X-ray-based measurement of composition during electron beam melting of AISI 316 stainless steel: Part I. Experimental setup and processing of spectra

An energy dispersive X-ray (EDX) detector mounted on a laboratory scale electron beam furnace (30 kW) was employed to assess the potential use of X-rays as a means of on-line liquid alloy composition monitoring during electron beam (EB) melting of alloys. The design and construction of the collimation and protection systems used for the EDX are described in Part I. X-ray spectra are obtained from a sample of AISI 316 stainless steel at both beam idle (in the absence of liquid metal) and high power (in the presence of liquid metal). Two different types of molds are employed: (1) a water-cooled copper mold and (2) a ceramic lined water-cooled copper mold. Various strategies for signal processing and filtration are presented and compared. Correction factors for beam voltage were developed and applied in order to develop correlations between the mole fraction and normalized X-ray intensity for Cr-K _α, and Fe-K _α based on an analysis of the vapor condensate. Correlations were also developed relating the change in the X-ray intensities to time for (a) Mo-L, (b) Cr-K _α, (c) Fe-K _α, and (d) Ni-K _α. The stability of the electron beam was found to be the principal source of error, and suggestions for further improvements are also discussed. The study confirms the feasibility of the method and is the first reported study of on-line analysis of a high-temperature liquid alloy. In Part II, the technique is applied to the study of the complex evaporation processes occurring during EB melting.     

Eutectic-gradient wear resistance in materials

New iron-base eutectic powder alloys have been developed (Fe−Mn−C−B−Si−Ni−Al−Sc, Fe−Mn−C−B−Si−Cr−Al−Sc, Fe−Mn−C−B−Si−Ni−Cr−Al−Sc) for wear-resistant coatings. The thermodynamic affinity for oxygen has been used in the technology for alloying the liquid with the necessary elements. Institute for Problems of Materials Science, Ukraine, National Academy of Sciences, Kiev, State University “L'vivska Polytechnika”, and Lublin Polytechical Institute, Poland. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(408), pp. 17–21, July–August, 1999.     

The effect of chromium on the activity coefficient of sulfur in liquid Fe−Cr−S alloys

The effect of chromium on the activity coefficient of sulfur in the ternary system Fe−Cr−S has been determined in the temperature range 1525° to 1755°C for chromium concentrations of up to 40 wt pct, using a levitation melting technique in H₂−H₂S atmospheres. The first order free energy interaction coefficient,e _S ^Cr , which is derived on the assumption that the thermal diffusion error is constant for both binary Fe−S and ternary Fe−Cr−S melts under controlled levitation conditions, is given by the relationship:e _S ^Cr =−94.2/T+0.040 The first order enthalpy and entropy interaction coefficients are found to beh _S ^Cr =−430±70 ands _S ^Cr =−0.183±0.007 respectively. These results are in good agreement with recently published data.     

Redetermination of the Fe-rich portion of the Fe−Ni−Co phase diagram

The iron rich portion of the Fe−Ni−Co ternary diagram (<10 pct Co, <15 pct Ni) was redetermined at four temperatures (800, 750, 700 and 650°C). The phase boundaries and tie-lines of the (α+γ) phase field were measured by analyzing the α and γ phases with an electron microprobe. Samples, whose compositions were located in the (α+γ) region of the phase diagram, were subjected to two different, long term heat treatments at the temperatures of interest. Grain boundary allotrimorphs of the α phase were observed in the polished and etched sections of samples which were step cooled from the γ phase into the (α+γ) region. Widmanstatten-type microstructures composed of γ-precipitates were observed in samples which were directly heated from room temperature into the (α+γ) region. The addition of cobalt to Fe−Ni alloys helps nucleate the alpha phase on cooling and also shifts the α/(α+γ) and the (α+γ)/γ phase boundaries to higher nickel contents. Diffusion controlled phase growth in the ternary Fe−Ni−Co system has also been investigated.     

