High Initial Dislocation Density Research Articles

Effects of rolling processing on the microstructures, mechanical properties and strain softening behavior of biodegradable Zn-0.06 Mg alloy

Micro-alloyed Zn–Mg alloys have attracted extensive attention in the field of medical implant due to their excellent biosafety and moderate degradation rate. The low Mg content could compromise the improvement of strength, therefore, the appropriate rolling process was investigated to enchance strength and inhibit strain softening behavior of Zn-0.06 Mg alloy. The observed changes in microstructures, mechanical properties, and strain softening behavior of the as-cast alloy after three different rolling processing were consistent with those exhibited by the solid-solution alloy. The cold rolled alloys showed excellent strength due to their small average grain size and high initial dislocation density, however, they also exhibited noticeable strain softening behavior. Hot rolling processing can result in coarsening of the grains, reduction in dislocation density and introducing a large number of twins, which inhibited strain softening behavior but also leaded to a decrease in the strength compared with cold rolling. The hot-cold rolling processing can induce the formation of heterostructure, wherein coarse grains encircled fine grains, while high density dislocation regions and low density dislocation regions were uniformly distributed, which effectively inhibited strain softening behavior on the basis of maintaining good mechanical properties.

Impact of laser powder bed fusion processing on the cyclic and fatigue properties of Ni20Cr alloy

Additive manufacturing is a new-age technology specialising in intricate fabrication in the manufacturing industry. However, extremely high cooling rates and far-from-equilibrium kinetics produce heterogeneous microstructure and induce high initial dislocation density, porosity, and residual stress. These unique microstructural features are known to severely impact cyclic and fatigue properties of alloys. In such a context, we investigated the cyclic and fatigue properties of a Ni-20 wt%. Cr alloy manufactured via laser powder bed fusion process (LPBF) and compared with its cast counterpart. The fatigue testing was carried out with three different strain amplitudes depicting Low, Medium and High Cycle Fatigue (LCF, MCF, HCF). LPBF samples exhibited higher fatigue resistance than that of cast samples, however experienced early failure in all the conditions. Despite strong planar glide, LPBF samples exhibited softening behaviour; the degree of softening is similar for LCF and MCF and less pronounced for HCF. The cast samples did not undergo any softening whatsoever indicating huge differences in cyclic strain mechanisms. Masing analysis was proposed to graphically analyse this softening behaviour. Further, the flow stress was categorised into backstress and effective stress for LPBF and cast samples. Majority of softening in LPBF samples for LCF and MCF occurred via backstress, whereas effective stress is associated to HCF. To investigate the origin of this softening for LPBF samples, microstructure characterisations were performed during the softening and at fracture. Post-fatigue microstructure indicates a clear modification of the dislocation structures from those inherited from LPBF to those linked to fatigue. This gradual change in microstructure is expected to induce the cyclic softening.

Effect of thermal-cold cycling treatment on microstructural stability of Al-Cu-Mg alloy hemispherical component

The microstructural stability of aluminum alloy hemispherical components in gyroscope navigation systems is the determinant for navigation precision. In this work, a thermal-cold cycling treatment (TCT) was applied to evaluate the microstructural stability of the hemispherical components. The results showed that the dislocations and precipitation of Al2CuMg in the component annihilated with the increasing TCT cycles, improving the microstructural stability. After 16 cycles, the dislocation density reached the minimum value of ∼1.51 × 1014 m−2 in the die fillet area (DFA), indicating an earlier stabilization than that in the bottom of the hemisphere (BOH) after 38 cycles. Such a difference was mainly attributed to the initial dislocation density. The high initial dislocation density in the DFA reduced the nucleation work and diffusion activation energy, which accelerated the nucleation of Al2CuMg phases, thereby reducing the stored energy and lattice distortion remarkably. Additionally, the high initial dislocation density increased the superimposed effect of two dislocations of opposite signs, accelerating the annihilation of dislocations. The annihilation of dislocations and precipitation of tiny Al2CuMg phases contributed to the optimized microstructural stability. In this sense, the TCT may serve as an effective method for the fabrication of aluminum alloy components with high stability for gyroscopes.

