Isothermal precipitation behavior of copper sulfide in ultra low carbon steel

Copper and sulfur are typical residual elements or impurity elements in steel.Sufficient removal of them during steelmaking process is difficult for copper and costly for sulfur.Utilization of copper and sulfur in steel, especially in steel scrap,has been an important issue for a long period for metallurgists.Copper and sulfur may combine to form copper sulfide,which may provide a prospect to avoid the detrimental effects of copper and sulfur in steel.Unfortunately the formation mechanism of copper sulfide in steel has not been completely clarified so far. In the present paper,solution treatment of samples containing copper and sulfur are firstly performed at 1623 K for 2.7×10~3 s followed by quenching into water.The samples are then isothermally heat-treated at 673 K,873 K, 1073 K,1273 K and 1373K for different time followed by quenching into water again.The size,morphology, constituent and crystallography of sulfide precipitates in these samples are investigated by scanning electron microscope(SEM) and TEM equipped with EDS.Fine copper sulfides(less than 100 nm) are observed to coexist with silicon oxide in samples even isothermally heat-treated at 1 373 K for 1.44×10~4 s;Film-like copper sulfides are generally observed to co-exist with iron sulfide in all samples;Plate-like copper sulfides are observed especially in sample isothermally heat-treated at 1 073 K for 1.44×10~4 s.The formation mechanisms of these copper sulfides have been discussed.     

Interstitial precipitation in Fe-Cr-Al alloys

Two separate stages of precipitation have been identified during the aging of ternary Fel8Cr3Al and Fel8Cr5Al alloys at temperatures in the vicinity of 475 °C. The first stage involves the formation of interstitial precipitates resulting from C and N impurities; the second and slower stage is the formation of the Cr-rich α′ phase. Transmission electron microscopy (TEM) results show that carbonitride precipitation occurs preferentially at dislocations, stacking faults, and grain boundaries, and also uniformly through the matrix. Aging for times in excess of 400 hours at 475 °C promotes coarsening of the heterogeneous precipitates and dissolution of the uniformly distributed matrix particles. A resistometric analysis shows that the kinetics of the initial stages of precipitation can be described by a (time)^2/3 relation. This kinetic behavior is explained in terms of stress-assisted diffusion in the highly stressed matrix resulting from coherency strains accompanying carbonitride precipitation. Experimental values of the activation energy for the first stage reaction correlate closely with those reported for the interstitial diffusion of C and N in alpha iron.     

Discontinuous precipitation in aluminum-base zinc alloys

The kinetics of discontinuous precipitation in Al-21.6 at. pct. Zn and Al-22 at. pct. Zn-0.01 at. pct Sn alloys was determined by quantitative metallographic measurements on solution-treated and aged specimens. The growth rate of the discontinuous precipitate appears to be controlled by interfacial diffusion. It was found that the addition of a small amount of tin reduced the overall reaction rate. Nucleation of the discontinuous precipitate was confined to grain edges or boundaries, filling the available sites (site saturation) at a very early stage in the reaction. It was observed that a variation in quench rate markedly affected the growth rate of the precipitate and the site-saturating-dimensionality during a room-temperature aging treatment.     

Laves phase precipitation in Fe-Ta alloys

Development of an iron-base alloy hardened by particles of an intermetallic compound rather than a carbide is a desirable goal because of the greater thermal stability of such a dispersion. As a first step in the development of iron-base alloys hardened with the Laves phase, structural studies of binary Fe-Ta alloys have been undertaken. The structures of two phase Fe-Ta alloys have been studied by means of optical and transmission electron microscopy, X-ray diffraction, electron beam microprobe analysis, and scanning electron microscopy. The hardness change as a function of time at 600°, 700°, and 800°C has been determined for binary alloys with 1 at. pct Ta and 2 at. pct Ta in iron. Also, the uniaxial tensile strengths of solution treated, quenched, and aged samples have been determined. These studies suggest that the compound, Fe₂Ta, is isomorphous with the structure type, MgZn₂, (C14) and has a range of compositional homogeneity. The latter results correspond with the predictions of the Engel-Brewer correlation. Also, it has been found that precipitation occurs at grain boundaries, dislocations, and randomly throughout the matrix. Particles which form at dislocations have a (100)α habit plane; whereas a (110)α habit plane has been reported by others^1,3 for the hexagonal Laves phase in α iron. Hypereutectoid composition alloys quenched from the ö phase field have a completely retained § structure. Isothermal decomposition at 600°, 700°, and 800°C of alloys with the retained § structure results in a sizable hardness increase in 2 at. pct Ta alloys but only a modest increase in 1 at. pct Ta alloys. Brittle fracture of aged tensile specimens tested at room temperature reveals that the ductile-brittle transition temperature in tension is above room temperature.     

