Order-Strengthening in a nickel-base superalloy (hastelloy alloy S)

HASTELLOY* alloy s is a commercial, solid solu* HASTELLOY is a registered trademark of Cabot Corporation tion-strengthened, nickel-base superalloy developed for applications where oxidation resistance, low thermal expansion and retained ductility after long-time exposure at elevated temperatures are prerequisites. Its typical heat treatment consists of annealing at 1340 K (1950 °F) followed by air cooling to produce an essentially single phase material. When specimens from annealed heats were aged at 810 K (1000 °F) for 1000 to 8000 h and then tensile tested at room temperature, it was found that relative to the annealed condition, the 0.2 pct yield strength had nearly doubled while about 70 pct of the tensile elongation was retained. It is the objective of this note to report on the formation of a long-range ordered phase that caused the observed strengthening.     

Effects of helium on the mechanical properties and microstructure of niobium

Mechanical property specimens of niobium (Cb) were doped with helium by the tritium trick to concentrations as high as 500 appm. The tritium decays by the reaction³H →³He +β ⁻ at a rate that produces about 7 appm per day in the host microstructure. Tensile properties were measured from room temperature to 800°C, and creep properties from 700 to 1000°C at stresses from 45 to 75 MPa. Transmission electron microscopy was used to study the microstructure of the helium doped specimens, and the observations were correlated with the mechanical property results. The results of this investigation showed that niobium has a high tolerance to helium trapped in the microstructure. The tensile and creep strengths of niobium increased as helium concentration increased. The ductility decreased significantly as the helium concentration increased, but niobium retained substantial ductility even at a high helium concentration of 500 appm. This amount of helium would be generated by (n,α) reactions in the microstructure of a niobium first wall after a 20 y exposure in a D-T fusion reactor. Thus, niobium and niobium alloys are potential candidates for high temperature structural materials in D-T fusion reactors.     

Radiation-induced strengthening and embrittlement in aluminum

Commercially pure aluminum (1100 grade) in the annealed condition was irradiated to fast fluences in the range 1 to 3 × 10²⁶ neutron/m² (E > 0.1 MeV) and to similar thermal fluences at temperatures of 318 to 328 K, and was then tensile tested at temperatures between 298 and 643 K. This irradiation doubled the ultimate tensile strength and more than tripled the flow stress at all test temperatures. The work hardening exponent was severely reduced and there was a large loss in ductility. Strengthening is shown to be due to a fine precipitate of transmutation-produced silicon and an associated dislocation structure. At test tempera-tures below about 423 K the fracture mode was transgranular. Above 473 K grain boundary cavities were observed, the fracture mode become predominantly intergranular, and ductil-ity was further reduced. Unirradiated specimens containing cyclotron-injected helium showed no change in strength but displayed a loss in ductility at elevated temperatures. Concurrently holes were formed on the grain boundaries. Embrittlement in the neutron-irradiated specimens arises from two sources. One is through the defect structure which reduces the work-hardening exponent. The other is an additional effect at elevated temper-atures involving grain boundary failure by cavity growth and coalescence. Helium encour-ages cavity nucleation, the degree of cavitatlon increasing with increasing tensile strength.     

The effect of helium on the stress-rupture behavior of type 316 stainless steel

Flat tensile samples of type 316 stainless steels with helium contents of 1.5 × 10⁻⁶ and 4.0 × 10⁻⁵ atom fraction, were stress-rupture tested in vacuum at 700°C. The presence of helium caused large reductions in rupture life and in elongation at failure. The amount of strain produced within the grains of helium-containing samples was a small fraction of the measured total elongation. The difference in these values is accounted for by extensive intergranular cracking.     

Hydrogen in iron

The applicability of advanced permeation techniques to the study of hydrogen and deuterium in iron and iron alloys is described. Time lag measurements lead to detailed information about hydrogen transport processes, including lattice diffusivities and trap binding energies and densities. The experimental technique couples gas phase charging of palladium coated specimens with the sensitive electrochemical detection method. In both annealed and deformed iron the trap binding energy for hydrogen and deuterium is 50 to 58 kJ/mol, while the trap density varies from about 10²⁰ m⁻³ for annealed iron to over 10²³ m⁻³ for heavily deformed iron. For the metallic glass Fe₄₀Ni₄₀P₁₄B₆ hydrogen transport occurs between energetically equivalent sites, with no evidence of trapping. The site density was estimated as about 6 × 10²⁹ m⁻³ . The hydrogen concentrations studied were several orders of magnitude less. Hydrogen and deuterium in iron differs only in their lattice diffusivities. The diffusivity ratio conforms nearly to the classical inverse square root of mass ratio, but shows a slight temperature dependence. The solubilities, trap binding energies, and partial atomic volumes of the two isotopes in iron are identical.     