Precipitation-strengthened austenitic Fe−Mn−Ti alloys

The precipitation of intermetallic compounds in the Fe−20Mn−2Ti and Fe−28Mn−2Ti alloy systems has been investigated over the temperature range 700 to 900°C by hardness measurements, optical and scanning electron microscopy, and X-ray diffraction. In both systems only the equilibrium Laves phase was observed. The precipitate was identified as C14(MgZn₂) type hexagonal Laves phase with a chemical composition close to Fe₂ (Ti, Mn). In an as-annealed sample precipitation occurred in a heterogeneous manner, predominantly along grain boundaries. The effect of a cold deformation between the solution annealing and aging processes was also investigated. In addition to a high density of dislocations, martensitic phases were induced by deformation: a γ→∈ transformation occurred in the Fe−28Mn−2Ti alloy while a γ→α′ transformation was predominant in the Fe−20Mn−2Ti alloy. Subsequent aging was conducted at temperatures above theA _f . A large number of very fine precipitates formed randomly in the matrix after a short aging period. This cold work plus aging treatment resulted in an increase in yield strength. The enhancement of mechanical properties is due to the randomly distributed precipitates combined with the high defect density and fine substructure.     

The role of thermal stabilization in martensite transformations

The variation of the kinetics of the martensite transformation with carbon content and martensite habit plane has been investigated in several Fe−Ni based alloys. Transformation in an Fe-25 wt pct Ni-0.02 wt pct C alloy exhibits predominantly athermal features, but some apparently isothermal transformation also occurs. In a decarburized alloy, on the other hand, the observed kinetic features, such as the dependence ofM _s on cooling rate, were characteristic of an isothermal transformation. In contrast, Fe-29.6 wt pct Ni-10.7 wt pct Co alloys with carbon contents of 0.009 wt pct C and 0.003 wt pct C transform by burst kinetics to {259}_γ plate. At both these carbon levels, theM _b temperatures of the Fe−Ni−Co alloys are independent of cooling rate. It is proposed that the change in kinetic behavior of the Fe-25 pct Ni alloy with the different carbon contents is due to the occurrence of dynamic thermal stabilization in the higher carbon alloy. Dynamic thermal stabilization is relatively unimportant in the Fe−Ni−Co alloys which transform by burst kinetics to {259}_γ plate martensite. P. J. FISHER, formerly with the University of New South Wales D. J. H. CORDEROY, formerly with the University of New South Wales     

Evidence of a thin sheet toughening mechanism in Al−Fe−X alloys

A toughening mechanism, dubbed thin sheet toughening, is proposed for improving the fracture resistance (K _IC and tearing modulus) of powder-metallurgy alloys of limited ductility. The basis of this approach is the recognition that internal delamination of thick-section components or cracked specimens into thin sheet ligaments in the fracture process zone leads to a drastic reduction in triaxial stresses, with the consequence of enhancing the critical fracture strain, fracture toughness (K _IC orJ _IC ), and tearing modulus. Theoretical analyses indicate that a factor of , increase in theK _IC value, and even greater increases in the tearing modulus are possible for idealized conditions. The predicted results are compared with experimental results of tensile,K _IC , andJ tests conducted on four powder-metallurgy Al−Fe−X alloys at 25 and 316°C. The comparison reveals that thin sheet toughening is a contributor to the highK _IC value observed in a state-of-the-art Al−Fe−V−Si alloy. Increasing the critical strain to fracture is also shown to be a possible method to improve the fracture toughness of Al−Fe−X alloys, independent of the thin sheet toughening effect.     

