An investigation of the generality of incomplete transformation to bainite in Fe-C-X alloys

The overall kinetics of the isothermal transformation of austenite to bainite and to pearlite in high-purity Fe-C-3 at. pct X alloys (X = Mn, Si, Ni, or Cu) containing 0.1 wt pct C and 0.4 wt pct C were investigated with quantitative metallography and transmission electron microscopy (TEM) to ascertain the presence or absence of the incomplete reaction phenomenon. The incomplete transformation of austenite to bainite was not observed in the Fe-C-Si, Fe-C-Ni, Fe-C-Cu, or Fe-0.4C-Mn alloys. It was found, however, in the Fe-0.1C-Mn alloy. Transmission electron microscopy results indicate that sympathetic nucleation of ferrite without carbide precipitation is a necessary but not a sufficient condition for the development of the incomplete reaction phenomenon. Transformation resumes following stasis in the low-carbon Fe-C-Mn alloy with the formation of a nodular bainite. The results support the view that the incomplete transformation of austenite to bainite is a characteristic of specific alloying elements and is not an inherent trait of the bainite reaction. Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University. Formerly Visiting Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University. Formerly Undergraduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Orientation relationships in graphitic cast irons

Ferrite/graphite and martensite/graphite interfaces in three commercial cast irons have been analyzed using transmission electron microscopy. Two recurring orientation relationships have been found to account for over 60 pct of the ferrite/graphite interfaces analyzed. A similar pair of relationships discovered in martensitic material strongly suggests that the prior austenite/graphite interface was also ordered. The same relationships were prominent in gray and ductile irons. One of the relationships observed can be transformed through the Kurdjumov-Sachs relationship to a previously-reported austenite/graphite relationship. Formerly with the Department of Metallurgy and Mining Engineering, University of Illinois, Urbana, IL. Formerly with the Department of Metallurgy and Mining Engineering, University of Illinois, Urbana, IL.     

A noncrystalline Pt-Sb phase and its equilibration kinetics

Utilizing splat cooling, a metastable noncrystalline phase was obtained in Pt-Sb alloys with 30 to 43 pct Sb and in Pt-Si alloys with the eutectic composition Pt-68 Si. These nonequilibrium structures can be retained indefinitely at room temperature. The kinetics of the decomposition of the metastable noncrystalline phase in a Pt-34 pct Sb alloy was studied in detail by X-ray diffraction, electron diffraction, and electron transmission microscopy. The decomposision starts at an appreciable rate at about 210°C, and progresses gradually with the precipitation of relatively equiaxed grains of crystalline phases. From isothermal kinetic data, average activation energies from 55.5 to 49.5 (±4) kcal per mole were derived for the overall decomposition process. Formerly Research Assistant, Department of Metallurgy and Materials Science, Massachusetts Institute of Technology, Cambridge, Mass. Formerly Research Associate, Department of Metallurgy and Materials Science, M.I.T.     

The influence of intercritical heat treatment on the temper embrittlement susceptibility of aP-Doped Ni-Cr steel

Temper embrittlement of a Ni-Cr steel doped with 0.06 wt pct P aged at 480°C after an intercritical heat treatment (IHT) has been compared with that of the conventionally heat treated steel with a range of prior austenite grain sizes. The IHT virtually eliminated the embrittlement susceptibility, but low temperature brittle fracture was still intergranular. It appears that most of the benefit of IHT in this steel comes from microstructural refinement; however, IHT also reduced the amount of segregation of Ni and P to grain boundaries. This is believed to be connected with partitioning of Cr during IHT. Formerly Post Doctoral Fellow, Department of Metallurgy and Materials Science, University of Pennsylvania Formerly on the Research Staff, LRSM, University of Pennsylvania     

Isothermal fatigue of an aluminide-coated single-crystal superalloy: Part II. effects of brittle precracking

The effect of brittle coating precracking on the fatigue behavior of a high-activity aluminide-coated single-crystal nickel-base superalloy has been studied using hollow cylindrical specimens at test temperatures of 600 °, 800 °, and 1000 °. Three types of precrack were studied: narrow precracks formed at room temperature, wide precracks formed at room temperature, and narrow precracks formed at elevated temperature. The effect of precracking on fatigue life at 600 ° was found to depend strongly on the type of precrack. No failure was observed for specimens with narrow room-temperature precracks because of crack arrestvia an oxidation-induced crack closure mechanism, while the behavior of wide precracks and precracks formed at elevated temperature mirrored the non-precracked behavior. Crack retardation also occurred for narrow room-temperature precracks tested at 800 °—in this case, fatigue cracks leading to failure initiated in a layer of recrystallized grains on the inside surface of the specimen. A significant reduction in fatigue life at 800 ° relative to non-precracked specimens was observed for wide precracks and elevated temperature precracks. The presence of precracks bypassed the initiation and growth of coating fatigue cracks necessary for failure in non-precracked material. No effect of precracking was observed at 1000 °. Formerly Research Student, Department of Materials Science and Metallurgy, University of Cambridge. Formerly Lecturer, Department of Materials Science and Metallurgy, University of Cambridge CB2 3QZ, United Kingdom.     