An investigation of superplasticity in a thermomechanically processed U-2Mo (α + δ) alloy

A uranium-2 molybdenum (U-2Mo) alloy was shown to exhibit superplastic behavior over the β + γ two-phase field temperature regime and over a limited temperature span in the α + γ field. At Oak Ridge, two distinct processes were developed that evolved microstructures conducive to superplasticity. These microstructures were shown to exhibit superplasticity (elongations >500 pct) over a broad range of strain rates, from 2.5 × 10^-4 to 1 × 10^-2 s^-1. A maximum value of 700 pct elongation was reached at 695 °C and a true constant strain rate of 2.5 × 10^-3 s^-1. This study details the processing sequences, microstructures, strain-rate sensitivity, and maximum elongation data generated to characterize the superplastic U-2Mo alloy. In addition, the fracture and cavitation analyses conducted on constant strain-rate tensile test specimens are discussed.     

Corrosion Compatibility Studies of Stainless Steel 304L in Flowing Liquid Lead Bismuth Eutectic

Stainless steel 304L is being considered as a structural material for some components in the lead–bismuth eutectic (LBE) target facility operating at a temperature of about 250°C. Data available on the corrosion of SS 304L in LBE for long duration exposure at these temperatures are scanty. This paper gives the corrosion behaviour of SS 304L after the exposure of 6500 h in LBE in a non-isothermal loop with oxygen concentration of ~4 × 10^?10 wt% at temperatures of 250 and 350°C and with flow velocity of 16 cm s^?1. The composition of the surface film was analysed by energy dispersive X-ray (EDAX) and X-ray photoelectron spectroscopy techniques. To record the changes in the mechanical properties of SS 304L upon exposure tensile tests of the tensile specimens exposed to LBE were performed in air at room temperature and the fractured surfaces were examined by scanning electron microscope (SEM). Changes in microstructure and elemental composition of the exposed surface of SS 304L were studied using SEM and EDAX. While no changes were observed in the mechanical properties and microstructure due to prolonged exposure to LBE at either temperature, the specimens exposed to 350°C after 6500 h showed a corrosion rate of 1.2 × 10^?3 μm h^?1. While no penetration of LBE into the grain boundaries was observed at either temperature after an exposure of 6500 h, a minor depletion of nickel was noted in specimens exposed at 350°C.     

The effect of high pressure hydrogen gas on the mechanical properties of three Ni based eutectic composites

Smooth and notched tensile bars of directionally solidified Ni-45.5W, Ni-19.7Nb-6.0Cr-2.5Al, and Ni-21.75Nb-2.55Al were tested to failure in 34.5 MN/m² hydrogen gas and 34.5 MN/m² helium gas. Decreases in the ultimate tensile strength, uniform elongation, and reduction in area values are found for the samples tested in hydrogen as compared to those tested in helium. The most marked and consistent changes in properties are found for the notched tensile bars. Since the presence of the hydrogen appears to have little effect on the shape of the stress strain curve at small strains, the notched bar test is the better test of hydrogen-induced damage in these materials. Hydrogen damage in the three materials is evidenced by cracking not usually found in samples tested in helium. The damage in the Ni-W eutectic is the most dramatic with extensive transverse and longitudinal cracking prior to failure observed. Additionally, the presence of hydrogen greatly weakens the reinforcement/matrix interface in each case, so that the interface becomes a preferred path for longitudinal cracking. In the other two alloys, grain boundary longitudinal cracking is also found. While no mechanisms have been conclusively established, the most likely source of the degradation is hydrogen trapping at the interfaces related to the presence of segregated trace element S or to the disordered nature of the interfacial region. G. GARMONG is member Technical Staff, formerly with Science Center, Rockwell International, Thousand Oaks, CA     

Peculiarities of helium porosity formation in the surface layer of the structural materials used for the first wall of fusion reactor

