On the substructure of athermal and isothermal martensites formed in an Fe-21 Ni-4mn alloy

Transmission electron microscopy and diffraction analysis have been made on the athermal and isothermal martensites formed in an Fe-21Ni-4Mn alloy, with particular attention to details of the martensite crystallography and substructure. In general, a variation in the martensite substructure has been found, comparing adjacent regions of near proximity in the same specimen, but nevertheless general conclusions can be drawn. Plates of martensite were observed after both isothermal and athermal treatment. At an early stage of growth, the isothermal martensite plates exhibit similar crystallographic and substructural features compared to those found in the athermally formed plates. In both cases, the overall macroscopic martensite habit plane is near (252)_f, but such macroscopic plates actually consist of many small plates (termed subplates) of the same orientation with a (121)_f habit plane. In the case of isothermal plates an apparent rotation of the habit plane to ward ( 111 )_f occurs as the plates thicken. This rotation was not observed for athermally formed plates. The observations made in this work suggest that temperature dependent relaxation of transformation strains may affect the growth and autocatalytic nucleation of martensite subplates, thus causing the microstructural differences occurring between isothermal and athermal martensite plates. Formerly with the Department of Metallurgy and Mining Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 Formerly with the Department of Metallurgy and Mining Engineering, University of Illinois     

The propagation of transgranular stress-corrosion cracks in admiralty metal

An investigation has been made of the propagation of transgranular stress-corrosion cracks in monocrystals and polycrystals of Admiralty Metal tested in ammoniacal solutions. Studies were made of the topography and orientation of the fracture surfaces, the acoustic emission during crack propagation, and the stress dependence of the crack velocity. Cracking was found to occur on parallel but displaced {110 facets, and the steps between the fracture facets also were crystallographic in nature, consisting of alternating segments of two {111 planes. The overall direction of the steps was found to be perpendicular to the crack front, and examination of opposing fracture surfaces indicated that the surface features are matching and interlocking. While not generally resolvable with the SEM, crack-arrest markings could be detected on the fracture faces under certain conditions, and this, together with the occurrence of discrete acoustic-emission events, indicated that propagation is discontinuous. Based on these and other observations, it is suggested that propagation occurs by discontinuous cleavage on {110 planes and that the steps are produced by shear on {111 planes. Possible mechanisms of embrittlement are discussed. J. A. BEAVERS formerly Research Assistant, Department of Metallurgy and Mining Engineering, University of Illinois, Urbana, IL 61801. E. N. PUGH formerly Professor, Department of Metallurgy and Mining Engineering, University of Illinois.     

Spinodal decomposition of a Co-Ti-Nb alloy

The microstructural evolution on aging a Co-3 wt pct Ti-2 wt pct Nb alloy has been followed by transmission electron microscopy and diffraction to show that the solid solution decomposed by the spinodal mode. The strengthening observed has been correlated with the differences in lattice parameters of the coexisting phases. The several stages of coarsening have been documented to yield information about their kinetics and morphological changes. Formerly Visiting Assistant Professor, Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, 1206 West Green Street, Urbana, IL 61801, is with .     

The formation of austenite in a low-alloy steel

Austenite formation from different microstructural conditions has been studied in an Fe-lV-0.2C steel. The starting microstructures of ferrite, bainite, and martensite, and the morphology of austenite formation have been examined in detail by light microscopy and transmission electron microscopy. Retained austenite in quenched martensitic areas has been used to establish the crystallographic relationships during austenite nucleation, and to indicate the behavior of the initial vanadium carbide dispersion present in the ferrite starting microstructure. Limited measurements were also made of the kinetics of austenitization. Formerly Research Student, Department of Metallurgy and Materials Science, University of Cambridge. Formerly of Royal Society, Warren Research Fellow, Department of Metallurgy and Materials Science, University of Cambridge.     

