Σ3 Grain Research Articles

In situ observation and temperature profile study of silicon Thin-sheet growth on quartz and silicon nitride substrates

Silicon wafer solidification on a substrate remains an important research topic. One of the major applications is in the production of kerf-free silicon wafers. In this study, we carried out in situ observations and temperature measurements to better understand the growth mechanism of silicon wafers on substrates. Silicon nitride and quartz substrates at cooling rates ranging from 5 K/min to 100 K/min were considered. Visual observations revealed that the growth of silicon proceeded with nucleation of solid strips, i.e., dendrites, which initially grew axially at high rates before expanding laterally. Furnace cooling parameters and the extent to which the substrate can sustain the undercooling were found to dictate the ensuing mode of growth and the trends experienced therein. Undercooling estimates, recalescence rates, solidification densities, and growth kinetics were discussed to discern their interrelation and explain their effects to the resulting nucleation and wafer growth behavior. Increasing cooling rate and undercooling for nucleation were found to consistently increase grain frequency as confirmed by electron backscatter diffraction analysis. Prevalence of certain grain orientation and the consequent increase in Σ3 grain boundaries was also discerned due to the preference of dendritic strips growth.

Segregation of P and S Impurities to A Σ9 Grain Boundary in Cu

The segregation of P and S to grain boundaries (GBs) in fcc Cu has implications in diverse physical-chemical properties of the material and this can be of particular high relevance when the material is employed in high performance applications. Here, we studied the segregation of P and S to the symmetric tilt Σ9 (22¯1¯) [110], 38.9° GB of fcc Cu. This GB is characterized by a variety of segregation sites within and near the GB plane, with considerable differences in both atomic site volume and coordination number and geometry. We found that the segregation energies of P and S vary considerably both with distance from the GB plane and sites within the GB plane. The segregation energy is significantly large at the GB plane but drops to almost zero at a distance of only ≈3.5 Å from this. Additionally, for each impurity there are considerable variations in energy (up to 0.6 eV) between segregation sites in the GB plane. These variations have origins both in differences in coordination number and atomic site volume with the effect of coordination number dominating. For sites with the same coordination number, up to a certain atomic site volume, a larger atomic site volume leads to a stronger segregation. After that limit in volume has been reached, a larger volume leads to weaker segregation. The fact that the segregation energy varies with such magnitude within the Σ9 GB plane may have implications in the accumulation of these impurities at these GBs in the material. Because of this, atomic-scale variations of concentration of P and S are expected to occur at the Σ9 GB center and in other GBs with similar features.

Analytic description of grain boundary segregation, tension, and formation energy in the copper–nickel system

In this theoretical study, a recently proposed segregation model is applied to segregation data of an exemplary Σ5 grain boundary (GB), which is investigated using a copper–nickel embedded-atom method potential. Segregation in the semi-grandcanonical ensemble is systematically studied by varying the chemical potential in order to explore the full composition range for temperatures from 500 K to 1000 K. As a major thermodynamic feature, the mentioned segregation model avoids the usage of interface compositions, for which an arbitrary volume must be defined, but rather models the thermodynamically unambiguous solute excess. It was shown that the solute excess and the interface formation energy could be described very accurately over a wide range of temperatures and over the entire composition range based on a composition-dependent, but temperature-independent effective energy of segregation. However, the model was initially derived for systems without lattice mismatch so that interface tensions were neglected. Since the copper–nickel system exhibits a moderate lattice mismatch of roughly 2.7%, the copper–nickel system is chosen in this study to further extend the segregation model by a linear-elastic theory to also account for the interface tensions. Using this extended model, it will be shown that the solute excess, GB tensions, and GB formation energies can be derived from the effective energy of segregation for all temperatures and over the whole composition range. As in principle only one single segregation isotherm is sufficient to determine the effective energy of segregation and derived thermodynamic interface quantities, the application of this model is especially interesting for experimental evaluations.

Grain boundary character distribution in the CoCrMo alloy processed by selective laser melting and post-heat treatment

The evolution of grain boundary character distribution (GBCD) in the CoCrMo alloys processed by SLM and post-heat treatments (aging and solid solution) has been investigated by the electron backscatter diffraction (EBSD) and transmission electron microscope techniques. A high fraction of Σ3n (n = 1, 2, 3) grain boundaries and the corresponding clusters occur in the solid solution-treated samples at 1150 °C, which indicates GBCD optimization is somewhat realized. The coherent and incoherent Σ3 boundaries were distinguished via the single-section trace analysis method based on EBSD measurements. Σ3 boundaries in the SLMed and aged samples were determined as incoherent, while those in the solid solution-treated sample are coherent. Regular e precipitation did not occur in the solid solution-treated samples as in the aged samples. Instead, stacking fault bands associated with coherent Σ3 twin boundaries are dominant. It suggests that post-heat treatment has a significant influence on the GBCD, especially the Σ3 grain boundary characteristics.

