Showing papers in "Metal science in 1976"
TL;DR: In this article, it is shown that a constraint on growth rate can occur in polycrystals because creep cavities are inhomogeneously distributed amongst the various grain boundaries, and that the rate of unconstrained cavity growth is directly proportional to the maximum principal tensile stress, whereas constrained growth is proportional to nth power of the octahedral shear stress.
Abstract: There have been a number of rate equations proposed in the literature for the growth of grain-boundary cavities during creep. Implicit in all these analyses is the assumption that growth occurs freely and that no constraints exist that would attenuate the predicted rates. It is shown that a constraint on growth rate can occur in polycrystals because creep cavities are inhomogeneously distributed amongst the various grain boundaries. This attenuation of growth rate is predicted to be greatest when high-strength alloys with large cavity populations are deformed slowly and least when pure metals with low cavity populations are deformed quickly. The application of these ideas to multiaxial stress states and extrapolation procedures is discussed in the knowledge that the rate of unconstrained cavity growth is directly proportional to the maximum principal tensile stress, whereas constrained growth is proportional to the nth power of the octahedral shear stress.
283 citations
TL;DR: In this article, a creep theory for precipitation-hardened alloys has been described, leading to a relationship of the type έ = A(έ-έp)4, where έp represents a back stress due to the particle dispersion.
Abstract: Recent experimental data of the stress/creep rate relationship for precipitation-hardened alloys have been surveyed. These materials display typically a higher stress-sensitivity of the creep rate than is observed for pure metals, and solid solutions. Furthermore, their stress/creep rate curves show pronounced breaks, with a stress-sensitivity of the creep rate, ∂ ln έ/∂ ln σ, of ∼4 below the break. The precipitation dispersion affects the creep rate such that it reaches a minimum at an intermediate interparticle spacing, with higher rates for both larger and smaller spacings. Based on the principles of recovery creep, a creep theory for precipitation-hardened alloys has been described, leading to a relationship of the type έ= A(έ-έp)4, where έp represents a back stress due to the particle dispersion. At high creep stresses έp is constant and defined by the stress to operate a particle-cutting mechanism or the Orowan mechanism. At lower stresses the dislocations are able to bypass the particles, s...
264 citations
TL;DR: In this paper, the authors compare the conditions under which hole growth without vacancy condensation is faster than hole growth by diffusion and show that low values of the ratio σ/e, where σ is stress and e is the strain rate, as well as large voids favour the strain process.
Abstract: In a creeping solid holes may grow by vacancy condensation or by the action of the applied stress producing strains at the surface of the hole which cause it to grow. The latter mechanism does not involve a vacancy flux to the hole. A comparison of the two processes indicates the conditions under which hole growth without vacancy condensation is faster than hole growth by diffusion. Low values of the ratio σ/e, where σ is stress and e is the strain rate, as well as large voids favour the strain process. Such conditions usually arise in tertiary creep but may also occur earlier in the creep life. Experimental examples of cavitation in which vacancy condensation is shown to be the minor process are given, and the relevance of such a mechanism to hole growth in grain-boundary sliding and regions of localized flow is indicated.
221 citations
TL;DR: The relation between the minimum creep rate and the time to fracture was investigated in this article, where the authors showed that the latter is positively correlated with the rate of the creep rate.
Abstract: (1976). The relation between minimum creep rate and time to fracture. Metal Science: Vol. 10, No. 11, pp. 382-384.
200 citations
TL;DR: In this article, the ways in which the cleavage-fracture stress and fracture toughness vary with grain size in mild steel have been studied and a statistical argument has been advanced to explain the grain-size dependence of this distance.
Abstract: The ways in which the cleavage-fracture stress and fracture toughness (K IC) vary with grain size in mild steel have been studied. The fracture toughness is related to the cleavage-fracture stress by a microstructural characteristic distance. A statistical argument has been advanced to explain the grain-size dependence of this distance.
