated during the holding time, and the discontinuous dynamic recrystallization was enhanced. It is found that the compression curves of twisted sample and as-extruded sample tended to be coincident at 300 °C. Related mechanisms were discussed in detail.

Atomistic understanding of incipient plasticity in BCC refractory high entropy alloys

Understanding the incipient plastic behavior of refractory high-entropy alloys (RHEAs) is crucial for their high-temperature applications. In this study, the initial dislocation nucleation and motion mechanisms in the TaTiZrV RHEA and their dependence on temperature were investigated upon nanoindentation by molecular dynamics (MD) simulations. Compared with the high stress-driven homogeneous nucleation criterion in pure BCC metals, the Zr-V short-range orders in the RHEA facilitate preferential inhomogeneous nucleation at low stress. The local compositional fluctuation not only causes intermittent slipping of screw dislocations by the trapping-detrapping mechanism but also severely reduces the moving rate of edge dislocations. In particular, the initial dislocation nucleation in the RHEA becomes more difficult with the increasing temperature. Based on the competitive mechanism between multiple-element-induced lattice distortion and point-defect-induced lattice distortion, the reason for their excellent retained mechanical properties at high temperatures was revealed. This study would provide theoretical support for the development of RHEAs in high-temperature technological applications.

Natural History of First-Time Anterior Shoulder Dislocation in Patients Older Than 50 Years: A Study of 179 Patients With a Mean Follow-up of 11 Years.

There is a dearth of knowledge on anterior shoulder instability in older patients. The purposes of this study were to describe the incidence and epidemiology, injury characteristics, and treatment and outcomes in patients ≥50 years old with first-time anterior shoulder instability. We also describe the historical trends in diagnosis and treatment. It was hypothesized that the rates of obtaining a magnetic resonance imaging (MRI) scan and surgical intervention have increased over the past 20 years. Descriptive epidemiology study. An established geographic database was used to identify 179 patients older than 50 years who experienced new onset anterior shoulder instability between 1994 and 2016. Medical records were reviewed to obtain patient characteristics, imaging characteristics, and surgical treatment and outcomes, including recurrent instability. Comparative analysis was performed to identify differences between age groups. Mean follow-up time was 11 years. The incidence of first-time anterior shoulder dislocation in our study population was 28.8 per 100,000 person-years, which is higher than previously reported. Full-thickness rotator cuff tears were found in 62% of the 66 patients who underwent MRI scans. Of all patients, 26% progressed to surgery at a mean time of 1.6 years after injury; 57% of all surgical procedures involved a rotator cuff repair, and 17% included anterior labral repair. All patients who underwent a labral repair also underwent concomitant rotator cuff repair. The rate of recurrent instability for the cohort was 15% at a median of 176 days after the initial instability event. There were no instances of recurrent instability after operative intervention. At an average of 7.5 years after the initial instability event, 14% of patients developed radiographic progression of glenohumeral arthritis. The rate of surgical intervention within 1 year of initial dislocation increased from 5.1% in 1994 to 1999 to 52% in 2015 to 2016. The incidence of first-time anterior shoulder instability in patients aged ≥50 years was 28.8 per 100,000 person-years. Full-thickness rotator cuff tears (62%) were the most common condition associated with anterior shoulder instability, followed by Hill-Sachs lesions (56%). The rate of recurrent instability for the entire cohort was 15%, with no instances of recurrent instability after operative intervention.

Comparing structure-property evolution for PM-HIP and forged alloy 625 irradiated with neutrons to 1 dpa

The nuclear power industry has growing interest in qualifying powder metallurgy with hot isostatic pressing (PM-HIP) to replace traditional alloy fabrication methods for reactor structural components. But there is little known about the response of PM-HIP alloys to reactor conditions. This study directly compares the response of PM-HIP to forged Ni-base Alloy 625 under neutron irradiation doses ∼0.5–1 displacements per atom (dpa) at temperatures ranging ∼321–385 °C. Post-irradiation examination involves microstructure characterization, ASTM E8 uniaxial tensile testing, and fractography. Up through 1 dpa, PM-HIP Alloy 625 appears more resistant to irradiation-induced cavity nucleation than its forged counterpart, and consequently experiences significantly less hardening. This observed difference in performance can be explained by the higher initial dislocation density of the forged material, which represents an interstitial-biased sink that leaves a vacancy supersaturation to nucleate cavities. These findings show promise for qualification of PM-HIP Alloy 625 for nuclear applications, although higher dose studies are needed to assess the steady-state irradiated microstructure.

