Steady-state electrotransport of carbon in nickel and in nickel-copper and iron-copper alloys

For comparison with previous data on iron-carbon alloys, steady-state electrotransport experiments were performed on three alloys: Nickel-0.15 wt pct carbon, nickel-20 copper-0.13 carbon, and iron-7.2 copper-0.10 carbon. The current density was 2.20 to 2.85 × 10⁷ AJm² and the temperature was 1300 K (and also 1200 K for the nickel-carbon alloy). The Z* values obtained were +5.7, +5.7, and +4.5 respectively, for the three alloys. Present theory appears to be unable to explain the relation of these data to similar data on iron-carbon alloys.     

The density of liquid iron-carbon alloys

The density of liquid Fe-C alloys at temperatures ranging from 1250 °C to 1550 °C was measured by the sessile drop profile method. An accurate method of digital image processing was de-veloped to capture, enhance, and determine the coordinates of the X-ray shadow image of the droplet. Laplace's equation was then solved to obtain the volume and density of the droplet. The density of iron-carbon alloys was then determined as a function of carbon content (from 0 to 4 wt pct) and temperature (1250 °C to 1550 °C). A least-squares analysis of our data points gives an equation for the density of liquid iron-carbon alloys as a function of temperatureT [K] and carbon content [pct C] [wt pct]: ρ[g/cm³] = (7.10 - 0.0732[pct C]) - (8.28 - 0.874[pct C]) × 10^-4(T - 1823) These results will give a value to within ±1.5 pct of the data of Lucas, [3] Widawski and Sauerwald, [2] and the present work.     

Thermodynamics of the iron-carbon-zinc system

A two-zone isopiestic experimental technique was used to determine the solubility of zinc vapor in liquid and solid iron-carbon alloys as a function of zinc partial pressure (0.1 to 1 atm), carbon content (0 to 4.6 wt Pct), and temperature (1473 to 1873 K). The solubility of zinc at a given partial pressure decreases with both increasing temperature and carbon content in both liquid alloys and solid austenite; its activity in these solutions, and in pure δ-ferrite, deviates more positively from ideality than previous model-based predictions have suggested. The Bale-Pelton unified interaction parameter formalism was successfully applied to the results of liquid-alloy experiments, but the degree of experimental scatter in the austenite equilibrations was too great to allow its application in the calculation of solid-solution iron-carbon-zinc thermodynamic parameters. Using the available results, values were calculated for the equilibrium partition coefficientK _zn in solidifying iron-carbon alloys as a function of alloy carbon content; the results suggest that significant segregation of zinc between solid and liquid phases is not likely.     

Rate of nitrogen desorption from liquid iron-carbon and iron-chromium alloys with argon

The rate of nitrogen desorption from inductively stirred liquid iron, iron-carbon, and iron-chromium alloys with argon carrier gas has been measured by the sampling method for a wide range of nitrogen, carbon, and chromium contents mainly at 1600 °C. The results obtained by the present work and other data of previous investigators are used to clarify the reaction mechanism of nitrogen desorption from liquid iron. The rate of nitrogen desorption from liquid iron and iron alloys is second order with respect to nitrogen content in the metal under the present condition, and mutual relationships among interfacial chemical reaction, liquid-phase mass transfer, and gas-phase mass transfer are elucidated. The effects of oxygen and sulfur on the rate of nitrogen desorption are given byk _' ^{c ’} = 3.15?_N ² [1/(1 + 300a₀ + 130a_s)]. Carbon dissolved in iron increases the rate of nitrogen desorption, and chromium decreases it. The effects of these alloying elements can be explained by the change of the nitrogen activity in the metal.     

The diffusion of carbon in iron-carbon alloys at 1560°C

Measurements have been made of the rate of absorption of carbon from CO-CO₂ mixtures into stagnant liquid iron and of the rate of unidirectional dissolution of graphite into stag-nant liquid iron and iron-carbon alloys at 1560°C. Total absorptions and concentration pro-files were found to be consistent with a composition dependent chemical diffusivity given byD = 1.1 (1 + wt pct CJ5.3) X 10∼⁴ cm²Js with an uncertainty of about ±0.2 x 10^-4. It is shown that many of the previous determina-tions of the chemical and self-diffusivity are consistent with this suggested dependence. Formerly graduate student at the University of Pennsylvania     

Rate of nitrogen desorption from liquid iron-carbon and iron-chromium alloys with argon

