Nanoengineering Opens a New Era for Tungsten As Well

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 µm, 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 temperature of interest divided by the melting 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 nanoengineering to commercial-purity tungsten, 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 methodology was adopted in which tungsten with a UFG microstructure was processed 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 µm), 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. The strain reduction, ECAP was performed at 1,000°C. To alleviate oxidation of the tungsten work level at which ASB kicks in is around piece, the tungsten rod was encapsulated in a stainless-steel canister. The die angle (or 0.1 (10%). The ASBs are fully developed connecting angle between the channels) is 120° to avoid cracking of tungsten. Another at a strain level of ~15%. The onset of SPD technique used is high-pressure torsion (HPT),14,34 where a disk of tungsten (~10 mm in diameter and ~1.0 mm in thickness) is subjected to high pressure (~4 GPa) while secondary shear bands was also observed 24 torsion is applied to it, allowing a tremendous amount of plastic strain to be pumped via high-speed photography. into the material (a full turn gives an equivalent strain of ~18 at the edge of the disk with Figure 5a displays a post-dynamic the above dimension). In this work, HPT was performed at 500°C, much lower than the loading optical micrograph of the HPT recrystallization temperature of tungsten. Details can be found in Reference 33. nano-tungsten. Again, instead of axial Scanning-electron microscopy and optical microscopy are used to examine the post-loading specimen surface to identify the deformation and failure mechanisms. cracking, a localized shear band is Transmission-electron microscopy is used to analyze grain size, defects, and their observed, subtending an angle of ~45° to distributions, and the grain boundary structure. the loading direction (horizontal in this The processed tungsten was tested under uni-axial quasi-static (strain rate 10–4 s–1 to case). The SEM micrograph of Figure 0 –1 10 s ) and dynamic compression (using split-Hopkinson pressure bar (SHPB), strain 5b shows the severe curving of the pre- rate ~103 s–1).35 For the HPT-processed tungsten, due to the limited dimension of the disk, a miniaturized SHPB system (or desk-top Kolsky bar)36 was used to obtain the dynamic existing scratches introduced during data. specimen preparation prior to dynamic loading. In this case, the shear band width (about 5 µm) is much smaller compared at 1,000°C (six passes) has increased the curves of HPT tungsten (data recorded to that of the UFG tungsten (ECAP+RW). quasi-static strength to 1.5 times that of using the miniature split-Hopkinson In other UFG/NC bcc metals such as the control CG tungsten sample (appar- pressure bar SHPB technique). The most iron, a change of shear band width with ent compressive yield strength of CG salient feature in these curves is the very grain size is also observed; the underlying tungsten is ~1.0 GPa). Further low-tem- early precipitous stress collapse, which mechanism is still under investigation. perature rolling has increased the is one of the most desirable properties Figure 5b also shows a crack as a con- strength to nearly twice its CG value. for a penetrator material. sequence of highly localized adiabatic Second, work hardening of the SPD- Under quasi-static and dynamic com- shearing. processed tungsten is considerably pression, conventional CG tungsten and As pointed out earlier in this paper, reduced. Furthermore, under dynamic extruded tungsten (extrusion at 1,200°C) because of their high mass density, compression, SPD tungsten shows flow that shows strength levels close to the tungsten and tungsten heavy alloys have softening at quite small plastic strains, ECAP tungsten presented here exhibits been investigated extensively for the pro- in sharp contrast to the CG tungsten axial cracks (cracks parallel to the load- duction of anti-armor KE penetrators in where under dynamic loading slight ing axis).43 The ECAP tungsten in this place of DU. However, most of the