Glass-Forming Ability and Competitive Crystalline Phases for Lightweight Ti-Be Based Alloys

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1 Glass-Forming Ability and Competitive Crystalline Phases for Lightweight Ti-Be Based Alloys Y. ZHANG, W.G. ZHANG, J.P. LIN, G.J. HAO, G.L. CHEN, and P.K. LIAW The glass-forming ability (GFA) for the Ti-Be based alloys in the Ti-Be-Zr ternary system is systematically studied. It was found that the best GFA obtained at a composition of Ti 41 Be 34 Zr 25 (at. pct) in the Ti-Be-Zr ternary system, and the bulk-metallic-glass (BMG) rod samples with a diameter of 5 mm were fabricated by Cu-mold casting. The competitive crystalline phases around the composition of the best GFA materials were determined by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). The GFA of the ternary alloys was further improved by an addition of 4 at. pct vanadium (V). The largest supercooled liquid region, DT x (DT x = T x T g, T g is the glass-transition temperature, and T x the crystallization temperature), in the ternary alloy system reaches about 110 K (110 C) for the Ti 35 Be 32 Zr 33 alloy. DOI: /s Ó The Minerals, Metals & Materials Society and ASM International 2010 I. INTRODUCTION BULK-METALLIC GLASSES (BMGs) have attracted extensive interest due to their unique physical, chemical, and mechanical properties attributed to the randomatomic configurations. Owing to their high glassforming ability (GFA), good processing ability, and exceptional stability with respect to the crystallization, along with many promising properties, such as high strengths, elastic strain limits, wear resistance, fatigue resistance, and corrosion resistant, BMGs have been the focus of the scientific research over the past 20 years. [1 19] Much more attention was paid to Ti-based amorphous alloys because of their high specific strength and relatively low cost. However, most currently available Ti-based BMGs with high GFA contain a high portion of late transition metals (LTMs) (e.g., Ni, Cu, and Sn), which are necessary for glass formation, and the specific strength of these BMGs is compromised of the high density of these LTMs. In this article, we attempted to exclude the LTMs in Ti-based BMGs to further improve their specific strength. The reported scheme based on the binary eutectic clusters is applied to predict a base alloy in the Ti-Zr-Be system. As a result, there is a class of Ti-based bulk amorphous alloys excluding LTMs with Y. ZHANG, J.P. LIN, and G.L. CHEN, Professors, and G.J. HAO, Research Associate, are with the State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing , China. Contact yongzhangustb@gmail.com W.G. ZHANG, formerly Graduate Student, State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, is with the Chinese Academy of Space Technology, Beijing , China. P.K. LIAW, Professor, is with the Department of Materials Science and Engineering, The University of Tennessee, Knoxville TN This article is based on a presentation given in the symposium Bulk Metallic Glasses VI, which occurred during the TMS Annual Meeting, February 15 19, 2009, in San Francisco, CA, under the auspices of TMS, the TMS Structural Materials Division, TMS/ASM: Mechanical Behavior of Materials Committee. high GFA, exceptional thermal stability, very high specific strength, and large compressive plasticity. Tanner [20 23] reported that amorphous ribbons (typically 30-lm thick) in the Ti-Be, Zr-Be, and Ti-Zr-Be systems could be prepared by splat-quenching or meltspinning techniques at high cooling rates of ~10 6 Ks 1. Recently, Wiest et al. [24,25] reported a series of ZrBeTi ternary BMGs with high GFA. By tailoring the primary dendrite phase, a tensile ductility was achieved in the Ti-based BMG composite. [26] In the present work, the binary eutectic-cluster-rule [27] and eutectic-coupled-zone methods [3,4] were applied to optimize the GFA in the Ti-Be-Zr alloy system. The article intends to clarify the glass-formation zone and its crystal phases. II. EXPERIMENTAL PROCEDURES Alloys were prepared using elements with purity better than 99.9 wt pct. The elements were weighed to be within ±0.1 pct of the calculated mass to ensure accurate compositions and were ultrasonically cleaned in acetone and ethanol before melting. Typically, 10-g ingots were arc melted on a Cu hearth in a Ti-gettered argon atmosphere and flipped at least 3 times to ensure the chemical homogeneity. The