Correlation of structure and superconducting properties in Nb(Cb)-Ti quaternary alloys

The aging behavior of a Nb-Ti alloy containing 60 wt pct Ti and small additions of oxygen and erbium or scandium was characterized and related to superconducting properties. The ternary and quaternary alloys were cold reduced and aged for various times at temperatures between 250° and 1000°C. ω and α phase transformations and oxide precipitation processes were followed by lattice parameter, diffraction intensity, resistivity, and metallographic studies, and correlated with superconducting critical magnetic field and critical current density measurements. The optimum 1 hr aging temperatures for producing ω and α phase precipitation were found to be 400° and 500°C, respectively. Aging at 1000°C produced only oxide precipitation. It was found that oxygen, erbium, and scandium stabilize the α phase but have little effect on ω precipitation. The ω phase proved the most effective fluxoid pinning precipitate. The fine scale dispersoid provided an extremely high number density of pinning sites. Formerly Research Assistant, Department of Metallurgy and Materials Science, M.I.T., Cambridge, Mass. Formerly Research Assistant, Department of Metallurgy and Materials Science, M.I.T. Formerly Research Associate, Department of Metallurgy and Materials Science, M.I.T.     

Infiltration of fibrous preforms by a pure metal: Part III. capillary phenomena

Infiltration experiments on aluminum cast into SAFFIL alumina fiber preforms containing a silica binder and of fiber volume fraction varying from 10 to 25 pct are reported. Data are compared with the theory presented in Part I and used to characterize wettability of the preforms by plotting the infiltrated length of composite divided by the square root of time as a function of applied pressure. The intercept of the resulting curves with the abscissa axis is shown to be a measurement of the capillary pressure needed to fully infiltrate the fiber preforms. Resulting experimental values of this capillary pressure are then used with Brunauer, Emmett, and Teller (BET) adsorption isotherm measurements of the preform’s specific surface to derive an apparent wetting angle of the fibers by aluminum during infiltration. In this manner, the effective wetting angle of pure aluminum on the alumina/silica fibers is found to be 106 ±5 deg, independent of fiber preform temperature. We also propose a mechanism for preventing preform compression during infiltration. Formerly Undergraduate Student with the Department of Materials Science and Engineering, Massachusetts Institute of Technology.     

Structure and properties of a splat cooled 2024 aluminum alloy

Pound lots of splat cooled 2024 aluminum flake materials were produced by rapidly quenching the atomized melt against a rotating copper disc. Three flake sizes were selected, cold compacted into aluminum cans, and extruded at 300°C at a reduction ratio of 20 to 1. The extruded rods were reduced 50 pct by cold swaging, solution treated at 495°C, water quenched, and naturally aged. The splat cooled 2024 alloy had constituent particles of 1 fim and finer (compared to 5 to 20 μm for the commercial alloy); further, one of the complex constituent phases (AlCuFeMn) was essentially eliminated by the rapid quench. Compared to commercial 2024-T4, the splat cooled 2024 alloys showed 14 to 17 pct increase in yield and tensile strength (no loss of ductility) a seven-fold increase in fatigue life at 30,000 psi, and a large improvement in the 300°F (150°C) stress rupture life in tests beyond 100 h. The fracture characteristics of the splat alloys, while exhibiting excellent to superior ductility, appear inhomogeneous due to the presence of finely dispersed oxide films scattered in the structure. Formerly Research Assistant, Department of Metallurgy and Materials Science, Massachusetts Institute of Technology     

The effect of interstitials on phase equilibria in P[Ti,Zr, Hf]-X[β-stabilizer (s)] alloy systems

Disagreements existing in the literature concerning phase equilibria inP [Ti,Zr,Hf]-X [β-stabilizer(s)] alloy systems are explained on the basis of three features believed to be characteristic ofP [Ti,Zr,Hf]-X [β-stabilizer(s)]-I [P,N,C] systems. Formerly Research Associate, Department of Materials Science, University of Southern California, Los Angeles, CA. Formerly Assistant Professor, Department of Materials Science, University of Southern California, Los Angeles     

Measurements of the electrical conductivity of Wood's alloy and other low melting point alloys

Formerly Graduate Student Research Assistant, Department of Materials Science and Mineral Engineering, University of California.     

