Key Engineering Materials Vols. 280-283

Abstract: In this paper, the oxidation process of the VFD array material in wet hydrogen at high temperature was studied. By the aid of SEM, EDAX, and XRD, the effect of temperature, duration, volume and dew point of hydrogen flow during oxidation on weight gain, percentage of the oxide phase, constitution and morphology of oxide scale were investigated, and a suitable oxide technology was obtained, it was as follows: 950°C, 40 ~ 60 min, D.P. 35°C, 8 l/min.

Abstract: In the present study, Fe and Si powders (3:1 in atomic proportion) were mechanical alloyed and subsequently treated under different anneal conditions. The changes of phase composition and structure during the mechanical alloying and following heat treatment processes were mainly studied. The results showed that the XRD peaks of Fe were gradually broadened and weakened with the increase of the milling time associated with the shifting from low angles to high angles, and the main phase changed into α-Fe (Si) solid solution after milling for 20h. When the product was heat treated below 400oC, the main phase remained α-Fe (Si). While it was treated above 400oC, ordered Fe3Si phase was found and the XRD peaks of Fe3Si became sharper with the further increase of the temperature.

Abstract: In the present study, Fe-Si sheets were formed by powder rolling of Fe-6.5wt%Si mixture powders, then were sintered in flowing Ar+5vol%H2 atmosphere. The changes in microstructure and density during the formation process were mainly discussed. The results show that Fe-Si mixture powders can be rolled into comparatively compact sheets. During subsequent sintering and re-rolling, the density of the sheet gradually increased and Si diffused into Fe lattice step by step with the increasing of temperature.

Abstract: Lecithin is a natural amphipathic surfactant. In this paper, lecithin was experimented to improve the suspension property of non-plastic materials with aluminum oxide slurry. The suspension property was compared by adding different types and amounts of lecithin into the aluminum oxide slurry in acid and alkaline conditions. The experiments showed that the 6090-type lecithin with the optimal amount (0.2%) of dry base best modified the suspension property of the aluminum oxide slurry.

Abstract: The effects of Cemfiber on the workability, mechanical properties, resistance to chemical corrosion and drying shrinkage of High-Performance Concrete (HPC) have been studied systematically. The results show that: (1) the effects of Cemfiber on the workability of HPC are related to parameter of mix proportion. Generally speaking, Cemfiber does not reduce the workability of HPC; (2) the effect of Cemfiber on the compressive strength and flexural strength is very small; (3) the HPC incorporated with Cemfiber has high resistance to corrosion of hydrochloric acid; (4) Cemfiber may reduce drying shrinkage of HPC significantly; (5) Cemfiber may increase cracking resistance of HPC resulting from drying shrinkage.

Abstract: At present, the concrete applied over the world is ordinary concrete with a compressive strength less than 50 MPa, whose service life is generally 40 ~ 50 years. How to get high performance, high durability, and make its service life extend from 40 ~ 50 years to 70 ~ 80 years is of great significance in saving resources, energy and money. From the point of view of material science, the durability of concrete is mainly affected by its components, inner structure and performance. This paper studied how to adjust the components and add special mineral ultra-finest powder to make the ordinary concrete get higher durability and performance.

Abstract: Channelled hollow ceramic cylinders have been sliced into discs of equal thickness and submitted to an adapted diametral compression, or brazilian, test such as to evaluate their reliability. The mean Weibull modulus, of m » 18, is representative of a rather good homogeneity of the ceramic material. The shapes of the distributions reveal a probable multimodality. This is analyzed in superimposing possible unimodal distributions of given characteristic value, Weibull modulus and number of items, and comparing to the experimental plot. Iterative modifications are made until a convincing superposition is attained. Complementary numerical simulations on “thermomechanically equivalent microstructures” have been created on the computer observing actual stereological data. The micro-mechanical model accounts for cracking of grain interfaces until specimen separation. Weibull plots for model structures under pore pressure suggest multimodal distributions with moduli ranging as in the measurements. The larger scatter at higher rupture pressures may indicate a varying degree of quasi-brittleness.

Abstract: A unified model for fracture of brittle solid based on crack opening displacement is presented. The model allows the prediction of elastic and fracture response of brittle materials containing spherical and cylindrical pores and polycrystalline solids containing anisotropic residual stresses. The analysis can also be used to predict spontaneous cracking and fracture of two phase systems possessing mismatch stresses.

Abstract: A novel estimation for the critical size of the frontal process zone of ceramics is proposed using a single-edge V-notched beam (SEVNB) technique. A three-point flexure test is carried out on aluminum titanate ceramics containing a sharp V-shaped notch with different depth. An exact solution of the critical local stress is analyzed at a critical distance from the notch tip. The critical frontal process zone size is determined as the distance between the notch tip and the point where the critical local stress equals the flexural strength of specimens without notches, based on the local fracture criterion and the Griffith-Irwin criterion. The critical size of the frontal process zone, the fracture toughness and the flexural strength were also estimated for several materials, such as, alumina, porous alumina, and alumina-based nanocomposites. The relationship between these mechanical properties indicated that there was an almost linear relationship between the fracture toughness and the resultant of strength and square root of the critical frontal process zone size, and that both of them must be increased to improve the fracture toughness of ceramics.

Abstract: A tensile test is proposed for evaluating the stress enhanced corrosion-coefficient of ceramics with open porosity. The analysis of load at fixed grip conditions bases on the assumption that stress corrosion results in a spread of pre-existing generalised damage seen as micro-crack starter due to open porosity. Sub-critical crack extension of microcracks departs from pores and provokes a time dependant diminution of the Young’s modulus, and of the stiffness, of the tested sample. The description derives the number and the size of pre-existing micro-cracks with respect to time. The density of microcrack-nuclei results from the process and belongs to the microstructure, i.e., the number of channels or crack starters, and is considered to remain constant. The rate of diminution of the load is considered, either with respect to the initial one or to a deduced stress intensity factor. Simple relationships yield then an approximate diagram for stress enhanced corrosion. Samples resulting from sintering a mixture of alumina and titania powders were tested under tempered water flow. A stress corrosion exponent, n = 16, near to that of alumina was obtained.