mass deviation after melting was maintained to be within ±0.1 pct from the originally weighed alloy before melting. Alloy rods with diameters of 3, 5, and 8 mm were prepared by means of copper-mold-suction castings. The phase structural identification of these alloys was carried out by means of the X-ray diffractometry (XRD) on the sections of the rods using Cu K a radiation (k = nm). The thermal properties associated with the glass transition and crystallization of the amorphous phase were measured using differential scanning calorimetry (DSC). The DSC traces were monitored during continuous heating from 373 to 1273 K (100 to 1000 C) using a constant heating rate of 0.33 K/s (0.33 C/s). The melting process of the

2 alloys was studied by Perkin Elmer differential temperature analysis (DTA), DTA-7, with a heating rate of 0.33 K/s (0.33 C/s); the experimental detail can be seen in Reference 28. Specimens with a diameter of 2 mm and an aspect ratio of 2 were compressed between two SiC platens at an engineering strain rate of 10-4s-1. The density of bulk glassy alloys was measured by the Archimedean principle. III. RESULTS As for exploring the glass formation of Ti-Be based alloys, 20 Ti-Be-Cu ternary alloys, 10 Ti-Be-Si ternary alloys, and 10 Ti-Be-Y ternary alloys were cast into 3-mm-diameter-rod samples. However, no glass phase formed after being detected by the XRD analysis. After that, the study focused on the ternary Ti-Be-Zr alloys. Totally, more than 50 alloys along 8 composition lines in the Ti-Be-Zr ternary alloy system were studied by arc melting and suction casting into 3-mm-diameter-rod samples, in which 17 alloys can form BMGs in 3-mm rods. The glass-formation compositions were shown in Figure 1. The alloy marked by red circles was found to have the best GFA, which can form BMG with at least 5-mm-diameter-rod samples. The typical alloys studied in this work were also listed in Table I. Figure 2 shows the glass-formation zones of 3 and 5 mm in the partial ternary-phase diagram. It is obvious that there are two glass-formation zones in the Ti-rich Ti-Be-Zr ternary phase diagram. The first one with higher Be and Ti contents can form BMGs in a 5-mm diameter, while the other one with the more Zr content can only form 3-mm BMGs. Figure 3 presents the scanning electron microscopy (SEM) pictures of the casting 3-mm-diameter-rod samples of the alloys of (a) and (b) Ti 55 Be 30 Zr 15, (c) Ti 39 Be 30 Zr 31, (d) Ti 52 Be 32 Zr 16, (e) Ti 54 Be 34 Zr 12, and (f) Ti 55 Be 35 Zr 10. Figures 3(a) and (b) indicate that the casting 3-mm rod of the alloy of Ti 55 Be 30 Zr 15 is mainly of the eutectic of a-ti + BeTi. Figure 3(c) shows that the alloys of Ti 39 Be 30 Zr 31 are mainly composed of the b-ti solid-solution dendrite + amorphous phase. Figure 3(d) demonstrates that the alloy of Ti 52 Be 32 Zr 16 consists of the phases of a-ti + BeTi + Be 2 Zr. Figure 3(e) shows that the alloy of Ti 54 Be 34 Zr 12 is mainly composed of a-ti + BeTi + amorphous phases. Figure 3(f) shows that the alloy of Ti 55 Be 35 Zr 10 mainly consists of the a-ti + BeTi phases. The amorphous phase formation with the competitive crystalline phases can be presented in Figure 4. Figure 4(a) presents the amorphous phase and its competitive primary phases of a-ti, BeTi, and Be 2 Zr, while Figure 4(b) shows the amorphous phase and its competitive crystalline phases of a-ti, Be 2 Zr, and b-zr. The first glass-formation zone contains two intermetallic compounds and one solid solution. The second one includes only one intermetallic compound and two solid solutions. The GFA of the first one is better than that of the second one. This trend will be further discussed later. [27,29] Fig. 1 Alloys studied exhibiting GFA of forming 3-mm-diameterrod BMGs, and the two lines were the connections of the binary glass-formation region in the Ti-Be and Zr-Be binary alloys. Fig. 2 Glass-formation zones of 3-mm and 5-mm rods in the Ti- Be-Zr ternary alloy system. Table I. Amorphous Parameters for the Typical Ti-Be Based Alloys Composition Phase T g (K) T X1 (K) DT X (K) Size (mm) Ti 45 Be 30 Zr 25 Amorphous (293.1 C) (343 C) 50 (50 C) 3 Ti 43 Be 32 Zr 25 Amorphous (312.7 C) (385.8 C) 73.1 (73.1 C) 5 Ti 47 Be 34 Zr 19 Amorphous (317.1 C) (372.5 C) 55.4 (55.4 C) 5 Ti 41 Be 34 Zr 25 Amorphous (300.9 C) (354.9 C) 54.0 (54.0 C) 5 Ti 35 Be 32 Zr 33 Amorphous 588 (315 C) 702 (429 C) 113 (113 C) 3 Ti 41 Be 30 Zr 25 V 4 Amorphous (299.3 C) (372.2 C) 72.9 (72.9 C) 8

