Slip Bands Research Articles

Crack bifurcation behavior of coarse-grained copper under cyclic torsion combined with axial static loading

Because the growth behaviors of fatigue cracks are crucial for the safe assessment of structural components, the crack propagation behaviors of coarse-grained copper (CG Cu) subjected to cyclic torsion combined with different axial static stresses were studied. The crack bifurcation behavior is related to the strain amplitude applied. When the strain amplitude is lower, both the type and the magnitude of axial stress have no significant effect on the direction in which the primary crack branches, which is mainly determined by the position of the maximum normal plane. However, when the strain amplitude is higher, the bifurcated crack deviates visibly from the maximum normal plane, which can be attributed to the high degree of plastic deformation and microcracks caused by slip bands along longitudinal direction.

Revealing the dynamic behavior and micromechanisms of enhancing the formability of AA1060 sheets under high strain rate deformation

Developing high strain rate (HSR) forming technologies is significant for efficient and high-quality aerospace sheet metal parts manufacturing. Research on the deformation behaviors and micromechanism of materials under HSRs will contribute to the further application of HSR forming technologies. In this study, the mechanical behavior of AA1060 pure aluminum at low strain rates (LSRs, 0.001 s−1 and 0.1 s−1) and high strain rates (HSRs, 2400 s−1 and 3300 s−1) was tested utilizing quasistatic tensile tests and Hopkinson bar tests at room temperature. The results of the mechanical properties showed that the elongations were 30.01 %, 37.91 %, and 46.61 %, corresponding to strain rates of 0.1 s−1, 2400 s−1, and 3300 s−1, respectively, which were 29.86 % at a strain rate of 0.001 s−1. The maximum amplification of AA1060 elongation reached 56.10 % at the HSR. Microstructure characteristics, including the Schmid factor distribution, geometrically necessary dislocation (GND) distribution, slip band, and dislocation cell, were observed by electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM) under the condition of the same elongation with strain rates of 0.001 s−1 and 3300 s−1. In addition, the reasons for the plastic improvements during HSR loading were also investigated. The hard-oriented grains participate in plastic deformation at HSRs, providing additional space for dislocation multiplication and movement, increased dislocation density, and interaction between dislocations. In addition, the deformed hard-oriented grains also coordinated the strain gradient with soft-oriented grains. This improved the overall deformation uniformity, thus avoiding the premature occurrence of strain localization, namely, the elongation increase.

Low cycle fatigue behaviors of CrCoNi medium-entropy alloy with different grain sizes

Low-cycle fatigue behaviors of CrCoNi medium entropy alloy with different grain sizes were investigated. It is found that grain refinement improved deformation homogeneity to delay fatigue crack initiation by dispersing slip bands and fatigue damage. Yet, the increase in grain size promoted deformation twinning to retard fatigue crack growth by dissipating plastic work. With enough plastic deformation accumulated, extensive dislocation slip and deformation twinning significantly refined the grains localized near fatigue cracks. The grain refinement could trade off the cyclic softening and help to retard fatigue crack growth. These findings indicate that refined grains with certain twinning ability would extend the fatigue life of CrCoNi alloy the most and suppress cyclic softening at the same time.

Fabrication of Al/Al-TiB2 laminate composites via hot press sintering process: An insight into the mechanical properties and fracture behavior

The Al/Al-TiB2 laminated composites with different numbers of layers was fabricated by hot press sintering method. The results show that Al/Al-TiB2 interface is well bonded without obvious micropores, cracks, or other defects, while the stacking faults and dislocation loops are observed at the interface. The average grain sizes of Al and Al-TiB2 layers are 45.17 and 5.51 μm in the composite with 3 layers, while they greatly increase to 80.07 and 6.51 μm in the composite with 5 layers. The hardness increases gradually along Al layer, Al/Al-TiB2 interface, and Al-TiB2 layer. TiB2 particles mainly contributes to the enhancement of bending strength, and the maximum strength of 110.0 MPa is achieved in the composite with 3 layers due to the fine grain size, small particle spacing, and more geometrically necessary dislocations in Al-TiB2 layer. During bending deformation, Al layer exhibits ductile fracture with shear lip, slip band, fatigue striation and dimple, while Al-TiB2 layer shows typical brittle fracture with micro-voids and cleavage. The stress concentration and crack always appear in Al-TiB2 layer, while Al layer can transfer strain and inhibit strain localization. An increasing ductility of the laminated composites is achieved due to crack deflection, interfacial delamination, and strain transferring.