Radiation-induced solute segregation and precipitation in alloys

Solute segregation and precipitation in dilute alloys during irradiation have been studied by means of the Johnson-Lam kinetic model. The model is based on a combination of chemical reaction rates and diffusion equations for free defects, solutes, and bound defect-solute complexes. The enrichment of solute at sink surfaces and solute depletion in the matrix have been calculated as functions of temperature, damage rate, defect-solute bind-ing energy, and initial solute concentration. Using parameters appropriate for Be in Ni, significant solute segregation is found in the temperature range from 0.2 to 0.7 Υ_m. The temperature for maximum segregation is higher for the high displacement rates typically used in charged-particle bombardment experiments than for the low displacement rates used in fast-reactor irradiations. The solute concentration at the sink surface builds up at high temperatures, without surpassing the solubility limit, until a steady state is at-tained. However, at lower temperatures solute enrichment at sinks becomes larger and the solubility, in general, becomes lower. Precipitation will occur when the local solute concentration reaches that of the phase boundary. The solute concentration at the precipi-tate-matrix interface is determined by the solubility limit, and precipitation continues until the matrix is sufficiently solute-depleted to achieve a steep concentration gradient that will balance the defect-induced solute flow by back-diffusion. Hence, the steady-state matrix composition is determined by radiation conditions and is independent of the initial alloy composition when precipitation occurs. The solute depletion at steady state is more severe at low displacement rates than at high rates. The calculations are quali-tatively compared with recent experimental observations of the temperature and com-positional dependence of solute precipitation in the Ni-Be system. This paper is based on a presentation made at a symposium on “Radiation In-duced Atomic Rearrangements in Ordering and Clustering Alloys” held at the annual meeting of the AIME, Atlanta, Georgia, March 7 to 8, 1977, under the sponsorship of the Physical Metallurgy and Nuclear Metallurgy Committees of The Metallurgical Society of AIME.     

The Bauschinger effect in precipitation strengthened aluminum alloys

The Bauschinger effect in precipitation strengthened Al-Cu-Mg, Al-Zn-Mg and Al-Cu polycrystals was measured as a function of applied strain. Alloys heat treated to contain easily shearable precipitates,i.e., GPB, GP and θ″ exhibited a small Bauschinger effect, on the order of that in pure aluminum. In contrast, alloys with nonshearable precipitates, S′, η and θ′ showed an anomolously large effect. A unique hysteresis loop shape, with a region of convex curvature between sharp inflection points, was observed in the nonshear-able precipitate alloys. The large Bauschinger effect and unusual hysteresis loop shape are due to internal elastic or back stresses exerted by the strong precipitates on the matrix. A nonlinear elastic hardening model is proposed in which the overall work harden-ing is partitioned into an elastic back stress component and a frictional dislocation forest hardening term. Plastic relaxation around the precipitates and inhomogeneous deformation in the polycrystal reduces the level of the internal stresses below that predicted theoretically by the Brown and Stobbs hardening theory.     

Electrochemical oxidation of nickel sulfide metallic alloys

The sequence of the electrochemical oxidation of phase components of the converter matte is confirmed and variations in the composition of its surface during the anode oxidation are revealed. It is shown that regularities of this process that are characteristic of its phase components, namely, nickel and its sulfides, are inherent to the converter matte. The oxidation of nickel sulfides in sulfide-metallic alloys starts at lower potentials (1.1–1.2 V) than that of synthesized nickel sulfide (1.2–1.3 V). During the oxidation of the converter matte, metal nickel initially oxidizes and, only as this stage finishes, the oxidation of sulfides accompanied by the isolation of elemental sulfur is developed. Copper impurities weakly affect the oxidation of the converter matte, although the shift of potentials takes place as the Cu/Ni ratio increases.     

