Electron-microscopic and XPS study of the phase transitions in a Co70Mo30 alloy

Electron-microscopic analysis and X-ray photoelectron spectroscopy are used to study the structure of the Co₇₀Mo₃₀ alloy formed at various temperatures. It is found that, at temperatures below the solidus temperature, the structure-phase transformations in the alloy have a tendency toward phase separation, which results in the formation of a cellular structure in the form of two types of clusters. These clusters are strongly enriched in molybdenum or cobalt and exist in the general fcc lattice of the solvent (cobalt). The allotropic α-Co → ?-Co phase transition occurs in the alloy at 532°C, and the transition-induced deformation contrast is superimposed on the cellular structure.     

A study of retained austenite in a fine-grained Fe-12Ni-0.25Ti alloy

The behavior of retained austenite in a fine-grained Fe-12Ni-0.25Ti cryogenic alloy system was investigated. When the alloy was held at a temperature within the two phase (α + γ) field austenite reversion occurred by a diffusion-controlled process. The redistribution of solute elements appeared to control the stability of the reverted austenite. The microstructural appearance of the retained austenite was examined using transmission electron microscopy. A preferential distribution of the austenite phase along martensite lath boundaries was observed. A precipitate-correlated austenite was also found to occur. The beneficial effect of introducing retained austenite appeared in the improvement in the tensile elongation as well as in the Charpy impact toughness at low temperatures. The retained austenite in this system did not improveK _IC at ?196°C. On the contrary, samples containing retained austenite showed increased susceptibility to unstable crack propagation in low temperature fracture toughness tests.     

A kinetic study of the β to a massive transformation in a Cu-Zn alloy

Transformation kinetics of the Cu-Zn β to α massive transformation have been studied by combining resistivity measurements with pulse heating techniques. Samples previously quenched to retain the β phase were pulse heated with a capacitor discharge apparatus to temperatures where they transformed to α phase. Quantitative measurements of transformation kinetics were obtained by taking into account also the dependence of the measured resistivities on the temperature change produced by the transformation. A prominent feature of the kinetics curves is that they do not show any evidence of the delay times observed with the use of metallographic techniques. Large nonsystematic variations in the reaction rates precluded comparison of the data with theoretical models. Formerly with the Department of Metallurgy and Materials Formerly with the Department of Metallurgy and Materials     

η toG phase transformation in electrodeposited iron-zinc alloy coatings

Theη toG phase transformation in electrodeposited iron-zinc alloy coatings is a rapid diffusional phase transformation occurring in the nanometer-scale microstructure of the coatings. Iron-supersaturated zinc, orη phase, undergoes a decomposition process to an iron-deficient Γ-like, orG, phase at temperatures near 150 it°C. The phase transformation is preceded by a polygonization process, driven by strain energy and/or supersaturation in the material. TheG phase nucleates on boundaries within theη phase and grows into 30- to 80-nm equiaxed grains. The activation energy for the transformation varies with iron content, which may indicate that solute effects influence boundary diffusion of zinc. C.A. Drewien, formerly Graduate Student, Lehigh University, Bethlehem, PA. J.I. Goldstein, formerly Professor, Materials Science and Engineering Department, Lehigh University, Bethlehem, PA.     

Numerical modeling of diffusion-controlled phase transformations in ternary systems and application to the ferrite/austenite transformation in the Fe-Cr-Ni system

An implicit finite-difference analysis has been used to model the diffusion process in a two-phase ternary system. The system was of finite length, and the interface between the two phases, α and γ, was allowed to move as the α phase grew or dissolved. Equilibrium at the interface was assumed. For simplicity, the diffusion coefficients were assumed to be independent of composition, and the cross-coefficient diffusion terms were assumed to be negligible. The details of the computational process are described. The accuracy of the approach and the major sources of error are examined in detail. The numerical results are in excellent agreement with analytical predictions under the limited conditions for which analytical solutions are available. The computational model was applied to the detailed study of ferrite growth/dissolution in the iron-chromium-nickel ternary system to examine ferrite stability in austenitic stainless steel welds. The starting compositions of the ferrite and austenite were typical of these phases in the as-welded condition, and the isothermal equilibration was followed at 700 °C to 1300 °C. It was found that the transformation to the equilibrium state could be divided into two major stages: the first corresponded to rapid diffusion in the ferrite phase, and the second was due to the more sluggish diffusion within the austenite. The path toward equilibrium was often indirect; sometimes the ferrite grew first before eventually shrinking to the final equilibrium size, whereas at other temperatures the ferrite initially dissolved before growing at a later stage. The results are compared to the limited available experimental data, and behavior that was found experimentally was accurately reproduced by the computations. The results provide hitherto unavailable data on the kinetics of the equilibration process at elevated temperatures near the solidus temperature and also provide insight into details of the transformation behavior. formerly at Oak Ridge National Laboratory     

Solidification of a ternary metal alloy: A comparison of experimental measurements and model predictions in a Pb-Sb-Sn system

Experiments were performed with a Pb-5 pct Sb-35 pct Sn system to experimentally validate simulations made with a continuum mixture model for ternary alloy solidification. Temperature histories were measured during casting, and composition profiles were found in the solidified ingot. Dendritic arm spacings were found from optical micrographs of the alloy microstructure and used to determine a constant in the Blake-Kozeny submodel for the mushy zone permeability in the liquid-solid interaction term of the momentum equations. Scaling analysis from a previous work and a large uncertainty in the permeability constant suggested that predictions of the composition are extremely sensitive to the choice of a permeability model. Three simulations were performed using the permeability constant as a parameter, and measured temperatures and compositions were compared with predictions based on different model constants. While the thermal histories had good agreement, the results near the liquidus interface, where all of the significant advection of solute occurs, suggest that the Blake-Kozeny model significantly underpredicts the resistance of the dendritic array to fluid flow.     