efforts apparent hardening is observed. This study exhibits similar behavior.24 Most have had limited success, mainly due to flow softening, especially in the ECAP+R of the cracks are along the pre-existing the lack of plasticity and poor propensity tungsten, is due to a change in deforma- grain boundaries, consistent with tensile for ASB in conventional CG tungsten. tion mode, as detailed later in this behavior of such tungsten where failure On one hand, plastic deformation is paper. occurs at a stress level about only half needed to introduce adiabatic heating, Microhardness measurement on the that of compression and with no evidence a prerequisite for the development of HPT tungsten shows that the nano-tung- of plastic deformation, similar to that of ASB. On the other hand, uniform plastic sten is super strong with a hardness value most ceramics. deformation of the penetrator material is around 11 GPa at the rim of the disk.33 Figure 4a is a post-dynamic loading to be avoided so that the kinetic energy This hardness value is remarkable even optical micrograph of an ECAP+R tung- can be used primarily for penetration. when compared with some strong ceram- sten, with the corresponding stress-strain Uniform plastic deformation of the ics (e.g., the hardness of sintered Si3N4 response shown in Figure 3a. Instead penetrator material is responsible for ceramic is about 20 GPa).41 Quasi-static of axial cracks, clear evidence of shear “mushrooming” of the penetrator head, compression of the HPT tungsten showed bands is observed. Higher-magnification in contrast to “self-sharpening” where a yield strength ~3.5 GPa, in accordance SEM imaging (Figure 4b, for example) the head remains sharp and failed pen- with the hardness value if the Tabor indicates severe and localized adiabatic etrator material is discarded away via relation42 between Vicker’s hardness shear flow within the ASBs. Polishing adiabatic shearing, thus leaving a small (VHN) and yield strength of an isotropic of the roughened surface followed by penetration channel within the target. The material is assumed (VHN≈3σy). Figure chemical etching reveals the detailed self-sharpening effect of the penetrator is 3b displays a few dynamic stress-strain microstructure of the shear bands, includ- rooted in the material property of shear

42 JOM • September 2006 band susceptibility in that under dynamic as SPD. It has been borne out that the loading, the plastic deformation is highly GBs induced by SPD have some special concentrated in a narrow region.30 properties.39 For instance, they are of Soluble interstitial impurities such as non-equilibrium and high-energy nature. carbon, nitrogen, and sulfur31 segregate Such GBs are ideal hosting sites for along the GBs to render the GBs the interstitial impurities. It has also been weak links under mechanical straining, proposed that the peculiar nature of the a 1 mm leading to grain boundary embrittle- SPD-induced GBs explains the paradox ment. However, brittle behavior is not of many UFG/NC metals where signifi- intrinsic to tungsten, since experimental cant ductility is observed concurrently results have shown that single-crystal with high strength.46 tungsten can be deformed to significant Another contributing factor to the plastic strain under tension even at 77K.44 reinstated ductility of the SPD nanoen- Therefore, if the pre-existing detrimental gineered tungsten might be the numerous impurities at the GBs can somehow be edge dislocations observed in HRTEM in depleted, ductility may be improved. the proximity of the GBs.33 Systematic One strategy to furnish this is by means work by Gumbsch et al.47 on the con- b 100 mm of grain boundary engineering.45 If more trolling factors for the fracture tough- Figure 4. (a) An optical micrograph of post- GBs are created in addition to the pre- ness and brittle-to-ductile transition in loading (uni-axial dynamic compression) ECAP+R tungsten showing the ASBs. existing GBs, and if appropriate kinetic single-crystal tungsten indicates that pre- Loading is vertical in this figure. (b) An condition is provided for the pre-existing plastic-deformation of the specimen can SEM image showing the severe and highly GB impurities to diffuse