Adjacent Grains Research Articles

Microstructure regulation and optimizing magnetic properties of (La, Ce, Pr)17Fe78B6 alloy via short-range grain boundary diffusion

Increasing the utilization ratio of La and Ce in rare earth permanent magnets is an effective measure to solve the balance utilization of rare earth resources. In this work, nanocrystalline melt spun [(Ce0.7La0.3)0.3Pr0.7]17Fe78B6 powder and low melting point PrTbCu alloy were used as the initial material and the diffusion source for short-range grain boundary diffusion (SRGBD) to improve the coercivity. High coercivity of up to 18.21 kOe can be achieved, enhancing by 108.82% compared with the initial powder, and meanwhile, the remanence was only slightly reduced by 2.13%. After that, nanocrystalline bulk magnets with excellent magnetic performance were produced through spark plasma sintering (SPS). TEM observation reveals that the bcc-RE2O3 intergranular phase that induced by short-range grain boundary diffusion is distributed between adjacent matrix phase grains, which is conductive to the improvement of magnetic isolation effect. EDS results show that the Tb-rich shell with high magnetocrystalline anisotropy field has formed. Moreover, it is found that Tb diffuses into the 2:14:1 phase more easily than Pr element during the initial stage of diffusion. Enhanced magnetic isolation effect and the formed Tb-rich shell in grain surface contributed to high coercivity.

Influence of intergranular friction weakening on rock avalanche dynamics

In this paper, we investigated the significance of intergranular friction weakening on the dynamics of the Jiweishan rock avalanche. First, high-velocity-friction tests simulating the shearing between the surfaces of adjacent grains were conducted to obtain a model of friction weakening of grains inside granular debris during transport. Then, the model was incorporated into an improved discrete element method that could consider intergranular friction weakening to simulate the dynamic course of the Jiweishan rock avalanche. Finally, we compared the results from the numerical models with and without granular friction weakening to investigate its influence on the dynamics of the rock avalanche. The results of the high-velocity-friction tests revealed that intergranular shearing could significantly reduce the friction of granular surfaces; this was determined by the overall results for normal stress, shear velocity and shear displacement. Our modeling results indicated that the rapid increase in normal stress and shear velocity were the main reasons for the rapid decrease in intergranular friction coefficients in the falling and collision stages. However, the friction coefficients further decreased due to increasing shear displacement between rock debris in the flowing stage. Furthermore, obvious intergranular friction coefficient distribution characteristics in rock avalanche deposits could be observed from our modeling; they tended to decrease with increased depth in the deposit and increasing distance from the source. With intergranular friction weakening, a faster velocity (∼10.0–15.5 m/s) and a longer transport time (∼12–44 s) of the rock debris were modeled due to the lower frictional energy consumption caused by intergranular friction weakening. Finally, the transport distance considering intergranular weakening was ∼ 395–711 m longer than that without weakening. Further analysis showed that intergranular friction weakening may be applied to explain the high mobility of rock avalanches. The results of our study may improve predictions of the speed and travel distance, and consequently the risk assessment, of widely distributed rock avalanches worldwide.

High temperature plasticity at twin boundary in Al: An in-situ TEM perspective

Coherent twin boundaries are of special importance in structural materials due to their strengthening ability by impeding dislocation motion while maintaining possible glide along their plane. Interactions of lattice dislocations with twin boundaries have been extensively studied experimentally and numerically but relaxation and deformation processes at high temperature are not fully understood. The reason is related to complex mechanisms of dislocation decompositions into disconnections, their further motion and possible reactions. Moreover, as in shear-coupled grain boundary (GB) mechanisms, motion of disconnections in the interface plane is expected to lead to twin motion perpendicular to its plane. Here, shear-coupled motion of a coherent twin in pure Al is explored during in situ straining in a transmission electron microscope (TEM) in a favorable geometrical configuration. Surprisingly, the twin boundary does not couple to shear but slightly migrates by propagating nanoscale incoherent facets. Although, disconnections of Burgers vector 1/6〈112〉 have been extensively observed moving in the interface plane, their motion did not lead to migration as expected but presumably to GB sliding. This was interpreted by the motion of disconnections with opposite single and double steps in the [111] direction. Extensive dislocation/GB interactions were observed and reactions following absorption are interpreted by contrast analysis and motion observations. Incoming dislocations do not always decompose but often react with the GB disconnection microstructure which may lead to intergranular motions or dislocation emissions in the adjacent grain. As these processes usually involve disconnection climb in the interface plane, direct transmission, i.e. spatially and temporally correlated absorption/emission processes, is not observed as revealed by large scale observations. Finally, the internal disconnection microstructure of the twin boundary often forms networks which although flexible are found to slow down intergranular plasticity. Such networks should strongly control stress induced mechanisms in realistic GBs.

