Showing papers in "Metal science in 1980"
TL;DR: In this article, the growth of grain-boundary cavities by power-law creep, under multi-axial stress states, is estimated by approximating the growth time to fracture at constant stress by t f =t n+ɑ/n+1)ess1n(1/(n+ 1)f i ) where t n is the nucleation time (the time at which the voids first appear), ess is the steady-state creep rate, n the creep exponent and f i is the original area fraction of cavities.
Abstract: Approximate methods are used to calculate the growth of grain-boundary cavities by power-law creep, under multi axial stress states. The time to fracture at constant stress is given by t f =t n+ɑ/n+1)ess1n(1/(n+1)f i )where t n is the nucleation time (the time at which the voids first appear), ess is the steady-state creep rate, n is the creep exponent and f i is the original area fraction of cavities. The quantity α is defined by:ɑ=1/sinh-{2(n-½)/(n½)P/σe}where p is the hydrostatic pressure and σe the von Mises equivalent stress. Differential equations are given which allow the times and the strains to failure under variable loading histories to be calculated.
391 citations
TL;DR: In this paper, the basic rate equation from chemical reaction rate theory is modified to reflect physical and chemical processes which occur at crack tips, based on the assumption that the crack tip can be modelled as an elastic continuum, an assumption that is supported by a simple atomistic model of crack growth.
Abstract: At normal temperatures and pressures water is known to have a strong influence on the strength of ceramics and glasses. Behaving as a stress corrosion agent, water causes these materials to fail prematurely as a consequence of subcritical crack growth. A basic premise of this paper is that stress corrosion cracking of ceramics is a chemical process which involves a stress-enhanced chemical reaction between the water and the highly stressed ceramic near the crack tip. Plastic deformation is believed to play no role in this fracture process. After a brief survey of chemical reaction rate theory, the basic rate equation from this theory is modified to reflect physical and chemical processes which occur at crack tips. Modification of the rate equation is based on the assumption thatl the crack tip can be modelled as an elastic continuum, an assumption that is supported by a simple atomistic model of crack growth. When tested against experimental data collected on glass the theory was found to be consi...
190 citations
TL;DR: In this article, the mechanisms by which failure is initiated in small-scale yielding and full plasticity are discussed with particular reference to micro void nucleation, growth, and coalescence in the multiaxial states of stress ahead of cracks in plane-strain conditions.
Abstract: The mechanisms by which failure is initiated in small-scale yielding and full plasticity are discussed with particular reference to micro void nucleation, growth, and coalescence in the multiaxial states of stress ahead of cracks in plane-strain conditions.
107 citations
TL;DR: In this article, the deformation-controlled growth of intergranular creep cavities was studied in a Type 347 austenitic steel and it was found that the number of cavities per unit volume is linearly related to the time-dependent creep strain, being independent of stress or temperature.
Abstract: The nucleation and growth of intergranular creep cavities has been studied in a Type 347 austenitic steel tested at 550 and 650°C over a limited range of stress. It was found that the number of cavities per unit volume is linearly related to the time-dependent creep strain, being independent of stress or temperature over the conditions tested. The results indicate that cavity nucleation is controlled by a deformation process. Studies on cavity growth show that the cavity diameter increases approximately linearly with time, with a stress dependence for growth similar to that for secondary creep. These results are consistent with a diffusion growth model when a simple deformation-controlled nucleation law is incorporated. Using this growth model the observed stress and time dependence for the total void volume can be predicted, and the rupture life has been estimated reasonably well using a simple fracture criterion.
102 citations
TL;DR: The micromechanisms of cleavage fracture in ferritic steels are reviewed in this paper, where it is concluded that cleavage in steels containing discrete carbide particles is nucleated by micro-cracks formed in the carbide particle when fracture obeys a critical tensile stress criterion.
Abstract: The micromechanisms of cleavage fracture in ferritic steels are reviewed. Mechanisms of crack nucleation by dislocation interaction are compared with those involving the formation of stable microcracks. It is concluded that cleavage in steels containing discrete carbide particles is nucleated by microcracks formed in the carbide particles when fracture obeys a critical tensile stress criterion. The cleavage fracture stress in these steels is seen to depend largely upon the size of the cracked carbide particle. No micromechanisms have been established for cleavage in martensitic or low-carbon bainitic steels but the packet size seems to control cleavage fracture in these materials.Two models of the relationship between the micromechanism of cleavage and fracture toughness are discussed in some detail. The statistical competition between different sized crack nuclei at a loaded crack tip can be represented by the requirement that the fracture stress be exceeded over a microstructurally determined ch...
