摘要:

AbstractThe composition and crystallography of small nitride precipitates in a 17Cr–12.5Ni low-carbon austenitic stainless steel, containing 0·5 at.-% V + Nb and 0·4 at.-%N, have been studied using atom-probe field-ion microscopy and transmission electron microscopy. Precipitates resulting from static aging at 750°C for 8,100, or 1170 h were all found to contain N, V, Nb, and Cr, and small amounts of Fe and C. The stoichiometry, close to MX0·5, remained almost constant, but the Nb content decreased and the V and Cr contents increased with longer aging times. Electron diffraction showed that the precipitates analysed had the fcc NaCI structure, but with a superlattice with a lattice parameter (a =0·838±0.004 nm) twice as large as that of the subcell. The appearance of the long-range-ordered MX0·5phase, its relationship to other phases, and the compositional changes occurring during aging are discussed and compared with earlier observations.MST/170

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.5.337

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractThe behaviour of point defects during oxidation has been re-examined in terms of the process of vacancy injection, the oxide-induced stress, and the oxygen-carbon gas reaction. In zinc and magnesium, the growth of dislocation loops as a function of oxygen pressure for specimens of different orientation is incompatible with the concept of oxidation-induced stress, but is consistent with vacancy injection. In nickel containing carbon, the amount of voiding on oxidation is prolific and is associated with the production of CO2. However, the measurement of oxide-induced tensile stress and the metallographic examination of oxidized single crystals both support a process of vacancy injection rather than vacancy generation by oxide-induced stress. In decarburized nickel, the oxidation rate increases dramatically compared with carbon-containing nickel, because the vacancies produced by oxidation are not required to relieve the stresses set up by the gas pressure arising from the carbon-oxygen gas reaction and are annihilated at the oxide/metal interface giving rise to spalling. In the absence of an oxide film, the source of vacancies is dislocations. The role of induced stress, whatever its source, is simply to aid or inhibit the precipitation of vacancies as appropriate.MST/180

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.5.344

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractThe effect of manganese on the microstructure developed by isothermal transformation of an Fe-Nb-C alloy was examined over a wide range of temperatures. A well defined discontinuity was found at about 750°C in the time-temperature-transformation curve of an Fe–0·033Nb–0·07C–1·07Mn (wt-%) alloy. The discontinuity appears close to the temperature at which the interphase precipitation of NbC begins to occur, this temperature being about 50 K lower than that for a corresponding Fe–Nb–C alloy. Carbide-free ferrite is principally formed between 850 and 780°C in the Mn-containing alloy, a wider temperature range than that for the alloy without manganese. The amount of NbC dispersed in ferrite is decreased by the addition of manganese. The changes that develop in the microstructure after the addition of manganese are explained in terms of the reduction of ferrite growth rate and the decrease in supersaturation of NbC in ferrite.MST/45

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.5.351

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractSpecimens of both as-deposited and fine-grained, reheated, manual metal arc C–Mn weld metal have been subjected to various strain and static strain-aging treatments in an attempt to simulate the thermomechanical cycles imposed upon the root region of a multipass weld as subsequent passes are made. The toughness was then measured, as a function of severity of treatment, using a crack opening displacement test. The strain aging treatments are found to lower markedly the cleavage resistance of the as-deposited and reheated microstructures. Non-metallic inclusions within the crack-tip plastic zone are found to be active as cleavage initiation sites in both types of microstructure. While the general shift in toughness can be explained by considering the changes in flow properties brought about by the various treatments, the observed variations in size and distance from the crack tip of the initiating inclusions are thought to be responsible for the associated experimental scatter.MST/157

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.5.357

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractDamage-mechanics concepts are incorporated into a fracture-mechanics interpretation of multiple grain-boundary cracking in uniaxial tensile creep specimens that undergo power–law creep deformation. Initially, methods of evaluating the fracture–mechanics creep parameter C*for a structure containing a single macroscopic crack are extended to a uniform distribution of grain-boundary voids. Damage-mechanics models are then applied to describe damage accumulation. It is shown that, when the voids grow as disc-shaped cavities, the analysis yields predictions of growth rates which are consistent with characterizations of macroscopic crack growth behaviour involving the exhaustion of the available creep ductility in a process zone at the crack tip.MST/195