The rate of nitrogen desorption from inductively stirred liquid iron, iron-carbon, and iron-chromium alloys with argon carrier gas has been measured by the sampling method for a wide range of nitrogen, carbon, and chromium contents mainly at 1600 °C. The results obtained by the present work and other data of previous investigators are used to clarify the reaction mechanism of nitrogen desorption from liquid iron. The rate of nitrogen desorption from liquid iron and iron alloys is second order with respect to nitrogen content in the metal under the present condition, and mutual relationships among interfacial chemical reaction, liquid-phase mass transfer, and gas-phase mass transfer are elucidated. The effects of oxygen and sulfur on the rate of nitrogen desorption are given byk _' ^{c ’} = 3.15ƒ_N ² [1/(1 + 300a₀ + 130a_s)]. Carbon dissolved in iron increases the rate of nitrogen desorption, and chromium decreases it. The effects of these alloying elements can be explained by the change of the nitrogen activity in the metal. This paper is based on a presentation made at the G. R. Fitterer Symposium on Nitrogen in Metals and Alloys held at the 114th annual AIME meeting in New York, February 24–28, 1985, under the auspices of the ASM-MSD Thermodynamic Activity Committee.     

Solid-solution strengthening in Ni-C alloys

“Pure” nickel and Ni-C solid-solution alloys of various grain sizes (ASTM no. 2 to 10), with eight different carbon concentrations in the range 0.008 to 0.304 wt pct, were strained in tension between 4° and 474°K at a strain rate of 8.3 × 10⁻⁵per sec. The critical resolved shear stress (CRSS) was independent of temperature in the range 200° to 474° K (athermal region). Below 200°K, the CRSS increased sharply with decreasing temperature, the increase being larger for alloys of high carbon concentration. Both the temperature-dependent and the athermal alloy hardening were found to be linear functions of carbon concentration. The strain-rate sensitivities of flow stress of alloys did not change with strain and were larger for alloys of higher carbon concentrations. The Hall-Petch relation was used to calculate the CRSS of Ni-C single crystals, Τ₀ ^f, so that the data can be compared with existing alloy hardening theories. The data are compatible with the solid-solution theory of Friedel in which the hardening is attributed to both elastic and electrical interactions between dislocation cores and solute atoms. Formerly with the Edgar C. Bain Laboratory for Fundamental Research, U.S. Steel Corp., Research Center, Monroeville, Pa. Formerly with the Edgar C. Bain Laboratory for Fundamental Research.     

Unstable flow in martensite and ferrite

It is proposed that unstable plastic flow of iron alloys containing carbon or nitrogen occurs when (? lnν/?τ ^*) _T becomes negative (v is the average velocity of the dislocations andτ ^* the effective stress acting upon them) at the maximum in the force-velocity curve deduced from either the Snoek or the Cottrell steady-state drag model. In addition, a nucleating event is necessary. A model is developed which predicts relationships between the applied stress, the strain rate and the temperature at the onset of unstable flow, and also the activation energy associated with the event. Experiments have been carried out on iron-carbon and iron-nickel-carbon alloys covering a range of carbon concentration in solution. The alloys were either annealed ferrite or martensite and, thus, the extremes of the range of possible dislocation density were examined. Two distinct types of plastic instability were identified: jerky flow, the result of Snoek interaction, and serrated flow due to Cottrell drag. All the qualitative and quantitative features of the phenomenon which were examined were found to be in complete accord with the model. The average velocity of the dislocations as a function of the temperature at the start of unstable flow has been deduced from the model and an estimate of the density of dislocations moving at the time has been made using measured values of the critical strain rate. The strain rate and temperature at which unstable flow disappears were measured as a function of the carbon concentration and the two types of substructure. The data show that an explanation based on the assumption that the phenomenon involved is the same as that producing the Köster internal-friction peak is untenable. An alternative suggestion, based on the relative stability of a Cottrell cloud and a carbide precipitate, is discussed briefly and qualitatively.     

The effect of microstructure on fatigue crack propagation in iron-carbon alloys

The dependence of fatigue crack growth rate on the cyclic stress intensity factor was determined for six iron-carbon alloys ranging in carbon content from 0.23 to 1.08 wt pct carbon. Both ferrite/pearlite and ferrite/free iron carbide microstructures were studied. Scanning electron microscope fractography studies correlated the fatigue mechanism with microstructure. It was found that when the predominant mode of crack growth was ductile, the crack growth rateda/dN could be related to the cyclic stress intensity factor ΔK by an equation of the formda/dN = (ΔK)_m where andm are constants. The constantm was approximately equal to four when the crack growth mechanism presumably was the blunting and resharpening of the crack tip by slip processes. The constantm was greater than four when the crack growth mechanism was void coalescence in the interlamella ferrite of pearlite colonies. The preferred fatigue crack path through the pearlitic alloys was through the free ferrite phase. formerly Research Assistant at Materials Science and Engineering Department and Materials Research Center, Northwestern University.     