98 citations
TL;DR: In this paper, the deformation of an ultrafine-grained aluminium alloy has been examined in tension and torsion, and the results show that at grain sizes below about 3 μm the alloy exhibits inhomogeneous yielding but this is absent at larger grain sizes.
Abstract: The deformation of an ultrafine-grained aluminium alloy has been examined in tension and torsion. At grain sizes below about 3 μm the alloy exhibits inhomogeneous yielding but this is absent at larger grain sizes. If the lower yield strength values are plotted versus grain size the strengths are inversely dependent on grain size whereas the usual plot versus d −½ is non-linear and shows an enhanced strength at the finer grain sizes. However, if the inhomogeneous yielding region is avoided by extrapolating the work-hardening portion of the curve back to the elastic line, all the data can be well represented by the Hall–Petch relationship. Torsion tests, which allow the investigation of a large strain range, show that. the gredient of the grain size plots decreases slowly wIth strain but that the grain boundaries remain effective barriers to flow at strains up to at least 1.0. The deformation behaviour does not appear to agree with the current models for the influence of grain size on the flow stre...
94 citations
TL;DR: In this article, the decomposition of ∆-ferrite to austenite has been studied in a 26%Cr-5%Ni stainless steel, and it was shown by STEM microanalysis that Ni partitions to the Austenite and Cr partitions to ferrit.
Abstract: The decomposition of ∆-ferrite to austenite has been studied in a 26%Cr-5%Ni stainless steel The decomposition takes place via two mechanisms: by nucleation and growth in the high temperature range (650–1200°C) and by an athermal martensitic process in the low temperature range (300–650°C) The high temperature reaction exhibits growth kinetics which are intermediate between volume diffusion control and growth by a ledge mechanism The morpho-logical variants of the austenite mirror those described in the Dube morphological classification for low-alloy steels, and the steel was used as a ‘model system‘ for the study of bcc ⇄ fcc transformations In the low temperature range (400–600°C) the formation of α′, which gives rise to 475°C embrittlement, is shown to occur by a nucleation and growth process, and the subsequent dissolution of the α′ is explained by the precipitation of austenite Finally, it is shown by STEM microanalysis that Ni partitions to the austenite and Cr partitions to the ferrit
88 citations
TL;DR: In this article, the influence of Mo on reversible temper embrittlement of a low-alloy 2¼Cr-lNi steel with variableP and Mo contents was studied by establishing the relationships among the Charpy impact transition temperature shifts, the nominal P and Mo concentrations, and their intergranular concentrations as measured by Auger electron spectroscopy.
Abstract: The influence of Mo on reversible temper embrittlement(RTE)of a low-alloy 2¼Cr-lNi steel with variableP and Mo contents was studied by establishing the relationships among the Charpy impact transition temperature shifts, the nominal P and Mo concentrations, and their intergranular concentrations as measured by Auger electron spectroscopy. Segregation was also studied in ternary Fe-Mo-P alloys.Embrittlement in the steels exhibits a deep minimum for a nominal Mo content of 0.7 wt-%,while at 1.1 wt-% it resumes its value in the Mo-free steel. The de-embrittling action of Mo has two distinct origins. When dissolved in the ferrite matrix, Mo decreases the segregation of P atoms by tying them up in the grain interior. When segregated at the boundaries, Mo counteracts the embrittling effect of segregated P atoms. The former effect is dominant in low-P steels and ternary Fe-Mo-P alloys, while the latter operates in higher-P steels where the segregation of Mo is enhanced by that of P due to the strongly at...
80 citations
TL;DR: In this paper, it is shown that the γ/α interfaces are preferred sites for the precipitation of alloy carbides (interphase precipitation), which occurs on both planar and curved boundaries.
