Interdiffusion in Co-Mn alloys

Interdiffusion coefficient in cobalt-manganese alloys has been determined by Matano's method in the temperature range between 1133 and 1423 K on (pure Co)-(Co-30.28 at. pct Mn alloy) and (pure Co)-(Co-51.76 at. pct Mn alloy) couples. This, ∼D, has been found to increase with the increase of manganese content. However, the activation energy (∼Q) and frequency factor ( ₀) show a maximum at about 10 at. pct Mn. The concentration dependence of and has been discussed taking into account the thermodynamic properties of the alloy. The difference in between the ferro- and paramagnetic phases in Co-5 at. pct Mn alloy has been found to be 24 kJ/mol, which is larger, than that for the diffusion of Mn⁵⁴ in this alloy. Further it has been found that the Kirkendall marker moves toward manganese-rich side, showing that manganese atoms diffuse faster than cobalt atoms. From the marker shift, the intrinsic diffusion coefficients,D _Co andD _Mn, at 33 at. pct Mn have been determined as follows:D _Co=0.22×10⁻⁴ exp(−263 kJ mol⁻¹/RT) m²/s, andD _Mn=0.98×10⁻⁴ exp(−229 kJ mol⁻¹/RT) m²/s.     

Diffusivity and solubility of hydrogen as a function of composition in Fe-Ni alloys

An electrochemical method has been used to determine the permeability,P, diffusion coefficient,D, and solubility,c, of hydrogen in alloys of the Fe-Ni system. The heats of activation for diffusion and the heats of solution have been derived.D falls from ≃10⁻⁴ sq cm per sec for pure iron to ≃10⁻¹⁰ sq cm per sec for 40 wt pct of Ni in the alloy. Thereafter it rises slightly to that for pure nickel,c rises by about 10³ between pure iron and 40 wt pct Ni, then remains constant up to pure nickel. The resultantP doubles at 5 wt pct Ni and then falls by 10³ times up to 40 wt pct Ni, afterwards rising slightly to that for pure nickel. Between 0 and 40 wt pct Ni the dominant factor in controlling the value ofP is the fall of the mole fraction of the α phase in the alloy. This hypothesis gives a reasonable quantitative calculation of theP-composition relation. Between 40 and 100 wt pct, the crystallographic phase is allγ and the major effect is the bonding of hydrogen in the alloy, the small changes noted being reasonably calculable. The negligible change of solubility in this region reflects the negligible change ind character of the alloy from 40 to 100 wt pct Ni. The hydrogen permeability of Fe-Ni (5 wt pct) is greater than that of palladium atT > 200°C. The corrosion rate and hydrogen permeability (hence, susceptibility to hydrogen embrittlement) pass through a minimum at about 50 wt pct Ni. A remarkable parallelism exists between corrosion rate and hydrogen permeation in Fe-Ni alloys. An interpretation is suggested. Formerly with the University of Pennsylvania Formerly with the University of Pennsylvania Work carried out by P. K. SUBRAMANYAN in partial fulfillment of the requirements for the degree of Doctor of Philosophy, University of Pennsylvania, 1970.     

Diffusion of cobalt,chromium, and titanium in Ni3Al

Diffusion studies of cobalt, chromium, and titanium in Ni₃Al (γ′) at temperatures between 1298 and 1573 K have been performed using diffusion couples of (Ni-24.2 at. pct Al/Ni-24.4 at. pct Al-2.91 at. pct Co), (Ni-24.2 at. pct Al/Ni-23.1 at. pct Al-2.84 at. pct Cr), and (Ni-24.2 at. pct Al/Ni-20.9 at. pct Al-3.17 at. pct Ti). The diffusion profiles were measured by an electron probe microanalyzer, and the diffusion coefficients of cobalt, chromium, and tita-nium in γ′ containing 24.2 at. pct Al were determined from those diffusion profiles by Hall’s method. The temperature dependencies of their diffusion coefficients (m[su2]/s) are as follows: ~D_(Co) = (4.2 ± 1.2) × 1O^-3exp {-325 ± 4 (kJ/mol)/RT} ~D_(Cr) = (1.1 ± 0.3) × 10^-1 exp {-366 ± 3 (kJ/mol)/RT} and D_(Ti) = (5.6 ± 3.1) × 10¹ exp {-468 ± 6 (kJ/mol)/RT} The values of activation energy increase in this order: cobalt, chromium, and titanium. These activation energies are closely related to the substitution behavior of cobalt, chromium, and titanium atoms in the Ll₂ lattice sites of γ′; the cobalt atoms occupying the face-centered sites in the Ll₂ structure diffuse with the normal activation energy, whereas the titanium atoms oc-cupying the cubic corner sites diffuse with a larger activation energy that includes the energy due to local disordering caused by the atomic jumps. The chromium atoms which can occupy both sites diffuse with an activation energy similar to that of cobalt atoms.     

