Dislocation-particle Interaction Research Articles

Ultrahigh-strength and ductile CoCrFeNi-based high-entropy alloys manufactured by laser powder bed fusion with multiple strengthening mechanisms

Laser powder bed fusion (L-PBF) has been proposed as a potential manufacturing process of high-entropy alloys involving rapid cooling rate and compositional element tuning. It, to some extent, compensates for the lack of strength of face-centered cubic (FCC) CoCrFeNi alloy by introducing high-density dislocations and generating hierarchical structures. In this study, the CoCrFeNi-based alloy was manufactured by L-PBF with the minor addition of B4C micro-particles and their properties and microstructure were evaluated. Nano-sized precipitates consisting of the boride and varying carbides were found in the form of a core-shell structure. An in-site transformation of carbides appeared around the core of the precipitate due to a large temperature gradient in the melt pool during manufacturing. The tensile yield and ultimate strength of CoCrFeNi–B4C alloy (1249.5 MPa and 1421.0 MPa) are double that of CoCrFeNi matrix (624.5 MPa and 691.1 MPa) at room temperature with competitive ductility. The exceptional mechanical properties can be mainly ascribed to precipitate hardening, dislocation hardening and grain refinement, as well as the particle-dislocation interaction.

A comparative study on the microstructure and strengthening behaviors of Al matrix composites containing micro- and nano-sized B4C particles

Two types of Al matrix composites (AMCs) containing micro-sized B4C particles (m-B4C/Al) and nano-sized particles (n-B4C/Al) were prepared by hybrid accumulative roll-bonding. The influences of m-B4C and n-B4C particles on the microstructure and mechanical properties of the AMCs were investigated systematically. Microstructure observations showed that B4C particles were uniformly distributed in the matrix with well-bonded interfaces in both the two types of composites. The n-B4C/Al composites presented finer grain size compared to the m-B4C/Al, indicating that n-B4C particles are more effective in enhancing the grain refinement and suppressing the grain growth than m-B4C particles. While the n-B4C/Al composites exhibited higher microhardness and strength, the m-B4C/Al composites showed higher elongation. Moreover, the strain hardening rate of the n-B4C/Al composites was higher at the initial stage of plastic deformation, but lower than that of the m-B4C/Al composites at larger strain. Furthermore, nanoindentation tests showed an obvious increment of nanohardness with the increase of deformation in the m-B4C/Al composites. The different strengthening behaviors of m-B4C and n-B4C particles can be attributed to the particle-dislocation interactions at different stages of plastic deformation. This work demonstrated the significant differences between the deformation and strengthening behaviors of AMCs reinforced by micro- and nano-sized B4C particles, providing novel insights for the design of AMCs with high strength and toughness by taking advantages of the hybrid-sized reinforcement particles.

Deformation behavior of a newly-developed T4-treated Al–Si die cast alloy

The aim of this study was to identify the monotonic and cyclic deformation behavior of a high-pressure die cast Silafont®-36 alloy in a naturally-aged T4 state. The T4 heat treatment led to a significant microstructural change from the as-cast coralloid-like eutectic Si particles to spheroidal Si particles embedded in the Al matrix, while the prior Fe-, Mn-rich inclusions remained. Although the strength became lower, the ductility of the T4 alloy was higher. Low cycle fatigue life was observed to be longer than its as-cast state at lower strain amplitudes. The T4 alloy exhibited strong cyclic hardening, where the degree of cyclic hardening was found to increase linearly with increasing strain amplitude along with the absence of Masing behavior, due to the strong particle-dislocation interactions during deformation. Strain energy density-based model by incorporating the microstructural aspects in the form of intrinsic fatigue toughness can be used to predict the fatigue life of the T4 alloy.

Dislocation dynamics modelling of the creep behaviour of particle-strengthened materials.

Plastic deformation in crystalline materials occurs through dislocation slip and strengthening is achieved with obstacles that hinder the motion of dislocations. At relatively low temperatures, dislocations bypass the particles by Orowan looping, particle shearing, cross-slip or a combination of these mechanisms. At elevated temperatures, atomic diffusivity becomes appreciable, so that dislocations can bypass the particles by climb processes. Climb plays a crucial role in the long-term durability or creep resistance of many structural materials, particularly under extreme conditions of load, temperature and radiation. Here we systematically examine dislocation-particle interaction mechanisms. The analysis is based on three-dimensional discrete dislocation dynamics simulations incorporating impenetrable particles, elastic interactions, dislocation self-climb, cross-slip and glide. The core diffusion dominated dislocation self-climb process is modelled based on a variational principle for the evolution of microstructures, and is coupled with dislocation glide and cross-slip by an adaptive time-stepping scheme to bridge the time scale separation. The stress field caused by particles is implemented based on the particle–matrix mismatch. This model is helpful for understanding the fundamental particle bypass mechanisms and clarifying the effects of dislocation glide, climb and cross-slip on creep deformation.

