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Showing papers in "Acta Metallurgica Et Materialia in 1990"


Journal ArticleDOI
TL;DR: In this paper, a model for the critical strains associated with the Portevin-Le Châtelier effect (PLC) in terms of the strain dependence of the densities of mobile and forest dislocations is proposed.
Abstract: A model is proposed for the critical strains associated with the Portevin-Le Châtelier effect (PLC) in terms of the strain dependence of the densities of mobile and forest dislocations. The classical critical condition for the onset of the PLC effect, viz. that of vanishing of the strain rate sensitivity of flow stress under the influence of dynamic strain aging is reexamined. The analysis takes into account the strain dependence of a key quantity: the elementary strain produced when all mobile dislocations perform a successful thermally activated step through the forest obstacles. This elementary strain is estimated by studying a system of coupled differential equations for the evolution of the two densities. Results are obtained in semi-quantitative form and compared with available data. It is shown that the following effects are consistently explained: the occurrence of critical strains for the onset and termination of jerky flow, occasional observation of two PLC regimes within the same deformation curve, the behaviour of the critical strains at high strain rates and low temperatures and, possibly, the particular behaviour exhibited by some alloys at low strain rates and high temperatures. Consequences for the “friction” and “forest” models of dynamic strain aging are discussed.

466 citations


Journal ArticleDOI
TL;DR: In this article, the glass transition is caused by a rather sudden increase of free volume towards equilibrium near T g, which can be calculated by making use of the well known kinetics of freevolume annilation and production in some metallic glasses.
Abstract: The glass transition, observed in glasses during DSC experiments at a constant heating rate, is described as a kinetic phenomenon caused by the continuous approach of free volume towards equilibrium during the warming up. At a certain temperature below the glass temperature T g , the amount of free volume becomes smaller than the equilibrium value at that temperature. The glass transition is caused by a rather sudden increase of free volume towards equilibrium near T g . This can be calculated by making use of the well known kinetics of free volume annilation and production in some metallic glasses. Calculations performed on this basis are in good agreement with available experimental data, both on structural relaxation and the glass transition.

391 citations


Journal ArticleDOI
TL;DR: In this article, a Brazil-nut-sandwich with a crack on a substrate/interlayer interface is developed for fracture testing, and the fracture loading phase is controlled by the angle of diametral compression.
Abstract: A Brazil-nut-sandwich with a crack on a substrate/interlayer interface is developed for fracture testing. The fracture loading phase is controlled by the angle of diametral compression. Interfacial fracture mechanics is summarized and adopted in reporting data. Experiments are conducted with aluminum, brass, steel and plexiglass as substrates and epoxy as interlayer. Interfacial toughness curves are measured for large range of loading phase. Effects of the roughness of the surfaces prior to bonding on the interfacial toughness are demonstrated. Failure patterns for the adhesive structure under different loading modes are observed with a scanning electron microscope. For the metal/epoxy systems, when the remote loading is predominantly mode I, cracks tend to kink out of interfaces and run within the epoxy layer, although the bulk epoxy fracture energy is much higher than the interfacial toughness. At large loading phases, abnormally high apparent toughness is measured. These observations are discussed in the light of crack path selection criteria in adhesive joints and large scale contact zone of crack faces.

390 citations


Journal ArticleDOI
TL;DR: In this paper, process modelling techniques are used to describe the changes in yield strength due to age hardening of heat-treatable aluminium alloys and a model for the isothermal ageing curve is developed.
Abstract: Process modelling techniques are used to describe the changes in yield strength due to age hardening of heat-treatable aluminium alloys. A model for the isothermal ageing curve is developed. This is demonstrated for a number of alloys and the success of the approach is assessed. Applications and a new diagram, showing the variation of strength with temperature and time, are described in an accompanying paper.

366 citations


Journal ArticleDOI
TL;DR: In this paper, a new creep equation is developed which considers as the rate-controlling event the thermally activated detachment of dislocations from dispersoid particles exerting an attractive force.
Abstract: The strongly stress-sensitive and temperature-dependent creep behaviour of dispersion strengthened materials cannot be described satisfactorily by current creep laws. In this paper a new creep equation is developed which considers as the rate-controlling event the thermally activated detachment of dislocations from dispersoid particles exerting an attractive force. The approach is motivated by recent TEM observations and theoretical calculations which strongly suggest that the “classical” view, according to which particles merely force dislocations to climb around them, is inadequate. The creep equation is applied to a dispersion-strengthened superalloy, two aluminium alloys and bubble-strengthened tungsten. Practical conclusions, regarding the optimum dispersoid size and alloy development, are drawn.

