K. M. Liang

Evaluation by indentation of fracture toughness of ceramic materials

A transition fracture mode from Palmqvist to median has been observed in a number of ceramic materials. A new expression to determine the fracture toughness (KIC) by indentation is presented. The KIC values calculated by this formula are independent of the crack profile (median or Palmqvist) and of the applied load. This formula has been obtained by modifying the universal curve of Evans and Charles to incorporate Palmqvist and median cracks over a wide range of loads in the case of brittle materials with different mechanical properties (elastic properties: E, v, KIC).

Review Functional materials of porous metals made by P/M, electroplating and some other techniques

Porous metals used as functional materials have been developed greatly and swiftly in recent years. In this paper, some preparative methods and relative applications are reviewed primarily for these materials. These methods concentratively deal with powder metallurgy (PM) and electroplating techniques, and these applications cover filtration and separation, fluid distribution and control, energy absorption, electromagnetic shielding, heat exchangers, electrode matrixes, reaction materials, biomaterials, and so on.

High-temperature oxidation behavior of aluminide coatings on a new cobalt-base superalloy in air

The high temperature oxidation behavior of aluminide coatings on the Co-base superalloy DZ40M by low-pressure vapor deposition has been investigated. The results showed that the isothermal oxidation process of the coatings in air at 900-1100 degreesC has three stages, including the initial, the stable and the accelerative. Adding Ti into aluminisor quickened the oxidation and degradation of the coating, and made the oxidation stage advance from the stable to the accelerative. The oxidation weight gains of coatings at lower temperatures of 900 and 1000 degreesC with theta -Al2O3 emerging were more than that at higher temperature of 1050 degreesC with only alpha -Al2O3 forming

Effect of oxygen vacancies on transformation of zirconia at low temperatures

The effect of oxygen vacancy on the stability of phase structures and phase transformation of zirconia at low temperatures was studied using an electrochemical technique. It is suggested that the decrease of oxygen vacancies decreases the stability of metastable tetragonal zirconia and promotes its transformation to the monoclinic phase.

The effect of an electric field on the phase separation of glasses

The electromagnetic theory is used in order to characterize the polarization processes in glasses. The change in free energy owing to phase transformation of glasses in the presence of an electric field is . The analysis shows that the free energy may be reduced or enhanced by an electric field depending on the ratio of the dielectric constants of the old and new phases. This means that the electric field can either stimulate or inhibit the nucleation process. The theory is qualitatively consistent with the experimental results. Furthermore, we may extend it to the phase transformation of all dielectric materials upon application of an electric field. These results suggest a possible method for the preparation of nanophase glass ceramics.

Effect of Ce4+-modified amorphous SiO2 on phase transformation towards α-cordierite

Abstract The sintering temperature of cordierite ceramic is dependent on the transformation temperature to α-cordierite. In the present work, the effect of Ce 4+ -modified amorphous SiO 2 on the transformation behavior of the ceramics were studied using the techniques such as X-ray Diffraction, Differential Thermal Analysis (DTA) and Infrared Spectrometry so as to decrease the sintering temperature. Experimental results show that, Ce 4+ -free amorphous SiO 2 has no effect on the transformation, while Ce 4+ -modified amorphous SiO 2 can obviously decrease the onset temperature. This is correlated to that Ce 4+ addition, which makes the structure of tetragonal SiO 2 loosened and decreases the transformation temperature of SiO 2 to tetragonal and so, improves the condition of solid solution of Al 3+ and Mg 2+ into tetragonal SiO 2 . Moreover, Ce 4+ addition has little effect on the conversion rate, because it cannot effectively promote the diffusion of Al 3+ and Mg 2+ into tetragonal SiO 2 .