Microstructure and ordering of L12 titanium trialuminides

The present article reports and discusses the results of the microstructural characterization of various modifications of Ll₂ trialuminides containing various titanium contents, including the first ever report on their degree of ordering. The Ll₂ trialuminide alloys Al₃Ti + X, where X = Cu, Fe, Cr, and Mn were studied. The as-cast structure contains a very low level of porosity, and the amount of second phase depends on the particular alloy. After homogenization, the second phase is reduced in almost all the alloys to the level less than 0.5 pct, except for the Mn-high Ti alloy in which it remains at about 20 pct and its composition is 67.9 ± 0.6 at. pct Al, 2.2 ± 0.6 at. pct Mn, and 29.9 ± 0.3 at. pct Ti. In almost all the alloys, porosity after homogenization increases about twofold, except in the Al₃Ti + Cr alloy in which it remains at almost the as-cast level. Limited transmission electron microscopic observations have revealed the existence of very fine (≈10 nm) unidentified precipitates in the homogenized Al₃Ti + Cu alloy. The homogenized Al₃Ti + Cr and Mn alloys have greater lattice parameters than the Al₃Ti + Fe and Cu alloys. It is also found that the long-range order parameterS of the ho- mogenized Ll₂ Al₃Ti + X alloys dramatically decreases with increasing titanium content.     

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.     

Intrinsic diffusion coefficients and the vacancy flow factor in Dilute Cu-Zn Alloys

Interdiffusion coefficients in copper-rich copper-zinc solid solutions containing up to 8 at. pct of Zn at 1168 K have been determined by Matano's analysis using semi-infinite diffusion couples consisting of pure copper and Cu-Zn alloys with Kirkendall markers. From the marker shift and Darken's relation, intrinsic diffusion coefficients, D_Zn and D_Cu, in the alloys containing 3.2 and 4.7 at. pct of Zn have been determined. Further, using thin plate couples, D_Zn and D_Cu in Cu alloys containing 0.9, 2.3, 3.5, and 4.6 at. pct of Zn at 1168 K have been determined by Heumann's method. The ratio of the intrinsic diffusion coefficients, D_Zn/D_Cu, has been found to be about two for all the compositions examined. Using the values of the intrinsic diffusion coefficient of copper at infinite dilution of zinc obtained by extrapolating the concentration dependence of D_Cu, and the self- and impurity diffusion coefficients in pure copper, the vacancy flow factor has been estimated to be - 0.22_-0.15 ^+0.06 at 1168 K. By combining this value of the vacancy flow factor with the solute enhancement factor of solvent diffusion determined by Peterson and Rothman, the correlation factor for impurity diffusion of Zn in Cu at 1168 K has been evaluated to be 0.5, which is in good agreement with the value of 0.47 determined by Peterson and Rothman based on the isotope effect measurement. KAZUTOMO HOSHINO, formerly Graduate Student, Tohoku University is now with Materials Science Division, Argonne National Laboratory, Argonne, IL 60439. YOSHIAKI IIJIMA, Instructor, and KENICHI HIRANO, Professor, are both with the Department of Materials Science, Faculty of Engineering, Tohoku University, Sendai 980, Japan.     

