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Stable U-based metallic glasses
Journal of Alloys and Compounds 684 (2016) 75e83
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Stable U-based metallic glasses H.G. Huang a, H.B. Ke a, Y.M. Wang b, **, Z. Pu a, P. Zhang a, P.G. Zhang a, T.W. Liu a, * a b
Institute of Materials, China Academy of Engineering Physics, Mianyang 621900, China Key Laboratory of Materials Modification (Ministry of Education), Dalian University of Technology, Dalian 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2016 Received in revised form 11 May 2016 Accepted 13 May 2016 Available online 15 May 2016
A series of U-based U-Co-Al metallic glasses with greatly improved glass-forming ability and thermal stability were obtained by the alloying addition of Al into U-Co alloys. The fragility of these glasses approached 28, indicating strong liquid behavior. The corrosion resistance and mechanical hardness of the glasses were substantially enhanced in comparison with crystalline uranium materials. The roles of delocalization of 5f electrons and their bonding states are stressed in the discussion of the glass formation, thermal stability and the deviation of elastic modulus from the rule of mixtures. The finding not only adds a new member to the family of metallic glasses, but also provides a category of potentially useful uranium materials. © 2016 Elsevier B.V. All rights reserved.
Keywords: Metallic glass Uranium alloy Corrosion resistance Glass-forming ability Mechanical property
1. Introduction The formation, structures and properties of metallic glasses have been investigated in a wide variety of binary and multicomponent alloys [1e7]. Some criteria for easy glass formation (or glass-forming ability) are now well established in these systems [8e13]. In general, the ease of glass formation is determined by both thermodynamic factors that favor the stability of undercooled alloy liquids rather than their crystalline counterparts and kinetic conditions that inhibit the crystallization of the liquids. All these factors are dependent on the energetic characteristic of a metallic solid, and hence at a fundamental level, are correlated with the strength and directionality of electronic bonding and the electron density of states at the Fermi level (EF). Early actinide metals exhibit unique electronic structures. In these elements 5f electron states are partially delocalized and the electron density of states across EF is narrow and high [14,15]. A narrow band is prone to hybridize with other bands close in energy, and the electrons in such band are highly sensitive to perturbations. This would cause strong chemical activity of early actinide metals in a solid state [16], and also lead to unusual glass formation behavior in their alloys [17].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y.M. Wang), [email protected] (T.W. Liu). http://dx.doi.org/10.1016/j.jallcom.2016.05.124 0925-8388/© 2016 Elsevier B.V. All rights reserved.
The rapid liquid quenching preparation and thermal stabilities of binary actinide-TM (transition metals: V, Cr, Mn, Fe, Co, Ni) glasses were reported in the early studies of Ray and Musso [18], Giessen and Elliott [19], and Drehman and Poon [20]. U-LTM (late transition metals: Fe, Co, Ni) alloys among them were found more readily to form metallic glasses. Especially, the composition ranges of uranium glasses covered unusually low LTM concentrations, e.g., U85.7Fe (or Co)14.3 (in atomic percentage; at.%), which is not consistent with the prediction of atomic size criteria [13], and is also in contrast to experimental observations in the common binary glass-forming alloy systems [21e23]. With regard to thermal stability, heat release upon the crystallization of uranium-LTM glasses was determined in the range of 3e7 kJ/mol [17,19,20,24] which is comparable to those of other metallic glass formers. Glass transition, however, was not observed in the calorimetric analysis of previous uranium glasses [17,18,20]. Corrosion has been a serious problem in the practical applications of crystalline uranium alloys [16], so the corrosive property of uranium metallic glasses deserves a comprehensive study. In previous studies, a preliminary evaluation of corrosion resistance for U-(Cr, V) glasses was made merely from the color change of their sample surface [18]. Our recent study of rapidly quenched U-Co alloys showed that their glassy samples were much more corrosively resistant than depleted uranium in a 0.002 M NaCl solution [24]. The previous findings on glass formation and limited property studies for uranium alloys suggest that a stable U-based glass is quite appealing for a systematic investigation of thermal stability, mechanical and
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corrosive properties, etc. Bulk metallic glasses are sufficiently stable against crystallization to explore supercooled liquid state over a wide temperature span above the glass transition temperature Tg [5,8,25]. Large complexities of element species and atomic size, and their favorable combination are recognized to account for high thermal stability and glass-forming ability of such glasses. In light of chemical factors involved in bulk-glass-forming alloys, stable uranium metallic glasses may be obtained in a multi-component system. Considering the radioactivity and chemical toxicity of uranium alloys, the extensive trial-and-error procedure may not be appropriate. It would be desirable to first consider potential alloy compositions in a ternary system. Wang [26] proposed that the ternary systems possessing MgCu2-and MgNi2-type equal-atomicratio Laves phases, e.g. Zr1/3(Ni, Co, or Cu)1/3Al1/3 and Ln (rare earth metals)1/3(Ni, Co, or Cu)1/3Al1/3 are possible easy glass formers, and that metallic glasses are most likely to form at the compositions enriched in large atomic size constituents. Therewith, the formation of a stable glass is expected to occur in U-rich U-(Co, Fe, or Ni)-Al alloys far apart from U1/3(Ni, Co, or Fe)1/3Al1/3 Lave phase compositions [27,28]. In the present work, we report the formation, thermal stabilities, mechanical and corrosive properties of U-Co-Al metallic glasses. It is found that stable glasses with calorimetrically accessible Tg were formed in the U-rich U-Co-Al alloys, and the new glasses exhibited excellent corrosion resistance, high mechanical hardness and elastic modulus, and a rather small fragility parameter characteristic of strong glass-forming liquids. 2. Experimental Our previous study indicated that metallic glasses in a continuous ribbon form are more readily formed within the Co concentration range of 30e40 at.% in U-Co system [24]. By taking U69.2Co30.8 and U62.5Co37.5 as two basic compositions and Al as the alloying element, three series of ternary alloys, namely, (U69.2Co30.8)100xAlx, U69.2Co30.8xAlx and U62.5Co37.5xAlx (x ¼ 0e17.5) were produced. The purities of the starting metals were 99.5 wt% for depleted uranium, 99.9 wt% for cobalt and 99.99 wt% for aluminum. Alloy ingots with the designed compositions were prepared by arc-melting mixtures of the constituent metals in a water-cooling copper crucible in a Ti-gettered Ar atmosphere with a background vacuum level of about 5 103 Pa. Each ingot with about 5 g in mass was remelted four times to ensure compositional homogeneity. These ingots were placed in quartz tubes to conduct liquid-quenching and continuous ribbon samples were made by using a single melt-spinner at a surface velocity of ~50 m/s. The cross-section dimensions of as-spun samples were about 1 mm wide and 20e30 mm thick. The structures of U-Co-Al ribbons were examined with an X-ray diffractometer (Cu-Ka radiation) by means of the standard q2q scan within a diffraction angle range of 2q ¼ 20e100 . Differential scanning calorimetry (DSC) studies were carried out at a constant heating rate of 0.333 K/s. For a special stable uranium glass, DSC measurement was also done at different rates from 0.167 K/s to 2.667 K/s for the derivation of both the fragility parameter of the undercooled liquid and the activation energy for the glass transition. The room temperature electrochemical performance of the ribbons was compared with depleted-uranium metal and U-5.5 wt % Nb alloy in a 0.002 M NaCl aqueous solution. Potentiodynamic polarization measurement using a platinum counter electrode and a saturated calomel reference electrode (SCE) was performed during the electrochemical performance examination. Prior to testing, ribbon samples were washed with acetone and distilled water, and then dried in air. The electrochemical polarization was driven at a
potential sweep rate of 1 mV/s after about 1 h open circuit immersion in the solution freely exposed to air. A micro-compression test was carried out for ribbon specimens under displacement control on a Triboscopee Nanoindentation System equipped with a 5 mm Berkovich diamond indenter. The maximum loads of 3 mN and 2 mN were applied for the typical and cycle loading tests, respectively. For each specimen, the test was performed at least ten times to ensure the reliability of the indentation data. The load-depth (P-h) curves of the specimens were also obtained. 3. Results 3.1. Phase identification An amorphous phase was identified for the rapidly quenched UCo-Al alloys in the composition region of 55e75 at.% U, 15e40 at.% Co and less than 20 at.% Al. The compositions and numbers of the alloys are summarized in Table 1, and their locations are shown in the composition chart in Fig. 1a. The X-ray diffraction results indicate that the Nos.3~6, 8, 9 and 13 samples are fully amorphous, and in the other samples a minor crystalline phase coexists with the amorphous phase. The typical XRD patterns of fully and partially amorphous samples are shown in Fig. 1b. It is seen that a tiny but sharp crystal peak appears at 2q z 28 , which cannot be indexed by any of the known binary U-Co and ternary U-Co-Al phases. Under nearly the same quenching conditions, fully amorphous structure was not achieved in U-Co binary system [24]. The addition of Al therefore facilitates glass formation in U-Co-Al system. 3.2. Thermal stability and fragility The DSC traces of all the U-Co-Al ribbons studied are presented in Fig. 2. The crystallization process of the fully amorphous ribbons is dominated by a single exothermic event, whereas double-staged crystallization appears in some partially amorphous ones. Upon melting, most of the U-Co-Al samples exhibit two endothermic peaks except for U62.5Co20Al17.5 and U64Co34Al2 which are associated with a single peak melting process. The data of the onset crystallization temperature (Tx), onset melting temperature (Tm), liquidus temperature (TL) and crystallization enthalpy (DHc) are listed in Table 1. Also included are the values of the reduced temperature (Tx/TL). Tx is found to increase with the Al concentration in each series of the (U69.2Co30.8)100xAlx, U69.2Co30.8xAlx and U62.5Co37.5xAlx (x ¼ 0e17.5) alloys. For the fully amorphous alloys, by comparison with U62.5Co37.5 and U69.2Co30.8 glasses, an increment of about 100 K in Tx was reached, indicating the great enhancement of thermal stability. The DHc values of 3e5 kJ/mol are about a half of those of the two U-Co glasses. Both Tm and TL were raised due to the introduction of Al. Because of the absence of Tg, the Tx/TL parameter has been used to evaluate the glass-forming ability of U-Co alloys [20,24]. As seen from the data in Table 1, the Tx/TL values of the U-Co-Al samples are considerably larger than those of the U-Co glasses. Moreover, Tg is accessible in the fully amorphous U-Co-Al glasses. As shown by the DSC traces of the U64Co28.5Al7.5 glass in Fig. 3a, the feature of Tg becomes more and more distinct and its value increases as the heating rate is raised, which is a kinetic characteristic of glass transition. The Tg value ascended from 612 K at the heating rate of 0.167 K/s to 650 K at 2.667 K/s. At the commonly used heating rate of 0.333 K/s, the Tg value was determined equal to 616 K and therefore the reduced glass transition temperature Trg (¼Tg/TL) was calculated to be 0.567. This Trg value of the U64Co28.5Al7.5 glass is close to those of Zr66Al8Ni26 (0.57), Mg65Cu25Y10 (0.58), La66Al14Cu20 (0.58), Pr60Cu20Ni10Al10 (0.58) bulk metallic glasses [5].