away and be increase both low- and high-temperature localized flow in the ASB. The roughened surface has been polished and etched to relocated to the newly created GBs, the fracture toughness. Particularly, much reveal the microstructure of the ASB. average impurity concentration at the improved high-temperature fracture GBs can be reduced. This will increase toughness has been achieved by pre-plas- ˆ the GB strength and reinstate the duc- tic-deformation. They pointed out that under isothermal conditions ( σ0 is a tility. The most efficient way to induce if a dislocation (either generated at the normalizing stress), and σ0 is the yield more GBs without worsening impurity crack tip or a pre-existing one) moves in strength. Ultra-fine grain/NC metals contamination is a top-down route such the stress field along the crack tip, it will have much higher yield strength than generate dislocation segments of non- their CG counterparts (the well-known screw (and thus highly mobile) character Hall-Petch effect). Recent experimental 3,000 parallel to the crack tip. High-resolution results and theoretical analyses have 2,500 TEM shows a presence of residual edge shown that SRS of UFG/NC bcc metals 33 2,000 dislocations in the HPT nano-tungsten. is considerably reduced compared to the Such edges may work together with other CG counterparts.21 Calculations based on 1,500 pre-existing dislocations that are highly the experimental results and the physical

True Stress (MPa) True 1,000 mobile so as to provide highly efficient properties of tungsten show that suscep- shielding of the crack tip, thus resulting tibility to ASB of the UFG/NC tungsten 500 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 in relatively ductile failure.33 is several orders of magnitude higher a True Strain Finally, the enhanced propensity for than that of conventional CG tungsten. ASB in nano-engineered tungsten must CONCLUSIONS 4,000 be considered. Mechanistic models have been articulated to predict this propen- This study has applied nanoengineer- 3,000 sity in visco-plastic materials. Wright48 ing to produce tungsten with unusual 2,000 derived Equation 1 (see the Equations microstructure and mechanical behavior

table on page 41), where χSB is the suscep- by using a progressive methodology to True Stress (MPa) True 1,000 tibility to ASB, a is the non-dimensional refine the grain size. Ultrafine-grained 0 thermal softening parameter defined by tungsten and nanocrystalline tungsten 0 0.05 0.1 0.15 0.2 0.25 0.3 a=(–∂σ / ∂T) / ρc (σ is the flow stress, were processed using SPD under various b True Strain T the temperature, ρ the density, and c conditions. Mechanical testing under Figure 3. (a) The true stress-strain curves the specific heat of the material), n the uni-axial dynamic loading showed that of ECAP tungsten and ECAP+R tungsten under quasi-static and dynamic uni-axial strain hardening exponent, and m the such UFG and NC tungsten exhibit the compression. The significant flow softening strain rate sensitivity (SRS). long-sought-after localized shearing, in the ECAP+R tungsten is of particular For a perfectly plastic material (no rather than uniform plastic deformation interest. (b) Dynamic stress-strain curves of HPT tungsten as measured by desktop strain hardening) such as the SPD and/or axial cracking. This work shows Kolsky bar technique; HPT tungsten nano-engineered tungsten presented that nanoengineering is opening a new exhibits much more elevated flow strength here, the susceptibility reduces to Equa- era for tungsten, particularly for its with much earlier stress collapse compared ˆ to the ECAP+R tungsten. tion 2, where λ=–(1/ σ0)∂σ/∂T is the application in anti-armor kinetic energy thermal softening parameter evaluated penetrators.