The Microstructures, Mechanical Properties, and Deformation Mechanism of B2-Hardened NbTiAlZr-Based Refractory High-Entropy Alloys.

The NbTiAlZrHfTaMoW refractory high-entropy alloy (RHEA) system with the structure of the B2 matrix (antiphase domains) and antiphase domain boundaries was firstly developed. We conducted the mechanical properties of the RHEAs at 298 K, 1023 K, 1123 K, and 1223 K, as well as typical deformation characteristics. The RHEAs with low density (7.41~7.51 g/cm3) have excellent compressive-specific yield strength (σYS/ρ) at 1023 K (~131 MPa·cm3/g) and 1123 K (~104.2 MPa·cm3/g), respectively, which are far superior to most typical RHEAs. And, they still keep appropriate plastic deformability at room temperature (ε > 0.35). The superior specific yield strengths are mainly attributed to the solid solution strengthening induced by the Zr element. The formation of the dislocation slip bands with [111](101_) and [111](112_) directions and their interaction provide considerable plastic deformation capability. Meanwhile, dynamic recrystallization and dislocation annihilation accelerate the continuous softening after yielding at 1123 K.

A refractory medium-entropy alloy with ultrahigh specific yield strengths for elevated-temperature applications

Refractory high- or medium-entropy alloys (RHEAs or RMEAs) with excellent high-temperature mechanical properties and softening resistance have been proven to be the potential candidates for advanced engineering applications. However, room temperature brittleness and high density have become an important challenge that needs to be addressed. In this work, the tensile mechanical behavior and the underlying deformation mechanisms of lightweight Ti40Zr20Nb13.33V26.67 RMEA at 298 and 873 K were investigated systematically. The results showed that the as-cast RMEA has a single body-centered cubic phase and low density (5.88 g/cm3) and exhibits excellent mechanical properties at 298 K, with yield strength of 1033.9 MPa, specific yield strength of 175.8 MPa·cm3/g, and tensile fracture strain of 5.3%. More importantly, it also exhibits ultrahigh strength and sufficient ductility at 873 K, with yield strength of 783.2 MPa, specific yield strength of 133.2 MPa·cm3/g, and tensile fracture strain of 5.7%. It showed that a large number of slip bands and dislocation bands are the main deformation products at 298 K, leading to excellent ductility. In comparison, high dislocation density was found between the slip bands in the samples deformed at 873 K, which can effectively hinder the motion of dislocations, resulting in strain hardening and the increase in strength. This work can provide a route for the design and fabrication of high-performance lightweight alloys, which would be beneficial for engineering applications.

IBC-Oxford University2023_BJSOral_20Video Presentation: Fundectomy and Indocyanine green fluorescence angiography in revisional laparoscopic Roux-en-Y gastric bypass

Abstract Introduction Revisional bariatric procedures are increasingly prevalent. These procedures are more complex with higher risk of complications as compared to primary procedures. The potential indications for revisional laparoscopic gastric bypass (R-LRYGB) may include weight regain and gastroesophageal reflux disease (GORD). This video abstract presents role of fundectomy and Indocyanine green fluorescence angiography (ICG-FA) for perfusion assessment of gastric pouch in R-LRYGB. Methods and Operative technique A 40-year-old female with previous laparoscopic adjustable gastric band (LAGB) surgery underwent R-LRYGB procedure following approval at bariatric multidisciplinary team meeting. LAGB was removed due to band slippage and chronic pouch dilatation, 2 years prior to the revisional procedure. Preoperative BMI was 53 kg/m2. Pre-operative upper gastrointestinal endoscopy was normal. The operative technique included dissection of angle of His, access into lesser sac along lesser curve below 2nd vessel off left gastric artery (LGA) and division of short gastric vessels to gain access to leaser sac from lateral aspect. The stomach was divided horizontally with linear stapler (EndoGIA 60 Gold cartridge), level with second vessel off LGA followed by creation of vertical gastric pouch over a size 34 Oro-gastric tube, that included excision of gastric fundus. ICG was administered intravenously according to an agreed protocol for the study. Karl Storz Image1 STM Rubina system with overlay and monochromatic modes was used for perfusion assessment. ICG-FA was utilised for qualitative assessment of gastric pouch perfusion before and after linear stapled gastro-jejunal anastomosis. Results Operative time was 240 minutes. Adhesions were encountered in the lesser sac at the site of previous LAGB. Fundectomy allowed better visualisation of lesser sac, adhesiolysis and facilitated creation of gastric pouch. ICG perfusion of gastric pouch was satisfactory. The postoperative length of stay was 2 days. There were nopostoperative complications at 6 months follow-up. The patient achieved 42% excess weight loss (BMI 43) over 6 months period. Conclusions Fundectomy is a safe and feasible operative step in R-LRYGB in patients with previous primary LAGB. ICG-FA is a promising qualitative assessment tool in revisional bariatric procedures.

