Papers by Keyword: HCP

Abstract: Co-Cr-Mo alloys are currently applied as major materials for orthopedic implant because of their excellent wear resistance. The main strengthening mechanism of this alloy is the transformation of γ-phase Co-Cr-Mo alloys to the ε-phase. In this study, the evolution of the microstructure of a Co-28Cr-6Mo-0.08C-0.2N alloy has been investigated during isothermal aging. Solution treatment at 1275°C for 15 hours was carried out for as-cast alloy, followed by aging treatments at 700°C and 800°C for up to 15 hours. Microstructure evaluation, XRD analysis and micro-hardness test were carried out for both as-cast and aged alloys. From XRD analysis showed that the transformation of γ-phase to ε-phase occurred during the isothermal aging. The amount of ε-phase increased with increasing aging temperature and time, while the hardness of the alloy increased with increasing the amount of ε-phase. This led to the increasing of the hardness of alloy at higher aging temperature and time. In addition, the very fine precipitation in the cobalt matrix was observed in the aged-specimens. Aging at 800°C caused the progressive formation of very fine precipitates along intra-granular striation and the matrix. It is suggested that the precipitation was took place in the grain on the dislocations and the stacking faults.

Abstract: Strain hardening behavior is known to strongly affect the formability of metallic sheets. The effect of lattice rotation on the hardening behavior of hexagonal close-packed (HCP) metals is numerically investigated using a homogenization-based crystal plasticity model to represent the polycrystalline behavior. The effect of lattice rotation on strain hardening behavior evaluated using different initial textures, and the geometrical hardening effect of HCP metals is investigated. In addition, the critical resolved shear stress of each slip system is varied and is shown to affects the strain hardening in HCP metals. In this study, we further discuss the possibility to improve the formability of HCP metals.

Abstract: The Schmid law says that yielding takes place when resolved shear stress on slip plane reaches the critical value. It is valid for wide variety of materials. However, it is well known that breaking of Schmid law takes place in bcc materials due to non-planar splitting of dislocation cores. The non-Schmid behavior is also possible for plastic deformation of fcc and hcp materials. Particularly, it is sometimes reported for deformation twinning. Present paper demonstrates the non-Schmid phenomena in hcp magnesium by means of computer simulations. We consider influence of non-glide stress components on motion of screw <a> dislocation as well as migration of twin boundaries.

Abstract: The present authors recently presented the time-dependent Ginzburg-Landau (TDGL) formulation for L1₂ type ordering process in binary alloys, taking into account the symmetrical relationships of these ordered phases. Extending the formulation, the authors have developed the TDGL model for microstructural evolution of D0₁₉ type ordering. The D0₁₉ structure based on hcp is divided into four equivalent sublattices. The site occupation probabilities are given as a function of three order parameters and a composition parameter. Multiple types of variants of the structures are represented by the order parameters. Mean-field free energies are defined in a form of Landau type expansion with the order parameters and the composition parameter. Interfacial energies due to local variations of degrees of order and composition are given in a gradient square approximation. Kinetic equations for time-evolution of the order parameters and the composition one are derived from the Ginzburg-Landau type potential consisting of the mean-field free energies and the interfacial energy terms. Three-dimensional numerical simulations based on the kinetic equations have been performed, and the domain structures obtained are compared with a TEM image of Cu₃Sn alloy.

Abstract: {10 2} twin is common in pure hcp metals. In this study, initiation and development of {10 2} twin in hcp metal was simulated by molecular dynamics (MD) method. Two types of model crystals were stretched along the y-axis by applying displacement of 0.005a (a is lattice constant of the basal plane) every 1000 MD steps and relaxed atoms by molecular dynamics method. The Y-boundary of the model was applied fixed boundary condition and the X and the Z-boundary were free boundary condition. The Lennard-Jones type interatomic potential was employed in this simulation. In the single crystal, {10 1} pyramidal slip initiated at the crack tip and the pyramidal slip was stopped at Y-boundary atoms. After that, a {10 2} twin was initiated at a front of the slip. With increasing external strain, the twin grew with increasing external strain. In the bi-crystal, {10 0} prismatic slip occurred at the crack tip in Crystal 1 and a-dislocation of the slip stopped at a grain boundary between the Crystal 1 and 2. With increasing applied strain, {10 2} twin was initiated in the Crystal 2 at the grain boundary. From the simulation results, the ‘shuffling’ process of twin deformation was estimated.

Abstract: {111} fiber textured face centered cubic (fcc) titanium has been found to coexist with the {0002} fiber textured hexagonal close packed (hcp) titanium in polycrystalline titanium (Ti) thin films (thickness: 144 nm to 720 nm) deposited on Si (100) substrate by magnetron sputtering. X-ray diffraction investigation confirms that relative phase fraction of such metastable fcc Ti phase decreases with increasing film thickness indicating thickness dependent fcc-hcp phase transformation of titanium. Texture development in hcp Ti phase was due to film microstructure (thickness effect) rather than the phase trans-formation. Diffraction stress analysis (by d-sin2 method) indicates that fcc to hcp phase transformation is also accompanied by the reduction of compressive stress in the hcp Ti phase with increasing film thickness. Strain energy calculations for both phases of titanium indicate that fcc Ti is a more stable phase compared to hcp Ti at relatively low film thickness (144 nm to 432 nm). It has been concluded that film stress favours fcc to hcp phase transformation towards the higher film thickness. Reverse transformation (hcp to fcc) occurs towards the lower film thickness.

Abstract: Compaction behavior of two component slurry during High-speed Centrifugal Compaction Process (HCP) was observed. Slight amount of iron oxide powder is mixed into alumina slurry, then the slurry was sedimented in a centrifuge under rotational speed of up to 11,500 rpm. A “Y” letter shaped flow pattern was emerged in the cross section of the compact. The pattern was clearer with higher rotational speed, but indifferent to acceleration rate of rotation. A similar pattern was simulated when we presume bidirectional initial flow in centrifugal field, which indicated that the combination of Corioli’s force and bidirectional flows of powder and dispersing medium caused such flow pattern.

Abstract: Grain boundary character in samples of Zr701 annealed at two different temperatures has been investigated in terms of lattice misorientation. The main difference between the two samples was the extent of grain growth post-recrystallization. The textures were typical for the material. Differences between the texture-based misorientation distribution function (T-MDF) and the microstructure-based MDF (M-MDF) revealed significant preferences for certain grain boundary types, notably those with <11-20> rotation axes.

Abstract: Textured Mg alloys exhibit tension – compression strength asymmetry due to mechanical twinning. The distinction arises as the material deforms primarily by slip in one direction and by twinning in the other. In-situ neutron diffraction during cyclic loading in tension and compression of extruded bar allows study of the effect of twinning on subsequent load reversals. The diffraction data reveal the texture evolution and internal stress development as a function of deformation. De-twinning resulted in complete texture reversal during initial cycles, but eventually “fatigued” resulting in some residual twin component.