Structure formation during sintering in the presence of a liquid phase in Cr-Cu-metal of the iron family systems. II. Systems Cr-Cu-Ni(Co, Fe)

Features of structure and phase formation in Cr-Cu-Ni(Co, Fe) systems during liquid-phase sintering in a vacuum of (2–4) · 10⁻³ Pa at 1200 and 1270°C are studied. It is shown that with introduction of elements of the iron family into molten copper there is dispersion of refractory particles and formation at the boundary with the readily melting component of a transition layer whose structure and composition are determined by the nature of the addition. Growth of refractory particles during liquid-phase sintering of composites of the system Cr-Cu-Ni is controlled by diffusion of elements into the solid phase of the transition layer. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(450), pp. 19–25, July–August, 2006.     

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     

Oxide skin strength on molten aluminum

The oxide skin strength on molten aluminum has been measured as a function of temperature for pure aluminum and some aluminum alloys. The measured values fit well to previous published data of surface tension of liquid aluminum, σ _lg , combined with the wetting angle, ϑ, and the mechanical strength of the oxide. It is assumed that the work per area needed to stretch and rupture the oxide skin is a sum of the interfacial tensions and tensile strength of oxide, times oxide thickness:

β brass bicrystal stress-strain relations

The stress-strain relations of three isoaxial and one nonisoaxial bicrystal whose grain boundaries are parallel to the stress axis have been studied. From one set of isoaxial bicrystals it has been possible, for a given strain, to determine the average stress in the grain boundary deformation zone from the relationship where σ _T is the applied stress, σ _b is the single crystal flow stress for the given strain, andV _gb is the volume fraction of grain boundary deformation zone. was determined from measured values of σ _T , σ _b , andV _gb . From these data and it was possible to extrapolate to the grain boundary to obtain σ _gb , the grain boundary stress. From the nonisoaxial bicrystal series containing a hard and a soft component, it was possible to determine at a given strain the stresses in each component and therefore the stress-strain relations from the relationship where and are the average stress in the hard and soft components respectively, andV _H andV _S are the corresponding volume fractions. The remaining two isoaxial bicrystal series were used to evaluate strengthening effects of bicrystal boundaries. YII-DER CHUANG, formerly Graduate Student, Department of Metallurgy and Materials Sciences, School of Engineering, New York University, Bronx, N. Y. 10453 This paper is based on a thesis submitted by Yii-der Chuang in partial fulfillment of the requirements for the Ph.D. degree in metallurgy at New York University, New York, N. Y.     

Microstructure of W - Ni - Sn Composites in Relation to Amount of Liquid Phase on Sintering

Microstructure transformation is examined for W - Ni - Sn composites made by liquid-phase sintering at 1200 °C for 1 h; the liquid phase was a eutectic containing 67.5 mass% Ni and 32.5 mass% Sn. There is a nonmonotone dependence of the tungsten grain size in the sintered composites on the volume proportion of liquid phase in the range 6.7–36.8%. The mean grain size in relation to volume proportion of liquid phase in liquid-phase sintering is calculated from rheological sintering theory. The form of the empirical function f(v) is quantitatively demonstrated, which determines the rate of tungsten grain growth on liquid-phase sintering and is dependent on the proportion of matrix in the composite and the size of the liquid phase layer. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(444), pp. 53–57, July–August, 2005.     

Thermodynamics of the system NaF-Alf3. part ii: The free energies of formation of cryolite (Na3AlF6) and chiolite (Na5Al3F14)

The theory of the solid-electrolyte cells is given, and it is shown that cryolite itself with Ca²⁺ in solid solution is a suitable Na⁺-ion conductor. Experimental electromotive forces for the ranges 570° to 725°C and 570° to 670°C, r − 18,960 cal with a standard deviation of ±36 cal (based on a third-law calculation). For 5NaF(s) + 3AlF₃(s) = Na₅Al₃F₁₄(s), ΔG° = −38,560 − 7.081T with a standard deviation of ±130 cal. Combination of these results with recent values for Al + 3/2 F₂ = A1F₃ and for 6NaF + Al = Na₃AlF₆ + 3Na gives ΔH°_f298(Na₃AlF₆) = −792,400 cal and ΔH°_f298(NaF) = −137,530 cal. The latter is in excellent agreement with the most recent critical assessment.     