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Table 1 The compositions, thermodynamic parameters and mechanical properties of U-Co-(Al) metallic glasses. Alloy number
Composition
Tg(K)
Tx(K)
Tm(K)
TL(K)
Tx/TL
DHc (kJ mol1)
Er (GPa)
H (GPa)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 / /
U67.8Co30.2Al2 U65.7Co29.3Al5 U64Co28.5Al7.5 U60.5Co27Al12.5 U58.8Co26.2Al15 U69.2Co29Al1.8 U69.2Co25.8Al5 U69.2Co20.8Al10 U69.2Co18.3Al12.5 U62.5Co35Al2.5 U62.5Co30Al7.5 U62.5Co25Al12.5 U62.5Co20Al17.5 U64Co34Al2 U69.2Co30.8 [24] U62.5Co37.5 [24]
e e 617 e 625 595 e e 614 e e e 622 e e e
563 587 638 640 641 639 577 643 649 584 581 593 649 557 543 544
1007 1006 1056 1056 1057 1055 1008 1055 1105 1007 1008 1005 1105 1005 1010 1010
1058 1080 1087 1095 1096 1107 1089 1115 1119 1086 1074 1081 1119 1020 1026 1057
0.532 0.544 0.587 0.584 0.585 0.577 0.530 0.577 0.580 0.537 0.541 0.549 0.580 0.546 0.529 0.515
4.76 5.48 3.58 3.58 3.64 4.47 4.62 3.56 3.04 4.98 4.63 4.77 3.59 6.55 7.0 7.9
77.0 79.3 79.1 88.5 87.0 82.1 / 85.7 86.9 83.5 / 89.8 91.6 / / /
4.3 4.8 4.9 5.1 4.9 5.1 / 4.8 5.5 4.7 / 5.1 5.1 / / /
from heating-rate- dependent Tg data by using the Kissinger’s equation [29]:
. ln 4 Tg2 ¼ Eg RTg þ C
(1)
where 4 stands for the heating rate, R the gas constant and C a constant. A good linear relationship between ln (4/Tg2) and 1/Tg is found for the U64Co28.5Al7.5 glass (Fig. 3b). Accordingly, Eg ¼ 338.2 kJ/mol was worked out. The value is found to lie among those of Ca65Mg15Zn20 (~148 kJ/mol), La60Ni15Al25 (~510 kJ/mol) and Co43Fe20Ta5.5B31.5 (~590 kJ/mol) [30e33]. Also, a fragility parameter (m) can be deduced from Tg measurement with the following equation [34]:
m¼
dlog hðTÞ T ¼ Tg d Tg T
(2)
where h(T) is the viscosity and T the temperature. h(T) is defined by the equation [35]:
hðTÞ ¼ h0 exp
E kB T
(3)
where h0 means the high-temperature limit of the viscosity, E the energy and kB the Boltzmann’s constant. Then we can deduce
m¼
Eg ln 10 RTg
(4)
m z 28 was obtained for the U64Co28.5Al7.5 glass with Tg ¼ 616 K and Eg ¼ 338.2 kJ/mol. This value is slightly smaller than those (m ¼ 32e34) of Ce-based metallic glasses [36]. Generally, m is small for strong liquids such as directional bonding glasses such as SiO2 (m z 16), and large for fragile liquids such as o-terphenyl (m z 150) [35]. Metallic glass-forming liquids have an intermediate fragility of 30e70 [37]. According to this classification, U-Co-Al may belong to a strong glass-forming liquid system. Fig. 1. (a) The composition range of metallic glasses in the U-Co-Al ternary system. The red solid circle denotes fully glassy samples and the blue semi-solid circle is for partially crystallized samples; (b) the typical XRD patterns of Nos. 3, 6, 7 and 13samples. The existence of minor crystalline phase is indicated by the peak at 2q z 28 . (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The activation energy (Eg) for glass transition can be derived
3.3. Corrosion-resistant properties Fig. 4a shows the potentiodynamic polarization curves of the U64Co28.5Al7.5 and U64Co34Al2 glasses, U-5.5 wt% Nb crystalline alloy and pure depleted-uranium metal. The measured data of the corrosion potential (Ecorr) and current density (icorr) are given in Table 2. It is found that the two glasses possessed high Ecorr, which are 410e470 mV and 280e340 mV larger than those of uranium
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Fig. 3. (a) DSC traces of the U64Co28.5Al7.5 (No.3) glass obtained at different heating rates of 0.167~2.667 K/s; (b) the corresponding Kissinger fitting [29]. Fig. 2. DSC traces showing the crystallization (a) and melting behaviors (b) of the UCo-Al metallic glasses at a heating rate of 0.333 K/s.