Atomic structures of Ti‐doped α‐Al2O3 Σ13 grain boundary with a small amount of Si impurity

AbstractDoping is a fundamental technique to control sintering of α‐Al2O3, and many types of elements have been used to control grain growth and material properties. Such dopants tend to be concentrated at the grain boundaries and modify grain boundary properties. It is therefore important to investigate the variation of grain boundary atomic structures induced by additives. However, in the sintering process it is difficult to prevent small amounts of unintentional impurity (such as Si), and the potential influence of additives and impurities on the grain boundary structure should be taken into account. Here we show that two types of grain boundary atomic structures are formed in a Ti‐doped Σ13 []/() α‐Al2O3, which has been determined by atomic‐resolution scanning transmission electron microscopy. Combining with atomic‐scale spectroscopic analyses, we elucidate that the local concentration of Si impurities significantly alters the grain boundary atomic structures and the valence state of Ti additives. The present results suggest that the subtle change in concentrations of both additives and impurity should have a strong impact on sintering processes and resultant properties in α‐Al2O3.

Improving mechanical properties of selective laser melted Co29Cr9W3Cu alloy by eliminating mesh-like random high-angle grain boundary

Selective laser melting (SLM) has been attracting increasing attention as a suitable route for fabricating personalized orthopedic implants to address patient–prosthesis mismatches and has been used for producing Co29Cr9W3Cu alloys in our previous study. However, SLM technology can result in the formation of mesh-like random high-angle grain boundaries (indicated as molten pool boundaries) and accumulation of residual stress in the microstructure, possibly causing an unstable mechanical property. In this study, the research on the relationship between microstructural evolution and mechanical properties was performed on the SLM-produced Co29Cr9W3Cu alloys with different heat treatments to improve the reliable mechanical property of the SLM-produced Co29Cr9W3Cu orthopedic implants. It was found that the microstrucutre with mesh-like random high-angle grain boundaries could be eliminated during recrystallization, replaced by the one with equiaxial structure containing the Σ3 grain boundaries (annealing twin). Most importantly, the combined effect of eliminating the mesh-like random high-angle grain boundaries and residual stress and generating the Σ3 grain boundary contributed to an increase in elongation from 12.49% of the as-SLM-produced one to 23.38%. Proper heat treatment is considered to be an efficient strategy to improve the mechanical properties of the SLM-produced Co29Cr9W3Cu alloy with a desired tensile ductility.

Ultrasonic vibration assisted tungsten inert gas welding of dissimilar metals 316L and L415

Ultrasonic vibration assisted tungsten inert gas welding was applied to joining stainless steel 316L and low alloy high strength steel L415. The effect of ultrasonic vibration on the microstructure and mechanical properties of a dissimilar metal welded joint of 316L and L415 was systematically investigated. The microstructures of both heat affected zones of L415 and weld metal were substantially refined, and the clusters of δ ferrite in traditional tungsten inert gas (TIG) weld were changed to a dispersive distribution via the ultrasonic vibration. The ultrasonic vibration promoted the uniform distribution of elements and decreased the micro-segregation tendency in the weld. With the application of ultrasonic vibration, the average tensile strength and elongation of the joint was improved from 613 to 650 MPa and from 16.15% to 31.54%, respectively. The content of Σ3 grain boundaries around the fusion line zone is higher and the distribution is more uniform in the ultrasonic vibration assisted welded joint compared with the traditional one, indicating an excellent weld metal crack resistance.

Hydrogen induced vacancy clustering and void formation mechanisms at grain boundaries in palladium

Hydrogen has a significant impact on the formation of vacancies, clusters and voids in palladium and other metals. The formation of vacancy-hydrogen complexes in bulk Pd and at Σ3 and Σ5 grain boundaries was investigated using first-principles calculations and thermodynamic models. Equilibrium vacancy and cluster concentrations were evaluated as a function of temperature and hydrogen partial pressure based on the Gibbs energy of formation including vibrational and configurational entropies. Vacancies were found to be significantly stabilized by association with interstitial hydrogen, leading to enhanced concentrations by several orders of magnitude. Vacancy clusters were further stabilized at grain boundaries, with equilibrium concentrations reaching site saturation for clusters comprising up to three vacancies. Nanovoids were investigated based on Wulff constructions from calculated surface energies of the (0 0 1) and (1 1 1) terminations as a function of temperature and coverage of hydrogen adsorbates. The most stable termination changed from (1 1 1) in vacuum to (0 0 1) in H2, and the surface energies were lowered due to hydrogen adsorbates. Consequently, voids were also stabilized in the presence of hydrogen. Coalescing of vacancies into nanovoids was found to be thermodynamically unfavorable due to the loss of configurational entropy. It was therefore concluded that enhanced concentrations of vacancies and clusters does not directly cause the formation of voids. The formation of voids in Pd-based membranes was discussed in terms of microstructural characteristics, and strain due to chemical expansion and plastic deformation.

