Aging behavior of an Al−Li−Cu−Mg−Zr alloy

The aging processes in an Al-Li-Cu-Mg-Zr alloy have been examined by means of electrical resistivity measurements and high-resolution transmission electron microscopy coupled with an image-processing system. The specimens quenched in iced brine after solution treatment, were reheated at a constant rate of 1 K/min up to 773 K. Six reactions were clearly separated in the temperature derivative of the resistivity/temperature curve, i.e., there was a slight increase at temperatures around 333 K, a large decrease at around 368 K, a significant decrease at around 448 K, a large increase at around 538 K, a remarkable decrease at around 568 K, and a final broad increase at around 623 K. Each reaction observed by the electrical resistivity measurement was examined metallographically. In the as-quenched specimen, spherical undissolved β′ (AlZr₃ L1₂ structure) particles dispersed, but the matrix was already ordered congruently into an L1₂ structure. The first reaction at around 333 K is probably due to the increase of the degree in the congruent ordering, but the second one, at around 368 K, is thought to rise from the rearrangements of antiphase domain, boundaries (APDBs) such as the partition of Li atoms between an APDB and the matrix, the APDBs lying parallel to the {100} and {110} planes. Reheating to temperatures around 448 K induces the phase separation, with well-defined interfaces into Li-rich, ordered δ′ (L1₂) and Li-poor, less-ordered regions, and the Ost-wald ripening of the ordered regions follows. The reactions at 538, 568, and 633 K were identified as the dissolution of δ′ particles into the matrix, the precipitation of δ (AlLi B32) and S′ (Al₂CuMg orthorhombic) particles, and the dissolution of both δ and S compounds into the matrix, respectively. SADAYOSHIITO, formerly Graduate Student, Department of Materials Science and Engineering, Ehime University     

Phase equilibria in Ti−Fe−Nb alloys

Isothermal sections for 750 and 600°C have been constructed for the Ti?Fe?Nb ternary phase diagram in the region of titanium-rich alloys by the use of metallography, x-ray diffraction, and electron-probe microanalysis. At 750–600°C, 3.0–3.5 mass % niobium raises the solubility of iron in α-titanium from 0.05–0.06 mass % to 1.0–1.5 mass %.     

Activities in the system LiF−NaF−AlF3

Measurements have been made of the contents of Na and Li in Al in equilibrium with the molten fluorides at 1020 °C. The theory to calculate the activities of the three constituents is derived. Across the Li₃AlF₆-Na₃AlF₆ section the activity coefficients {ie409-1} are given in terms of mol fractionsN _i by $$ \begin{gathered} \log \gamma _{{\text{AIF}}_{\text{3}} } = - 3.034 + 3.342N_{LiF} - 0.848(N_{LiF} )^2 \hfill \\ \log \gamma _{{\text{NaF}}} = - 0.246 - 1.114N_{LiF} - 0.283(N_{LiF} )^2 \hfill \\ \log \gamma _{LiF} = 0.158 - 0.266N_{LiF} - 0.283(N_{LiF} )^2 \hfill \\ \end{gathered} $$ Across the LiF?Na_2.5AlF_5.5 section the activity coefficients for 0≤N _LiF≤0.45 are nearly constant at log \Gg_LiF \t~ 0.1, log \Gg_NaF \t~ 0.4, and log \Gg_{AIF 3} \t~ -2.6.The vapor over these melts is a mixture of LiAlF₄ and NaAlF₄, the pressures being given byp _{LiAlF 4}/bar=0.78a _LiF·a _{AlF 3} andp _{NaAlF 4}/bar=56.2a _NaF·a _{AlF 3}. Combination of these equations with those for the activity coefficients reproduces the maximum observed in the total pressure in the Li₃AlF₆?Na₃AlF₆ section. The increase in pressure observed when Li₃AlF₆ is added to Na₃AlF₆ is due, not to the appearance of LiAlF₄ in the gas, but to the increased pressure of NaAlF₄ following the rise in AlF₃ activity.     