Rp ¼ ba bc =½2:3ðba þ bc Þicorr and the UeNb alloy, respectively. The value of icorr can be deduced from the intersection of anodic and cathodic Tafel lines. For the U64Co28.5Al7.5, U64Co34Al2, U-5.5 wt % Nb and uranium samples, icorr was determined to be 20.5, 49.9, 141.1 and 415.5 nA/cm2, respectively. Moreover, a basically inverse relationship between icorr and Ecorr can be found for these materials, as displayed in Fig. 4b. Polarization resistance (Rp) can be deduced from the Tafel line in a polarization curve by using Stern-Geary equation [38]:
(5)
where ba and bc are the slopes of anodic and cathodic Tafel lines, respectively. The Rp data of these four materials are included in Table 2, and their varying tendency is plotted in Fig. 4c. Obviously, the U64Co28.5Al7.5 (167.6 kU/cm2) and U64Co34Al2 (76.8 kU/cm2) glasses exhibit much bigger Rp than U-5.5 wt%Nb (30.6 kU/cm2) and uranium (13.2 kU/cm2). Altogether, the room temperature corrosion resistance of Ubased metallic glasses in NaCl aqueous solution is far superior to crystalline uranium materials. It should be noted that the
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Table 2 Tafel extrapolation data for all investigated materials. Sample
Ecorr (mV)
icorr (nA/cm2)
Rp (kU/cm2)
Pure uranium U-5.5 wt%Nb U64Co28.5Al7.5 (No.3) U64Co34Al2 (No.14)
562 430 147 90
415.5 141.1 20.5 49.9
13.2 30.6 167.6 76.8
3.4. Mechanical property Fig. 5a shows three typical load-depth curves derived from three different indentions for the U58.8Co26.2Al15 glass. For each indention, the time duration in the loading, holding and unloading segments was set 5s (see inset of Fig. 5a). These curves were seen to superimpose closely with each other, indicating good reproducibility. Based on the loading and unloading curves, the hardness (H) and reduced Young’s modulus (Er) of the U-Co-Al glass were obtained by direct calculation [39]. The values of Er and H are presented in Table 1. The composition dependence of Er for the two series of U-Co-Al glasses containing 69.2 at.% and 62.5 at.% U is plotted in Fig. 5b. Er tends to increase with raising Al concentration and has a smaller value at a higher U concentration, however. The load-depth curves of 15 cycles loading of the U58.8Co26.2Al15 glass are shown in Fig. 5c. In each cycle, 50% of the maximum load was unloaded before the next loading (see inset of Fig. 5c). The changes of Er and H are plotted as a function of the indentation depth in Fig. 5d. A steep rise in the Young’s modulus is seen when the indentation depth is increased from 5 nm to 20 nm. The phenomenon may be attributed to the dynamic instability of surface nano-layers of a metallic glass [40], but may also be an instrumental false appearance occurring at the initial stage of the nanoindentation. When the indentation depth is greater than 20 nm, Er manifests a weak “softening” feature, while the H data have a stable indication. The “softening” phenomenon has been observed in Zr-based metallic glasses, which is mainly attributed to the size effect of an indenter tip [41]. To estimate the real Young’s modulus (E) of the U-Co-Al glasses, the following equation was adopted [39]:
1 1 n2 1 n2i þ ¼ Er E Ei
Fig. 4. (a) The potentiodynamic polarization curves of the U64Co28.5Al7.5 (No.3), U64Co34Al2 (No.14) glasses, pure uranium and U-5.5wt%Nb alloy; (b) the current density (icorr) versus polarization potential (Ecorr) plot; (c) the comparison of their electrical resistances (Rp).
(6)
where v denotes the Poisson’s ratio of a testing material, vi the Poisson’s ratio of a diamond indenter, Ei the Young’s modulus of the indenter. It can be taken in the present work that vi ¼ 0.07, Ei ¼ 1140 GPa. The v values of U, U-6wt%Nb alloy were 0.21 and 0.35, respectively [42], and those of metallic glasses are typically ~0.3 [43]. By taking v ¼ 0.3 in the present case, E z 0.9Er can be obtained for the U-Co-Al glasses. According to the results from Fig. 5d, it can be attained that E z 81 GPa and H z 5 GPa. The two values are basically close to those of Zr-, Pd- and Cu-based metallic glasses (E ¼ 90 ± 10 GPa, H ¼ 5~6 GPa) [43e45]. Compared with crystalline U-6wt%Nb (E z 51 GPa and H z 2 GPa [46]) and U-10 wt %Mo (H z 3 GPa [42]) alloys, the U-based metallic glasses have attained much higher elastic modulus and hardness. 4. Discussion 4.1. Glass formation and thermal stability
application of crystalline uranium alloys has been greatly limited by their poor corrosion resistance. The highly corrosion-resistant uranium glassy alloys would offer some chance for corrosion protection such as surface coating.
Like common ternary bulk metallic glass-forming alloys, U-CoAl system possesses some favorable thermodynamic factors for the glass formation. The Goldschmidt atomic sizes of U, Al and Co are 0.156 nm, 0.143 nm and 0.125 nm, respectively. These elements
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Fig. 5. (a) Three load-depth curves of typical nanoindentation testing for the U58.8Co26.2Al15 (No. 5) glass; (b) the dependence of reduced Young’s modulus (Er) on Al content for U69.2- and U62.5-series of the U-Co-Al glasses; (c) the load-depth curve of 15 cycles loading for the No.5 glass; (d) Er and hardness (H) as a function of penetration depth for the same glass.
exhibit negative mixing enthalpies, DHUCo ¼ 23 kJ/mol, DHUAl ¼ 30 kJ/mol and DHAlCo ¼ 19 kJ/mol [47]. The disparity in atomic size and the characteristic of mixing enthalpy satisfy the requirement of the Inoue’s empirical criterion [8] proposed for multi-component glass-forming systems. This would also lead to a beneficial thermodynamic condition for the glass formation by making a lower enthalpy for the amorphous state than for a single phase crystalline counterpart possibly available in the U-Co-Al alloys. Tg is a kinetic factor governing easy glass formation. A high Eg for glass transition can be ascribed to large-length-scale correlated motion or rearrangement of atoms in a metallic glass. The Eg value estimated for the U64Co28.5Al7.5 glass is comparable to other bulk metallic glass formers, therefore reflecting the atomic correlation within a large spatial scale in the amorphous structure. As implied by the relatively low fragility parameter (about 28) of this glass, atoms in the U-Co-Al strong liquids would be comparatively closepacked with short- or medium-range order [48]. Furthermore, it is known that early actinide metals including uranium have unusually high viscosity, which is relevant with their 5f electron structures, at the melting temperature and such a high viscosity characteristic would be expected for U-based glassy alloys [49,50]. The crystallization during the undercooling of U-Co-Al liquids could be well frustrated by the combination of the topologically dense atomic packing and the likely high viscosity. The remarkable changes in Tx,
Tm and TL due to the Al addition imply that the nature of the competing crystalline phases may change. The XRD patterns of the crystallized U-Co-Al samples (not presented here), however, are hardly indexed by the known uranium alloy phases. A further study is needed to consider the crystallizing kinetics, and the results will be reported elsewhere. All of these form the kinetic aspects for the ease of glass formation in the U-Co-Al alloys. Elliott and Giessen first pointed out that 5f electrons of uranium may bond more effectively in the liquid states of its alloys [17]. Noticing the narrowness of the f-band [14,15] in uranium, the 5f electrons are likely to hybridize with the extensive p and d partial states from Al or Co [51e53], and hence get more delocalized from the core in the U-Co-Al metallic glasses. Consequently, a high degree of short-range order may be developed by the formation of pseudo-molecules or cluster-like structure units [54] from unlike U-Co and U-Al pairs. Actually, the starting compositions U69.2Co30.8 and U62.5Co37.5 are very close to those of two icosahedral clusters U9Co4 and U8Co5, which are extracted from U-Co crystalline phases and are conjectured as dominant local structure units in the U-Co undercooled liquids. The introduction of Al is expected to decrease the energetic states of the local structures by changing the bonding states and by promoting the packing efficiency of the clusters. As suggested by the eutectic criterion for metallic glass formation [55], an alloy with high glass-forming ability usually has a low TL. However, all the present U-Co-Al alloys exhibited higher TL
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values than U69.2Co30.8 and U62.5Co37.5, even though the melting point of Al (~934 K) is much lower than those of U (~1406 K) and Co (~1770 K). This might be resulted from the enhancement of flow resistance in the U-Co-Al alloy melts, since liquid flow requires the rearrangement of atoms that may be quite difficult in the matrix having strong short- or media-range order. In addition, the great difficulty in destructing such ordered structures can explain larger Tx of the U-Co-Al glasses as compared with the U-Co ones. 4.2. Possible origins for the high corrosion-resistance The values of Ecorr (<400 mV), icorr (>140 nA/cm2) and Rp (<31 kU/cm2) were obtained for pure uranium metal and U-5.5 wt% Nb alloy in the corrosive media of 0.002 M NaCl. For the U-Co-Al glasses, Ecorr (150 ~ 90 mV) and Rp (75e170 kU/cm2) were much higher, while icorr (<50 nA/cm2) was lowered by one order of magnitude. The corrosion resistance of the glassy materials is evidently much better than the crystalline ones. Moreover, the U64Co28.5Al7.5 alloy in a fully glassy state was more anti-corrosive than the partially glassy U64Co34Al2. The possible reasons of these features are discussed from thermodynamic and kinetic viewpoints. First, higher Ecorr of the U-Co-Al glasses can be mainly attributed to lower U concentrations when compared with the U-Nb alloy and uranium, since the standard electrode potential of U (1.8 V) is much lower than Co (0.227 V) and Al (1.660 V). A larger Co content also contributes to higher Ecorr of the U64Co34Al2 glass relative to the other glass. This quality of Ecorr should contribute directly to small icorr observed in these glasses. Second, crystal defects, passivating elements and the adhesion of passivated films to the crystalline or amorphous matrix can be regarded as kinetic factors [56]. A metallic glass as a single phase alloy can trap a large number of solute elements including passivating ones, and is free of crystal defects such as dislocations, grain boundaries and secondary phases. Since Al is one of the most effective alloying elements favoring a high passivating ability of metallic glasses [57], the dissolution of a large amount of Al would result in the formation of a compact passivated Al oxide film on the sample surface. The super anti-corrosive performance of the U-CoAl glasses could be caused by the lack of crystal defects and the passivation. Since the U64Co28.5Al7.5 alloy was formed into a fully amorphous structure but the U64Co34Al2 was not, the inferior corrosion resistance of the latter can be mainly attributed to the presence of crystal defects as well as low Al concentration. Nb element has also been used through passivation for protecting uranium alloys [58,59]. Although the Nb concentration in U-5.5 wt %Nb alloy (¼U87Nb13; at.%) is much higher than the Al concentration in the U64Co34Al2 glass, the corrosion performance of the glass was found in a higher level, which may be a consequence of the good adhesion of the passivating film to the amorphous matrix. 4.3. Deviation of modulus from the rule of mixtures Elastic modulus (M) of a metallic glass can be well predicted by a weighted average of the elastic constants of the constituent elements, i.e., the rule of mixtures (ROM). It is expressed by the formula [43]:
X 1 1 ¼ fi M Mi
(7)
where Mi is the modulus of the ith component and fi the atomic percent of the ith component. In this equation, atomic volume is not considered because its effect on the variation trend of the calculated modulus (Ecal) is negligible [60]. As shown in Fig. 6, the
Fig. 6. Comparison between the calculated (Ecal) and experimental elastic modulus (EMG) of various types of metallic glasses. The conventional metallic glasses follow “the rule of mixtures” [43] while the U-Co-Al glasses exhibit a remarkable deviation from the rule.
