Boundary Migration Research Articles

Fast densification mechanism of highly transparent AlON ceramics pressureless sintered by co-doping Y2O3 and MgO additives

By co-doping 0.15 wt% Y2O3 and 0.60 wt% MgO, AlON ceramics with high transmittance in visible and infrared range (the maximum transmittance of 81.8%@3600 nm, 78.1%@400 nm) were successfully fast fabricated via pressureless sintering at 1880 °C for 2.5 h. The phase assemblage, microstructure and densification process of samples doped (Y2O3/MgO single- and co-doped) and undoped additives during heating were monitored to explore the densification mechanism. MgO was found to be an exceptional inhibitor for both AlON decomposition and grain growth. By incorporating MgO as one constituent of additives, the content of α-Al2O3 that decomposed from AlON in samples during heating was significantly reduced and Mg2+ illustrated inhibition to grain boundary migration, resulting in particle aggregation/coarsening were effectively suppressed at ≤ 1600 °C. Based on the obtained microstructure with compacted small particles and negligible particle aggregation/coarsening at early stage of sintering, facilitated by the acceleration of Y3+ on mass transfer, rapid densification of AlON was achieved.

Enhanced high temperature performance of two-scale network-structured (TiB+Ti5Si3)/TA15 composites by simultaneous adjustment of matrix and nano-scaled Ti5Si3

To improve the high temperature performance of the titanium matrix composite, solution and aging treatments were carried out on the (TiB + Ti5Si3)/TA15 composite with a two-scale network structure. The microstructure evolution during the heat treatment and high temperature performance at 650 °C were investigated. It is found that a fine matrix with basket-weave structure and nano-scaled Ti5Si3 can be obtained through a proper combination of solution and aging treatment. Optimal high temperature performance is achieved with yield strength and ultimate tensile strength improved by 15 % and 24 % respectively without sacrificing ductility. Matrix refinement is attributed to network-distributed TiB and fine martensite after solution treatment. During subsequent aging, fine secondary α phase and β phase are formed and then grow. The growth mechanisms of the secondary α phase and β phase are phase boundary migration and grain coalescence. The main strengthening mechanisms are boundary strengthening of the matrix and the precipitation strengthening of Ti5Si3, which are mainly related to the thickness of the secondary α phase and the fraction of Ti5Si3, respectively.

Accelerating and decelerating effects of Gd content and peak-aging treatment on the grain growth of magnesium

This short communication concerns the dependence of thermal stability of fine grains in Mg alloy on the Gd content. The enhanced thermal stability via increasing Gd content can be reflected by the SRX mechanism and corresponding boundary migration ability. In the low Gd-alloyed Mg, continuous SRX dominates the annealing process via dislocation re-arrangement. The grain boundaries composed by dislocations can easily migrate, since the uniform Gd solute distribution provides limited delaying effect. In the high Gd-alloyed Mg, discontinuous SRX dominates the annealing process via consuming dislocations for grain nucleation. The grain boundaries can hardly migrate due to the strong delaying effect provided by the Gd-segregation along grain boundaries. But unexpectedly, peak-aging can deteriorate the decelerated effect of high Gd-alloying owing to the absent Gd-segregation. Consequently, an accelerated grain growth rate is attained, even faster than that in the low Gd-alloyed Mg.

Microstructure evolution and EBSD analysis of multi-principal element alloy coatings after high-temperature oxidation