The Effect of Vacancies on Grain Boundary Segregation in Ferromagnetic fcc Ni.

This work presents a comprehensive and detailed ab initio study of interactions between the tilt Σ5(210) grain boundary (GB), impurities X (X = Al, Si) and vacancies (Va) in ferromagnetic fcc nickel. To obtain reliable results, two methods of structure relaxation were employed: the automatic full relaxation and the finding of the minimum energy with respect to the lattice dimensions perpendicular to the GB plane and positions of atoms. Both methods provide comparable results. The analyses of the following phenomena are provided: the influence of the lattice defects on structural properties of material such as lattice parameters, the volume per atom, interlayer distances and atomic positions; the energies of formation of particular structures with respect to the standard element reference states; the stabilization/destabilization effects of impurities (in substitutional (s) as well as in tetragonal (iT) and octahedral (iO) interstitial positions) and of vacancies in both the bulk material and material with GBs; a possibility of recombination of Si(i)+Va defect to Si(s) one with respect to the Va position; the total energy of formation of GB and Va; the binding energies between the lattice defects and their combinations; impurity segregation energies and the effect of Va on them; magnetic characteristics in the presence of impurities, vacancies and GBs. As there is very little experimental information on the interaction between impurities, vacancies and GBs in fcc nickel, most of the present results are theoretical predictions, which may motivate future experimental work.

Abnormal growth of columnar grains and formation of Σ3 grain boundaries in non-oriented electrical steels

The remarkably abnormal growth behavior of columnar grains and the formation of Σ3 grain boundaries (GBs) were studied via plastic deformation and annealing based on an electron backscattering diffraction (EBSD) investigation. Driven by the stored energy and GB mobility, coarse austenite columnar grains were formed by the abnormal growth of fine equiaxed grains with higher grain orientation spread (GOS) values in the reducing atmosphere of hydrogen. Moreover, extraordinarily coarse columnar grains with high GOS values and non-{1 1 1} texture were formed by microstructure inheritance during γ → α transformation. In addition, slender ferritic columnar or island grains with Σ3 GBs and low GOS values were generally obtained, and the formation mechanism was strictly associated with the Kurdjumov-Sachs (K-S) relationship, which was induced during variant selection driven by the GB mobility and surface effect in the reducing atmosphere of hydrogen.

First-principles study of the Σ3(112) grain boundary in Fe-rich Fe-Cr alloys

We report a first-principles investigation of the Σ3(112) grain boundary (GB) in body-centered cubic Fe-rich Fe1-xCrx alloy as a function of chemical composition and temperature. The equilibrium amount of Cr at the GB undergoes a sharp transition from slight enrichment in low-alloyed Fe-Cr (x ≤ 0.06–0.08) to complete Cr saturation for 0.08 ≤ x ≤ 0.15. The GB chemistry at room-temperature and below is characterized by miscibility gaps, destabilizing a Fe-Cr interfacial solid solution towards decomposition into Fe-rich and Cr-rich clusters. The Cr-rich clusters at the GB may serve as nucleation centers for secondary phases in Fe-Cr alloys.

Effect of grain boundary on the crack-tip plasticity under hydrogen environment: An atomistic study

It has been found that the plasticity is significantly affected by the hydrogen interstitials in metallic materials. However, the underlying physics responsible for the dislocation/hydrogen interactions is still poorly understood. Using molecular dynamics simulations, we study the emission of dislocations from a crack tip in fcc Ni single-crystal and bicrystal samples under a hydrogen environment. The results show that the critical mode-I stress intensity factor (SIF) is reduced due to the presence of hydrogen, but the existence of Σ5 grain boundaries (GBs, with an inclination angle ranging from 0 to π/4) almost does not alter the critical mode-I SIF for dislocation emission, compared with the single-crystal cases. These findings suggest that further large-scale investigations should be conducted to study the influence of various microstructural factors, such as the distance from the crack tip to GB and density of GB as well as the existence of other defects, e.g., voids and inclusions.

