Nanoengineering Opens a New Era for Tungsten As Well
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Research Summary Tungsten: Today’s Technology Nanoengineering Opens a New Era for Tungsten as Well Q. Wei, K.T. Ramesh, B.E. Schuster, L.J. Kecskes, and R.J. Dowding For the past century, tungsten has been shifted to other unusual properties of UFG/NC regimes. The most direct cause exploited for numerous applications due UFG/NC metals, including the retained of the loss of ductility is the introduction to its unique properties, including its ductility via some special processing of volume defects such as residual poros- extremely high melting point, mass den- routes.7–9,11,12 ity, poor inter-particle bonding due to sity, and mechanical strength. One There are a number of ways to make impurity contamination, etc.13 Many of specific potential application of tungsten UFG/NC metals. The bottom-up meth- these effects are a consequence of so- (owing to its high mass density and ods involve producing nanosize particles called “two-step” processing (usually strength) is the replacement of depleted followed by consolidation, while the bottom-up) where UFG/NC powders are uranium within kinetic energy anti- top-down methods start with a bulk CG produced followed by hot compaction. armor penetrators. Strenuous efforts in metal and refine the grain size into the One strategy to mitigate such detrimen- this direction have had limited success. tal effects is “one-step” processing where However, nanoengineering has been handling of powders in open air is applied recently to tailor the microstruc- avoided, or the starting material is in ture and properties of tungsten, leading fully dense, bulk forms (top-down). Koch to dramatic improvement with regard to and co-workers have demonstrated that this application. This paper provides NC copper with d~20 nm produced by some recent results on nanoengineered in-situ consolidation can have tensile tungsten and discusses the underlying elongation >10%.8 Another technique principles. It appears that nanoengineer- for the production of UFG/NC metals is ing is opening a new era for tungsten. severe plastic deformation (SPD), where a fully dense work piece is subjected to INTRODUCTION a 40 mm very large amounts of plastic strain.14 Nanoengineering is the practice of This avoids the handling of powders and engineering at the nanometer scale (a subsequent consolidation, and is there- nanometer is one-billionth of a meter, fore a one-step, top-down process. Very or 1 nm = 10–9 m). The history of nano- recently, a bi-modal grain size distribu- engineering can be traced back to the tion has manifested both significant end of the 1950s when the Nobel Prize strength and tensile elongation, where Laureate Richard Feynman gave his the NC grains serve as the strengthening famous speech “There is plenty of room medium and the large (d a few microm- at the bottom.”1 About one decade prior eters) grains accommodate the plastic- to Feynman’s speech, metallurgists had ity.10,11 This concept has since been used already recognized the strong effect of in other systems.15 reducing grain size on the behavior of The majority of nanoengineering metals and alloys.2,3 It is now almost efforts are on metals of face-centered- common sense that metals with grain cubic (fcc) structures such as aluminum, b 550 nm size in the ultrafine-grained (UFG, grain copper, and nickel.16 Much less work has Figure 1. (a) An optical micrograph of size d smaller than 500 nm but greater ECAP tungsten (four passes at 1,000°C) been conducted on body-centered-cubic than 100 nm) and nanocrystalline (NC, showing refined microstructure due to (bcc) metals. One reason is the difficulty d<100 nm) regimes have much greater severe plastic deformation. Notice that in refining the grain size of bcc metals the pre-existing grain boundaries are still strength than their coarse-grained (CG) visible. (b) After further rolling at relatively into the UFG/NC regimes. Recent efforts counterparts. Accompanying the high low temperatures, the microstructure is have shown that UFG/NC bcc metals strength is a loss of tensile ductility,4–6 refined into the UFG regime. Selected exhibit some very unique behavior com- area diffraction pattern (not shown here) 17–27 which has recently been considered to indicates that many grain boundaries are pared to their fcc counterparts. For be induced by artificial defects.7–11 The of the low-angle type. example, UFG/NC iron exhibits local- focus of investigations has recently ized shearing even under quasi-static 40 JOM • September 2006 Equations ~40 mm, grain size reduction by ECAP GB. First, it is of large-angle type. is obvious. However, ECAP at 1,000°C Second, it has a number of atomic facets, can only refine the grain size of tungsten steps or ledges, suggesting its high energy χSB 1 = min 1, (1) down to a few micrometers due to and non-equilibrium nature.37–39 