Understanding creep mechanisms of a Cu-Cr-Nb alloy by testing under constant structure conditions

Stress reduction creep tests conducted on a Cu-Cr-Nb alloy (GRCop-84) at 923 K have confirmed local dislocation climb to be the rate-controlling deformation mechanism. The activation energy of creep was measured to be consistent with that of self-diffusion in Cu matrix. An internal back stress of approximately -9 MPa was identified to act on the rate-controlling dislocations, which is believed to be the sum of the back stress for dislocation-particle interaction and the forward stress for dislocation-dislocation interaction. This back stress, however, does not lead to a true threshold in the framework of thermally activated dislocation motion.

Dislocation-particle interactions in magnesium alloys

The aim of this work is to investigate the fundamentals of dislocation-particle interactions in a Mg-1%Mn-1%Nd (wt.%) (MN11) alloy in order to better understand the limited precipitation strengthening exhibited by this material. A combined approach including micromechanical testing, slip trace analysis, and high resolution transmission electron microscopy (TEM) is put in place to analyze the interaction between basal and non-basal dislocations with the Mg-Nd plate precipitates that are characteristic of the investigated alloy. It is shown that basal dislocations can easily shear the precipitates, both when their long axis lays parallel and perpendicular to the matrix c axis. This is attributed to the high coherency between the matrix and the particles, which is characterized in detail. It is additionally shown that pyramidal ⟨c + a⟩ dislocations, on the contrary, interact with precipitates both in the form of shearing along the precipitate pyramidal planes, as well as by Orowan looping. This study reveals that pyramidal <c+a> dislocations are able to shear the plate precipitates along pyramidal planes when there is a good alignment between the slip plane of the incoming matrix dislocation and the outgoing precipitate pyramidal plane. Otherwise pyramidal <c+a> dislocations sliding on matrix planes with small geometric compatibility with the precipitate pyramidal planes bypass these particles by an Orowan looping mechanism.

Dislocation Dynamics Simulations of Dislocation-Particle Bypass Mechanisms

Effect of precipitation strengthening on metal is generally attributed to the dislocation interaction with the precipitate which acts as the barrier to the dislocation motion on the slip plane. In order to achieve better understanding of critical events of dislocation motion and evolution of dislocation microstructure, we have developed numerical simulation method of dislocation-dislocation and dislocation-particle interactions by means of discrete dislocation dynamics at mesoscopic scale. In this work, Green’s function method is utilized for the computation of the stress fields of dislocation and misfitting particle, and the interaction forces acting on the dislocation. We also proposed the efficient algorithm of the connectivity vector for the dislocation line elements, linked-list data structure, to deal with the flexible interaction of dislocation line elements. The geometrical effect of dislocation slip planes on the dislocation bypassing behaviors is tested by changing the relative height of dislocation slip plane against the center plane of spherical particle, where cross slip event is also taken into account for the dislocation motion. Simulation results show a wide variety of topological changes of dislocation during motion on the slip planes around the particle, which results from the stress field of the particle varied with the relative height between the dislocation slip plane and center plane of particle. The full analysis of the mechanisms of dislocation line bypassing misfitting particle has been explained in this study.

Novel amorphous SiOC dispersion-strengthened austenitic steels

A novel amorphous silicon oxycarbide dispersion-strengthened (SiOC-DS) austenitic steel has been fabricated via a powder metallurgy process. The microstructure of dispersion particles has been characterized by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD), revealing that amorphous SiOC nanoparticles with an average particle size of 30 nm were homogeneously distributed in the austenite grains with a sub-micrometer grain size. The high strength and hardness of SiOC-DS may be attributed to grain boundary strengthening, as well as dispersion strengthening via dislocation–particle interactions that were revealed by TEM investigations. In situ ion irradiation experiments showed that amorphous SiOC particles were stable after irradiation of 3.7 dpa, and the SiOC/steel interface can be an effective sink for the annihilation of irradiation defects. The excellent mechanical and irradiation properties of SiOC-DS austenitic steel make it a promising structural material for nuclear applications.

