K. T. Hartwig

Microstructure and mechanical properties of tantalum after equal channel angular extrusion (ECAE)

We have investigated the microstructure and mechanical properties of equal channel angular extruded (ECAE) Ta. Mechanical properties were measured both under quasi-static loading and dynamic loading (in the latter case, the compression Kolsky bar technique was employed to attain strain rates of ∼10 3 s −1 ). It is shown that four passes of ECAE with route C at room temperature, which results in an equivalent strain of ∼4.64, increases the strength of Ta by a factor of 2–3 under quasi-static loading, and by a factor of more than 1.5 under dynamic loading. Under quasi-static loading, the ECAE processed samples exhibit almost elastic-perfect plastic behavior; under dynamic loading, slight softening is observed, presumably due to adiabatic heating. It is found that ECAE decreases the strain rate sensitivity. Comparison of the X-ray diffraction (XRD) between the un-processed and ECAE processed Ta indicates significant broadening of the XRD peaks in the ECAE processed sample. Transmission electron microscopy reveals textured, elongated substructures with an average size of about 200 nm, and the substructures are separated by small angle grain boundaries. This work shows the potential for the production of ultra-fine grained or even nano-structured refractory metals with high melting points by using severe plastic deformation. Signs indicating increased shear localization tendancy were observed at high strain rates.

In situ composites processed by simple shear

Abstract A new deformation method for the fabrication of in situ composites is described. It is found that intensive simple shear, performed repeatedly in the same direction, transforms ductile second phase particles into aligned filaments. Processing is performed by multi-step equal channel angular extrusion (ECAE). Results of experiments on Cu-18Nb and Cu-25Ag alloys demonstrate the ability of this method to fabricate filamentary composites from cast bulk products with no change in the work piece cross-section.

Superior radiation-resistant nanoengineered austenitic 304L stainless steel for applications in extreme radiation environments

Nuclear energy provides more than 10% of electrical power internationally, and the increasing engagement of nuclear energy is essential to meet the rapid worldwide increase in energy demand. A paramount challenge in the development of advanced nuclear reactors is the discovery of advanced structural materials that can endure extreme environments, such as severe neutron irradiation damage at high temperatures. It has been known for decades that high dose radiation can introduce significant void swelling accompanied by precipitation in austenitic stainless steel (SS). Here we report, however, that through nanoengineering, ultra-fine grained (UFG) 304L SS with an average grain size of ~100 nm, can withstand Fe ion irradiation at 500°C to 80 displacements-per-atom (dpa) with moderate grain coarsening. Compared to coarse grained (CG) counterparts, swelling resistance of UFG SS is improved by nearly an order of magnitude and swelling rate is reduced by a factor of 5. M23C6 precipitates, abundant in irradiated CG SS, are largely absent in UFG SS. This study provides a nanoengineering approach to design and discover radiation tolerant metallic materials for applications in extreme radiation environments.

Consolidation of advanced WC–Co powders

Abstract WC–Co powders were consolidated using a new method of extrusion. The consolidation process included two equal channel angular extrusions through a 90° die at temperatures of 300°C, 1100°C or 1200°C on composite billets containing different WC–Co powder blends. The composition investigated was WC–14%Co with particle sizes ranging from 0.2 to 0.8 μm. The as-received powders and consolidated-annealed materials are characterized for structure, particle and grain size, impurity content, void fraction, composition and hardness. Full compaction is achieved after two extrusions at 1200°C and subsequent annealing in argon at 1400°C. The results compare favorably with results on commercially consolidated materials. The findings show that angular extrusion combined with post extrusion annealing can be used to successfully consolidate WC–Co powders into hard and strong cemented carbides of interest to the cutting tool and oil field industries.

Equal-channel angular extrusion of beryllium

The equal-channel angular extrusion (ECAE) technique has been applied to a powder metallurgy (P/M) source Be alloy. Extrusions have been successfully completed on Ni-canned billets of Be at approximately 425 °C. No cracking was observed in the billets, and significant grain refinement was achieved. In this article, microstructural features and dislocation structures are discussed for a single-pass extrusion, including evidence of 〈c〉 and 〈c+a〉 dislocations. Significant crystallographic texture developed during ECAE, which is discussed in terms of this unique deformation processing technique and the underlying physical processes which sustain the deformation.