Elastic interaction stresses: Part II. The influence of orientation on stresses generated in iso-axial bicrystals

The effects of four types of incompability on stresses generated in seven iso-axial bicrystals of 70-30 alpha brass subjected to tension were examined by finite element methods. The distribution of applied stress and resolved shear stresses was determined. The applied stress, { }, was higher at the interior than at the surface, while the resolved shear stresses were generally higher at the surface. For both applied { } and resolved shear stresses, the largest stresses occurred at the bicrystal boundaries. The effect of end constraints on a [213] oriented single crystal was found to create nonuniform stresses and strains, and an explanation for this behavior was proposed. The effect of orientation on the magnitude of grain boundary resolved shear stress, { }, and the relative extent (volume fraction) of enhanced grain boundary resolved shear stress,V _gb, were determined. The highest values of { } andV _gb were reached in different portions of the stereographic triangle. The elastic interactions producing this behavior were too complex to permit a simple interpretation. TZI-KANG CHEN formerly Graduate Student at Polytechnic University     

High temperature creep of alpha iron in terms of effective stress and dislocation dynamics

The dynamics of dislocations in both steady state and transient creep in alpha iron of about 99.5 pct purity was investigated in the temperature interval 773 to 923 K, and the applied stress range 24.5 to 220.5 MN m⁻². The applied stress sensitivity parameter of the steady state creep rate m∲ = (∂ In ε/∂ In σ) _T increased linearly with increasing applied stress σ from about 5 at σ = 24.5 MN m⁻² to about 12 at σ = 196 MN m⁻². The apparent activation energy of steady state creep rate was found to decrease linearly with stress from 89 K cal mol⁻¹ at σ = 98 MN m⁻² to 81 K cal mol⁻¹ at a = 147 MN m⁻². Measurements of the mean effective stress σ^* by the strain transient dip test technique led to a nonlinear relation between σ^* and σ, indicating a dependence of the ratio σ*/σ on the applied stress. The effective stress sensitivity parameter was lower than m′.However, the apparent activation energy was equal toQ. Using the stress sensitivity technique, the relation between transient creep rate and effective stress at various constant internal stresses and temperatures was obtained. The effective stress sensitivity of transient creep rate was found to be lessthan that of steady creep rate.     

Interdiffusion in Ni-rich,Ni-Cr-Al alloys at 1100 and 1200 °C: Part II. Diffusion coefficients and predicted concentration profiles

Ternary interdiffusion coefficients were measured in the Ni solid solution γ (fcc) phase of the Ni-Cr-Al system at 1100 and 1200 °C. Extensive use was made of both γ/γ and γ/γ + β (β-NiAl structure) diffusion couples. Two analysis techniques were employed to calculate the interdiffusion coefficients. When the Matano planes for Al and Cr were not coincident, numerous integral calculations were made to determine an average diffusion coefficient and to assess the effect of the noncoincidence of the Matano planes. The results of the diffusivity measurements showed that is approximately four times greater than , while and are of the same magnitude. For all concentrations, is two to three times greater than . Both and increase with increasing Al concentration, whereas and show little concentration dependence on Cr alone. A ternary, finite-difference interdiffusion model was employed to predict concentration profiles for the γ/γ couples utilizing the concentration dependence of the measured diffusivities. Good agreement was observed between the predicted and measured concentration profiles for both 1100 and 1200 °C.     

Isothermal Reaction of NiO Powder with Undiluted CH<Subscript>4</Subscript> at 1000 K to 1300 K (727 °C to 1027 °C)

In this study, isothermal reaction behavior of loose NiO powder in a flowing undiluted CH₄ atmosphere at the temperature range 1000 K to 1300 K (727 °C to 1027 °C) is investigated. Thermodynamic analyses at this temperature range revealed that single phase Ni forms at the input \( {{n_{{{\text{CH}}_{ 4} }}^{\text{o}} } \mathord{\left/ {\vphantom {{n_{{{\text{CH}}_{ 4} }}^{\text{o}} } {\left( {n_{{{\text{CH}}_{ 4} }}^{\text{o}} + n_{\text{NiO}}^{\text{o}} } \right)}}} \right. \kern-0pt} {\left( {n_{{{\text{CH}}_{ 4} }}^{\text{o}} + n_{\text{NiO}}^{\text{o}} } \right)}} \) mole fractions (\( X_{{{\text{CH}}_{ 4} }} \)) between ~0.2 and 0.5. It was also predicted that free C co-exists with Ni at \( X_{{{\text{CH}}_{ 4} }} \) values higher than ~0.5. The experiments were carried out as a function of temperature, time, and CH₄ flow rate. Mass measurement, XRD and SEM-EDX were used to characterize the products at various stages of the reaction. At 1200 K and 1300 K (927 °C and 1027 °C), the reaction of NiO with undiluted CH₄ essentially consisted of two successive distinct stages: NiO reduction and pyrolytic C deposition on pre-reduced Ni particles. At 1200 K (927 °C), 1100 K (827 °C), and 1000 K (727 °C), complete oxide reduction was observed within ~7.5, ~17.5, and ~45 minutes, respectively. It was suggested that NiO was essentially reduced to Ni by a CH₄ decomposition product, H₂. Possible reactions leading to NiO reduction were suggested. An attempt was made to describe the NiO reduction kinetics using nucleation-growth and geometrical contraction models. It was observed that the extent of NiO reduction and free C deposition increased with the square root of CH₄ flow rate as predicted by a mass transport theory. A mixed controlling mechanism, partly chemical kinetics and partly external gaseous mass transfer, was responsible for the overall reaction rate. The present study demonstrated that the extent of the reduction can be determined quantitatively using the XRD patterns and also using a formula theoretically derived from the basic XRD data.     