Interfacial reaction kinetics in the decarburization of liquid iron by carbon dioxide

The kinetics of decarburization of liquid iron have been studied between 1160 and 1600°C under conditions where mass transport of reactants is not rate determining. Studies with continuously carbon-saturated iron and of iron with varying carbon concentration have been used to show that the slow step at high concentrations of carbon is independent of carbon concentration and is first order with respect to the pressure of CO₂. For high purity iron, the forward rate constant, in mole cm² s^-1 atm^-1, is given by the equation ln k_f = -11,700/T-0.48. It is concluded that the data are consistent with the chemisorption process as the rate limiting step. A marked sensitivity of the rate to trace amounts of sulfur has been found and it is shown that this is consistent with ideal adsorption of sulfur and is in fair accord with the existing measurements of the depression of the surface tension of iron-carbon alloys by sulfur. D. R. Sain was formerly a Graduate Student.     

Theoretical and experimental investigations on macrosegregation caused by mixing in the liquid bulk melt

An improved model describing the macrosegregation caused by mixing in the liquid bulk melt is presented. This model includes the mass transfer between the interdendritic liquid and the bulk melt which takes place additionally to the mass transfer via the boundary layer in front of the dendrite tips. Further, laboratory experiments on macrosegregation of iron-carbon alloys are described. The experiments were carried out in a unidirectional solidification device with 1,5 kg melts. During the solidification process the liquid melt was stirred by a ceramic blade stirrer with rotational speeds between 180 and 720 min^?1. The melts were supperheated above the liquidus temperature by amounts between 3 and 80 K. The solidification velocities were 2 to 6 mm/min. The experiments could satisfactorily be described by the model using an exchange factor α for the amount of additional mass transfer. The exchange factor increases with decreasing amounts of superheating and solidification velocity and with increasing stirring speed.     

The volume expansion accompanying the martensite transformation in iron-carbon alloys

A dilatometric investigation was conducted to determine the effect of carbon on the volume expansion accompanying the martensite transformation in iron-carbon alloys. It was found that the volume expansion at theM _s temperature varies from 2.0 pct at 0.19 wt pct carbon to 3.1 pet at 1.01 pct carbon, largely due to the effect of carbon on lowering the temperature at which the transformation occurs. Also of importance is the solid solution effect of carbon on altering the lattice parameters of both the austenite and martensite phases at theM_s.     

The volume expansion accompanying the martensite transformation in iron-carbon alloys

A dilatometric investigation was conducted to determine the effect of carbon on the volume expansion accompanying the martensite transformation in iron-carbon alloys. It was found that the volume expansion at theM _s temperature varies from 2.0 pct at 0.19 wt pct carbon to 3.1 pet at 1.01 pct carbon, largely due to the effect of carbon on lowering the temperature at which the transformation occurs. Also of importance is the solid solution effect of carbon on altering the lattice parameters of both the austenite and martensite phases at theM_s.     

Thermodynamics of the Fe-Mo-C system at 985 K

The solubility of carbon and the composition of carbides in the ferritic Fe-Mo-C system were measured at 985 K by a gas flowing method and a sealing method. The composition of alloys ranged from 0.24 pct to 2.93 pct Mo. An iron-carbon binary alloy was included in the equilibration as a reference material. The molybdenum-carbon interaction in the α-phase was analyzed by the central atoms model. The Wagner interaction coefficient was determined as ε _c ^Mo = •100 ± 2, which is a higher negative value than that in the Fe-Cr-C system at the same temperature. The carbide phase was analyzed as a regular solution of two component carbides, FeC _x and MoC _x . M₆C carbide was in equilibrium with α in the carbon activity range from 0.045 to 0.156, and M₂C carbide was in equilibrium at the carbon activity 0.51. M₆C and M₂C carbides were present at the carbon activity 0.45. Molybdenum partitioning between α- and carbide phases was measured. The standard free energies of formation of two component carbides and the interaction energy parameters were determined for M₆C and M₂C carbides.     

Phosphorus interdiffusivity inα-Fe binary and alloy systems

The isothermal segregation kinetics of phosphorus to Fe and Fe-based alloy surfaces have been monitored from 783 to 923 K using Auger electron spectroscopy. The P segregation kinetics are consistent with a model which assumes bulk diffusion of P to be the rate controlling mechanism in the segregation process. The activation energy and preexponential for P diffusion in Fe calculated from the diffusion data are 314 ± 30 kJ · mol⁻¹ and 8 × 10⁵ cm²s in the temperature range studied. Similar results are found for the carbon free Fe-based alloys studied. Phosphorus segregation kinetics for a carbon containing alloy are more rapid, with evidence of a dependence on carbon concentration. The activation energy and pre-exponential calculated in this study for carbon-free alloys are considerably higher than published values measured at higher temperatures by conventional techniques. It is pointed out that an increase in activation energy with decreasing temperature is consistent with observations in several studies of Fe and other ferromagnetic alloys of an increase in activation energy for diffusion below the ferromagnetic transition temperature.     

