Coincidence Site Lattice Research Articles

Development of nanotwins in electroplated copper and its effect on shear strength of tin/copper joint

Cost-effective copper (Cu) electroplating is the primary technique used to fabricate wires/interconnects in microelectronics. Grain microstructure and impurity incorporation in electroplated Cu plays an important role in its mechanical, electrical, and thermal properties. In this study, a specific plating formula with a basic electrolyte and a suppressor, polyethylene glycol (PEG) and chloride ion (Cl−), is used to fabricate the Cu layers. An adjustment of plating current density dramatically alters the grain microstructure and impurity incorporation in the electroplated Cu. Scanning electron microscopy (SEM) and coincidence site lattice (CSL) analyses show that the Cu deposit plated at a low current density (<0.43 A/dm2) exhibits a lamellar nanotwinning structure with a large fraction of Σ3 (60° rotation at 〈111〉) twin boundary. Time-of-flight secondary ion mass spectrometer (TOF-SIMS) analysis shows that the impurity incorporation level in electroplated Cu is significantly reduced with the formation of twinning structure. The Sn/Cu joint benefits greatly from the formation of nanotwins in the electroplated Cu layer due to effective suppression of interfacial voids, which exhibits an improved shear strength as compared with that without the formation of nanotwins.

Atomistic Simulations of Carbon and Hydrogen Diffusion and Segregation in Alfa-Iron Deviant CSL Grain Boundaries

Polycrystalline materials’ mechanical properties and failure modes depend on many factors that include diffusion and segregation of different alloying elements and solutes as well as the structure of its grain boundaries (GBs). Segregated solute atoms to GB can alter the properties of steel alloys. Some of these elements lead to enhancing the strength of steel, on the other hand others can degrade the toughness of steel significantly. It is well known that carbon increases the cohesion at grain boundary. While the presence of hydrogen in steel have a drastic effects including blistering, flaking and embrittlement of steel. In practice during forming processes, the coincidence site lattice (CSL) GBs are experiencing deviations from their ideal configurations. Consequently, this will change the atomic structural integrity by superposition of sub-boundary dislocation networks on the ideal CSL interfaces. For this study, the ideal ∑3 (112) structure and its angular deviations in BCC iron within the range of Brandon criterion are studied comprehensively using molecular statics simulations. The GB and free surface segregation energies of carbon and hydrogen atoms will be quantified. Rice-Wang model is used to assess the strengthening/embrittlement impact variation over the deviation angles.

Effect of grain boundary character distribution on weld heat-affected zone liquation cracking behavior of AISI 316Ti austenitic stainless steel

In this work, the role of grain boundary engineering (GBE) on the weld heat-affected-zone (HAZ) liquation cracking resistance of austenitic stainless steel AISI 316Ti was investigated. Standard wrought-processed alloy 316Ti was cold rolled to 5% strain and subsequently annealed at 1373 K for 30 min to achieve the optimum grain boundary character distribution (GBCD). The GBE samples were found to consist of a significantly higher fraction (72%) of low Ʃ coincident site lattice (CSL) boundaries as compared to the as received (AR) samples (45%). To study the liquation cracking behavior, single-track longitudinal varestraint tests was performed on AR and GBE samples. Microstructural examination revealed that the grain coarsening and the liquation events were less prominent in the GBE samples as compared to the AR samples. It is believed that the reduced segregation of impurities to the special boundaries (coherent Ʃ3 twins, in particular) present in the GBE samples is responsible for the observed improvement in the HAZ liquation cracking resistance.

