Copper Single Crystals Research Articles

A molecular dynamics study on microscopic mechanism of nano-polished single crystal copper

Molecular dynamics simulations are performed to investigate the microscopic mechanism of diamond abrasive grain (DAG) polishing on the surface of single crystal copper (SCC). In this paper, the polishing parameters including the incident angle and the polishing force applied to the DAG are varied to analyze the surface morphology, friction and subsurface damage state of the workpiece. The polishing process on scratched SCC surface was also investigated. It is found that variations in the incident angle and the polishing force have significant effects on the swarf formation and internal structure of the workpiece. At small incident angle, unstable DAG motion and obvious friction fluctuations is observed. As the incident angle becomes larger, the sliding of the DAG on the workpiece surface becomes stable, and the friction force undergoes transition from the rapid growth to the stabilization phase. However, the fluctuation of dislocation density increases with the incident angle. As the polishing force increases, the sliding distance required to reach the peak of dislocation density becomes longer. In addition, the friction as well as the dislocation density show different characteristics on the scratched surface compared to the smooth surface.

Analysis of mechanical properties of single crystal copper under low-temperature indentation test

Abstract As a common superconducting material in the aerospace field, the mechanical properties of single-crystal copper at low temperatures have attracted much attention. Combined with the current low-temperature nanoindentation test technology and the research status of micromechanical properties of crystal copper at home and abroad, this paper puts forward the low-temperature nanoindentation test on single-crystal copper. It analyzes the changes in mechanical properties of the low-temperature environment by indentation curve. Before the formal test, we first analyzed the Oliver-Pharr micro and nanoindentation test theory and described briefly the advantages of using nanoindentation to test the mechanical properties. The final test results show that in the temperature range of 300K~150K, the hardness and elastic modulus of the single-crystal copper increase with the decrease in temperature. The lower the temperature is, the faster the increase of hardness and elastic modulus is. At the same temperature, the hardness and elastic modulus of single-crystal copper with three crystal faces are (111) > (110) > (100). The mechanical properties at low temperatures are effectively reflected by means of low-temperature nanoindentation, which can be used to guide the production of materials in real life.

Cell structure development during cyclic deformation of near-[001] and near-[011] copper single crystals

Dislocation cell structures significantly influence the fatigue failure process. Research on the cell structures in multiple-slip-oriented copper single crystals has focused primarily on the [1‾11] orientation, whereas formation mechanisms and development processes of cell structures in [001] and [011] copper single crystals remain unclear. Therefore, this study examines the dislocation structures of near-[001] and near-[011] copper single crystals under cyclic deformation at high strain amplitudes. The cyclic stress–strain curve of the [011] crystals showed a distinct plateau at plastic shear strain amplitudes ≤7.5 × 10−3. In the plateau region, the dislocation structures consisted of cell structures along the (111) primary slip plane and matrix wall structures. Furthermore, (111) cell structures were formed in the single-slip deformation bands (DBs). After the plateau region, several types of new DBs could be observed due to the activation of multiple-slip systems. Multiple-slip DBs have a higher degree of plastic strain concentration than single-slip DBs, leading to the preferential formation of cell structures. For the near-[001] crystals, (111) cell structures were uniformly formed throughout the matrix, without the formation of DBs. The formation mechanism of the (111) cell boundary was independent of the stress axis and resulted from the interaction between the primary and coplanar slip systems.

Growth of copper single crystal using cone die Czochralski method

Copper (Cu) and other metal single crystals are useful as substrates for the deposition of atomic layer materials for many electronic applications, but the growth of large-sized single crystals is difficult to achieve. Characteristics of the metal material, namely seed elongation and intense cooling radiation at high temperatures during crystal growth, are the main challenges encountered when growing ingots with large diameters. These problems can be resolved by optimizing the crystal growth parameters. By adjusting the shoulder formation angle of the ingot shape to approximately 20° to 40°, we are able to grow a large (1-inch diameter, 30 mm length) single crystal of metal Cu using the Czochralski (CZ) method. However, the generation of suspended solids and film impurities such as reactants and precipitates and their nucleation, growth, and solidification, limited the further increase in size of the Cu crystal. Using the cone-shape die CZ (CD-CZ) method solves this problem and a 2-inch diameter Cu single crystal is successfully grown. This is the world’s largest single crystal metal grown using this method and it paves the way for the growth of other metal crystals.