Hydrogen Induced Vacancy Clustering and Void Formation Mechanisms at Grain Boundaries in Palladium

Hydrogen has a significant impact on the formation of vacancies, clusters and voids in palladium and other metals. The formation of vacancy-hydrogen complexes in bulk Pd and at Σ3 and Σ5 grain boundaries was investigated using first-principles calculations and thermodynamic models. Equilibrium vacancy and cluster concentrations were evaluated as a function of temperature and hydrogen partial pressure based on the Gibbs energy of formation including vibrational and configurational entropies. Vacancies were found to be significantly stabilized by association with interstitial hydrogen, leading to enhanced concentrations by several orders of magnitude. Vacancy clusters were further stabilized at grain boundaries, with equilibrium concentrations reaching site saturation for clusters comprising up to three vacancies. Nanovoids were investigated based on Wulff constructions from calculated surface energies of the (0 0 1) and (1 1 1) terminations as a function of temperature and coverage of hydrogen adsorbates. The most stable termination changed from (1 1 1) in vacuum to (0 0 1) in H2, and the surface energies were lowered due to hydrogen adsorbates. Consequently, voids were also stabilized in the presence of hydrogen. Coalescing of vacancies into nanovoids was found to be thermodynamically unfavorable due to the loss of configurational entropy. It was therefore concluded that enhanced concentrations of vacancies and clusters does not directly cause the formation of voids. The formation of voids in Pd-based membranes was discussed in terms of microstructural characteristics, and strain due to chemical expansion and plastic deformation.

Effect of twin boundary formation on the growth rate of the GaSb{111} plane

The top surface of a crystal–melt interface was observed directly during directional solidification of polycrystalline GaSb at a constant cooling rate, and the effect of {111}Σ3 twin boundary formation on the rate of grain growth parallel to the 〈111〉 direction was investigated. A {111}Σ3 twin boundary was generated by the decomposition of another {111}Σ3 twin boundary with formation of a Σ9 grain boundary. The growth of the crystal–melt interface in the direction parallel to the <111>B direction was stalled during the nucleation and propagation of the new {111}Σ3 twin boundary. The growth rate of the crystal–melt interface after twin formation was approximately half of that before. This is considered to be due to {111} polarity reversal by twinning. The dangling bond density on the {111}A plane of GaSb is almost ten times of that on the {111}B plane, and the dangling bond density is related to the attachment energy of atoms of the plane; therefore, the growth rate in the <111>A direction would be significantly lower than that in the <111>B direction. These results indicate different growth rates in opposite 〈111〉 directions and a significant effect of the polarity on the competition of grain growth during the directional solidification of polycrystalline GaSb.

Microstructure and microtexture evolution of dynamic recrystallization during hot deformation of a nickel-based superalloy

Effect of strain rate route on the microstructure and microtexture evolution during the dynamic recrystallization (DRX) of a nickel-based superalloy subjected to the isothermal compression tests was investigated using electron back-scatter diffraction. The evolutions of microtexture components and fiber textures and the role of Σ3 boundaries in microstructure on the pole density in the partially and fully DRX processes were explored. The α-fiber in Euler space is regarded as the compression microtexture. The locally organized substructures in the matrix grains are formed on certain {111} slip planes with the high Schmid factor. Although the dislocation-free DRX grains show the random orientated distribution, the weak recrystallization 〈001〉 fiber parallel to the compression axis (ND) is developed at the high strain in the fully DRX processes. Whereas the compression 〈101〉 fiber parallel to ND in the partially DRX processes is sharpened. Moreover, the instantaneous decrease of strain rate leads to the highest fraction of Σ3 grain boundaries, indicating the lowest pole density. Cube-Twin component adjacent to Cube component contributes partly to the formation of Σ3 grain boundaries. The findings provide insights into the microtexture characteristics and enhance the understanding of the orientation dependence of the mechanical behavior of nickel-based superalloys.

