Dislocation Mechanism Research Articles

Ascertaining the microstructural evolution and strengthening mechanisms of the gradient nanostructured pure titanium fabricated by ultrasonic surface rolling process

This work utilized the ultrasonic surface rolling process (USRP) to construct a gradient nanostructured (GNS) layer in pure titanium. The microstructural characterization at different depths was conducted using a transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) to elucidate the grain refinement mechanisms. Results showed that the {10−12} extension twins (ETs) and {11−22} compression twins (CTs) and their intersections accommodate the strain at the early deformation stage. As the strain increased, the dense dislocation walls (DDWs) and dislocation cells (DCs) gradually evolved into laminated structures with low-angle grain boundaries (LAGBs). Compared to conventional severe plastic deformation (SPD) technology, the LAGB-dominated grain refinement mechanism delays the onset of the transformation process from LAGBs to high-angle grain boundaries (HAGBs). With the coaction of discontinuous dynamic recrystallization (dDRX) and amorphization, the grains are extremely refined to 11.7 nm. Tensile test results suggested that a considerable strength-ductility synergy is achieved. The enhanced strength was attributed to grain size, dislocation, and synergetic strengthening mechanisms, accounting for 55.5 %, 20.7 %, and 23.8 %, respectively. This work deepens the current understanding of the formation mechanism of the GNS layer in pure titanium processed by USRP and the relationship between microstructure and mechanical properties.

Revealing the interface-mediated dislocation behavior and its role in delocalizing strain band in laminated aluminum

This study investigated the plastic deformation behavior of fine-/coarse-grained laminated aluminum (Al) via in-situ tensile test coupled with digital image correlation (DIC) and electron backscatter diffraction (EBSD), aiming to unveil the relation between local strain behavior and dislocation mechanisms. The local strain analysis found that strain localization bands generating in hard but brittle fine-grained layers are effectively impeded under the constraint of neighboring coarse-grained layers. With the aid of novel misorientation mapping method, the dislocation structures, distinguished from those in the coarse grain interior, were found to be predominant within the propagation path of localized strain band. These interface-mediated slip systems, which are responsible for suppressing the strain band propagation, were further analyzed by integrating the dislocation structures and local orientation rotations. The dominant role of such mutual interactions between strain delocalization and dislocation behaviors in improving the strength-ductility synergy was also discussed. These findings are important for understanding the deformation behaviors of heterogeneous multi-layered metals, and also provide guidance for designing high-performance layered metals by regulating local strain behavior and underlying dislocation mechanisms.

Hot compression behavior and recrystallization mechanism of 1.5 wt% Ti3AlC2 ceramic-enhanced Mo alloys with two-dimensional layers

Improving the high temperature strength of Mo alloys is a key problem to expand the application range of Mo alloys. In this study, the Mo-1.5 wt%Ti3AlC2 alloy was designed by introducing new two-dimensional (2D)-layered ceramic particles Ti3AlC2 into Mo. The peak flow stress of the Mo-1.5 wt%Ti3AlC2 alloy reaches 308 MPa at 1200 °C. The diffusion activation energy Q of the Mo-1.5 wt%Ti3AlC2 alloy is as high as 533.38 kJ/mol, which is higher than that of pure Mo (480 kJ/mol). Sintering in the reducing atmosphere of hydrogen at high temperature, the Ti3AlC2 consumes a large amount of energy to form AlTi3 and Ti5O9, which leads to an increase in the diffusion activation energy. We have established the constitutive equation of the Mo-1.5 wt%Ti3AlC2 alloy, and studied the microstructure evolution and deformation mechanism under hot compression. The results show that the stress index of n1 > 3 indicates that the plastic deformation mechanism was a dislocation mechanism. With the addition of Ti3AlC2, a large number of dislocations were produced in the alloy at the early stage of hot deformation. With increasing temperature, the dislocation entanglement gradually cancels out, resulting in the decrease of dislocation density. With the increase of temperature and deformation rate, the dynamic recrystallization (DRX) mechanism of the Mo-1.5 wt%Ti3AlC2 alloy was mainly continuous dynamic recrystallization (CDRX) at 1200 °C/0.01 s−1 and discontinuous dynamic recrystallization (DDRX) at 1600 °C/10 s−1. This work provides a new research direction for designing high performance Mo alloys.