Thermal expansion in the Ni−Cr−Al and Co−Cr−Al systems to 1200°C determined by high-temperature X-ray diffraction

The effect of temperature on the lattice parameters of phases in twelve nickel-chromium-aluminum (Ni?Cr?Al) alloys and nine cobalt-chromium-aluminum (Co?Cr?Al) alloys was determined using high temperature diffraction (HTXRD). The temperature range was from 25 to 1200°C. The data for each phase of each alloy were computer fit to an empirical thermal expansion equation developed in this study: \(LP_T = LP_{25^\circ C} (1 + R)^{(1 + T/273)^{1.5} } \) and a value forR was derived for each. Excellent fits were obtained for nearly all cases. A comparison ofR values revealed that for a given phase (ψ/ψ′, β and α in Ni?Cr?Al and αCo and β in Co?Cr?Al),R was independent of alloy composition. In the Ni?Cr?Al systemR for γ/γ′ was 19.2×10^?4,R for β was 19.9×10^?4 andR for αCr was 13.4×10^?4. In the Co?Cr?Al system theR value for αCo was 20.9×10^?4 and theR for β was 17.8×10^?4. Of all of the phases only the αCr in the Ni?Cr?Al system had anR sufficiently low to reduce to an unimportant level the stress generated from thermal expansion mismatch between Al₂O₃ and substrate or coating and substrate with either Ni?Al or Co?Al coatings on a γ/γ′-δ substrate.     

New functionally gradient materials in the Ti−Si−C system

A new ceramic functionally gradient material (FGM) with controlled hardness and fracture toughness is presented. Very hard SiC and soft, fracture resistant Ti₃SiC₂ ceramics were manufactured as one multilayered composite. This composite was prepared by hot-pressing from SHS-derived powders. Microstructure and indentation tests proved the FGM structure. Mining and Metallurgy University, Krakow, Poland. Published in Poroshkovaya Metallurgiya, Nos. 3–4(406), pp. 42–45, March–April, 1999.     

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.     

Elevated-temperature deformation at forming rates of 10−2 to 102 s−1

In the hot working at constant strain rate ( ) of Al and α Fe alloys at 0.5 to 0.9 T _M (absolute melting temperature), steady-state deformation is achieved in similarity to creep, which is usually at constant stress. After an initial strain-hardening transient, the flow stress becomes constant in association with a substructure which remainsequiaxed and constant in the spacing of sub-boundaries and of dislocations in both walls and subgrains. All these spacings become larger at higher temperature (T) and lower values as well as with lower stress, being fully consistent with the relationships established in creep. Because hot working can proceed to a much higher true strain in torsion (∼100) and compression (∼2) as well as in extrusion (∼20) and rolling (∼5), it is possible to confirm that grains continue to elongate while the subgrains within them remain equiaxed and constant in size. When the thickness of grains reaches about 2 subgrain diameters (d _s), the grain boundaries with serrations (∼d _s) begin to impinge and the grains pinch off, becoming somewhat indistinguishable from the subgrains; this has been called geometric dynamic recrystallization (DRX). In polycrystals as at 20 °C, deformation bands form and rotate during hot working according to the Taylor theory, developing textures very similar to those in cold working. In metals of lower dynamic recoverability such as Cu, Ni, and γ Fe, new grains nucleate and grow (discontinuous DRX), leading to a steady state related to frequently renewed equiaxed grains, containing an equiaxed substructure that develops to a constant character and defines the flow stress. This article is based on a presentation made in the workshop entitled “Mechanisms of Elevated Temperature Plasticity and Fracture,” which was held June 27–29, 2001, in San Diego, CA, concurrent with the 2001 Joint Applied Mechanics and Materials Summer Conference. The workshop was sponsored by Basic Energy Sciences of the United States Department of Energy.     

