Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 1989"

Embrittlement of interfaces by solute segregation

Abstract: We discuss theoretical models of interfacial embrittlement by solute segregation. Of properties susceptible to alteration by segregation, the ideal work of interfacial separation, 2 γ int , is predicted to have an important but probably not exclusive role in controlling embrittlement. A thermodynamic framework for estimating 2 γ int from data available through free surface and grain boundary adsorption studies is outlined, and relevant segregation energies are given for carbon, phosphorus, tin, antimony and sulphur segregation in iron. Data from intergranular fracture experiments involving these same segregants is also summarized in an attempt to test the idea that segregation-induced embrittlement (or ductilization) can be understood in terms of the segregant's effect on 2 γ int . Uncertainties in present data do not allow a convincing test, but it is not implausible that the deleterious effects of phosphorus, tin, and sulphur in iron can be understood in this way. The effect of carbon does not seem to be similarly understandable, although that may be due to the inappropriateness of the only available surface segregation data in that case, which are for a (001) surface rather than a general polycrystalline surface created by intergranular fracture.

Theory of plastic deformation: - properties of low energy dislocation structures

Abstract: Work-hardening phenomena are based on the very fundamental principles (i) that at the position of every dislocation axis the respective resolved shear stress cannot exceed the friction stress, including the self-stress of bowing dislocations, and (ii) that always that structure forms which among those accessible by the dislocations minimizes stored energy per unit length of dislocation line. Such dislocation structures have been named LEDSs. The corresponding work-hardening theory, the mesh length theory, is applicable to all materials deforming via gliding dislocations and to all types of deformation. Results previously achieved with the mesh length theory are summarized, and a number of new developments are discussed. Depending on the dislocation structures formed, the work-hardening behavior differs. Easily intersecting glide causes dislocation cell structures with almost dislocation-free cell interiors delineated by dislocation rotation boundaries. Pronounced planar glide causes Taylor lattices characterized by local planar order parallel to the one or perhaps two most highly stressed glide plane(s), no systematic lattice rotations, and overall uniform dislocation density. The most widely observed basic features of work hardening are explained in general terms. Specific applications are indicated for layer-type crystals, h.c.p. single crystals, single-crystal and polycrystalline pure f.c.c. metals and α-brass-type alloys, precipitation-hardened materials and steels. Included are the different stages of work hardening, dynamical effects in low temperature plasticity, the general characteristics of grain boundary strengthening and the Hall-Petch relationship. In addition, proposed explanations for (i) glide system interactions in polyslip resulting in microbands and affecting texture formation, and (ii) creep without stress dependence of dislocation density, are discussed.

Sandwich test specimens for measuring interface crack toughness

Abstract: A crack lying along one interface on an elastic sandwich structure is analyzed. When the thickness of the middle layer is small compared with the other length scales of the structure, a universal relation is found between the actual interface stress intensity factors at the crack tip and the apparent mode I and mode II stress intensity factors associated with the corresponding problem for the crack in the homogeneous material. Therefore, if the apparent stress intensity factors are known, for example calculated from the applied loads as if the structure was homogeneous, this information can be immediately converted into the interface stress intensity factors with the universal relation. This observation provides the theoretical basis for developing sandwich specimens for measuring interface crack toughness. The universal relation reveals the extent to which the asymmetry inherent to a bimaterial interface induces asymmetry in the near tip crack field. In particular, the result of the study can be used to infer whether stress intensity factors for a homogeneous body can be used with good approximation in place of the actual interface stress intensity factors. A proposal for simplifying the approach to interfacial fracture is made which plays down the role of the so-called oscillatory interface singularity stresses.

