Effect of nitrogen on the dynamic strain ageing behaviour of type 316L stainless steel

The dynamic strain ageing (DSA) behaviours of type 316L stainless steels containing different nitrogen contents (0.01–0.15 wt% N) were studied in tension under varying strain rates (1 × 10^–2–2 × 10^–4s^–1) and the test temperatures (R.T.–1023 K). The temperature range for DSA was moved to higher temperature for increasing nitrogen contents. The critical strain, _c for the onset of serration increased with nitrogen content at 773 K and then became almost constant at 873 K. Type A and B serrations were observed at 873 K with the value of the strain required to effect the transition from type A to type B serration increasing for nitrogen contents upto 0.1 wt% and then becoming saturated. The activation energy for DSA was 23.4–26.2 kcal mol^–1 (97.8–109.5 kJ mol^–1) at the onset and 65.0–76.6 kcal mol^–1 (271–320.2 kJ mol^–1) at the end of serration. The lower activation energy was related to vacancy diffusion and the higher activation energy was attributed to the diffusion of chromium to dislocations. The activation energy for DSA was slightly increased with nitrogen addition. DSA was retarded by an increase in the nitrogen content since nitrogen reduced the chromium diffusion to dislocations due to a strong interaction between the nitrogen and chromium.     

Diffusion behaviour of the core-sheath structure in E-glass fibres exposed to aqueous HCl

The existence of a characteristic core-sheath morphology in E-glass fibres undergoing acid corrosion is well known. This effect is attributed to the removal of calcium and aluminium ions from the glass structure. Sheath growth measurements indicate that this abstraction involves a two-stage diffusion process, with a room-temperature diffusion coefficient of 1.4×10^–16 m ² sec^–1 initially, slowing to 1.5×10^–17 m² sec^–1 at later times. This two-stage process is related to fibre structure. It is also shown that there is an initial delay, during which the core-sheath structure does not develop.     

A study on titanium diffusion into LiNbO3 waveguides by electron probe analysis and X-ray diffraction methods

The diffusion of titanium into lithium niobate has been studied by X-ray rocking curve and topographic methods as well as electron probe microanalysis in an attempt to determine the diffusion mechanism and evaluate the crystallinity of the diffused layer. It was found that the titanium concentration along the depth in the diffused layer approximates to a Gaussian distribution, the activation energy for diffusion being 2.18 eV. We also found that the diffusion of titanium caused a marked lattice contraction along the a-axis (a/a –10^–3) resulting in the generation of misfit dislocations and cracks in the diffused layer. The high activation energy and the lattice contraction suggest that the titanium ions diffuse substitutionally into the lithium niobate crystal. Mechanisms causing refractive index changes in the diffused layer are also discussed.     

Dislocation morphology in graded heterojunctions: GaAs1−xPx

The details of the formation, propagation, interaction, and densities of misfit dislocations are combined into a simple model quantitatively predicting dislocation densities for both abrupt and graded heterojunctions. Three key concepts are introduced: (1) misfit dislocations are segmented; (2) accordingly, they must give rise to a density of inclined dislocations, n _I, that propagate through the growing layer; and (3) these inclined dislocations can bend in and out of any subsequently formed misfit plane to relieve the strain, and when bent in, serve as strain-relieving misfit dislocations. Thus, the value of n _I is expected to remain constant with thickness. Also, n _I is predicted to vary directly with the compositional gradient at the heterojunction. It is pointed out that there are two general classes of misfit dislocations, pure-edge and mixed and that their intersections, which cause the misfit dislocations to appear to bend within their plane, can be simply classified into three general types.Transmission electron microscopy was used for a comprehensive study of dislocations in a series of GaAs_1–xP_x heterojunctions prepared by a vapour phase growth technique. The main features of the above model were corroborated. The value of n _I was found to be constant with growth distance as postulated, and in quantitative agreement with prediction, n _I decreased from 4 × 10⁷ cm^–2 to 10⁶ cm^–2 as the compositional gradient decreased from 5% phosphorus/m to 0.2% phosphorus/m. Note that these values can far exceed the dislocation density of the substrates. Of particular significance, the inclined dislocations n _I were found to propagate through a constant-composition region grown on top of a compositionally graded region, so that formation of the heterojunction must affect subsequently grown layers. Finally, it is shown that the misfit dislocations are, indeed, a combination of pure-edge and mixed, and all three postulated general interactions between these dislocations are shown to occur.     