166 citations
TL;DR: In this article, the distribution of boron in samples of Type 316 austenitic steel (containing 18 ppm by weight of B), cooled at a rate of either 50 or 500 degC/s from solution-treatment temperatures in the range 900-1350°C, has been determined using an optical microscope autoradiographic technique.
Abstract: The distribution of boron in samples of Type 316 austenitic steel (containing 18 ppm by weight of B), cooled at a rate of either 50 or 500 degC/s from solution-treatment temperatures in the range 900–1350°C, has been determined using an optical microscope autoradiographic technique. Segregation of the boron to the grain-boundary regions was detected in the specimens cooled at the slower rate from the higher solution-treatment temperatures, the extent of the intergranular segregation diminishing with decreasing solution-treatment temperature; grain-boundary segregation of the boron was not detected in the specimens cooled at 500 degC/s irrespective of the solution-treatment temperature. A theoretical model is proposed to account for this non-equilibrium grain-boundary segregation of boron during cooling, based on the existence of mobile vacancy-boron complexes. The predictions of the model are in good agreement with the experimental observations if realistic values for the vacancy-boron binding ene...
155 citations
TL;DR: A ternary addition in the binary solution of an impurity in a metal can reduce the solubility and enhance the grain-boundary segregation of the impurity as mentioned in this paper, showing that both phenomena have a common t
Abstract: A ternary addition in the binary solution of an impurity in a metal can reduce the solubility and enhance the grain-boundary segregation of the impurity. Showing that both phenomena have a common t...
109 citations
TL;DR: In this article, the effects of prestrained QIN (HY-80) low-alloy steel on crack-opening displacement have been investigated and it was shown that the decrease in toughness with prestrain is accompanied by the development of zigzag fractures.
Abstract: Crack-opening displacement (COD) tests have been performed on prestrained QIN (HY-80) low-alloy steel. In specimens containing fatigue precracks the COD is largely insensitive to prestrains below 0.17, but a significant reduction in COD occurs at higher prestrains. Measurements of the yield-stress variation with tensile strain have enabled the effects of prestrain on fracture toughness to be estimated. Apart from a small initial increase, the general effect of prestrain is to reduce fracture toughness. The decrease in toughness with prestrain is accompanied by the development of zig-zag fractures. Crack growth appears to be intermittent and occurs by a process of shear decohesion along paths corresponding to either the spiral slip-lines emanating from a blunted cracktip, or, at lower displacements, the straight slip-lines emanating from a sharp crack.
93 citations
TL;DR: In this paper, it was shown that the general applicability of the expression with p ≃ 3.5 is confirmed on a variety of nickel and iron-base alloys encompassing a range of microstructures commonly found in engineering materials.
Abstract: It has previously been shown that, during creep deformation, secondary creep rates (es) can be related to stress (σ) by the equationes = A*(σ−σ0)p exp (−Qc*/RT).The exponent p ≃ 4, Qc*≃an activation energy for diffusion (usually self-diffusion), and σ0 is a friction stress. The parameter A* is material-dependent so that creep data for a variety of materials appear as a set of parallel lines on a log es/log (σ−σ0) graph. In the present paper, the general applicability of the expression with p ≃ 3.5 is confirmed on a variety of nickel- and iron-base alloys encompassing a range of microstructures commonly found in engineering materials. Further, a single universal equation to describe the creep behaviour of the various materials has been developed. It involves normalization of the effective stress (σ−σ0) by the proof (σ0.05) or yield (σy) stress of the alloy at the appropriate creep temperature, resulting in the expressiones = B(σ−σ0/σ0.05)3.5with B independent of material, crystal lattice, microstru...
87 citations
TL;DR: In this paper, the authors describe part of an investigation into differences in cavitation characteristics in commercial and high-purity Cr-Mo-V steels of the types used in rotors and pipes in steam power plant.