experimentally measured modulus (EMG) values of Zr-, Ti-, Pd-, Mg, Cu-, La- and Ni-based metallic glasses are well consistent with the Ecal data [43]. However, as indicated in Fig. 6, the EMG (¼ Er) values of the U-Co-Al metallic glasses are much smaller than those of Ecal calculated by taking 208 GPa as the modulus of U element, displaying a significant difference between EMG and Ecal. Recently, it has been found that metallic glasses can inherit elastic properties from the solvents [61,62]. Since elastic properties of solids including glass are largely determined by their electronic bonding states, abundant 5f electrons of uranium would play a crucial role for elastic response of U-based glasses. Once the 5f electrons are hybridized with the extensive p and d partial states from Al or Co, they would get more delocalized from the core of the uranium ion in the amorphous structure. Then the directionality of the 5f electron wave functions would be significantly disturbed, which could weaken the core-core interference among uranium ions. This naturally leads to a soft mode of correlated atomic vibration, but experimental evidence, for instance, the Debye temperatures of the U-Co-Al glasses, is necessary for further consideration. From the classical viewpoint of atomic motion, it has been proposed that a large difference in atomic weight of constituent atoms can lead to abnormal fast diffusion behavior of light atoms in a metallic glass at a temperature below Tg [63]. The decoupled fast diffusion [64] of Al and Co atoms in the disordered alloy matrix would cause microstructure inhomogeneity, and the volume faction of the so-called liquid-like zone in the matrix [65] can thus be significant. Therefore, the deviation of modulus from ROM could be resulted from the weakened core-core interference and the abundance of the liquid-like zone in the U-Co-Al glasses. 5. Conclusions Metallic glasses are discovered in U-Co-Al alloy system. The compositions, thermal stabilities, corrosive and mechanical properties of these new glasses are concluded in the following: (1) The addition of Al can significantly improve the glassforming ability of U-Co alloys. The formation of amorphous phase by rapid liquid quenching is identified in the composition region containing 55e75 at.% U, 15e40 at.% Co and 0e17.5 at.% Al.
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(2) The U-Co-Al glasses exhibit greatly enhanced thermal stability in comparison with U-Co glasses, and glass transition is accessible in their calorimetric analysis. In addition, U-Co-Al appears to be a strong glass-forming system due to its low fragility (m z 28 for U62.5Co20Al17.5). (3) The ternary glasses possess superior corrosion resistance in NaCl aqueous solution at room temperature, when compared with U-based crystalline materials. (4) The mechanical hardness of the U-Co-Al glasses is much greater than U-based crystals. The deviation of elastic modulus from ROM is observed in the new glass system. (5) The delocalization of 5f electrons and their bonding states are suggested to play a significant role for the improved glass formation, enhanced thermal stability as well as peculiar Young’s modulus behavior of the U-Co-Al glasses. Acknowledgement This research is supported by National Defense Basic Scientific Research (No.B1520133007) and Scientific and Technological Development Foundation of China Academy of Engineering Physics (No.2013A0301015). One of the authors, H. B. Ke would like to appreciate the financial support of National Natural Science Foundation of China (51501169). References [1] B.A. Sun, W.H. Wang, The fracture of bulk metallic glasses, Prog. Mater. Sci. 74 (2015) 211e307. [2] S.Y. Ding, Y.H. Liu, Y.L. Li, Z. Liu, S. Sohn, F.J. Walker, J. Schroers, Combinatorial development of bulk metallic glasses, Nat. Mater 13 (2014) 494e500. [3] J. Schroers, Bulk metallic glasses, Phys. Today 66 (2013) 32e37. [4] Y.Q. Cheng, E. Ma, Atomic-level structure and structure-property relationship in metallic glasses, Prog. Mater. Sci. 56 (2011) 379e473. [5] W.H. Wang, C. Dong, C.H. Shek, Bulk metallic glasses, Mat. Sci. Eng. R. 44 (2004) 45e89. [6] A.A. Mazilkin, G.E. Abrosimova, S.G. Protasova, B.B. Straumal, G. Schutz, S.V. Dobatkin, A.S. Bakai, Transmission electron microscopy investigation of boundaries between amorphous “grains” in Ni50Nb20Y30 alloy, J. Mater. Sci. 46 (2011) 4336e4342. [7] B.B. Straumal, A.R. Kilmametov, A.A. Mazilkin, S.G. Protasova, K.I. Kolesnikova, P.B. Straumal, B. Baretzky, Amorphization of Nd-Fe-B alloy under the action of high-pressure torsion, Mater. Lett. 145 (2015) 63e66. [8] A. Inoue, Stabilization of metallic supercooled liquid and bulk amorphous alloys, Acta Mater 48 (2000) 279e306. [9] Z.L. Long, H.Q. Wei, Y.H. Ding, P. Zhang, G.Q. Xie, A. Inoue, A new criterion for predicting the glass-forming ability of bulk metallic glasses, J. Alloys Compd. 475 (2009) 207e219. [10] W.L. Johnson, J.H. Na, M.D. Demetriou, Quantifying the origin of metallic glass formation, Nat. Commun. 7 (2016) 10313. [11] Z.W. Wu, M.Z. Li, W.H. Wang, K.X. Liu, Hidden topological order and its correlation with glass-forming ability in metallic glasses, Nat. Commun. 6 (2015) 6035. [12] K.J. Laws, D.B. Miracle, M. Ferry, A predictive structural model for bulk metallic glasses, Nat. Commun. 6 (2015) 8123. [13] J.H. Li, Y. Dai, Y.Y. Cui, B.X. Liu, Atomistic theory for predicting the binary metallic glass formation, Mat. Sci. Eng. R. 72 (2011) 1e28. [14] P. Modak, A.K. Verma, Role of 5f electrons in the structural stability of light actinide (Th-U) mononitrides under pressure, Phys. Chem. Chem. Phys. 8 (2016) 8682e8691. [15] K.T. Moore, G. van der Laan, M.A. Wall, A.J. Schwartz, R.G. Haire, Rampant changes in 5f5/2 and 5f7/2 filling across the light and middle actinide metals: electron energy-loss spectroscopy, many-electron atomic spectral calculations and spin-orbit sum rule, Phys. Rev. B 76 (2007) 073105. [16] M.A. Hill, R.K. Schulze, J.F. Bingert, R.D. Field, R.J. McCabe, P.A. Papin, Filiformmode hydride corrosion of uranium surfaces, J. Nucl. Mater 442 (2013) 106e115. [17] R.O. Elliott, B.C. Giessen, On the formation of metallic glasses based on U, Np or Pu, Acta Metall. 30 (1982) 785e789. [18] R. Ray, E. Musso, Amorphous Alloys in the U-Cr-V System, 1976. America, US3981722. [19] B.C. Giessen, R.O. Elliott, Properties of metallic glasses containing actinide metals, in: Proceedings of the 3rd International Conference on Rapid Quenching, 1978, pp. 406e414. Sussex, Brighton, England. [20] A.J. Drehman, S.J. Poon, Anomalous glass-forming ability of uranium-based alloys, J. Non-Cryst. Solids 76 (1985) 321e332.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Stable U-based metallic glasses H.G. Huang a, H.B. Ke a, Y.M. Wang b, **, Z. Pu a, P. Zhang a, P.G. Zhang a, T.W. Liu a, * a b
Institute of Materials, China Academy of Engineering Physics, Mianyang 621900, China Key Laboratory of Materials Modification (Ministry of Education), Dalian University of Technology, Dalian 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2016 Received in revised form 11 May 2016 Accepted 13 May 2016 Available online 15 May 2016
A series of U-based U-Co-Al metallic glasses with greatly improved glass-forming ability and thermal stability were obtained by the alloying addition of Al into U-Co alloys. The fragility of these glasses approached 28, indicating strong liquid behavior. The corrosion resistance and mechanical hardness of the glasses were substantially enhanced in comparison with crystalline uranium materials. The roles of delocalization of 5f electrons and their bonding states are stressed in the discussion of the glass formation, thermal stability and the deviation of elastic modulus from the rule of mixtures. The finding not only adds a new member to the family of metallic glasses, but also provides a category of potentially useful uranium materials. © 2016 Elsevier B.V. All rights reserved.