334 citations


Journal ArticleDOI
TL;DR: In this paper, friction stress effects of alloying on disolcation behavior are reviewed, as well as the role of stacking fault structure in inhibiting the clustering of dislocations in planar slip metal.
Abstract: While it is widely recognized that alloy factors other than stacking fault energy play a role in promoting planarity of slip, no detailed model has been advanced to explain the mechanism of planar vs wavy slip mode Therefore, friction stress effects of alloying on disolcation behavior are reviewed, as well as the role of stacking fault structure in inhibiting the clustering of dislocations in planar slip metal A model of cross-slip inhibition (and thus planar slip behavior) is developed from the idea that the joining of partials is resisted by frictional effects Planarity of slip is promoted not only by low stacking fault energy but by increase in shear modulus, atomic size misfit and solute content A critical solute concentration is predicted by the model for the transition from wavy slip to planar slip and this is shown to be in good agreement with observations for copper base solid solutions and other alloy systems

279 citations


Journal ArticleDOI
TL;DR: In this article, a micro-mechanical model is used to analyse the tensile properties of a ductile metal reinforced by short fibres, and the analyses focus on uniaxial tension in the fibre direction, but also the effect of stress triaxiality is investigated.
Abstract: A micro-mechanical model is used to analyse the tensile properties of a ductile metal reinforced by short fibres. It is assumed that fibre alignment has been obtained during processing, and the numerical analysis for a representative unit cell also relies on the assumption of a periodic array of fibres. The special unit cell model developed here represents neighbouring fibres shifted relative to one another. Comparison of results for rigid fibres with results accounting for fibre elasticity shows that apart from the initial elastic range the overall stress-strain behaviour is dominated by matrix plasticity. The analyses focus on uniaxial tension in the fibre direction, but also the effect of stress triaxiality is investigated. Quite good agreement is obtained between the model predictions and published experimental results. The sensitivity of the overall stress-strain behaviour to the fibre volume fraction, the fibre length, and fibre spacings is studied by a number of analyses.

261 citations


Journal ArticleDOI
TL;DR: In this article, the applicability of the CSL/DSC model of interface structure to intergranular corrosion susceptibility at grain boundaries in high purity (99.999%) polycrystalline nickel was evaluated.
Abstract: Electrochemical studies were conducted in 2 N H2SO4 at 303 K in order to assess the applicability of the CSL/DSC model of interface structure to intergranular corrosion susceptibility at grain boundaries in high purity (99.999%) polycrystalline nickel. Susceptibility to the initiation of localized corrosion at grain boundaries was manifested through characteristic overpotentials for passive film breakdown. These characteristic overpotentials were found to (1) decrease with increasing bulk sulphur concentration (0.3–50 ppm), and (2) be strongly dependent on interface structure (CSL/DSC). Boundaries close (Δθ) to low ΣCSL relationships were observed to be most resistant to the initiation of localized corrosion. A limiting structural field was determined, not extending beyond Σ25, and restricted to an angular deviation limit defined by a relation of the type: Δθ = 15° Σ−5/6. Results were determined to be consistent with a mechanism whereby susceptibility to intergranular corrosion is dictated by the (1) geometry, and (2) chemistry (i.e. solute concentration) of intrinsic grain boundary dislocations.