Abstract: The morphology of ferrite grains formed from austenite is first considered, then the refinement of ferrite grain size by controlled rolling as exemplified by HSLA steels. The influence of alloying elements is briefly examined, followed by a detailed discussion of the precipitation of carbides during and after the formation of ferrite. It is shown that the γ/α interfaces are preferred sites for the precipitation of alloy carbides (interphase precipitation), which occurs on both planar and curved boundaries. In the former, the precipitation is associated with the movement of small ledges across the γ/α interface, whereas the curved bands of precipitate are formed on high-energy boundaries which move by bowing around the particles. A change in the nature of the γ/α interface often leads to fibrous carbide growth, while more rapid rates of cooling lead to the formation of supersaturated ferrite and subsequent precipitation of carbides on dislocations. The dislocations in the ferrite are shown to play...
75 citations
TL;DR: In this paper, a model is developed to describe the growth of voids in superplastic alloys based on analyses by Hancock, Beere and Speight, and Edward and Ashby for the case of creep cavity growth controlled by plastic deformation of the matrix.
Abstract: A model is developed to describe the growth of voids in superplastic alloys. This is based on analyses by Hancock, Beere and Speight, and Edward and Ashby for the case of creep cavity growth controlled by plastic deformation of the matrix. It is shown that published data for the development of cavitation in several superplastic alloys fit the predictions of the model.
68 citations
TL;DR: In this paper, the application of available two-phase deformation models to dual-phase steels is examined by testing them against available data published in the literature and an experimental high-silicon-containing steel.
Abstract: The application of available two-phase deformation models to dual-phase steels is examined by testing them against available data published in the literature and an experimental high-silicon-containing steel. It is shown that the successful prediction of mechanical properties by the models depends critically on the strain distribution between the two phases of the steels examined. A detailed characterization by transmission electron microscopy of the hard phases obtained in the high-silicon steel is also carried out. Hard-phase structures consisting either of platelet martensite or of carbide-free upper bainite were identified, and both of these structures contained significant amounts of retained austenite.
TL;DR: In this article, the microstructural changes which occur during the extrusion of a commercial aluminium alloy (AA 5456) have been investigated at macroscopic, microscopic, and substructural magnifications.
Abstract: The microstructural changes which occur during the extrusion of a commercial aluminium alloy (AA 5456) have been investigated at macroscopic, microscopic, and substructural magnifications. Light microscopy has established that the deformation zone is not fully established until a short time after the peak or breakthrough pressure has occurred and that thereafter cylinder wall/billet shear is important in determining deformation. Light microscopy also reveals that recrystallized grains are present both at original grain boundaries and within grains when the deformation is advanced. Electron microscopy reveals that those grains contain a substructure and are thus dynamically recrystallized grains. The bulk of the material continues to deform with dynamic recovery as the operative mechanism. The deformation is thus duplex with dynamic recrystallization considerably modifying the structure normally observed in aluminium alloys. It is concluded that this occurs primarily because of the high solute addi...
TL;DR: A dynamic compaction process has been used to make large, three-dimensional pieces of Metglas 2826 alloy, but makes use of the shock pressures generated by gas-gun-fired projectile impacts as discussed by the authors.
Abstract: A dynamic compaction process has been used to make large, three-dimensional pieces of Metglas 2826 alloy. This process is similar to explosive compaction, but makes use of the shock pressures generated by gas-gun-fired projectile impacts. Careful control of the compaction process is required to cause interfibre bonding and hence make a sound compact while avoiding simultaneous crystallization. Bulk mechanical properties measured on specimens made of compacted glass are presented here and compared with the properties of the as-solidified fibre.
TL;DR: In this article, the nucleation and propagation of cracks in aligned continuous glass fibre reinforced polyester composites in the presence of hydrochloric acid have been studied using ring specimens subjected to parallel plate compression.
Abstract: The nucleation and propagation of cracks in aligned continuous glass fibre reinforced polyester composites in the presence of hydrochloric acid have been studied using ring specimens subjected to parallel plate compression. The appearance of the fracture surfaces depends on the initial strain but is characterized by planar cracks normal to the fibre direction with little fibre pull-out. The planar cracks are separated by interlaminar cracks. A model is presented for the nucleation and growth of cracks in stress and strain corrosion conditions.
TL;DR: A method for determining the local tensile stress σ* required to initiate brittle fracture at a grain boundary as a function of the concentration of segregated impurity on that boundary is presented in this article.
Abstract: A method for determining the local tensile stress σ* required to initiate brittle fracture at a grain boundary as a function of the concentration of segregated impurity on that boundary is presente...