Epitaxy and surface melting of aluminum alloys by high powered directed energy

Aluminum alloy surfaces were melted by beams from high powered directed energy sources. A variety of techniques were employed to examine the resulting alloy structures and it was found that the melted regions resolidify epitaxially onto the unmelted crystalline substrate. It is demonstrated that there is a region of strained material lying close to the maximum depth of fusion. Resolidification is always epitaxial and does not depend on the power setting of the instrument used to cause the melting (laser or electron beam) or the rate of travel of the point source on the sample. A qualitative explanation for the epitaxial growth phenomenon is given. Formerly Research Associate, Department of Metallurgy and Mining Engineering, University of Illinois, Urbana-Champaign, IL.     

Rapid melting and solidification of surface due to stationary heat flux

A general two-dimensional computer heat flow model is developed in an oblate spheroidal coordinate system for rapid melting and subsequent solidification of the surface of a semiinfinite solid subjected to a high intensity heat flux over a circular region on its bounding surface. Generalized numerical solutions are presented for an aluminum substrate subjected to both uniform and Gaussian heat flux distributions. Temperature distributions, melt depth and geometry, and melting and solidification interface velocities are calculated as a function of applied heat flux, radius of the circular region, and time. It is shown that the important melting and solidification parameters are a function of the product of the absorbed heat flux, q, and the radius of the circular region, a. General trends established show that melt depth perpendicular to the surface is inversely proportional to the absorbed heat flux for a given temperature at the center of the circular region. Dimensionless temperature distributions and the ratio of liquid-solid interface velocity to absorbed heat flux,R/q, as a function of dimensionless melt depth remain the same if the productqa is kept constant, whileq anda are varied. For a given total power absorbed melting and solidification parameters are compared for uniform and Gaussian heat flux distributions. For a given temperature at the center of the circular region both melt depth and width are smaller for the Gaussian distribution while temperature gradients and interface velocities are larger. Formerly Graduate Research Assistant, Department of Mechanical and Industrial Engineering, University of Illinois. Formerly Research Associate, Department of Metallurgy and Mining Engineering,University of Illinois. Formerly Professor at the University of Illinois, Urban, IL.     

The transgranular SCC of a Mg-Al alloy: Crystallographic,fractographic and acoustic-emission studies

A study has been made of the propagation of transgranular stress-corrosion cracks in a Mg−7.5 wt pct Al alloy tested at room temperature in an aqueous NaCl-K₂CrO₄ solution; the studies were carried out on single cracks which initiated and propagated within single grains in coarse-grained bend specimens. The resulting fracture surfaces were flat, and two-surface analysis established their orientation to be {ie1155-01}. Specimens fractured at liquid-nitrogen temperature cleaved on {ie1155-02}, indicating that the {ie1155-03} are not the normal cleavage planes in this material. The stress-corrosion fracture surfaces were cleavage-like in appearance, containing shallow steps which were matching and interlocking on opposite faces. Discrete acoustic signals were emitted during crack propagation, and these are considered to result from discontinuous crack advance. It is concluded that stress-corrosion cracking in this system occurs by discontinuous cleavage on {ie1155-04} planes. Formerly a Research Assistant in the Department of Metallurgy and Mining Engineering, University of Illinois, Urbana, Ill. 61801.     

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.     

Transitions between type A flake,type D flake,and coral graphite eutectic structures in cast irons

Directional solidification experiments were used to measure the transition velocities between the type A and coral eutectic structures in high-purity cast irons and between the type A and type D eutectic structures in S and Te doped cast irons. Introduction of O into the gas atmosphere was found to have little effect on the A ⟹ D transition velocities in S doped alloys, but it produced a strong reduction in the A ⟹ coral transition velocities in high-purity irons. Transmission electron micros-copy revealed interesting variations in the defect structures of the graphite in the flake ironsvs the type of flake (A or D) and the type of doping element. Scanning Auger microscopy demonstrated that both S and Te segregate to the iron/graphite interface. In the S doped alloys, type A flakes are generally covered with a monolayer of S with patches of 0 in the form of iron oxide having a thickness on the order of 2 nm. A series of experiments, including examination of fracture surfaces at the quenched solid/liquid growth front, have shown that S segregates to the iron/graphite interfaces from the liquid at the growth front, but O forms at these interfaces during the cooldown. These results are discussed in relation to current models of eutectic growth in cast irons. Formerly Graduate Student,Department of Materials Science and Engineering, Iowa State University.     