Experimental Research on Pollution-Free Alcohol Cutting Fluid in Scratching of Single-Crystal Copper Material

Cutting fluid can improve the heat dissipation and lubrication in the cutting process and thus increase the machining quality. In this work, a pollution-free alcohol solution was proposed as the cutting fluid in an ultra-precision cutting process to explore green cutting fluids. The scratching experiments were conducted with the alcohol cutting fluid to study its effect on the cutting process. It is found that the use of an alcohol cutting fluid, on average, reduces the tangential and normal force about 27–53%, but exhibits few effects on the friction coefficient in the cutting process. Compared to dry cutting, the alcohol cutting fluid reduces the exposed shear slip steps on the outside surface of the chip, which implies the decreased chip deformation degree of workpiece material in the cutting process. The alcohol cutting fluid can reduce microburrs and decrease the machined surface roughness Ra from 21 nm to 9.9 nm in the ultra-precision turning application on single-crystal copper material.

Hole-Carrier-Dominant Transport in 2D Single-Crystal Copper.

In 2D noble metals like copper, the carrier scattering at grain boundaries has obscured the intrinsic nature of electronic transport. However, it is demonstrated that the intrinsic nature of transport by hole carriers in 2D copper can be revealed by growing thin films without grain boundaries. As even a slight deviation from the twin boundary is perceived as grain boundaries by electrons, it is only through the thorough elimination of grain boundaries that the hidden hole-like attribute of 2D single-crystal copper can be unmasked. Two types of Fermi surfaces, a large hexagonal Fermi surface centered at the zone center and the triangular Fermi surface around the zone corner, tightly matching to the calculated Fermi surface topology, confirmed by angle-resolved photoemission spectroscopy (ARPES) measurements and vivid nonlinear Hall effects of the 2D single-crystal copper account for the presence of hole carriers experimentally. This breakthrough suggests the potential to manipulate the majority carrier polarity in metals by means of grain boundary engineering in a 2D geometry.

Effect of vacuum diffusion bonding on the mechanical and conductive properties of bonded bulk copper single crystals

Recrystallization-free atomic-level (AL) bonding between bulk copper single crystals was achieved by combining chemical mechanical polishing (CMP) and vacuum diffusion bonding. Under various bonding conditions, the bonded copper specimens can be classified into three categories: AL-bonded, HH-bonded, and L-bonded specimens. AL-bonded copper single crystals were obtained by performing bonding under 20 MPa at 600 °C for 60 min, achieving the mechanical and electrical properties that are closest to those of the parent copper single crystal. The AL-bonded interface is characterized by an asymmetrically tilted sub-grain boundary with a 2° misorientation and consists of two sets of edge dislocations with mutually perpendicular Burgers vectors. The formation of the AL-bonded interface depends on two critical factors: the creation of ultra-flat bonded surfaces and performing diffusion bonding within the elastic deformation range. Achieving an ultra-flat surface that eliminates residual stress allows for bonding under relatively low pressure. With a moderate bonding temperature and holding time, it is possible to simultaneously avoid the formation of voids and recrystallizations at the bonding interface, thereby achieving a high-quality AL-bonded interface between bulk copper single crystals.

Atomic sawtooth-like metal films for vdW-layered single-crystal growth

Atomic sawtooth surfaces have emerged as a versatile platform for growth of single-crystal van der Waals layered materials. However, the mechanism governing the formation of single-crystal atomic sawtooth metal (copper or gold) films on hard substrates (tungsten or molybdenum) remains a puzzle. In this study, we aim to elucidate the formation mechanism of atomic sawtooth metal films during melting–solidification process. Utilizing molecular dynamics, we unveil that the solidification of the liquid copper initiates at a high-index tungsten facet with higher interfacial energy. Subsequent tungsten facets follow energetically favourable pathways of forming single-crystal atomic sawtooth copper film during the solidification process near melting temperature. Formation of atomic sawtooth copper film is guaranteed with a film thickness exceeding the grain size of polycrystalline tungsten substrate. We further demonstrate the successful growth of centimeter-scale single-crystal monolayer hexagonal boron nitride films on atomic sawtooth copper films and explore their potential as efficient oxygen barrier.

