Development of tough, low-density -based bulk metallic matrix composites with tensile ductility

Douglas C. Hofmann1, Jin-Yoo Suh, Aaron Wiest, Mary-Laura Lind, Marios D. Demetriou, and William L. Johnson

Keck Laboratory of Engineering Materials, California Institute of Technology, Pasadena, CA 91125

Edited by J. C. Phillips, Rutgers University, Piscataway, NJ, and approved November 11, 2008 (received for review September 10, 2008) The mechanical properties of bulk metallic (BMGs) and their Two- composites based in titanium and can be composites have been under intense investigation for many years, readily produced, owing to the extremely low solubility of many owing to their unique combination of high strength and elastic and metalloids with the body centered cubic (b.c.c.) titani- limit. However, because of their highly localized deformation um/zirconium phase. For example, , , and all mechanism, BMGs are typically considered to be brittle materials exhibit low solubility in b.c.c. titanium, and additions of these and are not suitable for structural applications. Recently, highly- elements to molten alloys typically causes solute-poor b.c.c. Ti- toughened BMG composites have been created in a Zr–Ti-based based dendrites to precipitate and grow in a solute-rich . In system with mechanical properties comparable with high- cases where the remaining liquid is a good glass former, a 2-phase performance crystalline alloys. In this work, we present a series of BMG matrix composite is readily produced. Recently, it has been low-density, Ti-based BMG composites with combinations of high shown that with proper control over the (G)ofthe strength, tensile ductility, and excellent fracture toughness. dendritic phase and the of the dendrites, extensive toughening and ductility can be achieved (1). ecent progress in ductile-phase-reinforced bulk metallic To design a Ti-based BMG composite with tensile ductility, Rglass (BMG) composites has demonstrated that with proper several criteria must be satisfied: (i) identify a highly-processable design and microstructural control enhanced toughness and Ti-based BMG matrix , (ii) create a 2-phase partially tensile ductility can be achieved in 2-phase alloys with a glassy crystallized microstructure of liquid plus b.c.c. dendrites, (iii) matrix phase that exhibits large glass-forming ability (GFA) (1, lower the shear modulus, G, of the dendritic phase relative to the 2). This work has opened potential structural applications for glass matrix, and (iv) coarsen and homogenize the microstruc- BMG composites that are not possible in monolithic BMGs, ture to the length scale of deformation in the glass phase. To because of shear localization and subsequent catastrophic failure make such alloys competitive with crystalline titanium alloys, during unconfined loading (2). Although the problems associ- density and cost must be minimized. Low-density monolithic ated with unlimited extension of shear bands and the resulting titanium-based BMGs have recently been discovered that exhibit catastrophic failure seen in monolithic BMGs can be mitigated densities that range among common engineering titanium alloys by adding soft crystalline inclusions to the glass, current BMG (4.59–4.91 g/cm3) but with approximately double the specific matrix composites that exhibit this structure are based on the strength (20). These monolithic BMGs, based on the ternary relatively dense element zirconium (1–4). Reducing the cost and Ti–Zr–Be glass-forming system, possess substantial advantages density of current Zr-based composites is beneficial for the over Zr-based BMGs in terms of cost and density, but have commercialization of these new alloys. Low-density components relatively lower GFA compared with other Be-containing with high toughness and strength could be particularly useful in BMGs. Two titanium-based BMGs, Ti45Zr20Be35 and the aerospace and aeronautics industry as replacements for some Ti40Zr25Be35, exhibiting a critical casting dimension (for glass crystalline hardware. formation) of 6 mm have been reported (20). With this motivation, the current work explores Ti-based (in both and atomic percentage) BMG matrix composites Results and Discussion with mechanical properties and low density matching or sur- To design a composite structure around the ternary Ti–Zr–Be passing those of common engineering titanium alloys. Herein, system, we first increased the atomic percentage (at%) of Ti–Zr we report BMG composites composed of 42–62 weight percent- to Ϸ70%. At and above this concentration, the equilibrium alloy age (wt%) titanium, with densities ranging from 4.97 to 5.15 at high (Ϸ800–1,000 °C) partitions into a b.c.c. g/cm3, and all exhibiting at least 5% tensile ductility. We vary the plus solute (Be)-enriched glass-forming liquid. fraction of the glass phase from 20% to 70%, investigate The volume (or molar) fraction of dendrites embedded in the aluminum additions to lower density, and report 1 alloy with glass matrix is directly related to the percentage of beryllium Ͻ 1.0 wt% of beryllium. The current work demonstrates that (and Cu, Ni, or Co). For example, Ti40Zr30Be30 forms a BMG Ti-based BMG composites are competitive with conventional matrix composite having GFA up to 7 mm. Despite the appro- titanium alloys for some structural applications where high priate microstructure for tensile ductility, this composite is strength and toughness are a necessity. apparently more brittle than a monolithic BMG because of the Ti-based, ductile-phase-reinforced matrix composites have small size of the dendrites and their relatively high G. Significant been the subject of significant recent research (5–14). In other tensile ductility has been achieved only in BMG matrix com- systems, nano-crystalline composites have been reported that display significant compressive plasticity (15–19). These alloys are designed in a similar manner to BMG-matrix composites Author contributions: D.C.H. and W.L.J. designed research; D.C.H., J.-Y.S., A.W., and M.-L.L. with the significant difference being that the continuous matrix performed research; D.C.H., J.-Y.S., M.D.D., and W.L.J. analyzed data; and D.C.H. and W.L.J. material is comprised of a nanostructured eutectic instead of a wrote the . metallic glass. To improve tensile ductility and fracture tough- The authors declare no conflict of interest. ness, we investigated alloys that have a continuous glass matrix This article is a PNAS Direct Submission. and precipitated crystalline dendrites characterized by a rela- 1To whom correspondence should be addressed. E-mail: [email protected] tively low shear modulus (compared with the glassy matrix). © 2008 by The National Academy of Sciences of the USA