Influence of an oxygen-enriched layer on the tensile properties of an alpha titanium alloy

The manuscript focuses on establishing the role of an oxygen-enriched layer (OEL) on the tensile properties of an alpha titanium alloy, Ti-Al-Zr. The samples were heat-treated in the inert and air atmosphere for different durations up to 700 h. The air oxidation led to the formation of the OEL near the surface of the specimen. The specimens were characterized by different metallurgical techniques like a stereo microscope, optical microscopy, micro-hardness measurement and grain size measurement. The oxygen profiling was done by the glow discharge optical emission spectroscopy technique. The tensile testing was done at room temperature at a strain rate of 2.5 × 10−4 s−1. The fracture surface was examined under scanning electron microscopy. The tensile properties-yield strength and ultimate tensile strength were in the same range for the specimens oxidized in the inert and air atmosphere. However, the specimens oxidized in the air atmosphere showed a drastic decrease in ductility with increasing oxidation duration. The entire specimens oxidized in the inert atmosphere showed dimples features, typical of a ductile failure. The specimens oxidized in the air atmosphere showed fracture features like intercrystalline fracture, slip bands and cleavage facets in addition to dimples features. The OEL showed brittle failure and the regions other than OEL showed ductile failure. The role of oxygen on the tensile properties and the development of different fracture features during air oxidation are discussed.

Effect of Isothermal Annealing on Residual Stresses and Fatigue Properties of LPBF 316L Steel

This study aims to investigate the influence of isothermal annealing on the residual stresses and fatigue properties of a 316L austenitic stainless steel, manufactured by the laser powder-bed fusion (LPBF), possessing a high density of 99.98%. Residual stresses were evaluated using the X-ray diffraction techniques. High-cycle fatigue tests were performed on cylindrical samples manufactured in both horizontal and vertical orientations, subjected to force-controlled axial fully reversed loading. Following fabrication, the samples underwent isothermal annealing in a furnace either at 600 °C for 120 minutes or at 900 °C for 30 minutes. Subsequently, the samples were machined to their final dimensions and electropolished to a mirror surface finish. Preliminary findings revealed that increasing the annealing temperature effectively reduced the surface residual stresses. However, this reduction did not lead to an improvement in the fatigue resistance of this nearly fully dense material in the high-cycle fatigue regime. Interestingly, the structure annealed at 600 °C exhibited a higher fatigue strength compared to the structure annealed at 900 °C, with no discernible difference between the printing directions. Fracture surfaces and microstructural features examined using light and electron microscopy revealed that cracking was primarily initiated at surface defects or slip bands. These results highlight the complex interplay between residual stresses, microstructure, strength, and fatigue behaviour of LPBF 316L austenitic stainless steel. Further analysis and investigations are required to fully understand the underlying mechanisms and develop strategies for enhancing the fatigue performance of additive manufactured components.