Abstract: This paper describes part of an investigation into differences in cavitation characteristics in commercial and high-purity Cr-Mo-V steels of the types used in rotors and pipes in steam power plant....
70 citations
TL;DR: In this paper, the interplay between carbon and ferrite-stabilizing alloying elements during the peritectic transformation in high-alloy steels has been studied in unidirectionally solidified samples.
Abstract: The interplay between carbon and ferrite-stabilizing alloying elements during the peritectic transformation in high-alloy steels has been studied in unidirectionally solidified samples. During the experiments the samples were quenched in order to freeze the microstructure present during the unidirectional solidification. The sequence of solidification and transformation could thus be studied metallographically and by microprobe analysis. It was found that ferrite-stabilizing elements can segregate strongly to the ferrite during the austenite → ferrite transformation. This segregation can be described by the normal laws of segregation. It was further shown that the peritectic transformation in a steel containing a ferrite-stabilizing alloying element can occur in two different ways. The new phase, austenite, grows into ferrite as well as into the liquid phase. In one case, the growth into the liquid phase occurs by a eutectic transformation. In the other case, the growth into ferrite occurs by a m...
TL;DR: An Al-6 wt.-%Mg alloy containing Zr, Cr, and Mn was found to deform highly superplastically when produced in a heavily cold-rolled and recrystallized condition.
Abstract: An Al-6 wt.-%Mg alloy containing Zr, Cr, and Mn was found to deform highly superplastically when produced in a heavily cold-rolled and recrystallized condition. Grain-boundary sliding was detected during superplastic flow of this alloy and dislocations were observed to interact with precipitate particles within grains. The alloy specimens, obtained at 0°, 45°, and 90° to the rolling direction, exhibited anisotropy with respect to their flow stresses and textures. The superplastic deformation process in this alloy can be well explained in terms of grain-boundary sliding accommodated by dislocation motion within grains.
TL;DR: In this paper, a metastable miscibility gap has been found which explains the decomposition behavior of γ-Mn-Cu alloys, and a tweed structure was observed which coarsened upon annealing.
Abstract: A metastable miscibility gap has been found which explains the decomposition behaviour of γ-Mn-Cu alloys. Hardness measurements as well as results of other authors were used to outline the boundaries of the two-phase region. Electron microscopy provided direct evidence for the phase separation prior to α-Mn precipitation. A tweed structure was observed which coarsened upon annealing. The existence of the miscibility gap gives rise to the observed increase of the Neel temperature upon annealing as well as to the microstructural changes affecting the variations in specific damping capacity upon ageing, i.e. an initial increase and subsequent decrease of the damping capacity with ageing time.
TL;DR: In this article, the superplastic deformation characteristics of a microduplex stainless steel were examined over the temperature range 700 −1020°C and the strain rate sensitivity index, m, was observed to increase with the temperature.
Abstract: The superplastic deformation characteristics of a microduplex stainless steel were examined over the temperature range 700–1020°C. The strain rate sensitivity index, m, was observed to increase wit...
TL;DR: In this paper, structural changes in ball-bearing steels caused by rolling contact fatigue have been studied and the decay process is described, the transformation products identified, and the mechanisms discussed.
Abstract: Structural changes in ball-bearing steels caused by rolling contact fatigue have been studied. Attention is focused on changes that occur in the various carbide constituents. Large spherical primary cementite particles gradually break up and disappear during the fatigue process. At the same time, fine carbide constituents present in either a tempered martensite or lower bainite matrix also disappear. The decay process is described, the transformation products identified, and the mechanisms discussed.
TL;DR: In this article, a detailed study of grain-boundary damage in a 2¼%Cr-1%Mo steel after creep at 565°C was made, where large numbers of cavities were distributed heterogeneously on prior austenite grain boundaries and an association with coarse carbide particles was evident.