20136–20140 ͉ PNAS ͉ December 23, 2008 ͉ vol. 105 ͉ no. 51 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0809000106 Downloaded by guest on September 29, 2021 posites when G of the crystalline inclusion is lower than G of the nickel have restricted solubility in equilibrium b.c.c. Ti (and Zr) glass (1). The underlying reason for this can be understood by and are thus expected to chemically partition preferentially to examining how a propagating shear band in the matrix interacts the glass matrix during solidification. In contrast, other metals with an inclusion having a strong interface with the matrix. with larger (e.g., aluminum) have extensive solu- Noting the similarity between an operating shear band and a bility in b.c.c. Ti (Zr) and also increase the shear modulus, G,in mode II or mixed mode crack, one can follow an analysis similar the b.c.c. phase. For this work, we chose copper as a late- to that of He and Hutchinson (21) to show that the shear band transition addition because it has been shown to enhance will be attracted to and penetrate the inclusion if the Dundurs’ the fracture toughness of monolithic BMGs (compared with Co or Ni). The alloys reported here contain only 3–5 at% copper, parameter ␣ ϭ (Gdendrite Ϫ Gmatrix)/(Gdendrite ϩ Gmatrix) Ͻ 0. For a significantly positive ␣, the shear band is expected to be yet GFA (for the corresponding BMG–matrix composites) is Ͼ deflected away from the inclusion. A penetrating shear band will found to be 2 cm in solidified ingots. Addition of small amounts be arrested in the inclusion if deformation therein is stabilized of late-transition metals (Cu, Ni, Co, etc.) dramatically enhances by work hardening, whereas a deflected shear band will continue GFA of the matrix and thus the processability of composites. In contrast to late-transition metals, we note that the refrac- to extend by propagating in the matix. In Ti Zr Be , the 40 30 30 tory b.c.c. metals (e.g., Nb and V) partition preferentially to the dendrite is a b.c.c. Ti–Zr phase with G Ͼ 40 GPa, whereas the dendrite during solidification, The Nb and V concentrations glass is a Ti–Zr–Be phase with G Ϸ35 GPa. In this alloy, shear observed in the glass matrix are generally Ͻ5 at%, which implies bands are expected to be deflected or only weakly attracted by that their addition can dramatically reduce G of the dendrite the dendrite. To arrest shear band extension and increase the ␤ phase without compromising GFA or the fracture toughness of tensile ductility and fracture toughness, -stabilizers must be the BMG matrix phase. We have previously reported Zr–Ti- used that reduce G in the dendritic phase. Additions of Nb and based ductile phase composites containing the ␤-stabilizers V to b.c.c. Ti-based (or Zr-based) solid to a , , and . These ‘‘high-density’’ shear-softening phenomenon of electronic origin that dramati- elements raise the overall density of the composites substan- cally lowers G in the solid solution [typically G is reduced to as tially. In this work, we used the lower-density ␤-stabilizer low as 20–30 GPa (3)]. . A composite with Ϸ50% glass phase by volume can It is well known from our work with Vitreloy-type BMGs (e.g., be achieved when the at% of beryllium and copper sum to Ϸ20% Zr41.2Ti13.8Cu12.5Ni10Be22.5; Vitreloy 1) that the addition of late and the Ti/Zr ratio is approximately unity. We systematically add SCIENCES transition metals such as copper, nickel, , and in- vanadium and find that G of the dendrite drops below G of the creases the GFA from several mm to several cm (22) in glass. Substantial toughening is observed in the composite. We APPLIED PHYSICAL monolithic Zr–Ti–Be-based glasses. We notice that copper and then further increase the Ti/Zr ratio to lower density.





