Phase relationships in the Fe-Cr-Ni system at solidification temperatures

The phase relationships between the liquid phase and the primary solid phases were investigated in the iron-rich corner of the Fe-Cr-Ni system as part of a larger study of the Fe-Cr-Ni-C system. The investigation consisted of measurements and modeling of tie-lines and the liquidus surfaces of the liquid-delta (bcc) and liquid-gamma (fcc) equilibria and the peritectic surface involving all three phases in the iron-rich corner of the Gibbs triangle bounded by 0 to 25 wt pct Cr and 0 to 25 wt pct Ni (bal Fe). The temperature ranged from the melting point of iron (1811 K) to about 1750 K. Compositions for tie-lines were obtained from liquid-solid equilibrium couples and temperatures for the surfaces were obtained by differential thermal analysis. Parameters for modeling the system were then selected in the subregular solution model to minimize the square of the difference between experimental and calculated tie-lines. With one ternary parameter employed for each phase, calculations by the model are in excellent agreement with the tie-line and liquidus measurements and in fair agreement with the temperatures for the peritectic surfaceL + δ/L + δ + γ. The usefulness of the model is demonstrated by calculation of the solidification paths of selected alloys in the composition field investigated for the limiting cases of (a) complete equilibrium followed by the alloy system, and (b) no solid diffusion (i.e., segregation) with equilibrium maintained at the solidifying front and complete mixing in the liquid phase.     

Phase relationships in the iron-rich Fe-Cr-Ni-C system at solidification temperatures

The phase relationships between the liquid phase and the primary solid phases were investigated in the iron-rich comer of the Fe-Cr-Ni-C system as part of a larger study of the Fe-Cr-Mn-Ni-C system. The investigation consisted of measurements of tie-lines for the liquid-delta (bcc) and the liquid-gamma (fcc) equilibria in the iron-rich corner of the Gibbs tetrahedron bounded by 0 to 25 wt Pct Cr, 0 to 25 wt Pct Ni, and 1.2 wt Pct C (bal. Fe). The temperature ranged from 1811 to 1750 K. Compositions for the tie-lines were obtained from liquid-solid equilibrium couples and the temperatures of the equilibrium, by differential thermal analysis (DTA). A mathematical procedure was employed on the experimental data to obtain parameters for a thermodynamic model of the alloy system. This involved minimization of an error function. The details of this analysis are discussed fully in this paper. Calculations by the model employing the “best-set” parameters are in good agreement with the experimental results. The usefulness of the model is demonstrated by calculation of the three-phase equilibrium in the quaternary system as a function of temperature. Formerly Research Fellow, Massachusetts Institute of Technology, is Senior Research Engineer, Armco Inc., Middletown, OH 45043     

FeS-MnS phase relationships in the presence of excess iron

The FeS-MnS system is reexamined, both with and without excess iron. When excess iron is present, as is true for sulfide inclusions within steel, the pseudobinary reveals a peritectic rather than the previously assumed eutectic invariant. The maximum solubility limits (997 ± 3°C, or 1270 K) in the two solid phases are: a) 7.5 wt pct MnS in FeS, and b) 73.5 wt pct FeS in MnS. The peritectic liquid contains 66 wt pct Fe, ∼34 wt pct S, and ∼0.4 wt pct Mn. The two solid sulfide phases are nearly stoichiometric in the presence of excess iron; the Fe-richer sulfide is metal-deficient in the absence of a metallic iron phase. Based on this study, it is possible to be more specific than heretofore about the Fe-FeS-MnS-Mn region of the Fe-Mn-S ternary. In addition to the presence of a peritectic, it was concluded that the miscibility gap does not cross the univariant line between primary metal and (Mn,Fe)S phases. The peritectic liquid and the Mn-richer solid sulfide equilibrate with a metal containing ≤ 0.36 wt pct Mn. These data help explain the Mn/s ratios required to avoid hot-shortness in regular and resulfurized plain-carbon steels. G. S. MANN, formerly Graduate Student This is a part of the dissertation submitted by G. S. Mann for his Ph.D. at the University of Michigan     