Effects of matrix microstructure and particle distribution on fracture of an aluminum metal matrix composite

Abstract: This paper presents the results of a study on the effects of matrix microstructure and particle distribution on the fracture of an aluminum alloy metal matrix composite containing 20% by volume SiC particulate. The matrix microstructure was systematically varied by heat treating to either an under- or over-aged condition of equivalent strength, and was characterized using a combination of techniques. Quantitative metallographic techniques were utilized to characterize the material with respect to size, size distribution, and particle clustering, while transmission electron microscopy was utilized to characterize the details of the matrix microstructure in addition to the effects of aging on the character of the particle/matrix interfaces. Fracture experiments were conducted on smooth tensile, notched bend, shortrod toughness, and on specimens designed to permit controlled crack propagation, in an attempt to determine the effects of matrix microstructure and clustered regions on the details of damage accumulation. Large effects of microstructure on the notched properties were obtained with little effect of microstructure on tensile ductility. It is shown that the micromechanisms of fracture are significantly affected by the details of the matrix microstructure, interface character, and degree of clustering in the material. Fracture of the SiC was predominant in the underaged materials, with a preference for failure in the matrix and near the interface in the overaged material. Metallographic and fractographic analyses revealed that clustered regions were preferred sites for damage initiation in both the aging conditions tested, while preliminary results additionally indicate that damage accumulation ahead of a propagating crack also tended to occur in clustered regions.

The reactive element effect in high-temperature corrosion

Abstract: When small quantities (typically 1 wt.% or less) of a number of reactive elements are added to high-temperature alloys containing chromium or aluminum, a number of beneficial effects result. This is called the “reactive element effect” (REE). Some or all aspects of the REE can be developed when the reactive element is added as an alloy addition, when it is present as a dispersion of oxide particles in the alloy, when it is ion implanted into the surface, or when a coating of the reactive element oxide is applied to the surface of the alloy. In this paper, the experimental evidence is briefly summarized, and different models that have been proposed to account for one or more aspects of the REE are assessed in terms of this evidence.

On microstructural evolution and micromechanical modelling of deformation of a whisker-reinforced metal-matrix composite

Abstract: The precipitation characteristics, the mechanisms of accelerated aging, and the variation of uniaxial tensile stress-strain behavior in response to controlled variations in matrix microstructure were investigated for a 2124 AlSiC whisker composite. The yield strength of the composite was found to be independent of matrix aging condition. However, the overall ductility decreased monotonically with an increase in aging time. A finite element analysis of the constitutive response of the composite is presented. The results of these calculations, as well as the predictions of several models for composite strengthening available in the literature, were compared with the experimental results. The presence of brittle whiskers in aluminum leads to a significant build-up of hydrostatic stresses in the matrix during plastic deformation. Void formation in the matrix of the composite as well as at the whisker-matrix interface appears to play an important role in controlling the overall failure mechanisms. Transmission electron microscopy observations of void formation a whisker ends are described for composite specimens strained in tension at room temperature and at 300°C. A detailed discussion of matrix deformation and interfacial debonding is presented in an attempt to identify the origins of low ductility in discontinuously reinforced metal-ceramic composites.

Microstructural aspects of aluminium-silicon carbide particulate composites produced by a casting method

Abstract: When metal matrix composites are produced by molten metal methods there are some unique factors which have to be considered. In this paper, the microstructure of SiC-reinforced aluminium alloys produced by this method are considered. It is shown that the stability of SiC in the melt is dependent on the matrix alloy involved and that only alloys with high silicon contents have a low reactivity with this reinforcement. With other alloy matrices, SiC reacts to form Al 4 C 3 , and the nature of this reaction and its kinetics are considered in this paper. Initially, the reaction rates are very rapid but almost saturate after about 1 h. It is also shown that the distribution of the reinforcing particles is dependent on the solidification rate because particles are rejected and pushed ahead of the meniscus. At low solidification rates, and hence for large cell sizes, the reinforcing particles are clustered and form a network which delineates the cell walls. Because the SiC particles are in the interdendritic regions they will be associated with any coarse intermetallic particles present and this can influence the fracture behaviour.