Thermodynamic properties of the liquid Sn−Ge and Sn−Au systems by mass spectrometry

The activities and partial molar heats of mixing have been determined for the liquid Sn?Ge system at 1773 K and the liquid Sn?Au system at 1873 K. The experimental technique consisted of analyzing Knudsen cell effusates with a TOF mass spectrometer. The ion current ratios for the monomeric vapor species were measured as a function of temperature and composition and the thermodynamic properties calculated using a modified form of the Gibbs-Duhem equations. In addition to exhibiting very slight positive deviation from ideal behavior, the Sn?Ge system displayed parabolic solution behavior over the entire composition range. The results for the excess partial molar free energies and partial molar heats of mixing for the Sn?Ge system can be represented by $$G_1^E = 3.06X_2^2 kJ/g \cdot mol$$ and $$H_1^M = 5.86X_2^2 kJ/g \cdot mol$$ at 1773 K. The Sn?Au system exhibited negative deviation from ideal behavior and the results can be partially represented by $$\begin{gathered} \log _{10} \gamma Au = - 0.388 - 0.650 X_{Sn}^2 (0.00 \leqslant X_{Au} \leqslant 0.30) \hfill \\ \log _{10} \gamma Sn = 0.658 - 2.63 X_{Au}^2 (0.00 \leqslant X_{Sn} \leqslant 0.25) \hfill \\ \end{gathered} $$ and $$H_1^M = - 61.7 X_2^2 kJ/g \cdot mol$$ at 1873 K. Comparison of the results with other investigations indicates the heat of mixing for the system becomes more exothermic with increasing temperature above 1100 K. An experimental technique is presented for determining the effect of dissociative ionization of molecular species on the activity coefficient. The effect of dissociative ionization of the molecular species present in the Knudsen cell effusate was determined to be negligible.     

A mass-spectrometric study of the thermodynamics of the Fe−Cu and Fe−Cu−C(sat) systems at 1600°C

The Knudsen cell-mass spectrometer combination has been used to study the Fe?Cu and Fe?Cu?C_(sat) alloys at 1600°C. Activity coefficients in the Fe?Cu system are closely represented by the equations $$\begin{gathered} \ln \gamma _{Fe} = 1.86N_{Cu}^2 + 0.03, (0< N_{Fe}< 0.7) \hfill \\ \ln \gamma _{Cu} = 2.25N_{Fe}^2 - 0.19, (0.7< N_{Fe}< 1.0) \hfill \\ \end{gathered} $$ with an uncertainty in the quadratic terms of about 5 pct. For the iron-rich carbon-saturated alloys, the activity coefficient of copper is given by the equation $$\ln \gamma _{Cu} = 2.45(N'_{Fe} )^2 + 0.3N'_{Fe} + 0.03, (0< N'$$ to within an uncertainty of about 10 pct. N _Fe ^′ represents the fraction N_Fe/(N_Fe+N_Cu), etc. The activity coefficient of iron in this region is found to be essentially constant at 0.69±0.05.     

Correlation of yield strength with internal coherency strains for age-hardened Cu−Ni−Fe alloys

The maximum yield strengths for a given aging temperature were measured for age-hardened Cu−Ni−Fe alloys. The yield strengths were found to be proportional to the difference in cubic lattice parameters of unstressed precipitating phases and independent of other factors such as precipitate particle size and precipitate volume fraction. The yield strength dependence on lattice parameter differences alone indicated coherency stresses controlled the yield strengths. An analysis of the yield strength based only on internal coherency strains and stresses subsequently led to the derivation of an equation for the yield strength,i.e., where is the Taylor factor for converting from single crystal shear stress to polycrystalline tensile stress results,C _ijare single crystal elastic stiffness constants and Δa is the difference in, anda ₀ the average of the cubic lattice parameters of the precipitating phases. The equation indicates the yield strength is dependent only on the internal coherency strains and independent of particle size and precipitate volume fraction, as observed. The correlation of the experimentally measured yield strengths with the equation was quite good.     

Microstructure of A γ−α2−β TiAl alloy containing iron and vanadium

Ordered ß-phase (B2 CsCl structure) has been identified in a Ti-45.5 at.% Al-1.6 at.% Fe-1.1 at.% V-0.7 at.% B alloy being developed for high temperature applications. Selected area electron diffraction has been used to identify the orientation relationships and interface planes of the ß-phase with the bulk γ-TiAl and α₂-Ti₃Al phases. Energy dispersive X-ray spectroscopy indicates that the ß-phase has approximately the same Ti/Al ratio as the α₂-Ti₃Al phase, and is stabilized within the microstructure by Fe and V. Channeling enhanced microanalysis studied (ALCHEMI) show that the Fe and V both occupy the same sublattice as the Al atoms in the cubic ß-phase structure, and that Fe occupies the Al sites in the γ-phase. Fe and V are concentrated in the ß-phase, which effectively getters these dopants from the γ and α₂ phases. Titanium ferride precipitates, probably FeTi, have been observed within the ß-phase.     