Enhancing the creep resistance of a cast AZ91 alloy via minor Sc addition

The influence of 0.2 wt% Sc addition on the microstructure and creep characteristics of a cast AZ91 magnesium alloy was studied by impression creep tests under the constant punch stresses ranging from 175 to 700 MPa in the temperature range 425–525 K. The findings indicated that creep rates decreased at all stress levels and temperatures after Sc addition. This was ascribed to the formation of the thermally stable Al3Sc and Mg5Al4Sc particles, reduction in the quantity of the unstable β-Mg17Al12 phase, and solid solution strengthening of Al in the Mg matrix. The creep stress exponent and activation energy values obtained from the studied alloys ranged from 5–6 and 95–117 kJ/mol, respectively. The detected drop in creep activation energy with increasing stress implies that two concurrent mechanisms of dislocation climb controlled by lattice and pipe-diffusion are competing with each other. The former one was the governing mechanism at low stress levels, while the latter one was the prevailing mechanism in the high stress regimes.

Wear of a Fe2.5Ni2.5CrAl Multiprincipal Element Alloy: A Combined Experimental and Molecular Dynamics Study

The Fe2.5Ni2.5CrAl multiprincipal element alloy (MPEA) is a promising material for engineering applications because of its high strength and plasticity values. To evaluate the friction and wear performance, reciprocating dry sliding tests and molecular dynamics (MD) simulations are performed to determine the wear mechanism over a range of length scales. This alloy exhibits a lower average friction coefficient and wear volume loss during dry sliding compared with the well‐known Fe2Ni2CrAl alloy. The worn surface morphology reveals abrasive scratches, grooves, and delamination. The fine wear debris possesses high oxygen content, leading to higher wear resistance. The wear mechanism involves abrasive, adhesive, and oxidative wear. The wear, atomic stress and shear strain, dislocations, and lattice structure are analyzed by MD simulations. Point defects, atomic clusters, and stacking faults are identified in the nanowear process. The behavior of the (Shockley)‐type dislocations is identified as the main dislocation mechanism during sliding. Stacking faults produced during the stress release process are present in the indentation. This work provides a deep insight into the friction and wear behavior of Fe2.5Ni2.5CrAl MPEAs.

Distinct nucleation and propagation of prismatic dislocation loop arrays in Ni and medium-entropy CrCoNi alloy: Insights from molecular dynamics simulations

This paper utilizes molecular dynamics simulations to investigate the mechanisms of nucleation and propagation of prismatic dislocation loop (PDL) arrays in single-crystal medium-entropy CrCoNi alloys and pure Ni, with a particular focus on understanding the different size-dependent plasticity. To explore this phenomenon, we varied the size of PDLs. Our findings suggest that the anticipated size effects (i.e., smaller is stronger) are observed in both the elastic and plastic stages of Ni samples, but they are less pronounced in CrCoNi samples. Furthermore, it is observed that PDLs in CrCoNi alloys are often trapped in stable pileups, which results in a surprisingly significant strengthening effect. In contrast, PDLs in similar Ni configurations glide easily and exit from the free bottom. The observed differences between the behavior of CrCoNi and Ni can be attributed to the dislocation-length-dependent lattice friction in CrCoNi, as well as a significant stress gradient resulting from localized loading during nanoindentation. A simple dislocation mechanics model is used to rationalize the abnormal size effect in CrCoNi. These findings provide an insightful understanding of the strengthening mechanisms due to complex dislocations in multi-principal element alloys.