2006 September • JOM 43 line and Ultrafine-Grained Metals,” Scripta Mater., 49 crystalline and Ultrafine Grain Sizes on Constitutive (2003), pp. 663–668. Behavior and Shear Bands in Iron,” Acta Mater., 51 (2) 6. E. Ma, “Controlling Plastic Instability,” Nature Materi- (2003), pp. 3495–3590. als, 2 (2003), pp. 7–8. 30. L.S. Magness, “An Overview of the Penetration 7. K.M. Youssef et al., “Ultratough Nanocrystalline Copper Performances of Tungsten and Depleted Uranium Alloy with a Narrow Grain Size Distribution,” Appl. Phys. Lett., Penetrators: Ballistic Performances and Metallographic 85 (6) (2004), pp. 929–931. Examinations,” Ballistics 2000 (Lancaster, PA: DEStech 8. K.M. Youssef et al., “Ultrahigh Strength and High Publications, Inc., 2002), CD-ROM.. Ductility of Bulk Nanocrystalline Copper,” Appl. Phys. 31. E. Lassner and W.-D. Schubert, Tungsten-Proper- Lett., 87 (9) (2005), p. 091904. ties, Chemistry, Technology of the Element, Alloys and 9. S. Cheng et al., “Tensile Properties of In Situ Consoli- Chemical Compounds (Dordrecht, the Netherlands: dated Nanocrystalline Cu,” Acta Mater., 53 (5) (2005), Kluwer-Academic/Plenum Publishers, 1998). pp. 1521–1533. 32. B.C. Allen, D.J. Maykuth, and R.I. Jaffee, “The Recrys- 10. Y.M. Wang and E. Ma, “Three Strategies to Achieve tallization and Ductile-Brittle Transition Behavior of Tung- Uniform Tensile Deformation in a Nanostructured Metal,” sten,” J. Institute of Metals, 90 (1961), pp. 120–128. Acta Mater., 52 (2004), pp. 1699–1709. 33. Q. Wei et al., “Microstructure and Mechanical 11. Y.M. Wang et al., “High Tensile Ductility in a Nanostruc- Properties of Super-Strong Nanocrystalline Tungsten a 0.3 mm tured Metal,” Nature, 419 (2002), pp. 912–915. Processed by High-Pressure Torsion,” Acta Mater., 54 12. E. Ma, “Eight Routes to Improve the Tensile Ductility (2006), in press. of Bulk Nanostructured Metals and Alloys,” JOM, 58 (4) 34. A.P. Zhilyaev et al., “Experimental Parameters Influ- (2006), pp. 49–53. encing Grain Refinement and Microstructural Evolution 13. C.C. Koch and J. Narayan, “The Inverse Hall-Petch during High-Pressure Torsion,” Acta Mater., 51 (2003), Effect—Fact or Artifact?” Structure and Mechanical Prop- pp. 753–765. erties of Nanophase Materials—Theory and Computer 35. P.S. Follansbee, “High Strain Rate Compression Test- Simulations vs. Experiments (Warrendale, PA: MRS, ing,” ASM Metals Handbook (Metals Park, OH: American 2001), ID# 37343 . Society of Metals, 1985), p. 190. 14. R.Z. Valiev, R.K. Islamgaliev, and I.V. Alexandrov, “Bulk 36. D. Jia and K.T. Ramesh, “A Rigorous Assessment of Nanostructured Materials from Severe Plastic Deforma- the Benefits of Miniaturization in the Kolsky Bar System,” tion,” Prog. Mater. Sci., 45 (2000), pp. 103–189. Experimental Mechanics, 44 (5) (2004), pp. 445–454. 15. D.B. Witkin and E.J. Lavernia, “Synthesis and Mechan- 37. R.Z. Valiev, V.Y. Gertsman, and R. Kaibyshev, “Grain ical Behavior of Nanostructured Materials via Cryomill- Boundary Structure and Properties under External Influ- ing,” Prog. Mater. Sci., 51 (2006), pp. 1–60. ence,” Physica Status Solidi A, 97 (1986), pp. 11–56. b 5 mm 16. K.S. Kumar, H. Van Swygenhoven, and S. Suresh, 38. R.Z. Valiev, “Nanomaterial Advantage,” Nature, 419 “Mechanical Behavior of Nanocrystalline Metals and (2002), pp. 887–889. Figure 5. (a) An optical image of a post- Alloys,” Acta Mater., 51 (2003), pp. 5743–5774. 39. A.A. Nazarov, A.E. Romanov, and R.Z. Valiev, “On the loading (via desktop Kolsky bar) HPT 17. Q. Wei et al., “Evolution and Microstructure of Shear Structure, Stress Fields and Energy of Nonequilibrium nano-tungsten showing a single shear Bands in Nanostructured Fe,” Appl. Phys. Lett., 81 (7) Grain Boundaries,” Acta Metall. Mater., 41 (4) (1993), band (indicated by an arrow). The loading (2002), pp. 1240–1242. pp. 1033–1040. is horizontal. (b) An SEM image showing 18. Q. Wei et al., “Microstructure and Mechanical Prop- 40. J.W. Christian, “Some Surprising Features of the severe curving of the pre-existing scratches erties of Tantalum after Equal Channel Angular Extru- Plastic-Deformation of Body-Centered Cubic Metals and approaching the shear band. The crack is sion (ECAE),” Mater. Sci. Eng. A, 358 (1-2) (2003), pp. Alloys,” Metall. Trans. A, 14A (1983), p. 1237. a result of severe and highly concentrated 266–272. 