The elimination of defects in Czochralski grown lithium niobate

A method is described for growing large LiNbO₃ single crystals from the melt, completely free of low-angle grain boundaries. This crystalline perfection was achieved by eliminating localized cellular structures, which were introduced by thermal supercooling due to faceted growth. These defects were distributed only near the developed (012)_h and (0 –1 –2)_h planes when the growth of these planes as facets could be reduced rapidly in the conical part of the boule. Low-angle grain boundaries along the z-axis and polygonization of dislocations were induced by the stresses around the cellular structure. Voids and a corrugated interface have also been observed as a cell-boundary groove trail. The observation of cellular structures indicated that their formation was strongly dependent on growth in the radial direction and the pulled rate. Furthermore, to eliminate these structures, it was found most effective to keep the crystal growth rate, G, at less than 10 mm h^–1.     

A kinetic study of the silver-mercury contact reaction

The solid-liquid diffusion couple technique was employed to determine the interdiffusion coefficient of the gamma phase in the Ag-Hg contact reaction. Diffusion coefficients were calculated with the aid of an equation given by Wagner. The composition range of the gamma phase was determined to be between 55.3 and 57.5 at% by electron microprobe analysis, and values for the average interdiffusion coefficient of the gamma phase were found to beD _av(cm²sec^–1)=3.181×10^–5exp (–32539 (J mol^–1)/RT) in the temperature range 40 to 115° C. The amalgamation reaction between silver and liquid mercury proceeded with the formation of gamma phase and a solid solution of Ag-Hg. The growth of gamma phase followed a parabolic rate law. The penetration of liquid mercury into grain boundary of the gamma phase caused the gamma to be crumbled off. The possibility of short-circuit diffusion is discussed.     

Electrical conduction in quartz powder

Observations have been made on the direct current behaviour of crushed natural quartz at temperatures of the order of 1300 K in order to investigate the effect of applied electric fields on the impurities present in the samples. Migration of the alkaline impurities, leaving an alkali free layer adjacent to the anode, has been detected by analysis of the specimens. It is suggested that the electrical conduction in quartz is due almost entirely to the motion of these impurities. Finally, an estimation of a mean diffusion coefficient for these metals was made and was found to be of the order of 10^–l6m² sec^–1.     

Oxygen self-diffusion in single and polycrystalline magnesio-ferrites

Self-diffusion coefficients of oxygen ion in polycrystalline and single-crystal magnesioferrites have been measured by a gas-solid isotopic exchange technique using¹⁸O as a tracer at temperature in the range 975 to 1465° C. A new method was considered for the oxygen volume diffusion in polycrystalline magnesio-ferrite, and its reliability is discussed. The volume diffusion coefficients of polycrystalline magnesio-ferrite (MgO/Fe₂O₃=0.95 in mole ratio) can be expressed asD=1.51×10^–1exp (–78 600/RT) cm² sec^–1 (1135 to 1465° C) andD=1.2×10^–7exp (–38 000/RT) cm²sec^–1 (975 to 1135° C). The volume diffusion coefficients of the polycrystal in the high temperature range were very close to those of single crystal ferrite of the same composition as the polycrystal. The activation energy of grain-boundary diffusion in this polycrystal was expected to be greater than that of the volume diffusion. A possible interpretation of this unusual behaviour is given in terms of an increased enrichment of Fe²⁺ ion along the grain boundary with temperature elevation, by which oxygen vacancies increase.     

Salt diffusion in brick structures

Salts causes surface damages and efflorescence for masonry walls. This has increased the interest to understand the salt transfer phenomena taking place in porous matrix. There is still however a lack of published transport data in the literature. Especially the diffusion coefficients of salts in different brick structures are lacking. In this work a method for measuring the rate of diffusion of salt in ceramic material is presented and applied to the measurement of diffusivity of NaCl in three different brick materials. Fick's first law of diffusion was applied to calculate the diffusion coefficients in a pseudo stationary state by means of linear regression analysis. The result for the diffusivity of 0.05 molar NaCl in water in new Finnish red brick was (0.499 ± 0.004 ) * 10^–5 cm²/s in the temperature of 25 ± 0.05°C. The corresponding values for the old light brick and old dark brick was (0.453 ± 0.008) * 10^–5 cm²/s and (0.337 ± 0.009) * 10^–5 cm²/s respectively. The diffusion coefficients are given as an effective diffusion coefficients calculated with the porosity value measured to each of the specimen. The concept of salt diffusion and diffusion mechanism inside the porous matrix are also discussed.     