Modelling the mechanical behavior of cellular materials

Abstract: Materials with a cellular structure are widespread; they include natural materials such as wood and cork as well as man-made honeycombs and foams. Their cellular structure gives rise to unique properties which can be exploited in engineering design. The selection of a specific honeycomb or foam for a particular engineering application is guided by models which describe their mechanical behavior in terms of the cell geometry and the mechanisms of deformation and failure. In this review, models for the mechanical behavior of cellular materials are first described and then used to select the optimum foam for two engineering applications, the design of packaging and of light-weight structural sandwich panels.

Deformation structures in lightly rolled pure aluminium

Abstract: The dislocation structure of lightly rolled aluminium is free of substantial long-range stresses and thus is a low-energy dislocation structure (LEDS). It consists of ordinary cell walls, dense dislocation walls (DDWs) and microbands (MBs) which are stretched out along DDWs and are composed of small pancake-shaped cells. In one particular sample studied, MBs were found in the orientation of shear bands, although they are not observed macroscopically. Since the DDWs and MBs appear together as if forming one general feature they have been dubbed DDW-MBs. The structure can be explain on two hypotheses: (i) All volume elements enclosed by DDWs, including MBs, are blocks of dislocation cells sharing the same combination of active glide systems which, however, are fewer in number than would be needed to satisfy the Taylor condition fully, for the reason that the rate of work hardening increases with increasing number of simultaneously activated glide systems. (ii) MBs are formed later than the DDWs on which they are situated. They complement the deformation due to the earlier cell blocks towards a better approximation of the Taylor condition. It is considered that MBs assume the orientation of shear bands when the strain in them leads to geometrical softening.

Plasticity-induced martensitic transformation during cyclic deformation of AISI 304L stainless steel

Abstract: Specimens of AISI 304L stainless steel were deformed cyclically at room temperature at various plastic strain amplitudes Δϵpl/2. In addition to tests at constant plastic strain amplitudes, incremental step tests were also carried out. The cyclic deformation behaviour was investigated and related to microstructural changes observed by transmission electron microscopy (TEM). At low values of the plastic strain amplitude a saturation state was reached, whereas specimens fatigued at plastic strain amplitudes Δϵpl/2 > 0.3% exhibited, after initial cyclic hardening, an extensive secondary hardening stage which persisted until fracture. The reasons for this behaviour were clarified by TEM observations. Planar dislocation arrays with faults were observed after cycling at Δϵpl/2 = 0.02%. However, martensitic phases were not found. The specimens fatigued at Δϵpl/2 = 0.5% or in the incremental step test (0.02%<Δϵpl2<0.5%) displayed cell structures with a considerably higher number of stacking faults. Moreover, deformation-induced martensitic phases were observed and identified as ϵ- and α′-martensite by electron diffraction pattern analysis.

The role of interfacial dislocation networks in high temperature creep of superalloys

Abstract: The dislocation networks generated during high-temperature creep of several single-crystal nickel-based superalloys are analyzed. The networks continually evolve during creep at relatively low temperatures or eventually reach a more stable configuration at high temperatures. Specifically, the role of these networks in directional coarsening processes are studied, along with their formation kinetics, characteristics, and stability during creep. The results of this study combined with previous findings suggest that the directional coarsening process is strongly influenced by elastic strain energy. The dislocation networks formed during primary creep are found to be stable during all subsequent creep stages. Aspects of these dislocation networks are determined to be a product of both the applied creep stress and coherency strains.

Dislocation substructures in mild steel deformed in simple shear

Abstract: Simple shear tests are performed on mild steel samples predeformed in tension. Specific attention is paid to the transient mechanisms produced by the change of loading mode. The “length” of the transition is comparable to the prestrain. Both macroscopical behaviour and microscopial features appear to depend drastically on the angle (α) between the tensile and the shear directions. Yield stress at reloading is minimum for α = 135° and is followed by a stagnation of the work hardening. Dissolutions of cell walls, typical of Bauschinger tests, are observed for this case. By contrast, for α = 90° the yield stress is maximized; the stress-strain curve then exhibits softening which is shown to be related to microband formation. Finally, a pseudomonotonic behaviour is noted for α = 45°. These different features are connected to forest and mobile dislocation densities as well as to intragranular structure destabilization.

