Deformation of Nb-W single crystals: Athermal solid solution strengthening

The athermal component of the flow stress of single crystals of niobium and four substitutional solid solution strengthened Nb-W alloys (1, 3, 6, and 15 at. pct) has been measured. The magnitude of the athermal stress was obtained by the method utilizing the temperature dependence of the flow stress and by a stress relaxation method. The agreement between these two methods is good. Athermal strengthening is a linear function of concentration over most of the composition range. No current athermal hardening theory is completely consistent with the experimental results. Formerly with the Advanced Materials Research and Development Laboratory, Pratt and Whitney Aircraft, Middletown, Conn.,     

Plastic deformation of Ni-Cr single crystals

Single crystals of nickel containing up to 21.5 at pct Cr have been deformed in tension over the temperature interval 78 to 623°K. The critical resolved shear stress of alloys containing up to 9 at. pct Cr in the region of the athermal plateau can be fitted into aC ^1/2 andC ^2/3 relationship whereC is the concentration of chromium. The observed magnitude of hardening, however is larger than that predicted by the theory of Fleischer or that of Labusch. Rapid solution hardening in alloys having 9 to 21.5 pct Cr arises due to the presence of short range order. The work hardening parameters are examined and the temperature and strain rate dependence of the stress at the onset of dynamic recovery has been used to evaluate the stacking fault energy as a function of chromium content.     

High-temperature strength and ductility increases of Ti-Mo-Al alloys by step aging

Step-aging programs, based on principles of particle-dislocation interactions, were developed systematically to obtain increases in the high-temperature strength and ductility properties of Ti-7 at. pct Mo-Al alloys. A triple-step aging program applied to Ti-7 Mo-16 Al produced a yield stress σ_0.2 = 1,500 MN/m², elongation to fracture ε _F = 4 pct at room temperature, and σ_0.2 = 900 MN/m², ε _F = 12 pct at 600°C. A two-step aging program resulted in σ_0.2 = 1,350 MN/m², ε _F = 5 pct at room temperature; σ_0.2 = 800 MN/m², ε _F = 20 pct at 600°C. Formerly Assistant Research Professor, Materials Research Laboratory, Rutgers University     

Ordering transformations and mechanical properties of Ti3Ai and Ti3Al-Nb alloys

Phase transformations and the kinetics of domain growth were studied in near stoichiometric Ti₃Al and in a similar alloy containing about 5 at. pct Nb (Cb). The alloys were quenched from the β and from the α+ β fields and were subsequently annealed in the α₂ field to study the ordering transformation. The critical temperature (T _c) for ordering was found to be between 1125 and 1150° for both alloys. When quenched from aboveT _c the microstructure of the stoichiometric compound contained massive martensite with small antiphase domains of average size 8 × 10^— μm. On annealing the quenched structures in the range 700 to 1000°, domain coalescence occurred, the domains growing approximately as the square root of the annealing time. The activation energy for the domain growth process was found to be 64.6 ± 6 Kcal/mole (2.68 ± 0.25 × 10⁵ J/mole). On quenching the alloy containing Nb the β transforms to a fine acicular martensite. On annealing, antiphase domain coalescence within the martensite plates and the simultaneous recrystallization of the martensite resulted in a fine subgrain structure even after annealing at 900° for up to 3 h. The mechanical properties and the fracture modes of the two alloys tested at 700° were correlated with the observed microstructural changes. The effects of Nb in this alloy are to slow the domain growth kinetics, to reduce the planarity of slip, and to increase nonbasal slip activity. Formerly NRC Research Associate in the Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH     

The thermally activated dislocation motion in neutron irradiated iron-nickel alloys