Investigation of fine-scale dislocation distributions at complex geometrical structures by using HR-EBSD and a comparison with conventional EBSD

Dislocation distribution is one of the elementary factors that directly influence deformation processes in structural components. Electron backscatter diffraction (EBSD) analysis is one of the experimental techniques that allow for a semi-quantitative evaluation of the Geometrically Necessary Dislocation (GND) distribution in polycrystals. Owing to its noise floor the accuracy of conventional Hough transformed-based GND analysis is restricted to relatively large plastic strains. This problem can be overcome by using the cross-correlation-based high angular resolution electron backscatter diffraction (HR-EBSD) method. The present study demonstrates the improvements in characterizing localized dislocation distribution using the HR-EBSD method compared to the conventional approach. To demonstrate the efficacy, we investigated two more extreme examples of deformation conditions in polycrystalline zirconium (Zr), 1) a plastically deformed crack tip in presence of applied strain, and 2) a misfit plastic field around a hydride precipitate. The edge, screw and total GND components are compared for both cracked and hydride specimens. Dissimilarities in the identification of screw dislocations and subtle features with misorientation uncertainties are reported using HR-EBSD and conventional EBSD. A direct correlation between slip bands and the HR-EBSD estimated GNDs is presented, enhancing the scope of this approach to identify individual slip bands, which is traditionally challenging for any misorientation-based characterization technique.

Effect of microstructure on low cycle fatigue behavior of Inconel 706 at 650°C

AbstractFour different types of specimens were prepared from the center and the periphery of two large IN706 forged discs of commercial scale, and low cycle fatigue (LCF) tests were conducted at 650°C.The IN706 specimens with lower LCF lives showed relatively large fraction of cleavage‐like fracture along linearly aligned η (Ni3Ti) precipitates in the area of crack propagation. The amount of η precipitates for those specimens with greater LCF lives was negligible, and the fracture mode of crack propagation was dominantly intergranular. Crack initiation was mainly by persistent slip band (PSB) cracking at the surface, and no notable difference was found for each specimen. The correlation between tensile properties, grain size, and LCF lives of IN706 at 650°C appeared to be not as significant as expected.

The interface effect on crack nucleation under dwell fatigue loading in dual-phase Ti alloy

The microstructure of dual-phase Ti alloys crucially influences the nucleation of dwell fatigue cracks, which is of significance in determining the dwell fatigue life of Ti alloys. However, the microstructure effect on crack nucleation in dwell fatigue is still in dispute, and direct experimental investigation is desired. Here, we report that the probability of dwell fatigue cracks nucleating at grain boundaries between colonies and primary α grains is approximately one-fifth of that nucleating at grain boundaries between primary α grains. Three dislocation reaction mechanisms are determined for dislocation transmission through αs/β interfaces in colonies, two of which strongly obstruct and disperse dislocations, while the third mechanism leads to concentrated slip bands of dislocations during dwell cycle loading. As decreasing the localization of dislocation slips can effectively reduce the stress concentration at grain boundaries in colonies, it significantly decreases the possibility of crack nucleation. This demonstrates a quantitative relationship between the localization of dislocation slips and the probability of dwell-fatigue crack nucleation. The results provide insights into the quantitative analysis of the microstructure effect on crack nucleation in Ti alloys.

Investigation of the high-temperature fatigue mechanism related to altered local short-range order in AL6XN austenitic stainless steel

An increase in crystal local short-range order (SRO), characterized by obvious diffuse intensity maxima at 1/3{422} in the [111]-axis selected area diffraction pattern, is observed in AL6XN austenitic stainless steel subjected to high-temperature low-cycle fatigue deformation. SRO is an ordered fcc structure with nanometer size, which can be well explained by the ordered arrangement of Cr/Mo atomic layers segregated on the {422} crystal plane in austenite. Many planar slip bands are observed in the fatigue sample, the distribution of SROs is highly related with the distribution of dislocations. In situ high-energy X-ray diffraction (HE-XRD) experiments are performed to investigate the deformation behavior of both as-received and fatigued samples under uniaxial loading. The lattice strain evolution reveals an interesting lattice expansion in the initial elastic stage, which provides direct evidence of a decrease in SRO during further uniaxial tensile deformation, indicating that the deformation modes greatly affect the SRO transition. We believe that the high-temperature fatigue damage originates mainly from the accumulation of planar slip bands, accompanied by a local increase in SRO, which promotes work hardening near the final fatigue fracture.