Deformation and failure by weaving dislocation networks in body-centered-cubic tungsten

Dislocation is the primary carrier for accommodating plastic deformation in metals. It is generally envisaged that metals with low initial dislocation density have high strain-to-failure and good formability, and vice versa. However, contrary to most metals, body-centered-cubic pure tungsten becomes brittle after reducing its dislocation density by recrystallization, while its formability increases after heavy deformation. To elucidate the mechanism of this counterintuitive mechanical behavior, by combining ex-situ indentation tests and dislocation reconstruction techniques, we find that tungsten’s deformation and failure processes are accompanied by the formation of a specific dislocation pattern. Dislocation networks are governed by a mechanism similar to the weaving process of the spider’s orb web. After heavy deformation, subgrains are generated due to the weaving process. The recurring weaving of dislocations reduces the mean free path for crack propagation, which is considered as the main reason for the increased formability of tungsten after deformation.

The work hardening and softening behaviors of Mg–6Zn-1Gd-0.12Y alloy influenced by the VR/VD ratio

The effects of the VR/VD ratio (the volume fraction ratio of recrystallized grains and coarse deformed grains) on the work hardening and softening behaviors of Mg–6Zn-1Gd-0.12Y alloy were studied in detail by detecting the variation of dislocation density. The alloys with different VR/VD ratios were successfully fabricated by rolling and subsequently different annealing processes. It was shown that the higher VR/VD ratio closely related to the nucleation and growth of recrystallized grains could be obtained by an increase in annealing temperature. An increase in the VR/VD ratio from 0.16 to 24 reduced the work hardening rate in stage Ⅲ, while increased the work hardening rate in stage Ⅳ, which was closely related to the initial dislocation density and dislocation evolution. Low VR/VD ratio in the alloy, with high initial dislocation density, was identified to be beneficial for improving the work hardening rate in stage Ⅲ. And an increased VR/VD ratio could promote dislocation multiplication by stimulating the yield phenomenon due to the lack of initial dislocation density, and improve the work hardening rate in stage Ⅳ. With the VR/VD ratio increased, the softening behavior of the alloys increased in stage Ⅲ, and first decreased and then increased in stage Ⅳ. This was because the alloy with the lower dislocation density was easy to deform due to few obstacles, which accelerated the softening in stage Ⅲ, and improved dislocation density after yielding provided the strong driving force for the softening in stage Ⅳ. Meanwhile, the cycle stress relaxation tests further manifested that the stress drop values (Δσp) of the alloys were in close contact with the initial dislocation density and dislocation evolution influenced by the VR/VD ratio. As a result, when the VR/VD ratio was 1.6, the Mg–6Zn-1Gd-0.12Y alloy had excellent strength and toughness match owing to a dynamic balance between the work hardening rate and softening rate.

Twinning behavior in deformation of SLM 316L stainless steel

The present work investigates the microstructures, mechanical properties and deformation behaviors of the 316L stainless steel fabricated by selective laser melting (SLM). The initial dislocation density of as-SLM and conventional samples is measured using x-ray diffraction (XRD). It is found that the high dislocation density is obtained by SLM, contributing to the enhancement of yield strength. Twinning has occurred at the early stage of SLM deformation, which undergoes obvious grain rotation and twinning development during the tensile tests. However, only a few transformed martensite is presented in the deformed samples, and no significant grain orientation changes are observed.