Effects of preinjected helium in heavy-ion irradiated nickel and nickel-copper alloys

Pure nickel and two nickel-copper alloys (Ni-10 at. pct Cu and Ni-50 at. pct Cu) containing 50 appm preinjected helium have been irradiated with 14 MeV nickel ions at a constant homologous temperature of 0.45T _m to 3 to 5 dpa at a depth of 1 μm from the surface. The radiationinduced lattice defects have been analyzed by transmission electron microscopy (TEM) with samples prepared in cross section. In the helium preinjected region of the pure nickel specimen, a substantial density of voids with an average diameter of 35 nm was observed. The nickelcopper alloys were found to contain only a high density of small helium bubbles (under 5 nm in diameter) and dislocation loops. The density of both dislocation loops and helium bubbles increases with increasing copper content, while the size decreases. The observed resistance of the nickel-copper alloys to void formation regardless of the presence of helium bubbles is considered to be the result of local clustering of like atoms which generates a high density of traps, and, thereby, disperses vacancies and gas atoms. Formerly Research Assistant, Materials Science Program, University of Wisconsin-Madison. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, Illinois, September 25-29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

The structure and properties of aluminum alloys produced by mechanical alloying: Powder processing and resultant powder structures

Mechanical alloying of two aluminum alloy powders to form composite A1-A1₂O₃ powders has been studied. Changes in powder microstructure with processing are reported and interpreted. Mechanical alloying proceeds by the continual cold welding and fracturing of the constituent powder particles when subjected to the large compressive forces of a high speed mill. A suitable organic surfactant must be added so that a balance between cold welding and fracturing is obtained. The organic surfactant is embedded and finely distributed in the powder particles during mechanical alloying and is converted to discrete A1₄C₃ particles after hot pressing. The establishment of steady state processing conditions, characterized by equiaxed powder particles, a constant particle size distribution and a saturation hardness, is found to depend on the size distribution of the initial powders. The oxide particles formed and distributed during mechanical alloying are equiaxed, small (30 nm) and homogeneously distributed with a volumetric center to center distance of about 60 nm. Formerly Graduate Research Assistant in the Department of Materials Science and Engineering at Stanford.     

On the characteristics of the threshold stress for superplastic flow in Zn-22 Pct Al

Formerly Graduate Research Assistant, Department of Chemical and Biochemical Engineering, Materials Science and Engineering, University California-Irvine.     

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

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

Effect of grain boundaries on isothermal solidification during transient liquid phase brazing

The influence of liquid penetration at grain boundary regions on the rate of advance of the solid-liquid interface during isothermal solidification of transient liquid phase (TLP) brazed nickel joints has been examined. The test samples used in this study were Ohno-cast nickel with a grain size of >4 mm and a fine-grained nickel with a grain size of around 40 μm. Both Ni-base materials had the same chemical composition. The rate of isothermal solidification was greater when fine-grained nickel was employed during TLP brazing using Ni-11 wt pct P filler metal at 1200 °C. Liquid penetration at grain boundaries accelerates the isothermal solidification process by increasing the effective solid-liquid interfacial area and increasing the rate of solute diffusion into the base material. An analysis of electron channeling patterns has confirmed that random high-angle boundaries have a greater influence on the rate of isothermal solidification than ordered boundaries including small-angle or twin boundaries. Formerly Visiting Scientist, Department of Metallurgy and Materials Science, University of Toronto. Formerly Postdoctoral Fellow, Department of Metallurgy and Materials Science, University of Toronto     

A noncrystalline Pt-Sb phase and its equilibration kinetics

Utilizing splat cooling, a metastable noncrystalline phase was obtained in Pt-Sb alloys with 30 to 43 pct Sb and in Pt-Si alloys with the eutectic composition Pt-68 Si. These nonequilibrium structures can be retained indefinitely at room temperature. The kinetics of the decomposition of the metastable noncrystalline phase in a Pt-34 pct Sb alloy was studied in detail by X-ray diffraction, electron diffraction, and electron transmission microscopy. The decomposision starts at an appreciable rate at about 210°C, and progresses gradually with the precipitation of relatively equiaxed grains of crystalline phases. From isothermal kinetic data, average activation energies from 55.5 to 49.5 (±4) kcal per mole were derived for the overall decomposition process. Formerly Research Assistant, Department of Metallurgy and Materials Science, Massachusetts Institute of Technology, Cambridge, Mass. Formerly Research Associate, Department of Metallurgy and Materials Science, M.I.T.     