TL;DR: In this paper, the relative rates of particle coarsening of the three vanadium compounds in ferrite can be ranked in the ascending order VN ≃ V(CN) ≪ VC, with vanadium carbide coarsens approximately fifty times more rapidly than vanadium carbonitride and nitride.
Abstract: Detailed particle coarsening experiments have been done on a series of 0.25% vanadium steels containing various amounts of carbon and nitrogen. The relative rates of particle coarsening of the three vanadium compounds in ferrite can be ranked in the ascending order VN ≃ V(CN) ≪ VC, with vanadium carbide coarsening approximately fifty times more rapidly than vanadium carbonitride and nitride. A possible explanation for this result is given. During the aging of all five alloys investigated, a changeover from (time)½ to (time)⅕ particle coarsening kinetics occurred. This is interpreted as a changeover from interface reaction controlled coarsening to dislocation coarsening (controlled by pipe diffusion), a view supported by detailed electron microscopy of the alloys. A mechanism for this coarsening process is suggested. Estimates of the relative activation energies indicate that this is lower for dislocation coarsening than for interface reaction controlled coarsening. The effect of increasing the dis...
TL;DR: In this paper, it is shown that cracks in composite materials are inhibited by the presence of interfaces both at the microstructural level between fibres and matrix and at the macroscopic level as planes of weakness between separate laminations in a multiple laminate.
Abstract: Typical engineering composites consist of brittle fibres, such as glass or carbon, in a weak, brittle, plastic matrix such as epoxy or polyester resin. An important characteristic of these composites, however, is that they are surprisingly tough, largely as a result of their heterogeneous nature and the manner of their construction. During deformation, microstructural damage is widespread throughout the composite, but much damage can be sustained before load-bearing ability is impaired. Beyond some critical level of damage, failure may occur by the propagation of a crack which usually has a much more complex character than cracks in homogeneous materials. Crack growth is inhibited by the presence of interfaces both at the microstructural level between fibres and matrix and at the macroscopic level as planes of weakness between separate laminations in a multiple laminate. The fracturing of a composite therefore involves not only the breaking of the load-bearing fibres and the weak matrix, but a co...
TL;DR: In this article, it was shown that while being a necessary condition, a critical tensile stress is not sufficient to describe cleavage fracture occurring at the tip of a sharp crack.
Abstract: Cleavage fracture in steels obeys a critical tensile stress criterion; fracture occurs when a microstructurally determined tensile stress is locally exceeded. As a first approximation, the cleavage of a steel can be regarded as the propagation of a Griffith defect, this defect being provided by a cracked carbide particle. Dimensional considerations show that while being a necessary condition, a critical tensile stress is not sufficient to describe cleavage fracture occurring at the tip of a sharp crack. Ritchie, Knott, and Rice (hereafter referred to as RKR) postulated that the fracture stress must be exceeded over a microstructural ‘characteristic distance’ ahead of the crack tip before a steel will cleave. This model of cleavage fracture was used to predict the temperature dependence of the fracture toughness of mild steel, the predictions being in good agreement with experimental results when the characteristic distance was taken as being equal to two grain diameters. Although originally propos...
TL;DR: In this article, the role played by transgranular and intergranular modes of failure in the intermediate and low ∆K regions is considered, and the development of irregular fracture faces due to out-of-plane crack trajectories with the associated non-closure of the fatigue cracks is suggested as a substantial contributor to the factors determining ∆-K threshold levels.
Abstract: Results from investigations of the fatigue crack growth behaviour and ∆K threshold levels in α-titanium, α-brass, and low-carbon ferritic steels are presented. The observations show that microstructure has a substantial influence on the mechanisms and modes of fatigue crack growth. The role played by transgranular and intergranular modes of failure in the intermediate and low ∆K regions are considered. The development of irregular fracture faces due to out-of-plane crack trajectories with the associated ‘non-closure‘ of the fatigue cracks is suggested as a substantial contributor to the factors determining ∆K threshold levels.
TL;DR: In this paper, a model in which cracks propagate by nucleation, growth, and coalescence with cavities in front of the crack tip is analyzed in detail and the applied stress field is assumed to be elastic or elastic-plastic, and the relaxation of stresses from the growth of cavitation damage is included in the analysis.