Rutherford backscattering study of high temperature oxidation of melt- spun glassy fe- 22.5ai- 10zr

We have applied Rutherford backscattering spectroscopy to investigate the high temperature oxidation of melt-spun glassy Fe-22.5Al-10Zr. Oxide thicknesses and elemental depth distributions were measured at five times during oxidation at 500 °C and on both surfaces of the melt-spun ribbon. Oxidation kinetics were found to be inconsistent with oxidation by diffusion through a uniform oxide layer. The glassy metal oxidized slower on the rapidly cooled side of the ribbon. This was found to be associated with compositional differences that were present in the as-spun ribbons. The oxide that formed was composed of a thin iron-rich outer oxide beneath which was an aluminum-rich oxide that thickened with time. Formerly with Materials Science Division, Argonne National Laboratory Formerly with the Department of Civil Engineering, Mechanics, and Metallurgy, University of Illinois at Chicago, Chicago, IL.     

An electron microscopy study of carbide precipitation in vanadium

The precipitation of carbon from supersaturated solid solution in vanadium has been investigated using transmission electron microscopy techniques on thin foils. High purity vanadium strip was doped to a carbon content of about 0.2 at. pct and rapidly quenched in high vacuum, followed by aging treatments for various times in the temperature range from 300° to 600°C. The strip was then thinned for transmission electron microscopy. The carbon is observed to precipitate initially as very finely dispersed carbides visible through structure factor contrast. With increasing aging the precipitate distribution coarsens, and the carbides appear as very thin coherent platelets on {310} planes, showing a pronounced displacement fringe contrast. The coherent precipitate appears to have a bcc structure closely related to that of the matrix. With further aging these platelets are observed to thicken and partially lose coherency, punching out prismatic dislocation loops and helices having axes in 〈111〉 directions. This semicoherent precipitate is found to be the hexagonal V₂C phase described by other researchers, and its orientation relationship with the matrix may be expressed: (110)v‖(00.1)v₂c, [•111]v‖[•110] v₂C. The dislocation loops are a result of the specific volume difference between the V₂C precipitation and the matrix, and have Burgers vector b = a/2〈111〉. Formerly Research Assistant, Department of Metallurgy and Mining Engineering, University of Illinois at Urbana-Champaign, Urbana Ill.     

Inclusion behavior and heat-transfer phenomena in steelmaking tundish operations: Part II. Mathematical model for liquid steel in tundishes

Fluid flow, heat transfer, and inclusion flotation have been modeled mathematically for several types of industrial tundish designs. Computations are presented to illustrate the importance of thermal natural convection currents in mixing the upper and lower layers of steel. The use of flow modification devices was shown to be reasonably effective in further reducing inclusion density levels at the intermediate to larger size ranges. Small inclusions (≦40 μm) were not readily removed with or without flow controls because of their low Stokes rising velocities. Formerly Doctoral Candidate Formerly Postdoctoral Fellow, Department of Mining and Metallurgical Engineering, McGill Metals Processing Centre, McGill University. Macdonald Professor of Metallurgy     

Hydrogen embrittlement in a Mg-Al alloy

Experiments are described which demonstrate that a Mg-7.5 wt pct Al alloy is embrittled by both cathodically-generated and gaseous hydrogen. Immersion of unstressed specimens in an aqueous NaCl-K₂CrO₄ solution is shown to lead to absorption of hydrogen and to a form of internal hydrogen embrittlement similar to that observed in BCC metals. Thus embrittlement was manifested by loss of ductility in tensile tests and by cleavage-like fracture surfaces; vacuum annealing after immersion led to a reduction in hydrogen concentration and to a partial recovery of ductility; and embrittlement was not observed at high strain rates. Embrittled specimens also exhibited delayed failure in both constant-load tensile tests and constant-deflection bend tests. In a second series of experiments, slow crack growth was observed when edge-notched tensile specimens were stressed in ~ lpsig (~ 6.9 × 10³ N/m²) dry hydrogen; the fracture surfaces were again cleavage-like in appearance. The significance of these observations is discussed in relation to the stress-corrosion failure of this alloy. Formerly a Research Assistant in the Department of Metallurgy and Mining Engineering, University of Illinois, IL,     