The effect of precipitates on low-cycle fatigue behavior of WE54 magnesium alloys under stress-controlled mode

The precipitates have a significant influence on fatigue behavior in magnesium (Mg) alloy. In this work, the effects of precipitates on deformation mode, cracking mode and mechanical behavior during low-cycle fatigue under stress-controlled mode in WE54 Mg alloy at room temperature (RT) was analyzed quantitively and statistically via quasi-in-situ electron backscattered diffraction (EBSD), slip trace analysis, coupled with transmission electron microscopy (TEM). The results reveal that precipitates promoted the activation of pyramidal < c + a > dislocation slip and suppressed the activation of tension twinning in the T6 alloy. In the T4 alloy, persistent slip band (PSB) induced cracking together with intergranular cracking dominated fatigue damage, while intergranular cracking was the primary cracking mode in the T6 alloy. The obvious cyclic hardening behavior of the T4 alloy was attributed to dynamic precipitation, whereas suppressed dynamic precipitation, restrained multiplication of dislocations, together with the appearance of PFZ generated the marginal cyclic hardening behavior in the T6 alloy. Both the T4 and T6 alloys exhibited ratcheting behavior, and precipitates suppressed ratcheting deformation in the T6 alloy. Moreover, the underlying mechanism for the higher fatigue resistance in the T6 alloy was also discussed.

Extrusions and intrusions in the cyclic plastic zone of the growing fatigue crack in polycrystalline copper

Surface relief in the tip and along the path of a growing fatigue crack in polycrystalline copper has been observed and analyzed. A crack started from a semicircular notch in a prismatic specimen cycled with constant strain amplitude. In the notch tip and along the crack path on both sides of the specimen the surface relief of the grains was characterized by a high density of the PSMs belonging to two, up to three slip systems. Extrusions and intrusions of these PSMs intersect forming specific surface relief. Similar surface relief, corresponding to the grain boundary surface, has arisen on the fracture surface. It means that in the persistent slip bands inside the crystal, the material is pushed towards the neighboring grains or removed from the grain boundary (analogically to extrusions and intrusions on the surface). This mass transfer locally increases stresses on the grain boundaries creates discontinuities and weakens grain boundaries. Such novel mechanism of intergranular fatigue crack growth is discussed. The suggestion of an intergranular crack growth mechanism agrees with the predominantly intergranular crack path of the growing fatigue crack.

Microstructure characterization and mechanical properties of electron beam welded joints of WRe alloy and CoCrFeNi high entropy alloys

In this study, vacuum electron beam welding was performed on WRe/CoCrFeNi alloy, resulting in joint interfaces resembling brazing. The research primarily focused on microstructure, phase composition, grain orientation and texture, interface bonding characteristics, mechanical properties, and fracture mechanisms of the welded joints. The weld zone consisted of columnar, cellular, and equiaxed regions, influenced by solidification rate. Employing the PHACOMP method, it is predicted that the introduction of W element in the weld zone tends to precipitate the FCC phase into the μ-phase. Grain competition during weld growth led to differences in grain orientation. A strong cubic texture {001} <100> was observed in the weld, enhancing its ductility. The impact of Kirkendall voids and IMCs on the bonding at the WRe/WZ interface was discussed. Microhardness of the weld increased due to grain refinement and precipitation strengthening. The joint exhibited a tensile strength of 189 MPa and an elongation of 10 %, with a fracture mechanism characterized by interface brittle fracture and fusion zone ductile fracture. The interface fracture was attributed to Kirkendall voids, IMCs, and significant residual stresses induced by material CTE mismatch. Furthermore, deep dimples and slip bands on cleavage planes at the fracture surface bottom, along with low-angle grain boundaries promoting localized slip, indicated good plasticity in the fusion zone.

Plastic deformation capacity obtained by the process of strain delocalization in Hf0.5Nb0.5Ta0.5Ti1.5Zr multi-principal-element alloy

Multi-principal element alloys (MPEAs) with body-centered-cubic (BCC) structures composed of elements of IVB, VB, and VIB usually exhibit high compressive strength and superior high-temperature performance. However, premature necking under tensile loading at ambient temperature limits their applications. Herein, we report the Hf0.5Nb0.5Ta0.5Ti1.5Zr MPEA with a single BCC phase, which performs considerable tensile plasticity by the process of strain delocalization. The formation of dispersed slip bands and two major strain localized regions suppress premature necking. The strain-localized region with a larger strain gradient realized strain delocalization during non-uniform deformation, resulting in considerable tensile plasticity (∼20%) with a yield strength of 922 MPa. Two dominated work hardening mechanisms were revealed. One is the geometrically necessary dislocations (GNDs) produced by non-uniform deformation which can coordinate deformation incompatibility, thus enhancing plastic deformation capability. The other is the lattice distortion which can provide an easy path for the cross slip of dislocations and realize strain delocalization. These two kinds of work-hardening mechanisms jointly contribute to the significant plastic deformation capacity of the Hf0.5Nb0.5Ta0.5Ti1.5Zr MPEA.