Ultrafine and Nanostructured Refractory Metals Processed by SPD: Microstructure and Mechanical Properties

Severe plastic deformation (SPD) has been demonstrated to be the most efficient method to produce bulk metals with ultrafine grained (UFG, 100 nm < grain size d < 500 nm) and nanocrystalline (NC, d<100 nm) microstructures. Such metals exhibit some unique properties owing to their unusual microstructures such as high-energy, non-equilibrium grain boundaries. Efforts in the past two decades have focused on metals with face-centered cubic (fcc) structures. Recent experimental results have shown that UFG/NC metals with body-centered cubic (bcc) structures have some properties that are distinct from their fcc counterparts. Further, the majority of the fcc metals are very ductile and have relatively low melting points, making them easier to process using SPD. On the contrary, many bcc metals are refractory, and are very sensitive to interstitial impurities, rendering them difficult to work via SPD. In this article, we attempt to summarize the state-of-the-art of UFG/NC refractory metals processed by SPD, with focus on the microstructure and mechanical properties. Comparisons with UFG/NC fcc metals are made where appropriate. Outstanding issues and future directions are also addressed.

Severe plastic deformation of bulk Nb for Nb/sub 3/Sn superconductors

Cast pure Nb with very large grains was processed by multipass equal channel angular extrusion (ECAE) to refine the microstructure. Extrusions were performed on 25 and 50 mm square cross section bars in a right angle die at room temperature following different extrusion routes to strains above nine. The hardness of Nb reaches a saturation level after eight extrusion passes where it is /spl sim/20% above that of fully worked Cu. Recrystallization bands appear to be absent in material processed by a new multipass route (E) which imparts shear on three intersecting planes and gives high processing yields. Hardness and optical microscopy measurements on recrystallized specimens are similar for 25 and 50 mm square bars which indicates a favorable scale-up response. Comparisons to commercially processed Nb and a cost estimate indicate that ECAE may be a viable method for manufacturing fine-grained, homogenous Nb for Nb/sub 3/Sn multifilamentary superconductor applications.

Microstructural refinement of tantalum for Nb/sub 3/Sn superconductor diffusion barriers

Cast pure Ta was deformation processed via equal channel angular extrusion and then recrystallized to produce a uniform, fine-grained bulk material. Extrusions were performed on 25/spl times/25/spl times/150 mm billets at room temperature in 90/spl deg/ tooling to strains of 9.3. The Vickers microhardness reaches near maximum levels after four extrusions. As-worked microstructures are composed of nonuniform submicron grains with apparent banding. A homogeneous and fine grained recrystallized microstructure (average grain size /spl sim/8 /spl mu/m) free of banding is obtained after four extrusions via route E multipass processing and a 1000/spl deg/C heat treatment. The high microstructural homogeneity of such a material may exhibit better co-reduction characteristics with surrounding Cu over conventionally processed Ta for superconductor diffusion barrier applications.

Microstructural Refinement of Niobium for Superconducting RF Cavities

The mechanical properties of commercial polycrystalline pure niobium sheet used for superconducting radiofrequency cavities are known to provide inconsistent yield, springback and surface smoothness characteristics when plastically formed into a radiofrequency cavity. These inconsistent properties lead to significant variations in cavity geometry and thus superconducting cavity performance. One approach to reduce these problems is to refine the microstructure so that its properties are more uniform. Microstructural refinement of Nb sheet for RF cavities using multi-axis severe plastic deformation via equal channel angular extrusion (ECAE) was examined. ECAE was performed on 25 mm square cross-section bars of Reactor Grade Nb in a right angle die at room temperature following different extrusion routes to true strains above nine. This heavily worked material was rolled to 4 mm thick sheet and recrystallized. Measurements of hardness, springback, texture, and microstructural uniformity are reported and compared to those of commercial RRR Grade Nb sheet. Preliminary results show noteworthy promise for bulk Nb processed via severe plastic deformation prior to sheet rolling.

Fine Grained Tantalum for Composite

Poor deformation behavior of commercial polycrystalline Ta sheet used for Sn diffusion barriers in Nb3Sn composite superconductors leads to the use of more Ta than may be necessary in these conductors, and to strand fracture during wire drawing. These problems arise because of nonuniform deformation of the Ta layer when co-drawn with Cu. The origin of the problem resides in the microstructure of the Ta and the code formation mechanics of relatively strong BCC Ta with weaker and more ductile adjacent FCC Cu. In an attempt to remedy this problem, we have processed 25 mm square bars of Ta by multi-axis severe plastic deformation (SPD) via equal channel angular extrusion, then rolled the bars to sheet and annealed the as-worked sheet. The processing is done to refine the microstructure and reduce nonuniformities in grain size and texture. Measurements of hardness, microstructure and mechanical properties are reported for bulk and sheet Ta. The results of SPD processing are encouraging.