David J. Alexander

Development of repetitive corrugation and straightening

In this paper, we present recent developments in repetitive corrugation and straightening (RCS), a new severe plastic deformation (SPD) technique. Two refinements of the original RCS method are presented and results are shown for commercial purity copper that illustrate the associated improvements in the effectiveness of nanostructuring. Second-generation tooling was implemented using a bench scale rolling mill for continuous processing of sheet and bar. We have found that this design does not produce enough plastic strain per RCS cycle for effective grain refinement prior to the formation and growth of fatigue cracks. Third-generation tooling was designed to process sheet and increase the amount of shear deformation per iteration. The third-generation tooling design introduced significant shear strain and was found to be effective in grain refinement.

Non-uniform microstructure and texture evolution during equal channel angular extrusion

Intense plastic deformation during equal–channel angular extrusion (ECAE) can occur in a broad region at the channel die intersection called the plastic deformation zone (PDZ). When the outer corner of the ECAE die is rounded, PDZs deviate from the model of single–plane simple shear, causing flow to be inhomogeneous. In this work, we explore the validity of using an analytical description of the PDZ by comparing model predictions against finite element (FE) simulations using various material and friction conditions and orientation imaging microscopy (OIM) measurements on ECAE one–pass copper. Inhomogeneous deformation divided the sample into two distinct regions across the sample thickness, wherein the accumulated strain, velocity gradient, texture evolution, and microstructural features are distinct. We demonstrate that intense deformation in the upper part is described well by sequences of simple shearing in a central fan and deformation in the lower part by a combination of low–intensity shear and rigid body rotation. Texture predictions by FE provided the same result as the PDZ model and OIM for the upper part, regardless of the friction and strain hardening variables considered. However, texture results for the lower part were sensitive to choices of friction and strain hardening. Though an idealization, this analytical two–region PDZ model, once characterized, can lend insight and be computationally efficient for multi-pass predictions.

The effect of tantalum on the mechanical properties of a 9Cr–2W–0.25V–0.07Ta–0.1C steel

Abstract An Fe–9Cr–2W–0.25V–0.07Ta–0.1C (9Cr–2WVTa) steel has excellent strength and impact toughness before and after irradiation in the Fast Flux Test Facility (FFTF) and the High Flux Reactor (HFR). The ductile–brittle transition temperature (DBTT) increased only 32°C after 28 dpa at 365°C in FFTF, compared to a shift of ≈60°C for a 9Cr–2WV steel the same as the 9Cr–2WVTa steel but without tantalum. This difference occurred despite the two steels having similar tensile properties before and after irradiation. The 9Cr–2WVTa steel has a smaller prior-austenite grain size, but otherwise microstructures are similar before irradiation and show similar changes during irradiation. The irradiation behavior of the 9Cr–2WVTa steel differs from the 9Cr–2WV steel and other similar steels in two ways: (1) the shift in DBTT of the 9Cr–2WVTa steel irradiated in FFTF does not saturate with fluence by ≈28 dpa, whereas for the 9Cr–2WV steel and most similar steels, saturation occurs at

Effect of chromium, tungsten, tantalum, and boron on mechanical properties of 5-9Cr-WVTaB steels

Abstract The Cr–W–V–Ta reduced-activation ferritic/martensitic steels use tungsten and tantalum as substitutes for molybdenum and niobium in the Cr–Mo–V–Nb steels that the reduced-activation steels replaced as candidate materials for fusion applications. Studies were made to determine the effect of W, Ta, and Cr composition on the tensile and Charpy properties of the Cr–W–V–Ta; steels with 5%, 7%, and 9% Cr with 2% or 3% W and 0%, 0.05%, or 0.10% Ta were examined. Boron has a long history of use in steels to improve properties, and the effect of boron was also examined. Regardless of the chromium concentration, the steels with 2% W and 0.05–0.1% Ta generally had a better combination of tensile and Charpy properties than steels with 3% W. Boron had a negative effect on properties for the 5% and 7% Cr steels, but had a positive effect on the 9% Cr steel. When the 5, 7, and 9Cr steels containing 2% W and 0.05% Ta were compared, the tensile and Charpy properties of the 5 and 9Cr steels were better than those of the 7Cr steel, and overall, the properties of the 5Cr steel were better than those of the 9Cr steel. Such information will be useful if the properties of the reduced-activation steels are to be optimized.

