Showing papers in "Metal Science Journal in 1969"
TL;DR: In this article, a grain-boundary sliding and grain rotation mechanism was proposed for superplastic extension of the aluminium-zinc eutectoid, where the strain rate (έ), flow stress (σ), grain size (L), and temperature were related empirically.
Abstract: Superplastic extension of the aluminium–zinc eutectoid results primarily from grain-boundary sliding and grain rotation. The strain rate (έ), flow stress (σ), grain size (L), and temperature (T) are related empirically: ɛ ˙ = K σ 2 L 2 exp [ − U k T ] where K is a constant, K is Boltzmann's constant, and U correlates with the activation energy for grain-boundary diffusion.The following proposed mechanism is quantitatively in agreement with our observations...
299 citations
TL;DR: In this article, a theory of precipitate dissolution has been proposed, in which it is implied that a previous treatment by Thomas and Whelan (Phil. Mag., 1961, 6, 1103), where dissolution was considered to be approximately the reverse of growth, is in error in this assumption.
Abstract: A theory of precipitate dissolution has recently been proposed by Aaron (Metal Sci. J., 1968, 2, 192) in which it is implied that a previous treatment by Thomas and Whelan (Phil. Mag., 1961, 6, 1103), where dissolution was considered to be approximately the reverse of growth, is in error in this assumption. Moreover, the time-dependence of the radius of a dissolving precipitate according to Aaron ( R = R 0 − K D t ) disagrees with that of Thomas and Whelan (dR2/dt = −kD). It is pointed out that the “disagreement” arises because the situations treated are themselves dissimilar. Aaron's result is essentially one-dimensional and is derived from the transient part of the diffusion field in one dimension. The result of Thomas and Whelan is for three-dimensional diffusion a...
183 citations
TL;DR: In this paper, the effect of a dispersion of particles on high-temperature creep has been examined by comparing the creep and recovery properties of Nimonic 80A with those of a single-phase nickel-20% chromium alloy at 750° C.
Abstract: The effect of a dispersion of particles on high-temperature creep has been examined by comparing the creep and recovery properties of Nimonic 80A with those of a single-phase nickel–20% chromium alloy at 750° C. Over most of the creep curve, the strain at any instant can be described accurately as ɛ = ɛ0 + ɛ T (l − e−mt) + έ s t, where ɛ0 is the instantaneous strain on loading, ɛ t the total transient strain, έ s the steady-creep rate, and m a constant relating to the rate of exhaustion of transient creep. Measurements of the rate of recovery during transient and steady-state creep suggest that this equation applies when the creep rate is proportional to the rate of recovery. Deviations from this equation occur during the initial 10–15% of the transient stage, and are attributable to a rapid increase in the coefficient of work-hardening. Although the activation energy for creep and the stress-dependence of the creep rate of the Nimonic alloy are higher than the values obtained for the single-phase...
93 citations
TL;DR: In this paper, the volume-diffusion rates of chromium and nickel in a 20 Cr/25 Ni/Nb austenitic steel in the temperature range 840-1290° C were measured.
Abstract: Standard radiotracer techniques have been used to measure the volume-diffusion rates of chromium and nickel in a 20 Cr/25 Ni/Nb austenitic steel in the temperature range 840–1290° C. Within the limits of experimental uncertainty, the results are described by D Cr = 0·19 exp (−58·8 × 103/RT), D Ni = 4·06 exp (−67·5 × 103/RT). Grain-boundary-diffusion measurements have been made in the range 550–900° C. The diffusion rates of chromium and nickel isotopes are not significantly different from previously reported values for iron. Combining the data for all three elements gives a mean Arrhenius equation δDB = 1·5 × 10−7 exp (−44 × 103/RT).
68 citations
TL;DR: In this article, microstructural changes that occur during superplastic deformation of the Al-Cu eutectic alloy are considered, and it is shown that considerable modification occurs to the shapes and distribution of the inter-metallic CuAl2 particles during deformation.
Abstract: The microstructural changes that occur during superplastic deformation of the Al–Cu eutectic alloy are considered. It is shown that considerable modification occurs to the shapes and distribution of the inter-metallic CuAl2 particles during deformation. This effect may be understood in terms of sliding and diffusion-controlled shape changes associated with grain boundaries within the intermetallic. The roles of interface sliding and of diffusion creep in superplasticity are discussed.
59 citations
TL;DR: In this paper, the authors show that the strain/time relation for transient and steady-state creep can be derived from first-order reaction-rate theory under the assumption that transient creep obeys first order reaction rate theory.