Keywords: Metallic glass Uranium alloy Corrosion resistance Glass-forming ability Mechanical property
1. Introduction The formation, structures and properties of metallic glasses have been investigated in a wide variety of binary and multicomponent alloys [1e7]. Some criteria for easy glass formation (or glass-forming ability) are now well established in these systems [8e13]. In general, the ease of glass formation is determined by both thermodynamic factors that favor the stability of undercooled alloy liquids rather than their crystalline counterparts and kinetic conditions that inhibit the crystallization of the liquids. All these factors are dependent on the energetic characteristic of a metallic solid, and hence at a fundamental level, are correlated with the strength and directionality of electronic bonding and the electron density of states at the Fermi level (EF). Early actinide metals exhibit unique electronic structures. In these elements 5f electron states are partially delocalized and the electron density of states across EF is narrow and high [14,15]. A narrow band is prone to hybridize with other bands close in energy, and the electrons in such band are highly sensitive to perturbations. This would cause strong chemical activity of early actinide metals in a solid state [16], and also lead to unusual glass formation behavior in their alloys [17].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y.M. Wang), [email protected] (T.W. Liu). http://dx.doi.org/10.1016/j.jallcom.2016.05.124 0925-8388/© 2016 Elsevier B.V. All rights reserved.
The rapid liquid quenching preparation and thermal stabilities of binary actinide-TM (transition metals: V, Cr, Mn, Fe, Co, Ni) glasses were reported in the early studies of Ray and Musso [18], Giessen and Elliott [19], and Drehman and Poon [20]. U-LTM (late transition metals: Fe, Co, Ni) alloys among them were found more readily to form metallic glasses. Especially, the composition ranges of uranium glasses covered unusually low LTM concentrations, e.g., U85.7Fe (or Co)14.3 (in atomic percentage; at.%), which is not consistent with the prediction of atomic size criteria [13], and is also in contrast to experimental observations in the common binary glass-forming alloy systems [21e23]. With regard to thermal stability, heat release upon the crystallization of uranium-LTM glasses was determined in the range of 3e7 kJ/mol [17,19,20,24] which is comparable to those of other metallic glass formers. Glass transition, however, was not observed in the calorimetric analysis of previous uranium glasses [17,18,20]. Corrosion has been a serious problem in the practical applications of crystalline uranium alloys [16], so the corrosive property of uranium metallic glasses deserves a comprehensive study. In previous studies, a preliminary evaluation of corrosion resistance for U-(Cr, V) glasses was made merely from the color change of their sample surface [18]. Our recent study of rapidly quenched U-Co alloys showed that their glassy samples were much more corrosively resistant than depleted uranium in a 0.002 M NaCl solution [24]. The previous findings on glass formation and limited property studies for uranium alloys suggest that a stable U-based glass is quite appealing for a systematic investigation of thermal stability, mechanical and
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corrosive properties, etc. Bulk metallic glasses are sufficiently stable against crystallization to explore supercooled liquid state over a wide temperature span above the glass transition temperature Tg [5,8,25]. Large complexities of element species and atomic size, and their favorable combination are recognized to account for high thermal stability and glass-forming ability of such glasses. In light of chemical factors involved in bulk-glass-forming alloys, stable uranium metallic glasses may be obtained in a multi-component system. Considering the radioactivity and chemical toxicity of uranium alloys, the extensive trial-and-error procedure may not be appropriate. It would be desirable to first consider potential alloy compositions in a ternary system. Wang [26] proposed that the ternary systems possessing MgCu2-and MgNi2-type equal-atomicratio Laves phases, e.g. Zr1/3(Ni, Co, or Cu)1/3Al1/3 and Ln (rare earth metals)1/3(Ni, Co, or Cu)1/3Al1/3 are possible easy glass formers, and that metallic glasses are most likely to form at the compositions enriched in large atomic size constituents. Therewith, the formation of a stable glass is expected to occur in U-rich U-(Co, Fe, or Ni)-Al alloys far apart from U1/3(Ni, Co, or Fe)1/3Al1/3 Lave phase compositions [27,28]. In the present work, we report the formation, thermal stabilities, mechanical and corrosive properties of U-Co-Al metallic glasses. It is found that stable glasses with calorimetrically accessible Tg were formed in the U-rich U-Co-Al alloys, and the new glasses exhibited excellent corrosion resistance, high mechanical hardness and elastic modulus, and a rather small fragility parameter characteristic of strong glass-forming liquids. 2. Experimental Our previous study indicated that metallic glasses in a continuous ribbon form are more readily formed within the Co concentration range of 30e40 at.% in U-Co system [24]. By taking U69.2Co30.8 and U62.5Co37.5 as two basic compositions and Al as the alloying element, three series of ternary alloys, namely, (U69.2Co30.8)100xAlx, U69.2Co30.8xAlx and U62.5Co37.5xAlx (x ¼ 0e17.5) were produced. The purities of the starting metals were 99.5 wt% for depleted uranium, 99.9 wt% for cobalt and 99.99 wt% for aluminum. Alloy ingots with the designed compositions were prepared by arc-melting mixtures of the constituent metals in a water-cooling copper crucible in a Ti-gettered Ar atmosphere with a background vacuum level of about 5 103 Pa. Each ingot with about 5 g in mass was remelted four times to ensure compositional homogeneity. These ingots were placed in quartz tubes to conduct liquid-quenching and continuous ribbon samples were made by using a single melt-spinner at a surface velocity of ~50 m/s. The cross-section dimensions of as-spun samples were about 1 mm wide and 20e30 mm thick. The structures of U-Co-Al ribbons were examined with an X-ray diffractometer (Cu-Ka radiation) by means of the standard q2q scan within a diffraction angle range of 2q ¼ 20e100 . Differential scanning calorimetry (DSC) studies were carried out at a constant heating rate of 0.333 K/s. For a special stable uranium glass, DSC measurement was also done at different rates from 0.167 K/s to 2.667 K/s for the derivation of both the fragility parameter of the undercooled liquid and the activation energy for the glass transition. The room temperature electrochemical performance of the ribbons was compared with depleted-uranium metal and U-5.5 wt % Nb alloy in a 0.002 M NaCl aqueous solution. Potentiodynamic polarization measurement using a platinum counter electrode and a saturated calomel reference electrode (SCE) was performed during the electrochemical performance examination. Prior to testing, ribbon samples were washed with acetone and distilled water, and then dried in air. The electrochemical polarization was driven at a
potential sweep rate of 1 mV/s after about 1 h open circuit immersion in the solution freely exposed to air. A micro-compression test was carried out for ribbon specimens under displacement control on a Triboscopee Nanoindentation System equipped with a 5 mm Berkovich diamond indenter. The maximum loads of 3 mN and 2 mN were applied for the typical and cycle loading tests, respectively. For each specimen, the test was performed at least ten times to ensure the reliability of the indentation data. The load-depth (P-h) curves of the specimens were also obtained. 3. Results 3.1. Phase identification An amorphous phase was identified for the rapidly quenched UCo-Al alloys in the composition region of 55e75 at.% U, 15e40 at.% Co and less than 20 at.% Al. The compositions and numbers of the alloys are summarized in Table 1, and their locations are shown in the composition chart in Fig. 1a. The X-ray diffraction results indicate that the Nos.3~6, 8, 9 and 13 samples are fully amorphous, and in the other samples a minor crystalline phase coexists with the amorphous phase. The typical XRD patterns of fully and partially amorphous samples are shown in Fig. 1b. It is seen that a tiny but sharp crystal peak appears at 2q z 28 , which cannot be indexed by any of the known binary U-Co and ternary U-Co-Al phases. Under nearly the same quenching conditions, fully amorphous structure was not achieved in U-Co binary system [24]. The addition of Al therefore facilitates glass formation in U-Co-Al system. 3.2. Thermal stability and fragility The DSC traces of all the U-Co-Al ribbons studied are presented in Fig. 2. The crystallization process of the fully amorphous ribbons is dominated by a single exothermic event, whereas double-staged crystallization appears in some partially amorphous ones. Upon melting, most of the U-Co-Al samples exhibit two endothermic peaks except for U62.5Co20Al17.5 and U64Co34Al2 which are associated with a single peak melting process. The data of the onset crystallization temperature (Tx), onset melting temperature (Tm), liquidus temperature (TL) and crystallization enthalpy (DHc) are listed in Table 1. Also included are the values of the reduced temperature (Tx/TL). Tx is found to increase with the Al concentration in each series of the (U69.2Co30.8)100xAlx, U69.2Co30.8xAlx and U62.5Co37.5xAlx (x ¼ 0e17.5) alloys. For the fully amorphous alloys, by comparison with U62.5Co37.5 and U69.2Co30.8 glasses, an increment of about 100 K in Tx was reached, indicating the great enhancement of thermal stability. The DHc values of 3e5 kJ/mol are about a half of those of the two U-Co glasses. Both Tm and TL were raised due to the introduction of Al. Because of the absence of Tg, the Tx/TL parameter has been used to evaluate the glass-forming ability of U-Co alloys [20,24]. As seen from the data in Table 1, the Tx/TL values of the U-Co-Al samples are considerably larger than those of the U-Co glasses. Moreover, Tg is accessible in the fully amorphous U-Co-Al glasses. As shown by the DSC traces of the U64Co28.5Al7.5 glass in Fig. 3a, the feature of Tg becomes more and more distinct and its value increases as the heating rate is raised, which is a kinetic characteristic of glass transition. The Tg value ascended from 612 K at the heating rate of 0.167 K/s to 650 K at 2.667 K/s. At the commonly used heating rate of 0.333 K/s, the Tg value was determined equal to 616 K and therefore the reduced glass transition temperature Trg (¼Tg/TL) was calculated to be 0.567. This Trg value of the U64Co28.5Al7.5 glass is close to those of Zr66Al8Ni26 (0.57), Mg65Cu25Y10 (0.58), La66Al14Cu20 (0.58), Pr60Cu20Ni10Al10 (0.58) bulk metallic glasses [5].
