As-processed State Research Articles

Structural and textural changes of CVD-SiC to indentation, high temperature creep and irradiation

The structure and microtexture of different SiC-based CVD coatings have been studied by RMS, in their as-processed state and after high temperature annealing, creep, indentation and irradiation. Both annealing and creep resulted in the same degree of SiC crystal growth and decrease of stacking faults. A slight influence of stress was however observed on the structure and texture of the co-deposited free-carbon, likely as a consequence of the intergranular creep mechanism. Room temperature indentation induces substantial structural disorders (dislocations, stacking faults, small grains) near the contact and more extended damages due to dislocation slip parallel to the compact Si–C planes. These structural changes were found to depend on the single crystal orientation and therefore, on the texture of polycrystalline SiC. Room temperature proton irradiation produced only small amounts of disorders. The specific alterations of the Raman features were explained using a phonon confinement model. This approach supports the existence of very low scale defects, likely as punctual defects.

Grain-Boundary Wetting-Dewetting in z= 1 SiAlON Ceramic

The grain-boundary structure of a model SiAlON polycrystal with nominal composition Si5AlON7 was characterized by transmission electron microscopy (TEM) both in an equilibrium (as-processed) state at room temperature and after quenching from elevated temperature. In addition, low-frequency (1–13 Hz) internal friction data were recorded as a function of temperature, showing a pronounced grain-boundary sliding peak positioned at 1030°C. High-resolution transmission electron microscopy (HRTEM) of the equilibrated low-temperature microstructure revealed residual glass only at multigrain junctions, but no amorphous intergranular films were observed. The detection of clean interfaces in the as-processed sample contradicts the internal friction data, which instead suggests the presence of a low-viscosity grain boundary phase, sliding at elevated temperatures. Therefore, a thin section of the as-sintered material was heated to 1380°C and rapidly quenched. HRTEM analysis of this sample showed, apart from residual glass pockets, wetted grain boundaries, which is in line with the internal friction experiment. This wetting-dewetting phenomenon observed in z= 1 SiAlON is expected to have a strong impact not only on high-temperature engineering ceramics but also on geological, temperature-activated processes such as volcanic eruptions.

TEM/HREM observations of nanostructured superplastic Ni 3 Al

Abstract Nanostructured Ni3Al intermetallic (Ni–8.5 Al–7.8 Cr–0.6 Zr–0.02 B; in wt%) with an average grain size of 50 nm was produced using the technique of severe plastic deformation (SPD). Previous studies of Ni3Al in the as-processed state demonstrated non-equilibrium grain boundaries, indicated by a high concentration of extrinsic defects, characteristic of material processed by SPD. Diffraction and high resolution electron microscopy (TEM/HREM) have been used here to examine specimens of the material annealed at 650°C, and 750°C, plus material taken from the gauge section of a specimen superplastically deformed to about 300% elongation at a temperature of 650°C. Examination of the sample annealed at 650°C showed that grain boundaries remained in high energy, nonequilibrium configurations. The sample annealed at 750°C contained grain boundaries with a high density of dislocations. By contrast, the superplastically deformed sample showed no visible distortions of the crystal lattice near the grain boundary. Additionally. TEM/HREM observations of twin boundaries in the deformed specimen showed no indication of trapped lattice dislocations, suggesting that grain boundary sliding was accommodated by diffusion.

Determination of the In Situ Fiber Strength in Ceramic‐Matrix Composites from Crack‐Resistance Evaluation Using Single‐Edge Notched‐Beam Tests

A model for the single‐edge notched‐beam (SENB) test is presented which allows the load–displacement curve of fiber‐reinforced ceramic‐matrix composite (CMC) specimens to be calculated using the crack‐opening function. This function considers the effects of the main parameters of the composite components that govern the fracture of the composite (e.g., fiber strength and interfacial properties). Then, in an experimental study, the SENB curves of an oxide–oxide composite are measured in the as‐processed state and after exposure at elevated temperatures. Comparison between the measured and calculated SENB curves indicates that the characteristic loss of the maximum load in the SENB curves due to the annealing is caused by the corresponding reduction of the in situ fiber strength. This method then is proven to be an excellent tool to evaluate the actual fiber strength in CMCs, which is, otherwise, barely measurable. It will be shown that the SENB test is very much qualified for a proper investigation of the relationship between in situ fiber strength and composite strength (the maximum SENB load), because of the limited influence of other parameters (Weibull modulus of the fiber strength and the friction coefficient) on the maximum SENB load.