State of the art on high-temperature corrosion-resistant coatings

Abstract: A high-temperature protective coating must meet several criteria: provide adequate environmental resistance, be chemically and mechanically compatible with the substrate, be applicable. Comprehensive reviews on high-temperature coatings have appeared regularly since the early 1970s. Our purpose is not to recapitulate the material covered therein but rather to focus on recent trends, and point out some research perspectives. Historically the development of high-temperature protective coatings has been linked with the evolution of demanding applications such as super-alloy components in gas turbine engines; the searches for better performance (higher inlet temperatures, longer lifetimes, etc.) and for cost-saving solutions (use of contaminated low-grade fuels) have been the main incentives for developing the different coatings now available in production: simple and “modified” diffusion coatings, overlay coatings, thermal barriers. In recent years research and development activity has been concentrating on the following points. 1. (a) Degradation mechanisms in high-temperature corrosion of metallic coatings; basic studies on the growth mechanisms of oxide scales, for example are still required, in particular to understand the role of addition elements such as platnum and palladium. 2. (b) Alternative techniques for depositing MCrAlY coatings; electrolytic codeposition and electrophoresis, for example, have been developed at the laboratory stage and these permit the deposition of MCrAlY coatings with claimed economic and technical advantages over processes already in production. 3. (c) Thermal barrier coatings; ceramic coatings have been applied to sheet metal combustor components for about 15 years; only recently have they been used in the turbine section. Two challenges remain though to exploit these coatings on turbine blades: improve their reliability and, in the case of stationary gas turbines, their hot corrosion resistance. Both structural and mechanical approaches are required to determine, in particular, the role of microstructure, microcracking, porosity, residual stresses, oxidation of the bond layer in the degradation mechanisms of these coatings. 4. (d) Mechanical properties of coated systems; the intrinsic mechanical properties of coating materials are still poorly described and the lack of information hinders the adequate modelling of the behaviour of coating-substrate composite systems. In parallel, an increasing activity is noted concerning the design and development of high-temperature coatings for protecting materials other than superalloys, for instance ceramic composites and titanium-based alloys.

Cracking and spalling of protective oxide layers

Abstract: This paper discusses the role of thermal stresses on the mechanical integrity of oxide layers. It is shown that the processes of cracking and spalling of oxides differ depending on the stress state (tensile or compressive) and on the relative strengths of oxide and oxide-metal interface. Under tensile conditions, through-thickness cracks develop from pre-existing defects in the oxide layer and these generate shear stresses along the interface which may result in decohesion. Under compressive conditions, spallation may result either from the growth of a tensile, wedge crack along the interface or by buckling and cracking of the oxide layer. Initial results of a finite element analysis are provided for both mechanisms.

Thermal barrier coatings for gas turbine use

Abstract: The use of thermal barrier coatings on high-pressure turbine components can improve gas turbine efficiency through reduction of cooling airflow. However, the risk involved in reducing cooling airflow requires a highly reliable thermal barrier coating. This increased reliability will be achieved through several complementary approaches; material and process development, life prediction method development and engine service experience. The two processes available for deposition of thermal barrier coatings (plasma spray and physical vapor deposition) are compared, and the advantages and disadvantages of each discussed as they apply to gas turbine components. The results of bond coat material development which has increased the thermal cycle life of plasma spray thermal barrier coatings are presented. Improvements were achieved by two methods: (1) the use of creep-resistant bond coat compositions and (2) overaluminiding of the bond coat. Results of engine testing of thermal barrier coatings in an environment that produces hot corrosion are also presented.