Interaction behavior of nitrogen in liquid niobium

The kinetics and mechanism of absorption/desorption of nitrogen in liquid Nb were investigated in the temperature range of 2470 °C to 2670 °C in samples levitated in a N₂/Ar stream with various nitrogen partial pressures. The nitrogen solution reaction in liquid Nb was found to be exothermic, with the standard enthalpy and entropy of solution of −236.4 ± 23.3 kJ/mol and −-5.3 ± 8.3 J/K · mol, respectively. Above the threshold flow rate of the N₂/Ar stream, the absorption process was determined to be second order with respect to nitrogen concentration, indicating that the rate-controlling step is either the adsorption of nitrogen molecules on the liquid surface or dissociation of adsorbed nitrogen molecules into surface-adsorbed atoms. The desorption process was found to be second order as well. At lower flow rates, however, the absorption rate was found to depend on the gas-phase mass transfer rate. The rate equation for nitrogen absorption in the range of 2470 °C to 2670 °C is given by with the value ofQ calculated to be −327.2 ± 20.6 kJ/mol, while nitrogen desorption at 2670 °C follows the relation      

Isothermal Decomposition of Ferrite in a High-Nitrogen,Nickel-Free Duplex Stainless Steel

The formation and crystallography of second phases during isothermal decomposition of ferrite (α) in a high-nitrogen, nickel-free duplex stainless steel was examined by means of transmission electron microscopy (TEM). At an early stage of aging, the decomposition of α started along the α/γ phase boundaries where sigma (σ) phase and secondary austenite (γ ₂) precipitated in the form of an alternating lamellar structure. The combined analyses based on the simulation of diffraction patterns and stereographic projection have shown that most of the σ phase was related to the γ ₂ by the following relation: (111)_g ||(001)_s (111)_{\gamma } \parallel (001)_{\sigma } and [10[`1]]<sub>g</sub> ||[110]<sub>s</sub> . [10\bar{1}]_{\gamma } \parallel [110]_{\sigma } . The intergranular and intragranular precipitation of Cr<sub>2</sub>N with trigonal structure were identified, and the orientation relationships (ORs) with α and γ matrix could be expressed as ( 110 )<sub>a</sub> ||( 0001 )<sub>\textCr2 \textN</sub> \left( {110} \right)_{\alpha } \parallel \left( {0001} \right)_{{{\text{Cr}}_{2} {\text{N}}}} , [ [`1]11 ]_a ||[[`1]100]<sub>\textCr2 \textN</sub>  ; (111)<sub>g</sub> ||(0001)<sub>\textCr2 \textN</sub> \left[ {\bar{1}11} \right]_{\alpha } \parallel [\bar{1}100]_{{{\text{Cr}}_{2} {\text{N}}}} \,;\,(111)_{\gamma } \parallel (0001)_{{{\text{Cr}}_{2} {\text{N}}}} , and [ [`1]10 ]_g ||[ [`1]100 ]_{\textCr2 \textN} , \left[ {\bar{1}10} \right]_{\gamma } \parallel \left[ {\bar{1}100} \right]_{{{\text{Cr}}_{2} {\text{N}}}} , respectively. The precipitation of intermetallic χ phase was also observed inside the α matrix, and they obeyed the cube-on-cube OR with the α matrix. Prolonged aging changed both the structure of matrix and the distribution of second phases. The γ ₂, formed by decomposition of α, became unstable because of the depletion of mainly N accompanied by the formation of Cr₂N, and it transformed into martensite after subsequent cooling. As a result, the microstructure of the decomposed α region was composed of three kinds of precipitates (intermetallic σ,χ, and Cr₂N) embedded in lath martensite.     