Thermodynamics of titanium and nitrogen in an Fe-Ni melt

The equilibrium solubility of titanium and nitrogen in Fe-Ni melts was measured in the presence of pure solid TiN under various nitrogen pressures in the temperature range of 1843 to 1923 K. The activity coefficients of titanium and nitrogen relative to a 1 mass pct standard state in liquid iron were calculated from the experimental results for Fe-Ni alloys of nickel contents up to 30 mass pct. Nickel decreases the activity coefficient of titanium, but it increases the activity coefficient of nitrogen in an Fe-Ni-Ti-N melt. Therefore, the effect of nickel on the solubility product of TiN is not significant. The first- and second-order interaction parameters of nickel on titanium (e _Ti ^Ni and r _Ti ^Ni , respectively) were determined to be −0.0115 and 0 at 1873 K, respectively. Similarly, the interaction parameters of nickel on nitrogen (e _N ^Ni and r _N ^Ni , respectively) were determined to be 0.012 and 0, respectively, at 1873 K. The temperature dependence of these interaction parameters was also determined.     

An evaluation of the flow behavior during high strain rate superplasticity in an Al-Mg-Sc alloy

An Al-3 pct Mg-0.2 pct Sc alloy was fabricated by casting and subjected to equal-channel angular pressing to reduce the grain size to ∼0.2 μm. Very high tensile elongations were achieved in this alloy at temperatures over the range from 573 to 723 K, with elongations up to >2000 pct at temperatures of 673 and 723 K and strain rates at and above 10⁻² s⁻¹. By contrast, samples of the same alloy subjected to cold rolling (CR) yielded elongations to failure of <400 pct at 673 K. An analysis of the experimental data for the equal-channel angular (ECA)-pressed samples shows consistency with conventional superplasticity including an activation energy for superplastic flow which is within the range anticipated for grain boundary diffusion in pure Al and interdiffusion in Al-Mg solid solution alloys.     

Influence of precipitates on the mechanical response and substructure evolution of shock-loaded and quasi-statically deformed Al−Li−Cu alloys

The mechanical response and substructure evolution of two Al−Li−Cu alloys (Al-2.90 wt pct Li-1.00 pct Cu-0.12 pct Zr and Al-2.30 pct Li-2.85 pct Cu-0.12 pct Zr) subjected to shock-loading (strain rate έ> 10⁶ s^-1), Split-Hopkinson-Pressure-Bar compression (έ ~ 5 × 10³ s^-1), and quasi-static compression (έ ~ 1.5 × 10^-3 s^-1) were examined. The strain levels achieved in these three deformation paths were desined to be comparable,i.e., all ∼15 pct. Both alloys were either naturally or artificially aged to yield an underaged or overaged condition. Various precipitates, such as theδ' andT ₁ phase, of different sizes and volume fractions were dispersed in the matrix and at the grain boundaries. The substructure in all of the shock-loaded, Split-Hopkinson-Pressure-Bar, and quasi-static compression samples was characterized by localized slip bands and microbands with the exception of the overaged alloys. The density of dislocations and dislocation loops was higher, independent of the aging condition, in the shock-loaded specimens. Well-defined cell structures were not observed in any of the samples, independent of strain rate. The influence of precipitates, shearable or not, on the substructure development in Al−Li−Cu alloys during shock-loading was seen to be pronounced, even though the size and volume fraction of precipitates was small and low, respectively. Flow stress measurements showed that the shock-loaded samples have flow strengths 3 to 8 pct higher than the quasi-statically deformed samples. This small, but reproducible, strength increment, for alloys deformed to equivalent strains at low and high rates, indicates that the Al−Li alloys studied have a small rate sensitivity. Based upon comparison of the results of the shock-loaded and quasi-static samples, it is concluded that the fundamental deformation mechanisms and substructure evolution in all three loading paths are not drastically different, corroborating previous investigations.     