Role of structural hierarchy on mechanics and electrochemistry of wire arc additive manufactured (WAAM) single phase titanium

The absence of understanding of correlations between structure, mechanics, and electrochemical behavior in single-phase additive-manufactured titanium components impedes the advancement of large-scale processing. The present study addresses this scientific knowledge gap by investigating the evolution of structural hierarchy and its impact on the hardness and corrosion resistance of wire arc additive manufactured (WAAM) commercially pure titanium (cp-Ti) along three mutually orthogonal deposition directions. Integrated X-ray diffraction and optical microscopy revealed that a single-phase α-Ti monolithic structure is devoid of observable deposition tracks and interfaces with traces of titanium oxide. Homogeneous nucleation in the single phase at low undercooling and slow solidification rates during WAAM resulted in coarse grains of size 1 mm. The grains are randomly oriented with slight preferential texture along the hexagonal close-packed basal {0001} planes due to their parallel orientation with the close-packed body-centered cubic {011} plane during the β – α phase transition. The single-phase structure results in a uniform distribution of microhardness along layer-by-layer buildup direction and the bulk with an average of 1.6 GPa, which is about 80 % of conventionally cast cp-Ti. An increased dislocation density at the surface and from thermal strain during WAAM enhances nanohardness to 3.0 GPa. The absence of galvanic potential difference between two adjacent grains and the coarse size imparts high corrosion resistance with a rate of 80 × 10−5 mm per year, less than 50 % of other additive-manufactured titanium alloys. Overall, this study advances the state-of-the-art in homogenous, single-phase WAAM processing as a potential large-sale additive manufacturing alternative to conventionally processed Ti components.

Unsupervised clustering of nanoindentation data for microstructural reconstruction: Challenges in phase discrimination

Automated clustering of input nanoindentation data using a K-means algorithm was performed on dual phase (DP) and high strength low alloy (HSLA) steels with the aim of accurate microstructural classification of the phases present. Electron microscopy and diffraction techniques were used to correlate the clustering result with the microstructure, and for the DP steel an accurate separation of polygonal ferrite, martensite and anomalous indents (i.e. located on grain boundaries or adjacent to martensite islands) was achieved. In direct contrast, K-means did not reveal an accurate separation of the HSLA constituent phases. After comparing the hardness level of selected HSLA grains and the minimum distinguishable hardness level of the DP steel, it is found that K-means input variables should differ from each other by at least 10% in order to achieve reasonable separation of clustering results. Further targeted nanoindentation experiments on polygonal ferrite grain pairs were then performed to investigate the impact of indentation near grain boundaries between two adjacent grains with similar mechanical properties on the K-means clustering analysis. It is shown that the optimal number of clusters could not converge, and consequently, for the HSLA steel and its complex microstructure the limiting factors to obtain a phase classification by using K-means clustering based on nanoindentation input variables are: (i) the influence on the measured hardness in the vicinity of grain boundaries; (ii) the similar mechanical properties of both phases; (iii) the inhomogeneous substructure of granular bainite.

Study on the phase structure of the interface zone of Cu–Al composite plate in cast-rolling state and different heat treatment temperatures based on EBSD