Diffusion of managenese and silicon in liquid iron over the whole range of composition

The measurement of the diffusivities of manganese and silicon in molten binary ferroalloys over the whole range of composition was undertaken to clarify existing but conflicting data at lower concentrations, to present new data at higher concentrations and to indirectly confirm the behavior of both systems observed in thermodynamic studies. The experiments were carried out under argon atmosphere in a Tammann furnace. The diffusion couples were held in 5 mm ID alumina tubes (98 pct Al₂O₃). Electron probe microanalysis of the samples led to a diffusion-penetration curve for the system under consideration. Results obtained over the whole range of composition showed a slight negative deviation for the Fe−Mn system and a very large positive deviation for the Fe−Si system. At lower concentrations (0 to 4 pct Mn), the temperature dependence of managanese diffusivity for the Fe−Mn binary alloy in the temperature range 1550° to 1700°C is as follows:D _Fe−Mn=1.8×10⁻³ exp (−13,000/RT) cm²/sec The concentration dependence of manganese diffusivity for the same system at 1600°C may be expressed asD _Fe−Mn={5.48−0.0137 (%Mn)+0.000276 (%Mn)²}×10⁻⁵ cm²/sec The temperature dependence of silicon diffusivity for the Fe−Si binary system in the temperature range 1550° to 1725°C at various concentrations is as follows:D _Fe−Si=2.8×10⁻³ exp (−11,900/RT) cm²/sec at 20 pct SiD _Fe−Si=2.1×10⁻³ exp (−13,200/RT) cm²/sec at 12.5 pct SiD _Fe−Si=5.1×10⁻⁴ exp (−9,150/RT) cm²/sec at 2.2 pct Si FELIPE P. CALDERON, formerly Graduate Student. University of Tokyo, Tokyo, Japan. This paper is based on a portion of a thesis submitted by FELIPE P. CALDERON in partial fulfillment of the requirements for the degree of Doctor of Engineering at University of Tokyo.     

Caustic Stress Corrosion Cracking of Mild Steel

The stress corrosion cracking (SCC) behavior of cold worked mild steel in hot, aqueous, 33 pct NaOH solutions was studied with prefatigue cracked double cantilever beam specimens. SCC kinetics were studied under freely corroding potentials (E _corr ≈ −1.00 V_SHE) and potentiostatic potentials of −0.76 V_SHE near the active-passive transition. The pH of the liquid within the crack was determined and fractography was studied by scanning electron microscopy. Cracking was transgranular atE _corr, intergranular at −0.76 V_SHE, and produced no detectable change in crack liquid pH from that of the bulk solution. Crack rates were dependent upon temperature, potential, and stress intensity (K ₁). The apparent activation energy in Region II, where crack growth rate was independent ofK, was ∼ 24kJ/mol for both cracking modes. This was considered to be due to mixed rate control involving activation polarization and mass transport processes. The mechanism of cracking was entirely consistent with metal dissolution at –0.76 V_SHE and may involve hydrogen embrittlement and/or dissolution effects atE _corr. DOUGLAS SINGBEIL, formerly Research Student, University of British Columbia, is Research Scientist, Pulp and Paper Research Institute of Canada, 570-Blvd. St. Jean, Pointe Claire, Quebec, Canada H9R 3J9.     

Phase Equilibria and Phase Transition of the Ni–Fe–Ga Ferromagnetic Shape Memory Alloy System

Phase equilibria and martensitic and magnetic transitions of the β (B2 and L2₁) phase in the Ni–Fe–Ga system were investigated. The b phase was found to be in equilibrium with the γ (A1 structure) or γ′ (L1₂ structure) phase. The Curie temperature, T _c , equilibrium temperature, T _o 5 (Ms + Af)/2, martensitic transition starting temperature, M _s , and reverse transition finishing temperature, Af , of the β single–phase alloys were sensitive to the Fe and Ga compositions. The Fe substitution for Ni decreased and increased the T _o and T _c , respectively. The Ga substitution for Ni or Fe decreased both the T _o and T _c . The entropy change accompanying the reverse martensitic transition showed compositional dependence due to the magnetic contribution. The saturation magnetization I _s of the Ni–Fe–Ga system showed a strong dependence on the magnetic valence Z _M . The Is values of the Ni–Fe–Ga alloys annealed at 1023 K showed the same Z _m dependence as other ferromagnetic shape memory alloy (FSMA) systems. This article is based on a presentation made in the symposium entitled "Phase Transformations in Magnetic Materials," which occurred during the TMS Annual Meeting, March 12-16, 2006, in San Antonio, Texas, under the auspices of the Joint TMS–MPMD and ASMI–MSCTS Phase Transformations Committee.     