Martensitic transformation in binary and ternary alloys based on the AuZn beta prime phase

Martensitic transformation temperatures have been measured in binary and ternary alloys based on the AuZn ß’ phase, using an electrical resistivity method and cold-stage optical microscopy. The third element additions were Ag, Cu, Mg, Cd, Ga, In, and Si. Attempts to identify the influence of factors such as degree of long range order, electron concentration, atomic size, ionic size, and electronegativity on the martensitic transformation temperatures did not prove successful, and it is apparent that the rationalizations attempted by previous workers are not of general applicability. It is concluded that an interplay of several of these factors considered affects elastic anisotropy, and hence theM _s temperature. Lattice parameters and density measurements made on slowly cooled alloys showed that the ß’ AuZn phase did not exhibit a composition dependent vacancy defective structure on the zinc-rich side of the equiatomic composition.     

Investigation of the phase equilibria in the Sn-Bi-In alloy system

This article presents an investigation of the phase equilibria in the Sn-Bi-In ternary alloy system, performed both by theoretical and experimental methods. Following the regular solution model and a standard thermochemical calculation, a theoretical evaluation of the phase equilibrium in the entire ternary system is conducted. The thermodynamic parameters required for the calculation are initially obtained by fitting the model to existing data available from prior studies. The theoretical results are then validated and further improved by experimental work in which alloys with several critical compositions were chosen and examined. In the experimental work, differential scanning calorimetry (DSC), X-ray diffraction (XRD), and energy-dispersive X-ray (EDX) spectroscopy were jointly used to identify the transition temperatures and the phases in the microstructure. The resulting phase diagram agrees well both with the existing data and with the data from the current experiments. However, different from previous findings, this study finds a nonbinary nature of the Sn-BiIn and Sn-BiIn₂ quasi-binaries and nine invariant reactions, one eutectic, six peritectic and two peritectoid. The phase-reaction scheme (Scheil diagram), the liquidus projection, and the phase diagram, covering entire compositional ranges, are established.     

Application of the diffusion couple to study phase equilibria in the Fe-Cr-S ternary system at 600 °C

The sulfidation of Fe-Cr alloys has a large financial significance for industries that use fossil fuels, such as the electric utility industry. Therefore, the sulfidation of a series of Fe-Cr alloys was studied at 600 °C using a solid-state diffusion couple technique. The diffusion couple technique combined Fe_0.95S powder and FeCr binary alloys together in a configuration that allowed for post-heat-treatment microanalysis using an electron probe microanalyzer (EPMA). The results showed that only two different diffusion couple microstructures formed in samples spanning the entire Fe-Cr binary range. The Fe-rich alloy diffusion couples contained a surface αFeCr layer and an internal sulfide precipitate layer that contained three different sulfide phases. The Cr-rich alloy diffusion couples also possessed an internal precipitate layer, as well as a thick, triplex interfacial scale. The ternary elemental diffusion was described using diffusion paths plotted on the 600 °C isothermal section of the Fe-Cr-S phase diagram. The results also showed that samples with less than 51 wt Pct Cr were more sulfidation resistant. The accuracy of the existing Fe-Cr-S 600 °C isothermal section was assessed, and it was determined that the τ phase field had a larger composition than previously published.     

Cellular decomposition in a Cu-25Ni-15Co side-band alloy

The process of decomposition in a Cu-25 mass pct Ni-15 mass pet Co alloy has been investigated by X-ray diffraction, optical and electron microscopy, and hardness measurements. The alloy first decomposedvia spinodal decomposition, followed by a set of complex cellular-like reactions. X-ray diffraction results showed that side bands exist even after the spinodal microstructure has been completely replaced by the cellular-like microstructure. TEM showed that the cellular-like microstructure had a fiber-like appearance with high degree of periodicity. It is this structure which gives rise to the side bands in the X-ray diffraction pattern late in the transformation process. The primary cells engulfed the entire matrix after a short time of aging and were followed by secondary and tertiary cells. During these later reactions the fiber spacing increased and the composition of the phases reached their equilibrium values. This cellular-like transformation is different from the conventional cellular precipitation, and is interpreted as the preferential growth of part of the spinodal microstructure which is near the grain boundary, due to grain boundary migration. We call this type of cellular transformation in the spinodal alloys “cellular decomposition”.     