Abstract: An analysis is presented which shows that the strain (ɛ)/time (t) relation ɛ = ɛ 0 + ɛ T { 1 − exp ( − t / τ ) } + ɛ ˙ s t where ɛ0 is the instantaneous strain on loading, ɛ T the magnitude of transient creep, τ the relaxation time of transient creep, and έ s the steady-state strain rate, can be derived by assuming that transient and steady-state creep obey first-order reaction-rate theory. The assumption is considere...
49 citations
TL;DR: In this article, the thermodynamics and diffusivity of oxygen in dilute solution in nickel in the range 1050-1200° C were studied by α-ray spectrometry in atmospheres of controlled oxygen partial pressure between 1295 and 1400° C.
Abstract: Measurements have been made of the thermodynamics and diffusivity of oxygen in dilute solution in nickel in the range 1050–1200° C. The dissolution of thoria in nickel was then studied by α-ray spectrometry in atmospheres of controlled oxygen partial pressure between 1295 and 1400° C. It was concluded that the rate-determining step in the dissolution process was the diffusion of thorium atoms in nickel, and that the presence of oxygen in solution in the metal enhanced this process.
39 citations
TL;DR: The mechanism of formation of G.P. zones and γ′ precipitate, as revealed by electron microscopy, was reported for a range of copper-beryllium alloys as discussed by the authors.
Abstract: The mechanism of formation of G.P. zones and γ′ precipitate, as revealed by electron microscopy, is reported for a range of copper–beryllium alloys. Individual zone images were obtained for alloys of lower Be content. The γ′ precipitate is nucleated by G.P. zones when these are present. Above the G.P. zone solvus (∼ 320° C) γ′ is heterogeneously nucleated.
32 citations
TL;DR: In this paper, the authors investigated temperature, applied stress and thorium dioxide particles effects on Ni-Cr alloys and their creep properties, investigating temperature and applied stress, and the effect of thorium carbon dioxide particles.
Abstract: Dispersion strengthened Ni-Cr alloys creep properties, investigating temperature, applied stress and thorium dioxide particles effects
32 citations
TL;DR: An analysis of results in the literature on density changes during creep in copper is described in this paper, where the data are treated systematically in order to isolate the effect of each of the variables strain, time, stress, and temperature on the accumulation of creep damage.
Abstract: An analysis of results in the literature on density changes during creep in copper is described. The data are treated systematically in order to isolate the effect of each of the variables strain, time, stress, and temperature on the accumulation of creep damage. It is shown that all the measurements of void volume/unit weight may be described by the relation V = const . ɛ t σ 2·3 exp ( − 22 500 R T ) . The theoretical implications of this empirically derived equation are considered. It is concluded that the behaviour may be e...
28 citations
TL;DR: In this article, the authors measured dislocation density and observations of dislocation structure on two 20% Cr-35% Ni steels, one solid solution and the other precipitation-hardened by Ni3 (Ti, Al), both creep-tested at 700°C.
Abstract: Measurements of dislocation density and observations of dislocation structure have been carried out on two 20% Cr–35% Ni steels, one (A) solid solution and the other (B) precipitation-hardened by Ni3 (Ti, Al), both creep-tested at 700° C. The dislocations were arranged in a three-dimensional network. Subgrains were formed only in the solid solution. A linear relationship between creep stress (τ) and (dislocation density)½ (ρ½) was established for both materials. This may be considered as supporting the view that the recovery-creep model applies to materials hardened by a second phase. The τ vs. ρ½ relationship had a positive stress intercept for steel B and may be regarded as a measure of the decrease in the driving force for recovery due to the precipitates. The lower creep rate of alloy B could be entirely accounted for by this decrease in driving force. The solid-solution alloy exhibited a negative stress intercept in the τ vs ρ½ relation and this is a consequence of the increase in dislocatio...
TL;DR: The modes of fracture in nickel and three nickel-iron alloys have been examined over a range of true strain rates from 2·06 × 10−3 to 3·96 s−1 at temperatures from 762 to 1279° C.
Abstract: The modes of fracture in nickel and three nickel–iron alloys have been examined over a range of true strain rates from 2·06 × 10−3 to 3·96 s−1 at temperatures from 762 to 1279° C. Fracture was initiated at triple points and at serrations in the original grain boundaries. Subsequent propagation was governed by the structural changes in the materials. If no recrystallization occurred, cracks spread rapidly along the grain boundaries and failure occurred at low strains. If recrystallization took place, the initial cracks were isolated and further growth occurred slowly in small increments to give failure at large strains. The initiation of cracking has been related to mechanisms proposed for creep rupture.