Effect of cerium addition on phase transformation and microstructure of cordierite ceramics prepared by sol-gel method

Low-temperature sintering of cordierite ceramic depends on the phase transformation into cordierite and the properties depend on its microstructure. In the present work, the effect of cerium on the phase transformation and microstructure of cordierite ceramics prepared by sol-gel method is studied by X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) in order to lower the sintering temperature and improve the properties of cordierite ceramic with the addition of cerium. It is observed that the cerium addition obviously lowers the crystallization temperature of α-cordierite while slightly raises that of μ-cordierite. The lowest temperature for μ → α cordierite transformation, which approaches the crystallization temperature of μ-cordierite, is achieved in the sample containing 4 wt% of cerium, implying a possibility to lower the sintering temperature of cordierite ceramics. The Ce-contained ceramics show a biphasic microstructure that is dependent on sintering temperature. Sintered below 1300°C, a cordierite-CeO2 microstructure is present; while sintered at the temperature above 1300°C, appears a cordierite-glass microstructure, of which the amount of glass phase is limited to a small extent. Since the addition of 4 wt% cerium to this MgO-Al2O3-SiO2 system substantially enhances the densification of cordierite ceramics and lowers the sintering temperature to the level of around 1000°C, it makes the ceramics suitable for such applications, where the low-temperature sintering is required, as the substrates for electronic circuit and the catalytic supports (with oxygen storage capacity) for cleaning of automotive exhaust emissions.

Phase separation of CaO·Al2O3 ·SiO2 glasses induced by electric field

By means of S-4200 SEM, phase separation of CaO-Al2O3-SiO2 glasses upon application of an electric field is investigated. The experimental results show that externally applied electric field promotes the phase separation of the glass, and it leads to a different size of the droplet phase. In the vicinity of the anode of the glass, the smaller droplet phase is induced compared with the cathode of the glass. The accelerating phase separation of the glass in the presence of an electric field is due to the decrease in the free energy forming a critical nucleus size. The different size of the droplet phase of the cathode is due to the increase in the driving force of nucleation and decrease in the viscosity of the glass compared with that of the anode of the specimen.

Influence of homogenization treatment in an electric field on the workability of 1420 Al-Li alloy during hot rolling

In A1-Li alloys after casting, a significant amount of dendritic segregation is present, which leads to collapse during bot rolling. Antes [1] shows that a dramatic improvement in ductility is achieved by eliminating dendritic segregation. The effect of undissolved second phase on ductility has been investigated by Singh and Flemings [2]. Their results show that the ductility during hot rolling depends on not only the volume fraction but also the shape and the distribution of second phase particles. For a given amount of second phase, spherical shape and random distribution of second phase particles are beneficial for the improvement of ductility. Purdy and Kirkaldy [3] have drawn attention to the advantages to be gained by homogenization at temperamre above the melting point of segregated alloys. However, for A1-Li alloys, high temperature homogenization treatment results in local overheating or local underheating and a heavy loss of Li. The homogenization process invented by David [4] is to heat slowly at rates not exceeding 50°C/h to temperature of at least 530°C up to a final homogenization. But, due to the difficulties in controlling temperature, the total content of Li and Mg is not permitted to exceed 6%. Deformation of segregated alloys promotes the homogenization of alloys [5]. However, for the as-cast A1-Li alloys, great risk of collapse exists during deformation. The heat treatment of 2091 AI-Li alloy with an electric field has been investigated [6, 7]. The results show a significant effect of the electric field on the diffusion and phase transformation of 2091 A1-Li alloy. In this letter, the effect of an electric field on the workability of 1420 A1-Li alloy is reported. 1420 A1-Li alloys were melted under an argon atmosphere. Table I gives the chemical compositions of the alloy. Ingots were treated either with or without an electric field at the same homogenization time (32 h) and temperature (470 °C). The effect of homogenization time (16 h, 24 h, 32 h) in an electric field on the volume fraction, shape and distribution of second-

DETERMINATION OF KISCC BY INDENTATION IN CERAMICS

A new method is presented for the determination of resistance to crack propagation of stressed ceramics in corrosive environments.