Fig. 1. Tensile ductility in titanium-based metallic glass composites. (A) Room tension tests for the 6 BMG composites developed in this work compared with commercial pure titanium and Ti-6Al-4V. A maximum stress of 1.6 GPa is obtained, and each alloy exhibits Ͼ5% tensile ductility. (Inset)An example of necking in the alloy DV1 is shown. (B) Optical images of necking in commercial pure titanium (Left) and Ti-6Al-4V (Right). (C) Dense shear band pattern on the tensile surface of DV1. (D and E) SEM micrographs showing necking in the 6 BMG composites (D) along with their respective microstructures (E).

Hofmann et al. PNAS ͉ December 23, 2008 ͉ vol. 105 ͉ no. 51 ͉ 20137 Downloaded by guest on September 29, 2021 We present results for several alloys. The first, fractions of glass to retain tensile ductility, 31% and 20% in 3 Ti48Zr20V12Cu5Be15 (DV1), with ␳ ϭ 5.15 g/cm , was produced DVAl1 and DVAl2, respectively. It is noteworthy that DVAl2 in large ingot form by the semisolid processing method as contains 62% titanium and only 0.9% beryllium by weight. This reported (1). DV1, which is the marquee alloy in this study, represents both the highest amount of titanium and the lowest partitions during solidification into 53 vol% of a glass phase with amount of beryllium that we have observed in BMG-dendrite composition Ti32Zr25V5Cu10Be28 and 47% b.c.c. dendritic phase composites with Ͼ5% tensile ductility. with composition Ti66V19Zr14Cu1, as determined through As a direct comparison between the BMG composites and energy-dispersive X-ray spectrometry. These values have an competing crystalline titanium alloys, we have included tension estimated error of 5%. DV1 exhibits 12.5% total strain to failure tests in Fig. 1A for commercial pure titanium (CP-Ti or grade 2) at 1.4-GPa maximum stress in room temperature tension testing, and Ti-6Al-4V (Ti-6-4 or grade 5 annealed). As expected, CP-Ti as seen in Fig. 1A. Despite having low density and large volume exhibits low ultimate strength (Ϸ400 MPa) but extensive elon- fraction of glass, DV1 exhibits extensive shear band stabilization, gation (Ϸ25%). Ti-6-4 in the annealed condition has ultimate evidenced by the serrated nature of the tension curve just before strength of Ϸ850 MPa combined with Ϸ16% total strain. Ti-6-4 failure. Each drop in stress is associated with a normally cata- was selected because of its low density (4.43 g/cm3) and because strophic shear band being arrested by the microstructure, leading it accounts for the majority of commercial titanium applications. to significant necking (43% reduction in area) shown in Fig. 1A Both tests were done in the same 3-mm diameter as the BMG Inset. The necking instability is associated with dense patterns of composites with the same sample geometry, shown in Fig. 1B.It primary, secondary, and even tertiary shear bands that are is clear that both crystalline alloys exhibit global deformation visible on the surface (see Fig. 1C). DV1 also displays a throughout the entire tensile gauge length while reduction in noticeable amount of overall work hardening before the necking area at the neck is Ͻ30%. In contrast, BMG composites exhibit instability, something that is not seen in previously reported a localized necking instability with larger reductions in area BMG composites. relative to their crystalline counterparts. We used the optimized structure of DV1 to design 3 more Although not shown, we notice that the tensile ductility and alloys that sample the Ti–Zr–V–Cu–Be system. First, we inves- yield strength of the titanium-based BMG composites is similar tigated the effect of increasing the of glass to