A discrete dislocation dynamics framework for modeling polycrystal plasticity with hardening

A two-dimensional discrete dislocation dynamics (DDD) framework is extended to model plastic deformation in polycrystalline materials having realistic grain morphology in finite geometries. The formulation includes constitutive rules to represent intra-grain dislocation mechanisms as well as dislocation/slip transmission across grain boundaries. Rules for intra-granular interactions mimic key three-dimensional dislocation mechanisms, such as line tension and dynamic junction and source formation, in addition to other short-range dislocation interactions. Various effects of idealized three-dimensional dislocation mechanisms on the mechanical response in tension are explored. The scaling of flow strength with grain size is investigated with and without slip transmission at grain boundaries. To aid in the analysis, spatial mappings of relevant stress components and geometrically necessary dislocation density are produced, which bring out competing dislocation-based strengthening mechanisms leading to the observed grain size effect in polycrystals.

Microstructure Evolution and Dislocation Mechanism of a Third-Generation Single-Crystal Ni-Based Superalloy during Creep at 1170 °C.

The present study investigates the creep behavior and deformation mechanism of a third-generation single-crystal Ni-based superalloy at 1170 °C under a range of stress levels. Scanning electron microscopes (SEM) and transmission electron microscopes (TEM) were employed to observe the formation of a rafted γ' phase, which exhibits a topologically close-packed (TCP) structure. The orientation relationship and elemental composition of the TCP phase and matrix were analyzed to discern their impact on the creep properties of the alloy. The primary deformation mechanism of the examined alloy was identified as dislocation slipping within the γ matrix, accompanied by the climbing of dislocations over the rafted γ' phase during the initial stage of creep. In the later stages of creep, super-dislocations with Burgers vectors of a<010> and a/2<110> were observed to shear into the γ' phase, originating from interfacial dislocation networks. Up to the fracture, the sequential activation of dislocation shearing in the primary and secondary slipping systems of the γ' phase occurs. As a consequence of this alternating dislocation shearing, a twist deformation of the rafted γ' phase ensued, ultimately contributing to the fracture mechanism observed in the alloy during creep.

Helical dislocation in twisted bilayer graphene

We observe helical dislocation network in twisted bilayer graphene (tBLG), which is accompanied by large out-of-plane deformation. By atomistic calculations, we demonstrate two distinct out-of-plane deformation modes, breathing mode with small out-of-plane deformation and bending mode with one order larger corrugation magnitude compared to the breathing mode. The out-of-plane deformation is caused by inhomogeneous interlayer coupling resulting from the periodic stacking order of the tBLG moiré superlattice. Instead of commonly observed screw dislocation in tBLG, we demonstrate a slip-induced helical dislocation network in the bending mode tBLG. We show that bending mode deformation is more stable at low twist angles, as the energy savings due to interface energy exceeds the energy penalty due to the strain energy caused by the large out-of-plane deformation. Our work provides a detailed picture of a new helical dislocation structure in tBLG and establishes a direct connection between the dislocation and deformation. Therefore, understanding the dislocation mechanics of tBLG may open up the possibility to control the corrugation, and reveal an opportunity to tune the intriguing physical properties of twisted bilayer graphene.

Effect of periodic image interactions on kink pair activation of screw dislocation

Periodic boundary condition along a dislocation line is commonly used in computing activation barriers or formation energies of kink pair of screw dislocation in BCC metals. Although the effect of periodic image interactions on the computation results is obvious, there had been no comprehensive analysis on such effect. In this work, we quantify it through combined nudged elastic band (NEB) simulations and theoretical analysis based on dislocation mechanics. The NEB calculation result demonstrates a non-negligible size dependence on the activation barrier at zero and low stresses. The theoretical analysis offers a practical approach to quantify such size effect without the need of time-consuming NEB simulations. Notably, a simple relationship between kink activation barrier and dislocation line length is derived at zero stress, offering a new approach to compute kink pair formation energy based on NEB simulation results.