Abstract: A model in which cracks propagate by nucleation, growth, and coalescence with cavities in front of the crack tip is analysed in detail. The applied stress field is assumed to be elastic or elastic–plastic, and the relaxation of stresses from the growth of cavitation damage is included in the analysis. The spacing of the cavities, the threshold stress for the nucleation of cavities, and the self-diffusion coefficients are the important material parameters. The model results in a threshold stress intensity and a K I n dependence of steady-state crack propagation rate when K I > K th. The stress distribution in front of the crack during the transient and the steady states is calculated. The size of the ‘damage zone’ varies from about 10 to 100 cavity spacings. Since the typical cavity spacing in engineering materials is about 1 μm, the damage zone is expected to be a few grain diameters in length.
TL;DR: In this article, a systematic study of the effects of tempering on AISI 410 steel was carried out, and reversible temper embrittlement was detected and related to the mechanical properties and microstructural variations and to the segregation of carbon and alloy elements.
Abstract: In order to explain the influence of carbon on the embrittlement of 12%Cr steels, a systematic study of the effects of tempering on AISI 410 steel was carried out. Reversible temper embrittlement was detected and related to the mechanical properties and microstructural variations and to the segregation of carbon and alloy elements. Carbon was directly localized on samples using high-resolution autoradiography and was detected whatever its chemical state (in solution or as precipitates). It was observed that embrittlement was associated with the concentration of carbon in solid solution at martensitic lath interfaces but also with a decrease in the elemental carbon content of the prior austenitic interfaces.
TL;DR: The micromechanisms of fatigue crack growth in metals in Stage II and Stage III are examined in detail in this paper, where the link between crack advance and crack tip opening is emphasized.
Abstract: The micromechanisms of fatigue crack growth in metals in Stage II and Stage III are examined in detail. The link between crack advance and crack tip opening is emphasized. Empirical crack growth laws are reviewed in the light of the micromechanisms involved.
TL;DR: In this article, annealing infinite diffusion couples between appropriate pairs of alloys and determining the resulting concentration profiles by electron-probe X-ray microanalysis was used to analyze the cobalt solid solution of the Co-Cr-AI system.
Abstract: The description of interdiffusion in a ternary system requires four composition-dependent diffusion coefficients which together form a coefficient matrix. The values of this matrix have been determined for a wide rangeof compositions in the cobalt solid solution of the Co-Cr-AI system at 1100°C. This was achieved by annealing infinite diffusion couples between appropriate pairs of alloys and determining the resulting concentration profiles by electron-probe X-ray microanalysis. The diffusion coefficients obtained agreed with expectations based on the results of previous studies of related systems. All four coefficients were found to vary systematically with composition, as illustrated by contour maps. However, this did not fit theoretical predictions based on the Wagner dilute solution model. Certain anomalous results were explained on the basis of the formation of non-equilibrium vacancy concentrations.
TL;DR: In this article, the elastic-constant variability in stainless steel 304 was studied and the results showed surprisingly small elasticconstant variations; about 1% in Young's and shear moduli and about 1.5% in the bulk modulus and Poisson's ratio.
Abstract: Recently, the author collaborated in studying the elasticconstant variability in stainless steel 304. Twenty samples acquired randomly were measured ultrasonically within an experimental uncertainty of 0.2% in the sound velocities. The results showed surprisingly small elastic-constant variations; about 1% in the Young's and shear moduli and about 1.5% in the bulk modulus and Poisson's ratio. This included all possible variation sources: chemical composition, thermomechanical processing, sample anisotropy, and experimental error.
TL;DR: In this article, a ledge mechanism of growth of discontinuous vanadium carbide precipitate in an austenitic Fe-Mn-V-C alloy has been established by observing the progress of the reaction using scanning electron microscopy.
Abstract: A ledge mechanism of growth of colonies of discontinuous vanadium carbide precipitate in an austenitic Fe-Mn-V-C alloy has been established by observing the progress of the reaction using scanning electron microscopy. The mobility of the risers of the ledges was measured and found to be at least an order of magnitude higher than that of the broad faces of the ledges. An explanation of this difference in mobility involving the nucleation of vanadium carbide has been proposed. Transmission electron microscopy of a ledged colony boundary confirmed that the planar density of coincident sites on the risers of ledges, where fibrous precipitation occurs, was greater than that on the broad faces at which particulate precipitation takes place.