Variation of interface compositions during diffusion controlled precipitate growth in ternary systems

Isothermal diffusion controlled phase growth in ternary systems has been modeled using the Crank-Nicolson finite difference equations. Local equilibrium at phase boundaries and one dimensional growth are assumed. The model includes a method of determining phase growth velocity and interface compositions consistent with the diffusion rates of both elements. It also considers the effects of finite or overlapping diffusion fields (impingement). The growth of phosphide, (FeNi)₃P, in α ferrite in the Fe−Ni−P system was chosen for the simulation. Interface compositions are predicted to change with time, controlled by the necessity to balance the two solute (Ni and P) fluxes which cause the precipitate to grow. Diffusion controls the growth process although during initial growth interface structure may be important. The ratio of the major ternary coefficientsD _PP ^Fe /D _NiNi ^Fe controls the amount of shift of the precipitate interface composition from the tie line through the bulk composition. During the major period of growth the Ni interface compositions in phosphide and α remain constant and a square root of time,t ^1/2, dependence for growth is predicted. The practical, effect of impingement is to decrease phase growth and to allow the interface compositions to shift towards the tie line through the bulk composition.     

Lattice spacings and stacking fault probabilities in Ag−Cd−In alloys

Lattice spacing values in ternary Ag−Cd−In alloys containing 5 to 30 at. pct Cd and 5 to 15 at. pct In have been determined from X-ray powder patterns using the Nelson-Riley extrapolation procedure. The values are shown to lie, in a ternary plot, on iso-parameter lines obtained by joining equal lattice spacing values in the relevant binary alloys. The intrinsic stacking fault probabilities (α) in these alloys obtained from measurement of X-ray peak displacements are found to follow the empirical relation α=α₀ expk|ΔZ|p, where α₀ is the intrinsic fault probability in Ag, |ΔZ| is the weighted mean of solvent-solute valence differences,p is the combined solute content, andk is a constant approximately equal to 0.08 in agreement with the value found earlier for binary alloys based on silver.     

Equilibrium partition coefficients in iron-based alloys

Accurate relationships between equilibrium partition coefficients and solute concentration are required for the prediction of solute redistribution during solidification. Thermodynamic analyses are presented to relate these coefficients to fundamental thermodynamic quantities. Using the most accurate data available, partition coefficients are calculated for ten Fe−X (X=Al, C, Cr, Mn, Ni, N, P, Si, S, Ti) binary systems and compared with literature values. Equations are presented to allow for prediction of these partition coefficients as a function of temperature, as well as liquidus temperature as a function of composition. In addition, partition coefficient values are examined for the ternary systems Fe−Cr−C, Fe−Mn−Ni, and Fe−Ni−S. THOMAS P. BATTLE, formerly Graduate Research Assistant, The University of Michigan This paper is based on a presentation made in the T.B. King Memorial Symposium on “Physical Chemistry in Metals Processing” presented at the Annual Meeting of The Metallurgical Society, Denver, CO, February, 1987, under the auspices of the Physical Chemistry Committee and the PTD/ISS.     