Comparative assessment of slow strain rate, 4-pt bend and constant load test methods for measuring environment stress cracking of polymers

The environment stress cracking (ESC) susceptibility of polycarbonate and acrylonitrilebutadiene-styrene (ABS) in a number of environments has been evaluated using slow strain rate, four-point bend and constant load tests. Testing was conducted for the materials in the as-processed and annealed state. For the as-processed materials, the longitudinal direction of the specimen was generally perpendicular to the melt flow direction. Nevertheless, a few tests were carried out with the specimen orientation parallel to the flow direction. The ESC techniques gave consistent ranking of aggressivity of environments although the extent of relaxation of the ABS when exposed to particular environments limited the significance of results from the bend tests. Otherwise, the results validate further the use of the slow strain rate technique as an accelerated test method for assessing the ESC of plastics, albeit primarily as a ranking technique. The threshold stress for crazing was highest for the slow strain rate method which can be attributed partly to the particular strain rate adopted. There was no effect of molecular orientation on ESC for the limited range of test conditions evaluated. The difference in threshold stress measured for the annealed and as-processed specimens indicated the existence of a net tensile residual stress in the very near-surface region, not detected by the layer removal method of residual stress measurement.

Origin of nonlinear surface impedance and intermodulation distortion in YBa2Cu3O7−x microstrip resonator

We present measurements of the nonlinear surface impedance Zs and the third-order intermodulation distortion (IMD) of a YBa2Cu3O7−x microstrip resonator, which is initially in the as-processed state and subsequently annealed at 450–600 °C. Zs of the as-processed sample in the high-dissipated-power range is explained in terms of the vortices in weak links. In the low-dissipated-power range, outputs of IMD are changed by annealing the resonator, and the minimum IMD generation is obtained with a sample annealed at 500 °C. By comparing the IMD generation of the samples with the crystalline structure measured by x-ray diffraction and transmission electron microscopy, it is shown that IMD of the YBa2Cu3O7−x film is generated at lattice distortions of the a−b plane in grains and grain boundaries.

Evaluating the effect of oxygen content in BN interfacial coatings on the stability of SiC/BN/SiC composites

Boron nitride was studied as a fiber-matrix interface coating for Nicalon/SiC composites. The effect of initial O-impurity content within the as-processed BN coatings on the long-term interface stability was investigated at elevated temperatures in flowing oxygen. Two types of Nicalon/SiC composites were used for this study; one composite had a BN coating with I 1% (high-O BN) in the as-processed state. The high-O BN is actually most representative of BN coatings available commercially. The BN coatings in both the high-O and low-O BN containing composites were structurally similar. The samples used here were thinned to < 200 μm before oxidation and the final preparation for electron microscopy examination of the interface region was done after the reactions were completed. Thin samples were used to simulate maximum corrosion effects that would occur at the surface of an actual part during service. Ech sample was exposed to flowing oxygen at temperatures as high as 950°C for times up to 400 h. After each oxidation experiment, the BN coatings were examined by TEM to quantify the extent of any reaction which occurred at either the fiber/BN and BN/SiC matrix interfaces. At 950°C for 100 h, there were no interface microstructural changes observed in the low-O BN but there was extensive silica formation at the fiber/BN interfaces in the high-O BN. After 400 h at 950°C, large voids formed at the fiber/BN interface in the high-O BN sample only. Oxygen present within the initial BN coating contributed significantly to the degradation of the interfacial properties of the composite. Several techniques, including transmission electron microscopy (TEM). Auger electron spectroscopy (AES ), energy-dispersive spectrometry (EDS). and electron energy-loss spectroscopy (EELS) were used to characterize changes in structure and chemistry of the fiber-matrix interface region and to elucidate and quantify composite degradation mechanisms.

Transmission electron microscopy of microstructures in ceramic materials

Based on selected examples from the area of Si 3N 4 ceramics, the value of utilizing transmission electron microscopy (TEM) as a technique to study ceramic microstructures as well as a characterization tool for the development of new materials is demonstrated. In the field of ‘new ceramics’, one Si 3N 4-based composite is discussed, which was processed via pyrolysis of liquid precursors (polysilazanes). Moreover, it is shown that TEM in general can helpfully accompany ceramic processing techniques. This applies to the characterization of ceramic starting powders as well as to the study of densified materials. The investigation of Si 3N 4 powders, in particular the influence of the addition of sintering aids via organometallic precursors, which leads to a homogeneous distribution of sintering additives in the powder compact, in contrast to the use of metal oxide powders, is shown. The variation of micro-structures during the densification process of liquid assisted sintering is also demonstrated. The most common application of the TEM technique is to characterize dense ceramic components in the as-processed (as-sintered) state. Post-sintering heat treatment can initiate secondary phase crystallization. However, the very important aspect of microstructure integrity at elevated temperatures, e.g. the stability of microstructures under severe service conditions, is also addressed. Emphasis is placed on the fact that ceramic microstructures, which are typically thought to be rather stable, can undergo serious microstructural changes when temperature and stress is applied simultaneously, which strongly limits potential applications of these materials.