Third, a/ m (/n m)/+ n m dynamic recrystallization and grain no GB phase, amorphous or crystalline, growth. To further refine the grain size, can be associated with the GB. In other a λσ0 χSB = = (2) the ECAP tungsten was rolled at 800°C words, the crystalline structure of the m cm ρ and below. Figure 1b displays the trans- constituent grains is disrupted only by mission-electron microscopy (TEM) the presence of the GB, and the GB is compression.17,28,29 Nanocrystalline micrograph of tungsten that was further clean and well defined. An HRTEM of vanadium fails in a manner similar to rolled at 600°C to introduce an additional the same specimen also reveals the metallic glasses under dynamic compres- strain of 1.8. The average grain size (or existence of a large number of edge sion.20 Such deformation and failure sub-grain size) is around 500 nm, thus dislocations in the vicinity of the GB modes are desirable for a kinetic energy in the UFG regime. (not shown here).33 This is unusual since (KE) penetrator material. There has been Figure 2a displays a typical micro- the plasticity of tungsten at such low tremendous research work in search of structure of high-pressure torsion (HPT) temperature is usually accommodated a replacement for depleted uranium (DU) processed tungsten. The average grain by screw dislocations by means of the for the making of such penetrators, and size derived from TEM micrographs is double-kink mechanism, and only among all the candidates, tungsten, a bcc about 100 nm. The selected area diffrac- straight screws were observed by TEM metal, seems to be the best in terms of tion pattern (Figure 2b) shows almost after finite plastic deformation.40 its mass density rivaling DU.30 continuous rings, indicating the absence Figure 3a shows the quasi-static and Body-centered-cubic metals are of preferential orientation of the grains dynamic stress-strain curves of tungsten known to be very vulnerable to soluble (texturing). Figure 2c is a typical high- specimens processed by ECAP and interstitial impurities which have been resolution TEM (HRTEM) image of a ECAP+rolling (referred to henceforth held responsible for their brittle failure grain boundary (GB). A few interesting as ECAP+R tungsten). The following at low homologous temperatures (the characteristics can be identified with this observations can be derived. First, ECAP homologous temperature is the tem- perature of interest divided by the melt- ing point of the specific metal).31 Tung- sten is the most notorious in this respect. Conventional powder metallurgy (P/M) CG tungsten exhibits a very high ductile- to-brittle transition temperature (DBTT) of around 150°C.32 Efforts to produce UFG/NC tungsten through P/M have had little success to this point. Very recently, the authors have shown that SPD-based nanoengineering may be an alternative for tungsten.23,24,33 This article reports and reviews the application of nano- engineering to commercial-purity tung- sten, demonstrating strong evidence of a 95 nm b adiabatic shear banding (ASB), which is the basis for the desired performance of a KE penetrator. A progressive meth- odology was adopted in which tungsten with a UFG microstructure was pro- cessed and examined, and then nano- engineered tungsten was investigated. See the sidebar for details on strategy and methodology. EXPERIMENTAL DEMONSTRATIONS OF c PRINCIPLE Figure 2. (a) A bright-field TEM image showing the nanocrystalline microstructure of HPT tungsten. (b) Selected-area diffraction pattern indicates continuous rings and apparent Figure 1a shows the microstructure absence of texturing. (c) A lattice image of a grain boundary in the HPT tungsten showing of tungsten after four passes of equal- a large-angle GB with atomic ledges and steps, suggesting its high energy and non- channel angular pressing (ECAP). Since equilibrium nature. the starting material has a grain size of 2006 September • JOM 41 STRATEGY AND METHODOLOGY ing density and direction of the shear Severe plastic deformation (SPD)14 is used to progressively refine the grain size of lines, width of the shear bands (around commercial-purity tungsten. First, equal-channel-angular-pressing (ECAP) is used 40 mm), and cracking along the central followed by low-temperature rolling for the production of ultra-fine grain tungsten. line of the shear bands (Figure 4 b). High- Details of ECAP can be found in, for example, Reference 14. In this technique, the speed photography (not shown here) work piece is pushed through two connecting channels with the same cross-sectional indicates that the stress-collapse in the area. With a right connecting angle the work piece experiences an equivalent strain of ~1.0 with each pass of ECAP. The recrystallization temperature of tungsten is around stress-strain curves roughly corresponds 24 1,250°C for moderate plastic deformation.31 To ensure the efficiency of grain size to the initiation of ASBs.