Multi-principal element alloy coatings (MPEACs) have great application potential for high-temperature diffusion barrier coatings. The sluggish diffusion and lattice distortion of this new coating material due to its own atomic size effect give the coating excellent resistance to high temperature oxidation and grain coarsening. In this study, the high-temperature oxidation properties, and microstructure evolution of quaternary, quinary, senary, and denary alloy coatings (whose lattice distortion magnitude is linearly increased) at 1100 °C were studied. The evolution of grain orientation, size, and grain boundaries of alloy coatings after high temperature exposure were analyzed by EBSD. The results showed that from quaternary to denary alloy coatings, with the rise in lattice distortion magnitude of the alloy system, the sluggish diffusion ability of the coating at high temperatures was more obvious. Specifically, the oxidative activation energy of quaternary to denary alloy coatings is 355.41, 480.96, 478.76, and 654.03 kJ/mol, respectively. A high distortion magnitude is beneficial to increase the activation energy of the alloy coating, as a result, grain growth of alloy coatings with high activation energy is significantly inhibited, and under the combined action of storage strain energy and solute dragging, the grain boundary migration rate of the coatings also declines. Finally, the denary alloy coating shows good resistance to high temperature softening. This study provides a positive reference for promoting the application of multi-principal alloys in diffusion barrier coatings.

Stability and growth kinetics of {112} twin embryos in [formula omitted]-Ti alloys

{112}〈111̄〉 deformation twin, shortened as {112} twin, is usually the dominant twinning mode in transition metal alloys in a body-centered cubic (BCC) lattice except for many BCC β titanium (Ti) alloys. To understand this twin-mode variation, we investigate stability and early-stage growth kinetics of {112} twin embryos with multiple atomic layers in a series of β-Ti alloys by applying density functional theory (DFT) and classical atomistic simulations. Both simulation methods demonstrate that, as average valence electron concentration (VEC measured in a unit of e/a) of Ti alloys decreases, β→ω phase transformations at {112} twin boundaries, which are confirmed by our transmission electron microscopy characterizations, increase the critical thickness of {112} layers as stable twin embryos, possibly raising {112} twin nucleation energy barriers. In simulations of twin embryo growths under applied shear stress on {112} planes, when VEC (or temperature) values are low (∼4.25e/a at 300 K), the applied shear stress results in β twin →α’/α” phase →β matrix phase transformations through an anti-twinning mechanism inside the existing twin embryos; concurrently, ω phases strongly impede the twin boundary migration; these combined effects result in eliminations of {112} twin embryos. However, when VEC increases slightly (∼4.34e/a at 300 K), ω phases at twin boundaries reduce the required stress for {112} twin embryo growth compared with the cases with high VEC values (e.g., ∼4.50e/a at 300 K). These changes of ω effects on {112} twin embryo growth kinetics could originate from free energy landscape variations of β matrix →ω phase →β twin phase transformations.

A novel low thermal conductivity refractory aggregate for high-temperature applications: Lightweight microporous alumina-rich spinel (Mg0.4Al2.4O4)

This study aimed to develop lightweight refractory aggregates with high closed porosity and low thermal conductivity. We report the preparation of a series of lightweight microporous alumina-rich spinel (Mg0.4Al2.4O4) aggregates with low open porosity, high closed porosity, small intragranular pore size, and low thermal conductivity. The sintering activity of the nano-zirconia sol increased the grain boundary migration rate under the action of surface tension during sintering; therefore, the pores were closed before they were eliminated. Simultaneously, Zr4+ replaced Mg2+ and Al3+, which increased the number of cation vacancies in the crystal and accelerated the ion migration, resulting in a faster formation of Mg0.4Al2.4O4. When 0.75 wt% nano-zirconia sol was added, it increased the diffusion rate of ions and the migration rate of grain boundary, which improved the sintering reactivity. The pores were mainly small-sized intragranular pores (median size: 1.89 μm), which reduced the thermal conductivity (2.5 W•m−1K−1 at 1000 °C).