Phase relationships in the fe-cr-mn-ni-c system at solidification temperatures

The phase relationships between the liquid phase and the primary solid phases were investigated in the iron-rich corner of the quinary system Fe-Cr-Mn-Ni-C. Of the five quaternary systems that comprise the quinary system, this study was limited to the three quaternary systems which contain both carbon and iron as two of the components;viz.: Fe-Cr-Mn-C, Fe-Cr-Ni-C, and Fe-Mn-Ni-C, as well as all of the binary and ternary subsystems that have iron as a component. This paper discusses the modeling efforts for these systems, with particular emphasis on the ternary systems Fe-Cr-Mn and Fe-Mn-Ni and the quaternary systems Fe-Cr-Mn-C and Fe-Mn-Ni-C. The experimental investigation consisted of measurements of tie-lines for the liquid-delta (bcc) and the liquid-gamma (fcc) equilibria in the iron-rich corner of the Gibbs simplex bounded by 0 to 25 wt pct Cr, 0 to 12 wt pct Mn, 0 to 25 wt pct Ni, and 0 to 1.2 wt pct C (bal. Fe). The temperature ranged from 1811 to about 1750 K. Compositions for the tie-lines were obtained from liquid-solid equilibrium couples, and the temperatures for the equilibrium by differential thermal analysis (DTA). Parameters were selected in a thermodynamic model of the alloy system to minimize the square of the difference between experimentally and calculated tie-lines, the latter being implicitly a function of the derived parameters in the model. Binary and higher-order parameters were generally required. Ternary parameters were obtained on ironcarbon base alloys Fe-Cr-C, Fe-Mn-C, and Fe-Ni-C, and for the Fe-Cr-Ni system, but not for the Fe-Cr-Mn and Fe-Mn-Ni systems. Of the quaternary systems investigated, quaternary parameters were required only for theL/δ equilibrium in the Fe-Cr-Ni-C system; the Fe-Cr-Mn-C and Fe-Mn-Ni-C systems were found to be represented adequately by employing only binary and ternary parameters.     

The influence of thermal treatment on the chemistry and structure of grain boundaries in inconel 600

Although it is widely accepted that certain heat treatments result in carbide precipitation accompanied by chromium depletion at the grain boundaries, no direct evidence of this phenomenon exists for Inconel 600. Using the Scanning Transmission Electron Microscope (STEM), the extent of grain boundary chromium depletion is quantitatively determined as a function of thermal treatment time at 700 °C following a 30 min solution anneal at 1100 °C. Results confirm the presence of grain boundary chromium depletion that varies in extent with time at temperature, the chromium concentration falling to values as low as 3 wt pct. The chromium depletion volume is characterized by a depletion parameter which is correlated with intergranular corrosion test results to determine a self-healing (desensitization) chromium concentration of 9 wt pct. Trace element segregation at grain boundaries is measured by Auger Electron Spectroscopy (AES) as a function of aging treatment. Results show that after thermally treating samples for various times at 700 °C, phosphorus is always present at the grain boundaries. Intergranular corrosion behavior as a function of thermal treatment appears to be governed more strongly by chromium depletion than trace element segregation. G. S. WAS, formerly Research Assistant, Nuclear Engineering Dept., Massachusetts Institute of Technology H. H. TISCHNER, formerly Postdoctoral Associate, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

Pressureless infiltration of aluminum metal-matrix composites

Pressureless infiltration of ceramic preforms by molten aluminum is described. The preforms are SiC with varying amounts of particulate Al, Ti, and Ni. Infiltrants employed are pure Al and Al-12.5 wt pct Si. It is shown that a pressure differential within the preform is required for infiltration, and measurements are made of pressure changes in the preforms during infiltration. Results indicate that atmospheric pressure is essential for infiltration but that capillarity may play a role as well. T. NUKAMI, formerly Research Assistant, Massachusetts Institute of Technology     

Monkman-grant analysis of creep fracture in dispersion-strengthened and particulate-reinforced aluminum