TL;DR: In this article, the authors used density changes in each section following creep to determine the effect of the test variables on V, the total void volume/unit weight, and demonstrated that some of the assumptions made in the past regarding cavitation damage are untenable.
Abstract: Stepped creep bars providing four different stresses on each specimen have been tested at 525° C for up to 850 h. Density changes in each section following creep are used to determine the effect of the test variables on V, the total void volume/unit weight. It is shown that the density changes are described by the relationship V = 1·52 × 10−38 ɛ t σ7·5, where ɛ is the strain, t the time in h, and σ the initial stress in lbf/in2. This equation is valid to the point of rupture and calculated rupture times are in good agreement with actual values. However, since a knowledge of the value of V at rupture is required for such a prediction, it is doubtful whether the approach will be suitable for purposes of extrapolation. It is demonstrated that some of the assumptions made in the past regarding cavitation damage are untenable. In particular, the amount of damage at failure or at any stage of the creep curve is dependent on the test conditions. There is no reason to suspect that the development of cavi...
TL;DR: In this article, a combined metallographic and microbeam X-ray analysis revealed the presence of { 10 1 ¯ 2 } -, { 11 2 ¯ 1 } - and [11 2 ¯ 2 ] type twins.
Abstract: Coarse-grained (∼0·3 mm) α-titanium has been fatigued in tension/compression about a zero mean load at a frequency of ∼ 100 Hz at room temperature. A combined metallographic and microbeam X-ray analysis revealed the presence of { 10 1 ¯ 2 } -, { 11 2 ¯ 1 } -, and { 11 2 ¯ 2 } - type twins. In internal grains, i.e. grains completely constrained by surrounding material, fatigue _damage was observed to be associated with the { 11 2 ¯
TL;DR: In this article, a correlation between particle size and hardness was established and observed qualitative changes in microstructure were related to the variation in titanium content of the particles, which are believed to be based on the DO3Fe3Si structure.
Abstract: The microstructural investigations outlined in this paper represent natural extensions of previous work in investigating precipitation in the Fe–Ti–Si system. The obvious value of the field-ion microscope in enabling pre-peak-hardness microstructures to be defined is established and a correlation between particle size and hardness is given. The observed qualitative changes in microstructure are related to the variation in titanium content of the particles, which are believed to be based on the DO3Fe3Si structure. Quantitative size-distribution data are presented that extend the work performed by other authors to shorter ageing times. For ageing times > 1 h at 600° C a linear time-dependence is found for the cube of the mean particle size, in agreement with bulk-diffusion-controlled coarsening theory and previous work. For ageing times < 1 h deviations in such a plot occur, which are shown to arise from an incomplete precipitation reaction.
TL;DR: A refined recovery-creep model, which takes into account the variation in strain-hardening and recovery with dislocation density, has been worked out in this article, which gives a realistic description of the strain/time relationship in the primary and secondary creep stages.
Abstract: A refined recovery-creep model, which takes into account the variation in strain-hardening and recovery with dislocation density, has been worked out. The model gives a realistic description of the strain/time relationship in the primary- and secondary-creep stages. Furthermore, it gives information about the dislocation-density/time relationship and the constant dislocation density attained in the steady state. The model contains two main constants, A 0 and B 0, that define the initial creep rate and the mobility of climbing dislocations. It has been proved that έ s /A 0, ρ s , and ɛ p (creep rate (έ s ) and dislocation density in the steady state (ρ s ), and primary-creep strain (ɛ p )) can be expressed as simple functions of the ratio B 0/A 0. Examples are shown where the agreement between experimental and computed creep values is excellent. Comparisons are made between the strain/time and the dislocation-density/time relationships for the refined recovery-creep model and various approximations...
TL;DR: A diffuse X-ray scattering study has been carried out on two aluminium-zinc alloys, one containing 15 wt% zinc and the other 25 wt.-%. Both were examined after water-quenching and ageing for different times at different temperatures as mentioned in this paper.
Abstract: A diffuse X-ray scattering study has been carried out on two aluminium–zinc alloys, one containing 15 wt.-% zinc and the other 25 wt.-%. Both were examined after water-quenching and ageing for different times at different temperatures. The results support a proposal made by Lasek (Physica Status Solidi, 1964, 5, K117) regarding the solvus curve for Guinier–Preston zones in aluminium–zinc alloys. The curve by Borelius (J. Metals, 1951, 3, 477) appears to be a lower limit for the transformation (after prolonged ageing) of completely coherent Guinier–Preston zones to partially coherent zones.