Transmission electron microscopy is used to study the formation of helium porosity in the nearsurface layer of ferritic–martensitic steels and vanadium irradiated by 40-keV He⁺ ions at a temperature of 923 K up to fluence of 5 × 10²⁰ He⁺/m² and, then, by 7.5-MeV Ni²⁺ ions at 923 K up to dose of 100 dpa. Large gas bubbles are found to form in the zone with the maximum concentration of radiation vacancies during He⁺ ion irradiation. Moreover, small bubbles form in some grains at the depths that are larger than the He+ ion range in the irradiated material. Sequential irradiation by He⁺ and Ni²⁺ ions leads to the nucleation of helium bubbles at still larger depths due to helium atom transport via recoil and/or ion mixing. The precipitation hardening of the steels by Y₂O₃ oxide nanoparticles is found to suppress helium swelling substantially.     

Superplastic behavior of thermomechanically treated P/M 7091 aluminum alloy

The superplastic behavior of thermomechanically treated P/M 7091 aluminum alloy was assessed in the temperature range of 573 to 773 K. The thermomechanical treatment (TMT) comprised of three steps of solution treatment, overaging, and warm rolling. There are large η-phase (MgZn₂) precipitate particles of average size of 1.30 μm in the overaged condition. The warm-rolled alloy undergoes continuous recrystallization at the test temperatures of 573 and 623 K, exhibiting a maximum tensile elongation of 450 pct at 573 K and a strain rate of 8 × 10⁻⁵ s⁻¹. The precipitate particles play a major role in the process of continuous recrystallization. For a given volume fraction of precipitate particles and constant amount of warm rolling (in the course of TMT), an optimum precipitate particle size is expected to maximize the rate of continuous recrystallization and render the finest recrystallized grain size. The warm-rolled alloy undergoes static recrystallization at temperatures above 673 K. The grain growth accompanying the deformation at these test temperatures limits the tensile ductility to a lower value. Irrespective of the test temperature and strain rate, the specimens undergo extensive cavitation when deformed at elevated temperatures.     

Short-range order effects on the tensile behavior of a nickel-base alloy

A nickel base weld filler metal alloy with nominal composition of 67 pct Ni, 20 pct Cr, 3 pct Mn, 3 pct Fe, and 2.5 pct Nb (Cb) is used to make austenitic-ferritic dissimilar metal joints. Tensile properties were determined for this alloy over the range 25 to 732°C at strain-rates of 3×10⁻⁶ and 3×10⁻⁴/s. Above about 450°C, both the yield strength and the ultimate tensile strength in the low strain-rate tests showed significant increases over the strengths at the higher strain-rate. The enhanced values for the yield strength persisted to the highest test temperature (732°C), whereas the ultimate tensile strength for the low strain-rate fell below the curve for the higher strain-rate at about 600°C. Above 600°C, the ultimate tensile strength dropped off rapidly and at 677°C approached the yield strength (i.e., the uniform elongation dropped to less than 1 pct). The strain-rate effects have been attributed to “K-state” formation, an effect that investigators have attributed to short range order in other Ni−Cr base alloys.     

HIGH-VELOCITY EXTRUSION FORGING OF BILLETS COMPACTED FROM IRON AND STEEL POWDERS

Abstract

Billets 25 mm in dia. and weighing ～135 g have been compacted from sponge-iron powder using a single-end-pressing technique. A range of densities from 5·6 to 7·2 g/cm³ was produced. The billets were then hot extrusion forged, at high speed, into tensile specimens of gauge-dia. 10 mm, gauge-length 28 mm, and head dimensions of 13 mm dia. × 12·5 mm long. About half the billets were pre-sintered before heating to forging temperature, while others were hot-forged without a pre-sintering stage. The tensile specimens were then tested and selected ones examined metallographically.

The work was extended to investigate the extrusion forging of alloy steel powder.     

