Subsequent Microstructural Evolution Research Articles

Molecular dynamics simulation study of void collapse mechanisms in cubic metals under shock compression

Molecular dynamics simulations have revealed the collapse mechanism of a void in cubic metals under shock compression along [100] direction. The results show that the void collapse is caused by emitting dislocations from its surface. The type of cubic metal plays a decisive role in the subsequent microstructural evolution around the void. Void in face-centered-cubic metals collapses by emitting Shockley dislocations from its surface when piston velocity exceeds the threshold; while void in body-centered-cubic metals collapses by emitting prismatic dislocation loops, which are formed by the reaction of edge dislocations. Simulation results also reveal that the type of dislocation that induces void collapse is independent of void size. Finally, based on the elastic theory, we find that the maximum resolved shear stress along the slip direction determines the types of dislocation emitted from the voids.

Prominent superelastic response induced by Ni4Ti3 phase in NiTi alloys fabricated via wire-arc directed energy deposition

In this study, a wall-shaped NiTi alloy component was manufactured using the wire-arc directed energy deposition (DED) technique. The superelasticity of the NiTi alloy was enhanced through aging treatment, and the subsequent microstructural evolution during the superelastic response was analyzed. Different aging treatments (at 200 °C, 300 °C, and 400 °C) on the NiTi alloy samples were performed, which significantly enhanced the superelastic recovery rate and stability by adjusting the size of the Ni4Ti3 phase. In detail, the samples aged at 200 °C exhibit significant residual strain during the cyclic loading. The 300 °C aged NiTi alloy displays prominent superelasticity, with a stable recovery rate of over 99.7% throughout 10 cycles. In contrast, the samples aged at 400 °C essentially do not demonstrate superelastic behavior. The significant changes in superelasticity are fundamentally related to dislocations and martensitic phases. The precipitates of Ni4Ti3 coherent with the matrix effectively hinder the generation of dislocations during cyclic loading, thereby enhancing the superelastic recovery rate. The appearance of the martensitic phase, however, leads to a decrease in superelasticity, which is related to the nature of superelasticity. The obtained results provide guidance for the fabrication of NiTi alloys with excellent superelasticity.

Development of an interatomic potential for L12 precipitates in Fe–Ni–Al alloys

Fe–Ni-based alloys have been considered as candidate structural materials for advanced nuclear reactors, and some of their excellent properties are particularly associated with L12 precipitates dispersed in the alloys. For a better understanding of the irradiation response, classical molecular dynamics is an essential tool to simulate the irradiation-induced defect formation and subsequent microstructural evolution. We here develop an interatomic potential for L12(Ni,Fe)3Al precipitates in the ternary Fe–Ni–Al system. The fitting parameters are tuned to describe the lattice and elastic constants of NiFe solid solutions and defect formation/vacancy migration energies in the fcc matrix. Furthermore, the lattice properties and defect energies of L12(Ni,Fe)3Al are considered for the fitting. We demonstrate the developed potential reproduces the lattice and defect properties with reasonable accuracy by comparison with density functional theory calculations, existing empirical potentials, and experimental data. This potential will be useful to investigate the defect formation and evolution of (Ni,Fe)3Al precipitates in the alloys under irradiation, which would provide critical insights into the materials design strategy to achieve enhanced properties.

Calculation of the recombination radii between various point defects and defect clusters in nickel via kinetic Activation Relaxation Technique

The recombination radii between various point defects and defect clusters are essential input parameters for mathematical and computational models of radiation-induced damage evolution. In this work, such recombination radii is calculated by the kinetic Activation Relaxation Technique (k-ART)—an off-lattice, self-learning kinetic Monte Carlo algorithm—in conjunction with molecular dynamics simulations. We found vacancy clusters exhibit larger average recombination radii with interstitials than single vacancies, reaching as large as 2.2a0 (a0 is the lattice parameter of nickel). Such recombination enhancement can be attributed to the intensified distortion near cluster surfaces. Unlike vacancy clusters, free surfaces feature smaller recombination radii than single vacancies. Their recombination radii are correlated with the angle between surface normal and <110> directions. Finally, the recombination radii of interstitial clusters are larger than vacancy clusters and strongly depends on the cluster configurations. Overall, the recombination radii of most internal defect clusters range from ∼2a0 to ∼2.4a0. Because previous research has found that recombination radii affect the density-size distribution of point defect clusters but not the fraction of a particular defect, our analysis suggests that given an accurate initial estimate of cluster density-size distributions, choosing a uniform recombination radius for different clusters in object kinetic Monte Carlo simulations should be sufficient to predict subsequent microstructural evolution.