Dislocation-mediated brittle-ductile transition of diamond under high pressure

Recent confirmation of room temperature dislocation plasticity in single-crystal diamond and the activation of {001}<110> slip system in experiments has opened up a new era to understand the deformation mechanism of brittle covalent crystals. However, few theoretical research and simulation has been conducted on the dislocation gliding in {001}<110> slip system. In this work, a new potential function is derived from the original Tersoff potential to the Tersoff High Pressure potential with a novel bond order term with the nearest neighbors. This model has been validated with series of molecular dynamic simulations of both {001}<110> and {111}<110> dislocation slip systems in diamond, successfully describing the dislocation mediated brittle-ductile transition under high hydrostatic pressure. The gliding mechanism of dislocation on {001} plane presents a jumping character with a large burgers vector under high hydrostatic pressure. Whereas, the diamond exhibits a zipper-like Griffith cleavage upon shearing under zero hydrostatic pressure. Similar deformation processes are also found in polycrystalline diamond. The current model is expected to prove useful in related simulations focusing on the strengthening mechanism in diamond and related research.

Injury mechanism of patellar dislocation in professional athletes: a video analysis study

Objective: Patellar dislocation (PD) is a devastating injury in professional athletes. An important aspect of injury prevention requires not only identifying the risk factors but also determining the responsible injury mechanism. Therefore, this study aimed to evaluate the injury mechanisms by examining the videos of PD injuries that occurred in professional athletes. Material and Methods: Injury videos of identified athletes and/or sports competitions where the injury occurred were detected on social media platforms (YouTube®, Twitter®, Facebook®). On January 1, 2021, 32 patella dislocation videos were found. A total of 28 PD that occurred in professional athletes between 1999 and 2020 were identified. Of these, 18 PD injuries with adequate video data were analyzed for injury mechanism, body posture, and player and sports characteristics. Three independent reviewers evaluated the videos. Results: There were 17 (94.4%) male and 1 (5.6%) female athletes. The mean age was 26.2±3.1 years. Distribution of athletic branches were such: four basketball (22.2%), two football (11.1%), nine rugby (50.0%), two soccer (11.1%), and one boxing (5.6%). In 13 cases (72.2%), the injury occurred by contact mechanism. Eight of these injuries (61.5%) occurred as a result of direct contact. The most important findings of this study were that patellar dislocation occurred when the trunk, hip, knee and ankle were slightly flexed. Dislocation occurred with the contraction of the quadriceps while the foot and tibia were performing external rotation. Conclusion: In professional athletes, PD most frequently occurs during a collision. The most common posture of the athlete who lost his balance is the trunk in flexion, knee and hip in flexion, ankle in plantar flexion.

Incorporating Dislocation Mechanisms into a Phenomenological Cyclic Plasticity Model for Structural Alloys

Incorporating Dislocation Mechanisms into a Phenomenological Cyclic Plasticity Model for Structural Alloys

Threading dislocation and lattice stress modulation of Si based GaN material with AlPN nucleation layer

An innovative AlPN nucleation layer is proposed for the growth of high quality GaN on Si. AlPN nucleation layer is prepared on Si substrate by metal organic chemical vapor deposition to explore the stress and dislocation adjustment mechanism. It is found that there are stress-related antisite defects in the P-doped lattice. In addition, the incorporation of P makes the dislocation densities of screw dislocations and edge dislocations have different trends, which can realize the independent control of the different types of dislocation densities. A hexagonal-cubic-phase mixed structure model is proposed to analyze and explain the variation trends of dislocation densities and stress. This work not only puts forward a new type of nitride material, which provides an idea for the growth of high-quality GaN on Si. On the other hand, it is of great significance to perfect the independent regulation mechanism of dislocations.