Thermal Behavior of a Multi-layered Thin Slab Carrying Periodic Signals Under the Effect of the Dual-Phase-Lag Heat Conduction Model

The thermal behavior of a two–layered thin slab carrying periodic signals under the effect of the dual-phase-lag heat conduction model is investigated. Two types of periodic signals are considered, a periodic heating source and a periodic imposed temperature at the boundary. The deviations among the predictions of the classical diffusion model, the wave mode, and the dual-phase-lag model are investigated. Analytical closed-form solutions are obtained for the temperature distribution within the slab. The effect of the angular frequency, thickness of the plate, dimensionless thermal relaxation time, dimensionless phase-lag in temperature gradient, thermal conductivity, and thermal diffusivity on the temperature distribution of the slab was studied. It is found that the deviations among the three models increase as the frequency of the signals increases and as the thickness of the plate decreases. It is found that the use of the dual-phase-lag heat conduction model is necessary when the metal film thickness is of order 10^–6 m and the angular frequency of the signals is of order 10¹²rad · s^–1.     

Fatigue crack propagation in polymeric materials

In order to gain a better understanding of matrix-controlled fatigue failure processes in non-metallic materials a series of fatigue tests were performed on several different polymer materials representing different classes of mechanical response. Fatigue crack propagation rates between 5×10^–6 in. cycle^–1 (127 nm cycle^–1) and 4×10^–4 in. cycle^–1 (10 300 nm cycle^–1) were measured in nylon, polycarbonate, ABS resin, low-density polyethylene and polymethyl methacrylate. A strong correlation was found between the fatigue crack propagation rate and the stress intensity factor range prevailing at the advancing crack tip. Whereas metals exhibit comparable fatigue growth rates for a given stress intensity range when normalised with respect to their static elastic modulus, the polymer materials exhibited a 1300-fold difference in crack growth rate for a given normalised stress intensity range. This observation dramatically illustrates the importance of understanding molecular motion and energy dissipation processes in polymer materials as related to their chemistry and architecture. The relative behaviour of the different polymer materials could be generally correlated with their reported damping characteristics.     

The effects of fluid flow on secondary arm coarsening during dendritic solidification

Although dendrites are the result of diffusion limited growth it has long been appreciated that flow within the parent melt can have a dramatic effect on these structures. A free boundary model of dendritic solidification is used to assess the effects on the secondary arm coarsening processes of fluid flow within the parent melt. It is found that for solutally controlled coarsening realistic interdendritic flow velocities of the order 10^–3–10^–2 m s^–1 give rise to ripening rates which are comparable to diffusive transport. However, only flows with a component aligned from the secondary tip towards the root enhance the ripening rate. Oppositely aligned flows actually reduce the ripening rate. Thus, due to the four-fold symmetry of dendrites in cubic metals, the actual effect on the secondary arm spacing could be quite small. The results are shown to be in general agreement with recent microgravity experiments on dendritic coarsening.     

Influence of current pulses on the mobility and multiplication of dislocations in Zn

For the purpose of clarification of the mechanisms of the electron-plastic effect the influence of current pulses with a length of 2·10^–4 sec and a density of more than 20 MA/m² on the mobility of pyramidal dislocations in Zn single crystals in the area of thermally activated movement at 77 and 293 K and also on their multiplication was studied by the method of selective etching. It was shown that the increase in the rate of movement of dislocations under the action of current pulses is accompanied by multiplication of the dislocations with current densities of more, than 10² MA/m². The rules established are discussed taking into consideration the pondermotive forces, thermal effects, electron-dislocation interaction, and the work of the Frank-Reid effect under the action of the electron wind.S. Ordzhonikidze Siberian Metallurgical Institute, Novokuznetsk. Translated from Problemy Prochnosti, No. 10, pp. 48–53, October, 1989.     

Effect of electrization and small direct currents on the wear of metals in sliding friction

It was shown that the magnitude and direction of thermally induced electric currents depend on both the friction conditions (loads, sliding speeds) and the friction pair material, and that the character of the influence of electrization on wear is complex. It was established that currents of the order of 10^–6–10^–4 A may have a noticeable effect on wear of metals which is probably due to electro-transfer phenomena taking place in the metal surface layers during friction.     

Analysis of structural defect annealing in copper-base alloys exhibiting the shape memory effect

The process of martensite stabilization is investigated by resistometric method and positron annihilation spectroscopy in Cu-Al-Zn and Cu-Al-Ni alloys exhibiting the shape memory effect. A strong stabilization of martensite observed in Cu-Al-Zn alloys is explained by diffusion of divacancies and monovacancies formed during quenching with the following values of the Arrhenius relation E _2V ^M = (0.69 ± 0.02) eV, K ₀₄ = 6.4 × 10^8.0±0.1s^–1 and E _1V ^M = (0.84 ± 0.02) eV, K ₀₃ = 6 × 10^9.00±0.15s^–1, respectively. It was found that the energy of vacancy formation in Cu-8.5Al-14.5Zn(%wt.) alloy was 0.56 eV. The proposed model has been confirmed by applying the positron annihilation technique. In Cu-Al-Ni alloys the disappearance of quenched-in vacancies is described by one recovery process only with the diffusion energy of monovacancy 0.74 eV.     