Loading-direction dependence of interaction types between different twin variants in AZ31 alloy

ABSTRACT In this research, Loading-direction dependence of interaction types between different twin variants in AZ31 alloy was investigated. The microstructure, twin evolution, and the deformation mechanisms of samples were characterised by quasi-in-situ electron backscatter diffraction. The result showed that during rolling direction (RD)-pre-compression and re-compression, the grains with co-zone twin–twin interaction (Type I) are in the majority, while during RD-pre-compression and transverse direction (TD)-re-compression, the grains with non-cozone twin–twin interaction (Type II) are dominant. Furthermore, the effect of different types of twin–twin interaction on subsequent microstructure evolution is clarified. Our experimental results may provide insights for tailoring different types of twin–twin interaction.

Effects of initial 3D printed microstructures on subsequent microstructural evolution in 316L stainless steel

Plastic deformation of 3D printed components may occur when they are in use. Here we analyze the effects of the initial 3D printed microstructure of 316 L stainless steel on the subsequent deformation behavior, where we apply 10% and 30% thickness reductions by cold rolling. The microstructures are characterized by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Compared to conventionally manufactured (solution treated) samples, deformation twinning is observed to occur at lower strains in 3D printed samples, and twins are observed to be thinner and intersecting inside some grains. These observations are linked to the pre-existing dislocation structure in the 3D printed samples, where dislocations of various Burgers vectors facilitate deformation twinning. Furthermore, cells with an average size of 125 nm are observed to form inside the initial cellular structure. The strength calculated based on microstructural parameters generally agrees with experimental results, showing a large strengthening contribution from twin–matrix lamellae. The present study provides fundamental ideas for microstructural engineering of 3D printed metals for even better mechanical properties.

Direct transformation of equilateral hexagonal Frank vacancy loops to stacking fault tetrahedra under thermal fluctuation

Stacking fault tetrahedra (SFTs) are highly interesting three-dimensional vacancy defects in quenched, plastically deformed or irradiated face-centered-cubic metals and have a significant impact on the properties and subsequent microstructural evolution of the materials. Their formation mechanism and stability relative to two-dimensional vacancy loops are still debated. Equilateral hexagonal Frank vacancy loops (faulted, sessile) observed in microscopy have been considered unable to directly transform to SFTs due to separation of Shockley partial dislocations as well as embryonic stacking faults. Here using sufficiently long (up to tens of nanoseconds) molecular dynamic simulations, we demonstrate that such a transformation can in fact take place spontaneously at elevated temperatures under thermal fluctuation, reducing potential energy of defected atoms by <0.05 eV/atom. The transformation becomes easier with increasing temperature or decreasing loop size.

Role of Hot Rolling in Microstructure and Texture Development of Strip Cast Non-Oriented Electrical Steel

In this study, the effect of the hot-cold rolling process on the evolution of the microstructure, texture and magnetic properties of strip-cast non-oriented electrical steel was investigated by introducing hot rolling with different reductions. The results indicate that hot rolling with an appropriate reduction, such as the 20% used in this study, increases the shear bands and {100} deformed microstructure in the cold roll sheet. As a result, in our study, enhanced η and Cube recrystallization texture and the improved magnetic induction were obtained. However, hot rolling with excessive reduction (36–52%) decreased the shear bands and increased the α-oriented deformation microstructure with low stored energy. It enhanced the α recrystallization texture and weakened the η texture, resulting in a decrease in the magnetic induction. In addition, hot rolling promoted the precipitation of supersaturated solid solution elements in the as-cast strip, thereby affecting the subsequent microstructure evolution and the optimization of its magnetic properties.

Influence of softening annealing on microstructural heredity and mechanical properties of medium-Mn steel

Softening annealing (SA) is often required for producing medium-Mn steels (MMS) as it lowers hardness so that they can be cold rolled to reduce thickness. The influences of different SA processes on the microstructural heredity during the processing route and the final tensile properties were studied. It was found that the SA process could either intensify or weaken the influence of the Mn segregation resulting from solidification on the subsequent microstructural evolution during the process, i.e., microstructural heredity. In the case when no SA was employed, both recrystallization and rapid growth of ferrite grains preceded the reverse austenitic transformation during the intercritical annealing (IA) in the Mn-lean regions, where very coarse ferrite grains were formed. This deteriorated ductility was due to the propagation of cracking along the boundary of the coarse-grained and fine-grained regions. In contrast, SA at a sufficiently high temperature could dissolve cementite, producing uniformly distributed austenite grains. They transformed to martensite during cold rolling but were reborn during IA. As a result, ultrafine austenite and ferrite grains were uniformly distributed, which improved ductility significantly. This study hints at a new approach to altering the microstructural heredity resulting from the heterogeneous Mn distribution in MMS.

