SiCf-SiBC composites: microstructural investigations of the as-received material and creep tested composites under an oxidative environment.

SiCf-SiBC composites fabricated by Snecma Propulsion Solide (St Médard en Jalles, France) were investigated by SEM and HRTEM in the as-received state and after creep tests performed in air, in a temperature range 1423-1573 K, under 170 and 200 MPa. These composites are reinforced by Hi-Nicalon fibres (Nippon Carbon). A pyrocarbon interphase was first deposited on the fibres. The matrix was then deposited on the fibrous preform by several chemical vapour infiltrations (CVI). As a result the matrix is multilayered and based on the Si-B-C ternary system. This matrix is self-sealing: this is due to the presence of boron inducing the formation of a sealant glass if the material is heated in an oxidative environment. This glass will protect fibres and fibre/matrix interphases against oxidation. Hi-Nicalon fibres as well as the different matrix layers were studied by HRTEM and EDX. Some investigations were carried out on the creep-tested specimens in order to characterize modifications observed in the different constituents of the composites, particularly at the interfaces between the matrix layers and at the fibre/matrix interface. It was shown that several matrix layers crystallized during the creep tests. Moreover, a thin silica layer was observed at the pyrocarbon/matrix interfaces. Differences between the behaviour of the same type of material creep tested under neutral atmosphere are discussed.

Microcracking mechanism in a SiCf-SiBC composite creep-tested in argon.

This paper addresses the creep behaviour of a woven SiCf-SiBC composite, tested in tension under a partial pressure of argon, between 1273 K and 1473 K. It appears that the creep strain begins from 1273 K and becomes larger at higher temperatures. Moreover, the shapes of the creep curves led to the assumption of the existence of two competing deformation mechanisms depending on the temperature domain. The creep mechanism involved is microcrack damage-creep. From higher resolution studies at higher scales (scanning electron microscopy, transmission electron microscopy (TEM) and high resolution electron microscopy (HREM)), many types of damage were observed, for example matrix microcracking, fibre/matrix debonding and fibre/matrix sliding. The observations via TEM and HREM enabled us to specify the existence or not of the classical creep mechanism of the constituents of the composite, and also to characterize the behaviour and the role of the different interfaces and especially of the pyrocarbon interphase. These multiscale observations will be discussed in order to highlight the creep-damage mechanism as a function of temperature of the SiCf-SiBC composites.

Transmission electron microscopy studies of squeeze cast Al-AlN composites.

Aluminium-matrix composites containing approximately 45 vol.% AlN particles were fabricated by melt infiltration of aluminium into an AlN preform under a pressure up to 130 MPa. Three types of aluminium alloy (2024, 6060 and 5754) were used. The as-prepared composites were studied by light microscopy, scanning and transmission electron microscopies, and energy-dispersive X-ray spectroscopy. As a result of the melt infiltration process, the composites are very dense and the microstructure shows a homogeneous distribution of the reinforcement. The interfaces are clean with very little porosity. Composites with 2024 and 6060 matrices were carefully studied by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) after heat treatments. Dislocation density in the matrix of the reinforced material increases due to the difference in thermal expansion coefficients of aluminium alloys and AlN. This can induce an accelerated ageing response of the coherent and semicoherent precipitations of age-hardened matrices. This behaviour has been studied in the 2024 and 6060 composites by using microhardness measurements and TEM. Reactions between the AlN reinforcement and aluminium matrices (6060 and 5754) were observed and analysed by TEM. Matrices containing some of magnesium display a MgAl2O4 spinel formation at the AlN/matrix interface. The spinel formation is probably due to the reaction between magnesium of the matrix and the thin Al2O3 layer on the AlN surfaces. This reaction can affect the mechanical behaviour of the composite infiltrated with the 5754 matrix. This has been confirmed by overageing some samples at high temperatures (300 degrees C and 550 degrees C) for 10 days in order to emphasize the interfacial reactions.

Automatic morphological sieving: comparison between different methods, application to DNA ploidy measurements.

The aim of the present study is to propose alternative automatic methods to time consuming interactive sorting of elements for DNA ploidy measurements. One archival brain tumour and two archival breast carcinoma were studied, corresponding to 7120 elements (3764 nuclei, 3356 debris and aggregates). Three automatic classification methods were tested to eliminate debris and aggregates from DNA ploidy measurements (mathematical morphology (MM), multiparametric analysis (MA) and neural network (NN)). Performances were evaluated by reference to interactive sorting. The results obtained for the three methods concerning the percentage of debris and aggregates automatically removed reach 63, 75 and 85% for MM, MA and NN methods, respectively, with false positive rates of 6, 21 and 25%. Information about DNA ploidy abnormalities were globally preserved after automatic elimination of debris and aggregates by MM and MA methods as opposed to NN method, showing that automatic classification methods can offer alternatives to tedious interactive elimination of debris and aggregates, for DNA ploidy measurements of archival tumours.

Influence of sample size on image cytometry of DNA ploidy measurements.

OBJECTIVE: To assess the number of nuclei required for significant image cytometry DNA ploidy measurements on one archival case of breast cancer. STUDY DESIGN: From one case of aneuploid DNA breast cancer, 18 subsets made up of 152-1,524 for the whole population of undamaged nuclei and made up of 74-735 epithelial nuclei had DNA measured. DNA ploidy type and five DNA ploidy indices, allowing DNA ploidy histogram interpretation were evaluated on each population. RESULTS: Three hundred nuclei were always sufficient for DNA typing, whereas reliable results for DNA ploidy indices required at least 750 nuclei. CONCLUSION: To DNA measure the above number of nuclei, fully automated image cytometry DNA ploidy measurements are required.

Influence of debris and aggregates on image cytometry DNA measurement of archival tumors.

OBJECTIVE: To investigate, for seven samples, the influence of unwanted elements (e.g., remains of erythrocyte cell membranes, sliced nuclei, damaged nuclei and aggregates) on image cytometry DNA ploidy measurements. STUDY DESIGN: Two normal reference tissues (brain and breast), one breast cancer and four brain tumors were studied. For each sample, the influence of the different classes of debris on DNA ploidy histograms and indices was evaluated. RESULTS: The influence differs regarding each class of debris and the index to be evaluated. CONCLUSION: Strict and precise elimination of debris and aggregates is required. Moreover, strategies and efforts that must be applied to automated elimination of these unwanted elements must be a direct function of the bias they introduce into DNA ploidy measurements.

Determination of size and molecular weight distributions of lipoproteins using automatic image analysis and density gradient ultracentrifugation.

A Leitz-Tas automatic image analysis system has been used to investigate the size distributions of human lipoprotein particles isolated by density gradient ultracentrifugation. Computerized image analysis enabled us to obtain more precise determinations of modal diameter and overall range in diameter for each lipoprotein subspecies than those determined by classical methods. In addition, lipoprotein molecular weight distributions were calculated from measurements of flotation density and particle size distributions. This method offers certain advantages over other procedures used to characterize mean molecular weight with regard to the ability to define lipoprotein polydispersity. The method and its limitations are discussed and illustrated with results obtained with human lipoproteins.