The influence of austenite stability on the hydrogen embrittlement and stress- corrosion cracking of stainless steel

A study has been made of the HE and SCC of a type 304 and a type 310 austenitic stainless steel, and the results correlated with the presence or absence of α′ martensite, determined by means of a ferrite detector. Hydrogen induced slow crack growth (SCG) was observed at room temperature when type 304 was stressed i) in 1 psig (∼10⁵ N/m²) gaseous hydrogen, ii) after high temperature charging, and iii) while undergoing cathodic charging. The fracture surfaces corresponding to SCG were primarily transgranular and cleavage-like, and were found to be associated with α′. Conditions i) to iii) did not produce SCG in the type 310 steel, in which α′ martensite was not detected, nor did SCG occur when type 304 was stressed in gaseous hydrogen above the M_D temperature (∼110°C). These observations indicated that the formation of the martensitic phase was a prerequisite for SCG under these test conditions. Stressing of type 310 while it was undergoing cathodic charging at room temperature was found to produce shallow, nonpropagating cracks, confirming earlier reports that austenite can be embrittled by hydrogen in the absence of α′. SCC occurred in both alloys in boiling aqueous MgCl₂ (154°C) with no evidence for α′ formation. The results are discussed in terms of the mechanisms of HE and SCC. Formerly Research Associate, Department of Metallurgy and Mining Engineering, University of Illinois. Formerly Corrosion-Control Analyst with the Physical Plant at the University of Illinois.     

Studies of the orientations of fracture surfaces produced in austenitic stainless steels by stress-corrosion cracking and hydrogen embrittlement

Orientation studies have been made on several different austenitic stainless steels, using photogrammetric and electron channeling techniques. The fracture facets produced by SCC in boiling aqueous MgCl₂ (155 °C) were large and relatively flat in the case of type 310 steels, and the fracture plane was found to be at or near {100}. The transgranular stress-corrosion fractures in type 304 steels were more complex, and there was considerably more scatter in the orientation determinations. However, the orientations of the fracture facets in these steels were clearly not {100}, but fell into two distributions, one near {211} and the other near {110}. Electron diffraction studies from the fracture surfaces indicated the presence of α′ and martensites in the type 304 but not in the type 310 cases; the possibility that this was responsible for the differences in fracture planes is discussed. Studies were also made of a type 304 specimen which had failed by SCC at 289 °C. No martensitic phases were detected at the fracture surfaces in this case, and the fracture facets were large and flat, similar to those for type 310. Cleavage-like fracture surfaces were also produced in type 304 steels by hydrogen embrittlement, using both gaseous hydrogen and cathodic charging, but the facets were too small for precise orientation determination. Formerly with the Department of Metallurgy, University of Illinois at Urbana-Champaign. Formerly with the Department of Metallurgy, University of Illinois at Urbana-Champaign. Formerly Professor of Metallurgy, University of Illinois.     

Steady state segregation and heat flow in ESR

A combined theoretical and experimental study of steady-state heat flow and segregation in ESR is presented. The segregation model permits prediction of pressure gradients, hence, interdendritic flow velocities responsible for macrosegregation in the “mushy≓ zone of axisymmetric ESR ingots. The heat flow model considers the solidus isotherm as a moving boundary. The relationships between power and slag temperature as well as power and heat transfer coefficient are experimentally measured and included in the heat balance equation for the slag. Experiments on both a low-temperature simulated ESR apparatus and on a 200 mm diam ESR ingot mold verify both models. S. CHAKRAVORTY formerly Research Associate, Department of Metallurgy and Mining Engineering, University of Illinois. J. D. NAUMAN formerly Engineering Associate, Cabot. Corporation     