Particle size effects on dislocation density, microstructure, and phase transformation for high-entropy alloy powders

Recrystallization and phase transformation reactions are driven by energy stored in the parent phase due to previous processing history. Grain boundaries, lattice strain, and dislocation networks may affect subsequent microstructural evolution, especially during metal powder consolidation processes. In this work, AlCrFe2Ni2 eutectic high-entropy alloy powders over a wide range of size distribution were used to investigate the correlation between dislocation density and microstructure as a function of particle size. Line profile analysis of the X-ray diffraction patterns of the as-received (quenched) samples shows that the dislocation density increases linearly with decreasing particle size. Based on microstructure analysis of the as-quenched and the annealed samples, it is found that the correlation is associated with the grain boundary length which increases with decreasing particle size, revealing that the key source of dislocation density is dislocations within the grain boundaries. The grain boundary energy acts as a driving force for the metastable to stable phase transformation of AlCrFe2Ni2, showing that the phase transformation kinetics is a function of particle size. This work shows a direct experimental observation and quantitative analysis that in metallic powder systems particle size is one of the key parameters which affects the dislocation density and the phase transformation kinetics most probably due to the different cooling rates achieved during powder production.

On the fracture response of shape memory alloys by void growth and coalescence

Although Shape Memory Alloys (SMAs) belong to a relatively brittle class of materials, that of intermetallics, their fracture surfaces are characterized by both cleavage and dimples, with the latter being indicative of ductile rupture. Two clear differentiators of SMAs from other intermetallics, which may contribute to the presence of ductile rupture are their ability to transform their crystallographic structure and the presence of precipitates in large volume fractions. In this paper, unit cell simulations are employed in an effort to quantify the relative importance of the two fracture mechanisms in the overall failure response of these materials. The numerical simulations involve a single pre-existing void, assumed to have initiated from a second phase particle, embedded in a SMA matrix material. Thus, the unit cell studies allow for an investigation of void growth and coalescence, ignoring void formation and its footprint in the subsequent microstructure evolution. The numerical studies show that void growth is rather limited before the peak stress-value in the effective stress–effective strain response of the unit cell for initial void-volume fractions representative of precipitation-hardened SMAs. Given the available experimental observations on notched round bars, which indicate that precipitation-hardened SMAs fail in a stress-controlled manner, the numerical studies suggest that void growth and coalescence should play a minor role in the formation of dimples, as compared to that of void nucleation and cleavage, and in turn into the fracture response of precipitated SMAs.

Effect of crystal orientation on the size effects of nano-scale fcc metals

The present work investigates the dominant mechanisms in the plasticity of nano-sized fcc metallic samples. Molecular dynamics simulations of nanopillar compression show that plasticity always starts with the nucleation of dislocations at the free surface, and the crystal orientation affects the subsequent microstructural evolution. The Schmid factor of leading and trailing partials plays a decisive role in leading to the twinning, or slip deformation. A significant difference is observed in the strength of pillars of the same size with different orientations. The power-law equation exponent is completely dependent on the crystal orientations, and a weak or no size effect is observed in the compression of [100]- and [110]-oriented nanopillars. The observed orientation based behaviour decreases by confining the free surface.

Impact of Si and Al on Microstructural Evolution and Mechanical Properties of Lean Medium Manganese Quenching and Partitioning Steels

Herein, the investigation of the tensile behavior of lean medium Mn Quenching and Partitioning (Q&amp;P) steels containing 0.2 wt% C, 4.5 wt% Mn, and additions of 1.5 wt% Si or 1.3 wt% Al is concentrated upon. By the variation of the quenching temperature (TQ), different volume fractions of primary martensite (α′prim) are adjusted, influencing the subsequent microstructural evolution during Q&amp;P processing. Using scanning electron microscopy (SEM), the final microstructure consisting of tempered martensite (α″), retained austenite (RA), partially bainitic ferrite (αB), and final martensite (α′final) is characterized. Furthermore, interrupted tensile tests at gradually increased strains are conducted to investigate the stability of RA against strain‐induced martensite transformation (SIMT) and overall tensile behavior of lean medium Mn Q&amp;P steels. The investigations manifest the formation of larger amounts of αB and consequently lower RA contents, when Si is substituted by Al. As an aftermath, the tendency to form α′final is significantly lower, compared with the Si‐alloyed composition, reflected in the overall stress–strain behavior. Especially in case of the Si‐alloyed samples containing RA fractions exceeding 15 vol%, an over‐accelerated SIMT is observed, inducing failures occurring prior to necking, whereas for the Al‐alloyed samples, a wider process window is obtained.