Effect of Deformation Temperature on Dynamic Recrystallization and CSL Grain Boundary Distribution of Fe-36%Ni Invar Alloy

This paper investigated the effect of deformation temperatures on microstructure evolution and grain boundary character distribution (GBCD) of Fe-36%Ni Invar alloy during hot compression tests. The isothermal hot deformation tests were carried out in a temperature range of 850-1100 °C and a strain rate of 0.01 s−1. The grain boundary character distribution was based on the Σ3 n (n = 1, 2, 3,…) coincidence site lattice (CSL) boundaries. The experimental results show that the extent of dynamic recrystallization significantly expanded at the strain rate of 0.01 s−1 with the increase in temperature and this phenomenon was consistent with the evolution tendency of Σ3 n boundaries. To some degree, the increasing proportion of typical low-ΣCSL boundaries, especially the increasing trend of Σ3, positively promoted the wider process of dynamic recrystallization.

The Evolution of Growth, Crystal Orientation, and Grain Boundaries Disorientation Distribution in Gold Thin Films

AbstractHerein, the growth and annealing effect of gold thin films are systematically studied by using an automated crystal orientation mapping (ACOM–TEM) technique. The crystallographic redistribution and structural changes in grain and boundaries due to recrystallization and grain kinetic processes are followed and described. The as‐grown films show a dominant &lt;111&gt; texture followed by &lt;110&gt;, &lt;100&gt;, &lt;112&gt;, &lt;102&gt;, and &lt;212&gt; orientations; having a noticeable reorientation of the grains after annealing along the &lt;111&gt; direction parallel to the electron beam, as well as the expected grain size enlargement. Moreover, the random high angle grain boundaries observed in as‐grown film transform towards low coincidence site lattice (CSL) boundaries after annealing. The consequences of recrystallization and grain kinetic processes, primarily grain rotation and coupled grain boundary migration on the evolution of the film structural properties are discussed based on the experimental results.

Epitaxial engineering of polar ε-Ga2O3 for tunable two-dimensional electron gas at the heterointerface

We predict the formation of a polarization-induced two-dimensional electron gas (2DEG) at the interface of ε-Ga2O3 and CaCO3, wherein the density of the 2DEG can be tuned by reversing the spontaneous polarization in ε-Ga2O3, for example, with an applied electric field. ε-Ga2O3 is a polar and metastable ultra-wide band-gap semiconductor. We use density-functional theory (DFT) calculations and coincidence-site lattice model to predict the region of epitaxial strain under which ε-Ga2O3 can be stabilized over its other competing polymorphs and suggest promising substrates. Using group-theoretical methods and DFT calculations, we show that ε-Ga2O3 is a ferroelectric material where the spontaneous polarization can be reversed through a non-polar phase by using an electric field. Based on the calculated band alignment of ε-Ga2O3 with various substrates, we show the formation of a 2DEG with a high sheet charge density of 1014 cm−2 at the interface with CaCO3 due to the spontaneous and piezoelectric polarization in ε-Ga2O3, which makes the system attractive for high-power and high-frequency applications.

Effects of Deformation Mode and Strain Level on Grain Boundary Character Distribution of 304 Austenitic Stainless Steel

In this study, the effects of deformation mode (rolling and tension) and strain level on grain boundary character distribution were systematically investigated in 304 austenitic stainless steel. The experimental results showed that the 〈110〉 component parallel to the normal direction orientation and the P(BND) {110}〈111〉 texture were predominant in the rolled specimens and the tensioned ones, respectively. For each mode of deformation, the fraction of low-Σ coincidence site lattice (CSL) boundaries, especially Σ3n (n = 1, 2, 3) boundaries decreased with the increasing strain level after annealing. At a lower strain level, the type of texture played a leading role in grain boundary reconstruction during annealing, and the 〈110〉 component parallel to the normal direction orientation facilitated the formation of low-Σ CSL boundaries during annealing compared with the P(BND) texture. However, for a higher strain level, the stored energy became dominant in grain boundary reconstruction during annealing, and a large stored energy was detrimental to the formation of low-Σ CSL boundaries, which resulted in a higher fraction of low-Σ CSL boundaries in the tensioned specimen than that in the rolled one after annealing.