Interdiffusion in the carbides of the Nb-C system

Interdiffusion coefficients in Nb₂C and NbC_1−x were measured using bulk diffusion couples in the temperature range from 1400 °C to 1700 °C. Marker experiments were used to show that carbon is the only component undergoing significant diffusion in both carbides. Carbon concentrations were measured by difference using electron probe microanalysis, and interdiffusion coefficients were taken from Boltzmann-Matano analyses of the resulting concentration profiles. This analysis clearly showed that, in NbC_1−x, interdiffusion coefficient varies with carbon concentration, and is expressed by

Anisotropic grain growth based on the atomic adsorption model in WC-25 pct Co alloy

The process of triangular prism formation and abnormal grain growth of WC was modeled using pseudo-Monte-Carlo simulation based on atomic adsorption and the coalescence mechanism. Grains of WC evolved into a triangular prism shape due to {10 0} and {1 10} planes of fast growth rate. Coalescence of {10 0} and {1 00} planes subsequent to the anisotropic evolution was the main reason for the abnormal grain growth. The probability of coalescence computed by the Monte-Carlo method agreed well with a theoretical prediction. Experimental evaluation of the computational model was made in sintered WC-25 wt pct Co alloy. The experimental alloy was made with WC powder of different particle size, 0.8 μm and −325 mesh, respectively, and with two different sintering conditions: solid-phase sintering and liquid-phase sintering. The sample made from the coarse powder (−325 mesh) showed the same morphological characteristics as those of the original milled state, whereas the sample made from the fine powder (0.8 μm) assumed a triangular prism shape quickly during solid-phase sintering. The anisotropic growth process of the latter sample could be explained by using the adsorption and coalescence model.     

Load relaxation in aluminum: I. Theory of plastic deformation. II. Plastic equation of state

According to the theory of thermally-activated deformation, the plastic strain rate equality will hold in a load relaxation experiment, wheret = 0 is de-fined as the time at which the crosshead stops. In this theory, plastic flow is intrinsically time dependent and its rate is controlled by interaction of glide dislocations with thermal obstacles(e.g. forest dislocations). The strain rate equation is of the form and att = 0 none of these variables changes instantaneously. Measurements reported here for [111] aluminum single crystals indicate that this prediction is wrong. The ratio is near zero at low stress and approaches unity only at high stress. This result is predicted if plastic strain itself is time-independent (athermal), as in the author’s recent theory. Time-dependent strain is then the result of thermal changes in structure, namely loss (recovery) and rearrangement of obstacle dislocations. Experi-ments were also done to test further the essential hypothesis of Hart’s recent formula-tion of an equation of state for plastic deformation-namely that each distinct σ-@#@ curve derived from load relaxation data corresponds to a unique “hardness” state and that re-covery does not occur. Significant differences were observed in the 77 K strsss-strain curves for 295 K relaxed and unrelaxed samples which indicate that substantial loss and some rearrangement of dislocations has occurred during the relaxation. It is concluded from both experiments that load relaxation in aluminum is a manifestation of recovery creep and cannot be taken as evidence for a plastic equation of state.     

The high temperature creep and fracture behavior of 70-30 alpha-brass

Constant stress creep tests have been carried out over a range of stresses at 324°, 413°, 503°, and 550°C. In all cases, over almost the entire creep curve, the time dependence of the true creep strain, ∈, can be described accurately as where ∈₀ is the instantaneous strain on loading, ∈_T the transient creep strain,m a parameter relating to the rate of exhaustion of transient creep, the steady creep rate, ∈_L andP are tertiary creep parameters, andt _t is the time to the onset of tertiary creep. A number of relationships exist between the transient and tertiary parameters in this equation and the steady creep rate, which suggests that the same basic deformation process operates throughout almost the entire creep life. Transient creep ends after a time,t _s, such thatmt _s is a constant (≃4). Similarly, the duration of tertiary creep (t _f – t_t), wheret _f is the time to fracture, depends on the parameter,p, asp (t_f − t_t) is a constant (≃4.5). The time to fracture is found to be related to the parametersm andp asmt _s +pt _f − t_s) =constant (−12) independent of stress and temperature. Formerly Research Student, University College     

Competition among basal,prism, and pyramidal slip modes in hcp metals

The competition of slip among , and slip modes of hcp metals has been analyzed geometrically in terms of a critical resolved shear stress, CRSS, criterion. Under the action of an applied stress slip systems of one or more modes may be activated depending on the value of the CRSS and on the orientation of the slip systems with respect to the applied stress. If the CRSS of a given slip mode should exceed a limiting value relative to the CRSS of the other modes, however, the given mode becomes inoperative even under the most favorably stressed conditions. It is found by an examination of the yield loci that basal slip is inoperative if α₂ < cos θ; prism slip is inoperative if α₂ < α₁ sin θ; and pyramidal slip is inoperative if α₂ > cos θ + α₁ sin θ where and are, respectively, the ratios of CRSS for prism and pyramidal slips relative to basal slip, and ϕ is the angle between the (0001) and normals. Since the value of ϕ is a function ofc/a, the limiting values of α₁ and α₁ depend on thec/a ratio of the crystal structure.     