Cooling dynamics of two titanium alloys during laser powder bed fusion probed with in situ X-ray imaging and diffraction

Metal parts produced by laser powder bed fusion (LPBF) additive manufacturing exhibit characteristic microstructures comparable to those observed in laser welding. The primary cause of this characteristic microstructure is rapid, localized heating and cooling cycles, which result in extreme thermal gradients where material solidification is followed by fast cooling in the solid state. The final microstructure and mechanical performance are also influenced by pore formation caused by melt pool fluid dynamics. Here, we use high speed, in situ X-ray diffraction to probe the kinetics of cooling and solid-solid phase transitions after laser melting in two aerospace titanium alloys: Ti-6Al-4V, an α + β alloy; and Ti-5Al-5V-5Mo-3Cr, a near-β alloy. We complement these diffraction studies with in situ X-ray imaging to probe melt pool dynamics and pore formation. From these two complementary probes, we quantify pore formation during melting and the subsequent microstructural evolution as the material rapidly cools after solidification. These results are critical for understanding defect formation and residual stress development in different titanium alloys under LPBF conditions and can help inform process models to predict final part performance.

Effects of boron content on the microstructure and mechanical properties of twin-roll strip casting borated steel sheets

High borated steel sheets containing 0.25 wt% to 4.0 wt% boron including hypoeutectic (0.28 wt% B and 2.11 wt% B), eutectic (2.43 wt% B) and hypereutectic (4.01 wt% B) compositions were tried to be fabricated by a novel strip casting technology, and the effects of boron content on sub-rapid solidification behavior and subsequent microstructural evolution together with mechanical properties was studied. It was found that the morphology of borides depended greatly on the boron content. When increasing boron, the morphological change of borides was network-like → grainy → cluster-like → plate-like. Various orientation relationships between different morphological borides and γ-Fe in as-cast steels were also identified. After subsequent hot-rolling and solution-treating, ultra-fine (<10 μm) borides were obtained in hypoeutectic and eutectic steels. Benefiting from that, excellent mechanical properties was achieved, which was much better than that of steels prepared by traditional ingot casting. Particularly, when the boron content approached eutectic composition, of which the effect on the microstructure and mechanical properties was more sensitive. The steels containing 2.11 wt% and 2.43 wt% boron exhibited different morphological borides with a total elongation of 14.1% and 8.0%, respectively. Moreover, a thin hypereutectic borated steel sheet was also prepared, but the borides were very large and the total elongation was very low. The correlation between microstructure and strength was clarified based on the Hall-Petch behavior and intermetallic strengthening effect. Smaller γ-grains and higher volume fraction of borides were responsible for higher strength. The plastic behavior of steels was influenced by strain-hardening rate that was determined by the volume fraction of γ-Fe matrix. A better sustainable strain-hardening capability was beneficial to extend the stage of uniform deformation and enhance the ductility. In addition, the mechanical properties of steels were also decided by the fracture mechanism that was dominated by the size of borides.

Effects of applied strain on defect production and clustering in FCC Ni

The structural materials of nuclear reactors are subjected to an environment of complex mechanical stresses, which has been reported to significantly affect the primary radiation damage suffered by the structure. In this study, we perform classical molecular dynamics simulations to study the effects of mechanical strains on the defect morphology due to primary radiation damage for a collision energy of 5 keV in face-centered cubic Ni. We consider the effects of four representative types of mechanical strains: hydrostatic, tetragonal shear, monoclinic shear, and uniaxial strains, and we discuss three aspects of the defect morphology: 1) time evolution of generated Frenkel pairs, 2) size distribution of defect clusters, and 3) fraction of clustered defects. We find that volumetric and anisotropic strains differently and significantly influence the primary radiation damage to the structure via affecting the intrinsic physical properties associated with the generation, formation, migration, or binding of Frenkel pairs; this result is generally consistent with those of many simulations. We believe our current study can aid in providing a fundamental mechanistic understanding of primary radiation damage in Ni-based alloys (for e.g., high-entropy alloys) as well as the subsequent long-term microstructural evolution.