The phase structure in the interface zone of Cu–Al composite plate was studied by EBSD after casting and rolling and different heat treatment temperatures. The grain sizes of the matrix and intermetallic compounds in the interface zone were analyzed statistically. The spatial distribution characteristics of the transverse, rolled, normal and various phases were characterized by inverse pole diagram. The results show that with the increase of heat treatment temperature, the phase of the interfacial area of the composite plate is rearranged, the residual stress can be released, and the zero resolution decreases. The interface zone of cast-rolled Cu–Al composite plate is composed of a layer of intermetallic compound, which is Al2Cu; Two layers of intermetallic compounds, both Al, were formed in the interfacial area after heat treatment at 300 °C and 400 °C Al2Cu and Al4Cu9;After heat treatment at 450 °C, the interface region is composed of three layers of intermetallic compounds, which is Al2Cu, AlCu, Al4Cu9;The grain size of copper matrix is obviously larger than that of aluminum matrix. Matrix aluminum, matrix copper, Al2Cu.The average grains of Cu are mostly large Angle grain boundaries, and the orientation difference of adjacent grains in aluminum layer is much smaller than that in copper layer. After heat treatment at 300 °C for 1 h, the recrystallization degree of the interface zone of Cu–Al composite plate is low. After heat treatment at 400 °C for 1 h, the recrystallization in the interfacial zone of Cu–Al composite plate is not complete; After annealing at 450 °C for 1 h, the subgrain boundaries in the aluminum layer grow up, and LAGB is basically converted to HAGB. The distribution characteristics of the characteristic appearance direction in the crystal space of each phase were characterized under the conditions of casting and rolling and different heat treatment temperatures. At the same time, the research results provide theoretical basis and scientific title for optimizing the preparation process and high-end application of Cu–Al composite plate.

Microstructure characteristics at different depths of 40CrNiMo steel after laser hardening

This study focuses on laser-hardened 40CrNiMo steel, which meets the requirements for use in sprag clutch wedges; specifically, the microhardness characteristics as well as the phase and element characteristics and formation mechanism at different depths are investigated after laser hardening. The results show that the hardness of the arc-shaped specimen with a laser-hardened layer fluctuates significantly after reaching a certain depth. With increasing depth, the original austenite grain size decreases exponentially, it becomes easier for acicular martensite to form near the surface, and the martensite bulge phenomenon near the surface is more pronounced. The proportion of small-angle grain boundaries decreases, while that of large-angle grain boundaries increases after laser hardening. The angle of the small-angle grain boundaries of the adjacent grains of the hardened layer is mostly below 5°, while that of the large-angle grain boundaries is predominantly between 50° and 60°. After laser hardening, the preferred orientation of the grains in the hardened layer is not clear. The aggregation of C atoms is an important factor determining the structural characteristics of the hardened layer.

Enhanced crack buffering of additively manufactured Ti–6Al–4V alloy using calcium fluoride particles

Ti-based alloys fabricated by wire and arc additive manufacturing (WAAM) exhibit continuous grain boundaries of α phase (GB-α), which easily initiate cracks and provide a continuous crack propagation path. Herein, an activating flux of CaF2 particles was introduced into WAAM-fabricated Ti–6Al–4V alloy to tailor the microstructure for crack buffering (i.e., inhibit smooth propagation of cracks). The results showed that the addition of CaF2 particles disrupted the continuous GB-α through the dowel-like lamellar α phase (DL-α), which was primary α lamella deeply embedded in the adjacent prior-β grains. This microstructure imparted an optimal combination of strain and strength even in the presence of pores in the WAAM-fabricated Ti–6Al–4V alloy. The enhanced mechanical properties can be attributed to uniform plastic deformations and tendency to transgranular fracture of tensile specimens. DL-α relieved stress concentration at the grain boundary and guided cracks into the prior-β grain interior (i.e., avoided inter-crystalline fracture), thereby promoting the crack buffering effect. The study can provide theory guidance and data support for improving the mechanical performance of WAAM-fabricated Ti–6Al–4V alloy.

Evidence of a Thermal Diffusivity Gap in Sintered Li-Co-Sb-O Solid Solutions.

In this work, the thermal properties of ternary Li3x Co7-4x Sb2+x O12 solid solutions are studied for different concentrations in the range 0 ≤ x ≤ 0.7. Samples are elaborated at four different sintering temperatures: 1100, 1150, 1200 and 1250 °C. The effect of increasing the content of Li+ and Sb5+, accompanied by the reduction of Co2+, on the thermal properties is studied. It is shown that a thermal diffusivity gap, which is more pronounced for low values of x, can be triggered at a certain threshold sintering temperature (around 1150 °C in this study). This effect is explained by the increase of contact area between adjacent grains. Nevertheless, this effect is found to be less pronounced in the thermal conductivity. Moreover, a new framework for heat diffusion in solids is presented that establishes that both the heat flux and the thermal energy (or heat) satisfy a diffusion equation and therefore highlights the importance of thermal diffusivity in transient heat conduction phenomena.

