Partition of Mn during the growth of proeutectoid ferrite allotriomorphs in an Fe-1.6 at. pct C-2.8 at. pct Mn alloy

A STEM analysis is made of the Mn distribution around grain boundary allotriomorphs of proeutectoid ferrite in an Fe-1.6 at. Pct C-2.8 at. Pct Mn alloy. Whereas the Mn enriched region is readily observed to extend along the austenite grain boundary, no substantial build-up or depletion of Mn near the ferrite : austenite interface is detected, consistent with the electron probe microanalysis previously reported. In the temperature range where the partition-local equilibrium (P-LE) mode has been proposed to prevail, measured parabolic growth rate constantsfall 1 to 2 orders of magnitude above that predicted from this model, but also below that calculated from the paraequilibrium (PE) model by roughly the same amount. A modification of the theory of grain boundary diffusion-aided growth of precipitates,i.e., the collector/rejector plate mechanism, on the other hand, accounts fairly well for the observed growth kinetics of ferrite allotriomorphs. However, only a slightly better accounting than the P-LE model is provided by this mechanism for the temperature dependence of Mn partition. Data on Ni partition, obtained in an Fe-0.5 at. Pct C-3.1 at. Pct Ni alloy, are also analyzed with the rejector plate model.     

Cu precipitation in a prestrained Fe-1.5 wt pct Cu alloy during isothermal aging

The control of Cu precipitation at low temperatures, e.g., bake hardening of Cu bearing steels, has recently attracted considerable attention due to the potential of achieving good formability and high strength. An Fe-1.5 wt pct Cu alloy, solution treated and 10 pct prestrained, exhibits a two-step age-hardening behavior, i.e., a smaller, but substantial hardening around 200 °C to 300 °C and a major hardening around 500 °C, while only the latter hardening occurs in undeformed specimens. The precipitation behavior of nanoscale Cu particles or bcc Cu clusters that plays a major role in age hardening was simulated by Cahn-Hilliard nonclassical nucleation theory and the Langer-Schwartz model. Simulation results are compared with the distribution of Cu particles observed under three-dimensional atom probe field ion microscope (3-D APFIM) and transmission electron microscope (TEM), and age hardening behavior as well. The increase in hardness in prestrained specimens at low temperatures (≤400 °C) can be ascribed to Cu particles nucleated preferentially at dislocations or to Cu particles that were formed in the matrix as early as at dislocations presumably due to excess vacancies introduced by prestraining.     

The tempering of martensite in an Fe-1.5 pct N alloy

The tempering of Fe 1.5 pct N martensite has been studied at temperatures up to 300°C using X-ray and electron microscope techniques. Stage 1 decomposition occurs below 270°C by the general precipitation, resembling spinodal morphology, of fine τa^" (Fe₁₆ N₂) lamellae on 001 habit planes in both matrix and twin crystals of the partially 112 twinned martensite plates. Yet, gaged by changes in the X-ray spectrum, the reaction is discontinuous, the tetragonal martensite doublets decaying in intensity without change in their Bragg positions. The anomaly and the failure to detect by electron microscopy regions exhibiting fractional stages of the fine scale α^’ → α + α^" reaction is attributed to its occurrence at different times in different martensite (or parts of martensite) plates. It is believed that transformation occurs in this manner because the nucleation of coherent α^" plates is controlled by the prevailing internal stress field. Thus the time exponent “n” for the reaction decays from a normal value between 1 and 0.67 to less than 0.3 as stress relief by recovery dominates the more protracted stages of the reaction. Above 200°C the more stable nitride γ^’ (Fe₄N) forms at an increasing rate as plates on 012 habit planes, accompanied by marked softening.     

Elevated temperature deformation behavior of an Al-8.4 wt pct Fe-3.6 wt pct Ce alloy

The elevated temperature deformation characteristics of a rapidly solidified Al-8.4 wt pct Fe-3.6 wt pct Ce alloy have been investigated. Constant true strain rate compression tests were performed between 523 and 823 K at strain rates ranging from 10⁻⁶ to 10⁻³ s⁻¹. At temperatures below approximately 723 K, the alloy is significantly stronger than oxide dispersion strengthened (ODS) aluminum. However, at higher temperatures, the strength of the Al-Fe-Ce alloy falls rapidly with increasing temperature while ODS aluminum exhibits an apparent threshold stress. It is shown that particle coarsening cannot fully account for the reduction in strength of the Al-Fe-Ce alloy at elevated temperatures. The true activation energy for deformation of the Al-Fe-Ce alloy at temperatures between 723 and 773 K is significantly greater than that for self-diffusion in the matrix. This is unlike the behavior of ODS alloys, which contain nondeformable particles and exhibit true activation energies close to that for self-diffusion in the matrix. Since abnormally high true activation energies for deformation are also exhibited by materials containing deformable particles, such as γ^′ strengthened superalloys, it is concluded that elevated temperature deformation in ythe Al-Fe-Ce alloy involves deformation of both the matrix and the precipitates. The loss of strength of the Al-Fe-Ce alloy appears to be related to a reduction in strength of at least some of the second phase particles at temperatures above 723 K. Formerly Research Assistant, Department of Materials Science and Engineering, Stanford University.     