Effect of forging temperature on the microstructure, subsequent aging precipitation behavior, and mechanical properties of Mg-Gd-Y-Zr-Ag alloy

The deformation mechanism, microstructure evolution, and precipitation behavior of a Mg-8.9Gd-1.8Y-0.5Zr-0.2Ag (wt.%) alloy multi-directionally forged at three different temperatures were investigated. As the forging temperature increases, the particle-stimulated nucleation (PSN) effect diminishes as the number of dynamic precipitates decreases, pyramidal slip is activated, grain boundary migration accelerates, and continuous dynamic recrystallization (CDRX) dominates. The microstructures varied greatly, although fine-grained structures were formed at all different forging temperatures. Competitive precipitation between dynamic precipitate growth, dislocation-induced precipitation, and homogeneous precipitation was observed after aging treatment. Among them, the medium temperature (748 K) forged and aged alloy exhibits the best mechanical performance, with an ultimate tensile strength of 436 MPa, and elongation of 16.3%. The calculation indicates that the mixed precipitation structure containing the β′ precipitate band provides a 35% higher strengthening contribution than the typical homogeneously distributed precipitates. The formation of precipitation-free zones (PFZs) ensures that aging will not cause a dramatic decrease in ductility, which provides a reference for the industrial preparation of high-performance wrought Mg-Gd series alloys.

Texture evolution during multi-pass cold rolling and annealing of Ti-2Al-1.5Mn alloy

In this contribution, the underlying mechanisms of microstructure evolution and texture development during the cold rolling and annealing of a Ti-2Al-1.5Mn alloy were investigated and elucidated comprehensively. As a result of the heightened dislocation density and grain boundary, the hardness ascended as the successive reductions applied. However, the annealing treatment reinstated the structure through complete recrystallization. During cold working, the dominant basal <a> and prismatic <a> glide inclined the c-axis and <101̅0> axis towards the compressive direction and the rolling direction (RD), respectively. The primary rolled orientation consisted of <101̅0> fiber, the orientation of {1̅21̅3}<101̅0> with Euler angle {10°, 45°, 0°}and {1̅21̅7}<101̅0> with Euler angle {5°, 25°, 0 °} were predominant. Schmid Factor (SF) analysis and orientation rotation related to various textures demonstrated that basal <a> slipping resulted in a descendingϕangle in the Bunge system and <101̅0> pole parallel to the RD was the stable orientation. It was revealed that the recrystallized grains preponderantly exhibited textures inherited from the rolled matrix, but a new T-type texture also emerged. This implies that both the strain-induced boundary migration (SIBM) mechanism and selective grain growth, which nucleates at different sites, manipulate the annealing process. The study enhances our comprehension of texture evolution mechanisms involved in the plastic forming and helps ascertain the most effective operational parameters for producing high-performance titanium plates.

Investigating the influence of Si on dynamic recrystallization during hot deformation behavior of ultra-high-strength lightweight steel

In this work, we systematically evaluated the effect of Si on the dynamic recrystallization (DRX) during hot deformation behavior of a novel high-strength rolled Medium Mn steel. To this end, the novel steel was subjected to compression test in the temperatures of range temperature of 950–1150 °C and the strain rate of range of 0.01–10 s−1. Numerical simulation, constitutive analysis, different models and processing maps were also developed to predict the volume fraction of DRX, which was further confirmed through electron backscatter diffraction (EBSD). The results show that some disparities are existed between the model predictions and the results. The EBSD micrographs of samples displayed prominent discontinuous DRX peak in the true stress-strain curves exhibit conventional equiaxed DRX particles. Most specifically, in initial stages the addition of Si increased the DRX and grain boundary migration, and the later stages of deformation the DRX was triggered after achieving the steady state. Therefore, the final microstructure appears as elongated austenite or localized DRX. Furthermore, the work hardening stage before the new round of DRX nucleation caused the stress to continue to increase after the steady state. These findings provide valuable insights into the complex behavior of materials under the influence of Si addition and enhancing our understanding of microstructural evolution during hot deformation.

Effect of boundary grooving on grain growth by Potts model simulations

Grain growth in thin films is a technologically important and scientifically fascinating topic: As the grain size influences many mechanical and optical properties of a material, any change in the grain structure and therewith in the grain size changes also the materials properties. There are many factors influencing coarsening of the microstructure such as grain boundary properties or the presence of particles; additionally the film thickness becomes rather important. It has been observed that when the grain size reaches the magnitude of the layer thickness, coarsening slows down and may even stagnate. For thin films, surface effects such as grain boundary grooving become important. In general, grooves are totally left out of implementations in the standard Potts model. In the current investigation, we introduce grain boundary grooves in the Potts model as physical entities. In the analysis, we show how the mere presence of such grooves reduces the driving force of grain boundary migration at the surface tremendously at any stage of coarsening.