Diffusivity of hydrogen in liquid nickel and copper

The diffusivity of hydrogen in liquid nickel was determined from 1468° to 1550°C by a capillary gas-reservoir technique. The diffusion cell was semiinfinite in length and consisted of Specpure nickel in an alumina capillary. An argon and hydrogen gas flow maintained a constant hydrogen potential at the metal/gas interface. A controlled furnace hot zone of length 23 cm was obtained using eight separate windings of Pt-40 pct Rh wire. The temperature profile in this zone was adjusted so that the top of the cell was hotter than the bottom, to eliminate convection. The experiments were terminated by a rapid nonaqueous quench. The diffusion columns were then sectioned and analyzed by vacuum extraction. Diffusivities were calculated using a solution to Fick’s Second Law. At 1468°C,D = 3.17 x 10⁻³ sq cm per sec with σ = ±0.76 × 10⁻³; at 1550°C,D = 3.48 × 10⁻³ sq cm per sec with σ = ±0.54 × 10⁻³. The diffusivity of hydrogen in liquid copper was determined using a shallow melt and analyzing for the total diffusate content; at 1101°C,D = 0.99 x 10⁻³ sq cm per sec with a = ±0.25 × 10⁻³; at 1201°C,D = 1.26 × 10⁻³ sq cm per sec with σ = ±0.16 × 10⁻³. Formerly with the Nuffield Research Group in Extraction Metallurgy at Imperial College, London, England This paper is based upon a thesis submitted by J. H. WRIGHT in partial fulfillment of the requirements of the degree of Doctor of Philosophy at the University of London.     

High temperature mechanical properties of vanadium alloyed γ base titanium-aluminides

High temperature mechanical properties of vanadium alloyed γ base Ti aluminides have been related to the microstructure. The microstructures were controlled by chemistry modification—vanadium alloying between 5.3 and 12.7 mol.% for phase distribution control—and isothermal forging. The properties were evaluated using tensile tests at strain rates between 3 × 10⁻⁴s⁻¹ and at temperatures between 1070 and 1420 K. The γ plus β phase intermetallic exhibited excellent hot workability with an elongation more than 600% and an m-value of 0.8. The mechanism of the superplastic deformation was critically discussed.     

Correlation of the microhardness with the tensile properties of neutron irradiated molybdenum

It is shown that the uniform elongation (ε_u) of molybdenum irradiated at reactor ambient temperature to a fast neutron fluence of about 1 X 10²⁴ nm^-2 can be correlated with parameters obtained from the contours of the plastically flowed metal adjacent to the indentation. The microhardness impression parametersh/r, h/t andt/r are based on the height(h), thickness (t) and impression radius (r) of the flowed metal. Once the tensile ductility starts to recover, following progressively higher temperature anneals of the irradiated specimens, the parameterh/r monotonically decreases with an increase in the uniform elongation. TEM observations in the region directly under the indentation show that dislocation channeling is associated with the observed nil ductility for those specimens annealed at temperatures of 700 °C and below.     

Influence of temperature transients on the hot workability of a two-phase gamma titanium aluminide alloy

The hot deformation behavior, microstructure development, and fracture characteristics of a wrought two-phase γ-titanium aluminide alloy Ti-45.5Al-2Nb-2Cr containing a fine, equiaxed microstructure were investigated with special reference to the influence of temperature transients immediately pre-ceding plastic deformation. Specimens were soaked at 1321 °C or 1260 °C, cooled directly to test temperatures of 1177 °C and 1093 °C, and upset under conditions of constant strain rate and tem-perature. Plastic flow behavior and microstructure evolution occurring in tests involving prior tem-perature transients were compared with those occurring in specimens which were directly heated to the test temperature and upset under identical deformation conditions. Flow curves associated with prior exposure at 1321 °C exhibited very sharp peaks and strong flow softening trends compared to those obtained under isothermal conditions,i.e., involving no temperature transients. During cooling from 1321 °C, the metastable α phase undergoes limited or complete decomposition into α/α₂ + γ lamellae, depending on the final temperature (1177 °C/1093 °C). Subsequent hot deformation leads to partial globularization of the lamellae together with extensive kinking and reorientation of lamellae. In contrast, isothermal deformation at 1177 °C/1093 °C preserves the fine, equiaxed microstructure, through dynamic recrystallization of the γ grains. Cracking observed in specimens deformed at 1093 °C and 1.0 s⁻¹ after exposure at 1321 °C has been attributed to the low rate of globularization as well as the occurrence of shear localization. Plastic flow behavior observed in this work is compared with that observed in several single-phase and two-phase gamma titanium aluminide alloys in order to identify mechanism(s) responsible for flow softening.     