Modification of the B2-type matrix of aluminide diffusion coatings on nickel-base superalloys—bulk aluminide analogues

Pack aluminide coating is a useful method for conferring oxidation resistance on nickel-base superalloys. Nominally, these coatings have a matrix composed of a Ni-Al based B2-type phase (commonly denoted as Β). However, following high-temperature exposure in oxidative envi-ronments, aluminum is depleted from the coating. Aluminum depletion in turn, leads to de-stabilization of the Β phase, resulting in the formation of a characteristic lathlike Β-derivative microstructure. This article presents a transmission electron microscopy study of the formation of the lathlike Β-derivative microstructure using bulk nickel aluminides as model alloys. In the bulk nickel aluminides, the lathlike microstructure has been found to correspond to two distinct components: L1₀-type martensite and a new Β derivative. The new Β derivative is characterized and the conditions associated with the presence of this feature are identified and compared with those leading to the formation of the L1₀ martensitic phase. T.C. TOTEMEIER, formerly Research Student, Department of Materials Science and Metallurgy, University of Cambridge J.E. KING, formerly Lecturer, Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, CB2 3QZ United Kingdom     

Temper embrittlement of Ni-Cr steel by Sn

Tin-induced temper embrittlement of 3.5 pct Ni, 1.7 pct Cr steels containing 0.2 and 0.4 pct C was studied by means of notched-bar testing, scanning electron fractography, and Auger electron spectroscopy of isothermally aged specimens. The ductile-brittle transition temperature varied linearly with Sn concentration on grain boundaries at a rate which increased with hardness of the steel. The rate of approach to the steady state level of embrittlement was faster in the higher C steel. The grain boundary concentrations of Ni and Sn bear a unique relationship in a manner analogous to the behavior found previously in Sb-doped steels. The potency of Sn as an embrittling element is somewhat less than that of Sb, but considerably greater than that of P. Formerly Research Fellow, Department of Metallurgy and Materials Science, University of Pennsylvania, Philadelphia,PA Formerly Post Doctoral Fellow, Department of Metallurgy and Materials Science, University of Pennsylvania H. C. Feng Formerly with the Research Staff, LRSM, University of Pennsylvania     

Correlation of structure and superconducting properties in Nb(Cb)-Ti quaternary alloys

The aging behavior of a Nb-Ti alloy containing 60 wt pct Ti and small additions of oxygen and erbium or scandium was characterized and related to superconducting properties. The ternary and quaternary alloys were cold reduced and aged for various times at temperatures between 250° and 1000°C. ω and α phase transformations and oxide precipitation processes were followed by lattice parameter, diffraction intensity, resistivity, and metallographic studies, and correlated with superconducting critical magnetic field and critical current density measurements. The optimum 1 hr aging temperatures for producing ω and α phase precipitation were found to be 400° and 500°C, respectively. Aging at 1000°C produced only oxide precipitation. It was found that oxygen, erbium, and scandium stabilize the α phase but have little effect on ω precipitation. The ω phase proved the most effective fluxoid pinning precipitate. The fine scale dispersoid provided an extremely high number density of pinning sites. Formerly Research Assistant, Department of Metallurgy and Materials Science, M.I.T., Cambridge, Mass. Formerly Research Assistant, Department of Metallurgy and Materials Science, M.I.T. Formerly Research Associate, Department of Metallurgy and Materials Science, M.I.T.     

The kinetics of ferrite nucleation at austenite grain edges in Fe-C and Fe-C-X alloys

Nucleation kinetics of proeutectoid ferrite allotriomorphs at the edges of austenite grains in Fe-C and Fe-C-X alloys, where X is successively Mn, Ni, Co, and Si, have been measured using a modification of the techniques previously developed to study nucleation at grain faces. Analysis of these data with classical heterogeneous nucleation theory has shown that ferrite nuclei formed at grain edges have low energy interphase boundaries. An equivalent conclusion was reached during our previous studies of ferrite nucleation at austenite grain faces. The influence of alloying elements on nucleation rates was also found to follow a pattern similar to that demonstrated for grain face nucleation. Formerly Graduate Student with the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University,     

Influence of Interfacial Structure upon Carbide Precipitation at Austenite : Ferrite Boundaries in an Fe-C-Mo Alloy