Abstract: A detailed study has been made of grain-boundary damage in a 2¼%Cr-1%Mo steel after creep at 565°C. By subjecting the material to specific heat-treatments, a quantitative description of the resultant structures together with an assessment of creep damage has enabled creep cavitation to be correlated with certain microstructural features. Large numbers of cavities were distributed heterogeneously on prior austenite grain boundaries and an association with coarse carbide particles was evident. The cavity population was relatively insensitive to changes in the prior austenite grain size but was found to be inversely related to the width of precipitate-free zones (PFZ's) at the prior austenite grain boundaries. This result was linked with surface observations of zone shear and suggested that stress concentrations necessary for cavity nucleation were induced by local accommodation strains adjacent to the grain boundaries. On these lines, a theoretical nucleation model has been proposed and discussed in...
TL;DR: Ferrite morphologies and carbide precipitate dispersions in a ½Cr-Mo-V creep-resisting steel have been studied after isothermal transformation at a number of subcritical temperatures in the ferrite and bainite ranges, and also after rapid quenching to room temperature to produce martensite.
Abstract: Ferrite morphologies and carbide precipitate dispersions in a ½Cr-Mo-V creep-resisting steel have been studied after isothermal transformation at a number of subcritical temperatures in the ferrite and bainite ranges, and also after rapid quenching to room temperature to produce martensite. It has been found that VC precipitates by the interphase mechanism during direct transformation to ferrite, and towards the latter stages of the transformation, some fibres of Mo2C also form. Bainite morphologies ranged from granular at the higher transformation temperatures to lath-like at lower temperatures. Tempering of bainite and martensite at 700°C resulted in VC precipitation on dislocations and the interparticle spacing tended to decrease as the initial transformation temperature in the bainite range was increased. However, the best resistance to overageing during tempering was provided by ferrite produced by direct transformation at 700°C. These differences in dispersion stability are explained in term...
TL;DR: The recrystallization method, a newly developed technique for measuring plastic strain, makes it possible to observe directly the intense strain region near the crack tip as discussed by the authors, and the shape of this zone and the plastic strain distribution obtained by this method are similar to the theoretical findings of Rice and Johnson.
Abstract: The recrystallization method, a newly developed technique for measuring plastic strain, makes it possible to observe directly the intense strain region near the crack tip. The shape of this zone and the plastic strain distribution obtained by this method are similar to the theoretical findings of Rice and Johnson. It is also confirmed that progressive crack-tip blunting under large-scale yielding, i.e. COD, is closely related to the extension of these regions of intense strain near the crack tip.
TL;DR: In this article, the effects of carbon, nitrogen, molybdenum, titanium, and niobium on the occurrence of 485°C embrittlement in 25%Cr ferritic stainless steels were examined.
Abstract: An examination has been made of the effects of carbon, nitrogen, molybdenum, titanium, and niobium on the occurrence of 485°C embrittlement in 25%Cr ferritic stainless steels. All these alloying additions increase the rate of precipitation, as measured by the rate of age hardening. In general the tensile strength and ductility reflected the effects of the α′ precipitation, but it is shown that a marked refinement of the ferrite grain size can offset the embrittling effects of even heavy α′ precipitation. Various suggestions are made to explain why the alloying elements studied accelerate the rate of α′ precipitation.
TL;DR: In this paper, an extensive study of fatigue-crack growth in BS15 mild steel has been made, in laboratory air at 21°C and at two cycling frequencies, namely 0.25 and 35 Hz.
Abstract: An extensive study of fatigue-crack growth in BS15 mild steel has been made, in laboratory air at 21°C and at two cycling frequencies, namely 0.25 and 35 Hz. A wide range of cyclic growth rates, starting from the region associated with fatigue thresholds (10−7 mm/cycle) up to the region of rapid fracture, was investigated using contoured DCB specimens which provided constant stress-intensity factors over the crack lengths of ∼150 mm. It was found that for BS15 a relationship between cyclic growth rate da/dN and the tensile loading levels of the form da/dN = aϕα was applicable. a and α are constants and ϕ a parameter dependent upon the loading conditions and material properties. The proposed equation is shown to be dependent upon the range of crack-opening displacement, the plastic zone size, and the J-contour integral. The relation has also been successfully applied in analysing published crack growth rate data obtained from four different steels as well as some Al alloys. Threshold values of stre...