Creep aging behavior of Al-Cu-Mg-Si alloy with elastic and plastic loads

The creep deformation, age hardening response and microstructural evolution of an Al-Cu-Mg-Si alloy under different stress levels from elastic to plastic loading have been systematically studied by creep aging test, tensile test, transmission electron microscopy and X-ray diffraction. It was found that the creep behavior of this alloy within elastic loads exhibit a multi-stage creep feature, which is related to the formation of solute atmosphere and solute-depleting mechanism. When stress loading exceeds the yield strength of the alloy, a two-stage creep behavior with the significantly increase in primary creep stage has been presented. The aging precipitation behavior varies greatly under different loads. Preferentially formed Q′phase was found in the elastic load creep aging process, while the plastic load creep aging precipitation process dominated by θ′ phase. Detailed microstructure analysis reveals that the initial dislocations induced by plastic load accelerate the formation of θ′ phases, and further restrain the precipitation of Q′ phases due to the existence of a competitive relationship between the two type precipitates. The θ′ phases with stress orientation effect and the performance under plastic load is more obvious. This contributes to the strength loss of the creep-aged alloy, especially under plastic load levels. Moreover, the dislocation density before the creep aging process affect the aging precipitation sequence of the Al-Cu-Mg-Si alloy, leading to the significant difference in the creep behavior under the elastic and plastic loads.

Extracorporeal shockwave therapy as an alternative treatment in cases of posttraumatic delayed bone union

Impaired posttraumatic bone healing is arelevant complication of fractures. Usually, the standard treatment is surgical revision. For about 30years extracorporeal shockwave therapy (ESWT) has emerged as an alternative treatment option with similar consolidation rates but less complications. This article aims to present our data in context to the current literature MATERIAL AND METHODS: From 2007 to 2016 a total of 97patients diagnosed with impaired posttraumatic bone healing were treated with ESWT. Clinical and demographic data of this population were retrieved and analyzed retrospectively. The general consolidation rate was60.8%. Multiple variables were analyzed. Apreinterventional bone gap ≥ 5 mm, initial dislocation > ½ of the bone shaft, nicotine consumption and along time span from fracture to ESWT (> 6months) were found as factors which significantly impair bone healing after ESWT. ESWT is asafe and promising alternative treatment option for delayed unions. Regarding risk factors of apoor outcome may be identified before and increase the rate of success.

Load partitioning behavior in a metastable austenitic stainless steel subjected to pre-deformation below and above Md temperature: Experimental and modeling studies

A physics-based model coupling the evolution of dislocation density in the austenite and strain-induced martensite phases with composite flow strength and kinetics of martensite transformation was developed to describe ambient temperature uniaxial tensile deformation behavior of mill annealed and pre-deformed 304 SS. The applicability of the model was evaluated by fitting the experimental stress-strain data and validated with the experimentally estimated dislocation density in the austenite phase of the steel. The modeling results revealed that the initial microstructure of the steel significantly influences the dislocation accumulation and annihilation rates as well as the load partitioning between austenite and martensite phases. Despite a lower initial dislocation density than the pre-deformed conditions, the mill annealed condition revealed a higher capacity (approximately two orders of magnitude) for dislocation accumulation in the austenite phase. The initial volume fraction of strain-induced martensite phase and the initial dislocation density of pre-deformed 304 SS are governed by the degree of pre-deformation and pre-deformation temperature, either below or above Md temperature (where Md is defined as the temperature above which plastic deformation does not result in austenite to martensite transformation). It is observed that storage, as well as annihilation rates in the austenite phase, are higher for the steel having a higher initial volume fraction of martensite and dislocation density. Though the pre-deformation altered the evolution of martensite volume fraction, the steel exhibited a minor variation in dislocation accumulation capability in the martensite phase for different conditions.