Understanding the microstructure evolution characteristics and mechanical properties of an AlCoCrFeNi2.1 high entropy alloy fabricated by laser energy deposition

In the present work, an AlCoCrFeNi2.1 high entropy alloy is fabricated by laser energy deposition. Specimens are heated to 700 °C (2#), 850 °C (3#) and 1000 °C (4#), respectively, held for 1 h and cooled in water to investigate the effect of heat-treating conditions on microstructure evolution and mechanical properties of the alloys. FCC(L12) + B2 dual-phase microstructure is acquired from all the four deposits. Ordered FCC (L12) disappears and transforms into B2 particles in 3# and 4#. Original B2 phase and newly formed B2 particles show K–S orientation relationship (OR) with adjacent FCC grains. Nano-sized phase m particles, which contain monoclinic lattice structure and have a composition similar with FCC phase, exist on FCC-BCC phase boundary in all the four deposits. FCC-m phase boundary is coherent. Existence of phase m in 4# indicates that it is stable at temperatures as high as 1000 °C. Precipitate hardening plays an important role in strengthening the materials, while the coarsening of precipitate particles weakens their strengthening effect. The high strength of this alloy group is also attributed to the two-phase interface strengthening effect. Possible reasons for the formation of m particles and their effect on the strength of the material are analyzed.

In-situ study of adjacent grains slip transfer of Inconel 718 during tensile process at high temperature

Adjacent grains slip transfer is an important mechanism for metal adaptation to incompatibilities in intergranular deformation. Thus, this paper investigates slip transmission at grain boundaries (GBs) and annealed twin boundaries (TBs) in Inconel 718 (IN718) alloy using in-situ tensile test at 650 °C by integrating slip trace analysis and the crystal plasticity finite element method (CPFEM). Results show that the mαβ criterion, based on the match of intersection lines between slip planes to GB planes and slip directions, is more reasonable in predicting induced activated slip systems than Luster-Morris mαβ′ criterion. In addition, slip transfer tended to occur at mαβ′>0.83 and Δb(1/b)<0.44 or mαβ>0.82 and Δb(1/b)<0.46 for general GBs; when mαβ′>0.92 or mαβ>0.93, continuous slip transfer is a crucial mechanism to coordinate strain and reduce stress concentration. Furthermore, the acceptable criteria for slip transfer at TBs are θ≈0 and Δb(1/b)<0.6 by experimental and computational results. This study deepens the understanding of the adjacent grains slip transfer behavior of IN718 and also provides a new strategy to study the deformation behavior of Nickel-based superalloys.

A machine learning study of grain boundary damage in Mg alloy

Understanding the initiation of grain boundary (GB) damage is useful for designing Mg alloys with improved ductility. To this end, in-situ tensile tests were conducted for two Mg alloy samples of different textures. Prior to the two samples’ fracture, damages were observed at 5.8% and 7.5% of the GBs, respectively. Statistical correlations between GB damage and some GB features were identified. A machine learning model was further built for predicting whether a GB would initiate damage based on its feature values. After parameter optimization, that model achieves AUC (area under the receiver operator characteristic curve) scores of 73.7% and 84.1% for predicting the observed GB damages in the two samples. The most influential features are the crystallographic c-axis misalignment between the two adjacent grains, the average size of the two grains, and the GB inclination with respect to the loading axis. The reasons for those unpredicted GB damages are also discussed.

Investigation of the effects of the magnetic field on the anodic dissolution of alloy 690 in SO42− + SCN− solution using digital holography

Digital holography has been employed for in situ observation of dynamic processes occurring at the electrode|electrolyte interface during the anodic dissolution of Alloy 690 in solutions containing SO42− + SCN− with or without magnetic field (MF). It was found that MF increased the anodic current of Alloy 690 in 0.5 M Na2SO4 + 5 mM KSCN solution but showed a decreased value when evaluated in 0.5 M H2SO4 + 5 mM KSCN solution. For each solution, as a result of the stirring effect due to Lorentz force, MF showed a decreased localized damage further preventing pitting corrosion. The content of nickel and iron at grain boundaries is higher than that on the grain body, in accordance with the Cr-depletion theory. MF increased the anodic dissolution of nickel and iron, which in turn increased the anodic dissolution at grain boundaries. In situ inline digital holography revealed that IGC begins at one grain boundary and progresses to adjacent grain boundaries with or without MF.