Intermetallic phases formed during DC-casting of an Al−0.25 Wt Pct Fe−0.13 Wt Pct Si alloy

The Al−Fe and Al−Fe−Si particles formed during DC-casting of an Al-0.25 wt pct Fe-0.13 wt pct Si alloy have been examined. The particles were analyzed by transmission electron microscopy (TEM) and energy dispersive spectroscopy of X-rays (EDS). Crystal faults were studied by high resolution electron microscopy (HREM). Samples for electron microscopy were taken at various positions in the ingot,i.e., with different local cooling rates during solidification. At a cooling rate of 6 to 8 K/s the dominating phases were bcc α-AlFeSi and bct Al _m Fe. The space group of bcc α-AlFeSi was verified to be Im3. Superstructure reflections from Al _m Fe were caused by faults on {110}-planes. At a cooling rate of 1 K/s the dominating phases were monoclinic Al₃Fe and the incommensurate structure Al _x Fe. In Al₃Fe, stacking faults on {001} were frequently observed. The structure of Al _x Fe is probably related to Al₆Fe. Some amounts of other phases were detected. For EDS-analysis, extracted particles mounted on holey carbon films were examined. Extracted particles were obtained by dissolving aluminum samples in butanol. Accurate compositions of various Al−Fe−Si phases were determined by EDS-analysis of extracted crystals.     

The thermodynamic studies of liquid copper alloys by electromotive force method: Part I. The Cu−O,Cu−Fe−O,and Cu−Fe systems

Oxygen activities in liquid Cu−O and Cu−Fe−O alloys were measured in the temperature range 1100° to 1300°C by the solid oxide electrolyte emf method with mixtures of Ni−NiO and Co−CoO as reference electrodes. The Cu−O and Cu−Fe−O alloys were analyzed for iron and/or oxygen content. The activity coefficient of oxygen at infinite dilution in liquid copper was found to be 0.115, 0.195, and 0.286 at 1100°, 1200°, and 1300°C, respectively. The results are compared with previous investigations on the Cu−O system. Based on this comparison, the best equation for the free energy of solution has been suggested. The standard free energy of formation of CoO(s) has been calculated at the experimental temperatures. In the liquid Cu−Fe−O system at 1200°C, a minima in oxygen solubility is reached at 1.1 at. pct Fe in the alloy. The value of interaction coefficient, , is −565 at 1200°C. Iron activities in the liquid Cu−Fe alloys have been calculated at 1100° and 1200°C, and a strong positive deviation from ideality is observed. Results of this study were combined with literature data at 1550°C to obtain the values of and at infinite dilution in liquid copper. A. D. KULKARNI, formerly with Chase Brass and Copper Co., Cleveland, Ohio     

Thermodynamics of the solid phases in the system Fe−Mn−C

Phase relationships in the Fe−Mn−C system in the temperature range 600 to 1100°C have been studied using metallographic and X-ray methods and the electron microprobe. Isothermal sections of the phase diagram of the system are reported based on the present results and those of earlier investigators. The fcc λ-phase (austenite) containing carbon is stable at all values ofy _Mn=x _Mn/(x _Mn+x _Fe) in the range 890 to 1100°C and in a more restricted composition range at lower temperatures. Its composition under conditions of equilibrium with the carbides (Fe, Mn)₃C, (Fe, Mn)₂₃C₆, ε, and liquid are shown for several temperatures. The free energy of formation of the cementite phase, (Fe, Mn)₃C, at 1000°C, from γ-Fe, γ-Mn (undercooled) and graphite is ΔG ₁₂₇₃=−35,790y _Mn−2760y _Fe+3RT (y _Mn lny _Mn+y _Fe lny _Fe). The data show that the alloyed cementite is essentially and ideal mixture of Fe₃C and Mn₃C,i.e., the metal atoms are distributed at random on the metal sites in the lattice. ROBERT BENZ, formerly of the Research Staff, Massachusetts Institute of Technology, Cambridge, Massachusetts     