Effects of deep cryogenic treatment on the microstructure evolution, mechanical and thermal fatigue properties of H13 hot work die steel

The service life of hot work die steel is significantly influenced by its microstructure and thermal fatigue properties. This paper explores the impact of deep cryogenic treatment (DCT) on H13 hot work die steel's microstructural evolution and mechanical as well as thermal fatigue properties. The results demonstrate that a portion of unstable retained austenite transforms into martensite during deep cryogenic treatment and numerous finely dispersed carbides precipitate from the matrix, thereby enhancing hardness. Analysis of the average crack length and distribution of thermal fatigue test samples reveals that the average crack length and crack density of DCT-treated samples are lower than those of non-DCT-treated samples. Thermal fatigue cracks in the specimens primarily originated from trigeminal grain boundaries and the carbides are found at the crack tips. DCT facilitates the precipitation of fine and dispersed carbides, imparting a robust pinning effect on boundary migration and thereby impeding the initiation and propagation of thermal fatigue cracks. Furthermore, deep cryogenic treatment results in a reduction in the number of M23C6 type carbides with large particles and an increase in smaller M6C type carbides during the thermal fatigue test. This indicates a positive influence on the carbide distribution and further contributes to the improved thermal fatigue resistance of the hot work die steel.

Microstructure evolution and constitutive analysis of nuclear grade AISI-316H austenitic stainless steel during thermal deformation

Compression experiments were performed on AISI-316H austenitic stainless steel using Gleeble-3800 at temperatures ranging from 900 °C and 1200 °C and strain rates ranging from 0.01 and 10 s−1, up to the actual strain of 0.69. The tests aimed to examine the material’s microstructure evolution and flow stress behavior. Based on OM and EBSD studies, it was found that thermal deformation mostly induces discontinuous dynamic recrystallization (DDRX). The proportion of recrystallization nucleation increases steadily with increasing deformation temperature, while the impact of strain rate on recrystallization is complex. At the same deformation temperature, the recrystallization volume fraction initially declines and rises as the strain rate rises. In low strain rate regime, the longer (deformation) time available for grain boundary migration, the higher recrystallization volume fraction. In high strain rate regime, the higher stored energy (and thus the increased boundary velocity) raises the probability of nucleation events, stimulating twin formation. As a result, the twin promotes a dynamic recrystallization (DRX) process. An abundance of Σ3 twins was notably observed in uniformly refined recrystallized grains at a true strain of 0.69, at a temperature of 1200 °C, and a strain rate of 10 s−1. As a result, it was discovered that DRX occurs at higher strain rates and deformation temperatures. In addition, the flow stress curves were modified to account for adiabatic heating at strain rates exceeding 1 s−1. The findings demonstrated that adiabatic heating increased when strain level and strain rate increased and deformation temperature decreased. The strain compensation Arrhenius model is developed following the given stress–strain curve while considering strain. The model exhibits high accuracy, with a correlation value of 0.986. According to a kinetic study, the average activation energy for hot deformation of the tested steel was 444.994 kJ/mol. These findings provide fundamental insights into the microstructure control technology and the outstanding mechanical properties of the austenitic stainless steel AISI-316H.