The effect of tempering temperature on near- threshold fatigue crack behavior in quenched and tempered 4140 steel

Fatigue crack growth in compact tension samples of high purity 4140 steel quenched and tempered to various strength levels was investigated. Tempering temperatures of 200, 400, 550, and 700 °C produced yield strengths from 1600 to 875 MPa, respectively. Crack propagation and crack closure were monitored inK-decreasing tests performed underR = 0.05 loading conditions in laboratory air. Results indicated that as the yield strength increased the crack growth rate increased at a given ΔK and ΔK_th decreased. Threshold values varied from 2.8 MPa m^1/2 (200 °C temper) to 9.5 MPa m_1/2 (700 °C temper). Cracks in the 200 °C tempered samples grew by an intergranular mechanism following prior austenite grain boundaries probably caused by hydrogen embrittlement or tempered martensite embrittlement. Tempering above 200 °C produced transgranular fatigue crack growth. The level of crack closure increased with tempering temperature and with crack propagation in a given tempered condition. Crack closure was caused by a combination of plasticity-induced and oxide-induced mechanisms. The use of an effective stress intensity range based on crack closure consolidated the fatigue crack growth curves and the threshold values for all tempering temperatures except 200 °C. Formerly Graduate Research Assistant, Department of Materials Science and Engineering, Stanford University, Stanford, CA. Formerly Professor, Department of Materials Science and Engineering, Stanford University, Stanford, CA.     

Retarded grain boundary mobility in activated sintered molybdenum

Rapid grain growth accompanies the enhanced sintering of molybdenum treated with nickel additions. Grain growth is detrimental to sintering kinetics and mechanical properties. A sintering model is developed to illustrate that reducing grain boundary mobility is a means to increase the densification rate. A fine silica dispersion is added to molybdenum powder which is activated by the addition of nickel. This powder exhibits a long term sintering benefit due to retarded grain growth which is attributed to dispersoid drag effects on grain boundaries. These experimental powders are further analyzed through precision dilatometry, showing a characteristic shift in shrinkage rate during constant heating rate experiments. The shrinkage rate of molybdenum is increased by a factor of 10 at 1000°C when activated with 0.37 pct Ni. The shrinkage rate of nickel activated molybdenum is further increased by 67 pct with 1400 ppm silica dispersed at the interparticle grain boundaries. This paper is based on a presentation delivered at the symposium “Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds” held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME. Formerly Research Assistant in the Rensselaer Polytechnic Institute, Materials Engineering Department     

Determination of the sodium activity in aluminum and aluminum silicon alloys using sodium beta alumina

The construction of a sodium beta alumina probe for the determination of the sodium activity in molten aluminum alloys is described. It was found that the emf at a given sodium concentration was a strong function of the silicon content. Henry’s law was obeyed in super purity aluminum and the activity coefficient of 350 at 998 K agrees with other determinations. Formerly Graduate Student in the Department of Metallurgy and Materials Science, University of Cambridge     

Thermodynamic properties and ordering in PdAl

The activity of aluminum in the Pd-Al system was determined by an isopiestic method from 30 to 80 at. pct Al between 1090° and 1490°K. It shows a strong negative deviation from ideality, with a decrease in activity of aluminum of four orders of magnitude around the stoichiometric composition of PdAl. The defect structure in the CsCl-structure PdAl compound has been determined by lattice parameter measurements. Equations were derived relating the degree of intrinsic disorder (α) to the shape of the activity curve. Excellent agreement between the calculated curve and the activity data was obtained for anα = 2.5 × 10⁻⁴. The results from this investigation and from previous studies indicate that the degree of intrinsic disorder in the equiatomic transition metal-aluminum compounds is controlled by electronic effects. Formely Department of Metallurgy and Materials Sciences, New York University, New York, N. Y. Formerly Department of Metallurgy and Materials Sciences, New York University Formerly Department of Metallurgy and Materials Sciences, New York University This paper is based on a Thesis submitted by M. ETTENBERG to the Graduate Division, School of Engineering and Science, New York University, in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Materials Science.     