Fabrication and mechanical properties of TiC reinforced CoCrFeMnNi high-entropy alloy composite by water atomization and spark plasma sintering

In this study, the TiC-reinforced CoCrFeMnNi high-entropy alloy (HEA) composite was fabricated using water atomization (WA), mechanical milling (MM), and spark plasma sintering (SPS). The microstructural evolution and mechanical properties of TiC-reinforced HEA composite are investigated using electron backscatter diffraction, transmission electron microscopy, and room temperature compression tests. The addition of 5 wt% of TiC nano-particles to CoCrFeMnNi HEA resulted in fine grain size, high yield strength, and high strain hardening. The average grain size achieved for alloys with and without TiC after sintering is 5.1 μm and 10.6 μm, respectively. The addition of TiC increases the compressive yield strength from ∼507 MPa to ∼698 MPa and compressive fracture strength from ∼1527 MPa to ∼2216 MPa, without sacrificing the ductility. The strengthening behavior of TiC-reinforced CoCrFeMnNi HEA composite is quantitatively discussed based on grain boundary strengthening, dislocation strengthening, and dispersion strengthening. The role of TiC nano-particles in the strain hardening improvement is investigated with respect to the dislocation-particle interaction and consequently increased dislocation density.

Oxide Particle-dislocation Interaction in ODS Ferritic Steel

Oxide Particle-dislocation Interaction in ODS Ferritic Steel

Microstructural Damage Mapping on Heat-Resistant Steel Due to High-Temperature Fatigue

This study aims to develop a damage map based on microstructure due to high-temperature fatigue load. To start with, Ni–Cr–Mo steel was used. The tensile and fatigue properties of it were investigated. The tests were performed at 750 °C. Microscopic evaluation after deformation revealed a variation of the damage mechanism, which was compared with damage map developed computationally. The dislocation activities that typically were homogeneously distributed at room temperature were localized near grain boundaries at elevated temperatures, similar to that of creep damage. A strong particle-dislocation interaction, usually called interfacial pinning, was observed and hereby mapped. Additional microstructural evolution such as the transformation of structure into coarse equiaxed grains and the carbide coarsening was also observed. A change in fracture mechanism from trans-granular at room temperature to intergranular fracture at elevated temperature similar to creep damage was also verified. The heart of this research was the ability to relate the finite element model that simulates both the equivalent stress and temperature distributions to the real experiment. Based on this estimation, the damage level was developed. Microstructural mapping corresponding to the damage level was subsequently made based on the experiment. It was concluded that the mapping was possible and this research could be thought as the embryo for the better understanding in damage theory bridging metallurgical approach to conventional approach using mesomechanics.

Heterogeneous [formula omitted] microstructures in nickel-base superalloys and their influence on tensile and creep performance

Mechanical properties of nickel-base superalloys can be greatly influenced by spatial variation of γ′ microstructure, calling for models at mesoscale to study additional complexity in the underlying microstructure/property relationship. We combine a FFT-based elasto-viscoplasticity (FFT-EVP) model with a phase-field (PF) model to study plastic deformation of γ/γ′ superalloys. The model is applied to Haynes 282 (H282), where the low volume fraction (<20%) of dispersed spherical γ′ particles results in a dislocation-particle interaction of either Orowan looping at tensile conditions or climb-bypass at creep conditions. The incorporation of these mechanisms is achieved through the framework of a dislocation density based constitutive model in FFT-EVP, together with the introduction of a location-dependent inter-particle spacing based on the k-nearest neighbor algorithm. Features of non-uniform γ′ microstructures, including γ′ volume fraction variation and non-uniform γ′ distribution due to element microsegregation observed in welded H282 samples, are modeled using the PF method and then passed to FFT-EVP to explore their influence on the tensile and creep properties in a parametric manner. It is found that if the particle distribution is uniform, γ′ volume fraction variation can exhibit a nonlinear effect on both the tensile strength and creep rate; for a given overall γ′ volume fraction, the degree of particle distribution non-uniformity can also exhibit nonlinear influence on the tensile strength and creep rate. It is also shown that the location-dependent inter-particle spacing serves as a better microstructure descriptor than the conventional analytical expression of the average inter-particle spacing in terms of establishing a homogenized but microstructure-sensitive constitutive microstructure-property relationship for fast-acting engineering applications.

