Key Engineering Materials Vols. 340-341

Abstract: This study investigated a thermal imprint technique to pattern parylene microstructures over an area of 25
25 mm2. A nickel mold having arrays of 25 m-high, 10 m-wide and 1 mm-long lines with 10 m spacing was fabricated using the deep RIE silicon etching followed by the electroplating process. Imprint tests were then carried out under different conditions of temperature, imprint-hold time and applied pressure to investigate a thermal imprint condition for the complete filling of parylene. Good release results without damage or deformation in parylene microstructures were achieved by the help of a release agent in the imprint temperature range of 160 oC to 250oC. With increasing temperature, the depths of imprinted structures increased and their distribution came to be homogeneous. Complete filling was obtained under the imprint temperature of 250oC, applied load of 195 kgf (3 MPa) and imprint hold time of 1800 s.

Abstract: A new grain refinement and strengthening technique by modified ECAP (equal-channel angular pressing) technique was proposed in this study. ECAP technique is an effective technique for grain refinement and strengthening of metal material. This technique gives high shear plastic strain to the material without geometric transformation of the specimen. However, traditional ECAP technique is restricted by material type and size, especially length. Such kinds of restrictions cause various problems in practical use. We modified traditional ECAP dies and processes to allow high plastic strain for long-length pure-aluminum specimens. Then, grain distribution was observed using a microscope, grain size was determined by the Jeffries and the Heyn methods, and strengthening was investigated by micro-Vickers hardness test. Then the effectiveness of proposed grain refinement or strengthening techniques was discussed.

Abstract: Electron backscattering diffraction, EBSD, technique as well as atomic force microscopy, AFM, was employed to investigate fatigue damage mechanism in ultrafine-grained copper processed by equal channel angular pressing, ECAP. The fatigue damage evolution under axial tension compression was investigated. The results show that linearly shaped fatigue damage was introduced in the scale of micrometers in spite of the average grain size of 300 nm. The linear damage was randomly oriented when the shear direction of the last ECAP-pressing in perpendicular to the loading axis. The orientation analysis by EBSD revealed that the linear damage is introduced in the area with the same crystallographic orientation in the direction of the maximum Schmid factor as in the slip deformation in coarse-grained materials. The comparison before and after fatigue tests shows the grain coarsening in the area where large linear fatigue damage was formed. It is considered that strain concentration at the edge of the slips introduced in a relatively coarse ultrafine grain causes the grain rotation and deformation in the adjacent nano-sized grains, resulting in the grain coarsening and subsequent propagation of the slips in the order of micrometers.

Abstract: Molecular dynamics (MD) simulations are performed to study the onset of fracture at the free edges of bi-material interfaces. The objective is to see whether a unified criterion could be formulated for crack initiation at interface edges with different angles or not. The simulations are facilitated with model bi-material systems interacting with Morse pair potentials. Three simulation models are considered, i.e. the interface edges with angles 45°, 90° and 135°, respectively. The simulation results show that, at the instant of crack initiation, the maximum stresses along the interfaces reach the ideal strength of the interface; also, the interface energies just decrease to below the value of the intrinsic cohesive energy of the interface. These findings revealed that the onset of fracture at the interface edges with different geometries could be controlled by the maximum stresses or the cohesive interfacial energy.

Abstract: This paper presents a concurrent multiscale method for the stress analysis of solids using a coupled meshless and molecular dynamic analysis. A new transition algorithm using transition particles was employed to ensure the compatibility of both displacements and their gradients. The equivalent continuum strain energy density was obtained locally based on the atomic potential and Cauchy-Born rule, and hence plasticity can be easily handled in not only the atomic domain but also the continuum domain. Numerical examples demonstrated that the present multiscale technique has a promising potential of application to multiscale systems subjected to deformation.