Modification of the B2-type matrix of aluminide diffusion coatings on nickel-base superalloys—bulk aluminide analogues

Pack aluminide coating is a useful method for conferring oxidation resistance on nickel-base superalloys. Nominally, these coatings have a matrix composed of a Ni-Al based B2-type phase (commonly denoted as Β). However, following high-temperature exposure in oxidative envi-ronments, aluminum is depleted from the coating. Aluminum depletion in turn, leads to de-stabilization of the Β phase, resulting in the formation of a characteristic lathlike Β-derivative microstructure. This article presents a transmission electron microscopy study of the formation of the lathlike Β-derivative microstructure using bulk nickel aluminides as model alloys. In the bulk nickel aluminides, the lathlike microstructure has been found to correspond to two distinct components: L1₀-type martensite and a new Β derivative. The new Β derivative is characterized and the conditions associated with the presence of this feature are identified and compared with those leading to the formation of the L1₀ martensitic phase. T.C. TOTEMEIER, formerly Research Student, Department of Materials Science and Metallurgy, University of Cambridge J.E. KING, formerly Lecturer, Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, CB2 3QZ United Kingdom     

Fatigue threshold studies in Fe,Fe-Si,and HSLA steel: Part I. Effect of strength and surface asperities on closure

The fatigue threshold was found to increase as the temperature decreased from 300 K to 123 K for a sequence of Fe and Fe-S binary alloys and an HSLA steel. A prominent closure was observed with the magnitude of closure stress intensity higher at lower temperature. The correlation between closure and threshold stress intensity was examined first with residual plasticity models. With geometrical asperity closure arguments, the closure stress intensity was also correlated with the yield stress and the grain size of the material. By considering the reverse slip, geometry, and oxide-induced closure simultaneously, a model for the closure at fatigue threshold was proposed and compared to the data from this study and the literature. All three mechanisms of residual plasticity, geometrical asperity, and oxide formation are shown to be relevant to the observed closure phenomena in this study. Formerly Research Assistant with the Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN. Formerly Research Assistant with the Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN.     

Influence of Interfacial Structure upon Carbide Precipitation at Austenite : Ferrite Boundaries in an Fe-C-Mo Alloy

During conventional isothermal transformation of an Fe-0.11 pct C-1.95 pct Mo alloy, eutectoid decomposition occurs by the interphase boundary carbide precipitation and the fibrous carbide mechanisms at 770° to 825 °C. When proeutectoid ferrite is formed and then recrystallized within the α + γ region, and subsequently further transformed at 770° to 825 °C, however, both of these eutectoid decomposition mechanisms are rendered inoperative. Carbide precipitation occurs instead entirely as isolated particles. This result supports the deduction that carbide precipitation at austenite : ferrite boundaries can occur only when these boundaries are locally immobilized by a partially coherent interfacial structure. A general approach to explaining the development of planar and curved interphase boundary precipitation, fibrous structure, and pearlite is developed in terms of two crystallographic factors. Formerly Research Associate in the Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931 and the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213 Formerly Graduate Student, Michigan Technological University, and Visiting Graduate Student, Camegie-Mellon University Formerly Professor at Michigan Technological University     

Capillarity in isothermal infiltration of alumina fiber preforms with aluminum

Models derived in petroleum engineering and soil science for flow of two immiscible fluids in a porous medium are extended to the infiltration of ceramic preforms by a liquid metal. SAFFIL alumina fiber preforms are infiltrated with an aluminum matrix in a series of interrupted unidirectional and isothermal experiments at various low applied pressures, to measure profiles of the volume fraction of metal along the length of the preforms. Comparison of experimental data with theory reveals the existence of a pressure-dependent incubation time for wetting of the alumina preforms by molten aluminum at 973 K. If this incubation time is taken into account, experimental curves of metal distribution are well predicted by theory, confirming the validity of the models after initiation of flow. Formerly Undergraduate Student, Department of Materials Science, Massachusetts Institute of Technology, Cambridge, MA     

In situ synthesis of TiC particulate-reinforced aluminum matrix composites

Particulate TiC-reinforced aluminum composite specimens were processed by compacting a mixture of titanium, carbon, and aluminum powders into preforms that were infiltrated with molten aluminum and subsequently heated in a differential thermal analyzer to about 1573 K under argon atmosphere. The onset of formation of TiC particles began at about 1150 K by reaction of TiAl₃ with Al4C₃. Subsequent formation of TiC particles at higher temperatures to approx-imately 1265 K occurred by direct reaction of carbon with TiAl₃. Above this temperature, the TiC particles coarsened with increasing temperature from an initial size of about 0.15μm. TiC particles were also produced in preforms that were not infiltrated; however, the presence of liquid aluminum in infiltrated specimens inhibited particle agglomeration and sintering. Infil-trated preforms could, therefore, serve as excellent "master alloys" for subsequent dilution in an aluminum melt and processing of metal-matrix composites (MMCs) reinforced with sub-micron TiC particulates. Formerly Research Scientist, Massachusetts Institute of Technology, Cambridge, MA 02139