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Table 1 The compositions, thermodynamic parameters and mechanical properties of U-Co-(Al) metallic glasses. Alloy number
Composition
Tg(K)
Tx(K)
Tm(K)
TL(K)
Tx/TL
DHc (kJ mol1)
Er (GPa)
H (GPa)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 / /
U67.8Co30.2Al2 U65.7Co29.3Al5 U64Co28.5Al7.5 U60.5Co27Al12.5 U58.8Co26.2Al15 U69.2Co29Al1.8 U69.2Co25.8Al5 U69.2Co20.8Al10 U69.2Co18.3Al12.5 U62.5Co35Al2.5 U62.5Co30Al7.5 U62.5Co25Al12.5 U62.5Co20Al17.5 U64Co34Al2 U69.2Co30.8 [24] U62.5Co37.5 [24]
e e 617 e 625 595 e e 614 e e e 622 e e e
563 587 638 640 641 639 577 643 649 584 581 593 649 557 543 544
1007 1006 1056 1056 1057 1055 1008 1055 1105 1007 1008 1005 1105 1005 1010 1010
1058 1080 1087 1095 1096 1107 1089 1115 1119 1086 1074 1081 1119 1020 1026 1057
0.532 0.544 0.587 0.584 0.585 0.577 0.530 0.577 0.580 0.537 0.541 0.549 0.580 0.546 0.529 0.515
4.76 5.48 3.58 3.58 3.64 4.47 4.62 3.56 3.04 4.98 4.63 4.77 3.59 6.55 7.0 7.9
77.0 79.3 79.1 88.5 87.0 82.1 / 85.7 86.9 83.5 / 89.8 91.6 / / /
4.3 4.8 4.9 5.1 4.9 5.1 / 4.8 5.5 4.7 / 5.1 5.1 / / /
from heating-rate- dependent Tg data by using the Kissinger’s equation [29]:
. ln 4 Tg2 ¼ Eg RTg þ C
(1)
where 4 stands for the heating rate, R the gas constant and C a constant. A good linear relationship between ln (4/Tg2) and 1/Tg is found for the U64Co28.5Al7.5 glass (Fig. 3b). Accordingly, Eg ¼ 338.2 kJ/mol was worked out. The value is found to lie among those of Ca65Mg15Zn20 (~148 kJ/mol), La60Ni15Al25 (~510 kJ/mol) and Co43Fe20Ta5.5B31.5 (~590 kJ/mol) [30e33]. Also, a fragility parameter (m) can be deduced from Tg measurement with the following equation [34]:
m¼
dlog hðTÞ T ¼ Tg d Tg T
(2)
where h(T) is the viscosity and T the temperature. h(T) is defined by the equation [35]:
hðTÞ ¼ h0 exp
E kB T
(3)
where h0 means the high-temperature limit of the viscosity, E the energy and kB the Boltzmann’s constant. Then we can deduce
m¼
Eg ln 10 RTg
(4)
m z 28 was obtained for the U64Co28.5Al7.5 glass with Tg ¼ 616 K and Eg ¼ 338.2 kJ/mol. This value is slightly smaller than those (m ¼ 32e34) of Ce-based metallic glasses [36]. Generally, m is small for strong liquids such as directional bonding glasses such as SiO2 (m z 16), and large for fragile liquids such as o-terphenyl (m z 150) [35]. Metallic glass-forming liquids have an intermediate fragility of 30e70 [37]. According to this classification, U-Co-Al may belong to a strong glass-forming liquid system. Fig. 1. (a) The composition range of metallic glasses in the U-Co-Al ternary system. The red solid circle denotes fully glassy samples and the blue semi-solid circle is for partially crystallized samples; (b) the typical XRD patterns of Nos. 3, 6, 7 and 13samples. The existence of minor crystalline phase is indicated by the peak at 2q z 28 . (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The activation energy (Eg) for glass transition can be derived
3.3. Corrosion-resistant properties Fig. 4a shows the potentiodynamic polarization curves of the U64Co28.5Al7.5 and U64Co34Al2 glasses, U-5.5 wt% Nb crystalline alloy and pure depleted-uranium metal. The measured data of the corrosion potential (Ecorr) and current density (icorr) are given in Table 2. It is found that the two glasses possessed high Ecorr, which are 410e470 mV and 280e340 mV larger than those of uranium
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Fig. 3. (a) DSC traces of the U64Co28.5Al7.5 (No.3) glass obtained at different heating rates of 0.167~2.667 K/s; (b) the corresponding Kissinger fitting [29]. Fig. 2. DSC traces showing the crystallization (a) and melting behaviors (b) of the UCo-Al metallic glasses at a heating rate of 0.333 K/s.