During conventional isothermal transformation of an Fe-0.11 pct C-1.95 pct Mo alloy, eutectoid decomposition occurs by the interphase boundary carbide precipitation and the fibrous carbide mechanisms at 770° to 825 °C. When proeutectoid ferrite is formed and then recrystallized within the α + γ region, and subsequently further transformed at 770° to 825 °C, however, both of these eutectoid decomposition mechanisms are rendered inoperative. Carbide precipitation occurs instead entirely as isolated particles. This result supports the deduction that carbide precipitation at austenite : ferrite boundaries can occur only when these boundaries are locally immobilized by a partially coherent interfacial structure. A general approach to explaining the development of planar and curved interphase boundary precipitation, fibrous structure, and pearlite is developed in terms of two crystallographic factors. Formerly Research Associate in the Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931 and the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213 Formerly Graduate Student, Michigan Technological University, and Visiting Graduate Student, Camegie-Mellon University Formerly Professor at Michigan Technological University     

Isothermal fatigue of an aluminide-coated single-crystal superalloy: Part I

The isothermal fatigue behavior of a high-activity aluminide-coated single-crystal superalloy was studied in air at test temperatures of 600 °, 800 °, and 1000 °. Tests were performed using cylindrical specimens under strain control at ≈0.25 Hz; total strain ranges from 0.5 to 1.6 pct were investigated. At 600 °, crack initiation occurred at brittle coating cracks, which led to a significant reduction in fatigue life compared to the uncoated alloy. Fatigue cracks grew from the brittle coating cracks initially in a stage II manner with a subsequent transition to crystallographic stage I fatigue. At 800 ° and 1000 °, the coating failed quickly by a fatigue process due to the drastic reduction in strength above 750 °, the ductile-brittle transition temperature. These cracks were arrested or slowed by oxidation at the coating-substrate interface and only led to a detriment in life relative to the uncoated material for total strain ranges of 1.2 pct and above 800 °. The presence of the coating was beneficial at 800 ° for total strain ranges less than 1.2 pct. No effect of the coating was observed at 1000 °. Crack growth in the substrate at 800 ° was similar to 600 °; at 1000 °, greater plasticity and oxidation were observed and cracks grew exclusively in a stage II manner. Formerly Research Student, Department of Materials Science and Metallurgy, University of Cambridge. Formerly Lecturer, Department of Materials Science and Metallurgy, University of Cambridge CB2 3QZ, United Kingdom.     

The stabilization of martensite in Cu-Zn-AI shape memory alloys

The martensitic transformation temperature in shape memory alloys can be affected differently by aging above and below the transformation temperature. Under such circumstances the normally reversible transformation can be prevented and the martensite structure “stabilized”. This effect has been studied using electron microscopy, differential scanning calorimetry, and mechanical testing. Evidence is given of an apparently martensitic high temperature transformation, and a careful comparison is made of the stabilized and unstabilized states of the alloy. Three possible models for stabilization are considered in the light of the results obtained, and it is concluded that no single mechanism can be responsible for all the phenomena observed. Formerly with the Department of Metallurgy and Materials Science, University of Cambridge, England     

Determination of the sodium activity in aluminum and aluminum silicon alloys using sodium beta alumina

The construction of a sodium beta alumina probe for the determination of the sodium activity in molten aluminum alloys is described. It was found that the emf at a given sodium concentration was a strong function of the silicon content. Henry’s law was obeyed in super purity aluminum and the activity coefficient of 350 at 998 K agrees with other determinations. Formerly Graduate Student in the Department of Metallurgy and Materials Science, University of Cambridge     

Instantaneous determination of hydrogen content in molten aluminum and its alloys

An electrochemical probe has been developed for determining hydrogen contents in molten aluminum alloys. The probe incorporates calcium hydride as a solid electrolyte and a calcium/calcium hydride mixture as the reference material. The testing of such probes under industrial conditions is described and the results are compared with existing analytical methods. R. GEE formerly Research Assistant, Department of Metallurgy and Materials Science, University of Cambridge     

Carbon solubility as carbide in calcium silicate melts

R.A. Berryman, Formerly Graduate Student, Department of Metallurgy and Materials Science, University of Toronto,     

High-temperature transmission electron microscopyin situ study of lower bainite carbide precipitation

In situ observation of the bainite carbide precipitation processes in 40CrMnSiMoV steel by means of high-temperature transmission electron microscopy (TEM) is conducted. It is evident that carbides can precipitate either in bainitic ferrite or from austenite when carbide-free bainite (meta-bainite) obtained by isothermal transformation is tempered at higher temperatures. In view of the quantity of carbides precipitated from ferrite in combination with the result of an X-ray diffraction analysis of the bainitic ferrite carbon content, it can be concluded that bainitic ferrite growth involves supersaturation of carbon content to some degree. Formerly with Northwestern Polytechnical University Formerly with Northwestern Polytechnical University This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Pearlite phase transformation in Si and V steel