First-principles investigation of grain boundary morphology effects on helium solutions in tungsten

Helium solutions at eight symmetric tilt grain boundaries (GBs) in tungsten (W) have been investigated through first-principles calculations. The GBs are constructed by the coincidence site lattice (CSL) model and all interstitial sites at GBs are identified with convex deltahedra. For each GB, helium atom prefers to dissolve in interstitial sites with the lowest charge density. Interactions between the helium atom and the GBs atoms are rather localized for all eight GBs studied here, and the helium solution explicitly relates to the local environment of the interstitial site. It is found out that the helium solution energies decrease as the Voronoi volumes of interstitial sites increase, and they can be quantitatively determined by helium solution energy in tungsten clusters Wn. Based on this quantitative relationship, it is easy to estimate the helium solution energy in various interstitial sites in tungsten without massive first-principles calculations. Our result provides a sound theoretical guide to design favorite grain boundaries to suppress helium segregations at GBs.

Grain Boundary Characteristics Optimization of 90Cu–10Ni Copper-Nickel Alloy for Improving Corrosion Resistance

The grain boundary character distribution and corrosion resistance of a 90% Cu 10% Ni alloy were investigated. When the rolling reduction was increased, the fraction of coincidence site lattice (CSL) grain boundaries with low-Σ index increased initially, and decreased thereafter. The highest fraction of low-Σ CSL boundaries (70%) was obtained through 9% secondary rolling reduction. Electrochemical testing indicated that the corrosion resistance improved significantly after intermediate deformation (7% and 9% reduction ratios) and high-temperature annealing. This improvement was attributed to an increase in the fraction of low-Σ CSL boundaries and the formation of triple junctions during recrystallization.

Nanoscale Intergranular Corrosion and Relation with Grain Boundary Character as Studied In Situ on Copper

The initiation of intergranular corrosion at various types of grain boundaries (GBs) was studied at the nanometer scale on microcrystalline copper in 1 mM HCl aqueous solution. In situ Electrochemical Scanning Tunneling Microscopy (ECSTM) and Electron Back-Scatter Diffraction analysis of the same local microstructural region were combined using an innovative methodology including micro marking performed with the STM tip. The results demonstrate that electrochemically-induced intergranular dissolution, at the surface termination of GBs, is dependent on the grain boundary character. It is found that random high angle boundaries as well as Σ9 coincidence site lattice (CSL) boundaries are susceptible to nanoscale initiation of intergranular corrosion while for Σ3 CSL boundaries the behavior is dependent on the deviation angle of the GB plane from the exact orientation. For the Σ3 twins, a transition from resistance to susceptibility occurs between 1° and 1.7° of deviation as a result of the increase of the density of steps (i.e. misorientation dislocations) in the coincidence boundary plane. The work emphasizes the precision needed in the design of the grain boundary network in applications where intergranular corrosion or its initiation must be controlled at the nanoscale.

Three dimensional modelling of grain boundary interaction and evolution during directional solidification of multi-crystalline silicon

The development of grain structures during directional solidification of multi-crystalline silicon (mc-Si) plays a crucial role in the materials quality for silicon solar cells. Three dimensional (3D) modelling of the grain boundary (GB) interaction and evolution based on phase fields by considering anisotropic GB energy and mobility for mc-Si is carried out for the first time to elucidate the process. The energy and mobility of GBs are allowed to depend on misorientation and the GB plane. To examine the correctness of our method, the known the coincident site lattice (CSL) combinations such as (∑a+∑b→∑a×b) or (∑a+∑b→∑a/b) are verified. We frther discuss how to use the GB normal to characterize a ∑3 twin GB into a tilt or a twist one, and show the interaction between tilt and twist ∑3 twin GBs. Two experimental scenarios are considered for comparison and the results are in good agreement with the experiments as well as the theoretical predictions.