246 citations


Journal ArticleDOI
TL;DR: In this paper, the brittle compressive fracture under uniaxial loading of fresh-water, granular ice Ih has been studied, and the results are explained in terms of the frictional crack sliding-wing crack model.
Abstract: The brittle compressive fracture under uniaxial loading of fresh-water, granular ice Ih has been studied. Measurements are reported of the fracture stress at temperatures from −10 to −50°C at strain rates of 10 −3 and 10 −1 s −1 for grain sizes from approximately 1 to 10 mm. Also a summary is reported of measurements by Jones et al . (unpublished) of the kinetic coefficient of friction for ice on ice at temperatures from −10 to −40°C at sliding velocities from 5 × 10 −7 m s −1 to 5 × 10 −2 ms −1 . Observations via high speed photography of internal cracking during loading are included. The strength, albeit scattered, increases with decreasing grain size, with decreasing temperature and at −10°C with decreasing strain rate. Similarly, the coefficient of friction increases with decreasing temperature and at −10°C with decreasing sliding velocity. Wing cracks were observed on some inclined cracks nucleated during loading. The results are explained in terms of the frictional crack sliding-wing crack model [as developed by Ashby and Hallam, Acta metall. 34, 497 (1986)] of compressive fracture. Finally, a simple model is presented for the transition from ductile to brittle behavior. It is based upon the competition between the building up and the relaxation of internal stresses within the vicinity of the internal cracks, and it leads to a transition strain rate which can be expressed in terms of the fracture toughness, the creep rate, the kinetic coefficient of friction and the microstructural scale of the material.

226 citations


Journal ArticleDOI
TL;DR: In this article, a correlation between the structure and zero-temperature energy of symmetrical tilt grain boundaries (STGBs) in f.c. metals is investigated using two embedded-atom-method potentials (for Cu and Au) and a Lennard-Jones potential fitted for Cu.
Abstract: The correlation between the structure and zero-temperature energy of symmetrical tilt grain boundaries (STGBs) in f.c.c. metals is investigated using two embedded-atom-method potentials (for Cu and Au) and a Lennard-Jones potential fitted for Cu. Similar to free surfaces, misorientation phase space associated with these simple planar defects consists of only two degrees of freedom, namely those associated with the GB plane. The sampling of this two-dimensional phase space in terms of the stereographic triangle shows energy cusps as its corners and along its edges. These cusps are shown to arise from GB geometries with particularly small planar unit cells. Similar to free surfaces, a good correlation is found between the number of broken nearest-neighbor bonds per unit area and the GB energy. Also, as in our earlier study of twist boundaries, a practically linear relationship is found between the GB energy and volume expansion at the boundary. Finally, a comparison with twist boundaries shows that the STGBs represent the endpoints of the energy vs twist-angle curves. This enables a direct comparison of the properties of twist and tilt boundaries.

223 citations


Journal ArticleDOI
TL;DR: In this article, the effects of changes in matrix microstructure on crack initiation and growth toughness were determined on an Al-Zn-Mg-Cu alloy containing 0, 15, 20 percent by volume of SiC particulates.
Abstract: The effects of systematic changes in matrix microstructure on crack initiation and growth toughnesses were determined on an Al-Zn-Mg-Cu alloy containing 0, 15, 20 percent by volume of SiC particulates. Materials were heat treated to underaged (UA) and overaged (OA) conditions of equivalent matrix microhardness and flow stress. Although both the fracture initiation and growth toughnesses, as measured by JIc and tearing modulus, were similar for the unreinforced materials in the UA and OA conditions, significant effects of microstructure on both JIc and tearing modulus were observed in the composites. SEM and TEM observations of fracture paths in the two conditions are utilized to rationalize these observations in light of existing theories of ductile fracture propagation.

Journal ArticleDOI
TL;DR: In this article, the authors studied the fracture properties of Al2O3-based ceramic specimens by instrumented in situ dynamic scanning electron microscopy (SEM) using the double torsion technique.
Abstract: The fracture micromechanics and underlying physical processes of fracture in Al2O3-based ceramic specimens have been studied as a function of grain size by instrumented in situ dynamic scanning electron microscopy (SEM) using the double torsion technique The toughness is found to increase with grain size Crack bridging is found to extend over hundreds of grain diameters behind the crack tip, resulting in R-curve behaviour Evidence is amassed which points to frictional energy dissipation, rather than distrubuted microcracking or crack-closure due to elastic ligaments, as the dominant contribution to toughening The friction occurs at grains which bridge the crack faces and are pulled out as the faces separate Restraining stresses, which constrain the bridging grains in their sockets, are believed to be the result of both grain morphology and the thermal expansion anisotropy of the material Simple modelling indicates that only a few percent of the grains need be involved in the frictional process to account for the toughening The conclusion is supported by hysteresis measurements