Effect of homogenization on sulfide inclusions in ferrous alloys

The effect of high temperature homogenization on sulfide inclusions was studied in directionally solidified AISI 4340 low alloy steel of various sulfur contents and in directionally solidified iron-carbon-manganese-sulfur alloys. The morphological modifications of the various types of inclusions caused by coarsening were investigated versus homogenization time. The decrease of the number of inclusions per unit volume of matrix, and that of the sulfide-matrix interface area per unit volume of sulfide, and the increase of the mean over-all inclusion size with homogenization time were quantitatively established. These variations were compared in a specific case with analytical predictions based on a coarsening model.     

Irradiation induced precipitation in tungsten based,W-Re alloys

Tungsten-base alloys containing 5, 11, and 25 pct Re were irradiated in the EBR-II reactor. Irradiation temperatures ranged from 600 to 1500 °C. All compositions were irradiated to fluences in the range 4.3 to 6.1 X 10²⁵ n/m² (E > 0.1 MeV), and three 25 pct Re samples were also irradiated to 3.7 X 10²⁶ n/m² at temperatures 700 to 900 °C. Postirradiation examination included measurement of electrical resistivity at room temperature and lower temperatures, X-ray diffraction, optical metallography, microprobe analysis, and transmission electron microscopy. Irradiation induced resistivitydecreases observed in most of the samples suggested second-phase precipitation. Complete results confirmed the precipitate formation in all samples, in disagreement with existing phase diagrams for the W-Re system. Electron diffraction showed the precipitates to be consistent with the cubic, Re-richX-phase and inconsistent with the σ-phase. Large variations in precipitate morphology and distribution were observed between the different compositions and irradiation conditions. For the 5 and 11 pct Re-alloys, spherically symmetric strain fields surrounded the equiaxed precipitate particles, and were observed even where no particles were visible. These strain fields are believed to arise from local Re enrichment. Thermoelectric data show that the precipitation can lead to decalibration of W/Re thermocouples.     

Kinetics of precipitation in aluminum alloys during continuous cooling

Rate of cooling from the solution treatment temperature has a pronounced effect on corrosion behavior and mechanical properties of precipitation-hardenable aluminum alloys. This effect is attributed to loss of vacancies and to precipitation of solute during the quench. Analysis of data for aluminum alloys indicates that precipitation rate is an explicit function of temperature and the amount of solute remaining in solution; consequently, precipitation during cooling is additive and can be evaluated by integrating this function. A method is presented whereby type of corrosion attack or mechanical properties can be accurately predicted from the cooling curve and a C-curve for the alloy. Because the actual cooling curve is considered, the method is applicable regardless of the complexity of the cooling conditions. Also presented is an improved method of accurately determining C-curves using either interrupted or continuous quenching techniques. C-curves describing type of corrosion attack in 2024-T4 sheet and loss in mechanical properties of 7075-T6 sheet are presented.     