Thermodynamic properties of alumina-saturated iron-carbon-bismuth alloys

Using direct and indirect equilibration methods, the solubilities of iron in molten bismuth and those of bismuth in molten iron-carbon alloys have been measured over the temperature range 1473-1873 K. The Henrian behaviour of bismuth in molten ferrous alloys allows the calculation of its activity coefficient, which in turn facilitates modelling as a function of temperature and carbon content, using the unified interaction parameter formalism developed by Pelton and Bale. The modelling results generate the expression: . A model has also been developed expressing bismuth content in molten iron-carbon alloys as a function of temperature and composition: .     

Mass transport of carbon in one and two phase iron-nickel alloys in a temperature gradient

The flux of carbon atoms induced by an applied temperature gradient on a specimen was investigated for an Fe-32.5 wt pct Ni alloy for six carbon concentrations. Carbon was found to migrate to the higher temperature region in the low carbon single phase alloys. However, in the higher carbon alloys an abrupt jump in carbon concentration results when a portion of the specimen is in a two-phase region while the portion in the one-phase region exhibits the usual solute migration toward the higher temperature. A value of -12.2 +- 0.4 kJ mol^-1 was obtained for the heat of transport of carbon in the γ-phase Fe-Ni alloys for a wide range of carbon concentrations. A model for diffusion and thermotransport in multiphase systems is presented to explain the observed results. I. C. I. OKAFOR, formerly with Ames Laboratory, DOE     

Thermomechanical properties of iron and iron-carbon alloys: density and thermal contraction

For the modelling of strains und stresses arising during solidification data on the mechanical and thermal properties of the steel are needed. Data on density and on related properties of iron and iron-carbon alloys in the literature were evaluated giving the thermal contraction as a function of temperature and carbon content.     

Nitrogen solubility in liquid Fe-Ta,Fe-Cr-Ta,Fe-Ni-Ta,and Fe-Cr-Ni-Ta alloys

The nitrogen solubility in liquid Fe-Ta, Fe-Cr-Ta, Fe-Ni-Ta, and Fe-18 pet Cr-8 pet Ni-Ta alloys was measured using the Sieverts’ method. The experiments covered the temperature range from 1782 to 2031 K, and tantalum contents from 2.0 to 20.0 wt pct Ta. Nitrogen solution obeyed Sieverts’ law and no nitride precipitation was observed in this concentration range. Tantalum increases the nitrogen solubility and the heat of solution of nitrogen is more negative at higher tantalum contents in these alloys. The excess enthalpy and entropy of solution of nitrogen were determined. The first and second order interaction parameters between nitrogen and tantalum were determined as a function of temperature, e _N ^Ta = -101.7/T + 0.018 and e _N ^TaTa = -3.27/T + 0.0022. The effects of alloying elements on the activity coefficient of nitrogen were measured and the second order cross-interaction parameters between nitrogen and Ta with Cr and Ni were determined at 1873 K as e _N ^CrTa = 0.00052 and e _N ^NiTa = 0.00045.     

Nitrogen solubility in liquid Fe-V and Fe-Cr-Ni-V alloys

Nitrogen solubility in liquid Fe, Fe-V, Fe-Cr-V, Fe-Ni-V and Fe-18 pct Cr-8 pet Ni-V alloys has been measured using the Sieverts’ method for vanadium contents up to 15 wt pct and over the temperature range from 1775 to 2040 K. Nitrogen solution obeyed Sieverts’ law for all alloys investigated. Nitride formation was observed in Fe-13 pet V, Fe-15 pet V and Fe-18 pet Cr-8 pet Ni-10 pet V alloys at lower temperatures. The nitrogen solubility increases with increasing vanadium content and for a given composition decreases with increasing temperature. In Fe-V alloys, the nitrogen solubility at 1 atm N₂ pressure is 0.72 wt pet at 1863 K and 15 pct V. The heat and entropy of solution of nitrogen in Fe-V alloys were determined as functions of vanadium content. The first and second order interaction parameters were determined as functions of temperature as: $$e_N^V = \frac{{ - 463.6}}{T} + 0.148 and e_N^{VV} = \frac{{17.72}}{T} - 0.0069$$ The effects of alloying elements on the activity coefficient of nitrogen were measured in Fe-5 pet and 10 pet Cr-V, Fe-5 pet and 10 pet Ni-V and Fe-18 pet Cr-8 pct Ni-V alloys. In Fe-18 pet Cr-8 pet Ni-10 pet V, the nitrogen solubility at 1 atm N₂ pressure is 0.97 wt pet at 1873 K. The second order cross interaction parameters, e _N ^Cr,V and e _N ^Ni,V , were determined at 1873 K as 0.00129 and ? 0.00038 respectively.