Rp ¼ ba bc =½2:3ðba þ bc Þicorr and the UeNb alloy, respectively. The value of icorr can be deduced from the intersection of anodic and cathodic Tafel lines. For the U64Co28.5Al7.5, U64Co34Al2, U-5.5 wt % Nb and uranium samples, icorr was determined to be 20.5, 49.9, 141.1 and 415.5 nA/cm2, respectively. Moreover, a basically inverse relationship between icorr and Ecorr can be found for these materials, as displayed in Fig. 4b. Polarization resistance (Rp) can be deduced from the Tafel line in a polarization curve by using Stern-Geary equation [38]:
(5)
where ba and bc are the slopes of anodic and cathodic Tafel lines, respectively. The Rp data of these four materials are included in Table 2, and their varying tendency is plotted in Fig. 4c. Obviously, the U64Co28.5Al7.5 (167.6 kU/cm2) and U64Co34Al2 (76.8 kU/cm2) glasses exhibit much bigger Rp than U-5.5 wt%Nb (30.6 kU/cm2) and uranium (13.2 kU/cm2). Altogether, the room temperature corrosion resistance of Ubased metallic glasses in NaCl aqueous solution is far superior to crystalline uranium materials. It should be noted that the
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Table 2 Tafel extrapolation data for all investigated materials. Sample
Ecorr (mV)
icorr (nA/cm2)
Rp (kU/cm2)
Pure uranium U-5.5 wt%Nb U64Co28.5Al7.5 (No.3) U64Co34Al2 (No.14)
562 430 147 90
415.5 141.1 20.5 49.9
13.2 30.6 167.6 76.8
3.4. Mechanical property Fig. 5a shows three typical load-depth curves derived from three different indentions for the U58.8Co26.2Al15 glass. For each indention, the time duration in the loading, holding and unloading segments was set 5s (see inset of Fig. 5a). These curves were seen to superimpose closely with each other, indicating good reproducibility. Based on the loading and unloading curves, the hardness (H) and reduced Young’s modulus (Er) of the U-Co-Al glass were obtained by direct calculation [39]. The values of Er and H are presented in Table 1. The composition dependence of Er for the two series of U-Co-Al glasses containing 69.2 at.% and 62.5 at.% U is plotted in Fig. 5b. Er tends to increase with raising Al concentration and has a smaller value at a higher U concentration, however. The load-depth curves of 15 cycles loading of the U58.8Co26.2Al15 glass are shown in Fig. 5c. In each cycle, 50% of the maximum load was unloaded before the next loading (see inset of Fig. 5c). The changes of Er and H are plotted as a function of the indentation depth in Fig. 5d. A steep rise in the Young’s modulus is seen when the indentation depth is increased from 5 nm to 20 nm. The phenomenon may be attributed to the dynamic instability of surface nano-layers of a metallic glass [40], but may also be an instrumental false appearance occurring at the initial stage of the nanoindentation. When the indentation depth is greater than 20 nm, Er manifests a weak “softening” feature, while the H data have a stable indication. The “softening” phenomenon has been observed in Zr-based metallic glasses, which is mainly attributed to the size effect of an indenter tip [41]. To estimate the real Young’s modulus (E) of the U-Co-Al glasses, the following equation was adopted [39]:
1 1 n2 1 n2i þ ¼ Er E Ei
Fig. 4. (a) The potentiodynamic polarization curves of the U64Co28.5Al7.5 (No.3), U64Co34Al2 (No.14) glasses, pure uranium and U-5.5wt%Nb alloy; (b) the current density (icorr) versus polarization potential (Ecorr) plot; (c) the comparison of their electrical resistances (Rp).
(6)
where v denotes the Poisson’s ratio of a testing material, vi the Poisson’s ratio of a diamond indenter, Ei the Young’s modulus of the indenter. It can be taken in the present work that vi ¼ 0.07, Ei ¼ 1140 GPa. The v values of U, U-6wt%Nb alloy were 0.21 and 0.35, respectively [42], and those of metallic glasses are typically ~0.3 [43]. By taking v ¼ 0.3 in the present case, E z 0.9Er can be obtained for the U-Co-Al glasses. According to the results from Fig. 5d, it can be attained that E z 81 GPa and H z 5 GPa. The two values are basically close to those of Zr-, Pd- and Cu-based metallic glasses (E ¼ 90 ± 10 GPa, H ¼ 5~6 GPa) [43e45]. Compared with crystalline U-6wt%Nb (E z 51 GPa and H z 2 GPa [46]) and U-10 wt %Mo (H z 3 GPa [42]) alloys, the U-based metallic glasses have attained much higher elastic modulus and hardness. 4. Discussion 4.1. Glass formation and thermal stability
application of crystalline uranium alloys has been greatly limited by their poor corrosion resistance. The highly corrosion-resistant uranium glassy alloys would offer some chance for corrosion protection such as surface coating.
Like common ternary bulk metallic glass-forming alloys, U-CoAl system possesses some favorable thermodynamic factors for the glass formation. The Goldschmidt atomic sizes of U, Al and Co are 0.156 nm, 0.143 nm and 0.125 nm, respectively. These elements
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Fig. 5. (a) Three load-depth curves of typical nanoindentation testing for the U58.8Co26.2Al15 (No. 5) glass; (b) the dependence of reduced Young’s modulus (Er) on Al content for U69.2- and U62.5-series of the U-Co-Al glasses; (c) the load-depth curve of 15 cycles loading for the No.5 glass; (d) Er and hardness (H) as a function of penetration depth for the same glass.
exhibit negative mixing enthalpies, DHUCo ¼ 23 kJ/mol, DHUAl ¼ 30 kJ/mol and DHAlCo ¼ 19 kJ/mol [47]. The disparity in atomic size and the characteristic of mixing enthalpy satisfy the requirement of the Inoue’s empirical criterion [8] proposed for multi-component glass-forming systems. This would also lead to a beneficial thermodynamic condition for the glass formation by making a lower enthalpy for the amorphous state than for a single phase crystalline counterpart possibly available in the U-Co-Al alloys. Tg is a kinetic factor governing easy glass formation. A high Eg for glass transition can be ascribed to large-length-scale correlated motion or rearrangement of atoms in a metallic glass. The Eg value estimated for the U64Co28.5Al7.5 glass is comparable to other bulk metallic glass formers, therefore reflecting the atomic correlation within a large spatial scale in the amorphous structure. As implied by the relatively low fragility parameter (about 28) of this glass, atoms in the U-Co-Al strong liquids would be comparatively closepacked with short- or medium-range order [48]. Furthermore, it is known that early actinide metals including uranium have unusually high viscosity, which is relevant with their 5f electron structures, at the melting temperature and such a high viscosity characteristic would be expected for U-based glassy alloys [49,50]. The crystallization during the undercooling of U-Co-Al liquids could be well frustrated by the combination of the topologically dense atomic packing and the likely high viscosity. The remarkable changes in Tx,
Tm and TL due to the Al addition imply that the nature of the competing crystalline phases may change. The XRD patterns of the crystallized U-Co-Al samples (not presented here), however, are hardly indexed by the known uranium alloy phases. A further study is needed to consider the crystallizing kinetics, and the results will be reported elsewhere. All of these form the kinetic aspects for the ease of glass formation in the U-Co-Al alloys. Elliott and Giessen first pointed out that 5f electrons of uranium may bond more effectively in the liquid states of its alloys [17]. Noticing the narrowness of the f-band [14,15] in uranium, the 5f electrons are likely to hybridize with the extensive p and d partial states from Al or Co [51e53], and hence get more delocalized from the core in the U-Co-Al metallic glasses. Consequently, a high degree of short-range order may be developed by the formation of pseudo-molecules or cluster-like structure units [54] from unlike U-Co and U-Al pairs. Actually, the starting compositions U69.2Co30.8 and U62.5Co37.5 are very close to those of two icosahedral clusters U9Co4 and U8Co5, which are extracted from U-Co crystalline phases and are conjectured as dominant local structure units in the U-Co undercooled liquids. The introduction of Al is expected to decrease the energetic states of the local structures by changing the bonding states and by promoting the packing efficiency of the clusters. As suggested by the eutectic criterion for metallic glass formation [55], an alloy with high glass-forming ability usually has a low TL. However, all the present U-Co-Al alloys exhibited higher TL
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values than U69.2Co30.8 and U62.5Co37.5, even though the melting point of Al (~934 K) is much lower than those of U (~1406 K) and Co (~1770 K). This might be resulted from the enhancement of flow resistance in the U-Co-Al alloy melts, since liquid flow requires the rearrangement of atoms that may be quite difficult in the matrix having strong short- or media-range order. In addition, the great difficulty in destructing such ordered structures can explain larger Tx of the U-Co-Al glasses as compared with the U-Co ones. 4.2. Possible origins for the high corrosion-resistance The values of Ecorr (<400 mV), icorr (>140 nA/cm2) and Rp (<31 kU/cm2) were obtained for pure uranium metal and U-5.5 wt% Nb alloy in the corrosive media of 0.002 M NaCl. For the U-Co-Al glasses, Ecorr (150 ~ 90 mV) and Rp (75e170 kU/cm2) were much higher, while icorr (<50 nA/cm2) was lowered by one order of magnitude. The corrosion resistance of the glassy materials is evidently much better than the crystalline ones. Moreover, the U64Co28.5Al7.5 alloy in a fully glassy state was more anti-corrosive than the partially glassy U64Co34Al2. The possible reasons of these features are discussed from thermodynamic and kinetic viewpoints. First, higher Ecorr of the U-Co-Al glasses can be mainly attributed to lower U concentrations when compared with the U-Nb alloy and uranium, since the standard electrode potential of U (1.8 V) is much lower than Co (0.227 V) and Al (1.660 V). A larger Co content also contributes to higher Ecorr of the U64Co34Al2 glass relative to the other glass. This quality of Ecorr should contribute directly to small icorr observed in these glasses. Second, crystal defects, passivating elements and the adhesion of passivated films to the crystalline or amorphous matrix can be regarded as kinetic factors [56]. A metallic glass as a single phase alloy can trap a large number of solute elements including passivating ones, and is free of crystal defects such as dislocations, grain boundaries and secondary phases. Since Al is one of the most effective alloying elements favoring a high passivating ability of metallic glasses [57], the dissolution of a large amount of Al would result in the formation of a compact passivated Al oxide film on the sample surface. The super anti-corrosive performance of the U-CoAl glasses could be caused by the lack of crystal defects and the passivation. Since the U64Co28.5Al7.5 alloy was formed into a fully amorphous structure but the U64Co34Al2 was not, the inferior corrosion resistance of the latter can be mainly attributed to the presence of crystal defects as well as low Al concentration. Nb element has also been used through passivation for protecting uranium alloys [58,59]. Although the Nb concentration in U-5.5 wt %Nb alloy (¼U87Nb13; at.%) is much higher than the Al concentration in the U64Co34Al2 glass, the corrosion performance of the glass was found in a higher level, which may be a consequence of the good adhesion of the passivating film to the amorphous matrix. 4.3. Deviation of modulus from the rule of mixtures Elastic modulus (M) of a metallic glass can be well predicted by a weighted average of the elastic constants of the constituent elements, i.e., the rule of mixtures (ROM). It is expressed by the formula [43]:
X 1 1 ¼ fi M Mi
(7)
where Mi is the modulus of the ith component and fi the atomic percent of the ith component. In this equation, atomic volume is not considered because its effect on the variation trend of the calculated modulus (Ecal) is negligible [60]. As shown in Fig. 6, the
Fig. 6. Comparison between the calculated (Ecal) and experimental elastic modulus (EMG) of various types of metallic glasses. The conventional metallic glasses follow “the rule of mixtures” [43] while the U-Co-Al glasses exhibit a remarkable deviation from the rule.