Influence essential of current on electrochemical plasticization of single-crystal copper

Despite the long-standing identification of electrochemical plasticization (EP) in the metal surface layer, the mechanisms governing the impact of electric current density on EP remain incompletely understood. This study investigates the effect of current density (ranging from 0 mA cm−2 to 10.0 mA cm−2) on EP during electrochemical cold drawing of single-crystal copper in a 0.35 mol L−1 dilute H2SO4 electrolyte through microstructural characterizations and reactive force field molecular dynamic simulations. The results indicate that with increasing density, more atoms in the high-energy {110} and {001} planes in the copper surface are selectively dissolved, causing the atoms in the low-energy uncorroded {111} planes to become more loosened. This activates more abundant dislocations in a multi-slip way, thereby facilitating dislocation entanglements to reconfigure into movable short-range wavy dislocations and decreasing dislocation density through reactions, ultimately resulting in improved EP. However, when the density exceeds 6.7 mA cm–2, an opposite effect is observed due to the dissolution of fewer atoms resulting from enhanced local passivation.

Unique damage process in micro-sized copper single crystal with double-slip orientation in response to near-[112] tension-compression fatigue

In order to clarify the fatigue process for a micro-sized copper (Cu) single crystal with a double-slip orientation, a tension-compression cyclic loading test was carried out with in situ FE-SEM observation. Although the primary slip system B4 finally brought about a crack from eminent intrusions, the process was complex due to characteristic interactions among multiple slip systems. The process was approximately divided into three stages; early slipping, cave-in and damaging. In the early stage, two slip systems with high Schmid factor (SF), B4 and C1, worked at first, however both immediately stopped working. Extrusions/intrusions were caused by the survived slip system B5 (third-highest SF). In the second stage, a triangular depression was brought about by the cross-slip from B5 to C5 and localized wavy undulation was formed by the continuous cross slips. In the final stage, though both of B4 and B5 worked, the slip system with the highest SF, B4, dominated to form the eminent extrusions/intrusions, resulting in the fatigue cracking. The applied resolved shear stress amplitude was a few megapascals, indicating significantly lower fatigue strength than that of the bulk counterpart while it was in same range for that of micro-sized Cu with a single slip orientation. The complex fatigue damage morphology of the specimen was explained by accounting for the interaction of dislocations between the slip systems.

Orientational alignment of semiconducting carbon nanotubes by the parallel steps of high-index copper foils

Numerous research efforts have focused on the deposition and morphology control of semiconducting single-walled carbon nanotubes (s-SWCNTs) to exploit their extraordinary properties to fulfill in building next-generation electronics. However, it is difficult to realize s-SWCNT arrays at the controlled densities and alignments desired for high-performance devices. Here, we developed a novel approach to control the morphology and alignment of s-SWCNTs. Random s-SWCNT networks came into being well-aligned s-SWCNT arrays (within the alignment of 16.9°) along with the recrystallization of copper foils from polycrystalline to single-crystalline. Grain boundary migration induced motion and alignment of s-SWCNTs. In this way, high-quality s-SWCNTs were assembled into well-aligned CNT arrays on single-crystal high-index copper foils. Furthermore, theoretical calculations confirm that the strong binding energy between the s-SWCNTs and the copper edge steps leads to the orientational alignment of the s-SWCNTs on the copper surface.

Atomic simulation study on the effect of defects on nano-cutting mechanism of single crystal copper

Due to the high surface roughness of the parts produced by additive manufacturing (AM), the subsequent mechanical processing is required. The combination of additive manufacturing and micro/nano processing is the development trend of hybrid manufacturing in the future. In this work, the molecular dynamics simulation is used to study the effect and mechanism of different types of defects on copper cutting performance, including pore, Cr precipitate and diamond inclusion. The results show that the presence of defects leads to a decrease in the quality of the machined surface, while pore and diamond inclusion could lead to the appearance of deep holes on the machined surface. The phenomenon of sticking knife caused by stress concentration and high temperature at the tool tip is observed during the simulation in the model with Cr precipiate. The addition of defects causes a change in the horizontal cutting force. The highest force emerges during the simulation in the model with diamond inclusion. Defects in the copper substrate could reduce surface quality. Precipitate and inclusion could increase the stress near the tool so as to exacerbate tool wear and reduce tool life. Therefore, it’s necessary to decrease the number of defects in the matrix and to improve its cutting resistance.