Effect of Rhenium and Osmium on Mechanical Properties of a 9Cr-2W-0.25V-0.07Ta-0.1C Steel

Abstract The nuclear transmutation of tungsten to rhenium and osmium in a tungsten-containing steel irradiated in a fission or fusion reactor will change the chemical composition of the steel. To determine the possible consequences of such compositional changes on the mechanical properties, tensile and Charpy impact properties were measured on five 9Cr–2W–0.25V–0.07Ta–0.1C steels that contained different amounts of rhenium, osmium, and tungsten. The mechanical properties changes caused by these changes in composition were minor. Observations were also made on the effect of carbon concentration. The effect of carbon on tensile behavior was minor, but there was a large effect on Charpy properties. Several of the steels showed little effect of tempering temperature on the Charpy transition temperature, a behavior that was tentatively attributed to the low silicon and/or manganese concentration of the experimental steels.

Equal Channel Extrusion Heterogeneities in Recrystallized Copper

Various processing routes have been studied to develop the most efficient route toward microstructure refinement and texture homogenization. Most studies have focused on the central shear zone in round or square cross-section rods. However, the utility or application of these materials is subject to conditions across the entire rod. This study begins to investigate the development of recrystallization textures and microstructures across copper ECAE processed via route Bc through 16 passes. Although the recrystallized condition appears to be reasonably homogeneous after 4 or 8 passes, additional passes lead to stronger, heterogeneous recrystallization textures and coarsened microstructures.

Production of Fine-Grained Beryllium-6 WT% Copper for Fusion Ignition Capsules by Arc Melting and Equal Channel Angular Extrusion

Abstract Beryllium doped with 6 weight % copper is the material of choice for fabrication of target capsules for the National Ignition Facility because of its combination of attractive neutronic, electronic, physical, and mechanical properties. The target capsules are 2 mm in diameter and thin-walled (150 microns) and must meet demanding dimensional specifications. The material must be fine-grained and of low inclusion content. Arc-melted Be-Cu is being produced to eliminate the oxide content that is inevitably present in conventional powdermetallurgy materials. Equal channel angular extrusion (ECAE) is being used to refine the as-cast grain structure. Be-Cu rods produced by the arc-melting process (5 mm in diameter by 30 mm in length) are enclosed in nickel cans with electron-beam welded plugs. The Be-in-Ni billets (9.5 mm in diameter by 45 mm in length) have been processed by ECAE at temperatures from 500 to 750°C in tooling with a 120° angle. Selected samples have been annealed for 1 hour at temperatures from 700 to 775°C. The ECAE processing creates a heavily deformed and finely subdivided structure, and the annealing can produce an equiaxed microstructure with a grain size of approximately 20 μm.

Unraveling the Age Hardening Response in U-Nb Alloys

Complicating factors that have stymied understanding of uranium-niobium’s aging response are briefly reviewed, including (1) niobium inhomogeneity, (2) machining damage effects on tensile properties, (3) early-time transients of ductility increase, and (4) the variety of phase transformations. A simple Logistic-Arrhenius model was applied to predict yield and ultimate tensile strengths and tensile elongation of U-4Nb as a function of thermal age. Fits to each model yielded an apparent activation energy that was compared with phase transformation mechanisms.

Progress in the Production of Materials and Fabrication of NIF Beryllium-Copper Ignition Capsules at Los Alamos National Laboratory

Abstract Work is underway at Los Alamos National Laboratory to fabricate machined-and-bonded target capsules of Be-6 wt% Cu for the National Ignition Facility. Significant progress has been made in producing material with the desired composition, purity, and homogeneity of composition, by arc melting. This material is thermomechanically processed by equal channel angular extrusion, to break down the coarse ascast structure and refine the grain size, to about 20 μm. Machining with diamond tooling results in a significant improvement of the as-machined roughness, that also results in improved bond strengths. Bonding with a sputtered layer of Al can achieve high strengths with a bond 1.2 μm thick, and thinner bonds are being investigated. Laser-drilled holes and fill-tube counterbores produced by electrodischarge machining appear to be feasible, but will require improvements in specimen positioning.

LANL Experience Rolling Zr-Clad LEU-10Mo Foils for AFIP-7

The cleaning, canning, rolling and final trimming of Low Enriched Uranium-10 wt. pct. Molybdenum (LEU-10Mo) foils for ATR (Advanced Test Reactor) fuel plates to be used in the AFIP-7 (ATR Full Size Plate In Center Flux Trap Position) experiments are summarized. Six Zr-clad foils were produced from two LEU-10Mo castings supplied to Los Alamos National Laboratory (LANL) by Y-12 National Security Complex. Details of cleaning and canning procedures are provided. Hot- and cold-rolling results are presented, including rolling schedules, images of foils in-process, metallography and local compositions of regions of interest, and details of final foil dimensions and process yield. This report was compiled from the slides for the presentation of the same name given by Duncan Hammon on May 12, 2011 at the AFIP-7 Lessons Learned meeting in Salt Lake City, UT, with Los Alamos National Laboratory document number LA-UR 11-02898.