Microstructural investigation of the oxidation of an Fe-3 pct Cr alloy

The microstructure and microchemistry of the oxide scale on an Fe-3 wt pct Cr alloy have been investigated after oxidation in the temperature range 700 to 800 °C. Transmission and scanning electron microscopy along with energy dispersive X-ray analysis and Auger electron spectroscopy techniques were used for the investigation. Multilayered scales are observed which vary in composition and structure; the innermost oxide is an Fe-Cr spinel of the type Fe(Fe_2•xCr_x)O₄. The intermediate layer and the outer oxide layer are both α-Fe₂O₃ hematite. The outer hematite layer is nonadherent and wrinkling is observed. Spallation occurs readily at the inner hematiteJspinel interface and at the spinel oxideJalloy interface. The poor oxidation resistance of the alloy is discussed in terms of these observations.     

Simultaneous creep and oxidation of an Fe-1 Wt pct Al alloy at 973 to 1073 K

Studies of the simultaneous creep and oxidation of an Fe-lwt pct Al alloy at constant stresses of 16 to 26 MN/m² in the temperature range 973 to 1073 K have shown that the steady state creep obeys a power law. The stress exponentn was found to be 6.9 for creep in an argon atmosphere(Po₂ = 10^-3 mbar). Values of the apparent activation energy for creep were in the range 225 to 351 kJ/mol and appeared to be dependent on theP _{o 2} of the test environment. Oxidationstrengthening (è decreasing) o°Curred at 998 K but only atP _{o 2} = 10^-3 mbar and was primarily due to intergranular oxidation and the mechanical constraint of a strongly adherent scale. Oxidationweakening (è increasing), however, o°Curred at 973 to 1073 K in environments of the lowestP _{o 2} (10^-9 and 10^-5 mbar) and in the highestP _{o 2} (162 to 1013 mbar). The factors contributing to weakening are believed to be oxidation-induced vacancies, weakly adherent scales and the loss of solute strengthening aluminum through selective oxidation. M. H. SHAHHOSSEINI, formerly a Research Student at the University of Manchester.     

Simultaneous creep and oxidation of an Fe-1 wt pct Ai alloy at 973 to 1073 K

Studies of the simultaneous creep and oxidation of an Fe-lwt pct Al alloy at constant stresses of 16 to 26 MN/m² in the temperature range 973 to 1073 K have shown that the steady state creep obeys a power law. The stress exponentn was found to be 6.9 for creep in an argon atmosphere (Po₂ = 10^?3 mbar). Values of the apparent activation energy for creep were in the range 225 to 351 kJ/mol and appeared to be dependent on the P_{0 2} of the test environment. Oxidationstrengthening (?e decreasing) occurred at 998 K but only at P_{0 2} = 10^?3 mbar and was primarily due to intergranular oxidation and the mechanical constraint of a strongly adherent scale. Oxidationweakening (?e increasing), however, occurred at 973 to 1073 K in environments of the lowest P_{0 2} (10^?9 and 10^?5 mbar) and in the highest P_{0 2} (162 to 1013 mbar). The factors contributing to weakening are believed to be oxidation-induced vacancies, weakly adherent scales and the loss of solute strengthening aluminum through selective oxidation.     

The kinetics of the internal nitriding of Fe-2 at. pct Al alloy

The kinetics of the precipitation of aluminum nitride on internal nitriding the Fe-2 at. pct Al alloy was investigated for cold-rolled and recrystallized specimens exhibiting “ideally weak” interaction behavior of the solutes Al and N. The kinetic analysis was performed using mass-increase data obtained for thin foils (thickness ⪯0.1 mm) upon nitriding in a NH₃/H₂ gas mixture at temperatures in the range 803 to 853 K. Activation-energy analysis revealed that precipitation of AlN in the recrystallized specimens is associated with a Gibbs free energy barrier for the formation of a precipitate of critical size; the precipitation rate is controlled by both nucleation and growth. On the other hand, precipitation of AlN in the cold-rolled specimens occurs without a Gibbs free energy barrier for formation of a precipitate of critical size; the precipitation rate is controlled by growth with kinetics governed by volume diffusion of alu-minum. Analysis of the total Gibbs free energy of formation of AlN in the α-Fe matrix showed that in the case of the recrystallized specimens, the formation of incoherent AlN precipitates with a hexagonal crystal structure is favored. In the case of the cold-rolled specimens, containing a high dislocation density, the formation of coherent AlN precipitates with cubic crystal structure is favored, at least in the beginning of precipitation.     