Study on the mechanism of influence of water molecule cooling and vacuum cooling on the machining of single crystal copper nanos

Aiming to clarify the influence mechanism of water molecule cooling on the machining of single-crystal Cu nanocutting, this study conducts an in-depth investigation through molecular dynamics simulations, incorporating both qualitative and quantitative analyses. The focal points of the research include the distribution of Von Mises stress, shear strain, cutting temperature, and dislocation formation. In terms of Von Mises stress, water molecule cooling significantly reduces the average stress at a cutting speed of 10 Å, and exhibits a turning point in stress reduction at a speed of 300 m/s. The average stress under water molecule cooling is consistently lower than that under vacuum cooling, and the difference diminishes at a cutting speed of 20 Å. Shear strain analysis reveals that at a cutting speed of 10 Å, the chip height under water molecule cooling is notably lower than that under vacuum cooling, while at 15 Å, it shows a trend of first decreasing and then increasing. The average chip height under vacuum cooling is 1.1 times that of water molecule cooling. At a cutting depth of 15 Å, speeds of 200 m/s and 300 m/s both demonstrate superior cooling performance, with cutting temperatures significantly lower than vacuum cooling, with temperature differences of approximately 85K and 180K, respectively. Dislocation analysis indicates that under water molecule cooling, the number of dislocations at greater cutting depths is significantly higher than under vacuum cooling, predominantly consisting of 1/6<112> (Shockley) dislocations. Overall, water molecule cooling shows significant advantages in several aspects, significantly reducing stress, chip height, and cutting temperature, while increasing the number of dislocations. These results provide quantified empirical data for ultra-precision nanocutting machining, offering important references for engineering practice and positively contributing to the optimization of nanocutting process parameters.

Corrosion Resistance of Atomically Thin Graphene Coatings on Single Crystal Copper

Designing minimally invasive, defect-free coatings based on conformal graphene layers to shield metals from both abiotic and biotic forms of corrosion is a persistent challenge. Single-layer graphene (SLG) grown on polycrystalline copper (PC-Cu) surfaces often have inherent defects, particularly at Cu grain boundaries, which weaken their barrier properties and worsen corrosion through grain-dependent mechanisms. Here, we report that an SLG grown via chemical vapor deposition (CVD) on Cu (111) single crystal serves as a high-performance coating to lower corrosion by nearly 4–6 times (lower than bare Cu (111)) in abiotic (sulfuric acid) and microbiologically influenced corrosion (MIC) environments. For example, the charge transfer resistance for SLG/Cu (111) (3.95 kΩ cm2) was 2.5-fold higher than for bare Cu (111) (1.71 kΩ cm2). Tafel analysis corroborated a reduced corrosion current (42 ± 3 µA cm−2) for SLG/Cu (111) compared to bare Cu (111) (115 ± 7 µA cm−2). These findings are consistent with the results based on biofilm measurements. The SLG/Cu (111) reduced biofilm formation by 3-fold compared to bare Cu (111), increasing corrosion resistance, and effectively mitigating pitting corrosion. The average depths of the pits (3.4 ± 0.6 µm) for SLG/Cu (111) were notably shallower than those of bare Cu (111) (6.5 ± 1.2 µm). Surface analysis of the corrosion products corroborated these findings, with copper sulfide identified as a major component across both surfaces. The absence of grain boundaries in Cu (111) resulted in high-quality SLG manifesting higher barrier properties compared to SLG on PC-Cu. Our findings show promise for using the presented strategy for developing durable graphene coatings against diverse forms of corrosion.