Effect of initial dislocation density on the plastic deformation response of 316L stainless steel manufactured by directed energy deposition

The relationship between the microstructural features (such as the solidification cells and initial dislocation densities) and the tensile properties of alloys additively manufactured (AM) using techniques such as laser powder bed fusion (L-PBF) and directed energy deposition (DED) is yet to be firmly established. In this work, a detailed investigation into the structure-property relations in DED 316L austenitic stainless steel (316L SS) was conducted. The microstructural parameters were varied systematically by changing the laser energy employed. Results show that while the sizes of grains and cells and the volume fraction of the oxide particles increase with increasing laser energy, the dislocation density decreases. Importantly, a uniform distribution of dislocations, instead of dislocation networks that are reported in many AM alloys, was observed. The connection between these microstructural features and the yield strength and work hardening capability of the DED 316L SS, which vary systematically with the laser energy, are explored. The correlation shows that a Hall-Petch type relation cannot capture the measured yield strength variation. Instead, the initial dislocation density dominates both the yield strength and the work hardening behavior. These results suggest a strategy for manipulating the mechanical performance in AM alloys through the control of dislocation densities and their distribution.

Poster 145: Posterior Glenoid Osteotomy with Capsulolabral Repair Restores Force in a Critical Bone Loss Biomechanical Model

Objectives:Bone loss in posterior shoulder instability is an increasingly recognized clinical phenomenon. There is no widespread consensus on the surgical treatment of posterior shoulder instability with critical posterior glenoid bone loss. Here, we study the effect of opening posterior wedge osteotomy on resistance force of soft tissue repair alone in the setting of 20% critical bone loss.Methods:Native glenoid retroversion was measured on 9 cadavers. The humerus was potted in 90 degrees of forward flexion and 30 degrees of internal rotation relative to the scapula and a posterior dislocation was performed in order to create a posterior capsulolabral injury model. The specimens were each taken through a fixed sequence of testing: (1) posteroinferior capsulolabral tear, (2) no glenoid bone loss with posteroinferior capsulolabral repair, (3) 20% posterior glenoid bone loss with posteroinferior capsulolabral repair, (4) and 20% glenoid bone loss with posterior glenoid opening wedge osteotomy and posteroinferior capsulolabral repair. The resultant peak forces with 1 centimeter of posterior translation were measured. A one-way repeated-measures analysis of variance (RM-ANOVA) was used to compare mean force values.Results:Following the initial dislocation event, all shoulder had a resultant posterior capsulolabral injury. Repairing the capsulolabral complex in the 20% posterior glenoid bone loss group did not result in a statistically significant increase in resistance force compared to the labral deficient group, p = 0.068. When 20% posterior bone loss was created, the posterior glenoid osteotomy with capsulolabral repair was significantly stronger than the posterior repair alone both with and without bone loss, p = 0.008 and p = 0.045, respectively.Conclusions:In the setting of critical posterior glenoid bone loss, an opening wedge posterior glenoid osteotomy with capsulolabral repair significantly improved resistance to posterior humeral translation compared to capsulolabral repair alone.

Arrangement and Decomposition of Grain Boundary Dislocations: Two-Mode Phase-Field Crystal Simulation

The grain-boundary dislocation arrangement and decomposition during constant-volume deformation of a nanoscale bi-crystal system in fcc-structured materials were studied by using the two-mode phase-field crystal (2PFC) method. The effects of different grain boundary misorientations (GBMs) and tensile deformation directions on the dislocation arrangement and decomposition are analyzed. In three different symmetrical tilt grain boundaries evaluated by PFC, the atomic density profile of grain boundaries changed periodically at equilibrium. The initial grain boundary dislocation arrangement of the three samples is almost the same when tensile deformation is applied to the samples in the x- or y- direction, and all are symmetrically arranged in a “bowknot ”structure. The stress at the grain boundary is concentrated with the increase of strain, and dislocation decomposition can effectively reduce the stress concentration. The time steps of dislocation decomposition at grain boundaries decreases with increasing strain rate. This work facilitates the application of PFC in the analysis of grain-boundary mechanics in an extended range of materials.