Polycrystalline samples of iron-nickel alloys with various interstitial impurity levels were irradiated to a fluence of 1.6 × 10¹⁹ n/cm² (E > 1 MeV). The temperature and strain rate dependence of the yield stress of the alloys have been investigated in the temperature range of 77 to 580°K. The activation parameters have also been evaluated and used to identify the rate controlling mechanisms of the dislocation motion. It was observed that above 300°K the thermally activated dislocation motion over the neutron-produced obstacles is the rate controlling mechanism for the irradiated samples. This observation is in agreement with a prediction made by Arsenault. The effect of post-irradiation annealing on the temperature dependence of the yield stress becomes significant above 400°K; therefore, by the single strain rate change tests there is no way to determine the athermal stress for the irradiated samples. The motion of double kinks in dislocations over the Peierls’ stress hill is the lower temperature rate controlling mechanism before and after neutron irradiation.     

Low cycle fatigue behavior of polycrystalline Ni3Al alloys at ambient and elevated temperatures

The low cycle fatigue (LCF) resistance of polycrystalline Ni₃Al has been evaluated at ambient, intermediate (300 °C), and elevated (600 °C) temperatures using strain rates of 10⁻²/s and 10⁻⁴/s. Testing was conducted on a binary and a Cr-containing alloy of similar stoichiometry and B content (hypostoichiometric, 200 wppm B). Test results were combined with electron microscope investigations in order to evaluate microstructural changes during LCF. At ambient and intermediate temperatures, the cyclic constitutive response of both alloys was similar, and the LCF behavior was virtually rate independent. Under these conditions, the alloys rapidly hardened and then gradually softened for the remainder of the life. Initial hardening resulted from the accumulation of dislocation debris within the deformed microstructure, whereas softening was related to localized disordering. For these experimental conditions, crack initiation resulted within persistent slip bands (PSBs). At the elevated temperature, diffusion-assisted deformation resulted in a rate-dependent constitutive response and crack-initiation characteristics. At the high strain rate (10⁻²/s), continuous cyclic hardening resulted from the accumulation of dislocation debris. At the low strain rate (10⁻⁴/s), the diffusion of dislocation debris to grain boundaries resulted in cyclic softening. The elevated temperature LCF resistance was determined by the effect of the constitutive response on the driving force for environmental embrittlement. Chromium additions were observed to enhance LCF performance only under conditions where crack initiation was environmentally driven. Formerly Postdoctoral Research Fellow, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA     

Low temperature strengthening in niobium-hydrogen single crystals

The mechanical behavior of high purity niobium and niobium-hydrogen single crystals at temperatures in the range 77° to 380°K was investigated. Thermally activated hardening by hydrogen in solution is appreciable at temperatures ≲150°K. However, hydride precipitation occurs at these temperatures, can be strain-induced during mechanical testing, and accounts for the largest part of the decrease in ductility in alloys with up to 720 at. ppm of hydrogen. The influence of hydrogen additions and the effects of purification on the low temperature flow stress of niobium demonstrate the large influence of interstitial solutes on the strength of bcc materials at low temperatures. Formerly Graduate Assistant, Division of Metallurgy and Materials Science, Case Western Reserve University, Cleveland, Ohio     

Solid Solution Strengthening for Mg-3.0 Mass?Pct (2.71 At.?Pct)Al and Mg-0.06 Mass?Pct (0.036 At.?Pct)Ca Alloys

Tensile tests were carried out at 123 K to 373 K (–150 °C to 100 °C) on pure Mg, Mg-3.0 mass pct (2.71 at. pct) Al alloy, and Mg-0.06 mass pct (0.036 at. pct) Ca alloy. Little decrease occurred in the yield stress of the pure Mg and the Mg-Ca alloy with increasing temperature from 223 K to 373 K (–50 °C to 100 °C). For the Mg-Al alloy, however, its yield stress decreased with increasing temperature from 223 K to 373 K (–50 °C to 100 °C). Analyses based on the existing solid-solution strengthening theories, focusing on the athermal component of stress, revealed that the dominant strengthening mechanism is the shear modulus effect for the Mg-Ca alloy and the chemical interaction for the Mg-Al alloy. It is suggested that the shear modulus effect is dominant at a low concentration and the chemical interaction is dominant at a high concentration for Mg alloys.     