Systematic research has been undertaken on the effects of single and combined additions of vanadium and silicon on the phase transformation and microstructure of pearlitic steels. Both alloy additions were found to result in the formation of nonlamellar products in the vicinity of austenite grain boundaries in hypereutectoid compositions (0.77 to 0.95 wt pct C). The products comprise discrete initial cementite particles and grain boundary ferrite, which is embedded with interphase precipitates of vanadium carbide. As the carbon content is increased further (up to 1.05 wt pct), the amount of grain boundary ferrite gradually decreases without any dramatic change in the morphology of the initial cementite particles. No continuous embrittling grain boundary cementite network was formed. The aspect ratios of the grain boundary cementite particles were decreased from 60:1 to 25:1 by the addition of the alloy elements. A compre-hensive model has been suggested to explain these effects. Other effects of these alloy elements on the microstructure of pearlitic steels have also been examined. For given austenitization conditions, an increase in carbon and vanadium content produced a decrease in austenite grain size. Silicon was found to increase the rate of interphase precipitation of vanadium carbides. Formerly Graduate Student, Department of Materials, Oxford University Formerly University Lecturer, Department of Materials, Oxford University     

Thermodynamic properties and ordering in PdAl

The activity of aluminum in the Pd-Al system was determined by an isopiestic method from 30 to 80 at. pct Al between 1090° and 1490°K. It shows a strong negative deviation from ideality, with a decrease in activity of aluminum of four orders of magnitude around the stoichiometric composition of PdAl. The defect structure in the CsCl-structure PdAl compound has been determined by lattice parameter measurements. Equations were derived relating the degree of intrinsic disorder (α) to the shape of the activity curve. Excellent agreement between the calculated curve and the activity data was obtained for anα = 2.5 × 10⁻⁴. The results from this investigation and from previous studies indicate that the degree of intrinsic disorder in the equiatomic transition metal-aluminum compounds is controlled by electronic effects. Formely Department of Metallurgy and Materials Sciences, New York University, New York, N. Y. Formerly Department of Metallurgy and Materials Sciences, New York University Formerly Department of Metallurgy and Materials Sciences, New York University This paper is based on a Thesis submitted by M. ETTENBERG to the Graduate Division, School of Engineering and Science, New York University, in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Materials Science.     

The influence of Mo on P-lnduced temper embrittlement in Ni-Cr steel

The effects of the addition of Mo on the temper embrittlement susceptibility of a P-doped Ni-Cr steel were studied. It is shown that P-induced temper embrittlement can be eliminated for aging times up to 1000 h at 475 and 500°C, presumably by the scavenging effect of Mo on P, if the Mo : P ratio is high enough and if the Mo is not precipitated by C. The latter reaction, which allows temper embrittlement to proceed, can occur more rapidly as the aging temperature or C content is raised. Formerly Research Fellow in the Department of Metallurgy and Materials Science, University of Pennsylvania H. C. Feng Formerly with Research Staff, LRSM, University of Pennsylvania     

Growth and overall transformation kinetics above the bay temperature in Fe-C-Mo alloys

The kinetics and morphology of isothermal transformation in the vicinity of the time-temperaturetransformation (TTT) diagram bay have been investigated with optical and transmission electron microscopy (TEM) in 19 Fe-C-Mo alloys at three levels of carbon concentration (approximately 0.15, 0.20, and 0.25 wt pct) and at Mo concentrations from 2.3 to 4.3 wt pct, essentially always at temperatures above or at that of the bay,T _b . Quantitative metallography yielded no evidence for incomplete transformation (stasis) in any of these alloys atT > T _b . Measurements of the thickening kinetics of grain boundary ferrite allotriomorphs (invariably containing either interphase boundary or fibrous Mo₂C) demonstrated four different patterns of behavior. The customary parabolic time law for allotriomorph thickening in Fe-C and in many Fe-C-X systems was obtained only at higher temperatures and in the more dilute Fe-C-Mo alloys studied. With decreasing temperature and increasing solute concentrations, a two-stage and then two successive variants of a three-stage thickening process are found. In the most concentrated alloys and at temperatures nearest the bay, the second stage of the three-stage thickening process corresponds to “growth stasis”—the cessation of allotriomorph thickening. Sufficient prolongation of growth stasis presumably leads to “transformation stasis.” A number of models for growth of the carbide-containing allotriomorphs were investigated during attempts to explain the observed kinetics. It was concluded that their growth is controlled by carbon diffusion in austenite but with a driving force drastically reduced by a very strong solute drag-like effect (SDLE) induced by Mo segregation at disordered-type austenite: ferrite boundaries. Carbide growth in the fibrous structure appears to be fed by diffusion of Mo along austenite: ferrite boundaries, whereas carbides in the interphase boundary structure grow primarily by volume diffusion of Mo through austenite. Formerly Republic Steel Corporation Fellow, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI, and Visiting Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA. Formerly Professor, Michigan Technological University. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.