Grain-boundary type and distribution in silicon carbide coatings and wafers

Silicon carbide is the main diffusion barrier against metallic fission products in TRISO (tristructural isotropic) coated fuel particles. The explanation of the accelerated diffusion of silver through SiC has remained a challenge for more than four decades. Although, it is now well accepted that silver diffuse through SiC by grain boundary diffusion, little is known about the characteristics of the grain boundaries in SiC and how these change depending on the type of sample. In this work five different types (coatings and wafers) of SiC produced by chemical vapor deposition were characterized by electron backscatter diffraction (EBSD). The SiC in TRISO particles had a higher concentration of high angle grain boundaries (aprox. 70%) compared to SiC wafers, which ranged between 30 and 60%. Similarly, SiC wafers had a higher concentration of low angle grain boundaries ranging between 15 and 30%, whereas TRISO particles only reached values of around 7%. The same trend remained when comparing the content of coincidence site lattice (CSL) boundaries, since SiC wafers showed a concentration of more than 30%, whilst TRISO particles had contents of around 20%. In all samples the largest fractions of CSL boundaries (3 ≤ Σ ≤ 17) were the Σ3 boundaries. We show that there are important differences between the SiC in TRISO particles and SiC wafers which could explain some of the differences observed in diffusion experiments in the literature.

Differentiating between intergranular and transgranular fracture in polycrystalline aggregates

The competition between intergranular (IG) and transgranular (TG) fracture in fcc polycrystalline aggregates with physically representative GB misorientation distributions comprised of random low-angle, random high-angle, and coincident site lattice (CSL) GBs has been investigated. Physically-based critical conditions for IG fracture, due to the formation of dislocation pileups, and TG fracture, due to the propagation of cracks on cleavage planes, were coupled to a dislocation-density-based crystal plasticity formulation and a computational fracture scheme for crack branching to investigate how dislocation–GB interactions influence dislocation transmission, pileup formation, and local failure modes. The predictions indicate that aggregates with a large fraction of random and CSL high-angle GBs are dominated by IG fracture, as low GB transmission leads to extensive dislocation-density pileup formation and localized stress accumulations that induce IG fracture. Aggregates with a majority of low-angle GBs are dominated by TG failure, which is consistent with experimental observations. This investigation provides a fundamental understanding of the physical mechanisms governing IG and TG fracture in polycrystalline aggregates.

Correlation of grain boundary extra free volume with vacancy and solute segregation at grain boundaries: a case study for Al

Grain boundary extra free volume (GB EFV) can be considered as fundamental microstructural parameter for polycrystalline or nano-crystalline materials. Here, we present a systematic first principles study on a group of representative symmetric tilt grain boundaries of Al with various EFVs subjected to vacancy formation and Mg segregation. All grain boundaries were constructed using the coincident site lattice (CSL) and the structural unit (SU) models. It was found that the SU model is superior to the CSL in describing FCC-Al GBs, the same as we previously revealed for BCC-Fe. The predicted relation between GB misorientation angle and EFV, and the predicted EFV criteria for a stable GB, both agree with available experimental observations. Vacancy formation and Mg segregation show stronger preference to those GBs with high EFV values, due to the resultant high levels of atomic disorder. These findings not only provide a new, atomistic perspective on the significance of EFV, but also suggest a viable means of predicting GB properties based on direct experimental characterisation of GB EFVs.

Effects of annealing on grain-boundary character distribution and texture evolution in hot-rolled Fe-6.5 wt% Si steel

In this paper, the effects of annealing on grain-boundary character distribution and texture evolution were investigated in hot-rolled Fe-6.5 wt% Si steel. An annealing treatment performed at different temperatures affected the recrystallization behavior and changed the volume fraction of low-angle and coincidence site lattice (CSL) boundaries. High frequencies of coincidence boundaries Σ3, Σ9, Σ13, and Σ27 were observed in all annealed samples, while the annealing temperature can lead to a large difference in frequencies for Σ13. The effect of crystallographic texture on the directional dependence of the specific magnetic energy in the rolling plane of investigated sheets was calculated and discussed. These results demonstrate the possibility of optimizing grain-boundary character distribution (GBCD) and texture to improve the magnetic and mechanical properties of Fe-6.5 wt% Si steel by controlling annealing temperature.