3 Fig. 3 SEM pictures of the casting alloys of (a) and (b) Ti 55 Be 30 Zr 15,(c) Ti 39 Be 30 Zr 31,(d) Ti 52 Be 32 Zr 16, (e) Ti 54 Be 34 Zr 12, and (f) Ti 55 Be 35 Zr 10. Figure 5 exhibits the typical DSC heating curves of (a) the Ti 68 x Be 32 Zr x alloys and (b) the Ti 66 x Be 34 Zr x alloys. All the curves present glass-transition and crystallization events. The glass-transition temperature (T g ) and first crystallization temperature (T x1 ) are (300.9 C) and K (354.9 C), respectively, for the Ti 41 Be 34 Zr 25 alloy. The data about the other glass formers are listed in Table I. The largest supercooled liquid region DT x (DT x =T x1 T g ) is obtained at the composition of Ti 35 Be 32 Zr 33, and the DT x is about K (113.5 C). It is interesting that after adding a small amount of vanadium (V), the GFA of the ternary alloys was further improved. Figure 6(a) presents the XRD patterns of Ti 41 Be 34 x Zr 25 V x alloys. It can be seen that the alloy of Ti 41 Be 30 Zr 25 V 4 can form BMGs in an 8-mm-diameter-rod sample. Thus, a small amount of the V addition enhanced the GFA of Ti-Be-Zr ternary alloys. Figure 6(b) presents the DSC heating curves of the crystallization process. It can be seen that the best glass former, Ti 41 Be 30 Zr 25 V 4, has the largest DT x value among the other V-containing alloys, which is about 73 K (73 C). Figure 6(c) exhibits the melting process of the Ti 41 Be 34 x Zr 25 V x alloys. It can be seen that the best glass former, the Ti 41 Be 30 Zr 25 V 4 alloy, has the

4 Fig. 4 Schematic pictures of the two glass-formation zones with its competitive crystalline phases: (a) a-ti, BeTi and Be 2 Zr; and (b) a-ti, b-zr, and Be 2 Zr. lowest liquidus temperature as well as the lowest solidus temperature. Figure 7 shows the typical compressive stress-strain curves of the TiBeZr ternary BMGs. It is clear that some of the TiBeZr ternary alloys can have compressive plasticity and high fracture strength (r b ). The highest specific strength can be 448 MPa/g/cm 3 for the alloy of Ti 45 Be 35 Zr 20. Figure 8 exhibits the typical shear bands on the side surface of the BMG samples. It can be found that the primary shear bands are along a direction, 45 deg, to the compressive stress, while the secondary shear bands are normal to the compressive stress. IV. DISCUSSION A. GFA of the Ti-Be Based Alloys Inoue [30] has proposed that the BMG-forming alloys should be composed of the LTM and early-transition metal (ETM) and simple metal (SM), namely, ETM + LTM + SM. A typical example should be TiCuBe. However, our experiments showed that the TiBeCualloy system has a lower GFA than that of the TiBeZralloy system. It has been reported that the elements of Y and Si are good for the GFA improvement in the Zr- and Cu-based alloys, [31 36] but the GFA of the TiBeY and TiBeSi is not so high to form the 3-mm BMGs. As for the ternary system of Ti-Be-Zr, it was found to have a much better GFA than the other three. The ternary Ti-Be-Zr can be written as a pseudo-binary Fig. 5 DSC heating curves of (a) Ti 68 x Be 32 Zr x and (b) Ti 66 x Be 34 Zr x BMG alloys. of (Ti, Zr)-Be, because Ti and Zr are of the same group in the element Periodic table. The TiBeZr alloys without containing the LTM would make the BMG lightweighted, because the LTM usually has a high density. For example, Cu, Fe, or Ni has a density greater than 7 g/cm 3. It is interesting that a small amount of V can improve the GFA of the TiBeZr greatly. Wiest et al. also reported that Cr has similar effects. [24,25] B. Compositions of the Ti-Be-Zr Based BMGs Tanner [20 23] has found the glass-formation regions for the binary of Ti-Be and Zr-Be alloys and, thus, linked them to the binary glass-formation terminal points, as shown in Figure 2. Tanner reported a ternary glassy alloy with a composition of Ti 50 Be 40 Zr 10, which has a trademark of METGLAS 2204 (Allied Chemical Corp., Morristown, NJ). Wiest et al. [24,25] observed the glass-formation composition of Ti 45 Be 35 Zr 20, which can form 6-mm-diameter-rod samples. The binary-eutectic

5 Fig. 7 Compressive stress-strain curves of the Ti 41 Be 32 Zr 27 Ti 37 Be 32 Zr 31 alloys. and Fig. 6 (a) XRD patterns, (b) DSC heating curves showing the glass transition and crystallization, and (c) DTA curves showing the melting process of Ti 41 Be 34 x Zr 25 V x alloys. Fig. 8 Typical shear bands on the side surface of the Ti 41- Be 30 Zr 25 V 4 BMGs.