Crystallographic details of precipitates in Fe-22Cr-21Ni-6Mo-(N) superaustenitic stainless steels aged at 900 °C

Crystallographic features of second phases and the effect of nitrogen addition on the microstructural evolution in superaustenitic Fe-22Cr-21Ni-6Mo-(N) (all in wt pct) stainless steels during isothermal aging at 900 °C were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Both alloys with and without nitrogen contained sigma phase and M₂₃C₆ carbides in the solution treated condition. While four phases (sigma, M₂₃C₆, M₆C, and chi) of intermetallics and carbides appeared sequentially as a function of aging time in the nitrogen-free alloy, two nitrides (Cr₂N and AlN) were additionally observed after long time aging of the nitrogen-containing alloy. The addition of nitrogen into Fe-22Cr-21Ni-6Mo steel promoted a finer and more uniform distribution of precipitates during isothermal aging. The exact identification and crystallography of various second phases were confirmed from the analyses of selected area diffraction patterns from various orientations, stereographic projection, and energy dispersive spectroscopy. The orientation relationships between the precipitates and austenite matrix can be summarized as follows: (1) two carbides (M₂₃C₆ and M₆C): cube-on-cube orientation relationship; (2) chi phase: Kurdjumov-Sachs (K-S) orientation relationship; (3) two nitrides (Cr₂N and (AlN): (11 0)_nitrides //(211) _γ and [0001]_nitrides //[ 11] _γ ; and (4) sigma phase: (1) ( 11) _γ //(00 ) _σ and [ 0] _γ //[ 0] _σ or (2) ( 10) _γ //( 10) _σ and [ 2] _γ //[113] _σ . For the sigma phase, the former orientation relationship was predominant throughout aging, and the latter orientation relationship was occasionally observed under limited aging conditions.     

Texture evolution and mechanical anisotropy in dual-phase Ti<Subscript>3</Subscript>Al-based alloy loaded at 700 °C to 1000 °C

The super α ₂ Ti₃Al-based alloy with a fine grain size of ∼2.2 μm exhibits superplastic elongations over 1000 pct at 920 °C to 1000 °C, 600 pct at 900 °C, 330 pct at 850 °C, and 140 pct at 750 °C. Mechanical anisotropy is observed in this alloy, and relatively lower flow stresses and higher tensile elongations are obtained in the 45 deg specimen loaded at 25 °C to 960 °C. The texture characteristics appear to impose significant influence on the mechanical anisotropy at temperatures below 900 °C (under the dislocation creep condition), and the {111}〈2 〉 and {0001} basal textures evolve in the β and α ₂ phases after tensile straining. At loading temperatures higher than 900 °C (under the superplastic flow condition), the anisotropy effect is less pronounced and the grain orientation distribution becomes basically random in nature. Rationalizations for the mechanical anisotropy in terms of the Schmid factor calculations for the major and minor texture components in the β and α ₂ phases provide consistent explanations for the deformation behavior at lower temperatures as well as the initial straining stage at higher temperatures.     

Orientation relationships between M2C carbide and the austenite matrix in an Fe-Mn-AI-Mo-C alloy

M₂C carbides were observed to precipitate within the austenite matrix of an Fe-24.6Mn-6.6Al-3. IMo-1.0C alloy after quenching from 1200 °C and aging at 750 °C. By means of transmission electron microscopy (TEM) and diffraction techniques, the orientation relationships between M₂C (p) and the austenite (γ) matrix were determined to be: (0001)p//(111)γ, (11– 0)p// (1 0)γ, ( 100)p//(11 )γ. M₂C carbide has been reported by many researchers to precipitate from the ferrite matrix or along austenite/ferrite boundaries in alloy steels containing Mo. However, little information concerning the formation of M₂C in the austenite matrix has been provided. This investigation presents the first evidence for the existence of M₂C carbide wholly within the austenite matrix and its relationship to the austenite. The energy-dispersive spectrometry (EDS) analyses were performed on M₂C carbides, and the results indicate that the solubility of the M₂C carbide for foreign atoms other than Mo is very limited.