TL;DR: In this paper, a mechanism was proposed whereby combined grain-boundary sliding and grain deformation gives rise to cavity growth, based on the observations from the fracture surfaces and from polished sections of creep specimens.
Abstract: Replicas have been examined both from the fracture surfaces and from polished sections of creep specimens of pure iron and copper, to determine the shape of grain-boundary cavities. From the observations a mechanism is proposed whereby combined grain-boundary sliding and grain deformation gives rise to cavity growth.
TL;DR: In this paper, the interdiffusion coefficient in the α solid solution of tungsten in nickel has been determined for the nickel-tungsten and TD (thoria-dispersed) systems over the range 1000-1316° C.
Abstract: The interdiffusion coefficient in the α solid solution of tungsten in nickel has been determined for the nickel–tungsten and TD (thoria-dispersed) Nickel–tungsten systems over the range 1000–1316° C. The Boltzmann–Matano analysis was employed to compute diffusivities from concentration vs. penetration curves measured with the electron microanalyser. The interdiffusion co-efficient is concentration-dependent in the nickel–tungsten system, decreasing with increasing tungsten content and can be represented by the equation: D ~ = 1·19 exp ( − 0·0469 C w ) exp ( − 3·69 × 10 4<...
TL;DR: In this article, the ambient and elevated-temperature tensile behavior of the directionally solidified fibrous Fe2B and lamellar Ni-Ni3Si eutectics has been studied.
Abstract: The ambient- and elevated-temperature tensile behaviour of the directionally solidified fibrous Fe–Fe2B and lamellar Ni–Ni3Si eutectics has been studied. The matrix in the fibrous composite displays enhanced strength characteristics owing to a constraint phenomenon caused by the combined effects of the closeness of the fibres and the condition of strain compatibility at the matrix/fibre interface. Expressions are derived to obtain the yield strain of the matrix phase in the composite. In polycrystalline specimens, a grain boundary inclined to the tensile axis, having no fibres crossing it, is found to be an easy shear path at elevated temperatures. This may limit the use of similar eutectic systems to single crystals.
TL;DR: In this article, the surfaces of cavities and cracks were observed by electron fractography using carbon extraction replicas and the cavities appeared to be polyhedral and were associated with particles of V4C3, 200-600 A in dia.
Abstract: Creep rupture, in the form of cavities and reheat cracks at the prior austenite grain boundaries, has been examined in the heat-affected zone and weld metal of a CrMoV steel weldment. The surfaces of the cavities and cracks were observed by electron fractography using carbon extraction replicas. The cavities appeared to be polyhedral and were associated with particles of V4C3, 200–600 A in dia. It is possible that the cavities were nucleated heterogeneously at the particles. There was no evidence of cavity nucleation at the usual nonmetallic inclusions. The surfaces of cavities and cracks were thermally faceted. The faceting of the crack surfaces had undulations corresponding to the size of the prior cavities.
TL;DR: In this article, tensile tests have been conducted at 300 and 500°C on specimens of unidirectionally grown aluminium-CuAl2 eutectic of various orientations.
Abstract: Tensile tests have been conducted at 300 and 500° C on specimens of unidirectionally grown aluminium–CuAl2 eutectic of various orientations and at 300 and 400° C on polycrystalline samples of this alloy. The tensile results, deformation and failure mechanisms, and spheroidization phenomena are described.
TL;DR: In this article, the mechanisms of cleavage fracture in coarse-grained 3% silicon iron have been studied between −196 and +60° C using uniaxial tensile and notch-bend tests.
Abstract: The mechanisms of cleavage fracture in coarse-grained 3% silicon iron have been studied between −196 and +60° C using uniaxial tensile and notch-bend tests. Emphasis was placed on determining the critical event for fracture under the various experimental conditions. In the tensile tests at the lowest temperatures (−196 to −125° C) fracture occurred below the slip yield stress and was twin-nucleated. Between −125 and ∼ −50° C twinning coincided with macroscopic yielding by slip and fracture occurred after an approximately constant plastic strain of < 1%. It was deduced that the critical event for fracture throughout this range (−196 to ∼ −50° C) was the nucleation of a suitable microcrack. Between ∼ −50 and +55° C the critical event was the growth of grain-size microcracks, themselves nucleated at cracks in grain-boundary carbides or pearlite colonies. The deformation and fracture mechanisms in the notched specimens were in sharp contrast to those described above. Throughout the range +40 to −160° ...
TL;DR: In this article, the effect of grain size on the high-temperature fatigue properties of an Mg-0·8% Al alloy has been studied, and it is shown that this effect is related to an increase in the initial plastic strain amplitude, caused by an increasing in the rate of grain-boundary sliding.