An ultimate tensile strength dependence on processing for consolidated metal matrix composites

The ultimate tensile strength (UTS) of metal and intermetallic matrix unidirectional composites can be significantly lower than expected from the rule of mixtures prediction. One possible explanation is that the fibers in the as-processed state are in a residual state of stress and in some cases are broken because of the inhomogeneous nature of the densification during manufacture. Three main results emerge from the effort to include the effect of this processing damage on the composite UTS. First is the development of a simple but accurate analytical version of Curtin's model for predicting the stress-strain response and UTS of this class of composites. Second is the generalization of Curtin's model to include both process induced fiber bending and fracture. Third is that the reduction in strength is a sensitive function of the consolidation conditions; thus a link is established between the quality of the composite and the conditions of its manufacture.

Auger Electron Spectroscopy Study of Grain Boundary Segregation in Alloy K-500: Part I. Behavior in As-Processed State

Increased interest has been paid to grain boundary segregation in alloy K-500 due to severe intergranular cracking recently observed in forged bars. However, little systematic study of this segregation has been performed so far. A detailed auger electron spectroscopy (AES) study of grain boundary segregation in alloy K-500 has been carried out as a function of alloy chemistry. To determine C segregation, the C and O contamination rates in a vacuum chamber were measured and the necessary condition for C grain boundary segregation determination was established. It has been found that severe C, Al, and Cu segregation to grain boundaries occurred and depended on alloy chemistry. High bulk Ni and low bulk Al promoted C and Al grain boundary segregation, and low bulk Ni and high bulk Al significantly enhanced Cu segregation to grain boundaries. The depth profiles of intergranularly segregated elements also showed different features for high and low Ni content alloys. In high Ni alloys, C and Al levels dropped continuously as a function of distance from the grain boundaries but the Cu level dropped only slightly. In low Ni alloys, the Al and C levels rose from relatively low grain boundary levels to a peak at a certain distance from the grain boundary where the high grain boundary Cu level dramatically dropped. Transmission electron microscope (TEM) observation revealed a grain boundaryγ′-depleted zone followed by a region with coarser and denserγ′ particles in low Ni and high Al alloys but quite uniformly distributedγ′ particles with no depleted zone in high Ni and low Al alloys. These can be explained by the observed segregation behavior. The occurrence of Cu segregation is explained according to available theories about surface segregation in binary Ni-Cu alloys, and the segregation of C and Al to grain boundaries is suggested to be probably due to their interaction with Ni and Cu.

Structure/oxidation behavior relationship in the carbonaceous constituents of 2D-C/PyC/SiC composites

Oxidation tests were performed (at 500 < T < 900° C), on 1. (a) as-received T300 fiber, 2. (b) PyC-infiltrated tows 3. (c) C/PyC/SiC composite. The goal was to assess which carbonaceous constituent exhibits the highest reactivity with oxygen in the composites. TGA and TEM were used to establish the relationship between the behaviour of the materials in oxygen and their nano/microtexture. In the as-processed state, the overall oxidation rate of the fiber is higher than that of the PyC interphase, whereas both constituents behave in a similar way after an annealing treatment at 1600°C. Beyond 700°C, the rate-determining step in the oxidation of uncoated fibers is diffusion transport in the external stagnant boundary layer. Below 700°C, it is a mixed in-pore diffusion/surface reaction mode for the as-received fibers and mainly surface reaction for the annealed fibers. The oxidation of the fiber is selective. In cross-section, it occurs more rapidly in the outer zone, owing to the high porosity of this specific zone. The occurrence of a high porosity zone very near the PyC interphase plays a key role in understanding the effect of environment on the mechanical behaviour of the composites.

Microstructural Studies of the Interfacial Zone of a SiC‐Fiber‐Reinforced Lithium Aluminum Silicate Glass‐Ceramic

Transmission electron microscopy studies have been conducted on interfaces in a lithium aluminum silicate/SiC‐fiberreinforced composite. In the as‐processed state, interphases of amorphous C and carbides of Nb have been confirmed, with circumferential thermal debonds evident in the C layer. After heat treatment in air at 800°C, the C is found to be replaced by amorphous SiO2, and the carbides of Nb replaced by oxides. The SiO2 thickens with exposure time and typically contains circumferential separations. Some Mg and Al diffusion also accompanies the heat‐treatment process and eventually leads to the formation of MgO and Mg silicates in the interfacial zone.