Effects of electron irradiation on precipitation in Ni-Al alloys

Binary Ni-Al alloys have been irradiatedin situ by high voltage electron microscopy (650 kV) in order to investigate the effects on precipitation. Nucleation and growth of dis-location loops are observed during irradiation in the temperature range 20 to 650°C. In solution-treated specimens, large faulted loops form during irradiation at 500°C. Con-trast analysis shows that loops are Frank-type with l/3 〈HI〉 Burgers vectors and in-terstitial in character. At 650°C and above, loop formation and growth are not observed once γ′ precipitates have formed. In preaged specimens, much smaller loops form dur-ing irradiations at below 500°C. At 500°C and above, no loops are observed in specimens containing large volume fractions of precipitates. The spatial distribution of loops ap-pears rather uniform regardless of the presence of γ′ precipitates except when large in-coherent precipitates exist. The growth of γ′ precipitates is enhanced by irradiation in the temperature range 300 to 750°C. Precipitate coarsening under irradiation follows radiation-enhanced diffusion kinetics. Quantitative analysis demonstrates that the radia-tion-enhanced diffusion coefficient responsible for coarsening of γ′ is independent of the temperature between 300 and 650°C. This result is in agreement with theoretical pre-dictions for the case in which irradiation-produced point defects are lost by their anneal-ing at fixed sinks. Formerly with Department of Metallurgy and Materials Science, Case Western Reserve University, is now with Glidden Metals Group, SCM Corpora-tion, Cleveland, OH 44106 This paper is based on a presentation made at a symposium on “Radiation Induced Atomic Rearrangements in Ordering and Clustering Alloys” held at the annual meeting of the AIME, Atlanta, Georgia, March 7 to 8, 1977, under the sponsorship of the Physical Metallurgy and Nuclear Metallurgy Committees of The Metallurgical Society of AIME.     

Dendritic morphology observed in the solid-state precipitation in binary alloys

The precipitation of γ ₂ phase in Cu-Al β-phase alloys has been observed to occur in the dendritic morphology. Such morphology is rarely observed in the solid-state transformations. Earlier it was reported that the γ precipitates were formed in the dendritic shape when Cu-Zn β-phase alloys were cooled from high temperature. The characteristics of these two alloy systems have been examined to find the factors promoting the dendritic morphology in the solid-state transformations. Rapid bulk diffusion and fast interfacial reaction kinetics would promote such morphology. The kinetics of atom attachment to the growing interface is expected to be fast when crystallographic similarities exist between the parent phase and the precipitate. We have predicted the dendritic morphology in the solid-state precipitation in many binary alloy systems simply based on such crystallographic similarities. These alloys include, in addition to Cu-Al and Cu-Zn, the β-phase alloys in Ag-Li, Ag-Zn, Cu-Ga, Au-Zn, and Ni-Zn systems, γ-phase alloys in Cu-Sn and Ag-Cd systems, and δ-phase alloys in Au-Cd system. Of these, the alloys in Ag-Zn, Ni-Zn, Ag-Cd, and Cu-Sn systems were prepared and it was indeed found that the precipitates formed in the dendritic shape.     

Internal oxidation of dilute Co-Ti alloys

The kinetics of internal oxidation of Co-Ti alloys and the crystal structures and morphologies of the internal oxide particles have been investigated in the temperature range 900 to 1080°C. Three oxides were observed:δ-TiO, TiO₂ (rutile) and CoTiO₃. The permeabilities of oxygen in Co were calculated from the observed internal oxidation kinetics using an analysis due to Meijering as well as an analysis based on a more rigorous solution of the diffusion equations. The factors affecting the size and morphology of internal oxide particles are discussed.     

Internal oxidation of dilute Cu-Ti alloys

Metallurgical and Materials Transactions A - The kinetics of internal oxidation of dilute Cu-Ti solid solutions, containing up to 1 wt pct Ti have been investigated over the temperature range 700...     

Internal oxidation of dilute Cu-Ti alloys

The kinetics of internal oxidation of dilute Cu-Ti solid solutions, containing up to 1 wt pct Ti have been investigated over the temperature range 700 to 900°C, and the oxide morphologies produced have been studied by electron metallography. Values of the solubility-diffusivity products for oxygen in copper (N_o ^(s)D_o) are in good agreement with those obtained for pure copper by electrochemical techniques. The TiO₂ particles formed in the Cu-matrix are extremely small (50-200?) and appear to be ellipsoidal in shape. The oxide particle size has been observed to increase linearly with distance below the specimen surface. In addition, electron metallography reveals TiO₂ particles both in the matrix and at grain boundaries which exhibit interesting fringe contrast or “stripes” which are perpendicular to the major axis of the ellipsoids. The origin and possible significance of these regions in terms of coherency of the TiO₂/Cu interfaces are discussed. The interfacial energies for incoherent TiO₂ /Cu interfaces have been found to be quite low, on the order of 400 to 700 ergs/cm². D. ADAMONIS is formerly Graduate Student Assistant