Electron microscope study of stacking fault energies in Cu-11.5 and 16.0 At. Pct Al alloys at temperatures between 20 and 700°C

The temperature dependence of stacking fault energies in Cu-11.5 and 16.0 at. pct Al alloys was determined by means of the extended node technique using a hot-stage in a 500 kV electron microscope. The temperature range was from 20 to 700°C. The stacking fault energy increased markedly with increasing temperature above ~200°C in both alloys (13.6 erg/cm² and 16.2 erg/cm² at 20°C and 500°C in Cu-11.5 at. pct Al alloy and 11,7 erg/cm² and 16.9 erg/cm² at 20°C and 700°C in Cu-16.0 at. pct Al alloy, respectively). The increasing rate of the stacking fault energies was 0.9 ~ 1.0 erg/cm²/100°C at temperatures above ~200°C in both alloys. Formerly Research Fellow at Tohoku University     

Characterization and modeling of the high temperature flow behavior of aluminum alloy 2024

Isothermal flow curves were determined for aluminum alloy 2024-0 at temperatures of 145 to 482 °C and at constant true-strain rates of 10^-3 to 12.5 s^-1 using compression tests of cylindrical specimens. The average pressure was corrected for friction and for deformation heating to determine the flow stress. At 250 °C and above, the isothermal flow curves usually exhibited a peak followed by flow softening. At 145 °C the flow curves exhibited strain hardening. For 250 °C≦ T<= 482 °C, 10^-3 s^-1 ≦ ≦ 12.5 s^-1, and ε ≦ 0.6 the flow behavior was represented by the constitutive equation σ =K (T, ε) where logK andm are simple functions of temperature and strain. The as-deformed microstructures generally supported the idea that flow softening in Al 2024-0 is caused by dynamic recovery. At the higher temperatures and strain rates, however, fine recrystallized grains were observed in local areas near second phase particles and at as-annealed grain boundaries. At 482 °C, there was evidence of re-dissolution of the CuMgAl₂ precipitate. Formerly Visiting Associate Professor, Wright State University, Dayton, OH 45435 Formerly a Mechanical Systems Engineering Student at Wright State University Formerly a Materials Engineering Student at Wright State University Formerly Director, Metallurgy Program, National Science Foundation, Washington, DC     

Steady-state electrotransport of carbon in nickel and in nickel-copper and iron-copper alloys

For comparison with previous data on iron-carbon alloys, steady-state electrotransport experiments were performed on three alloys: Nickel-0.15 wt pct carbon, nickel-20 copper-0.13 carbon, and iron-7.2 copper-0.10 carbon. The current density was 2.20 to 2.85 × 10⁷ AJm² and the temperature was 1300 K (and also 1200 K for the nickel-carbon alloy). The Z* values obtained were +5.7, +5.7, and +4.5 respectively, for the three alloys. Present theory appears to be unable to explain the relation of these data to similar data on iron-carbon alloys. Formerly Postdoctoral Fellow, Department of Metallurgical and Materials Engineering, University of Florida, Gainesville, Florida     

The effect of strain rate on the cyclic response of metals

Monotonie and cyclic strain hardening parameters were measured for five alloys: 7075-T6 aluminum; 6061-T651 aluminum, an α/β brass (alloy 365); copper-l. pct beryllium (alloy 172); and martensitic 4340E steel. Cyclic measurements were made under fully reversed strain control at constant strain rate. A maximum strain of ±0.015 and strain rates of 0.003, 0.03, 0.10, and 0.30 s^-1 were used. The cyclic stress-strain coefficientsK’ and exponentsn’ were found to be independent of strain rate and had the values: 7075-T6 (105 ksi, 0.06); 6061-T651 (60 ksi, 0.06); brass (132 ksi, 0.21); copper-beryllium (165 ksi, 0.13); 4340E steel (300 ksi, 0.19). Adiabatic heating imposed an upper limit < 0.3 s^-1) on the strain rate at which isothermal cyclic strain hardening tests could be performed. Formerly with Midwest Research Institute     