Magnetic alloys in nanoscale biomaterials

Fe−Co composition gradient and Fe−Pt multilayer alloy films were tested as catalysts for groving vertically aligned carbon nanofibers (VACNFs) by plasma-enhanced chemical vapor deposition (PECVD). The Fe−Co film yielded nanofibers with alloy tips in a wide compositional range varying from 8.15 pct Fe at the Co-rich end to 46.29 pct Fe in the middle of the wafer as determined by energy-dispersive X-ray analysis. Two Fe−Co cubic phases (SG Pm3m, were identified by preliminary X-ray diffraction (XRD) measurements. Magnetic measurements showed a substantially greater hysteresis loop area and coercivity in Fe−Co catalyst nanoparticles as compared to the asdeposited Fe−Co film. The Fe−Pt film did not break into FePt alloy nanoparticles under the applied processing parameters and thus the utility of FePt as a VACNF catalyst has been inconclusive. I.M. ANDERSON, formerly with the Microscopy, Microanalysis, Microstructures Metals and Ceramics Division. Oak Ridge National Laboratory This article is based on a presentation made in the symposium entitled “Fourth International Alloy Conference,” which occurred in Kos. Greece, from June 26 to July 1, 2005, and was sponsored by Engineering Conferences International (ECI) and co-sponsored by Lawrence Livermore National Laboratory and Naval Research Laboratory, United Kingdom.     

Formation of α″ iron nitride in FeN martensite: Nitrogen vacancies, iron-atom displacements, and misfit-strain energy

The precipitation of α″ iron nitride in FeN martensite (containing about 5.9 N/100 Fe) is studied as a function of tempering time successively at 333 and 373 K by means of X-ray diffractometry (XRD). On the basis of peak (=lattice parameter) shifts and intensity changes of main and superstructure reflections, it is concluded that, during tempering, the structure of the formed α″ precipitates changes. At 333 K, initially stoichiometric Fe₁₆N₂ is formed and, with further tempering, structural vacancies are introduced, i.e., Fe₁₆N_2−x is formed. At 373 K, the vacancies are partially filled. The change of the α″ structure at 333 K is caused by the change of the matrix from martensite to ferrite, i.e., the misfit between the α″ and the matrix is initially relatively small and then increases. By allowing nitrogen-deficient α″, the increase of the misfit can be diminished at the expense of some increase of the volume of α″. Gibbs free energy calculations (at 333 K), in which differences in the misfit energy are decisive, indeed show that, in a martensite matrix, Fe₁₆N₂ is favored, whereas, in a ferrite matrix, Fe₁₆N_2−x is favored. Further, in this article, methods are presented to calculate structure factors for nitrogen-deficient α″ and to obtain, from measured (=strained) lattice parameters, strain-free α″ lattice parameters from which the composition can be deduced.     

Standard gibbs energies of formation of the carbides of chromium by emf measurements

The standard Gibbs energies of formation of Cr₃C₂, Cr₇C₃, and Cr₂₃C₆ have been determined by electromotive force (emf) measurements using galvanic cells of the type (−) Cr, CrF₂, CaF₂ // CaF₂ // CaF₂, CrF₂, ‘Cr−C’ (+) The measurements have been carried out in the temperature range 1002 to 1176 K. The ΔG° values obtained in the present work are generally in agreement with some of the earlier emf measurements but differ significantly from those obtained by Cr₂O₃−CO equilibrium studies.     

Thermodynamic modeling of binary and ternary metallic solutions

A model is proposed for describing heat of mixing behavior in binary and ternary metallic solutions. The binary model, which has the form, ΔH ^M =α₁ X _A ² X _B +α₂ X _A X _B ² −α₃ X _A ² X _B ² , whereX _A andX _B are mole fractions of componentsA andB and α₁, α₂, and α₃ are constants, is applied to the heat of mixing values for 84 solid and liquid systems and the results are compared with the subregular model. The ternary model, which is composed of the sum of the binary equations and a ternary interaction term of the form α _ABC X _A X _B X _C , was applied to the Bi−Cd−Pb, Cd−Pb−Sn, and Cd−Pb−Sb systems. There was excellent agreement both as to the shapes of the isoenthalpy of mixing curves and as to the heat of mixing values in the ternary systems when the model was used to predict the experimental values.