Journal ArticleDOI
TL;DR: In this article, a metastable trigonal (P3m1) ω-related phase, designated ω″, forms along with small amounts of D019 and L10 phases.
Abstract: During cooling of an alloy of composition Ti4Al3Nb from a B2 phase field above 1100°C, a metastable trigonal (P3m1) ω-related phase, designated ω″, forms along with small amounts of D019 and L10 phases. The ω″ phase exhibits partial collapse of 111 planes and reordering relative to its B2 parent. An apparently equilibrium low temperature phase with the B82 structure was found after 26 days of annealing at 700°C. Both ω″ and B82 structures were verified by means of transmission electron microscopy and by single crystal X-ray diffraction. The latter permitted detailed analysis of the collapse parameters and site occupancies. The observed transformation path, B2(Pm3m)→ω″(P3m1)→B82 (P63/mmc), occurs in two steps. The first involveds a subgroup transition during cooling that is primarily displacive with reordering consistent with the trigonal symmetry imposed by the ω-collapse. The second involves a supergroup transition during prolonged annealing that is primarily replacive and constitutes a chemical disordering. The direct equilibrium transformation, B2→B82, without the formation of an intermediate trigonal phase, can only occur by a reconstructive transformation.

Journal ArticleDOI
TL;DR: In this paper, the Young's modulus, strength and fracture toughness of a brittle reticulated vitreous carbon foam was measured as a function of cell size at a constant density and compared to a theoretical model.
Abstract: The Young's modulus, strength and fracture toughness, of a brittle reticulated vitreous carbon foam, was measured as a function of cell size at a constant density and compared to a theoretical model Image analysis was used to characterize the macrostructure of the samples and provided a basis for evaluating the mechanical behavior It was determined that both the compressive and bend strength scale inversely with cell size The change in compressive strength is due to a change in the strut strength with cell size The bend strength behavior may be due to a reduction in the critical flaw size, as well as the increasing strut strength at smaller cell sizes The fracture toughness and elastic modulus were found to be independent of cell size Comparison of these results with previous work on open cell alumina clearly indicates a very different behavior and is attributed to a change in the microstructure of the solid phase with cell size in the alumina materials

Journal ArticleDOI
TL;DR: Neumann et al. as mentioned in this paper reported about plastic strain controlled high cycle fatigue (HCF) experiments on an austenitic stainless steel (26% Ni, 15% Cr, 23% Ti, 13% Mn, 12% Mo, 03% Al) for both annealed and age hardened conditions.
Abstract: We report about plastic strain controlled High Cycle Fatigue (HCF) experiments on an austenitic stainless steel (26% Ni, 15% Cr, 23% Ti, 13% Mn, 12% Mo, 03% Al) for both annealed and age hardened conditions Since twin boundary cracking phenomena were dominant for both conditions, most of the research focused on annealed steel The local orientation of more than 190 grains was measured by a modified electron channeling technique In a simplifying model [P Neumann and A Tonnessen, Proc Conf Fatigue, Charlottsville, USA, Vol 1, pp 3–22 (1987); P Neumann and A Tonnessen, Proc Int Conf Strength of Metals and Alloys, Tampere, Finland, Vol 1, p 743–748 (1987)] these orientation data were used to calculate the local stress concentrations near the twin boundaries By this method for 89% of the examined twin boundaries we could predict whether a crack would develop or not A laser interferometric method was used to measure the local plastic strain across different selected twin boundaries These experiments show that local plasticity is the most important parameter in HCF of fcc polycrystals

Journal ArticleDOI
Abstract: The creep deformation behavior of metal-matrix composites has been studied by a continuum mechanics treatment utilizing finite element techniques. The objective of the work has been to understand the underlying mechanisms of fiber reinforcement at high temperatures and to quantify the importance of reinforcement phase geometry on the overall deformation rate. Internal stress distributions are presented for a material that consists of stiff elastic fibers in an elastic, power law creeping matrix. Results indicate that large triaxial stresses develop in the matrix, and that these stresses have a strong effect on reducing the creep rate of the composite. Reinforcement phase geometry, as measured by the fiber volume fraction, aspect ratio, separation, and overlap, greatly influences the degree of constraint on the flowing matrix material and the overall deformation rate. Theoretical predictions from this modeling are compared to experimental results of creep deformation in metal-matrix composite systems with varying degrees of agreement.