Transmission electron microscopy of dislocations in nickel oxide single crystals

The dislocation structure of NiO single crystals used in diffusion studies has been examined by transmission electron microscopy. The crystals contain dislocations (the dislocation density being 4 × 10¹² m^–2) that are probably a result of the growth process. The dislocations have a Burgers vector ofa/2 110. On annealing at temperatures above 1400° C the density is reduced to 7 × 10¹¹ m^–2, most of the dislocations forming low angle boundary arrays. The dislocation density was found to be much greater in the vicinity (within 1m) of a mechanically polished surface.     

Strain and relaxation in Si-MBE structures studied by reciprocal space mapping using high resolution X-ray diffraction

High resolution X-ray diffraction measurements have been done on Si(001)-based structures grown by molecular beam epitaxy (MBE). By systematically varying the angle of incidence and the diffraction angle, the diffraction intensity data can be displayed in a two-dimensional X-ray diffraction intensity map that can be interpretted as a reciprocal space map of the reciprocal lattice points. The experimental technique is described and results for studies of the strain and the strain relaxation with high temperature annealing are described for the following material systems: strained Si/Si_1–xGe_x heterostructures, highly B-doped Si layers on Si and highly B-doped Si_1–xGe_x layers on Si. The strain relaxation in Si_1xGe_x layers occurs via generation of misfit dislocations creating a shift and a characteristic mosaic broadening of the layer peak in the reciprocal space maps. A summary of how the degree of relaxation, as measured from reciprocal space maps, depends on the annealing temperature and the layer thickness is given. The relaxation of the strain induced by B, for doping concentrations up to 5 × 10²⁰ cm^–3, is obtained by diffusion of B into the substrate. For a Si_0.82Ge_0.18 layer with partially compensated compressive strain due to a B concentration of 3 × 10²⁰cm^–3, the maps show a combination of strain relaxation via misfit dislocations and B diffusion into the substrate.     

High temperature compressive properties over a wide range of strain rates in an AZ31 magnesium alloy

High temperature compressive properties in AZ31 magnesium alloy were examined over a wide strain rate range from 10^–3 to 10³ s^–1. It was suggested that the dominant deformation mechanism in the low strain rate range below 10^–1 s^–1 was dislocation creep controlled by pipe diffusion at low temperatures, and by lattice diffusion at high temperatures. On the other hand, analysis of the flow behavior and microstructural observations indicated that the deformation at high strain rates of 10³ s^–1 proceeds by conventional plastic flow of dislocation glide and twinning even at elevated temperatures.     

Low-cycle fatigue of polycrystalline α-iron modified by mutually immiscible silver-ion implantation

Cyclic deformations of annealed pure polycrystalline -iron with and without further mutually immiscible silver-ion implantation (90 keV, 6×10¹⁶ ions cm^–2) were studied in a plastic strain-controlled tension-compression fatigue test (triangular loading waveform, frequency 0.02–0.3 Hz, and plastic strain range 3×10^–3–1.2×10^–2). The obtained plastic strain-life (_p-N _f) curves showed that the iron specimens could survive for a greater number of cycles before failure when implanted. Comparison of the cyclic stress-strain curves suggested that the implanted specimens had maintained a relatively more stable microstructural change than those unimplanted ones which had undergone a violent cyclic hardening during cyclic deformation. This is proposed to be a strong indication that the fatigue ductility has been improved and the cross slip of screw dislocations, which leads to the evolution of the persistent slip bands for fatigue damage, was hindered to some extent after ion implantation.     

Surface Diffusion of Solid Hydrogen

A number of experiments have shown that hydrogen molecules can migrate on films of solid hydrogen. This was thought to be a diffusive process but recent experimental findings have led others to a different conclusion, so a question remains about the process by which the hydrogen molecules migrate over their own solid surface. We report new measurements using a hole-burning technique which confirm these recent experiments but we interpret them differently. We have developed a model for the recovery of the hydrogen which accounts for these results and shows that the H₂ does move diffusively. The diffusion is thermally activated and we have determined the diffusion constant, D ₀=4.03×10^–4±2.5×10^–5 m²s^–1, and the activation energy, E=43±7 K, for hydrogen.