Mechanical behavior of Al−Li−SiC composites: Part I. Microstructure and tensile deformation

The microstructure and tensile properties of an 8090 Al−Li alloy reinforced with 15 vol pet SiC particles were investigated, together with those of the unreinforced alloy processed following the same route. Two different heat treatments (naturally aged at ambient temperature and artificially aged at elevated temperature to the peak strength) were chosen because they lead to very different behaviors. Special emphasis was given to the analysis of the differences and similarities in the microstructure and in the deformation and failure mechanisms between the composite and the unreinforced alloy. It was found that the dispersion of the SiC particles restrained the formation of elongated grains during extrusion and inhibited the precipitation of Al₃Li at ambient temperature. The deformation processes in the peak-aged materials were controlled by the S′ precipitates, which acted as barriers for dislocation motion and homogenized the slip. Homogeneous slip was also observed in the naturally aged composite, but not in the unreinforced alloy, where plastic deformation was concentrated in slip bands. The most notorious differences between the alloy and the composite were found in the fracture mechanisms. The naturally aged unreinforced alloy failed by transgranular shear, while the failure of the peak-aged alloy was induced by grain-boundary fracture. The fracture of the composite in both tempers was, however, precipitated by the progressive fracture of the SiC reinforcements during deformation, which led to the early failure at the onset of plastic instability.     

Mechanical milling of gas-atomized Al−Ni−Mm (Mm=misch metal) alloy powders

Al−14Ni−14Mm (Mm=misch metal) alloy powders rapidly solidified by the gas atomization method were subjected to mechanical milling (MM). The microstructure, hardness, and thermal stability of the powders were investigated as a function of milling time using X-ray diffraction (XRD), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC) methods. In the early stages of milling, a cold-welded layer with a fine microstructure formed along the edge of the milled powder (zone A). The interior of the powder remained unworked (zone B), resulting in a two-zone microstructure, reminiscent of the microstructures in rapidly solidified ribbons containing zones A and B. With increasing milling time, the crystallite size decreased gradually reaching a size of about 10 to 15 nm and the lattice strain increased reaching a maximum value of about 0.7 pct for a milling time of 200 hours. The microhardness of the mechanically milled powder was 132 kg/mm² after milling for 72 hours and it increased to 290 kg/mm² after milling for 200 hours. This increase in microhardness is attributed to a significant refinement of mcirostructure, presence of lattice strain, and presence of a mixture of phases in the alloy. Details of the microstructural development as a function of milling time and its effect on the microhardness of the alloy are discussed.     

Observations on the faceted fatigue fracture of cast Co−Cr−Mo alloy used for surgical implants

Fatigue crack propagation behavior of cast Co−Cr−Mo alloy used for surgical implants is studied at room temperature with the stress intensity factor range, ΔK, between 20 and , corresponding to a crack growth rate,da/dN, between 10⁻⁵ and 10⁻² mm/cycle. Faceted fatigue fractures are observed to be the dominant fracture features of the alloy under all tested conditions. The characteristics of the faceted fatigue fractures are discussed and identified. The development of facets seems to be a consequence of the low stacking fault energy of the alloy since this results in a dense structure of stacking fault intersections. Grain size has little effect on the occurrence of faceted fatigue fractures in cast Co−Cr−Mo alloy. Modifying the composition of the base alloy with Ni-additions increases the fracture ductility of the alloy.     

Effect of Heat Treatment in the Temperature Range 400−1300°C on the Properties of Nanocrystalline ZrO2−Y2O3−CeO2 Powders

Powder Metallurgy and Metal Ceramics - The properties of nanocrystalline powders of compositions (mol.%) 97 ZrO2–Y2O3, 95 ZrO2–3 Y2O3–2 CeO2, 92.5 ZrO2–2.5 Y2O3–5...     