Journal ArticleDOI
TL;DR: In this article, a series of fracture surfaces, namely transgranular cleavage, intergranular fracture, micro void coalescence, quasicleavage and intragranular microvoid coalescence are analyzed in terms of fractal geometry.
Abstract: To examine the usefulness of the fractal concept in quantitative fractography, a series of classical fracture surfaces, namely transgranular cleavage, intergranular fracture, microvoid coalescence, quasicleavage and intergranular microvoid coalescence, are analyzed in terms of fractal geometry. Specifically, the five brittle and ductile fracture modes are studied, from three well characterized steels (a mild steel, a low-alloy steel and a 32 wt% Mn-steel) where the salient microstructural dimensions contributing to the final fracture morphology have been measured. Resulting plots of the mean angular deviation, and Richardson (fractal) plots of the lineal roughness, as a function of the measuring step size, are interpreted with the aid of computer-simulated fracture-surface profiles with known characteristics. It is found that the ranges of resolution, over which the fractal dimension is constant, correspond to the pertinent metallurgical dimensions on the fracture surface, and thus can be related to microstructural size-scales.

Journal ArticleDOI
TL;DR: In this paper, the effect of stress on the creep properties of 30 vol.% silicon carbide particulate reinforced 6061 aluminum (SiC p -6061 Al), produced by powder metallurgy, has been studied in the temperature range of 618-678 K. The experimental data, which extend over seven orders of magnitude of strain rate, show that the creep curve exhibits a very short steady-state stage; that the stress exponent, n, is high (n > 7.4) and increases with decreasing the applied stress; and that the apparent activation energy for creep
Abstract: The effect of stress on the creep properties of 30 vol.% silicon carbide particulate reinforced 6061 aluminum (SiC p -6061 Al), produced by powder metallurgy, has been studied in the temperature range of 618–678 K. The experimental data, which extend over seven orders of magnitude of strain rate, show that the creep curve exhibits a very short steady-state stage; that the stress exponent, n , is high ( n > 7.4) and increases with decreasing the applied stress; and that the apparent activation energy for creep, Q a , is much higher than the activation energy for self-diffusion in aluminum. The above creep characteristics of SiC p -6061 Al are similar to those reported for dispersion strengthened (DS) alloys, where the high stress exponent for creep and its variation with stress are explained in terms of a threshold stress for creep that is introduced by the dispersoid particles. Analysis of the creep data of SiC p -6061 Al using the various threshold stress models proposed for DS alloys indicates that the threshold stresses introduced by the SiC particulates are too small to account for the observed creep behavior of the composite. By considering an alternate approach for the source of the threshold stress in SiC p -6061 Al, an explanation for the asymptotic behavior of the creep data of the composite is offered. The approach is based on the idea that the oxide particles present in the Al matrix, as a result of manufacturing the composite by powder metallurgy, serve as effective barriers to dislocation motion and give rise to the existence of a threshold stress for creep.

Journal ArticleDOI
TL;DR: In this article, the microstructure and deformation characteristics of a fine-grained superelastic yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) have been investigated.
Abstract: The microstructure and deformation characteristics of a fine-grained superelastic yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) have been investigated. Both hot indentation and tensile tests were carried out at temperatures between 1273 and 1923K over the strain rate range from 2.7 × 10−5 to 2 × 10−3 s−1. It was found that the material exhibited extensive plasticity at temperatures higher than 1473K; a maximum tensile elongation of over 800% was recorded. Microstructural examination did not indicate the presence of a glassy phase at grain boundaries. Yttrium, however, was found to segregate to the grain boundaries. The microstructure of the Y-TZP was thermally unstable and appreciable grain growth was observed at emperattures higher than 1723 K; the grain growth was enhanced by external stresses, i.e. dynamic grain growth was observed. Grain growth at elevated temperatures resulted in apparent strain rate sensitivity exponents of approximately 0.33 at 1723K. This value decreased with increasing temperature. The grain size-compensated strain rate, however, was found to depend approximately on the square of the flow stress, i.e. to exhibit a true strain sensitivity value of 0.5, which suggests a grain boundary sliding mechanism. Microstructures from samples that were deformed superelastically indicated that grains remained equiaxed; this observation is consistent with a grain boundary sliding mechanism. The activation energy for superplasticity, under the conditions of constant structure, in Y-TZP was calculated to be 720 kJ/mol.