Crack initiation mechanism of a silicide-containing high-strength Ti–5Al-7.5V alloy under low-cycle fatigue loading

In this work, the dual phase titanium alloy Ti–5Al-7.5V was subjected to low-cycle fatigue at room temperature to reveal the deformation mechanisms of fatigue and crack initiation. Transmission electron microscope analysis showed that planar dislocation slip, most of them localized at the basal and prismatic planes of α phase, is the primary deformation mode of low-cycle fatigue. Multiple slips operate concurrently in the high Schmid factor planes within a single grain. Moreover, a limited number of observed microcracks were formed along the basal slip bands. Electron back-scattered diffraction analysis evidenced that numerous microcracks were formed along basal planes. And, the microcracks were confined in primary α grains without propagation to surrounding transformed β matrix, indicating that the alloy exhibits a high tolerance to microcrack propagation. The α grain aggregate oriented for basal slip is a fatigue-critical microstructure configuration, which provides a high Schmid factor path by linking adjacent grains and potentially lead to internal crack initiation with the formation of a field of facets in the cycling process. While the silicides were observed in contact with the microcracks and slip bands, no evidence of crack initiation from silicides was detected. Given that the microcracks were formed along the pre-existing slip bands, it remains an open question whether the silicides can act as the dislocation sources for planar slips and facilitate crack nucleation from these silicides.

Microstructural evolution in graphene nanoplatelets reinforced magnesium matrix composites fabricated through thixomolding process

The distribution characteristic of reinforcements is a critical issue for the final performance of magnesium matrix composites (MMCs). Here, the microstructural evolution of graphene nanoplatelets (GNPs) reinforced MMCs fabricated by thixomolding process was investigated. The GNPs successfully penetrated through the grain boundaries and embedded in two adjacent grains. Those intragranular GNPs/Mg interfaces not only could provide efficient barriers to pin dislocations, but also contribute to the multiplying dislocations to accumulate rather than dissipate. In addition, the finer MgO particles and elongated Mg17Al12 were anchored around the GNPs, which could effectively improve the interfacial load transfer due to the increased load-bearing capability and the effective interfacial shear strength. These special distribution characteristic of GNPs was beneficial to the significant enhancement of mechanical properties for the composites. The concept of intragranular reinforcements anchored by finer particles in this work may bring positive and directive significance for the structural design of the composites.

Highly thermally stable Nd0.6Y0.4-Fe-B magnet with a spontaneous shell-strengthened structure by Tb alloying

Thermally stable Nd0.6Y0.4-Fe-B magnet with 0.83 at% Tb alloying was yielded with a coercivity of 1.58 T and a temperature coefficient (β20–120 ℃) of − 0.531%/°C. Microstructure analyses showed that the aggregation of Y in the core region essentially dominated the heterogeneous distribution of rare earth (RE) elements in the matrix phase grains, leading to the enrichment of Pr, Nd and Tb in the outer layers of matrix grains. The preferred regulation of Y for Tb distribution contributed to the spontaneous Tb-rich shells with strengthened anisotropy field, significantly mitigating the inevitable degradation of coercivity caused by the high Y-substitution. Continuous non-ferromagnetic grain boundary phases were also constructed to isolate the adjacent grains. The synergistic effect of above microstructural modification presents a reliable new strategy to compensate for the magnetic dilution effect of Y and promote the effective utilization of Tb.

