Characterization of NixCo1−xO/ZrO2(CaO) directionally solidified eutectic (DSE) ceramic composites with a ductile interphase

NixCo1−xO/ZrO2(CaO) directionally solidified eutectics (DSEs) form a ductile metallic interphase after they are chemically reduced at high temperatures. Vickers indentation tests have previously shown a significant change in the crack propagation behavior of the reduced composites due to plastic deformation and strain energy absorption mechanisms operating in the system after reduction. This paper focuses on structural and chemical characterization of NixCo1−xO/ZrO2(CaO) DSEs after reduction. Analytical transmission electron microscopy techniques such as energy dispersive x-ray spectroscopy (EDXS) and electron energy loss spectroscopy (EELS) show elimination of oxygen and formation of nanoscale Ni(Co) solid.

The Influence of a Ductile Interphase on the Overall Elastoplastic Behavior of a Fiber-Reinforced Composite

While there exist various homogenization theories for the plasticity of a fiber-reinforced composite, no such theories have been explicitly developed to account for the influence of a ductile interphase. In this paper a simple scheme is developed for such a purpose. The theory evolved out of the work of Qiu and Weng (1992) and Hu (1996), and bears an identical structure to Ponte Castan˜eda’s (1991) variational procedure and Suquet’s (1995, 1996) modified secant moduli approach. An exact solution under the plane-strain biaxial loading is also developed to assess the accuracy of the theory. It is found that, with either a soft or a hard interphase and with or without work-hardening, the homogenization theory can produce sufficiently accurate results under this condition. The theory is then used to examine the influence of the interphase volume concentration on the anisotropic behavior of the composite under axial tension, transverse tension, axial shear, and transverse shear, with both a soft and a hard interphase. The results indicate that, while the axial tensile behavior is not sensitive to the interphase concentration, the behaviors under other types of loading are greatly affected by its presence, especially when the interphase is softer than the matrix.

Plasticity of Particle-Reinforced Composites With a Ductile Interphase

A homogenization theory is developed to determine the overall elastoplastic behavior of a particle-reinforced composite with a ductile interphase. Unlike most existing homogenization theories which are primarily concerned with the ordinary two-phase composites, the present one is confronted with two ductile phases, with one enclosing the other. The theory is developed with the aid of a linear comparison composite using a field-fluctuation method to calculate an energy-based effective stress of the ductile phases. In order to examine its accuracy, an exact elastic-plastic analysis under dilatational loading is also developed, and it was found that, despite its simplicity, the theory could provide plausible estimates for the overall behavior of the three-phase composite. The theory is applicable to a composite system regardless whether the interphase is more ductile or stiffer than the matrix, and when the interphase is more ductile, it is shown that even the presence of a thin layer can have a very significant effect on the plasticity of the overall composite.