Byung-Nam Kim

Estimation of fracture resistance of Al2O3 polycrystals from single-crystal values

Abstract A crack deflection model was constructed to explain the fracture behavior of Al2O3 polycrystals. From a consideration of the competition between penetration and deflection at grain boundaries, a three-dimensional fracture mechanism was modelled, where the crack path is affected by adjacent crack planes. The cracks in polycrystals were divided into three types: transgranular, intergranular and a mixture of the two crack types. The fracture toughness and the transgranular fracture ratio were calculated as a function of the grain boundary toughness. However, for anisotropy within a grain, experiments on single-crystal Al2O3 were carried out and the results were used for the deflection model. The maximum toughness obtainable in Al2O3 polycrystals is estimated to be about 4.3 MPa m 1 2 , which is about three times that of single crystals, and the crack path restriction at triple or quadruple points was found to be the primary reason for the increased fracture toughness of polycrystals over that of single crystals. The calculated relationship between the fracture toughness and transgranular fracture ratio showed good agreement with the experimental results.

Simulation of fracture behavior in particle-dispersed ceramic composites

Abstract A two-dimensional crack path is simulated in particle-dispersed ceramic composites along with the related variation of fracture resistance with crack extension. The direction of crack propagation is influenced by the geometrical crack shape and localized residual stresses due to the thermal expansion mismatch between particle and matrix. The simulation is conducted for both a SiC matrix composite dispersed with Al 2 O 3 particles and an Al 2 O 3 matrix composite dispersed with SiC particles, under an assumption of hard particles. In the SiCAl 2 O 3 (p) composite, a crack propagating near the Al 2 O 3 particles has a tendency to be repelled due to the residual tensile stress in the radial direction around the particles, and the entire fracture resistance shows a lower value than the matrix toughness. In the Al 2 O 3 SiC (p) composite, a crack is attracted by the SiC particles. Due to the residual compressive stress in the radial direction around the SiC particles, the fracture resistance increases up to five times the matrix toughness when the crack propagates along the interface. The apparent fracture resistance of the Al 2 O 3 SiC (p) composite shows a higher value than the matrix toughness, and an increasing R -curve behavior with crack extension is predicted. The approximately estimated two-dimensional fracture toughness of the Al 2 O 3 SiC (p) composite increases with the volume fraction of particles, while it decreases in the SiCAl 2 O 3 (p) composite.

A new method based on simultaneous acoustic emission and in-situ SEM observation to evaluate the fracture behavior of metal matrix composites

In this study, the authors tried to combine the advantages of AE evaluation and in-situ SEM observation to analyze the fracture behavior of 6061 Al alloy matrix composites with different volume fractions and different reinforcement sizes of SiC particles. By using in situ SEM observations of crack initiation and growth together with two-channel acoustic emission (AE) linear location characterization the authors can analyze the effect of different parameters on fracture behavior of Particulate Reinforced Metal Matrix Composites (PRMMCs), and thereby lay the foundation for improving the fracture properties of this genre of composites.

Two-dimensional simulation of grain growth based on an atomic jump model for grain boundary migration

Abstract The kinetics and the topological phenomena during two-dimensional grain growth are studied by computer simulations based on an atomic jump model for grain boundary migration. The grain boundaries are assumed to be straight. The kinetics show the 1/2-power growth law for the average grain size, and the size and the side distributions are time-invariant. In particular, the simulated side distribution is well consistent with the theoretical prediction. The present simulation follows the Aboav–Weaire law for entire topological classes, the Lewis law for intermediate topological classes and the von Neumann–Mullins law for intermediate and high classes. The deviation from the von Neumann–Mullins law for low topological classes is reduced by taking account of the effects of curved grain boundaries. The other distinctive results of the simulation are also shown and discussed.

Strengthening mechanism of alumina ceramics prepared by precoarsening treatments

Abstract Effects of precoarsening treatments on strength and on subcritical crack growth are examined for Al 2 O 3 ceramics. Al 2 O 3 polycrystals sintered by conventional hot-pressing procedure yield a 4-point bending strength of 400–500 MPa, while the precoarsening treatments increase strengths up to 750 MPa. Fracture toughness remains at the value of 3.5 MPa√m regardless of the treatments and the kind of source powder. From observations of fracture origin, it is found that the precoarsening treatments suppress the subcritical growth of flaws during testing to give a relatively small flaw size. The grain boundary toughness is evaluated qualitatively by comparing the lengths of subcritically fractured regions, and quantitatively by measuring the percentage of transgranular fracture. It is concluded that the fracture toughness of the grain boundary is increased by the precoarsening, and the toughened grain boundary plays a major role in reducing the subcritical growth rate of sintering flaws, resulting in strengthening.

Atomic jump model for migration of curved grain boundary

Abstract The migration behavior of grain boundary was analyzed for 2D and 3D isolated grains by the atomic jump model, where the decrease of the grain boundary surface energy due to the reduction of surface area, not the pressure difference, is regarded as the driving force of the migration. The kinetics both in a 2D and 3D can be represented as an exponential function of grain size. When the grain size is large enough, the variation of the radius is nearly proportional to a square of time.