A theoretical and experimental study of aluminum alloy 6061-SiC metal matrix composite to identify the operative mechanism for accelerated aging

Abstract: Accelerated aging in metal matrix composites (MMCs) can be attributed to an increased dislocation density in the vicinity of the reinforcements or to the matrix residual stress field near reinforcements. Both mechanisms aid the diffusion of solute atoms, thereby leading to more rapid precipatation. In this work, the precipitation behavior of aluminum 6061 alloy reinforced with 10 vol.% SiC whiskers of variable aspect ratio was studied experimentally. The results were compared with the precipitation behavior of a control aluminum alloy 6061 in the unstrained and plastically strained conditions. It was found that the strained control alloy, with approximately the same expended plastic work as the composite, showed a similar β′ precipitation rate and activation energy as the composite. On the contrary, the unstrained alloy had a much higher activation energy for precipitation. A theoretical model was developed to predict the rate of precipitation in the residual stress field of the matrix. This rate was compared with the rate of precipitation on a regular edge dislocation array. It was found that, for realistic values of fiber radii and dislocation densities (about 0.25–1 μm and 1013–1014m−2 respectively), both mechanisms give comparable precipitation rates. However, solute atoms flowing towards the matrix-fiber interface under the influence of the residual stress field on encountering matrix dislocations are trapped, thereby lowering the activation energy to that of precipitation on dislocations. It was concluded that, for MMCs with large fibers and high dislocation densities, dislocation generation is the principal contibutor to accelerated aging while, in MMCs with small fibers and low dislocation densities, the residual stress mechanism predominates. For intermediate fiber radii and dislocation densities, both mechanisms could be important although, in real MMCs, dislocations seem to play the dominant role.

Electrical discharge machinable ceramic composites

Abstract: The recent improvements in mechanical properties of ceramics have led to the development of high-strength and high-toughness ceramic composites the machining of which, using conventional techniques, is a rather complicated and expensive routine especially when complex parts are manufactured. Electrical discharge machining is an attractive alternative machining technique but it requires electrically conductive ceramic materials. New conductive silicon nitride- and alumina-based composites with good mechanical properties were developed for this purpose by adding amounts of TiC and/or TiN particles to the ceramic matrix. The machinability and the physical properties of the composites are related to the content and the characteristics of the conductive TiC and/or TiN particles. The compositions can be adjusted to find the best compromise between high strength and electrical discharge machinability.

Strain hardening and substructural evolution in NiCo solid solutions at large strains

Abstract: Torsion deformation was used to investigate dislocation substructure evolution at large strains in high purity nickel and NiCo solid solutions. Observations of small strain dislocation structures formed in stage III revealed that the laminar dislocation structure observed after large strains in stage IV develops from short paired dislocation sheets within the tangled dislocations of an equiaxed cell wall. The development of these short paired dislocation sheets into long microbonds occurs gradually by a multiple-slip process in accordance with the principles of low energy dislocation structures and without the occurrence of a shear instability. The plane of these sheets and /or microbands does not correspond to a {111} slip plane. As these microbands form, a misorientation between the interior of the paired sheets and the surrounding matrix develops and increases with increasing strain.

Structure and chemistry of metal/ceramic interfaces☆

Abstract: The present state of knowledge is reviewed concerning the structure and chemistry of metal/ceramic interfaces. Experimental observations are described for several model systems and open problems concerning different aspects of structure and properties of heterophase boundaries are discussed.

The mechanical performance of fiber-reinforced ceramic matrix composites

Abstract: This article evaluates the current understanding of relationships between microstructure and mechanical properties in ceramics reinforced with aligned fiber. Emphasis is placed on definition of the micromechanical properties of the interface that govern the composite toughness. Issues such as the debond and sliding resistance of the interface are discussed based on micromechanics calculations and experiments conducted on both model composites and actual composites.

A simple development of the shear lag theory appropriate for composites with a relatively small modulus mismatch

Abstract: The standard shear lag analysis is known to give unreliable predictions for composite stiffness and other properties when applied to systems with a relatively small ratio between the elastic moduli of the two constituents, such as metal matrix composites. This has long been thought to be at least partly due to neglect of the transfer of normal stresses across the fibre end jaces. In this paper, a simple postulate is made that this stress will approximate to the mean between the peak value in the fibre and the average value in the matrix remote from the interface. On this basis, simple analytical expressions are derived for composite stiffness and applied stress at the onset of yielding. It is shown by comparison with the Eshelby predictions and experimental data that the analysis appears reliable over the complete range of fibre aspect ratios and volume fractions when applied to metal matrix composites.