TL;DR: In this article, the formation of supersaturated ferrite in a series of Fe-Cu and Fe-cu-Ni alloys which have been directly transformed from austenite is described.
Abstract: This paper describes the formation of supersaturated ferrite in a series of Fe-Cu and Fe-Cu-Ni alloys which have been directly transformed from austenite. It is shown that supersaturated ferrite can be formed at all temperatures of transformation and that although, within certain temperature limits, the ferrite produced is similar to the ‘massive ferrite’ observed in many alloy systems during continuous cooling transformations, not all the features of this reaction product are fully consistent with a massive phase.
TL;DR: In this article, a model of the climbing process is presented on the basis that climb is aided solely by the local normal stress; the average contribution of the shear stress through forward slip of a climbing dislocation is considered to be negligible.
Abstract: The bypass of particles by climb necessarily results in an increase in length of a dislocation, provided its ends are anchored at network nodal points. The energy associated with this length increase is provided by the applied stress. It follows that a critical threshold stress for the climb process, and hence for recovery creep, can be defined as that required to maintain the minimum length increase during bypass. A model of the process is presented on the basis that climb is aided solely by the local normal stress; the average contribution of the shear stress through forward slip of the climbing dislocation is considered to be negligible.The computed value of threshold stress is only weakly dependent upon applied stress and, for practical purposes, can be considered to be independent. There is a strong dependence on the dispersion parameter r/λ, where r is the particle radius and λ the surface-to-surface particle separation. A preliminary comparison of the theory is made with results on a TiN di...
TL;DR: In this paper, the aging characteristics of MC and M23C6 in a Ti-stabilized austenitic stainless steel with a balanced Ti and C content have been investigated for aging times of up to 5000 h at 750°C.
Abstract: The precipitation and aging characteristics of MC and M23C6 in a Ti-stabilized austenitic stainless steel with a balanced Ti and C content have been investigated for aging times of up to 5000 h at 750°C. It was found that MC precipitated on dislocations and stacking faults while M23C6 initially precipitated intergranularly. Quantitative measurements made by transmission electron microscopy showed that, after 5000 h aging, MC which had initially precipitated on dislocations partially transformed to intragranular M23C6. The MC precipitates on stacking faults were found to be more stable than those on dislocations so that denuded zones developed around the sheets of stacking fault precipitate at long aging times. Although the partial transformation of MC precipitated on dislocations resulted in a reduction in the volume fraction of this type of precipitate, it was found that the number density of these particles increased. These results are discussed in terms of expected changes in the composition of...
TL;DR: The inverse relationship between recrystallized grain size and twin density is not as simple as originally thought as discussed by the authors, which suggests that twins form because the reorientated matrix provides an easier reaction path for a rapid reduction in dislocation density and not because the twinning process may lead to a reduction in interfacial energy.
Abstract: Recrystallization studies were carried out in a photoemission electron microscope (PEEM) on OF copper and also on nickel and aluminium bronze. Grain sizes and prior cold work were varied over a wide range. The twin density, when expressed as the number of coherent twin interfaces per unit area, is found to correlate well witha parameter reflecting dislocation density. However, the inverse relationship between recrystallized grain size and twin density is not as simple as originally thought. The results show a large scatter which seems to be associated with the variation in recrystallization temperature incurred during heating runs in the PEEM. These variations, although not affecting final grain size, may have a considerable influence on the twin density. The results suggest that twins form because the reorientated matrix provides an easier reaction path for arapid reduction in dislocation density and not because the twinning process may lead to a reduction in interfacial energy.
TL;DR: In this paper, a range of concentration profiles and diffusion paths for the generalized diffusion couple A-10B/A-10C were plotted using three different models of diffusion coefficient behaviour.
Abstract: In order to illustrate the physical meaning of the four independent interdiffusion coefficients which describe a single-phase ternary system, a range of concentration profiles and diffusion paths for the generalized diffusion couple A-10B/A-10C were plotted using three different models of diffusion coefficient behaviour. The shapes of the diffusion paths predicted by these various models were compared with that of an experimentally determined diffusion path from the cobalt solid solution of the substitutional alloy system cobalt-chromium-aluminum. The results indicate that even the most sophisticated dilute solution model fails to predict completely the behaviour of the real system.