Achieving good tensile properties in ultrafine grained nickel by spark plasma sintering

In this work, the tensile properties of ultrafine grain Ni samples processed by spark plasma sintering are investigated. To this aim, a high purity Ni powder was nanostructured by ball milling using different processing conditions, and consolidation of the powder was performed by spark plasma sintering. Milling parameters were varied to produce samples with different microstructures. Each sample was characterised in terms of grain size and grain boundary character distribution to study the influence of milling parameters on the microstructure. Results show that suitable ball milling and subsequent sintering can be employed to obtain specimens with grain sizes in the ultrafine grain range with a high fraction of Σ3 grain boundaries. All samples exhibit a low internal stress state, which is evaluated in terms of grain orientation spread computed from electron backscatter diffraction and dislocation observation by transmission electron microscopy. Uniaxial tensile testing shows an increase in yield strength with grain refinement with a pronounced deviation from the Hall-Petch relation, for samples with grain sizes below 1.2 μm. Heterogeneity in deformation at yielding seems responsible for the deviation due to differences in the strain hardening behaviour between ultrafine grains and conventional coarse grains. All samples display high ductility with values of elongation to fracture above 35% as well as good toughness. The combination of ball milling with spark plasma sintering is therefore a promising tool to produce ultrafine grain Ni with good mechanical properties.

Coexistence of two different atomic structures in the Σ13 pyramidal twin boundary in α-Al2O3

ABSTRACTAn α-Al2O3 bicrystal with a (104)/[110] Σ13 grain boundary was fabricated by joining two pieces of single crystal at 1500°C in an Ar–H2 mixture gas. The resultant grain boundary was characterised by scanning transmission electron microscopy (STEM) at the atomic level. STEM observations revealed that the grain boundary consists of two regions with different atomic structures. These two structures were identified to be the most and second-most stable structures by theoretical calculations and STEM image simulation. The coexistence of two different structures in the Σ13 grain boundary is discussed.

Atomistic simulations of the interaction between transmutation-produced Re and grain boundaries in tungsten

High energy neutron irradiation not only causes the transmutation of tungsten (W), but also induces transmutation elements to segregate and precipitate at grain boundaries. In this work, the segregation of transmutation-produced rhenium (Re) and their clusters at a Σ5(130) grain boundary (GB) in bulk W has been investigated by using both molecular dynamics and molecular statics methods. It is found that, for single interstitial, Re atom diffuses from the bulk to the GB region via a three-dimensional rotation and migration in the form of an interstitial 〈1 1 1〉 Re-W mixed dumbbell, because the rotation and migration barriers of a Re-W dumbbell are both small. When Re atoms are absorbed by the GB, it is noteworthy that Re atoms tend to occupy the substitutional sites near the GB. The segregation energy of interstitial Re is ranged from −7.67 to −6.12 eV, and the effective interaction distance between the GB and Re interstitials is about 7 Å, which increases with increasing the cluster size. By modeling the uniaxial tensile test of W-GB structure containing Re substitution clusters, the fracture strength and elongation of the GB are significantly reduced. The present results provide important insights into the detailed mechanisms of Re segregation at GBs and its possible effects on the mechanical properties of W-based materials.

Effect of Twin Boundary Formation on the Growth Rate of the GaSb{111} Plane

The top surface of a crystal-melt interface was observed directly during directional solidification of polycrystalline GaSb at a constant cooling rate, and the effect of {111}Σ3 twin boundary formation on the rate of grain growth parallel to the direction was investigated. A {111}Σ3 twin boundary was generated by the decomposition of another {111}Σ3 twin boundary with formation of a Σ9 grain boundary. The growth of the crystal-melt interface in the direction parallel to the B direction was stalled during the nucleation and propagation of the new {111}Σ3 twin boundary. The growth rate of the crystal-melt interface after twin formation was approximately half of that before. This is considered to be due to {111} polarity reversal by twinning. The dangling bond density on the{111}A plane of GaSb is almost ten times of that on the {111}B, and the dangling bond density is related to the attachment energy of atoms of the plane; therefore, the growth rate in the A direction would be significantly lower than that in the B direction. These results indicate different growth rates in opposite directions and a significant effect of the polarity on the competition of grain growth during the directional solidification of polycrystalline GaSb.

Study of hydrogen-induced delayed fracture in high-Mn TWIP/TRIP steels during in situ electrochemical hydrogen-charging: Role of microstructure and strain rate in crack initiation and propagation

The initiation and propagation of hydrogen-induced cracks (HICs) in high-Mn twinning/transformation-induced plasticity (TWIP/TRIP) steels were explored by slow strain rate tension test. For TWIP/TRIP steels with 10% phase volume fractions of ε-martensite (PVFM), the initiation of HICs is dominated by dislocation slips at 1 × 10−6 s−1, but, at 1 × 10−5 s−1, it is dominated by deformation twins. For TWIP/TRIP steels with 45% PVFM, inconsistent deformation of ε-martensite promotes HICs initiation. Both Σ3 grain boundary and γ/ε phase interface with Nishiyama-Wassermann orientation could prevents HICs propagation. And ε-martensite with <0001>||RD microscopic orientation at the crack tip is more likely to induce transgranular crack propagation.