Viscosity of PbO-SiO2 melts

Viscosities (η) of PbO-SiO₂ melts that contained 25.0 to 48.8 mole pct SiO₂ were measured at temperatures 928 to 1273 K by a rotating cylinder method. The data were analyzed in terms of the conventional polymer theory. The results followed Arrhenius behavior in the different temperature ranges despite the general belief of non-Arrhenius behavior of viscosity in slag melts. The calculated activation energies were a function of temperature and composition of the melts. The activation energies for viscous flow, in general, at above approximately 850 °C were lower than those below 850 °C and varied between 25 and 150 kJ per mole. Formerly with National Research Council of Canada, Atlantic Research Laboratory, Halifax, NS, Canada     

Yielding behavior of a γy-precipitation strengthened Co Ni Cr Nb Fe alloy

The yielding behavior of a 35 Co, 35 Ni, 15 Cr, 12 Nb, 3 Fe alloy, precipitation strengthened by an ordered bct γ phase has been studied. It was observed that during the early stages of aging at 973 K γ precipitation particles are sheared by the glide dislocations which occur in pairs. During this stage both order as well as coherency strain hardening mechanisms seem to be operative. Upon continued aging, when γ phase starts to transform to a stable ordered orthorhombic β phase, this alloy does not lose strength but its ductility is reduced. The strengthening due to β phase is attributed to the dislocation bypassing mechanism. On sabbatical leave with Aluminum Company of Canada, Research Centre, Kingstron, Ontario until June 30, 1981 Formerly Research Associate in the Metallurgical Science Laboratory, Department of Mechanical Engineering, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2. Formerly Research Associate in the Metallurgical Science Laboratory, Department of Mechanical Engineering, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2.     

Effect of nitrogen on the strength of thorium

The effect of nitrogen in solid solution on the plastic deformation of polycrystalline thorium was studied by measuring the flow stress from 4.2 K to 773 K at several strain rates. Nitrogen was found to behave similarly to carbon in thorium and increased the thermally activated component of the flow stress. The activation energy was 1.3 eV for overcoming nitrogen atoms in solution. Aging the higher nitrogen alloys produced an age hardening contribution to the flow stress which was a thermally activated component at about 600 K but became an athermal component at lower temperatures. D. R. McLACHLAN, formarly a Graduate Assistant, Ames Laboratory, ERDA, Iowa State University.     

Athermal Solid Solution Hardening in Tantalum

The influence of solute atoms on the athermal component of the flow stress, determined by means of dip-tests (incremental unloading), has been investigated at room temperature and slightly above in binary Ta-Re, Ta-Mo, Ta-W, Ta-Hf, Ta-Zr, and Ta-Nb alloys and in ternary Ta-W-Re, Ta-W-Mo, Ta-W-Hf, and Ta-W-Nb alloys. Binary athermal substitutional solid solution hardening in tantalum is linear up to high concentrations of solute and is dominated by the atomic size misfit parameter, in agreement with the authors’ recent model for binary athermal solid solution hardening in bcc metals at temperatures where the Peierls stress is still important. In this model, solid solution hardening is caused by interactions of solute atoms having a size misfit with polarity reversing kinks and constrictions in 〈111〉 screw dislocations. The observed solid solution hardening in the ternary alloys is well described by the authors’ phenomenological model for multicomponent solid solution hardening. L.A. GYPEN, formerly with the Departement Metaalkunde, Katholieke Universiteit Leuven, Belgium     

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.     