The effect of friction-stir welding in hydrogen embrittlement of Fe-17Mn alloy

In present study, we suggest the friction-stir welding (FSW) is an effective welding technique for improving the resistance of hydrogen (H) embrittlement in Fe-17Mn (wt%) alloy with a dual phase of γ austenite and ε martensite. The FSW specimen exhibited the enhanced resistance of H embrittlement during H-charging/tensile deformation despite active α′ martensitic transformation which is apt to H embrittlement, relative to base metal. During H-charging, numerous ε phase and higher initial dislocation density, as potential sites for H trapping, after FSW reduced H diffusion into specimen, causing the shallow H-related brittle fracture. In addition, the increased γ/ε interface and boundary in the FSW specimen alleviated H accumulation. During tensile deformation, the grain rotation and higher cε/aε ratio by FSW reduced Schmid factor of finer ε grains, leading to stronger resistance of boundary decohesion and the suppression of H-induced crack growth.

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.

Beyond strength-ductility trade-off: 3D interconnected heterostructured composites by liquid metal dealloying

A breakthrough in the strength and ductility trade-off is crucial for the development of advanced metallic materials. Herein, we present a novel heterostructured composite composed of immiscible magnesium (Mg) and ferrochrome (FeCr) with a 3D interconnected morphology and synthesized by liquid metal dealloying. Soft Mg and hard FeCr zones mutually interlock with each other. This unique interpenetrating-phase configuration leads to a significant alternation of their intrinsic mechanical properties, especially in the soft Mg zone. It causes a strong forest hardening effect, resulting in a high initial dislocation density, and the surrounding hard zones create hydrostatic pressure at the soft zone under tension. The measured yield strength of the composite is close to the upper rule of mixture while its tensile elongation is larger than that of the mixture. These outstanding mechanical properties originate from the synergetic interaction between the soft and hard zones through the immiscible interface zone. The current 3D interconnected heterogeneous composite acts a guideline for the design of advanced materials possessing physical properties beyond expectations.

The possible role of nano sized precipitates on the mechanical properties of additively manufactured IN 718 superalloy

In the present study, three dimensional parts of Inconel 718® (IN 718) superalloy were fabricated using laser powder bed fusion (L-PBF) additive manufacturing (AM). The soundness of the as-built parts was checked by measuring the relative density using Archimedes method. Tensile tests were carried out at the strain rate of 4.6 × 10-3 s-1 at ambient temperature on cylindrical samples extracted from the large size 3D blocks of AM IN 718. Tensile tests were also carried out employing the same test parameters on the wrought counterpart of IN 718 (W IN 718) so as to compare their mechanical properties. The AM IN 718 exhibited about 15% increase in 0.2% yield strength (0.2%YS) as compared to W IN 718. Microstructures of AM IN 718 and W IN 718 were characterized using electron back scattered diffraction. The dislocation substructure of pre-deformed samples was examined using transmission electron microscope (TEM). Bright field TEM images revealed nano-sized precipitates in AM IN 718 and carbides in W IN 718. The marginal increase in 0.2%YS in AM IN 718 is attributed to the impediment to dislocation movements by nano-sized precipitates and high initial dislocation density.

Multiaxial plasticity and fracture behavior of stainless steel 316L by laser powder bed fusion: Experiments and computational modeling

The multiaxial large deformation and ductile fracture behavior of laser powder bed fusion (L-PBF) additively manufactured austenitic 316L stainless steel was experimentally measured. Data from tests in two orientations, under five dissimilar stress states (shear, combined shear/tension loading states, plane strain tension, and uniaxial tension) were used to calibrate and validate anisotropic plasticity and fracture models, with different specimen geometries used to probe plasticity versus fracture. Shear softening, hypothesized to be due to shear band formation in the material due to high initial dislocation density and sub-micron cellular structures, was observed in shear dominated tests, and modeled through the adoption of a shear damage criterion in an anisotropic plasticity model. Using a combined experimental and computational approach, isotropic and anisotropic Hosford-Coulomb and modified Mohr-Coulomb ductile fracture models were calibrated for both sample orientations. The calibrated anisotropic Hosford-Coulomb fracture model best captures the stress state dependent and anisotropic failure behavior of L-PBF 316L.

Formation mechanism of large grains inside annealed microstructure of GH4169 superalloy by cellular automation method

In authors’ previous work [Mater. Charact. 141 (2018) 212–222], it was found that the heterogeneous deformed microstructures can be replaced by the relatively homogeneous recrystallized grains through an annealing treatment. However, there are still some relatively large recrystallized grains. To find the reasons for the formation of large grains, some new annealing treatment tests were done, and the cellular automation (CA) simulations were carried out in the present work. The experimental results showed that the microstructural evolution during annealing treatment is significantly affected by the content of δ phase. So, the effects of δ phase on the nucleation and growth of grains are carefully considered in the CA model to accurately simulate the microstructural evolution behavior. By the CA simulation, it is found that the dislocation density rapidly decreases due to the nucleation of static recrystallization (SRX) and the growth of dynamc recrystallization (DRX) nuclei at the early stage of annealing. The high initial dislocation density can provide the high velocity for the growth of DRX nuclei, which is responsible for the formation of coarse grains. However, the growth rate of SRX nuclei is relatively small due to the low dislocation density and pinning effects of δ phase.