Microstructural and mechanical characterization of an equiatomic YGdTbDyHo high entropy alloy with hexagonal close-packed structure

The microstructural and mechanical characterization of an equiatomic YGdTbDyHo high entropy alloy with hexagonal close-packed structure was performed. The phase state and chemical homogeneity of the solid solution were analysed with respect to crystal structure, phase stability, and oxide formation. It was found that Y-rich precipitates form at grain boundaries and that the alloy is prone to oxidation, leading to a homogeneous distribution of ∼10 nm-sized oxides in the grain interiors. The plastic response at the sub-grain level was studied in terms of the activated slip systems, critical resolved shear stresses (CRSS), and strain hardening using micropillar compression tests. We observe plastic slip on the basal <a> system, with a CRSS of 196 ± 14.7 MPa. Particle strengthening and strength dependence on sample size are discussed on the basis of dislocation particle interaction and mechanical size effects.

Probing Novel Microstructural Evolution Mechanisms in Aluminum Alloys Using 4D Nanoscale Characterization.

Dispersions of nanoscale precipitates in metallic alloys have been known to play a key role in strengthening, by increasing their strain hardenability and providing resistance to deformation. Although these phenomena have been extensively investigated in the last century, the traditional approaches employed in the past have not rendered an authoritative microstructural understanding in such materials. The effect of the precipitates' inherent complex morphology and their 3D spatial distribution on evolution and deformation behavior have often been precluded. This study reports, for the first time, implementation of synchrotron-based hard X-ray nanotomography in Al-Cu alloys to measure kinetics of different nanoscale phases in 3D, and reveals insights behind some of the observed novel phase transformation reactions. The experimental results of the present study reconcile with coarsening models from the Lifshitz-Slyozov-Wagner theory to an unprecedented extent, thereby establishing a new paradigm for thermodynamic analysis of precipitate assemblies. Finally, this study sheds light on the possibilities for establishing new theories for dislocation-particle interactions, based on the limitations of using the Orowan equation in estimating precipitation strengthening.

Influence of Zr addition on the microstructures and mechanical properties of 14Cr ODS steels

Oxide dispersion strengthened (ODS) steel is one of the most promising candidate structural materials for the high-temperature nuclear reactor application. In this study, two compositions of ODS steels (14Cr-ODS and 14Cr-Zr-ODS) were prepared to investigate the influence of Zr addition on the microstructures and mechanical properties of ODS steels. The microstructures, including dispersion morphology and crystal structures of oxide particles, particle-matrix interface coherency and particle-dislocation interactions, were characterized using TEM, HRTEM, and SEM, and the mechanical properties at room and high temperatures were measured using uniaxial tensile tests. Results show that Zr addition leads to the formation of finer precipitated particles, which was identified as rhombohedral Y4Zr3O12, with denser dispersion in the matrix. The calculation results reveal that the lattice misfit, δ, at the interface between particle and matrix increases as the particle size increases. In addition, the strength and elongation of ODS steels are improved with Zr addition due to the stronger interface bonding force between fine particles and matrix as well as the larger pinning effect of small particles to dislocation movements.

High temperature deformation mechanisms in the 14% Cr ODS alloy

The stress relaxation behavior of the ODM401 ODS steel is tested at the temperature range 400–800°C in order to investigate the deformation mechanisms. The repeated relaxation tests are performed on the tensile specimens within the plastic strain range 10−4–10−1. The deformation mechanisms are analyzed in terms of an activation volume, dislocation density and velocity, a thermal stress component and a strain rate hardening. The deformation is driven by the dislocation glide and climb. Dispersed particle-dislocation interactions is a dominant effect at temperatures 400–600°C. Short-range interactions between dislocations and a lattice are most significant at 600°C. The non-conservative motion of dislocations becomes active at temperatures above 600°C. The majority of the mobile dislocations is not trapped and is able to move freely in the microstructure.