Modeling creep and fatigue of copper alloys

This article reviews expressions to quantify the thermal creep and fatigue lifetime for four copper alloys: Cu-Ag-P, Cu-Cr-Zr, Cu-Ni-Be, and Cu-Al₂O₃. These property models are needed to simulate the mechanical behavior of structures with copper components, which are subjected to high heat-flux and fatigue loading conditions, such as molds for the continuous casting of steel and the first wall in a fusion reactor. Then, measurements of four-point bending fatigue tests were conducted on two-layered specimens of copper alloy and stainless steel, and thermal ratchetting behavior was observed at 250 °C. The test specimens were modeled with a two-dimensional elastic-plastic-creep finite-element model using the ABAQUS software. To match the measurements, a primary thermal-creep law was developed for Cu-0.28 pct Al₂O₃ for stress levels up to 500 MPa and strain rates from 10⁻⁸ to 10⁻² s⁻¹. Specifically, (s⁻¹)=1.43×10¹⁰ exp (−197,000/8.31 T(K)) (σ(MPa))^2.5 (t(s))^−0.9.     

The diffusion of hydrogen in liquid iron alloys

Rates of absorption of hydrogen in stagnant liquid iron and ten (Fe-X) binary iron alloy systems were studied by an unsteady-state gas-liquid metal diffusion cell technique. These rates were found to be controlled by diffusion of hydrogen in the liquid phase. Chemical diffusion coefficients (D _h) were measured in pure iron and Fe-X alloys in the following (at. pct) composition ranges: Mn (0 to 5), Cr (0 to 25), V (0 to 25), Nb (0 to 10), Mo (0 to 25), W (0 to 5), Ni (0 to 75), Co (0 to 75), Sn (0 to 10), and Cu (0 to 25). All measuredD _H values at 1600°C lie between 7 × 10^-4 and 16 × 10^-4 sq cm per sec. The diffusion coefficients found for pure iron can be represented by D_H ^Fe = 4.37 × 10⁻³ exp (−4134 ± 1012)/RT cm²/sec where the uncertainty in the activation energy, Q, in cal per mole, corresponds to the 90 pct confidence level. A linear relationship was found between the logarithm of the hydrogen diffusion coefficient D_H ^Fe-X and the interaction parameter ε_H ^X for low and medium concentrations of alloying elementX, when applied to a fixed concentration ofX(5 or 25 at. pct) and to individual periods in the periodic table. A useful linear correlation also appears to exist between logD_H ^Fe-X and hydrogen solubility for fixed concentration ofX and with respect to the period in whichX is found. Formerly Research Assistant, Department of Mineral Engineering, Stanford University, Stanford, Calif. This paper is based upon a thesis submitted by P. J. DEPUYDT in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Stanford University and part of a presentation made at the 1970 Annual AIME Meeting.     

Thermodynamics of TiO x in blast furnace-type slags

Equilibrium studies between CaO-SiO₂-10 pct MgO-Al₂O₃-TiO_1.5-TiO₂ slags, carbon-saturated iron, and a carbon monoxide atmosphere were performed at 1773 K to determine the activities of TiO_1.5 and TiO₂ in the slag. These thermodynamic parameters are required to predict the formation of titanium carbonitride in the blast furnace. In order to calculate the activity of titanium oxide, the activity coefficient of titanium in carbon-saturated iron-carbon-titanium alloys was determined by measuring the solubility of titanium in carbon-saturated iron in equilibrium with titanium carbide. The solubility and the activity coefficient of titanium obtained were 1.3 pct and 0.023 relative to 1 wt pct titanium in liquid iron or 0.0013 relative to pure solid titanium at 1773 K, respectively. Over the concentration range studied, the effect of the TiO _x content on its activity coefficient is small. In the slag system studied containing 35 to 50 pct CaO, 25 to 45 pct SiO₂, 7 to 22 pct Al₂O₃, and 10 pct MgO, the activity coefficients of TiO_1.5 and TiO₂ relative to pure solid standard states range from 2.3 to 8.8 and from 0.1 to 0.3, respectively. Using thermodynamic data obtained, the prediction of the formation of titanium carbonitride was made. Assuming hypothetical ‘TiO₂,’ i.e., total titanium in the slag expressed as TiO₂, and using the values of the activity coefficients of TiO_1.5 and TiO₂ determined, the equilibrium distribution of titanium between blast furnace-type slags and carbon-saturated iron was computed. The value of [pct Ti]/(pct ‘TiO₂’) ranges from 0.1 to 0.2.     

