Acoustic emission during fatigue crack propagation in SiC particle reinforced Al matrix composites

The acoustic emission (AE) behavior during fatigue propagation in aluminum 6061 and aluminum 6061 matrix composites containing 5, 10, and 20 wt pct SiC particle reinforcement was investigated under tension-tension fatigue loading. The purpose of this investigation was to monitor fatigue crack propagation by the AE technique and to identify the source(s) of AE. Most of the AEs detected were observed at the top of the load cycles. The cumulative number of AE events was found to correspond closely to the fatigue crack growth and to increase with increasing SiC content. Fractographic studies revealed an increasing number of fractured particles and to a lesser extent decohered particles on the fatigue fracture surface as the crack propagation rate(e.g., †K) or the SiC content was increased. This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ ASM-MSD Composite Materials Committee.     

In vitro release characteristics of bioactive molecules from dextran dialdehyde cross-linked gelatin hydrogel films

Vinyl chloride monomer (VCM) is a human carcinogen. However, the exact mechanism of carcinogenesis remains unclear. VCM may be metabolized by cytochrome P450 2E1 (CYP2E1), aldehyde dehydrogenase 2 (ALDH2) and glutathione S-transferases (GSTs). Thus workers with inherited variant metabolic enzyme activities may have an altered risk of genotoxicity. This study was designed to investigate which risk factors might affect sister chromatid exchange (SCE) frequency in polyvinyl chloride (PVC) workers. Study subjects were 44 male workers from three PVC factories. Questionnaires were administered to obtain detailed histories of cigarette smoking, alcohol consumption, occupations, and medications. SCE frequency in peripheral lymphocytes was determined using a standardized method, and CYP2E1, GSTM1, GSTT1 and ALDH2 genotypes were identified by the polymerase chain reaction (PCR). Analysis revealed that smoking status and exposure to VCM were significantly associated with increased SCE frequency. The presence of ALDH2 1-2/2-2 genotypes was also significantly associated with an elevation of SCE frequency (9. 5 vs. 8.1, p<0.01). However, CYP2E1, GSTM1 or GSTT1 genotypes were not significantly associated with SCE frequency. When various genotypes were considered together, combination of CYP2E1 c1c2/c2c2 with ALDH2 1-2/2-2 showed an additive effect on SCE frequency. Similar results were also found for the combination of smoking with CYP2E1, or smoking with ALDH2. These results suggest that VCM workers with ALDH2 1-2/2-2 genotypes, who also smoke, may have increased risk of DNA damage.     

Cu-Zn-Al 形状记忆合金热弹性马氏体相变临界驱动力的估算

发展了新的估算 Cu-Zn-Al 合金热弹性马氏体相变临界驱动力方法。由测量合金的杨氏模量及估算马氏体内的储存能量(层错能),所得的临界驱动力数值与量热法测得热效应后换算的近似值非常接近。     

Thermoplasticity,pseudoelasticity and the memory effects associated with martensitic transformations

The macroscopic mechanical behaviour (stress-strain-temperature relations in tension, compression and internal friction) associated with pseudoelasticity and the memory effects is extensively reviewed. The particular features of the tension and compression curves (the stress to induce or reorient the martensite, total elongation, reversibility and hysteresis) are analysed and their dependence on temperature and crystal orientation is discussed.     

Stress-induced phase transformations and enhanced plasticity in copper-aluminium and copper-aluminium-zinc martensites

The effect of deformation on the structure of the-type martensite has been studied in copper-aluminium and copper-aluminium-zinc alloys by X-ray analysis and mechanical testing. This study has yielded information on the stability of the crystal structure of-type martensite, i.e. the tendency of the stress-induced transformation to the fcc structure, as zinc is added in increasing amount to copper-aluminium. The detailed study of the X-ray powder diffraction patterns of the as-quenched martensites showed that as zinc is added in increasing amounts, the structure of the-type martensite becomes distorted and shows an increasing degree of orthorhombic distortion (i.e. deviation from the trigonal symmetry). When these martensites are plastically deformed, the structure is transformed more and more easily into an fcc lattice, i.e. the volume fraction of the fcc phase increases with zinc content. Tensile tests showed that the martensite showed higher and higher elongation values, when plastically deformed, which is due to the progressive increase in fcc phase.     

The self-accommodating character of the β1 copper-aluminum martensite

The phenomenological martensite theory is applied to the β₁ to β₁ martensitic transformation in Cu-Al. Crystallographic and morphological aspects of the resulting martensitic microstructure are discussed and verified with X-ray pole figures, which are combined with a single surface trace analysis based on the use of polarized light. From the analysis of the orientation of any single martensite plate related to that of one or more neighboring plates, it can be proved that the martensite microstructure in each former β-grain is composed of at most six self-accommodating martensite plate groups, each of which consists of four different martensite plate variants. Formerly with KUL, Leuven,     

Fractographic study of the alumina and zirconia particles embedded in mullite prepared by reaction sintering

Four different mullite-alumina-zirconia composites have been prepared by reaction sintering between alumina and zircon powders using magnesia or spinel (MgAl₂O₄) to increase the sintering and reaction rates. The microstructure of these materials can be described as composed of two parts: the first one is the mullite matrix containing various kinds of zirconia and alumina particles, whereas the second part is an amorphous phase in which alumina submatrices, zirconia and spinel particles are embedded. Examination of fracture surfaces allows one to identify the crack paths and shows that the main differences are related to zirconia inclusions. Analysis of mechanical properties and fracture features leads to the conclusion that crack deflection and microcracking are operative toughening mechanisms for the various materials. Moreover, a crack bowing mechanism is proposed to explain the higher modulus of rupture found for the series of materials prepared with magnesia as a reaction sintering aid. On leave from Instituto de Ceramica Y Vidrio, CSIC, Arganda del Rey, Madrid, Spain.     

Thermodynamic Calculation of Segregation in Multicomponent Steels

The thermodynamic equation for segregation in multicomponent steels is extended from that in ternary system and the segregation amounts of Cr, C and P in the intergranular phase in a Cr-steel are estimated.     

Photocytotoxic effect of pseudohypericin versus hypericin

Pseudohypericin and hypericin, the major photosensitizing constituents of Hypericum perforatum, are believed to cause hypericism. Since hypericin has been proposed as a photosensitizer for photodynamic cancer therapy, the photocytotoxicity of its congener pseudohypericin has been investigated. The presence of foetal calf serum (FCS) or albumin extensively inhibits the photocytotoxic effect of pseudohypericin against A431 tumour cells, and is associated with a large decrease in cellular uptake of the compound. These results suggest that pseudohypericin, in contrast to hypericin, interacts strongly with constituents of FCS, lowering its interaction with cells. Since pseudohypericin is two to three times more abundant in Hypericum than hypericin and the bioavailabilities of pseudohypericin and hypericin after oral administration are similar, these results suggest that hypericin, and not pseudohypericin, is likely to be the constituent responsible for hypericism. Moreover, the dramatic decrease of photosensitizing activity of pseudohypericin in the presence of serum may restrict its applicability in clinical situations.     

Influence of thermal cycling on the martensitic transformation and shape memory effect of a Fe－16Mn－

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