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Site-occupation behavior and solid-solution hardening effect of rhenium in Mo5SiB2
Intermetallics 53 (2014) 85e91
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Intermetallics journal homepage: www.elsevier.com/locate/intermet
Site-occupation behavior and solid-solution hardening effect of rhenium in Mo5SiB2 Junya Nakamura a, *, Takahiro Kaneko b, Takashi Hara a, Kyosuke Yoshimi a, Kouichi Maruyama a, Hirokazu Katsui c, Takashi Goto c a b c
Department of Materials Science and Engineering, Tohoku University, 6-6-04 Aramaki Aza Aoba, Aoba-ku, Sendai 980-8579, Japan Department of Environmental Studies, Tohoku University, 6-6-20 Aramaki Aza Aoba, Aoba-ku, Sendai 980-8579, Japan Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 February 2014 Received in revised form 17 April 2014 Accepted 20 April 2014 Available online 15 May 2014
The site-occupation behavior of Re in Mo5SiB2 (T2) was studied both theoretically and experimentally, and the effect of Re on the solid-solution hardening of T2 was investigated by taking into account the offstoichiometry of the T2 phase. MoeSieB quaternary alloys containing 1.4 at.% Re were produced using a conventional Ar arc-melting technique, and the cast samples were homogenized at 1800 C for 24 h in an Ar atmosphere. High-resolution high-angle annular dark-field scanning transmission electron microscopy observations strongly suggest that Re preferentially occupies the Mo sites in the MoeB layers of the T2 unit cell, which was confirmed by the site-occupation behavior predicted by first-principles calculations. Nanoindentation measurements indicate that the hardness of the T2 phase is affected by both the off-stoichiometry and Re addition. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Intermetallics B. Solid-solution hardening D. Site occupancy E. Ab-initio calculations F. Nanoindentation
1. Introduction MoeSieB-based alloys are considered to be promising ultrahigh temperature materials capable of operating at temperatures higher than those of Ni-based SX superalloys. However, the high density and low room-temperature fracture toughness of these MoeSieB alloys hinder their practical application. To solve this problem, the inclusion of additional elements has been investigated as a way to improve the material properties of MoeSieB alloys [1e 6]. One elemental candidate, Re, has been shown to improve the yield strength, ultimate tensile strength, and elongation at room temperature of Mo with an Re addition of higher than 20 at.% [7]; the effects on the mechanical properties of Mo are shown in Fig. 1 and are collectively known as the “rhenium effect”. For Mo and W alloys, the “rhenium effect” is also effective in improving the hightemperature strength and room-temperature ductility, decreasing the ductileebrittle transition temperature (DBTT), and reducing recrystallization embrittlement [8,9]. Re is therefore an element capable of improving the mechanical properties of MoeSieB alloys.
* Corresponding author. Tel.: þ81 22 795 7326; fax: þ81 22 795 7325. E-mail address: [email protected] (J. Nakamura). http://dx.doi.org/10.1016/j.intermet.2014.04.009 0966-9795/Ó 2014 Elsevier Ltd. All rights reserved.