Catalyzed precipitation in Al-Cu-Si

The work reported here concerns the effect of Si on the precipitation of θ′ phase (metastable Al₂Cu) during the isothermal aging of Al-2Cu-1 Si (wt pct). The binary alloys Al-2Cu and Al-1 Si were studied for comparison. Only two precipitate phases were detected: essentially pure Si in Al-1 Si and Al-2Cu-1 Si, and θ′ (metastable Al₂Cu) in Al-2Cu and Al-2Cu-1Si. On aging the ternary alloy at 225 °C, Si precipitates first and catalyzes the θ′ phase. The precipitates in the ternary alloy are smaller, are more densely distributed, have lower aspect ratios, and coarsen more slowly than those in the binary Al-2Cu aged at the same temperature. While the shapes of individual θ′ precipitates in binary Al-2Cu are strongly affected by the kinetic problem of nucleating growth ledges, which produces a significant scatter in the aspect ratio for samples of given thickness, the overall evolution of particle shape with size follows the predictions of the Khachaturyan-Hairapetyan (KH) thermoelastic theory, which reduces to κ=L/d ∞ √L at large sizes. The KH theory provides an estimate for the interfacial tension of the broad Al-θ′ interface of 85 to 96 mJ/m², which is near the values for other low-energy interfaces in Al, such as the twin boundary energy (100 mJ/m²) and the antiphase boundary energy in δ′ Al₃Li (70 mJ/m²). Si and θ′ precipitates in Al-2Cu-1 Si have a strong elastic interaction because of their compensating strain fields. This elastic interaction promotes the nucleation of θ′ precipitates on Si, decreases the expected aspect ratio of θ′, and inhibits coarsening. Finally, Si precipitation in ternary Al-2Cu-1 Si differs from that in binary Al-1 Si in that the Si precipitates are coarser, more equiaxed, and more extensively twinned. These changes appear to be effects of Cu, which increases the solubility of Si in Al and adsorbs on the Si-Al interface, promoting twinning by a “step-poisoning” effect at the interface.     

Discussion of “effects of tensile stress on microstructural change of eutectoid Zn-AI alloy”

In a recent contribution,[1] Zhu and Orozco presented a phase transformation of the ternary alloy Zn-20.2 wt pct Al-1.8 wt pct Cu, studied under tensile stress by using X-ray diffraction and scanning electron microscopy techniques. The authors report the existence of three phases in the alloy at room temperature after furnace cooling,α,ε, and a newη _′ ^T instead of the zinc-rich solid solutionη, as appears in the phase diagrams. The reported parameters for this hcp metastable phase are^[1,2] a = 0.2663 andc = 0.4873 nm; these values are close to the parameters of pure zinc,^[3] witha = 0.2664 nm andc = 0.4946 nm. The difference betweenη _′ ^T and zinc in thea parameter is around 0.03 pct, and it is 1.47 pet for thec parameter. When zinc is saturated with aluminum in the Zn-AI alloys, thea parameter shrinks^[3] to 0.2660 nm. It is possible to see that the value ofa of theη _′ ^T phase lies in-between the values of pure zinc and zinc-aluminum solid solution. The solubility of Al and Cu in Zn^[4] at 100 °C is 0.3 wt pct Cu and 0.06 wt pct Al. The covalent radius of Cu (0.117 nm) is smaller than the covalent radius of Al (0.118 nm) and Zn (0.125 nm), so the introduction of Cu in the zinc structure can result in a reduction of thec parameter. These values suggest that the metastable phaseη _′ ^T could be the hcp zincrich solid solution with low aluminum and copper contents. The article of Zhu and Goodwin,^[5] cited by Zhu and Orozco in their Reference 14, is related not to the eutectoid alloy, as they argue, but to an alloy with 27 wt pct Al, and no reports about the transformation ofε intoT′ were found. The presence of the metastable e phase (CuZn₄, sometimes called CuZn₅) at room temperature and its transformation to the stable phaseT′ (rhombohedral intermetallic phase, Al₄Cu₃Zn) have been observed by other authors.^[6,7] Y.H. ZHU and E. OROZCO:Metall Mater. Trans. A, 1995, vol. 26A, pp. 2611-15.     