Journal ArticleDOI
TL;DR: In this article, the effects of grain-boundary grooving at the free surface of a thin film were modeled by introducing a stagnation condition on grainboundary migration, which resulted in a lognormal grain-size distribution.
Abstract: A computer simulation of grain growth in two dimensions has been used to model microstructural evolution in thin films. In particular, we have modelled the effects of grain-boundary grooving at the free surface of a film by introducing a stagnation condition on grain-boundary migration. This stagnation results in a lognormal grain-size distribution, which is similar to experimentally observed distributions.

Journal ArticleDOI
TL;DR: In this article, the authors investigated factors influencing the development of two-way shape memory behavior in NiTi and found that the magnitude of the twoway memory was dependent on whether the training procedure involves the formation of stress induced martensite or reorientation, the number of training cycles, the training stress and prior heat treatment.
Abstract: Factors influencing the development of two-way shape memory behaviour in NiTi have been investigated. The magnitude of the two-way memory is found to depend on whether the training procedure involves the formation of stress induced martensite or martensite reorientation, the number of training cycles, the training stress and prior heat treatment. The development of optimal two-way shape memory strains is associated with training conditions in which a full one-way transformation strain is achieved under conditions of minimum stress and permanent strain.

Journal ArticleDOI
TL;DR: In this article, the orientation relationship between lath martensite and the narrow films of retained austenite in low carbon, low alloy steels has been determined to an accuracy of approximately ± 1 2° using Kikuchi line patterns obtained with a 7 nm probe in a 200 kV STEM set up for microdiffraction.
Abstract: The orientation relationship between lath martensite and the narrow films of retained austenite in low carbon, low alloy steels has been determined to an accuracy of approximately ± 1 2° using Kikuchi line patterns obtained with a 7 nm probe in a 200 kV STEM set up for microdiffraction. The mean of the orientation relationship determinations was close to the Greninger-Troiano relationship with the extreme results showing a spread of ±2° and lying between the neighbouring Kurdjumov-Sachs and aishiyama-Wassermann relationships. Laths with the same habit plane formed packets and, in general, adjacent laths in such a packet were of the same orientation. However a few cases were observed where adjacent laths had markedly different orientations. In all these cases the two orientations represented a pair of orientation relationship variants in which the same close packed plane in the austenite was parallel to a close packed plane in the martensite and the same close packed direction in the austenite was approximately parallel to a close packed direction in the martensite, i.e. (111) γ nearly parallel to (101) α [11 1 0] γ 1.5 to 3° from [ 1 11] α and (111) γ nearly parallel to (101) α [1 1 0] γ 1.5 to 3° from [ 1 11]α . These two orientation relationship variants approximated to a twin relationship between the adjacent laths and in the less accurate conventional electron diffraction spot patterns could easily have been interpreted as a twin relationship. Single surface trace analysis showed that the long direction of the laths was always the [1 1 0] γ direction which is approximately 2.5° from [11 1 ] α , while the habit plane of the laths was consistent with (557) γ —i.e. an ( hhl ) γ plane approximately 10° from the (111) γ which is parallel to (101) α .

Journal ArticleDOI
TL;DR: In this article, the effects of reinforcement debonding and work hardening on ductile reinforcement toughening of γ-TiAl have been examined, and it is demonstrated that a high work rupture is encouraged by extensive debonding when the reinforcement exhibits high workhardening.
Abstract: The effects of reinforcement debonding and work hardening on ductile reinforcement toughening of γ-TiAl have been examined. Debonding has been varied by either the development of a brittle reaction product layer or by depositing a thin oxide coating between the reinforement and matrix. The role of work hardening has been explored by comparing a Nb reinforcement that exhibits high work hardening with a solution hardened TiNb alloy that exhibits negligible work hardening. It is demonstrated that a high work rupture is encouraged by extensive debonding when the reinforcement exhibits high work hardening. Conversely, debonding is not beneficial when the reinforcement exhibits low intrinsic ductility due to an absence of work hardening.