Spinodal decomposition and mechanical properties of an austenitic Fe-30wt.%Mn-9wt.%Al-0.9wt.%C alloy

Abstract: The microstructure and mechanical properties of an austenitic Fe-30wt.%Mn-9wt.%Al-0.9wt.%C alloy after aging in the temperature range between 743 and 873 K have been investigated. The occurrence of spinodal decomposition was confirmed by transmission electron microscopy observation of a modulated structure with superlattice reflections and identification of the X-ray sidebands. The rapid increase in the yield stress in the early stage of aging was proportional to the increase in the modulation amplitude and independent of the wavelength. The observed hardening has been examined using a theory dealing with the coherency strain produced by the spinodal decomposition.

Processing and properties of metal matrix composites containing discontinuous reinforcement

Abstract: Metal matrix composite materials which utilize discontinuous-type reinforcement undergo unit operations associated with ingot, powder or cast metal manufacturing in addition to their own unique operations. An attempt is made to summarize the scope of operations for both cast- and powder-based processing and identify processing issues which cause matrix-reinforcement interface and property variability in discontinuous metal matrix composites.

Hydrogen permeation behaviour in austenitic stainless steels

Abstract: The permeability, diffusivity and solubility of hydrogen in six types of austenitic stainless steel—316L, 316LN, 21-6–9, 21-9-9, 304 and 321—have been measured by a gaseous permeation technique in the temperature range 200–430°C. The effect of the cold-working and heat treatment conditions and the alloy composition of the materials on hydrogen permeation has been investigated. The results indicate that the permeability and diffusivity of hydrogen in various alloys obey Arrhenius relationships over the experimental temperature range and the hydrogen permeation behaviour is not significantly influenced by cold-working and heat treatment conditions of the materials but is slightly influenced by alloy composition. The difference between the hydrogen permeation behaviour of pure iron, general alloying steels and austenitic stainless steels has been discussed, and a comparison between the present work and data in the literature has been made.

Evolution of dislocation structures and cyclic behaviour of a 316L-type austenitic stainless steel cycled in vacuo at room temperature

Abstract: Dislocation structures formed during cyclic deformation at room temperature in vacuo of a 316L-type austenitic stainless steel are presented. It has been shown that for this material having a rather low stacking fault energy of about 28 mJ m−2, dislocation structures exhibit a planar slip or a wavy slip character depending on the cyclic plastic strain amplitude. In particular the existence of wall and channel, labyrinth or ladder structures has been shown; these structures have been observed to evolve progressively into cells during extensive cycling in vacuo. The volume fraction of each type of structure in the specimen has been evaluated quantitatively as a function of the cyclic plastic strain level and the number of cycles throughout the fatigue life in vacuo.

Triple-line corrosion in high purity nickel

Abstract: Electrochemical studies were conducted with high purity (99.999%) polycrystalline nickel in 2 N H 2 SO 4 and 0.3 M NaCl + 0.3 M Na 2 SO 4 solutions in order to assess the relative susceptibility of triple junctions to localized corrosion. The susceptibility was found to be strongly dependent on the specific crystallographic arrangement of crystals (as determined by electron channelling) at the triple junction, in accordance with Bollmann's criteria for a nodal balance of adjoining grain boundary dislocation arrays. Preferential triple-line corrosion was only evident at those junctions where these criteria were not satisfied. This enhanced susceptibility to localized corrosion (i.e. relative to adjoining grain boundaries and crystal surfaces) was examined in the light of the structural and chemical characteristics of these defects, and their influence on electrochemical surface film stability.