Ferrite and carbide morphologies in and below the “bay region” of an Fe-0.19 Pct C-2.30 Pct Mo alloy

Optical and transmission electron microscopy were used to study ferrite and carbide morphology from temperatures just above to well below the bay temperature in an Fe-0.19 pct C-2.30 pct Mo alloy. Suppression of idmanstätten ferrite morphologies at the bay and their reappearance below, initially in highly degenerate form, previously reported by Boswellet al. in Fe-C-2 pct Mo alloys, were confirmed. These effects were again explained in terms of a solute drag-like effect. Just above the bay, the usual fibrous and interphase boundary Mo₂C structures appear. However, fibrous carbides disappear immediately below the bay whereas interphase boundary carbides continue to form until approximately 60 K below the bay. In a narrow temperature range below the bay, large, widely spaced Mo₂C laths are also observed. Many laths are associated with terraces and risers of ferrite superledges; some, however, nucleate on dislocations within ferrite. Nucleation of Mo₂C on such dislocations occurs throughout the temperature range investigated (898 to 748 K). At the lower temperatures studied, (Fe, Mo)₃C appears as plates approximately 10³ longer than those of Mo₂C also formed at these temperatures. Explanations for these observations on carbides are offered in terms of interactions between ferrite and carbide precipitation and the concepts of ledgewise diffusional growth Orientation relationships between ferrite and carbides in several types of microstructure are reported, but the equivocal nature of the information which these data supply about transformation mechanism is emphasized.     

Relationship between austenite dislocation density introduced during thermal cycling and M s temperature in an Fe-17 wt pct Mn alloy

The reason why thermal cycling decreases the martensite start (M _s ) temperature of an Fe-17 wt pct Mn alloy was quantitatively investigated, based on the nucleation model of ε martensite and a thermodynamic model for a martensitic transformation. The M _s temperature decreased by about 22 K after nine cycles between 303 and 573 K, due to the increase in shear-strain energy (ΔG _sh ) required to advance the transformation dislocations through dislocation forests formed in austenite during thermal cycling. The ΔG _sh value increased from 19.3 to 28.8 MJ/m³ due to the increase in austenite dislocation density from 1.5 × 10¹² to 3.8 × 10¹³/m² with the number of thermal cycles (in this case, up to nine cycles). The austenite dislocation density increased rapidly for up to five thermal cycles and then increased gradually with further thermal cycles, showing a good agreement with the increase in austenite hardness with the number of thermal cycles.     

强磁场下退火温度对Fe-0．36C合金铁素体转变的影响

采用12T强磁场热处理装置研究了强磁场下不同奥氏体化温度退火对高纯Fe-0.36C合金先共析铁素体组织形貌的影响. 结果表明: 强磁场能够显著抑制针状铁素体的形成,并使先共析铁素体晶粒沿磁场方向伸长且呈链状排列. 这是由于强磁场增加的相变驱动力克服了由界面能和退磁场能构成的相变阻力所致. 随奥氏体化温度升高,先共析铁素体晶粒沿磁场方向伸长程度逐渐减弱,针状铁素体面积分数增加,这是由于奥氏体晶粒粗大化而引起的退磁场能的被削弱程度降低所致.     

Formation mechanism of bainitic ferrite in an Fe-2 Pct Si-0.6 Pct C alloy

The bainite transformation at 723 K in an Fe-2 pct Si-0.6 pct C alloy (mass pct) was investigated with transmission electron microscopy (TEM) and quantitative metallography to clarify the growth mechanism of the ferritic component of bainite. In early stages of transformation, the bainitic ferrite was carbide free. The laths of bainitic ferrite within a packet were parallel to one another and separated by carbon-enriched retained austenite. The average carbon concentration of the bainitic ferrite was estimated to be 0.19 mass pct at the lowest, indicating that the ferrite was highly supersaturated with respect to carbon. The laths did not thicken during the subsequent isothermal holding, although they were in contact with austenite of which the average carbon concentration was lower than the paraequilibrium value. In the later stage of transformation, large carbide plates formed in the austenite between the laths, resulting in the decrease in the carbon concentration of the austenite. Subsequently, the ferrite with a variant different from the initially formed ferrite in the packet was decomposed for the completion of transformation. The present results indicate that the bainitic ferrite develops by a displacive mechanism rather than a diffusional mechanism. Formerly Graduate Student, Kyoto University, Kyoto 606-01, Japan This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.     