A Numerical Analysis of Deformation Processes in Oxide Dispersion-Strengthened Materials - Influence of Dislocation-Particle Interactions

A recently developed 3D discrete dislocation dynamics (DDD) model is employed to study kinetics of dislocation ensembles subjected to high temperature creep in microstructures of metal matrix composites. We particularly focus on a migration of low angle tilt boundaries in a field of rigid impenetrable particles. This type of dislocation boundaries represents a typical microstructural feature mediating plastic deformation during the high temperature loadings. The article compares results of numerical studies that considered distinct dislocation-particle in-teractions in order to describe the response of dislocation structure to the applied stress. The resultssuggest that, regardless the details related to the dislocation-particle interactions, a critical applied stress always exists, below which the boundary migration process ceases [1,2]. The existence of crit-ical threshold is confirmed by creep tests of ODS materials. This critical threshold, contrary to theclassical Orowan stress, is proportional to the dislocation density. The displacements of individual dislocation segments on the micro-scale level reflect the changes in the dislocation-particle interactions quite sensitively. Atthemacro-scale level, the overall strain rate, which averages out velocities of all the individual dislocation segments, is also significantly influenced by the changes in dislocation-particle interaction

Atomic-scale strengthening mechanism of dislocation-obstacle interaction in silicon carbide particle-reinforced copper matrix nanocomposites

Using molecular dynamics (MD) simulations, the strengthening mechanism of silicon carbide (SiC) particle acted as barrier for the motion of edge dislocation in copper (Cu) matrix nanocomposites under shear loading is investigated. The dislocation glide behavior and the dislocation-particle interaction accounting for the effect of the temperature and the particle size are discussed in terms of the stress-strain relationship, the crystal energy change, the stress distribution and the dislocation evaluation. The results show that the critical depinning stress and the critical depinning strain are found to vary significantly depending upon the temperature and the particle size, consistent with previous studies. Higher temperature or smaller particle results in lower critical depinning stress to allow dislocation to bypass particle. Moreover, the critical depinning stress from the current atomic study is lower than the theoretical value due to the presence of cross-slip and thermal activation. At low temperature the mechanical activation controls the depinning stress of the dislocation bypassing particle, while at high temperature the thermal activation dominates that. In addition, the trapping sites of dislocations for the dislocation multiplication are observed at high temperature or large particle, due to the lattice mismatch of particle taken as the dislocation source. Compared to the dislocation interacting with void or metal-particle, the interaction of dislocation and SiC-particle in Cu + SiC nanocomposite reveals much complex factors, such as the multiple glide planes, the nanosize effect, as well as the coupling effect of mechanical and thermal activation, on the strengthening mechanism. It maybe apply these results to a high throughput experimental design which provides the dense and targeted material data, to accelerate material discovery with the large-scale and low-cost fabrication strategies.

Comparative Assessment of Strength Models for AA6063 Alloy

This report presents a comparative assessment of the suitability of two existing physical models for predicting yield strength of artificially aged AA6063 Al-alloy. One model is based on the modified Orowan mechanism of dislocation by-pass for non-shearable rod shaped precipitates rejuvenated by Zhu and Stark, and the other model is based on classical dislocation-particle interaction that incorporates both cutting and by-pass mechanisms for spherical shaped precipitates developed by Deschamps and Brechet. Using these models, simulation of yield strength values have been performed considering nucleation-growth as well as nucleation-growth-coarsening of precipitates during the entire period of ageing. Comparison of experimental and simulated results reveals that the model by Deschamps and Brechet predicts yield strength more accurately when nucleation-growth-coarsening of precipitates are incorporated.

Load-partitioning in an oxide dispersion-strengthened 310 steel at elevated temperatures

Here the high temperature tensile performance of an oxide dispersion-strengthened (ODS) 310 steel is reported upon. The microstructure of the steel was examined through both transmission electron microscopy (TEM) and synchrotron scattering. In situ synchrotron X-ray tensile investigation was performed at a variety of temperatures, from room temperature up to 800°C. Pyrochlore structure yttrium titanate and sodium chloride structure titanium nitride phases were identified in the steel along with an austenite matrix and marginal residual α′-martensite. The inclusion phases strengthen the steel by taking extra load through particle-dislocation interaction during plastic deformation or dislocation creep procedures. As temperature rises, lattice strain measurement implies that the load partitioning effect of conventional precipitate phases starts to diminish, whereas those ultra-fine oxygen-enriched nanoparticles continue to maintain a considerable amount of extra lattice strain. Introduction of oxygen-enriched nanoparticles in austenitic steel is shown to improve the high temperature performance, making austenitic ODS steels promising for advanced nuclear applications.