Abstract: The effect of grain size on the high-temperature fatigue properties of an Mg–0·8% Al alloy has been studied. A reduction in rupture life occurred when the grain size exceeded the specimen thickness. It is shown that this effect is related to an increase in the initial plastic strain amplitude, caused by an increase in the rate of grain-boundary sliding. It is also established that this deformation mode controls cavity growth and that failure occurs at a critical mean value of the sliding displacement, independent of applied stress and structure.
TL;DR: In this article, a detailed structural examination of extrusions in an Al-Zn-Mg alloy (Zn 4·0, Mg 2 ·0, Mn 0·33, Cr 0·18, Zr 0·15, Fe 0·26, Si 0·12%).
Abstract: A detailed structural examination has been carried out on extrusions in an Al–Zn–Mg alloy (Zn 4·0, Mg 2·0, Mn 0·33, Cr 0·18, Zr 0·15, Fe 0·26, Si 0·12%). The extrusions had been produced to cover a range of macro structures and so showed varying stress-corrosion lives. The peripheral zones of all the extrusions were recrystallized, and the regions within the cores that appeared to be “fibrous” on a macro scale were composed of a substructure in which sub grains of very similar orientation were situated in layer-like colonies. The particles precipitated along low-angle boundaries during ageing were finer and more numerous than those precipitated on high-angle boundaries, although the associated precipitate-free zones along both types of boundary were of similar width. In addition, some coarse intermetallic particles of a different origin were located preferentially on the high-angle boundaries. Observations of their paths indicated that the stress-corrosion cracks followed high angle boundaries pr...
TL;DR: In this article, it has been shown that boron does not dissolve interstitially in alpha-iron when the metal is very pure and, more generally, that the available data for BORON solubility are dependent on the content of impurities in the iron used.
Abstract: Damping tests at 1 c/s and dilatometric measurements of the α–γ transition temperature have been carried out on high-purity iron (99·9985%) and its dilute alloys with boron. Confirmation has been obtained of the hypothesis previously advanced that boron does not dissolve interstitially in alpha-iron when the metal is very pure and, more generally, that the available data for boron solubility are dependent on the content of impurities in the iron used.
TL;DR: In this paper, a theory for ductile fracture in a material with a strain-rate-sensitive yield stress is proposed, based on the hypothesis that fracture is the result of internal necking between adjacent microscopic cavities.
Abstract: A theory for ductile fracture in a material with a strain-rate-sensitive yield stress is proposed, based on the hypothesis that fracture is the result of internal necking between adjacent microscopic cavities. The theory is developed for a plane-strain element containing uniformly distributed cavities and this model is used to examine the effects of strain-rate-sensitivity on the strain to fracture in ductile materials. The results suggest possible explanations for both “blue-brittleness” behaviour and the “superplasticity” effect.
TL;DR: In this article, it was shown that at low temperatures, the decomposition of austenite in plain chromium steel results in a periodic structure consisting of walls of carbide embedded in a ferrite matrix, with the walls parallel to the ferrite/austenite interface.
Abstract: Especially at low temperatures, the decomposition of austenite in plain chromium steel results in a periodic structure consisting of walls of carbide embedded in a ferrite matrix, with the walls parallel to the ferrite/austenite interface. It has been shown that this periodic eutectoid structure forms through the growth of steps at the interface in a direction parallel to it. The mechanism is assumed to be a general one in phase transformations in various alloys.
TL;DR: In this article, field-ion micrographs showing two dissociated total dislocations have been analysed, one dislocation was dissociated on {110} while the other had dissociated to give a composite fault on { 110} and {112}.
Abstract: Field-ion micrographs showing two dissociated total dislocations have been analysed. One dislocation was dissociated on {110} while the other had dissociated to give a composite fault on {110} and {112}.
TL;DR: In this article, the authors investigated the factors that lead to the onset of intergranular failure in terms of the relative ease of nucleation and rate of growth of intragranuiar cracks and grain-boundary cavities.
Abstract: Tensile tests on a Mg–0·8% Al alloy have been conducted at a strain rate of 10−2h−1 between 25 and 400° C with grain sizes ranging from 0·009 to 0·300 in. It is shown that the growth of voids occurs by a grain-boundary sliding mechanism and the variation of ductility in the intergranular-failure range is interpreted by means of this parameter. The factors that lead to the onset of intergranular failure are discussed in terms of the relative ease of nucleation and rate of growth of intragranuiar cracks and grain-boundary cavities.