to some common high-strength crystalline titanium alloys. Ti-6-4 create a high-strength composite. By directly replacing titanium in the standard condition (as opposed to the annealed condi- with beryllium we can increase the volume fraction of glass to tion), for example, has an ultimate strength of Ϸ1.1 GPa and 70% and retain 9.5% total strain to failure, but increase the Ϸ10% elongation, leading to a of 264 MPa ultimate tensile strength to 1.6 GPa. This alloy, cm3/g, higher than 4 of the BMG composites presented here but Ti44Zr20V12Cu5Be19 (DV2), is the highest strength (combined lower than DV1 and DV2. The tension tests are similar despite with the largest fraction of glass) alloy we have yet observed that the significantly lower Young’s modulus of the BMG composites Ͼ still exhibits 5% tensile ductility. The high strength, combined (78–94 GPa versus Ϸ115 GPa) and the larger elastic limit (Ϸ2% 3 with low density (5.13 g/cm ), puts the specific strength of DV2 versus Ϸ1%). (defined here as maximum stress divided by density) at 314 MPa One processing advantage of the BMG composites over 3 cm /g, which approaches the specific strength of Vitreloy-type crystalline titanium alloys is their low solidus temperature. monolithic BMGs (22). Vitreloy 1, for example, has a specific strength of 285–330 MPa cm3/g (depending on the tensile yield strength, which varies from Ϸ1.7 to 2.0 GPa because of material processing). A B In an attempt to minimize density, we also report 2 alloys with increased volume fraction of b.c.c. phase, Ti56Zr18V10Cu4Be12 and Ti62Zr15V10Cu4Be9 (DV3 and DV4, respectively). DV3 and DV4 exhibit densities of 5.08 and 5.03 g/cm3 with 46% glass and 40% glass, respectively. SEM micrographs of the necking in DV1–DV4 can be seen in Fig. 1D in order of descending volume fraction of glass phase (DV2, DV1, DV3, and DV4). The typical microstructures of the composites are shown directly below the respective tension test in Fig. 1E. We note that there appears to be a limit of Ϸ5.0 g/cm3 in the minimization of density when using the Ti-Zr-V-Cu-Be system. Noticing the good GFA of nonberyllium-containing BMGs, such C D as Zr57Nb5Cu15.4Ni12.6Al10 (Vitreloy 106) (23), and high-strength crystalline titanium alloys containing vanadium, such as Ti- 6Al-4V (in wt%), we note that aluminum is a beneficial addition to both systems. Therefore, we attempted to add aluminum to our BMG composites to lower density. With the addition of up to 10 at% aluminum, GFA was not degraded. Unfortunately, aluminum is a potent ␣-stabilizer for titanium and small addi- tions also substantially increase G. If the aluminum content exceeds Ϸ3 at%, tensile ductility rapidly falls to 0. We further report 2 aluminum-containing alloys, Ti60Zr16V9Cu3Al3Be9 and Ti67Zr11V10Cu5Al2Be5 (DVAl1 and DVAl2, respectively). Alu- minum exhibits partial solubility in b.c.c. titanium so it divides Fig. 2. Bending ductility in the titanium-based BMG composites. (A) Twenty- evenly between glass and dendrite. Therefore, small additions of five- master ingot of DV1 after being produced on the arc melter. (B) aluminum can be used to supplement copper for improving Ingot of DV1 undergoing semisolid processing in a -cooled copper boat. 3 3 GFA, resulting in alloys with density Ͻ5 g/cm (4.97 g/cm for (C) Beams of DV4 (front) and DV3 (middle) bent in a vice, illustrating bending both DVAl1 and DVAl2). Minimizing density requires reduc- ductility. (D) Several samples of the alloy DVAl2 demonstrating large GFA and tions in the Zr content, which leads to alloys with lower volume bending ductility. This alloy contains only 0.9 wt% beryllium (5 at%).