Performance-based three-level fortification goal and its application in anti-dislocation countermeasures: A case study of Shantou Submarine tunnel

The dislocation momentum is the design basis for anti-dislocation to tunnel when a tunnel crosses an active fault. The influence of different dislocation levels on tunnel performances is not clear. Thus, based on seismic activity parameters at the site of interest and probability of fault dislocation, probability fault displacement hazard analysis (PFDHA) methodology was introduced in this paper to ascertain the fault dislocation level under different exceeding probabilities (63%, 10%, and 2%–3%). Then, based on the definition of different ground motion strength and fortification goals of the tunnel, a three-level fortification goal with different performance requirements of the tunnel was proposed. The first attempt to use the proposed indexes including the maximum dislocation of the tunnel and maximum relative deformation of the tunnel was tried to evaluate deformation and failure states with an experimental approach. Subsequently, the feasibility of the three-level fortification goal was further investigated according to the self-defined qualitative description and quantitative indexes in the segmental design and sectional expansion tunnels comprehensively. The results show that the fault dislocations relying on PFDHA at the site of the Shantou Submarine Tunnel are firstly ascertained as 0.04, 1.8, and 2.4 m respectively. Taking the fault dislocation as model input values into account, the dislocation mechanism of the tunnel under the three levels was revealed. More importantly, judging from the dislocation performance requirements of the three-level fortification goal, the tunnel deformation and failure states are mitigated by adopting the countermeasures. The sectional expansion design can well meet the requirements without the restriction of a strong earthquake, while the effectiveness of the segmental tunnel can be proved under frequently occurred and fortification earthquake. The final research results are expected to provide a new fortification goal for anti-dislocation hazard evaluation on expansion design in high-intensity seismic regions and segmental design in slight and moderate-intensity seismic regions.

A multiscale analysis of adjacent fault dislocation mechanism induced by tunnel excavation based on continuous-discrete coupling method

Tunnelling usually causes adjacent fault dislocation that further induces serious disasters and threats, such as earthquakes, while its physical mechanism is still not clear. In this paper, we establish a multiscale numerical model of tunnel excavation adjacent to a fault by a coupled continuous-discrete method. The discrete element method (DEM) is employed to simulate the meso-mechanical behaviours of the fault fracture zone, and the finite difference method (FDM) is used to describe the macrodynamic characteristics of the hanging wall and footwall. A synthetic rock mass (SRM) model is developed to build the fractured rock to present the discontinuity and heterogeneity of the fault zone. The macro and meso mechanism of fault dislocation induced by tunnel excavation are analyzed, respectively. Results show that the fault experiences a different local dislocation form changing from upward movement to downward movement along the fault surface at the macro-scale. The weak transfer ability of the fault for deformation between the hanging wall and footwall, the reduced macro shear strength of the fault during the unloading process, and the meso joint slip and intact rock failure in the fault fracture zone are three main reasons for fault dislocation. The random distribution characteristics of the joints have remarkable influence on fault deformation. The fault slips more easily when the joint dip is close to the fault orientation. A higher joint density and a larger joint length provide a favourable environment to accumulate more connected joint surface, resulting in a larger slip displacement. Sensitivity analyses are conducted on several critical parameters including tunnel location, fault thickness, and stress condition, with the purpose of evaluating the safety of a potential tunnel site and predicting the potential seismic risks when tunnelling adjacent to a fault.

Atomistic simulations of nano-fiber-confined metal plasticity

Atomistic simulations are used to explore dislocation mechanisms in nano-confined aluminium (Al) containing silicon (Si) nanofibers. Our simulations revealed that dislocations on four {111} slip planes nucleate at Al-Si interfaces, propagate in Al nano-channels, loop and bow around Si nanofibers. Due to multiple active slip systems and dislocation cross-slips in the confined volume, a high density of dislocations intersects to form dislocation junctions and promote the formation of dislocation forests, resulting in high flow strength and strain hardening. The plasticity of Si nanofibers is accommodated by local shears and fracture triggered by the accumulated dislocation loops. Simulations show that the cracking of Si nanofibers can be efficiently buffered by dislocation mobility in the surrounding Al. Our results are consistent with experiments on nano-fibrous Al-Si eutectic microstructure.