6 rule [27] was used to predict the glass-formation composition, C A, in the ternary eutectic of A-B-C: C A ¼ a½euða-bþšþb½euðb-cþšþc½euðc-aþš The (Ti, Zr)-Be pseudo-binary system can be regarded as an ideal glass-forming liquid, and its composition C A can be obtained as follows: C A ¼ a½euðbe-tiþšþb½euðbe-zrþš ½1Š where a and b are constants, a + b = 1, and Eu denotes the eutectic composition (e.g., Eu(Be-Ti) denotes the eutectic composition of the Be-Ti binary system). As suggested in Reference 27, a and b values can be determined in a way that clusters with a large negative heat of mixing should have a relatively low concentration. Thus, adh Ti-Be ¼ bdh Zr-Be ½2Š where DH Ti-Be and DH Zr-Be are heats of mixing for the atomic pairs Ti-Be and Zr-Be, respectively. From the binary-phase diagrams of Be-Ti and Be-Zr, Eu(Be-Ti) = Ti 70 Be 30 and [Eu(Be-Zr)] = Zr 65 Be 35. Then a and b can be calculated by substituting the heat of mixing of these atomic pairs into Eq. [2], and the predicted glass former is Ti 41 Be 32 Zr 27. The best ternary BMG alloy was found by experiments with a composition of Ti 41 Be 34 Zr 25, which is very close to the predicted composition. C. Competition with Crystalline Phases In this work, two glass-formation zones, glass-zone A (GA) and glass-zone B (GB), were found. Each one corresponds to a ternary-eutectic reaction (EA and EB). The EA is L fi a-ti + BeTi + Be 2 Zr, and the EB is L fi a-ti + b-zr + Be 2 Zr. In the glass-formation zone of GA, the largest glass-casting size is 5 mm, while the GB is 3 mm. This trend may be due to the fact that the EA is composed of two intermetallic compounds and one solid solution, while the EB contains only one intermetallic compound with two solid solutions. Previous work has shown that the intermetallic compounds are easier to be suppressed, as they are usually the ordered phases, [17,20] which need a longer time to order. The interface between the liquid and the compounds usually faceted, and the interface undercooling of the faceted interface was deeper than that of the unfaceted one. All of the preceding factors are beneficial for suppressing the crystallization, thus improving the GFA of the alloys. D. GFA Indicators for the Ti-Be Based Alloys There have been many indicators for predicting the GFA of the alloys. For example, Inoue [7 9] proposed that the larger the DT x, the higher the GFA of the alloy would be. However, in this work, the largest DT x is reached at a composition of Ti 35 Be 32 Zr 33, which is about 113 K (113 C). While the best GFA is at a composition of Ti 41 Be 34 Zr 25, its DT x is about 54 K (54 C). Moreover, after adding 4 at. pct V, the GFA of the alloy increases from 5-mm to 8-mm-diameter rod of BMG samples, but the DT x is about 73 K (73 C). From the preceding facts, it is concluded that the GFA does not have a direct relationship with the DT x, which has also been reported in the La-based BMGs. [6] V. CONCLUSIONS The Ti-Be-Zr ternary alloy system exhibits higher GFA than those of the Ti-Be-Cu, Ti-Be-Y, and Ti-Be-Si ternary alloy systems. The BMG composition predicted by the binary-eutectic rule is very close to the experimental results. The ternary eutectic consisting of two solid solution phases and one intermetallics exhibits lower GFA than that of the one solid solution and two intermetallics. The DT x has no direct relation with the GFA of the alloys. The quaternary alloys of Ti 41 Be 30 Zr 25 V 4 can form BMGs with a diameter of 8 mm. The specific strength of the Ti 45 Be 35 Zr 20 BMG is 448 MPa/g/cm 3. 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