TL;DR: In this article, the tensile yield strength of quenched and tempered low carbon martensite with 5% nickel was analyzed with respect to grain size and substructural parameters and at the same time effects from an addition of 0.3% molybdenum are investigated.
Abstract: The tensile yield strength of quenched and tempered low-carbon (< 0.10%) lath martensite with 5% nickel is analysed with respect to grain size and substructural parameters and at the same time effects from an addition of 0.3% molybdenum are investigated. Microstructures are studied in detail by transmission electron microscopy. The initial martensite packets control the high-angle boundary grain structure after normal tempering, thus contributing to the yield strength through an ordinary Petch-Hall relation for the packet size. The Petch-Hall coefficient for packets seems to decrease on tempering if extensive recovery and subgrain formation occur. Subgrain boundaries and dislocations within subgrains contribute substantially to the yield strength and account for the high strength of this type of martensitic steel. Softening on tempering is mainly a result of recovery through which the original lath structure is destroyed and the initially high dislocation density is reduced. The addition of 0.3%Mo...
TL;DR: In this paper, the conditions required for metastable formation of austenite are investigated experimentally and theoretically and the dendrite structure is discussed from a theoretical viewpoint, and the primary and secondary dendritic arm spacings are evaluated as functions of growth rate and temperature gradient.
Abstract: The solidification of low-carbon manganese steels has been studied by unidirectional solidification techniques. The primary and secondary dendrite arm spacings are evaluated as functions of growth rate and temperature gradient. The conditions required for metastable formation of austenite are investigated experimentally and theoretically and the dendrite structure is discussed from a theoretical viewpoint.
TL;DR: In this article, a theoretical determination of the amount of precipitate available in precipitation-strengthened materials has been made, and it has been shown that in certain circumstances a maximum in this amount is available when the elements which precipitate are added in the stoichiometric ratio for the compound they form.
Abstract: A theoretical determination of the amount of precipitate available in precipitation-strengthened materials has been made. It has been shown that in certain circumstances a maximum in this amount is available when the elements which precipitate are added in the stoichiometric ratio for the compound they form. The commercial significance of this observation is emphasized. Previous data on the creep properties of 18/12/Nb and 20/25/Nb steels have been analysed numerically. Progressive precipitation during creep which increases with increasing solution-treatment temperature is related to improved creep strengths and rupture lives. The optimum creep strengths are found in steels in which niobium has been added to carbon in the stoichiometric ratio. The analysis also shows that Nb-rich compositions are stronger than C-rich ones.
TL;DR: In this article, the authors investigated chromium diffusion in a duplex stainless steel containing ∼8% ferrite in the temperature range 873-1273 K using the standard serial sectioning technique.
Abstract: Diffusion of chromium in a duplex stainless steel containing ∼8% ferrite has been investigated in the temperature range 873–1273 K using the standard serial sectioning technique. The resulting concentration profiles exhibited up to four distinct regions. The two main regions are attributed to volume diffusion in the austenite and ferrite phases, the other zones being due to short-circuiting paths. Volume diffusion in the austenite phase is in good agreement with Cr diffusion in Type 316 steel. The Cr diffusion coefficient in the ferrite phase of approximate composition 25Cr-5 wt.-%Ni is given by:Dα = (6.0⁺¹¹₋₃) X 10⁻⁶ (- 212± 5/RT) m²s⁻¹the activation energy being expressed in kJ mol−1. Little evidence was found for enhanced Cr diffusion along austenite/ferrite interface boundaries.
TL;DR: In this article, the accumulated plastic zone (i.e., a zone with accumulated plastic strain > 0.02) around a fatigue crack in Type 304 stainless steel has been made using the recrystallization phenomenon.