Effect of impurity type on boundary sliding behavior in the superplastic Zn-22 pct Al alloy

The effect of impurity type on boundary sliding behavior in the superplastic Zn-22 pct Al alloy was investigated using two grades of the alloy: Zn-22 pct Al-0.13 pct Cu (grade Cu) and Zn-22 pct Al-0.14 pct Fe (grade Fe). In the investigation, boundary sliding offset measurements in both grades were made at strain rates ranging from 5×10⁻⁷/s to 10⁻¹/s. This range of strain rate covered region I (the low strain rate region), region II (the intermediate strain rate region), and region III (the high strain rate region) of the sigmoidal plot between stress and strain rate that was previously reported for grade Fe. The experimental results show that the contributions of boundary sliding to the total strain, ξ, in the two grades of Zn-22 pct Al are about 20 and 52 pct at high (region III) and intermediate (region II) strain rates, respectively. By contrast, the experimental data reveal that ξ in grade Cu at low strain rates (52 pct) is essentially equal to that at intermediate strain rates (region II), while ξ in grade Fe at low strain rates (24 pct) is considerably lower than that at intermediate strain rates (56 pct). It is demonstrated that the difference in sliding behavior between grade Fe and grade Cu at low strain rates corresponds well with the difference in superplastic behavior between the two grades. In addition, consideration of the present and earlier data on sliding behavior in Zn-22 pct Al provides a correlation between two roles played by boundaries during superplastic deformation: the ability of boundaries to contribute to deformation through the process of boundary sliding and their ability to serve as favorable sites for the accumulation of impurities, i.e., boundary segregation.     

A metallographic study of porosity and fracture behavior in relation to the tensile properties in 319.2 end chill castings

A metallographic study of the porosity and fracture behavior in unidirectionally solidified end chill castings of 319.2 aluminum alloy (Al-6.2 pct Si-3.8 pct Cu-0.5 pct Fe-0.14 pct Mn-0.06 pct Mg-0.073 pct Ti) was carried out using optical microscopy and scanning electron microscopy (SEM) to determine their relationship with the tensile properties. The parameters varied in the production of these castings were the hydrogen (∼0.1 and ∼0.37 mL/100 g Al), modifier (0 and 300 ppm Sr), and grain refiner (0 and 0.02 wt pct Ti) concentrations, as well as the solidification time, which increased with increasing distance from the end chill bottom of the casting, giving dendrite arm spacings (DASs) ranging from ∼15 to ∼95 /im. Image analysis and energy dispersive X-ray (EDX) analysis were employed for quantification of porosity/microstructural constituents and fracture surface analysis (phase identification), respectively. The results showed that the local solidification time(viz. DAS) significantly influences the ductility at low hydrogen levels; at higher levels, however, hydro-gen has a more pronounced effect (porosity related) on the drop in ductility. Porosity is mainly observed in the form of elongated pores along the grain boundaries, with Sr increasing the porosity volume percent and grain refining increasing the probability for pore branching. The beneficial effect of Sr modification, however, improves the alloy ductility. Fracture of the Si, β-Al₅FeSi, α- Al₁₅(Fe,Mn)₃Si₂, and Al₂Cu phases takes place within the phase particles rather than at the particle/Al matrix interface. Sensitivity of tensile properties to DAS allows for the use of the latter as an indicator of the expected properties of the alloy.     