The tensile creep fracture properties of coarse- and fine-grained dispersion-strengthened-cast aluminum (DSC-Al) with 25 vol pct of submicron alumina dispersoids are presented for temperatures between 335 °C and 500 °C and stresses between 30 and 100 MPa. The primary, secondary, and tertiary creep strains are analyzed in terms of the minimum creep rate, applied stress, and temperature. Good agreement with the original and the modified Monkman-Grant relationships is found for the failure time of DSC-Al and other aluminum materials reinforced with dispersoids or particulates. The origin of the Monkman-Grant relationships for these materials is discussed in terms of stress exponents, specific interfacial areas, and ratio of secondary strain to failure strain. D.C. DUNAND, Associate Professor, formerly with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 A.M. JANSEN, formerly Graduate Research Assistant, Department of Materials Science and Engineering, Massachusetts Institute of Technology, is Manager     

The constitution of metastable titanium-rich Ti-Fe alloys:An order-disorder transition

Single phase Ti-Fe alloys with up to 50 at. pct Fe were prepared by splat cooling. Alloys with less than about 35 at. pct Fe consisted of β-titanium solid solution, designated Ti(Fe); alloys with more than 35 at. pct Fe consisted of a partially disordered, off-stoichiometric TiFe solid solution. The degree of order of this phase was determined. A continuous phase transition between the ordered and the disordered metastable phase is possible. The ordered metastable phase fulfills known valence electron concentration criteria for the occurrence of the CsCl structure type. Formally Research Assistant, Department of Metallurgy and Materials Science, Massachusetts Institute of Technology, Cambridge, Mass. Formally Research Associate, Department of Metallurgy and Materials Science, Massachusetts Institute of Technology,     

Solidification of undercooled Fe-Cr-Ni alloys: Part I. Thermal behavior

Solidification of undercooled Fe-Cr-Ni alloys was studied by high-speed pyrometry during and after recalescence of levitated, gas-cooled droplets. Alloys were of 70 wt pct Fe, with Cr varying from 15 to 19.7 wt pct, balance was Ni. Undercoolings were up to about 300 K. Alloys of Cr content less than that of the eutectic (18.1 wt pct) have face-centered cubic (fee) (austenite) as their equilibrium primary phase, and alloys of higher Cr content have body-centered cubic (bcc) (ferrite) as their equilibrium primary phase. However, except at low undercoolings in the hypoeutectic alloys, all samples solidified with bcc as the primary phase; the bcc then transformed to fcc during initial recalescence for the lower Cr contents or during subsequent cooling for the higher Cr contents. The bcc-to-fcc transformation, whether in the semisolid or solid state, was detected by a second recalescence. In the hypoeutectic alloys, the growth of primary metastable bcc apparently results from preferred nucleation of bcc. The subsequent nucleation of fcc may occur at bcc/bcc grain boundaries. Formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

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.     

Solidification of M2 high speed steel

The freezing process in AISI type M2 high speed tool steel (6 pct W, 5 pct Mo, 4 pct Cr, 2 pct V, 0.8 pct C) was studied by metallographic and thermal analysis techniques. Unidirectional solidification of small laboratory melts in a modified crystal growing apparatus was employed to provide metallographic sections of known macroscopic growth direction. Also cooling curves were obtained on 40 g specimens solidified in thimble crucibles. X-ray microradiography, electron probe scanning techniques, and quantitative microanalysis of dendrites and interdendritic carbides were extensively used to supplement conventional metallography. Carbon and vanadium contents of M2 were varied in order to observe the effect of an austenite and ferrite stabilizer on the thermal analysis curves and microstructure. The nonequilibrium freezing process in M2 includes three major liquid-solid reactions: 1) Liquid → Ferrite, 1435°C; 2) Liquid + Ferrite → Austenite, 1330°C; 3) Liquid → Austenite + M₆C + MC, 1240°C. These reactions account for the as-cast structure of the commercial alloy. The addition of carbon depresses the liquidus (1) and solidus temperatures (3) and narrows the gap between the liquidus (1) and peritectic transformation (2). This gap is eliminated at > 1.39 wt pct C, where the initial freezing reaction is the crystallization of austenite. The accompanying microstructural change is the elimination of σ eutectoid dendrite cores. The addition of vanadium promotes ferrite formation by strongly depressing the peritectic reaction and thus widening the gap between the liquidus and the peritectic.     