Effect of carbon content and helium gas environment on creep crack growth properties of Ni-26 pct Cr-17 pct W-0.5 pct Mo alloy at 1273 K

Creep crack growth tests were conducted on Ni-26 pct Cr-17 pct W-0.5 pct Mo alloys with different carbon contents in air and in helium gas environment at 1273 K using the compact-type (CT) specimen, and the effects of carbon content and environment on creep crack growth rate are discussed. Creep crack growth rateda/dt is evaluated by theC* parameter. Theda/dt is faster in higher-carbon alloys than in lower-carbon alloys in each environment. This effect of carbon content is attributed to the lower creep ductility due to the increase of fine trans-granular carbides in higher-carbon alloys. The environmental effect on theda/dt vs C* relations is scarcely observed in higher-carbon alloys. In the 0.003 pct C alloy, however,da/dt is much lower in the He gas environment than in air. Carburization is observed ahead of the crack tip in the He gas environment at 1273 K. The intergranular carbides precipitated due to carburi-zation have a granular configuration and are considered to prevent the grain boundary sliding in lower-carbon alloys.     

The effect of structure on the deformation of as-quenched and tempered martensite in an Fe-0.2 pct C alloy

As-quenched and tempered martensite in an Fe-0.2 pct C alloy were subjected to tensile testing and structural characterization by light and transmission electron microscopy. The light temper, 400°C-l min, did not change packet morphology, but did reduce dislocation density, coarsen lath size and cause the precipitation of carbides of a variety of sizes. The yield strength of the as-quenched martensite was strongly dependent upon packet size according to a Hall-Petch relationship, but tempering significantly diminished the packet size dependency, a result attributed to packet boundary carbide precipitation and the attendant elimination of carbon segregation present in the as-quenched martensite because of autotempering. Examination of thin foils from strained tensile specimens showed that a well-defined cell structure developed in the as-quenched martensite, but that the random distribution of jogged dislocations and carbide particles produced by tempering persisted on deformation of the tempered specimens. The authors were formerly Research Assistant and Professor, respectively, at Lehigh University, Bethlehem, PA.     

The evolution of grain boundary character during superplastic deformation of an Al-6 pct Cu-0.4 pct Zr alloy

The evolution of microstructure, texture, and microtexture in an Al-6 pct Cu-0.4 pct Zr alloy was studied during mechanical testing at 480 °C and a strain rate of 5·10⁻⁴ s⁻¹. The as-processed material had an elongated, banded microstructure and a deformation texture with orientation distribution along the β-orientation fiber. The true strain vs true stress curve exhibited three stages: I, II, and III. Work hardening occurred in stages I and III, whereas nearly steady-state behavior was observed in stage II. A bimodal distribution of boundary disorientation angles was evident in as-processed material and persisted into stage I, with peaks at 5–15 deg in the low-angle boundary (LAB) regime and at 45–60 deg in the high-angle boundary (HAB) regime. An increase in strain rate sensitivity coefficient, m, in stage I was accompanied by fragmentation of the initial microstructure, leading to the formation of new grains. During stage II the strain rate sensitivity coefficient, m, attained a value of 0.5, which is consistent with the onset of grain boundary sliding. In stage III, the texture and the grain boundary disorientation distribution became randomized while the m value decreased. Grain elongation and cavity formation at second-phase particles and along grain boundaries were seen in samples deformed to failure. The as-processed microstructure is described in terms of deformation banding, and a model for the evolution of such a structure during superplastic deformation is proposed.     

Decomposition in Al-Zn alloys: Part I. Isothermal decomposition in an Al-22 at. pct Zn-0.1 at. pct Mg alloy

A small-angle X-ray scattering (SAS) study has been made on solution treated and isothermally annealed specimens of an Al-22 at. pct Zn-0.1 at. pct Mg alloy. The changes in peak position and integrated area of the SAS spectra with time and temperature indicate that de composition is nearly complete immediately after quenching, in agreement with the earlier interpretation that Gerold and Merz placed on the results of Rundman and Hilliard in the binary Al-22 at. pct Zn alloy. Furthermore, structural changes occurring during annealing are consistent with a coarsening or maturation of the fluctuations in the solution. The domi nant wavelength varies ast ^1/3 over a large time span and the temperature dependence of the coarsening process yields an activation energy of 94.2 kJ/mole. The effect of Mg is to re tard the formation of the equilibrium phases while having a small effect on the growth of the composition fluctuations during the coarsening process. Now on sabbatical leave to Caterpillar Tractor Co., Mapleton Plant, Mapleton, Ill. 61554.