20138 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0809000106 Hofmann et al. Downloaded by guest on September 29, 2021 Table 1. Mechanical properties of titanium-based metallic glass composites

␴max/r, K1C, BMG, bcc, r, ␴y, ␴max, ␧y, ␧tot, MPa RoA, MPa E, G, Ts, Name at% wt% % % g/cm3 MPa MPa % % cm3/g % m1/2 GPa GPa ␷ K

DV2 Ti44Zr20V12Cu5Be19 Ti41.9Zr36.3V12.1Cu6.3Be3.4 70 30 5.13 1597 1614 2.1 9.5 315 34 NA 94.5 34.8 0.358 956 DV1 Ti48Zr20V12Cu5Be15 Ti44.3Zr35.2V11.8Cu6.1Be2.6 53 47 5.15 1362 1429 2.3 12.5 277 43 43.8 94.2 34.4 0.368 955 DV3 Ti56Zr18V10Cu4Be12 Ti51.6Zr31.6V9.8Cu4.9Be2.1 46 54 5.08 1308 1309 2.2 8.6 258 31 47.4 84.0 30.5 0.379 951 DV4 Ti62Zr15V10Cu4Be9 Ti57.3Zr26.4V9.8Cu4.9Be1.6 40 60 5.03 1086 1089 2.1 9.8 217 42 61.6 83.7 30.4 0.377 940 DVAl1 Ti60Zr16V9Cu3Al3Be9 Ti55.8Zr28.4V8.9Cu3.7Al1.6Be1.6 31 69 4.97 1166 1189 2.0 9.3 239 27 NA 84.2 31.0 0.360 901 DVAl2 Ti67Zr11V10Cu5Al2Be5 Ti62.4Zr19.5V9.9Cu6.2Al1Be0.9 20 80 4.97 990 1000 2.0 8.4 201 28 NA 78.7 28.6 0.376 998 Ti-6–4a Ti86.1Al10.3V3.6 Ti90Al6V4 (grade 5 annealed) NA NA 4.43 754 882 1.0 16.4 199 42 100.0 113.8 44.0 0.342 1877 Ti-6–4 s Ti86.1Al10.3V3.6 [Ref] Ti90Al6V4 (grade 5 STA) NA NA 4.43 1100 1170 ϳ1 ϳ10 264 NA 43.0 114.0 44.0 0.330 1877 CP-Ti Ti100 Ti100 (grade 2) NA NA 4.51 380 409 0.7 25.5 91 46 66.0 105.0 45.0 0.370 ϳ1930

BMG composites are shown with crystalline titanium counterparts in both wt% and at%. Reported values are density (␳), yield stress (␴y), ultimate tensile stress (␴max), yield strain (␧y), total strain (␧tot), specific strength (␴max/ ␳), reduction of area (RoA), Young’s modulus (E), shear modulus (G), Poisson’s ratio (␷), and solidus temperature (K). NA, not available.

