Materials Science and Engineering最新文献

Tempered martensite embrittlement (TME) in AISI 4340 steel was studied for how variations in the test temperature and grain size affect the plastic flow.

The grain size was changed by varying the austenitizing temperature in the range of 870–1200°C. For the evaluation of TME with test temperature, Charpy impact testing was performed in the range of −196-23°C.

TME occurs because of an effective activation of intergranular brittle fracture in the 300°C tempered condition where grain boundary carbides are present, the ductile-brittle transition temperature (DBTT) increases with increasing grain size and the transition to brittle fracture is attributed to the occurrence of intergranular brittle fracture. This effect of grain size on the fracture behaviour indicates that the intergranular brittle fracture is controlled by the stress concentration susceptibility, i.e. the extent of dislocation pile-up at the grain boundaries, which increases with increasing grain size.

In the 300°C tempered condition (in the presence of grain boundary carbides), the DBTT is higher by 70–150°C, compared with the 200°C tempered condition (nearly devoid of grain boundary carbides) where the transition to brittle fracture results from transgranular brittle fracture. A critical test temperature below which intergranular TME can occur is reduced with decreasing grain size.

Therefore, intergranular TME can be produced by the occurrence of intergranular brittle fracture in the presence of grain boundary carbides, which can be more effectively activated as the stress concentration susceptibility increases with increasing grain size or with decreasing test temperature.

CuInSe₂ thin films were prepared on mica substrates using an evaporation technique which involved varying the distance between the source and the substrate. Films evaporated on substrates at distances less than 1 cm from the source exhibited preferential orientation in the (112) direction. The grain size increased from 0.5 to 1.0 μ m as the source substrate distance decreased from 3 to 1 cm. The conductivity increased with a decrease in the distance between the source and substrate. Electron probe microanalysis of these films showed that films coated on substrates farther away from the source had a slight selenium deficiency and an excess of indium. Three characteristic energy gaps of 1.01, 1.25 and 2.4 eV were obtained from an analysis of the optical absorption spectrum. The optical transition probability for valence band to conduction band transitions was estimated to be 10.91 eV which gives an admixture of copper d states to the valence band of 32%.

Stress-strain hysteresis loops obtained from cyclic deformation of Ni₃Ge single crystals under strain control were analyzed. The major component of the cyclic flow stress was found to be the friction stress. The back stress was responsible for, at most, several per cent of the cyclic flow stress. The back stress was almost constant regardless of the amount of cumulative plastic strain. The origin of the back stress is attributed to a small long-range stress field of nearly uniformly distributed screw dislocations produced in the cyclically deformed Ni₃Ge single crystals. The friction stress is considered to be developed by the interaction between straight screw dislocations on parallel (111) slip planes and by the pinning of dislocations due to the thermally activated cross-slip from (111) to (010).

Grain boundaries in sintered magnetite were characterized with Kikuchi patterns from the neighboring grains, obtained by transmission electron microscopy. The misorientations were catalogued according to their Σ values, and their relative frequencies were compared with those for a random distribution (obtained by computer simulation). The experimentally determined distribution was essentially random; there was no dominance of low Σ boundaries.

The paper presents the results of studies on plasma nitriding of titanium in nitrogen at 1030°C. For the nitriding process, flat specimens were placed on the cathode and, after the treatment had been carried out, they were analyzed for differences between the layers produced on their upper and lower surfaces. The nitrided layers have the same thickness on both sides of the specimens. Microhardness and corrosion resistance in 15% H₂SO₄ of the layers produced on the upper surfaces are higher than those of the layers formed on the lower surfaces of the specimens.

The reason for these differences is the contamination of the lower layer by carbon. Carbon also causes an increase in the value of the lattice parameter of the TiN nitride.

The action of nitrogen ions on the upper surface of the specimens entails sputtering of the surface and removes the contamination by carbon.

For this reason, the nitrided layer produced on the upper surface contains less impurities and has better properties.

This work outlines the fracture properties of partially annealed low carbon mild steel and attempts to predict them using X-ray diffraction techniques. For this purpose sheets 2.5 mm thick with cold reductions of 0%, 28% and 50% were used to produce partially annealed samples (610 °C for 0, 60 and 300 s), the zero condition being the initial condition in each case. For each combination of percentage cold reduction and time of annealing, duplicated tests were performed to determine X-ray line broadening, microstructure, static tensile fracture properties and fatigue crack growth rates. From the results obtained it can be concluded that there is a definite relationship between the line broadening and the fracture characteristics of partially annealed low carbon mild steel. Furthermore, these relations can be used to predict fracture properties from the line broadening results.

The relation between the elastic strain sensitivity coefficient of resistance and the hydrostatic pressure coefficient of resistance is not simple, owing to the tensor nature of the resistivity of solids. We discuss this relation and derive a rigorous expression which demonstrates that both coefficients have to be measured in order to characterize a given material. Simple relations which often appear in the literature should be dealt with cautiously because they usually treat the resistivity of metals as a scalar, thus eliminating one of the piezoresistance coefficients.

The heterogeneous phase in commercial AlZnMgCu alloys has been studied using transmission electron microscopy. It is found that Fe₄Al₁₃ is the main constituent phase, in which multiple twins are frequently observed. Twinning planes are (200) and $\text{(20}\text{1}\text{)}$. The orientation relationship of neighbouring twins can be equivalently considered to rotate 2π/10 about [010]. A selected area diffraction pattern with tenfold symmetry has been recorded and is discussed in relation to quasi-crystals.

Using Auger electron spectroscopy, site competition in surface segregation of sulphur and phosphorus in a polycrystalline Co-5.4at.%Ru alloy was investigated. Annealing of the sample at 1250 K results in sulphur desorption. Then the phosphorus segregation starts to increase. A model of the surface which explains the observed phenomenon is presented.

An analysis of the kinematics of liquid-drop solidification is developed using a nucleation/growth model consisting of multiple nucleation events, occurring simultaneously on the drop surface, followed by progression of solidification fronts from each of these nuclei into the drop interior. The rate at which the fronts move may vary with time, but at any given instant it is the same for every point on every front. On this basis, closed-form expressions are derived for the time-dependent average volume-fraction that has solidified both as a function of position within the drop and for the drop as a whole. Some specific examples are considered; among these are certain limiting cases, the results for which are shown to be consistent with studies reported by other investigators.