Physical metallurgy of metastable bcc lanthanide-magnesium alloys for R = La,Gd, and Dy

Bcc La-Mg, Gd-Mg, and Dy-Mg alloys have been prepared by an ice water/acetone quench from liquid melts. Single-phase alloys could be retained in a window around the eutectoid composition: 13 to 22 at. pct Mg, 23.6 to 29 at. pct Mg, and 27 to 29 at. pct Mg for La, Gd, and Dy alloys, respectively. At the center of the windows, X-ray diffraction peaks are extremely sharp as in equilibrium bcc structures; however, as alloy composition is moved away from the eutectoid, line broadening is observed. Reversion of the bcc phase to the equilibrium micro-structure for R-Mg alloys (R = La, Gd, or Dy) has been characterized by differential thermal analysis (DTA) or differential scanning calorimetry (DSC) and isothermal annealing. La-Mg alloys revert directly to c~La (dhcp) + LaMg at about 350 °C when heated at 10 °C/min. In contrast, the Gd and Dy alloys revert by a two-step process: first, a transition to an intermediate distorted hcp phase between 300 °C and 400 °C and, second, the relaxation of this phase to αR (hcp) + RMg at about 490 °C when heated at 10 °/min. Isothermal annealing and high temperature X-ray diffraction confirm the nature of these reactions. Formerly Graduate Student, Ames Laboratory and Department of Materials Science and Engineering, Iowa State University Formerly Postdoctoral Associate, Ames Laboratory     

A model to predict carburization profiles in high temperature alloys

A mathematical model has been developed which, by means of finite difference computation techniques, permits the prediction of carbon concentration profiles in carburized high temperature alloys. It is assumed that a proportion of the carbon which diffuses into the alloy reacts with elements such as chromium to form carbide precipitates. The amount of carbon remaining in solution is determined from the solubility product of the carbide. Only this carbon in solution is able to diffuse through the alloy matrix, and thus the carbide precipitation process reduces the rate of carburization. Applying the model, the diffusion coefficient of carbon in Alloy 800 H at 900 °C has been determined as (3.3 ± 0.5) × 10⁻⁸ cm²/s. The model can also treat the carburization of an alloy containing two carbide-forming elements, but application to alloys containing both chromium and niobium (columbium) was successful only to a limited extent, probably as a result of the slow, complex kinetics of carbide precipitation. The model can be used to adapt carbon concentration profiles from one geometrical configuration to another. On the basis of profiles determined experimentally on small, cylindrical test specimens, carbon concenration profiles have been predicted for thick section tubes of Alloy 800 H exposed to a carburizing environment for up to 100,000 h. Formerly of the Institute of Reactor Materials, Nuclear Research Centre (KFA), Jülich     

Plastic flow in binary substitutional alloys of BCC iron-effects of wire drawing and alloy content on work hardening and ductility

The strength of cold-drawn, titanium-gettered iron wires can be substantially increased by substitutional solutes. For the elements studied, strengthening is progressively less in the order Si, Pt, Mn, Ni, Cr, and Co. The strengthening effect of the solute increases with strain, but at a greatly diminishing rate for true strains greater than unity. Six at. pct Si reduces the strain necessary to achieve a tensile strength of 200,000 psi (1380 MN/m²) from 7.3 for iron to 3.7. This effect of alloying on strain hardening appears to be related to the strength of the annealed alloy rather than to the specific alloying element used to achieve that strength. Also, the reduction-of-area ductility of the drawn wires is more closely related to the tensile strength of the wire than to its alloy content or degree of cold work. A fibrous cellular substructure is formed in all the alloys, but the formation of these cells is displaced to higher strains, the greater the strengthening effect of the solute. The transition from homogeneously distributed, tangled dislocations to a cellular substructure has no effect on the rate of strain hardening of the alloy-alloying can be used effectively as a substitute for cold work without adversely affecting the resistance of the alloy to ductile failure. Formerly with the U.S. Steel Research Laboratory, Monroeville, Pa. Formerly with the U.S. Steel Research Laboratory