Owing to the presence of a glass matrix, typically designed plates in conformance to the relaxed thickness requirements of around a deep eutectic, the BMG composites exhibit a solidus ASTM International standards. K1C values of 43.8, 47.4, and temperature that is Ϸ900 K lower than their crystalline coun- 61.6 MPa m1/2 were recorded for DV1, DV3, and DV4, terparts. In the BMG composites, the solidus temperature is that respectively. The high-strength crystalline alloy Ti-6-4 in the 1/2 of the eutectic liquid that forms the glass on undercooling. Above standard condition exhibits K1C ϭ 43.0 MPa m , similar to the solidus, the dendrites coexist in equilibrium with the glass- that observed in the BMG composites. Other mechanical and forming liquid, creating a semisolid that can be formed acoustical data on the BMG composites are summarized in

into net shapes (into copper molds for instance). In contrast, Table 1. Fig. 2D shows several large samples of DVAl2 having SCIENCES titanium alloys cannot be easily die-cast and must typically be significant bending ductility in up to Ϸ4-mm-thick beams, APPLIED PHYSICAL machined (postcasting) to achieve net-shaped parts. Machining despite being nearly beryllium free. of Ti-alloy components is generally costly. Compared with previously reported BMG composites, the As a further comparison between Ti-based BMG composites titanium-based BMG composites reported here greatly extend and crystalline titanium alloys, plane-strain fracture toughness the potential applications by reducing alloy cost and lowering measurements were performed on 3 of the best alloys, DV1, overall density, while maintaining the ability to suppress the DV3, and DV4. Fig. 2A illustrates the arc-melting procedure catastrophic shear failure endemic to monolithic BMGs. These used to create the master ingots of the composites. It should composites maintain desirable characteristics of monolithic be noted that the ingots produced in the arc-melter have an BMGs (high strength, high elastic limit, low processing tem- amorphous matrix before semisolid processing, demonstrating peratures, etc.) but simultaneously achieve combinations of the large GFA. The semisolid processing method used to engineering properties (toughness and tensile ductility) that homogenize and coarsen the dendrites is shown in Fig. 2B. The compare favorably with common high-strength crystalline difference in the scale of the microstructure before and after titanium alloys. semisolid processing can be seen clearly by comparing the Materials and Methods ingot in Fig. 2A with Fig. 2B. Previously, maximum bending The alloys used in this work were prepared from titanium and zirconium thickness of BMGs was correlated with fracture toughness, crystal and other elements with purity Ͼ99.5%. Master ingots of titanium– K1C. To demonstrate the influence of K1C on bend ductility, we vanadium were prepared by plasma arc melting, and other elements were took ingots of the composites, clamped them in a vice, and bent prealloyed and added later. Semisolid processing was done as described (1). them with repeated hammer strikes. The maximum thickness Tension tests and fracture toughness tests were done in accordance with for which significant bending is observed can be related to K1C ASTM International standards, where applicable, as described (1). Samples of (through the size of the zone at the tip of a crack). From crystalline titanium alloys were supplied by McMaster Carr. Fig. 2C we see that DV4 (in the front of Fig. 2C) can be bent to Ͼ90° at 3 mm, whereas at 4 mm, significant cracking is ACKNOWLEDGMENTS. We thank C. E. Hofmann for insightful discussions. This work was supported by the U.S. Office of Naval Research. D.C.H. was sup- observed. DV3 (in the middle of Fig. 2C) fractures closer to 30° ported by the U.S. Department of Defense through the National Defense at 3 mm. K1C measurements were performed in 3-mm-thick Science and Engineering Graduate fellowship program.

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20140 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0809000106 Hofmann et al. Downloaded by guest on September 29, 2021