Theoretical study including physic-based material model to identify underlying effect of vibration amplitude on residual stress distribution of ultrasonic burnishing process

The potential of distribution of compressive residual stress at deep surface layers caused by ultrasonic-based surface sever plastic deformation (SSPD) has attracted researchers' attention to deeply study the mechanism of the process by simulation models. In majority of the researches, macro-mechanical Johnson-Cook (JC) model was utilized to govern material constitutive behavior. However, the large differences between measured and predicted values of residual stress of some materials imply that the underlying mechanism is still unclear and merits further studies. In the present work, a new physics-based constitutive equation incorporating micromechanics of initial grain size, dislocation evolution, dislocation drag and acoustic softening has been developed to obtain plastic flow stress. Then, the mechanic of the ultrasonic burnishing process incorporating superposition of static and dynamic loads was simulated using concept of expanding cavity model (ECM). The residual stress has been further calculated through combination of developed models aiming to discover the undying mechanism of vibration amplitude effect on stress fields. In order to confirm the obtained results, series of ultrasonic burnishing experiments have been carried out on AA6061-T6 under different vibration amplitude. The obtained results revealed that the developed analytical models of residual stress field are considerably more accurate than those previously established that took the macro-mechanical constitutive models to calculate plastic flow stress. Through the developed simulation model, it was studied in depth that how the vibration amplitude as the most adjustable ultrasonic vibration parameter determine variation of residual stress field. It has been demonstrated that the acoustic softening because of presence of ultrasonic energy field has the greatest impact in generation of compressive residual stress in surface and beneath layers.

Impact response of pre-strained pure vanadium

The effect of modest, 0.6% and 5.5%, pre-straining on the impact response of 2 mm thick samples of annealed polycrystalline vanadium of commercial purity was studied in a series of planar impact tests. The loading of the samples by 0.5 mm thick copper impactors having velocities varying between 300 and 610 m/s was accompanied by continuous laser Doppler velocimetry of their rear surface. Based on the recorded velocity histories, the dynamic compressive σY and tensile (spall) σsp strengths and the strength σYsc of vanadium in the shock-compressed state were determined. Adjacent to the impact surface part of the cross sections of the softly recovered samples, the number of twins Ntw per unit area was counted. It was found that the main parameter governing both the strength σY of pristine (in the shock sense) material and that in the shock-compressed state, σYsc, was the initial dislocation density η0. Moreover, the dislocation surplus caused by pre-straining was responsible for complete suppressing of twinning in the 0.6% and 5.5% pre-strained samples. In undeformed vanadium, the twinning was partially suppressed by the presence of impurity atoms which, however, did not affect the twinning stress, which was equal to approximately 0.7 GPa.

Transmission Electron Microscopy Study of Single Shockley Stacking Faults in 4H-SiC Expanded from Basal Plane Dislocation Segments Accompanied by Threading Edge Dislocations on both Ends

Double-rhombic shaped single Shockley stacking faults (1SSFs) were considered to have a converted threading edge dislocation (TED) on the shallower side of the initial basal plane dislocation segments. However, the structural analysis using transmission electron microscopy (TEM) revealed other possible configuration of the double-rhombic 1SSFs expanded from basal plane dislocations (BPDs) of which both ends were connected with two TEDs.

The critical grain size for optimal strength–ductility synergy in CrCoNi medium entropy alloy

In metals with homogeneous microstructure, whether there is a critical grain size, and exactly at which, that enables the optimal strength-ductility synergy alongside the trade-off relation? Here, this issue is investigated in the recrystallized CrCoNi medium entropy alloys across a wide grain size range of 0.23-108 μm. Results reveal that there is indeed a critical grain size of ∼2 μm at which the product of yield strength (∼800 MPa) and uniform elongation (>30%) reaches the maximum, i.e., achieving the optimal strength-ductility synergy. Physics behind this critical grain size are explored by microstructure examination: (i) reduced grain size renders short dislocation slip path for high strength; (ii) low initial dislocation density and dense grain/twin boundaries enables effective accumulation of defects for consistent work hardening during plastic deformation.