Bombardment-induced compositional change with alloys, oxides, oxysalts and halides III. The role of chemical driving forces☆

Abstract: Recent experiments suggest that weak chemical driving forces play a role in bombarded targets even when, for reasons of extreme energy disparity, it might be difficult to understand why this should be so. With bombardment-induced segregation the driving force, assuming the process to resemble equilibrium segregation, is 0.06–0.52 eV atom −1 . We suggest that the segregation is the result of a certain fraction of the ballistic trajectories terminating near the target surface being followed by one or more low energy, chemically guided steps. With bombardment-induced mixing the driving force, according to Cheng et al., is typically 1.3 eV atom −1 or less. We propose a very similar model to that of segregation, namely ballistic trajectories with terminal steps which are chemically guided. Finally, with bombardment-induced decomposition, as when oxides lose oxygen or sulfates lose sulfur and oxyten, there appears to be a role for what are effectively phase changes in which the system appears to tolerate an energy increase of up to 0.7–0.8 eV atom −1 but not a larger amount. The changes, which resemble those analyzed by Fecht and Johnson, are aided by a number of factors, including amorphization, point-defect accumulation, volatility, diffusional transport and segregation. We tend to avoid in all cases a thermal spike type of argument, if only because the inequality (driving force) ⪆ kT must be obeyed.

The influence of hydrostatic pressure on the ductility of AlSiC composites

Abstract: Tensile tests with superimposed hydrostatic pressures were performed on two types of metal matrix composite: 2014 Al with 20% SiC particles and 2124 Al with 14% SiC whiskers. In the materials with SiC particulate, the ductility increases rapidly with pressure and the mode of damage initiation is by particle fracture. Materials containing SiC whiskers exhibit a different fracture mode involving whisker matrix decohesion, and strain localization which results in shear fracture.

Phase transformations of a nitrogen-implanted austenitic stainless steel (X10 CrNiTi 18-9)☆

Abstract: The available results of research work performed on nitrogen-implanted austenitic CrNi steels are contradictory. The following characteristics have been observed: formation of martensite in the austenite as well as complete retransformation of martensite into austenite; solution of nitrogen up to high concentrations as well as precipitation of nitrides at low concentrations; an increase as well as a reduction of the wear resistance. The transformation of austenitic CrNi steels under nitrogen bombardment is apparently not in accordance with a fixed pattern but can go in different directions. Therefore a systematic phase analysis in conjunction with hardness and corrosion tests is indispensable for process development. On the other hand, this system is of particular interest as a model for studying phase reactions in steels with nitrogen implantation because of the variety of transformations. In this paper we first give an outline of the problems encountered in the treatment of austenitic steels with nitrogen ion beams and introduce our programme for the investigation. We then discuss conditions for γ ⇌ α transformations in either direction. In this context we differentiate between pure irradiation impact and alloying effects caused by the constitutional changes. Further investigations cover the formation of nitrides in pure austenite and the development of surfaces with different pretreatment. The analytical methods used are conversion electron Mossbauer spectroscopy, X-ray diffraction, and depth profiling by nuclear reaction analysis.

The poison effect in cork

Abstract: The Poisson effect in cork was studied experimentally for compressive strains up to 30% in each of the principal directions (radial, axial and tangential). Because the axial and tangential directions are equivalent in cork, there are only three fundamental cases depending on the direction of compression (radial or perpendicular to the radial direction, i.e. non-radial) and on the direction in which the transverse strain is measured (radial or non-radial). The Poisson ratios are as follows (the first subscript in the directional of compression): ν R NR = 0.097, ν NR R = 0.064 and ν NR NR = 0.26 where R indicates radial and NR non-radial. At large strains the Poisson effect, measured by an incremental Poisson ratio, decreases in all cases. In the NR/R mode the effect becomes negative, the dimension parallel to the radial direction eventually becoming smaller than before compression. The effects are qualitatively explained in terms of the cellular structure of cork: prismatic cells with staggered cell bases and undulated lateral walls. An observed correlation between the undulations and the location of the cell bases is responsible for the peculiar Poisson effect in the NR/R mode.