experimentally measured modulus (EMG) values of Zr-, Ti-, Pd-, Mg, Cu-, La- and Ni-based metallic glasses are well consistent with the Ecal data [43]. However, as indicated in Fig. 6, the EMG (¼ Er) values of the U-Co-Al metallic glasses are much smaller than those of Ecal calculated by taking 208 GPa as the modulus of U element, displaying a significant difference between EMG and Ecal. Recently, it has been found that metallic glasses can inherit elastic properties from the solvents [61,62]. Since elastic properties of solids including glass are largely determined by their electronic bonding states, abundant 5f electrons of uranium would play a crucial role for elastic response of U-based glasses. Once the 5f electrons are hybridized with the extensive p and d partial states from Al or Co, they would get more delocalized from the core of the uranium ion in the amorphous structure. Then the directionality of the 5f electron wave functions would be significantly disturbed, which could weaken the core-core interference among uranium ions. This naturally leads to a soft mode of correlated atomic vibration, but experimental evidence, for instance, the Debye temperatures of the U-Co-Al glasses, is necessary for further consideration. From the classical viewpoint of atomic motion, it has been proposed that a large difference in atomic weight of constituent atoms can lead to abnormal fast diffusion behavior of light atoms in a metallic glass at a temperature below Tg [63]. The decoupled fast diffusion [64] of Al and Co atoms in the disordered alloy matrix would cause microstructure inhomogeneity, and the volume faction of the so-called liquid-like zone in the matrix [65] can thus be significant. Therefore, the deviation of modulus from ROM could be resulted from the weakened core-core interference and the abundance of the liquid-like zone in the U-Co-Al glasses. 5. Conclusions Metallic glasses are discovered in U-Co-Al alloy system. The compositions, thermal stabilities, corrosive and mechanical properties of these new glasses are concluded in the following: (1) The addition of Al can significantly improve the glassforming ability of U-Co alloys. The formation of amorphous phase by rapid liquid quenching is identified in the composition region containing 55e75 at.% U, 15e40 at.% Co and 0e17.5 at.% Al.
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(2) The U-Co-Al glasses exhibit greatly enhanced thermal stability in comparison with U-Co glasses, and glass transition is accessible in their calorimetric analysis. In addition, U-Co-Al appears to be a strong glass-forming system due to its low fragility (m z 28 for U62.5Co20Al17.5). (3) The ternary glasses possess superior corrosion resistance in NaCl aqueous solution at room temperature, when compared with U-based crystalline materials. (4) The mechanical hardness of the U-Co-Al glasses is much greater than U-based crystals. The deviation of elastic modulus from ROM is observed in the new glass system. (5) The delocalization of 5f electrons and their bonding states are suggested to play a significant role for the improved glass formation, enhanced thermal stability as well as peculiar Young’s modulus behavior of the U-Co-Al glasses. Acknowledgement This research is supported by National Defense Basic Scientific Research (No.B1520133007) and Scientific and Technological Development Foundation of China Academy of Engineering Physics (No.2013A0301015). One of the authors, H. B. Ke would like to appreciate the financial support of National Natural Science Foundation of China (51501169). References [1] B.A. Sun, W.H. Wang, The fracture of bulk metallic glasses, Prog. Mater. Sci. 74 (2015) 211e307. [2] S.Y. Ding, Y.H. Liu, Y.L. Li, Z. Liu, S. Sohn, F.J. Walker, J. Schroers, Combinatorial development of bulk metallic glasses, Nat. Mater 13 (2014) 494e500. [3] J. Schroers, Bulk metallic glasses, Phys. Today 66 (2013) 32e37. [4] Y.Q. Cheng, E. Ma, Atomic-level structure and structure-property relationship in metallic glasses, Prog. Mater. Sci. 56 (2011) 379e473. [5] W.H. Wang, C. Dong, C.H. Shek, Bulk metallic glasses, Mat. Sci. Eng. R. 44 (2004) 45e89. [6] A.A. Mazilkin, G.E. Abrosimova, S.G. Protasova, B.B. Straumal, G. Schutz, S.V. Dobatkin, A.S. Bakai, Transmission electron microscopy investigation of boundaries between amorphous “grains” in Ni50Nb20Y30 alloy, J. Mater. Sci. 46 (2011) 4336e4342. [7] B.B. Straumal, A.R. Kilmametov, A.A. Mazilkin, S.G. Protasova, K.I. Kolesnikova, P.B. Straumal, B. Baretzky, Amorphization of Nd-Fe-B alloy under the action of high-pressure torsion, Mater. Lett. 145 (2015) 63e66. [8] A. Inoue, Stabilization of metallic supercooled liquid and bulk amorphous alloys, Acta Mater 48 (2000) 279e306. [9] Z.L. Long, H.Q. Wei, Y.H. Ding, P. Zhang, G.Q. Xie, A. Inoue, A new criterion for predicting the glass-forming ability of bulk metallic glasses, J. Alloys Compd. 475 (2009) 207e219. [10] W.L. Johnson, J.H. Na, M.D. Demetriou, Quantifying the origin of metallic glass formation, Nat. Commun. 7 (2016) 10313. [11] Z.W. Wu, M.Z. Li, W.H. Wang, K.X. Liu, Hidden topological order and its correlation with glass-forming ability in metallic glasses, Nat. Commun. 6 (2015) 6035. [12] K.J. Laws, D.B. Miracle, M. Ferry, A predictive structural model for bulk metallic glasses, Nat. Commun. 6 (2015) 8123. [13] J.H. Li, Y. Dai, Y.Y. Cui, B.X. Liu, Atomistic theory for predicting the binary metallic glass formation, Mat. Sci. Eng. R. 72 (2011) 1e28. [14] P. Modak, A.K. Verma, Role of 5f electrons in the structural stability of light actinide (Th-U) mononitrides under pressure, Phys. Chem. Chem. Phys. 8 (2016) 8682e8691. [15] K.T. Moore, G. van der Laan, M.A. Wall, A.J. Schwartz, R.G. Haire, Rampant changes in 5f5/2 and 5f7/2 filling across the light and middle actinide metals: electron energy-loss spectroscopy, many-electron atomic spectral calculations and spin-orbit sum rule, Phys. Rev. B 76 (2007) 073105. [16] M.A. Hill, R.K. Schulze, J.F. Bingert, R.D. Field, R.J. McCabe, P.A. Papin, Filiformmode hydride corrosion of uranium surfaces, J. Nucl. Mater 442 (2013) 106e115. [17] R.O. Elliott, B.C. Giessen, On the formation of metallic glasses based on U, Np or Pu, Acta Metall. 30 (1982) 785e789. [18] R. Ray, E. Musso, Amorphous Alloys in the U-Cr-V System, 1976. America, US3981722. [19] B.C. Giessen, R.O. Elliott, Properties of metallic glasses containing actinide metals, in: Proceedings of the 3rd International Conference on Rapid Quenching, 1978, pp. 406e414. Sussex, Brighton, England. [20] A.J. Drehman, S.J. Poon, Anomalous glass-forming ability of uranium-based alloys, J. Non-Cryst. Solids 76 (1985) 321e332.
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