On twinning and shear banding in copper single crystals with rolling texture orientations plane strain compressed at high strain rate

Crystal lattice rotations developed in copper single crystals with (110)[1–1–2](Br), (110)[001](G), (112)[11–1](C), and S (346)[63–5](S) orientations have been examined to assess the role of an impact loading on twinning, deformation banding and shear banding. The single-crystalline specimens were channel-die compressed to logarithmic strains of 0.9 with a strain rate of 4.7 × 105 s−1. The microstructures were characterised using scanning (SEM) and transmission (TEM) electron microscope, whereas electron backscattered diffraction (EBSD) in SEM and X-ray diffraction have been applied to evaluate the texture evolution. The results showed that a very high strain rate promoted the formation of deformation twins in all orientations. However, a strong variation in the twin density and number of twin families was observed depending on the orientation of the single crystal. In the Br- and G-oriented crystals, only single needle-shaped twins were observed, whereas in the C- and S-oriented crystals, the formation of compact clusters of deformation twins of two generations on different twinning planes was identified. SEM/EBSD measurements revealed that deformation bands were formed in the crystallites with G and Br orientations, whereas shear bands (SBs) were observed in the C- and S-oriented crystals. This study demonstrates that the rotation of twin and matrix platelets in narrow areas of SBs developed in C- and S-oriented crystallites, combined with deformation twinning in the re-oriented primary matrix platelets can result in the formation of texture components near the {110}<001> orientation. Finally, a crystallographic model of the formation of SBs in twinned structures of face-centred cubic metals deformed at high strain rates is proposed.

Forest hardening and Hirth lock during grinding of copper evidenced by MD simulations

Through the use of molecular dynamics (MD) simulation, grinding process of a single crystal copper with two scratch configurations (i) near spacing (NS) between adjacent scratches, and (ii) far spacing (FS) between adjacent scratches were simulated and compared to the control samplei.e.,a single scratch (SS). FS configuration revealed the highest material removal, whereas NS configuration showed that the material removal is affected by various types of defects in the sub-surface which include FCC intrinsic stacking fault, a coherent twin boundary next to an intrinsic stacking fault and two adjacent intrinsic stacking faults. The formation of a Stair-rod 1/6 〈110〉 due to the reaction between two Shockley partial dislocations 1/6 〈112〉 was seen as a distinct feature of the NS configuration which forms the onset of hardening.

Comparison of Linear and Nonlinear Twist Extrusion Processes with Crystal Plasticity Finite Element Analysis.

The mechanical characteristics of polycrystalline metallic materials are influenced significantly by various microstructural parameters, one of which is the grain size. Specifically, the strength and the toughness of polycrystalline metals exhibit enhancement as the grain size is reduced. Applying severe plastic deformations (SPDs) has a noticeable result in obtaining metallic materials with ultrafine-grained (UFG) microstructure. SPD, executed through conventional shaping methods like extrusion, plays a pivotal role in the evolution of the texture, which is closely related to the plastic behavior and ductility. A number of SPD processes have been developed to generate ultrafine-grained materials, each having a different shear deformation mechanism. Among these methods, linear twist extrusion (LTE) presents a non-uniform and non-monotonic form of severe plastic deformation, leading to significant shifts in the microstructure. Prior research demonstrates the capability of the LTE process to yield consistent, weak textures in pre-textured copper. However, limitations in production efficiency and the uneven distribution of grain refinement have curbed the widespread use of LTE in industrial settings. This has facilitated the development of an improved novel method, that surpasses the traditional approach, known as the nonlinear twist extrusion procedure (NLTE). The NLTE method innovatively adjusts the channel design of the mold within the twist section to mitigate strain reversal and the rotational movement of the workpiece, both of which have been identified as shortcomings of twist extrusion. Accurate anticipation of texture changes in SPD processes is essential for mold design and process parameter optimization. The performance of the proposed extrusion technique should still be studied. In this context, here, a single crystal (SC) of copper in billet form, passing through both LTE and NLTE, is analyzed, employing a rate-dependent crystal plasticity finite element (CPFE) framework. CPFE simulations were performed for both LTE and NLTE of SC copper specimens having <100> or <111> directions parallel to the extrusion direction initially. The texture evolution as well as the cross-sectional distribution of the stress and strain is studied in detail, and the performance of both processes is compared.