The oxidation behavior of Ni-Cr-Al-2ThO2 alloys at 1093° and 1204°C

A pack diffusion process has been developed which permits the introduction of nearly 6 wt pct Al into solid solution in the near surface region of TDNiCr (Ni-20 wt pct Cr-2 vol pct ThO₂) and Ni-20Cr. Alumina scales, adherent under cyclic heating and cooling conditions, were produced on TDNiCr-5.86A1 upon exposure to an environment of 1.33 × 10³N/m² (10 torr) or 1.01 × 10⁵N/m² (760 torr) air at temperatures of 1093° and 1204°C. While the same oxidation kinetics were observed in isothermal tests for Ni-14.6Cr-5.86Al as were obtained for the TDNiCr-5.86A1, the dispersion strengthened alloy exhibited superior oxide scale adhesion during cyclic testing. At 1204°C continuous weight gains were observed under all test conditions for TDNiCr-5.86A1, in contrast to the weight loss with time which occurred several hours after exposure of TDNiCr to an oxidizing environment. TDNiCr with an initial aluminum surface concentration of 4.95 wt pct has nearly comparable oxidation resistance to the TDNiCr-5.86Al alloy. Specimens with 4.3 wt pct Al at the surface have inadequate aluminum to form Al₂O₃ scales, and weight losses are observed after 40 h upon exposure of these specimens to 1.01 × 10⁵N/m² (760 torr) air at 1204°C.     

Gas trapping and release in polycrystalline nickel preimplanted with helium

Hydrogen profiling using nuclear reaction analysis (NRA) and a thermal desorption technique coupled with scanning electron microscopy (SEM) observations have been used to study the gas trapping and release in high-purity polycrystalline nickel. The effect of the preimplanted helium dose on both deuterium and helium desorption was investigated over a wide range of helium doses (1 x 10²¹ to 4 x 10²¹ ions/m²). A computer code, DIFFER, was used to simulate the deuterium flux curves, and the trapping characteristics were evaluated. The simulation results clearly show that a wide distribution of trapping energies exists. This can be explained using a stress-field trapping model. The effective binding energy, E _h ^eff , was estimated to be in the range of 0.4 to 0.5 eV. For samples which were irradiated with helium ions to high doses, a massive helium release was also observed. Thermal charging with deuterium was found to reduce the helium self-trapping energy as was expressed by lower temperature helium release. For the high dose samples, deuterium or hydrogen gas charging and thermal ramping were also found to induce blisters growth and surface exfoliation.     

On the hot tensile deformation behavior of an AISI 316LN stainless steel

The hot deformation characteristics of AISI 316LN stainless steel were studied in the temperature range of 1123–1323 K and strain rate range of 10⁻⁴–10⁻¹s⁻¹ by carrying out tensile tests. The flow stress, ultimate tensile stress and percentage elongation were found to be strongly dependent on the temperature and strain rate. The critical strain required for the initiation of dynamic recrystallisation and peak strain were determined at each condition and their variation with temperature and strain rate studied. The deformation behavior was analyzed using a generic model for high temperature deformation and deformation parameters were computed. The variation of the true activation energy with strain for rate controlled high temperature tensile deformation was obtained. Microstructural studies were carried out on tested samples and the results of all the above studies are presented.     

Mixing and solid-liquid mass-transfer rates in a creusot-loire uddeholm vessel: A water model case study

The process of mixing and solid-liquid mass transfer in a one-fifth scale water model of a 100-ton Creusot-Loire Uddeholm (CLU) converter was investigated. The modified Froude number was used to relate gas flow rates between the model and its protoype. The influences of gas flow rate between 0.010 and 0.018 m³/s and bath height from 0.50 to 0.70 m on mixing time were examined. The results indicated that mixing time decreased with increasing gas flow rate and increased with increasing bath height. The mixing time results were evaluated in terms of specific energy input and the following correlation was proposed for estimating mixing times in the model CLU converter: T _mix=1.08Q ^−1.05 W ^0.35, where Q (m³/s) is the gas flow rate and W (tons) is the model bath weight. Solid-liquid mass-transfer rates from benzoic acid specimens immersed in the gas-agitated liquid phase were assessed by a weight loss measurement technique. The calculated mass-transfer coefficients were highest at the bath surface reaching a value of 6.40 × 10⁻⁵ m/s in the sprout region. Mass-transfer coefficients and turbulence parameters decreased with depth, reaching minimum values at the bottom of the vessel.