41. P.J. Blau, R.L. Martin, and E.S. Zanoria, “Effects of shear flow within the shear band. 19. Q. Wei et al., “Adiabatic Shear Banding in Ultrafine Surface Grinding Conditions on the Reciprocating Fric- Grained Fe Processed by Severe Plastic Deformation,” tion and Wear Behavior of Silicon Nitride,” Wear, 203 Acta Mater., 52 (7) (2004), pp. 1859–1869. (1997), pp. 648–657. ACKNOWLEDGEMENTS 20. Q. Wei et al., “Nano-Structured Vanadium: Process- 42. D. Tabor, The Hardness of Metals (Oxford, U.K.: ing and Mechanical Properties under Quasi-Static Clarendon Press, 1951). The authors are grateful to Drs. T. Jiao, and Dynamic Compression,” Scripta Materialia, 50 (3) 43. A.M. Lennon and K.T. Ramesh, “The Thermovisco- (2004), pp. 359–364. plastic Response of Polycrystalline Tungsten in Com- H. Zhang, and Y. Li, who assisted with 21. Q. Wei et al., “Effect of Nanocrystalline and Ultrafine pression,” Mater. Sci. Eng. A, 276 (2000), pp. 9–21. dynamic experiments, and Drs. E. Ma Grain Sizes on the Strain Rate Sensitivity and Activation 44. A.S. Argon and S.R. Maloof, “Plastic Deformation (Johns Hopkins University), T.W. Wright, Volume: fcc versus bcc Metals,” Mater. Sci. Eng. A, 381 of Tungsten Single Crystals at Low Temperature,” Acta (1-2) (2004), pp. 71–79. Metallurgica, 14 (1966), pp. 1449–1462. and J.W. McCauley (U.S. Army Research 22. Y.J. Wei and L. Anand, “Grain Boundary Sliding and 45. T. Watanabe, “An Approach to Grain Boundary Design Laboratory [ARL]) for illuminating Separation in Polycrystalline Metals: Application to for Strong and Ductile Polycrystals,” Res. Mechanica, discussions. This work was sponsored Nanocrystalline fcc Metals,” J. Mechanics and Physics 11 (1984), pp. 47–84. of Solids, 52 (2004), pp. 2584–2616. 46. R.Z. Valiev et al., “Paradox of Strength and Ductility by ARL through JHU-CAMCS under the 23. Q. Wei et al., “Plastic Flow Localization in Bulk-Tung- in Metals Processed by Severe Plastic Deformation,” J. ARMAC-RTP Cooperative Agreement sten with Ultrafine Microstructure,”Appl. Phys. Lett., 86 Mater. Res., 17 (1) (2002), pp. 5–8. #DAAD19-01-2-0003. The ECAP and (10) (2005), p. 101907. 47. P. Gumbsch et al., “Controlling Factors for the Brittle- 24. Q. Wei et al., “Mechanical Behavior and Dynamic to-Ductile Transition in Tungsten Single Crystals,” Sci- high-pressure torsion were performed Failure of High-Strength Ultrafine Grained Tungsten ence, 282 (5392) (1998), pp. 1293–1295. by Professor R.Z. Valiev’s group. under Uniaxial Compression,” Acta Mater., 54 (1) 48. T.W. Wright, The Physics and Mathematics of Adia- (2006), pp. 77–87. batic Shear Bands (Oxford, U.K.: Cambridge Press, References 25. T.R. Malow and C.C. Koch, “Mechanical Properties 2002). in Tension of Mechanically Attrited Nanocrystalline Iron Q. Wei is with the Department of Mechanical 1. R.P. Feynman, “There is Plenty of Room at the Bottom,” by the Use of Miniaturized Disk Bend Test,” Acta Mater., Engineering at the University of North Carolina- Engineering & Science (Pasadena, CA: Caltech Public 46 (18) (1998), pp. 6459–6473. Charlotte. K.T. Ramesh is with the Department Relations, 1959), cited 2006; www.zyvex.com/nano- 26. T.R. Malow et al., “Compressive Mechanical Behavior of Mechanical Engineering at the Johns Hopkins tech/feynman.html. of Nanocrystalline Fe Investigated with an Automated University in Baltimore, Maryland. B.E. Schuster, 2. E.O. Hall, “The Deformation and Ageing of Mild Steel: Ball Indentation Technique,” Mater. Sci. Eng. A, 252 L.J. Kecskes, and R.J. Dowding are with the U.S. III. Discussion of Results,” P. Phys. Soc. B, 64 (1951), (1998), pp. 36–43. Army Research Laboratory at Aberdeen Proving pp. 747–753. 27. J.E. Carsley et al., “Mechanical Behavior of Bulk Ground, Maryland. 3. N.J. Petch, “The Cleavage Strength of Polycrystals,” J. Nanostructured Iron Alloy,” Metall. Mater. Trans. A, 29A Iron and Steel Institute, 174 (1953), pp. 25–28. (1998), pp. 2261–2271. For more information, contact Q. Wei, University 4. C.C. Koch, “Optimization of Strength and Ductility in 28. D. Jia, K.T. Ramesh, and E. Ma, “Failure Mode and of North Carolina-Charlotte, Department of Nanocrystalline and Ultrafine Grained Metals,” Scripta Dynamic Behavior of Nanophase Iron under Compres- Mechanical Engineering, 362 ERB, 9201 University Mater., 49 (2003), pp. 657–662. sion,” Scripta Mater., 42 (2000), pp. 73–78. City Blvd., Charlotte, NC 28223; (704) 687-8213; 5. E. Ma, “Instabilities and Ductility of Nanocrystal- 29. D. Jia, K.T. Ramesh, and E. Ma, “Effects of Nano- fax: (704) 687-8345; e-mail: [email protected].

44 JOM • September 2006