Influence of Initial Defects on the Mechanical Properties of Single Crystal Copper: Discrete Dislocation Dynamics Study

In this paper, three dimensional discrete dislocation dynamics method was used to quantitatively investigate the influence of initial defects on mechanical response of single crystal copper. Both the irradiation defects (interstitial loops) and random dislocation lines with different densities are considered. The simulation results demonstrate that the yield strength of single crystal copper is higher with higher initial dislocation density and higher interstitial loop density. Dislocation density increases quickly by nucleation and multiplication and microbands are formed during plastic deformation when only the random dislocation lines are initially considered. Characteristics of microbands show excellent agreement with experiment results. Dislocation multiplication is suppressed in the presence of interstitial loops, and junctions and locks between dislocations and interstitial loops are formed. Dislocation density evolution shows fluctuation accompanied with strain-stress curve fluctuation.

Effect of recrystallization on hot deformation mechanism of TA15 titanium alloy under uniaxial tension and biaxial gas bulging conditions

To investigate the effect of recrystallization on hot deformation mechanism of TA15 titanium alloy, rolled sheet and laser-welded tubes before and after recrystallization annealing were tested by uniaxial tensile tests and biaxial gas bulging respectively. The results show that both of the initial rolled sheet and the as-welded tube consisted of equiaxed α microstructure with high initial dislocation density. During uniaxial tensile tests, a quasi-steady flow state was achieved at 800°C, 0.001s−1, which demonstrated the best formability with a maximum elongation of 536%. Significant recrystallization occurred at the early stage of tensile deformation and the grain size was found to decrease first and then increase. Significant recrystallization also occurred at the early stage of the biaxial gas bulging of the as-welded tube. During the bulging of the annealed tube consisting of equiaxed α with low initial dislocation density, dynamic recrystallization (DRX) was the main microstructure evolution characteristic, but most of the DRX occurred at the middle-late stage. The relatively coarse equiaxed α and increasing fraction of low angle grain boundaries during deformation may restrict the occurrence of grain boundary sliding (GBS), which resulted in the relatively worse formability of the annealed tube. However, recrystallization which happened widely at the early stage, increased the fraction of high angle grain boundaries, refined the microstructure, promoted the occurrence of GBS and enhanced the formability of the initial rolled sheet and as-welded tube. It was also found that the stress state had no effect on the DRX mechanism for TA15 at 800°C, which was confirmed to be discontinuous dynamic recrystallization under both uniaxial tension and biaxial gas bulging conditions.

Discrete dislocation plasticity analysis of the effect of interfacial diffusion on the creep response of Ni single-crystal superalloys

Discrete dislocation plasticity (DDP) analysis of the high temperature creep deformation of a single crystal Ni superalloy comprising Ni3Al precipitates (γ′) in a Ni matrix (γ) is presented. The γ′ precipitates remain elastic but can also deform due to the stress-driven inter-diffusion of the Al within the Ni on the γ/γ′ interface while plastic deformation of the γ phase occurs by a combination of dislocation glide and dislocation climb coupled to the diffusion of vacancies. At relatively low applied uniaxial tensile stresses, the creep strain rates are very low in the absence of interfacial diffusion. This is due to the stress-induced pile up of dislocations at γ/γ′ interfaces that serves to inhibit further nucleation and suppresses continued plastic flow in the γ phase. When interfacial diffusion is permitted, the creep rates not only increase but the superalloy also exhibits distinct secondary and tertiary creep regimes. While this change in behaviour is a result of interfacial diffusion, the contribution of the average γ′ strain to the deformation of the superalloy is small. Rather, the diffusional deformation at the interface results in the development of a wavy interface which relaxes the back-stresses of dislocations piled-up at the γ/γ′ interfaces. This permits continued dislocation activity within the γ phase with dislocations arranging themselves into low energy cell-structures in the γ phase via dislocation climb. The formation of these structures results in an increase in the creep strain rate and the onset of the tertiary creep regime. At high applied stresses, the high initial dislocation density within the γ phase results in the continued climb motion of dislocations and an evolving spatial distribution of vacancies within the superalloy. Thus, creep deformation occurs even in the absence of interfacial diffusion although the creep rates are significantly increased when interfacial diffusion is present. The DDP analysis presented here demonstrates the critical role of interfacial diffusion in controlling the creep rates of Ni superalloys and suggests that interface engineering to reduce interfacial diffusion rates will aid in improving the creep performance of these alloys.