Solidification of highly undercooled Sn- Pb alloy droplets

Experimental work is described on undercooling and structure of tin-lead droplets emulsified in oil. The droplets, predominantly in the size range of 10 to 20 μm, were cooled at rates (just before nucleation) ranging from about 10^-1 K per second to 10⁶ K per second. The higher cooling rates were obtained by a newly developed technique of quenching the emulsified droplets in a cold liquid. Measured undercoolings (at the lower cooling rates) ranged up to about 100 K. Structures obtained depend strongly on undercooling, cooling rate before and after nucleation, and alloy composition. Droplets containing up to 5 wt pct Pb were apparently single phase when undercooled and rapidly quenched. Droplets in the composition range of about 25 wt pct to 90 wt pct Pb solidified dendritically, even at the most rapid quench rates employed, apparently because these alloys undercooled only slightly before nucleation of the primary phase. Formerly Graduate Research Assistant and Postdoctoral Associate in the Department of Materials Science and Engineering, Massachusetts Institute of Technology.     

Slag-metal equilibrium during submerged arc welding

A thermodynamic model of the equilibria existing between the slag and the weld metal during submerged arc welding is presented. As formulated, the model applies only to fused neutral fluxes containing less than 20 pct CaF₂, however some results indicate that the model may be useful in more general cases as well. The model is shown to be capable of predicting the gain or loss of both Mn and Si over a wide range of baseplate, electrode and flux compositions. At large deviations from the predicted equilibrium, the experimental results indicate considerable variability in the amount of Mn or Si transferred between the slag and metal phases, while closer to the calculated equilibrium, the extent of metal transfer becomes more predictable. The variability in metal transfer rate at large deviations from equilibrium may be explained by variations between the bulk and the surface concentrations of Mn and Si in both metal and slag phases. Formerly a Graduate Research Assistant, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139     

Determination of the Fe-rich portion of the Fe-Ni-C phase diagram

The iron-rich portion of the Fe-Ni-C phase diagram has been determined in the composi-tion range from 0 to 20 wt pct Ni and 0 to 6.67 wt pct C for four temperatures, 773, 873, 923 and 1003 K. Long term heat treatments were used to grow the ferrite plus austenite assemblages, while slow cooling heat treatments (25 K/h) were used to grow the metal plus carbide assemblages. Other types of heat treatments produced metal plus graphite. The two phase tie-lines and three phase tie-triangles were measured using electron mi-croprobe techniques. In samples where bulk equilibration had not been achieved, tie-lines were obtained by using extrapolated interface compositions, on the assumption of local equilibrium at the interface. The tie-lines lie at higher Ni contents than the equilibrium tie-line through the bulk composition. The tie-line shift was required to produce match-ing growth rates of Ni and C for the carbides. The addition of Ni slightly reduces the solubility of carbon in austenite and decreases the stability of the carbide phase. In addi-tion, the carbide is always Ni-poor relative to the coexisting metal phase(s).     

Solidification of undercooled Fe-Cr-Ni alloys: Part II. Microstructural evolution

Results are reported on microstructures of Fe-Cr-Ni alloys, solidified over a range of undercoolings and quenched during or after recalescence. Alloys studied contained 70 wt pct Fe and with Cr varying from approximately 15 to 20 wt pct. The three lower Cr alloys were hypoeutectic (with fee as primary phase in equilibrium solidification); the two higher Cr alloys were hypereutectic (with bcc as primary phase in equilibrium solidification). Results obtained are in agreement with predictions based on thermal analyses previously presented; they confirm and extend the understanding gained in that work. The primary phase to solidify in the hypoeutectic alloys is bec when undercooling is greater than an amount which decreases with increasing Cr content. At the lower Cr contents, the stable fcc phase then forms by solid-state transformation of the metastable phase and its subsequent engulfment by additional fcc. At the higher Cr content, transformation is by a peritectic-like reaction in the semisolid state, except near the surface at higher undercoolings where the transformation is massive. In the hypereutectic alloys, primary solidification at all undercoolings is the stable bcc phase. Partial transformation to fcc occurs in the semisolid or solid state, depending on composition and undercooling. Formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