Abstract: Direct observation of the accumulated plastic zone (i.e., a zone with accumulated plastic strain > 0.02) around a fatigue crack in Type 304 stainless steel has been made using the recrystallization phenomenon. Annealing at 950°C for 24 h followed by electrolytic etching in Morris solution was used to reveal the accumulated plastic zone. It is shown that the larger the zone, the larger the crack-growth rate, and that the fatigue crack growth rate is proportional to the second power of the accumulateci plastic zone size.
TL;DR: In this paper, the effect of boron concentration on the surface and grain-boundary energies of AISI 316 steel at temperatures in the range 950-1250°C has been examined.
Abstract: The effect of boron concentration on the surface and grain-boundary energies of AISI 316 steel at temperatures in the range 950–1250°C has been examined. Four ingots were used with B contents of 0.001 and 0.006 or 0.0065 wt.-% and C contents of 0.12 and 0.05 or 0.06 wt.-%. The grain-boundary energies were decreased by 12–39% on increasing the B concentration but the effect on the surface energies was less clear. Grain-boundary grooves were formed by a surface diffusion process at 950–1150°C but no consistent effect of B on the diffusion coefficient was apparent.
TL;DR: The main cause of hot cracking in fully austenitic welds is sulphur segregation as mentioned in this paper, which occurs when manganese remains in solution and when the last of the S-rich melt solidifies Mn is simply not available to break up the sulphide films.
Abstract: Advanced quantitative chemical and image analysis techniques based on scanning and scanning-transmission electron microscopy have been used to study hot cracking and micro-segregation in an 18%Cr-10%Ni weld metal. It is found that the main cause of hot cracking is sulphur segregation. In fully austenitic welds, manganese remains in solution and when the last of the S-rich melt solidifies Mn is simply not available to break up the sulphide films. The present practice of substantially increasing the amount of Mn in austenitic welds, or adding cerium, would thus seem to be justified. Phosphorus also aggravates the problem of hot cracking, but fortunately it does not appear to segregate so markedly as sulphur, being much more soluble in austenite. If the weld metal is so alloyed that solidification to the two-phase austenite plus ferrite occurs, hot cracking does not occur. The two main reasons are concluded to be: (1) manganese is less soluble in ferrite than austenite and a sufficient quantity of Mn...
TL;DR: In this paper, the creep resistance of a two-dimensional system of hexagonal grains is analyzed for both Nabarro-Herring and grain-boundary sliding rate-controlling mechanisms.
Abstract: The creep resistance of a two-dimensional system of hexagonal grains is analysed for both Nabarro-Herring and grain-boundary sliding rate-controlling mechanisms Both mechanisms are shown to operate sequentially When one is much faster than the other the shear and normal stresses on the boundaries are shown to differ from those in an elastic body The creep rate is related to the boundary stresses and is shown to be independent of orientation of the uniaxial applied stress to the hexagonal array The grains are allowed to roll over neighbouring grains and it is shown that this can increase the creep rate The conditions under which grains can exchange neighbours are discussed, and also the effect of the resulting irregular shapes on the creep rate
TL;DR: In this paper, the surface oxide structure of several metals and alloys has been examined by X-ray photoelectron spectroscopy (ESCA), which facilitates the measurement of oxide thickness, when less than ∼5 nm, by a combination of chemical shift and electron escape depth.
Abstract: The surface oxide structure of several metals and alloys has been examined by X-ray photoelectron spectroscopy (ESCA). The technique facilitates the measurement of oxide thickness, when less than ∼5 nm, such as Al2O3 on Al and SnO on Sn, by a combination of chemical shift and electron escape depth. The oxidation state of multivalent metals such as Cu, Pb, and Sn is determined by utilizing the chemical shift or the multiplicity of core states of excited ions. In addition ESCA is shown to be a potentially useful technique for studying bulk-to-surface diffusion phenomena in alloys.