Crack initiation mechanism of laser powder bed fusion additive manufactured Al-Zn-Mg-Cu alloy

Crack is a serious defect limiting the additive manufacturing of metal materials. The morphologies and crystallographic orientation of cracks for Al-Zn-Mg-Cu alloy prepared by laser powder bed fusion (LPBF) were clarified. The mechanisms of crack initiation and propagation were expounded. Results show that the cracks were crisscrossed horizontally and propagated through multilayers along the building direction in the as-printed specimens. The cracks were classified as solidification cracks, which occurred in the final stage during solidification when the liquid fraction was <20%. Al-Zn-Mg-Cu alloy has a wide temperature range between the liquidus and solidus, leading to high cracking susceptibility. Solidification cracks were prone to initiate at high-angle grain boundaries (HAGBs) and terminate at low-angle grain boundaries (LAGBs) or within grains. LPBF process could affect crack initiation and propagation by refining the grain size and inhibiting the growth orientation. Solidification and thermal shrinkage of adjacent grains during solidification provided subjective factors for cracking initiation. Directional preferential growth of dendrites along the 〈001〉 crystal orientation and the untimely filling of the liquid phase provided secondary factors. This study can provide theoretical guidance for crack suppression in additive manufacturing of 7075 aluminum alloy and other metals with high cracking susceptibility. • Crack initiation and propagation mechanisms of Al-Zn-Mg-Cu alloy in LPBF were investigated. • Internal cracks of as-printed specimens are solidification cracks. • Cracks occur in the final stage during solidification. • Oriented crystallization and texture affect the initiation and propagation of cracks. • Cracks initiate at straight HAGBs and terminate at LAGBs or within grains.

Toward developing Ti alloys with high fatigue crack growth resistance by additive manufacturing

Fatigue crack growth behaviors were investigated by three-point bending tests for TA19 alloy fabricated by laser metal deposition and four kinds of heat-treated samples. The crack growth resistance of the TA19 samples in the near-threshold regime and Paris regime was evaluated through the experimental characterization and theoretical analysis of the interaction between fatigue crack and α/β phase interface, columnar prior-β grain boundary and colony boundary. The results show that in the near-threshold regime, the fatigue crack propagation threshold and resistance increase with the increase of widths of lamellar αp phases and colonies, and the decrease of the number of α laths with an angle (φ) relative to the applied stress direction ranging from 75° to 90°. In the Paris regime, the fatigue cracking path can be deflected at colony boundaries or columnar prior-β grain boundaries. The larger the deflection angle, the more tortuous the cracking path and the lower the fatigue crack growth rate. The angle (γ) of the columnar prior-β grain growth direction relative to the build direction affects not only φ of different α variants, but also the fatigue cracking path deflection angle (θij) at columnar prior-β grain boundaries. An optimal combination of γ = 0°-15°-0°-15° for several adjacent columnar prior-β grains is derived from the theoretical analysis, and that can effectively avoid φ being in the range from 75° to 90° and make θij as large as possible. Such findings provide a guide for the selection of scanning strategies and process parameters to additively manufacture Ti alloys with high fatigue damage tolerance.

Microstructure Evolution in a GOES Thin Strip

This paper focuses on the evolution of the microstructure in a grain-oriented electrical steel (GOES) thin strip after casting. After solidification, the microstructure consisted of delta-ferrite. A small fraction of austenite was formed during the cooling of the thin strip in the two-phase region (gamma+delta). Fine Cr2CuS4 particles precipitated in the ferrite and along the delta/gamma interfaces. Laths of primary Widmanstätten austenite (WA) nucleated directly on the high-angle delta-ferrite grain boundaries. The formation of WA laths in both adjacent ferritic grains resulted in a zig-zag shape of delta-ferrite grain boundaries due to their local rotation during austenite nucleation. Based on the EBSD results, a mechanism of the formation of the zig-zag grain boundaries has been proposed. Besides the Widmanstätten morphology, austenite also formed as films along the delta-ferrite grain boundaries. Sulfide precipitation along the delta/gamma interfaces made it possible to prove that austenite decomposition upon a drop in temperature was initiated by the formation of epitaxial ferrite. Further cooling brought the decay of austenite to either pearlite or a mixture of plate martensite and some retained austenite.

Visualization of magnetization reversal processes by dynamic Kerr microscopy

Abstract Dynamic Kerr microscopy enables the analysis of structural defects in magnetic materials and their impact on the demagnetization behavior. Using new visualization techniques, it is possible to image the demagnetization process in the microstructure. It has been found that residual stresses and inclusions in soft magnets, for example, will impede magnetization. In permanent magnets, abnormally large grains, for example, will demagnetize first and then cause an accelerated demagnetization of adjacent grains.