Texture Inheritance on Phase Transition in Low-Carbon, Low-Alloy Pipe Steel after Thermomechanical Controlled Processing

Orientation microscopy (electron back scatter diffraction, EBSD) is used to investigate the structural and textural states of low-carbon, low-alloy pipe steel (resembling 06Г2MБ steel) after thermomechanical controlled processing (TMCP): heating to 1000°C with subsequent quenching in water; isothermal quenching with holding at 300°C; and slow cooling in the furnace. The heat treatment is associated with double phase recrystallization: α → γ → aht, where aht is martensite, bainite, or ferrite. The texture obtained after TMCP is mainly formed by two strong scattered orientations from {112}〈110〉and two weaker scattered orientations close to {110}〈223〉. Despite the double phase recrystallization, the main crystallographic orientations of the bainite after TMCP and after isothermal quenching are the same. That indicates structural and textural inheritance in the material. The structures obtained after other thermal treatments of the structure (both martensite and ferrite) also include complex multicomponent textures, which are nevertheless distinct. Some of the main textural components of martensite and ferrite are the same as bainitic components. All the structures after heat treatment have a similar spectrum of large-angle boundaries, with strongly expressed boundaries of the coincidence site lattices (CSL): Σ3, Σ11, Σ25b, Σ33c, and Σ41c. The orientations forming the texture of all the structures obtained are related to the main orientation of the deformed austenite grains formed on hot rolling in TMCP, in accordance with orientation relations intermediate between the Kurdjumov–Sachs and Nishiyama–Wasserman types. In all cases, the orientation relationship of the textural components of the initial material and the structure obtained by heat treatment may be explained in terms of the onset of phase transformations (both shear and diffusional transition) at crystallographically determined boundaries (including special boundaries) similar to the CSL boundaries Σ3 and Σ11.

Grain boundary engineering for control of tellurium diffusion in GH3535 alloy

The effect of grain boundary engineering (GBE) on the Te diffusion along the surface grain boundaries was investigated in GH3535 alloy. It can be found that GBE treatment increases obviously the fraction of low-Σ coincidence site lattice (CSL) boundaries, especially the Σ3 ones, and introduces the large-size grain clusters. When the as-received (AR) and GBE-treated (GBET) specimens were exposed to Te vapor, only Σ3 boundaries were found to be resistant to Te diffusion. From the cross section and the surface, the fewer Te-attacked grain boundaries and the thinner corrosion layer can be observed in the GBET sample. The improvement of resistance to Te diffusion in the GBET sample can be attributed to the large size grain-clusters associated with high proportion of the Σ3n boundaries.

Tracking the Evolution of Annealing Textures from Individual Deformed Grains in a Cross-Rolled Non-oriented Electrical Steel

The evolution of microtexture and microstructure of a cross-rolled 0.88 wt pct Si non-oriented electrical steel was investigated using a quasi-in situ electron backscatter diffraction (EBSD) technique, where individual deformed grains with various initial orientations were tracked during annealing at the same temperature for different times. The textures recrystallized from different deformed grains were compared, and the observations were examined against the preferential nucleation and selective growth theories. Although the cold deformed 〈111〉//ND (normal direction) grains recrystallized first during annealing, they started with significantly different nucleation textures, i.e., γ-fiber (〈111〉//ND) in {111}〈112〉 deformed grains, and cube ({001}〈100〉) in {111}〈110〉 deformed grains. Both recrystallization textures were quite stable until the steel was completely recrystallized. Significant grain growth in these grains was only observed after the recrystallization was complete, which resulted in considerably different final textures as compared to the initial nucleation textures. Deformed grains with a rotated cube ({001}〈111〉) orientation were the last to recrystallize, and the recrystallization was accomplished mainly through the “invading” of neighboring grains into the deformed matrix. Analysis of the misorientations between the rotated cube grain (the matrix) and their neighboring recrystallized grains showed that the preferred growth of some of the grains can be attributed to the high grain boundary mobility associated with the coincident site lattices (CSL). During the course of recrystallization, some $$ \{ 11{\text{h}}\} \left\langle {12\frac{1}{h}} \right\rangle $$ and rotated cube grains also formed, but they disappeared quickly when the annealing time was increased.