Non-isothermal finite diffusion-controlled growth in ternary systems

A mathematical model has been developed for diffusion controlled phase growth in ternary systems. Local equilibrium at phase boundaries and one dimensional diffusion controlled growth is assumed. The model includes a method of determining phase growth velocity and interface compositions consistent with the diffusion rate of both solute elements. This method also accounts for the effects of overlapping diffusion fields and nonisothermal growth. Initial conditions can be any curvilinear composition gradients and boundary conditions can be fixed or vary with time and/or temperature. The Crank-Nicolson finite difference equations are used to provide numerical stability and flexibility. Other capabilities of the model include treatment of finite systems, of nonisothermal phase growth and of off-diagonal ternary coefficients (D ₂₁ ³,D ₁₂ ³). Several sample simulations of the constant cooling of a 2.1 wt pct P, 4.1 wt pct Ni, 93.8 wt pct Fe alloy are presented. Three cooling rates are used: 5×10⁻³, 5×l0⁻⁴, and 5×l0⁻⁵ °C/s. An Fe-Ni-P alloy of this same composition was cooled in the laboratory for five days at 5×lo⁻⁴ °C/s from 900 to 685°C. Excellent agreement was found for the predicted and measured composition gradients and precipitate sizes.     

Tensile properties of directionally solidified Al-4 wt pct Cu alloys with columnar and equiaxed grains

The tensile properties of directionally solidified Al-4 wt pct Cu-0.15-0.2 wt pct Ti alloys with equiaxed grains were determined and compared with the properties of directionally solidified Al-4 wt pct Cu columnar structures. The tensile properties of the equiaxed structure were isotropic, but varied with the distance from the chill face. The mechanical properties of the equiaxed structure were generally between those of the longitudinal and transverse columnar structures. The 0.2 pct offset yield stress(σ _y, MPa) is represented as a function of the grain size,d (mm), the average concentration, C_o (wt pct), and the local concentration, C (wt pct), by σ_y = [(15.7 + 22.6 C_o) + (1.24 + 1.04 C_o)d ^-1/2] + [15.7 △C], where △C = C - C_o. The equiaxed structure exhibits inverse segregation similar to that in the columnar structure.     

Structural superplasticity in a fine-grained eutectic intermetallic NiAl−Cr alloy

A rapidly solidified and thermomechanically processed fine-grained eutectic NiAl−Cr alloy of the composition Ni₃₃Al₃₃Cr₃₄ (at, pct) exhibits structural superplasticity in the temperature regime from 900°C to 1000°C at strain rates ranging from 10⁻⁵ to 10⁻³ s⁻¹. The material consists of a B2-ordered intermetallic NiAl(Cr) solid solution matrix containing a fine dispersion of bcc chromium. A high strain-rate-sensitivity exponent of m=0.55 was achieved in strain-rate-change tests at strain rates of about 10⁻⁴ s⁻¹. Maximum uniform elongations up to 350 pct engineering strain were recorded in superplastic strain to failure tests. Activation energy analysis of superplastic flow was performed in order to establish the diffusion-controlled dislocation accommodation process of grain boundary sliding. An activation energy of Q _c=288±15 kJ/mole was determined. This value is comparable with the activation energy of 290 kJ/mole for lattice diffusion of nickel and for ⁶³Ni tracer selfdiffusion in B2-ordered NiAl. The principal deformation mechanism of superplastic flow in this material is grain-boundary sliding accommodated by dislocation climb controlled by lattice diffusion, which is typical for class II solid-solution alloys. Failure in superplastically strained tensile samples of the fine-grained eutectic alloy occurred by cavitation formations along NiAl‖‖Cr interfaces.     

Activities of chromium in molten copper at dilute concentrations by solid-state electrochemical cell

In order to obtain the activities of chromium in molten copper at dilute concentrations (<0.008 chromium mole fractions), liquid copper was brought to equilibrium with molten CaCl₂ + Cr₂O₃ slag saturated with Cr₂O₃ (s), at temperatures between 1423 and 1573 K, and the equilibrium oxygen partial pressures were measured by means of solid-oxide galvanic cells of the type Mo/Mo + MoO₂/ZrO₂(MgO)/(Cu + Cr))alloy + Cr₂O₃ + (CaCl₂ + Cr₂O₃)_slag/Mo. The free energy changes for the dissolution of solid chromium in molten copper at infinite dilution referred to 1 wt pct were determined as Cr (s) = Cr(1 wt pct, in Cu) and ΔG° = + 97,000 + 73.3(T/K) ± 2,000 J mol⁻¹.