Dynamic recrystallization during creep in a 45 Pct Ni-35 pct Fe-20 pct Cr alloy system

A combined 3.5 wt pct Mo + 1.2 wt pct Ti imparted dynamic recrystallization in a 35 wt pct Fe-45 wt pct Ni-20 wt pct Cr alloy system during creep at 700 °C, whereas 3.5 wt pct Mo addition alone did not initiate recrystallization. Dynamic recrystallization substantially increased the creep elongation and produced a high ductile fracture topography in the present alloy system. A subgrain coalescence nucleation mechanism for dynamic recrystallization mechanism was operative during creep. The critical initiation strain requirements are also discussed.     

Precipitation in a Cu-30 Pct Ni-1 Pct Nb alloy

Decomposition of a Cu-30 pct Ni-1 pct Nb alloy on aging in the range of 866 K (600°C) to 1073 K (800°C) was investigated. The initial decomposition, concomitant with age hardening, occurred through the precipitation of body centered tetragonal metastable Ni₃Nb-γ” precipitates on the 100 matrix planes. Equilibrium orthorhombicβ phase formed either through a grain boundary cellular reaction at low temperature (≤973 K (700°C)) or as Widmanstaettenplatelets on the 1ll planes at higher temperatures (≥1073 K (800°C)) with the following crystallographic relationship: (0l0)_β//111_γ [100]β//[1•11]_γ. Based on the observations, a schematic transformation sequence is presented.     

Microstructural effects of martensitic transformation cycling of a Cu-Zn-Al alloy: Vestigial structures in the parent phase

Martensitic transformation kinetics and microstructural effects are correlated in a study of transformation cycling in a Cu-Zn-AI alloy. During the first few cycles of martensitic transformation and reversion, vestigial features develop in the parent phase. During these same cycles, the parent-to-martensite transformation temperature range shifts upward by several degrees and the martensite-to-parent reversion temperature range shifts down by about a degree, in effect reducing the transformation hysteresis. The vestigial ridge features are associated particularly with burst martensite phenomena, and this and other manifestations of imperfect thermoelastic martensite behavior during initial cycling lead to the kinetics changes. These changes virtually stabilize after 20 to 25 transformation cycles as the martensitic transformation achieves a completely reproducible pattern, both in terms of microstructure and kinetics.     

Lattice constants and compositions of the metastable Ni3Nb phase precipitated in a Ni-15Cr-8Fe-6Nb alloy

The lattice constants of γ and metastable γ″ (Ni₃Nb) phases and the composition of the γ″ phase in a nickel-based superalloy (a modified JIS NCF 3-type alloy) are investigated using an X-ray diffractometer and an analytical electron microscope with an energy-dispersive X-ray (EDX) system. The lattice constant of the γ phase decreases and those of the γ″ phase increase during the aging time. The γ″ phase is composed of nickel, niobium, chromium, and iron. While the content of nickel in the γ″ phase increases and reaches a constant composition during the aging time, the chromium and iron content decrease and reach a constant value during aging. The niobium content maintains a constant value regardless of the time that the alloy has aged. The relationship between the lattice constants and the composition of the γ″ phase is discussed in detail. In the early stages of aging, there is the possibility that nickel (including chromium and iron) and niobium atoms mutually substitute for one another, resulting in a somewhat lower degree of order than would normally be expected.     

A comparative study of the microstructures observed in statically cast and continuously cast Bi-In-Sn ternary eutectic alloy

A ternary eutectic alloy with a composition of 57.2 pct Bi, 24.8 pct In, and 18 pct Sn was continuously cast into wire of 2 mm diameter with casting speeds of 14 and 79 mm min⁻¹ using the Ohno Continuous Casting (OCC) process. The microstructures obtained were compared with those of statically cast specimens. Extensive segregation of massive Bi blocks, Bi complex structures, and tinrich dendrites was found in specimens that were statically cast. Decomposition of γSn by a eutectoid reaction was confirmed based on microstructural evidence. Ternary eutectic alloy with a cooling rate of approximately 1 °C min⁻¹ formed a double binary eutectic. The double binary eutectic consisted of regions of BiIn and decomposed γSn in the form of a dendrite cell structure and regions of Bi and decomposed γSn in the form of a complex-regular cell. The Bi complex-regular cells, which are a ternary eutectic constituent, existed either along the boundaries of the BiIn-decomposed γSn dendrite cells or at the front of elongated dendrite cell structures. In the continuously cast wires, primary Sn dendrites coupled with a small Bi phase were uniformly distributed within the Bi-In alloy matrix. Neither massive Bi phase, Bi complex-regular cells, nor BiIn eutectic dendrite cells were observed, resulting in a more uniform microstructure in contrast to the heavily segregated structures of the statically cast specimens.     

Nucleation of recrystallization in a Co- Cr- Mo alloy

Incoherent twin boundaries and twin-twin intersections have been identified as nucleation sites for recrystallization in a Co-Cr-Mo alloy. The recrystallized grain is a region of hep and has a twin orientation with respect to the fee matrix in its early stages of growth. This crystallographic relationship is discussed in terms of a martensitic embryo nucleation model for the fee → hcp transformation.