Journal ArticleDOI
TL;DR: In this article, the authors evaluate quantitatively the effect of different types, frequencies and configurations of grain boundaries, so-called the grain boundary character distribution (GBCD), on the toughness of a 3D polycrystals.
Abstract: The structural dependence of intergranular fracture processes in bicrystals and polycrystals of metals and alloys is first reviewed. It is shown that even in polycrystals, grain boundary structure plays a significant role in controlling the fracture properties of the material. Next, we evaluate quantitatively the effect of different types, frequencies and configurations of grain boundaries, so-called the grain boundary character distribution (GBCD), on the toughness of a three-dimensional (3D) polycrystals. The results show that the toughness of a polycrystals increases monotonically with increasing overall fraction of fracture-resistant low-energy boundaries in the material. A brittle-ductile transition, corresponding to a change of fracture mode from predominantly intergranular with low toughness to predominantly transgranular with high toughness, is observed when the overall fraction of low-energy boundaries reaches a critical value. For a 3D polycrystals with a non-random GBCD such that the fraction of low-energy boundaries on the inclined boundary facets is maximised, a smaller critical overall fraction of low-energy boundaries is needed to bring about the brittle-ductile transition. Similar effect is also found if the grains are made elongated and aligned with the stress axis. The results are discussed in relation to the concept of grain boundary design for strong and tough polycrystals proposed by one of the present authors (T.W.).

Journal ArticleDOI
TL;DR: In this paper, hydrogen embrittlement in AISI type 316 austenitic stainless steel has been investigated by in situ straining in a highvoltage electron microscope (HVEM) equipped with an environmental cell.
Abstract: The mechanisms of hydrogen embrittlement in AISI type 316 austenitic stainless steel have been investigated by in situ straining in a high-voltage electron microscope (HVEM) equipped with an environmental cell. Hydrogen effects on strain-induced phase transformations, the generation rate and velocity of dislocation, and crack propagation rates were studied. The salient features of the fracture were similar for cracks propagating in vacuum and in hydrogen gas. In each case, e and α′ martensite formed at the crack; the e phase extended ahead of the crack while the α′ phase was restricted to high stress regions near the crack tip. The principal effect of hydrogen was to decrease the stress required for dislocation motion, for phase transformation of the austenite, and for crack propagation.

Journal ArticleDOI
TL;DR: The process model for ageing of aluminium alloys, developed in Part I [H. R. Shercliff and M. F. Ashby, Acta metall. mater.38, 1789 (1990), is applied to a number of heat treatments, establishing a basis for such problems as the prediction of the strength loss in the heat-affected zone of welds as mentioned in this paper.
Abstract: The process model for ageing of aluminium alloys, developed in Part I [H. R. Shercliff and M. F. Ashby, Acta metall. mater.38, 1789 (1990)], is applied to a number of heat treatments, establishing a basis for such problems as the prediction of the strength loss in the heat-affected zone of welds. First, the reheating of previously aged material is considered. Heat treatment using a parabolic thermal cycle is then modelled in terms of an equivalent isothermal treatment, and extension to weld thermal cycles is considered. Finally the isothermal models are presented as novel “iso-yield diagrams”, which are useful for evaluating the data from thermal cycles and have potential as process diagrams.

Journal ArticleDOI
TL;DR: In this paper, it was shown that the existence of directional Al p-Ti d covalent bonds causes this compound to be brittle and the addition of those elements into TiAl which weaken p-d interactions but enhance d-d interaction is most effective in improving the ductility.
Abstract: Alloying effects on the electronic structure of a TiAl compound have been investigated systematically using the discrete variational (DV) Xα cluster calculation. It has been shown that the existence of directional Al p-Ti d covalent bonds causes this compound to be brittle. The addition of those elements into TiAl which weaken p-d interactions but enhance d-d interactions is most effective in improving the ductility.