Metal matrix composite fabricated from electrospun PAN, EGNS/PAN nanofibers and AL 5049 alloy by using friction stir processing

This work is an attempt to fabricate aluminum (AA 5049) matrix composites (AMCs) reinforced with electrospun polyacrylonitrile (PAN) nanofibers and consisting of exfoliated graphite nanosheets (EGNS/PAN) by utilizing friction stir processing (FSP) to improve the mechanical characteristics of AA 5049. The electrospinning method was used for fabricating PAN and EGNS/PAN nanofibers. The average diameter of the electrospun PAN nanofibers is 195 ± 57 nm, and after EGNS incorporation is 180 ± 68 nm. Dynamic recrystallization was the main process in the microstructure evolution of the stir zone during the FSP with PAN and EGNS/PAN nanofibers. According to PAN and EGNS/PAN nanofibers were used in the FSP procedure, the grain size reduced as a result of the pinning effects. PAN and EGNS/PAN nanofiber reinforcement enhanced the hardness to 89 and 98 Hv, respectively. Also, the ultimate tensile strength was raised to 291 MPa and 344 MPa, respectively. Tensile strength and hardness of the stir zone increased during the FSP with PAN and EGNS/PAN nanofibers due to the higher density of the strengthening mechanisms of grain boundaries and dislocations. The mechanical characteristics of AA5049 can be enhanced by the procedure of incorporating nanofibers, making them an ideal choice for applications in the automotive and aerospace industries.

Role of geometric dynamic recrystallization in nanocrystalline alloys

Nanocrystalline and near-nano-grained metals are plagued with an overall lack of ductility and formability. This is in part due to their inability to support the dislocation mechanisms or undergo the same types of transformations that conventional coarse-grained metals access inorder to increase their workability. Therefore, despite their advanced properties such as high strength, the manufacturing of such fine-grained metals, which exposes them to multi-axial states of stress in operations such as rolling, drawing, extrusion, forging, and bending, has not been possible. In this work, we address this limitation and highlight an important aspect of geometric dynamic recrystallization (GDRX) in nanocrystalline materials as a way to improve manufacturability. In particular, using two different alloy compositions and two different processing techniques, this study points to the fact that if nanocrystalline or near nano metals can be truly stabilized, they can repeatedly undergo the same types of processes that conventional coarse-grained metals endure, increasing both their formability and hence manufacturability. The results reveal that due to a large number of initial high-angle grain boundaries and large applied compressive strains, GDRX is activated in the absence of any discontinuous dynamic recrystallization (DDRX) or continuous dynamic recrystallization (CDRX). This is a unique aspect of stabilized nanocrystalline materials, as traditionally fine-grained metals are not microstructurally stable enough to accommodate such an event.

Interface-Dominated Plasticity and Kink Bands in Metallic Nanolaminates

The theoretical and computational framework of finite deformation mesoscale field dislocation mechanics (MFDM) is used to understand the salient aspects of kink-band formation in Cu-Nb nano-metallic laminates (NMLs). A conceptually minimal, plane-strain idealization of the three-dimensional geometry, including crystalline orientation, of additively manufactured NML is used to model NMLs. Importantly, the natural jump/interface condition of MFDM imposing continuity of (certain components) of plastic strain rates across interfaces allows theory-driven ‘communication’ of plastic flow across the laminate boundaries in our finite element implementation. Kink bands under layer parallel compression of NMLs in accord with experimental observations arise in our numerical simulations. The possible mechanisms for the formation and orientation of kink bands are discussed, within the scope of our idealized framework. We also report results corresponding to various parametric studies that provide preliminary insights and clear questions for future work on understanding the intricate underlying mechanisms for the formation of kink bands.