The transgranular SCC of a Mg-Al alloy: Crystallographic, fractographic and acoustic-emission studies

A study has been made of the propagation of transgranular stress-corrosion cracks in a Mg-7.5 wt pct Al alloy tested at room temperature in an aqueous NaCl-K₂CrO₄ solution; the studies were carried out on single cracks which initiated and propagated within single grains in coarse-grained bend specimens. The resulting fracture surfaces were flat, and two-surface analysis established their orientation to be 31–40. Specimens fractured at liquid-nitrogen temperature cleaved on 10•10, indicating that the 31•40 are not the normal cleavage planes in this material. The stress-corrosion fracture surfaces were cleavage-like in appearance, containing shallow steps which were matching and interlocking on opposite faces. Discrete acoustic signals were emitted during crack propagation, and these are considered to result from discontinuous crack advance. It is concluded that stress-corrosion cracking in this system occurs by discontinuous cleavage on (3140) planes. D. G. CHAKRAPANI, formerly a Research Assistant in the Department of Metallurgy and Mining Engineering, University of Illinois, Urbana, Ill. 61801.     

Inclusion behavior and heat-transfer phenomena in steelmaking tundish operations: part III. applications—computational approach to tundish design

The attributes of various tundish designs and flow modifiers have been studied based on the computational and physical modeling efforts developed in parts I and II. The importance of thermal natural convection effects in such vessels has been demonstrated. Flow patterns and residual ratios (RR) for various tundish designs of equal volume and metal throughput showed that a conventional biflow, twin-strand, trough-type tundish fitted with flow modifiers allows for the greatest removal of inclusions, while a one-way flow, twin-strand, wedge-shaped tundish performs best in the absence of flow modifiers. More detailed analyses of the conventional trough-type tundish were performed to assess the effects of double-weir-double-dam and single-weir-double-dam arrangements. The effectiveness of steeply sloping sidewalls for enhanced removal of inclusions was demonstrated; however, this measure was associated with penalties in the form of greater metal temperature losses. Formerly Doctoral Candidate. Formerly Postdoctoral Fellow, Department of Mining and Metallurgical Engineering, McGill Metals Processing Centre, McGill University. Macdonald Professor of Metallurgy.     

Ledges and carbides in lower bainite in a hypereutectoid steel

The ferrite/austenite interfaces and carbides in lower bainite have been observed using transmission electron microscopy (TEM) on isothermally reacted specimens of a hypereutectoid steel. Superledges are found to exist at the broad faces of ferrite plates. The height and the width of the superledges are approximately 5 to 40 nm and 15 to 80 nm, respectively. Multiple arrangements of carbides in lower bainite have been observed. This is inconsistent with the single variant of carbides oriented at an angle to the sheaf axis repeatedly reported in lower bainite. The experimental results show that carbide precipitation occurs in austenite at ferrite/austenite boundaries located in gaps between the ferrite plates and/or between the ferrite subunits. Therefore, the conclusion is incorrect that carbides associated with lower bainite precipitate from wholly supersaturated ferrite. In a word, all of these observations suggest that lower bainite forms by the diffusion-controlled movement of ledges. Formerly Graduate Student, Tsinghua 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.     

Inclusion behavior and heat-transfer phenomena in steelmaking tundish operations: Part I. Aqueous modeling

An in-house computer code, METFLO 3D, which can model three-dimensional (3-D) turbulent flow, heat transfer, and inclusion flotation, has been developed for steelmaking tundishes. Also, sensor equipment has been developed for the continuous detection of particles suspended in aqueous systems. The probe, based on the resistive pulse principle, withdraws a continuous sample of the feed water to be monitored. A full-scale (isothermal) water model tundish (Stelco Research Centre, Burlington, Canada) was used to test experimental data on particle separation within the tundish. These experimental data were compared with predictions calculated by the mathematical model developed for tundish flows, and satisfactory agreement was achieved. Formerly Doctoral Candidate, Department of Mining and Metallurgical Engineering, McGill Metals Processing Centre, McGill University. Macdonald Professor of Metallurgy