Journal ArticleDOI
TL;DR: In this article, a homogeneous bimodal distribution of faceted Pb particles embedded in a matrix of Al, with chill-cast Pb particle sizes of 1-2 μm and 5-50 μm, was examined by a combination of optical microscopy, scanning and transmission electron microscopy and electron probe microanalysis.
Abstract: Hypermonotectic alloys of Al-5 wt% Pb and Al-5 wt% Pb-0.5 wt% X where X = Mn, Cu, Zn, Fe and Si have been manufactured by chill-casting and melt-spinning. The resulting microstructures have been examined by a combination of optical microscopy, scanning and transmission electron microscopy, and electron probe microanalysis. The as-solidified hypermonotectic alloys exhibit a homogeneous bimodal distribution of faceted Pb particles embedded in a matrix of Al, with chill-cast Pb particle sizes of 1–2 μm and 5–50 μm, and melt-spun Pb particle sizes of 5–10 nm and 50–100 nm. The larger Pb particles are formed during cooling through the region of liquid immiscibility while the smaller Pb particles are formed during monotectic solidification of the Al matrix. The Pb particles exhibit a cube-cube orientation relationship with the Al matrix, and a truncated octahedral shape with {111} and {100} facets. The as-solidified Pb particle distributions are resistant to coarsening during post-solidification heat treatment. The equilibrium Pb particle shape and therefore the anisotropy of solid Al-solid Pb and solid Al-liquid Pb surface energies have been monitored by in situ heating in the transmission electron microscope over the temperature range between room temperature and 550°C. The anisotropy of solid Al-solid Pb surface energy is constant between room temperature and the Pb melting point, with the {100} surface energy 14% greater than the {111} surface energy, in good agreement with geometric near-neighbour bond energy calculations. The {100} facets disappear when the Pb particles melt, and the anisotropy of solid Al-liquid Pb surface energy decreases gradually with increasing temperature above the Pb melting point, until the Pb particles become spherical at about 550°C. The kinetics of Pb particle solidification have been examined by heating and cooling experiments in a differential scanning calorimeter. Pb particle solidification is nucleated catalytically by the Al matrix on the {111} facet surfaces, with an undercooling of 22K and a contact angle of 21°C. Ternary additions of Mn, Cu, Zn and Fe do not influence the Pb particle solidification behaviour, but Si is a potent catalyst and stimulates the Pb particles to solidify close to the equilibrium Pb melting point.

Journal ArticleDOI
TL;DR: In this paper, the critical resolved shear stress (CRSS) for MoSi2 with the C11b structure has been shown to increase at about 1000-1100°C and the anomalous strengthening is interpreted by the cross-slip mechanism as similar to that in L12 ordered alloys.
Abstract: Single crystals of MoSi2 with the C11b structure have been deformed in compression at 900–1500°C. Slip of the {013}〈3 3 1〉- and/or the {013}〈3 3 1〉- type is activated depending on crystal orientation. The critical resolved shear stress (CRSS) for {013}〈3 3 1〉- slip is much higher than that for {013}〈3 3 1〉- slip . At about 1300°C where ductility is remarkably improved, 〈100〉- and 〈110〉-type ordinary dislocations are observed and the climb mobility of these dislocations is very high. The CRSS for {013}〈3 3 1〉- slip , which depends on orientation exhibits a slight anomalous peak at 1000–1100°C. The anomalous strengthening is interpreted by the cross-slip mechanism as similar to that in L12 ordered alloys. Ductility improvement of MoSi2 above about 1200°C is assisted by the formation of stacking fault which is closely related to the instability of the C11b against to the C40 structure.

Journal ArticleDOI
TL;DR: In this paper, the coarsening behavior of the gamma-prime precipitate has been examined in high volume fraction nickel-base alloys aged at elevated temperatures for times of up to 5000 h.
Abstract: The coarsening behavior of the gamma-prime precipitate has been examined in high volume fraction nickel-base alloys aged at elevated temperatures for times of up to 5000 h. Although the cube rate law was observed during coarsening, none of the presently available coarsening theories showed complete agreement with the experimental particle size distributions (PSDs). These discrepancies were thought to be due to elastic coherency strains which were not considered by the available models. Increasing the Mo content significantly influenced the PSDs and decreased the coarsening rate of the gamma-prime cubes, as a result of increasing the magnitude of the lattice mismatch. After extended aging times, the gamma-prime cubes underwent massive coalescence into plates at a rate which was much faster than the cuboidal coarsening rate. Once the gamma-prime plates were formed, further coarsening was not observed, and this stabilization of the microstructure was attributed to the development of dislocation networks at the gamma-gamma-prime interfaces.