Evolution of in-plane strain fields onset of a plastically deforming crack in zirconium

Understanding the fracture behaviour of a material is essential for the sustainable design of structural components. The complex nature of microscale mechanical testing techniques makes identifying the fracture behaviour experimentally challenging at a microscopic scale. Validating microscale simulations with experimental studies can be difficult using traditional experimental techniques. High-resolution digital image correlation combined with electron backscatter diffraction describes the total in-plane deformation field at a microstructural scale, which can be compared against finite element models for verification. The present study determines the in-plane strain distribution ahead of a plastically deforming crack during an in-situ experiment. A crystal plasticity-based finite element model is used to simulate the experimental crack tip behaviour, after which quantitative and qualitative comparisons are made. A concentration of normal strains ahead of the crack tip is observed in both experimental and simulated results, defining the fracture process zone. Slip localization behaviour around the crack is also evaluated using Schmid factor analysis. A good correlation between the experimental slip bands and theoretical predictions has been observed. The strain path dependence on slip activation is also identified. Strain localization in deforming zirconium is initially limited with dense and low-intensity slip occurring up to a maximum in-plane shear strain of 0.2. Formation of well-defined equidistant slip bands occurs with further deformation. Equidistant slip bands are observed for prismatic <a> and pyramidal <c+a> slip systems, having an interplane distance of ∼0.60±0.05 µm and ∼0.46±0.04 µm, respectively.

Simulation of intragranular plastic deformation localization in FCC polycrystals by Discrete Dislocation Dynamics

Strain localization mechanisms taking place in polycrystal grains are investigated using Discrete Dislocation Dynamics (DDDs) simulations. First, elastic Finite Element Method simulations are used to calculate the intragranular stress distribution linked to strain incompatibilities between grains. Many configurations are tested to evaluate the stress heterogeneity and constitute a database for DDD simulations. From the analysis of these microstructures, a criterion is proposed to identify the grains where the emergence of the localization of the deformation is the most likely. Then, DDD simulations are used to explore the plastic strain localization phenomenon at the grain scale. Those simulations show that stress concentrations close to a polycrystal quadruple node can play a fundamental role in plastic strain localization. This work paves the way for future investigations to be made thanks to DDD simulations regarding slip band initiation and strain relaxation phenomena.

Towards understanding the crack suppression mechanism in brittle materials with high grinding speed at different temperatures

Ductile-regime grinding has been used to eliminate the formation of subsurface cracks by setting an extremely small depth of cut (DOC). The critical DOC is affected by multiple factors, including the grinding speed and material temperature. The underlying mechanism of DOC affected by the grinding speed is still unclear. To reveal the role of grinding speed and material temperature during the formation of cracks, we conducted a series of single-point grinding experiments with the different grinding speeds (26.7–192.3 m/s) and the initial material temperatures (25–200 °C). The experimental results showed that cracks were suppressed with an increase in the grinding speed and initial material temperature even when the DOC was much deeper than the critical DOC determined by the ductile-regime grinding. To understand the mechanisms underlying crack nucleation and suppression, we conducted systematic molecular dynamics simulations. Both simulation and experimental results showed that a crack can be formed by a single slip band. The crack nucleates from a microvoid within the slip band. With the aid of the local tensile stress on one side of the slip band tip, the crack nucleation forms an opening crack. The crack suppression is primarily caused by the high‐pressure field during high‐speed grinding, where the high‐pressure field superposes the local tensile stress to forming a compressive stress state that prevents crack nucleation. In addition, the brittle‐ductile transition is induced by the high temperature on the surface during high‐speed grinding. This study provides insights into building the DOC criterion for different grinding speeds and temperatures based on a ‘bottom-up’ approach.