Interfacial reaction of Sn-1.5Ag-2.0Zn low-silver lead-free solder with oriented copper

High density packaging technology reduces the pad size and the number of grains contained in the pad. When the polycrystalline pad turns into a single crystal pad, the grain orientation has an important impact on the formation of the intermetallic compound (IMC) at the interface. The growth of IMC at the interface between the solder and the single-crystal copper substrate is investigated by selecting the prospective Sn-1.5Ag–2Zn as the solder alloy. Sn-1.5Ag-2.0Zn lead-free solder joints soldered with single crystal (111) copper substrate and polycrystalline red copper substrate are reflowed at 250 °C for 5 min. Samples are subsequently aged at 160 °C. The uneven scallop like Cu6Sn5 IMC layer grows rapidly when the alloy solder contacts with the copper substrate. The Cu6Sn5 grain size formed on the surface of single crystal copper is larger than that of polycrystalline copper. Single crystal copper has no grain boundary to block atomic diffusion, which affects grain nucleation and growth. The growth rate of Cu6Sn5 formed by alloy solder and the single crystal (111) copper solder joint after aging treatment at 160 °C for 20 h is about twice that of the polycrystalline copper solder joint. Then it grows slowly with the increase of aging treatment time. The thick layer Cu6Sn5 breaks due to crack diffusion after 600 h of aging treatment, and the thickness of IMC remains at 3.5 μm. Cu5Zn8 generated at the solder and polycrystalline copper solder joint during aging treatment acts as a barrier layer, preventing the solder from contacting the copper substrate and inhibiting the formation of Cu6Sn5. Cu5Zn8 is broken and decomposed after 300 h of aging treatment, and Cu6Sn5 grows rapidly after the barrier layer disappeared. The thickness of Cu6Sn5 is about 2.8 μm. The thickness of IMC of solder joint on single crystal copper is 0.7 μm more than that on polycrystalline copper. After aging treatment, the IMC formed at the interface between alloy solder and single crystal copper has better compactness and basically no pores, while there are obvious pores between IMC grains at the interface between alloy solder and polycrystalline copper, which can predict that the soldering quality of the interface between alloy solder and single crystal copper is better. This will provide application prospects for solder joint interface reliability research.

Size effects in spherical indentation of single crystal copper

A study of the size effects in the spherical indentation test of a copper single crystal is carried out. The main novelty of the approach is the analysis of a wide spectrum of parameters measured in the test that are predicted by the proposed model, and the prediction is verified experimentally for six different tip radii. Load-penetration depth curves, nominal hardness, pile-up and sink-in profiles, and the rotation and rotation gradient of the crystallographic lattice in the cross-section beneath the indenter have been measured and also calculated using 3D finite element simulations on the micro- and nanometer scale. Two gradient-effects are examined numerically within the Cosserat elastoplasticity framework with the gradient-enhanced hardening law. It is shown that a good prediction of the experimentally observed size effect on nominal hardness is achieved using the conventional power-hardening law, calibrated from the standard uniaxial compression test, enhanced with a term dependent on the lattice spin gradient term with no adjustable parameter. Furthermore, it has been found that the observed distribution of lattice rotation and decrease in the rotation magnitude with decreasing indenter radius can be qualitatively modelled by adjusting the coefficient of accumulated lattice curvature energy within the same framework.

Orientation-dependent plastic flow in nanoscratching of copper surfaces

The characterization of frictional behavior in ductile metals is a crucial aspect of materials science, as it relates to the plastic deformation of the material. However, due to the intricate dislocation evolution at the submicron/nanometer scale, the isolation of the basic plastic mechanisms remains challenging. In this study, nanoscratching technology is employed to investigate submicron/nanometer-level plastic deformation in individual grains of polycrystalline copper. To achieve this, a crystal plastic constitutive model for copper materials is developed by incorporating geometrically necessary dislocation effects. The calibration of the constitutive model is conducted with great precision and specificity, leveraging mechanical responses, surface accumulation morphology, and indentation size effects from nanoindentation experiments of 110-oriented single crystal copper. Through the combination of crystal plastic finite element simulation and nanoscratching experiments, the plastic flow of the material is thoroughly analyzed. The results demonstrate that the anisotropic scratching behavior is influenced by different orientations of individual grains, taking into account both statistical storage dislocations and geometrically necessary dislocations. The insights gleaned from this study offer a valuable contribution to our understanding of the plastic mechanisms of metals at a small scale, enhancing the design and performance of advanced materials for a wide range of applications.