Microstructure and texture evolution of electrodeposited coatings of nickel in the industrial electrolyte

Nickel is an important strategic metal material, which has been widely used in modern industry. The different electrolyte systems and main parameters have a great effect on microstructure and mechanical properties of electrodeposited coatings. In order to investigate the evolution of texture and preferential orientation of nickel electro-crystallization in sulfide nickel soluble anode/sulfate solution, and grain size, microstructure of electrodeposited coatings of nickel were characterized by electrochemical workstation, XRD and SEM methods.The results show that nickel electro-crystallization conforms to the classical nucleation theory and crystal growth theory. The preferential orientation of electrodeposited coatings of nickel can be changed initially from (111) and (200) planes to (220) plane after 16h. The surface morphology changes slowly from pyramid to cellular structure, while the growth mechanism changes from screw dislocation growth to cumulative growth model. The cross sectional microstructure of electrodeposited coatings is non-uniform and mostly grains are columnar and perpendicular to the sheet. The distribution of grains abides by stepwise distribution along the thickness. The microstructure of electrodeposited coatings is fine equiaxed grain region and coarse columnar grain region, which are perpendicular to starting sheet when the electrodeposition time is 144h. Angle grain boundary is related to grain size, whereas the coincidence site lattice (CSL) is associated with preferential orientation. Therefore the grain boundary misorientation are mainly above 15°, being more than angle grain boundaries and coincidence site lattice Σ3 appearing highest frequency.

Electrochemical Scanning Tunnelling Microscopy Study of Early Stages of Intergranular Corrosion of Copper

Electrochemical scanning tunnelling microscopy (EC-STM) was used to investigate in situ and locally the topographical alterations of copper surfaces under electrochemical control. The aim of this work was to better understand the early stages of intergranular corrosion and the relationship between local structure of grain boundaries emerging at the solid/liquid interface and corrosion properties. A prerequisite for this approach is the use of a high purity material, in order to exclude the segregation of impurities, such as sulphur, at grain boundaries, which would strongly affect the intergranular corrosion behaviour and prevent us from studying the intrinsic effect of grain boundary structure This was achieved in this work by using high purity cast electrolytic tough pitch (ETP-) Cu, cryogenic rolled and post-annealed at relatively low temperature (200°C). This process also produces a grain size compatible with the limited field of view of STM, as confirmed by electron back scatter diffraction (EBSD) analysis. EC-STM data obtained in situ after a surface preparation producing oxide-free surfaces with emerging grain boundaries show that performing cycles of anodic dissolution in the absence of any surface oxide reveals a grain boundary structure-dependent corrosion behaviour. Localized attack is initiated at grain boundaries that can be assigned to either random-type boundaries or non-coherent coincidence site lattice (CSL) grain boundaries whereas, under the same conditions, coherent twin grain boundaries are not attacked. Direct coupling of EC-STM and EBSD, using a re-positioning procedure allowing the analysis of the same local surface area by both techniques, show that even for the corrosion resistant coherent twin grain boundaries, the intergranular corrosion behavior measured at the emergence of the grain boundaries at the solid/liquid interface is dependent on the local surface structure. Further combined EC-STM and EBSD analysis will be discussed to link grain boundary type, local surface structure and susceptibility to intergranular corrosion.