Approach to Enhance Gas Bulging Formability of TA15 Titanium Alloy Tube Based on Dynamic Recrystallization

Abstract In order to further improve the gas bulging formability of laser-welded TA15 titanium alloy tube, studies about the effects of initial microstructures on the formability were carried out at 800 ℃.Vacuum annealing procedures were performed to obtain different microstructures. The formability of these TA15 titanium alloy sheets with different microstructures was subsequently investigated by hot tensile test and hot gas free bulging. The results show that after the recrystallization annealing in the two phases field, the initial dislocation density of the material was reduced greatly due to recrystallization and the grains became coarser. After the double annealing at the near β phase field, phase transformation of α to β happened, secondary α formed in the β matrix and the equiaxed α grew coarser. The strength of material at elevated temperature increased and the ductility decreased after the annealing. Under the uniaxial tensile deformation, the material consisting of equiaxed α microstructure with high initial dislocation density exhibited the best formability. During its deformation, lots of dynamic recrystallization (DRX) happened due to its high initial dislocation density at the early stage of deformation, which increased the fraction of high angle grain boundaries, refined the microstructure, promoted the occurrence of grain boundary sliding and enhanced the formability of the material. The bulging results are consistent with the tensile results. The annealed tubes deformed more slowly due to the higher strength at elevated temperature. The tube consisting of equiaxed α microstructure with high initial dislocation density also exhibited the best formability. Therefore increasing the occurrence of DRX at the early stage of deformation by increasing the initial dislocation density in the material is an efficient approach to improve the gas bulging formability.

Grain size effects on microstructural stability and creep behaviour of nanotwinned Ni free-standing foils at room temperature

Creep tests were performed on the high stacking fault energy (SFE) nanotwinned (NT) Ni free-standing foils with nearly the same twin thickness at room temperature (RT) to investigate the effects of grain size and loading rate on their microstructural stability and creep behaviour. The grain growth mediated by the twinning/detwinning mechanism at low applied stresses (<800 MPa) and grain refinement via the detwinning mechanism at high applied stresses (>800 MPa) were uncovered in the present NT-Ni foils during RT creep, both of which are attributed to the interactions between dislocations and boundaries. It appears that a higher initial dislocation density leads to a faster primary creep strain rate and a slower steady-state creep strain rate. Unlike the non-twinned metals in which grain growth often enhances the creep strain rate, the twinning/detwinning-mediated grain growth process unexpectedly lowers the steady-state creep strain rate, whereas the detwinning-mediated grain refinement process accelerates the creep strain rate in the studied NT-Ni foils. A modified phase-mixture model combined with Arrhenius laws is put forward to predict the scaling behaviour between the creep strain rate and the applied stress, which also predicts the transition from grain growth-reduced to grain refinement-enhanced steady-state creep strain rate at a critical applied stress. Our findings not only provide deeper insights into the grain size effect on the mechanical behaviour of nanostructured metals with high SFE, but also benefit the microstructure sensitive design of NT metallic materials.

Non-equilibrium grain boundaries in titanium nanostructured by severe plastic deformation: Computational study of sources of material strengthening

A computational model of ultrafine grained (UFG) or nanostructured titanium (Ti), based on a finite element (FE) unit cell model of the material and a dislocation density based model of plastic deformation has been developed. FE simulations of tensile deformation of UFG Ti with different fractions and properties of the grain boundary (GB) phase have been carried out. The effect of different degrees of deviation from the equilibrium state of the grain boundaries (GBs) on the mechanical behaviour of nanostructured Ti have been investigated using the combined composite/dislocation dynamics based model. In particular, the effects of different diffusion coefficients in the GB phase, of a high initial dislocation density in the grain boundaries, as well as of atomic scale precipitates are investigated for affecting the deformation behaviour of UFG or nanostructured Ti.