Annealing behavior of AlSi10Mg alloy fabricated by laser powder bed fusion

The AlSi10Mg alloy fabricated by laser powder bed fusion was subjected to isochronal annealing for 2h at temperatures in the range of 100–520 °C. Recovery and grain growth of the annealed samples were investigated by combining EBSD and in situ TEM techniques. A bimodal grain size distribution with grain sizes of 0.9 μm and 2.3 μm was developed in the initial sample, where large grains were found in the interiors of melt pools and most of small grains were distributed along melt pool boundaries. In-situ TEM investigations revealed that nucleation that occurs at the melt pool boundaries is as a consequence of migration of high angle boundaries during annealing. A slight increase in grain sizes was discovered during high-temperature heat treatment, indicating the remarkable thermal stability of the alloy compared to conventional cast Al-Si alloys. This is explained by the fact that the stable eutectic Si structure, which was originally formed during rapid solidification, can effectively retard grain boundary migration during recrystallization.

Σ3 grain boundary dynamics studied by atomistic spherical bicrystal modeling

In this study, we propose an atomistic spherical bicrystal model that employs the synthetic driving force method to investigate the dynamics of curved interfaces. The model allows tracking the migration and faceting of spherical grain boundaries. As a demonstration case, a simulation for a grain boundary with Σ3 misorientation is performed. A subsequent analysis of interface energy and stiffness is performed, whereby it is found that the dynamics of [111] and segments in the Σ3 boundary plane fundamental zone follow energy and stiffness trends, while others, e.g., the segment, do not.

On the basic mechanism of grain growth: the role of texture on grain boundary migration rate

In this paper we will review some of the many achievements of Statistical Theory of Grain Growth (which is 40 years old, this year!), based on the classic law of grain growth. For instance, we will show that grain growth in presence of texture (which is the usual feature in a real microstructure) is not exclusively depending on the curvature of the grain boundary, as in this case the effect of curvature at the vertexes (in 2-D case) contributes as well (which, depending on the texture pattern, sometimes may be more relevant than curvature along the boundary). Such phenomenon requires a reformulation of Von Neuman’s equation in 2-D by involving, beside the different boundary mobilities, the different energies of the “third” grain boundary, which is pulling the vertex and inducing an extra curvature which is not balanced along the grain perimeter (for simplicity’s sake we are referring to a 2-D case, but the analysis can also be done in 3-D). We can then observe that boundary movement is not only depending on a couple of grains in contact (with the boundary curvature acting like in a bicrystal) but also includes the network of “third” grains operating at the vertexes. The overall consequence is that the growth kinetics and the grain size distribution shape are unique and variable for each texture component which contradicts classic growth laws as well as the expected results for the whole grain size distribution.

Nucleation of recrystallization

This paper reviews the mechanisms of nucleation during recrystallization of cold deformed, single phase polycrystalline metals including metals with large particles. The classic nucleation theories and conceptions are shortly summarized, while the main focus is on our results from recent 3D studies of nucleation. The novel results are related to the classic nucleation ideas and agreement/disagreement as well as new suggestions are discussed. More specifically the paper covers recovery leading to intragranular nucleation, bulge nucleation (which is often referred to as strain induced boundary migration), and particle stimulated nucleation. Also, effects of clustered nucleation, crystallographic orientation relationships and residual stress are considered. Finally future studies are suggested, which we consider key to advancing the understanding of nucleation during recrystallization.

Exceptional improvement in the yield strength of AZ61 magnesium alloy via cryo-stretching and its implications on the grain growth during annealing

This study explored the influence of pre-stretching temperature on the microstructural characteristics and deformation behavior of AZ61 magnesium alloy and its implications on grain growth during subsequent annealing. AZ61 alloy was stretched to 5% plastic strain along rolling (RD) and transverse direction (TD) at room (RT) and cryogenic temperature (−150 °C, CT) followed by annealing at 320 °C for 1 h to investigate the twinning and dislocation evolution and its consequent effect on the flow stress, plastic strain and strain hardening rate. Compared to RT-stretched samples, significant improvement in yield stress, strain hardening rate and moderate reduction in elongation to failure were witnessed for CT-stretched samples along RD and TD. The subsequent EBSD analysis revealed the increased fraction of fine {10−12} twins and nucleation of multiple {10−12} twin variants caused by higher local stress concentration at the grain boundaries in CT-stretched samples as manifested by the kernel average misorientation. This higher twin fraction and twin-twin interaction imposed the strengthening by restricting the mean free path of dislocations leading to higher flow stress and strain hardening rate. During annealing of the RT/CT-stretched samples, the residual strain energy and twin boundaries were decreased due to static recovery leading to a coarse-grained twin-free microstructure. Strain induced boundary migration (SIBM) was found to be the predominant mechanism governing the grain growth during annealing via movement of high angle grain boundaries.