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. MINORU NEMOTO, formerly Professor, Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University.     

Copper solubility in FeO−Fe2O3−SiO2−Al2O3 slag and distribution equilibria of Pb,Bi, Sb and As between slag and metallic copper

The solubility of copper in silica-unsaturated fayalite slags containing an average of about 8 pct Al₂O₃ was measured by equilibrating the slag with metallic copper at 1200 and 1300°C under CO?CO₂ atmospheres with oxygen potentials in the rangep _{O 2}=10^?6 to 10^?11 at. The copper solubility, which was found to be dependent upon the oxygen potential, was expressed in terms of the Fe/SiO₂ ratio, temperature and activity of CuO_0.5. The distribution of Pb, Bi, Sb and As between copper and slag was measured concurrently by doping the metallic copper. The distribution coefficient was defined by (mole fractionX in metal)/(mole fractionX in slag) assuming the FeO?FeO_1.5?SiO₂?AlO_1.5?CuO_0.5 system slag. The distribution coefficient for lead was found to be a function of the oxygen potential, and a PbO activity in the slag of 0.07±0.01 was measured over the range of 1200 to 1300°C. Dissolution of Bi, Sb and As in the slag was found to be independent of the oxygen potential suggesting atomic rather than oxidic dissolution. The observed distribution coefficient for Bi and Sb was 30. The observed distribution coefficient for As was 300, but this is subject to error up to one order of magnitude due to analytical uncertainty of slag. The data are useful in analyzing minor element behavior in copper smelting processes.     

Multiphase diffusion in Fe−Ni−Al system at 1000°C: II. Interdiffusion coefficients for β and γ alloys

Ternary interdiffusion coefficients were determined at 1000°C at several Fe−Ni−Al alloy compositions with multiphase β(bcc)vs γ (fcc) diffusion couples which developed planar β/γ-interfaces. The coefficients, (i,j=Al or Ni) were calculated at compositions corresponding to points of intersections of diffusion paths with Fe taken as the dependent component. These coefficients varied with composition by 1 to 2 orders of magnitude in the β-phase but relatively little in the γ-phase. Empirical relations were derived to describe the composition dependence of the main coefficients. and . Interdiffusion coefficients with either Al or Ni as the dependent component were also evaluated. The relative diffusivities of the elements increase in the order, Fe, Ni, Al for both β- and γ-alloys. The ternary diffusion data were consistent with binary interdiffusion coefficients for Fe−Al and Fe−Ni alloys. G. H. CHENG, formerly a Graduate Student at Purdue University     

Lattice correspondence and fivefold twins of the orthorhombic (2/1, 1/1) and (1/0, 2/1) approximants in a Ga−Fe−Cu−Si alloy

In the rapidly solidified and annealed Ga₄₆Fe₂₃Cu₂₃Si₈ alloy, the base-centered orthorhombic (2/1, 1/1) and (1/0, 2/1) approximants of the decagonal quasicrystal, with lattice parameters of a=2.00 nm, b=1.25 nm, and c=1.40 nm and a=0.76 nm, b=1.25 nm, and c=2.40 nm, respectively, were found to coexist with a definite lattice correspondence relationship. At the interphase boundary, their {101} planes and the 〈101〉 directions in these planes coincide. There are four possible types of lattice correspondences which have been confirmed by electron diffraction patterns as well as by high-resolution electron microscopy (HREM) images. Moreover, such lattice correspondence relationships can be accounted for by the structural subunits with angles of n×72 deg in these two orthorhombic approximants: flattened hexagons in the (1/0, 2/1) approximant, and octagons called “crown” subunits in the (2/1, 1/1) approximant. Various tilings of the crown subunits are discussed. In addition, fivefold rotational twins occur abundantly in both of these approximants and intergrowth between them is common. This can also be explained by their respective characteristic structural subunits. S.P. GE, formerly Graduate Student, and K.H. KUO, Professor, Department of Materials Physics, University of Science and Technology Beijing, 100083 Beijing, China