Activities of boron in the binary Fe-B,Co-B,and Cu-B melts

Activities of boron in the binary Fe-B, Co-B, and Cu-B melts have been directly determined by the electromotive force (emf) measurement. Boron-saturated liquid Cu-B alloy was used as the reference electrode and a ternary 28 wt pct Al₂O₃-29 wt pct B₂O₃-43 wt pct CaO oxide melt was used as the electrolyte. Deviations of the boron activities from Raoult's law have been found largely negative for the Fe-B and Co-B systems but largely positive for the Cu-B system. Boron activities calculated from the literature data have not been in good agreement with the measured data. Activities of iron, cobalt, and copper have been calculated from the obtained boron activities by means of the Gibbs-Duhem equation. Some modifications to the liquidus curves on the Fe-B and Co-B phase diagrams have been presented. M. YUKINOBU, formerly Graduate Student, Department of Metallurgy, The University of Tokyo S. GOTO, Professor, formerly with the Department of Metallurgy, The University of Tokyo     

FeSe-MnSe phase relationships in the presence of excess iron

The subsolidus phase relationships of iron selenide and manganese selenide are examined both with and without excess metallic iron. In the presence of excess metal, where the situation is similar to that existing in steels, the FeSe-MnSe pseudobinary has a peritectic at 976 ±3°C (1249 K) with the following phases: Fe (<0.3 wt pctMn) (s), (Fe,Mn)Se(2.5 wt pet MnSe) (s), (Mn,Fe)Se(48.5 wt pct FeSe) (s), and a selenidiliquid (4.3Mn, 37.7Fe, 58 wt pet Se). A minimum is observed in the liquidus surface at ∼920°C and FeSe-9 wt pet MnSe. A qualitative liquidus surface is proposed for the Fe-FeSe-MnSe-Mn section on the basis of these results. This is a part of the Dissertation submitted by G. S. Mann for his Ph.D. at the University of Michigan.     

Thermodynamics of the fcc Fe-Mn-C and Fe-Si-C alloys

The activity of carbon in austenitic Fe-Mn-C and Fe-Si-C alloys has been studied by equilibration with controlled CH₄-H₂ atmospheres at temperatures in the range 848° to 1147°C and for composition up to about 60 pct Mn and 7 pct Si. The activity coefficient of carbon is diminished by manganese and is increased by silicon. Activity coefficients and derived values of the partial molar free energy, enthalpy, and entropy of solution of graphite in the alloy are expressed in mathematical form. The heat of solution of graphite, which is positive in the Fe-C binary alloys, decreases with increasing manganese and increases with increasing silicon concentrations. The partial molar entropy is independent of manganese, but is decreased by silicon. TSUGUYASU WADA, formerly of the Research Staff, Massachusetts Institute of Technology, Cambridge, Mass. HARUE WADA, formerly of the Research Staff, MIT, Cambridge.     

Infiltration of fibrous preforms by a pure metal: Part II. Experiment

In a previous paper, a theory was developed to describe the flow of a pure metal into a fibrous preform. This paper presents experimental data to test the results of the theory for pure aluminum flowing into fibrous alumina preforms. An apparatus was designed and built for unidirectional infiltration under constant pressure and carefully controlled temperature parameters. A sensor was also developed to measure the position of the liquid metal in the fibrous preform during the experiment. This technique enabled quantitative comparison of theory and experiment. Experimental data are reported for the infiltration by 99.999 and 99.9 wt pct pure aluminum of SAFFIL alumina fibers fabricated into two-dimensionally random preforms. Fiber volume fraction was varied from 0.22 to 0.26, fiber preheat temperature was varied from approximately 483 to 743 K, and metal superheat was varied from 20 to 185 K. Infiltration pressure was varied from 1 to 4.5. MPa (145 to 650 psi). Agreement between theory and experiment was very good under all the experimental conditions studied for the 99.999 wt pct pure matrix. The impurity level of the metal was found to influence infiltration significantly. The measured perform permeability for 99.9 wt pct aluminum was much lower than that for 99.999 wt pct aluminum. L.J. MASUR, formerly a Graduate Student with, the Department of Materials Science and Engineering Massachusetts Institute of Technology.