Dynamic compression properties and microstructural evolution of ZCW612 alloy under different strain levels

In this study, a novel Mg-6Zn-1Cu-2Y-0.6Zr (ZCW612) magnesium alloy is prepared using a hot extrusion-shearing process. The mechanical properties, microstructural evolution, and deformation mechanisms of the extruded ZCW612 alloy under dynamic compression are studied at different temperatures (25 °C, 100 °C, 200 °C, 300 °C) and strain levels (3%, 6%, 8%, 12%) along the extrusion direction (ED). The experimental results show that the initial extruded alloy has an apparent (0001) basal texture and an average grain size of 4.5 µm. Meanwhile, the alloy consists mainly of α-Mg, MgZnCu phase, and Mg3Y2Zn3 phase. During dynamic compression at a strain rate of 2000 s−1, the peak stress of the ZCW612 alloy decreases with increasing temperature. The deformation mechanisms of the ED samples are mainly basal slip, pyramidal slip, and extension twinning. In addition, dynamic recrystallisation and low-angle grain boundaries resulting from the accumulation of a large amount of dislocations in grains are induced, leading to grain refinement of the ZCW612 alloy after dynamic deformation. At the same time, after dynamic compression at 300 °C, a certain degree of grain growth occurs in the ED samples due to grain boundary migration.

Microstructure evolution and creep-rupture behaviour of a low-cost Fe-Ni-based superalloy

ABSTRACT The pursuit of low-cost superalloys with excellent creep property is of great importance for practical engineering application. In this study, the creep performance, microstructure evolution and deformation mechanism of a low-cost Fe-Ni-based superalloy under the condition of 750°C/130 MPa and 700°C/200 MPa were investigated. The creep rupture life reached 14,715 h and 12,716 h, respectively, under the aforementioned conditions. After long-term creep, carbides and γ′ particles (Ni3(Al,Ti)) with spherical shape grew gradually due to the Ostwald ripening. Abnormal coarse needle-like γ′ phase presented at grain boundaries and grew along preferred orientation under the synergy effect of grain boundary migration, stress, and diffusion. Influence of precipitate-free zones, carbides, and coincidence site lattice (CSL) boundary on the creep properties were discussed. The creep deformation mechanism of the Fe-Ni-based superalloy were dominated by dislocations slip. Dislocations moved via by-passing the γ′ phase and formed dislocation bows around the precipitates.

Promoting strengthening and grain refinement of aluminum alloy during wire and arc additive manufacturing by adding TiB2 particles

The influence of submicron-sized TiB2 particles on the microstructure and properties of deposited 2319 Al has been investigated. The Al matrix demonstrated a small undercooling (ΔTfg≈1.12 K « ΔTAl-Cu≈13 K) satisfying its own free growth because of the addition of TiB2 particles. This promoted the nucleation and growth of an extremely large number of grains and hindered the migration of grain boundaries, thereby refining the grains from 96.5 to 47.42 μm. The texture features reduced with the decrease of the pole density of Al d from 1.997 to 1.417, and the maximum pole density value of CuAl2 decreased from 8.953 to 3.597. The plentiful θ′ phases precipitated near the grain boundary which restrained the formation of reticular eutectic phase, with a semi-coherent interface with the Al matrix, (112)CuAl2//(111)Al. Resulted in excellent precipitation strengthening of being the best strengthening effect by θ′ phases, the strength of the deposited sample was increased by 30 %, reaching 304 MPa.