Abstract: Molecular dynamics simulations are performed to verify the effect of grain boundary on nanolithography process. The model with about two hundred thousand copper (Cu) atoms is composed of two different crystal orientations of which contact surfaces are (101) and (001) planes. The grain boundary is located on the center of model and has 45 degree
angle in xz-plane. The tool is made of diamond-like-carbon with the shape of Berkovich indenter. As the tool is indented and plowed on the surface, dislocations are generated. Moreover, during the plowing process, the steps as well as the typical pile-ups are formed in front of the tool. These defects propagate into the surface of the substrate. As the tool approaches to the grain boundary, the defects are seen to be accumulated near the grain boundary. The shape of the grain boundary is also significantly deformed after the tool passes it. We observed the forces exerted on the tool by the contact with substrate, so that the friction coefficients can be obtained to address the effect of the grain boundary on the friction characteristics.

Abstract: Fine-grained polycrystalline metals have a very high yield stress and excellent workability. Hence, numerous researchers are trying to develop an efficient process to obtain such materials. Our goal is to develop an efficient severe plastic deformation (SPD) process through investigating grain-refinement mechanisms in Equal Channel Angular Pressing (ECAP). In this paper, a series of molecular dynamics (MD) simulations of severe simple-shear deformations, which are ideally equivalent to SPD applied by typical ECAP processing routes, is performed using three-dimensional models that are thin and have a square shape with a periodic-boundary condition. We analyze the influences of the processing route and initial texture on the microstructural evolution. It is shown that twinning deformations are dominant under the calculated conditions, and that the structural evolution is notably affected by the relationship between the applied simple-shear direction and the characteristic crystal orientation, which can easily cause a twinning deformation. We conclude that Route A, without a rotation of the billet between processes, is the most efficient route. This is because twinning deformations along the simple-shear direction interact with the twin boundaries developed by the stress-component conjugate to the simple-shear. Furthermore, we demonstrate that the influence of the initial texture difference remains in force during multiple processes that have the same sliding plane.

Abstract: The interactions between edge dislocations and the grain boundary have been studied by using quasicontinuum simulations. With an increase in the shear strain, dislocation pile-up is created and local stress concentration occurs at the head of the pile-up. The relationship between the stress concentration and the number of dislocations in the pile-up is discussed.

Abstract: Molecular dynamics simulation is conducted to investigate the effect of notch depth on the deformation and fracture behavior of a single crystal copper which is expected to a conductive material of micro-devices. In the stress – strain relationship, a normal stress increases with increasing applied strain. Then, the normal stress decreases rapidly. When the stress decreases, the dislocation emits from a notch root and the stacking fault is formed on the {111} plane, which is slip plane of the fcc crystalline structure. The maximum stress decreases with notch depth. The non-damaging defect size is quite small. The shear stress in the slip direction at dislocation emission is constant irrespective of the notch depth. The criterion of the dislocation emission is given by the critical value of the resolved shear stress in the sliding direction.

Abstract: In recent years, nano-crystalline materials have attracted many researchers’ attention, but the fracture mechanism has not been fully clarified. In a molecular dynamics (MD) simulation, grain size and crystal orientation can be chosen, and their effects on the mechanical properties of nano-crystalline materials can be evaluated clearly. This research first compares the results of crack growth behavior in single crystalline Fe for three typical interatomic potentials (Embedded Atom Method (EAM), Finnis Sinclair (FS), and Second Nearest Neighbor Modified EAM (2NNMEAM) potentials) and a Hybrid potential method, which uses FS potential for bcc structure atoms and 2NNMEAM potential for non-bcc structure atoms. The 2NNMEAM potential is accurate, but the computation time is dozens of times that of FS potential, which is the simplest of the three interatomic potentials. Therefore, the 2NNMEAM potential requires too much calculation for the purpose of this research that analyzes the crack growth behavior in nano-crystalline metals. However, Hybrid potential is able to give results similar to those of the 2NNMEAM potential, and the calculation time is close to that of the FS potential. From these results, the crack extension behavior in relatively large nano-crystalline models is analyzed using the Hybrid potential, and we demonstrate the grain-size dependency of the fracture behavior.