Quench Temperature-Dependent Phase Transformations During Nonisothermal Partitioning

An attempt has been made to estimate the amount and composition of different phases formed during the simulated quenching and nonisothermal partitioning (Q&P) process in a dilatometer by matching the experimental dilation data with empirically determined dilation curve. The result highlights the carbon enrichment of austenite, as well as its partial transformation to secondary martensite and/or bainite, during the partitioning step. Also, an increase in quench temperature (QT) led to enhanced bainite or secondary martensite formation, as a result of reduced carbon enrichment of remaining austenite. Further Q&P experiments on bulk samples were carried out to understand the dependence of carbon diffusion and subsequent microstructure evolution with QT. Although the change in experimentally obtained retained austenite (RA) content with QT corroborates with the existing model predictions, the maximum amount of RA was observed at QT lower than predicted. The half-thickness of RA films increased with increasing QT, which substantiates the theoretical prediction of the diffusion distance of carbon atoms in austenite.

Microstructure evolution during thermomechanical processing in 3Mn-0.1C medium-Mn steel

ABSTRACTMedium-Mn steels are energetically investigated as a candidate of the third generation advanced high strength steels (AHSSs). However, their phase transformation and microstructaure evolution during various heat treatments and thermomechanical processing are still unclear. The present study first confirmed the kinetics of static phase transformation behaviour in a 3Mn-0.1C medium-Mn steel. Hot compression tests were also carried out to investigate the influence of high-temperature deformation of austenite on subsequent microstructure evolution. It was found that static ferrite transformation was quite slow in this steel, but ferrite transformation was greatly accelerated by the hot deformation in austenite and ferrite two-phase regions. Characteristic dual-phase microstructures composed of martensite and fine-grained ferrite were obtained, which exhibited superior mechanical properties.This paper is part of a Thematic Issue on Medium Manganese Steels.

Improving magnetic properties of non-oriented electrical steels by controlling grain size prior to cold rolling

Abstract The 0.5 mm-thick Fe-0.9 wt%Si-0.3 wt%Al non-oriented electrical steel sheets were successfully produced with one-stage and two-stage cold rolling methods. The comparative investigations were conducted on the relationships between the processing routes, microstructures, textures and magnetic properties. This study mainly focused on how to increase the grain sizes prior to cold rolling and its influences on subsequent microstructure and texture evolution. The results showed that the fine microstructure prior to final cold rolling led to the pronounced γ-fiber texture, fine recrystallized grains and deteriorated magnetic properties after final annealing. Nevertheless, by introducing a low-reduction cold rolling and intermediate annealing, coarse grains could be generated prior to the final cold rolling. When the cold rolling reduction was 11%, a relatively homogeneous microstructure composed of fully coarse grains was produced. The increased grain sizes promoted the generation of dense shear bands during final cold rolling, which served as the nucleation sites for λ-grains and Goss grains. This resulted in the improved magnetic properties due to the weakened γ-fiber texture, strengthened λ-fiber texture and Goss texture and increased grain sizes in the final annealed sheets. This work provided a new way to improve the magnetic properties of non-oriented electrical steels by controlling the grain sizes prior to final cold rolling.

Numerical microstructure prediction by a coupled finite element cellular automaton model for selective electron beam melting

Selective Electron Beam Melting (SEBM) refers to an additive manufacturing process of building near net shaped components layer wise by iteratively melting of metal powder using an electron beam. Due to the preheating of the material and very high scan velocities of the electron beam, the process allows large processing windows defined by different process strategies. This variety enables the adjustment of part properties by, e.g., targeting certain microstructures. However, this variety has its disadvantage in the need for costly trial and error experiments. Therefore, numerical predictions are inevitable in locating promising process parameter combinations.We present the weak coupling of a Finite-Element (FE) model with a cellular automaton (CA) model to predict the microstructure evolution during SEBM. We use an FE model for the computation of the heat input, since the temperature history mainly influences the subsequent microstructure evolution. The FE model provides precomputed temperature fields for our sophisticated CA-based crystal growth model, which successfully predicts bulk material microstructures. The compound is validated by accurately reproducing the microstructure of an additively build cylinder from CMSX-4 powder, a nickel based superalloy. Special emphasize is laid on the accurate prediction of the microstructure both in the shell as well as the core of the cylinder.