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.5.366

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractTests have been performed on a cold-worked 18Mn–4Cr austenitic steel to measure rates of slow crack growth under constant load in gaseous hydrogen. The behaviour of the steel differs from that of quenched-and-tempered low-alloy steels in a number of ways: (i) slow crack growth occurs in vacuum over a wide range of stress intensity K, and hydrogen appears to accelerate this crack growth by a factor which does not depend strongly on K; (ii) the dependence of crack growth rate on K does not follow a three-stage curve, as is commonly observed in a wide range of steels; (iii) there is a strong temperature dependence of the threshold stress intensity Kthbelow ~30°C; (iv) a relatively large environmental addition of oxygen is only partially effective in inhibiting the effect of the hydrogen. Increased crack growth rates and scatter in the data are thought to be associated with bands of segregation and relatively high concentrations of phosphorus on some grain boundaries.MST/132

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.5.373

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractThe dependence of the flow stress on the orientation of the stress axis in the plane of textured zinc sheet has been investigated, both experimentally and theoretically. The theoretical investigation consisted of simulating the deformation process by using a generalized Taylor model (based on the minimum-work condition) and the Sachs model (based on the Schmidt shear-stress law). The deformation modes considered were slip and mechanical twinning. Only with the Taylor model was the orientation distribution consistent with the experimentally determined pole figures. The work done and the flow stress for a given plastic strain were therefore calculated for this orientation distribution using the Taylor model. The calculated and measured quantities exhibited the same directional dependence. This anisotropy can be attributed to the different activations for basal and pyramidal slip systems.MST/80

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.5.380

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractCentrifugally cast 0·4C–25Cr–20Ni (wt-%) steel (HK 40) has been subjected to detailed microscopic examination both in the as-cast condition and after long-term creep. The as–cast alloy contained a eutectic of M7C3and austenite, which was subjected to energy-dispersive X-ray analysis. Observations by transmission electron microscopy revealed high dislocation densities near cell boundaries. After creep in the range 750–1000°C, the eutectic carbides coarsened and transformed from M7C3to M23C6; moreover, the M23C6carbide precipitated predominantly on dislocations as a finer dispersion within the grains. During coarsening, the M23C6particles generated further dislocations in the austenite. The higher creep strength of the cast HK 40 compared to wrought alloys is discussed in terms of the detailed microstructural observations, in particular the M23C6dispersion.MST/135

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.5.385

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractThe microstructures of several centrifugally cast stainless steels containing strong carbide formers (Nb, Ti, and Zr) have been examined as cast and after prolonged creep in the range 800–1000°C. These additions refine the eutectic carbide, changing the morphology and composition as illustrated by the behaviour of IN 519 (Fe–Ni–Cr–Nb) and IN 519 TZ (Fe–Ni–Cr–Nb–Ti–Zr). The carbides present have been identified by electron diffraction and by energy-dispersive X-ray analysis. During creep, the precipitation of both MC and M23C6carbides was observed, the former being very much finer and very resistant to coarsening. The role of both the grain-boundary carbide networks and the matrix precipitation in determining the creep properties is discussed.MST/136

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.5.390

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor

摘要:

AbstractThe extent and duration of flow (casting fluidity and fluid life) of liquid tin along tubular Pyrex moulds has been measured as a function of melt superheat, mould bore, and mould material for injection under reduced pressure, gravity, and positive pressure, both with and without prior stirring of the injected melt. Macrostructural examination of the resulting fluidity samples showed that cessation of flow was triggered by sufficient inward penetration of columnar growth (vein-closure mechanism) at some distance from the mould entrance except when stirring of the melt had been carried out at low superheat before injection. In that case, growth was fully equiaxed and within the flowing stream so that cessation of flow was then expected to be governed by sufficient solidification at the advancing tip of the flowing stream. Comparison of measured fluid life with predictions of time to complete freezing in a cylindrical mould suggests that freezing occurred under heat-transfer conditions intermediate between Newtonian and ideal.MST/79

ISSN:0267-0836    

DOI:10.1179/mst.1985.1.5.398

出版商:Taylor&Francis    

年代:1985

数据来源: Taylor