Low-temperature bainite in low-carbon steel

The microstructures and the mechanical properties of 30MnSiCrAlNiMo low-carbon steel were systematically optimized by a series of heat-treatment processes, and the heat-treatment process of low-temperature bainite in low-carbon steel was explored. Results showed that the microstructure of low-temperature bainite in the low-carbon steel, containing a fine plate of carbide-free bainitic ferrite and a thin film of retained austenite, could be produced by continuous cooling transformation around the Ms temperature from Ms+10°C to Ms−20°C at a cooling rate of 0.5°Cmin−1. A new model was proposed to evaluate the comprehensive mechanical properties of steel, which found that the low-temperature bainite had the best comprehensive mechanical properties compared to any other microstructures for the low-carbon steel. The higher dislocation density and finer bainitic ferrite plate in the low-temperature bainite resulted in the higher yield strength and the higher toughness, but relatively lower ultimate tensile strength owing to the lower work-hardening rate caused by the higher initial dislocation density. There were some very fine particles in the bainitic ferrite of the steel after isothermal treatment at higher temperature. The ultimate tensile strength and the low-temperature impact toughness of the steel decreased with the volume fraction of the retained austenite increasing, while the elongation initially increased with an increase in the volume fraction of the retained austenite (<10%) and then remained constant.

Dynamic balance between grain refinement and grain growth during high-pressure torsion of Cu powders

High-pressure torsion (HPT) was applied to unmilled coarse-grained (CG) Cu powders with low initial dislocation density and cryomilled nanocrystalline (nc) Cu powders with high initial dislocation density, with identical processing parameters. HPT of unmilled CG Cu powders resulted in exceptional grain refinement and increase in dislocation density, whereas significant grain growth and decrease in dislocation density occurred during HPT of cryomilled nc Cu powders. Equilibrium structures were achieved under both conditions, with very similar stable grain sizes and dislocation densities, suggesting dynamic balances between deformation-induced grain refinement and grain growth, and between deformation-induced dislocation accumulation and dislocation annihilation. The equilibrium structures are governed by these two dynamic balances.

Positron annihilation study of effects of Ti and plastic deformation on defect accumulation and annealing in electron-irradiated austenitic steels and alloys

Positron annihilation spectroscopy was used to study the formation and annealing of vacancy clusters in 16Cr15Ni3Mo austenitic steels and Fe–36%Ni model alloys and in the same compounds containing 1.02 and 2.5 wt% titanium respectively. Defects were induced by electron (5 MeV) irradiation at 270–573 K. How a developed initial dislocation structure influenced the accumulation and annealing of vacancy defects in these steels and alloys was analyzed. It was shown that vacancies interacted with titanium atoms. As a result, the Ti-containing steels and alloys exposed to radiation at temperatures from 270 to 423 K had an enhanced concentration of fine vacancy clusters decorated with titanium, which were thermally stable up to 450 K. A high initial dislocation density in the deformed steels and alloys led to a several-fold decrease in the concentration of vacancy clusters as compared to their concentration in the solution annealed state. The formation of fine TiC particles in Ti-modified deformed steel was monitored at the annealing temperatures from 850 to 1070 K.

Misfit dislocation dissociation and Lomer formation in low mismatch SiGe/Si heterostructures

Using transmission electron microscopy we observe the dissociation of 60° misfit dislocations at the interface of SiGe/Si multilayers, extending into the substrate for distances of 5.0–7.5 nm. Analysis using elasticity theory shows that this dissociationis the equilibrium configuration for individual 60° misfit dislocations, as it is for 60° mixed dislocations in bulk Si, and that the compressively strained multilayer film serves mainly to position the partial dislocations and stacking fault with respect to the free surface. We observe both undissociated 60° and Lomer edge dislocations after annealing, and conclude that these result from dislocation climb in the interface. Since the dislocations move off their slip plane during climb, they cannot remain dissociated. Significant climb and Lomer dislocation formation for these low misfit layers is observed at temperatures above 850 °C and for samples with a high initial dislocation density, such as found in thicker as-grown samples. The dislocation configuration formed during annealing is distinct from that reported to form during growth of higher mismatch films: the Lomer dislocations tend to be segmented, with the segments connected by perfect 60° dislocations.

Strengthening via the formation of strain-induced martensite in stainless steels

The strengthening that results from the low-temperature formation of strain-induced martensite in austenitic stainless steel was studied. Specifically, the work hardening behaviour was characterized, as well as the spatial distribution of the martensite as a function of prior strain. Neutron diffraction measurements revealed the degree of elastic strain partitioning between the austenite and martensite. It was found that a sufficiently high initial dislocation density leads to a localization of the martensite transformation in the form of a Luders front. The martensite acts as an elastic reinforcing phase as it supports a higher stress than the austenite tensile loading, even though the martensite co-deforms plastically with the austenite. A model was developed that predicts the volume fraction of martensite formed as a function of plastic strain. © 2004 Elsevier B.V. All rights reserved.