Recently, the partitioning behavior of Re in Mo solid solution (Moss) þ Mo3Si þ Mo5SiB2 (T2) three-phase alloys were studied by Yang et al. [10], who reported that Re partitioned mainly into the Moss phase and indicated that the addition of Re also enhanced the solubility of Si in Moss. Furthermore, they clarified that Re had a low solubility in the Mo3Si, Mo2B, and T2 phases at 1600 C [10]. While the effect of Re on the mechanical properties of Moss was discussed, the effects were unclear for the other phases. The role and behavior of Re in the T2 phase is thought to be one key factor in controlling the mechanical properties and thus should be studied in detail. Therefore, the purpose of this work was to assess the Re effect in the T2 phase of MoeSieB multiphase alloys, and to do this, the siteoccupation behavior of Re in T2 was studied both theoretically and experimentally. The effect of Re on the solid-solution hardening of T2 was investigated by considering the off-stoichiometry of the T2 phase. 2. Experimental procedure Five quaternary Re-added MoeSieB alloys (Q1 to Q5) were prepared with 99.99 wt.% Mo, 99.999 wt.% Si, 99 wt.% B, and 99.9 wt.% Re using a conventional arc-melting technique in an Ar atmosphere. The added amount of Re was about 1.4 at.% in all the quaternary alloys. In addition, five ternary MoeSieB alloys (S1 to
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J. Nakamura et al. / Intermetallics 53 (2014) 85e91
where E (ApBq), Eeq (A), and Eeq (B) are the total energies per atom of the compound ApBq and the constituents A and B in their equilibrium (zero-pressure) geometries, respectively, and xA and xB are the concentrations of A and B, respectively, where xA ¼ p/(p þ q) and xB ¼ q/(p þ q). Microstructure observations and phase identification were performed using scanning electron microscopy (SEM) (Philips XL30FEG) with energy-dispersive X-ray spectroscopy (EDS). Highangle annular dark-field (HAADF) images were obtained using a field-emission-gun scanning transmission electron microscope (STEM) (Jeol ARM200F) operated at 200 keV. HAADF images were simulated with the MacTempas X software (Total Resolution, USA). The nanoindentation hardness of the T2 phase in each alloy was measured at room temperature with a maximum load of 100 mN.
3. Results and discussion
Fig. 1. Effect of Re on the mechanical properties of MoeRe alloys at room temperature.
S5) were prepared using the same technique for comparison. The prepared ingots were then subject to a heat treatment at 1800 C for 24 h in an Ar atmosphere, and subsequently furnace-cooled. Temperature rapidly decreased to 1450 C for 2 min, and reached room temperature for about 3 h. The weighed compositions of these alloys are listed in Table 1 and shown on the MoeSieB ternary phase diagram in Fig. 2 [11]. As shown in the figure, the composition of each alloy was chosen around the T2 composition; the composition of T2 in a sample corresponds to that at one of the corners of the T2 single-phase region. First-principles calculations were performed to assess the phase stability and site-occupation behavior of the Re-added T2 using the “Advance/PHASE” software package (Advance Soft Corp., Japan). All the calculations were conducted using projector augmented wave (PAW) pseudopotentials within the generalized gradient approximation (GGA) of density functional theory, and all the examined crystal structures were fully relaxed with respect to the cellinternal coordinates on periodic boundary condition. In the case of the T2 phase, a 13 13 5 MonkhorstePack mesh was used for the k-point sampling, and the cutoff energy of 800 eV was used. The equilibrium formation enthalpy DHeq (ApBq) is one key indicator of the stability of the constituent phases and that of an ApBq phase is given by the energy difference between the ApBq phase and the composition-weighted average of the constituent elements A and B in their equilibrium crystal structures [12]:
DH eq Ap Bq ¼ E Ap Bq ½xA Eeq ðAÞ þ xB Eeq ðBÞ
(1)
The atomic arrangement of the unit cell in the T2 phase is shown in Fig. 3. This phase has a D8l-type structure with lattice parameters of a ¼ 0.60 nm and c ¼ 1.10 nm [13e15], and it consists of 2 MoeB layers, 4 Mo layers, and 2 Si layers, as shown in Fig. 3(bef), stacked in the [001] direction. The crystal structure data used in this study are listed in Table 2. Also, initial atomic coordinates and refined parameters due to atomic structure relaxation by the first principle calculation are shown in Table 2. The site-occupation behavior of Re in T2 was predicted from first-principles using the atomic structure shown in Fig. 3(a) with a composition of Mo20Si4B8 and by assuming periodic boundary conditions. Replacing one atom in the T2 unit cell with Re corresponds to an Re concentration of approximately 3.1 at.%. There are four possible atomic positions for Re site-substitution, namely (1) a Mo-1 site in the MoeB layer (Fig. 3(b and f)), (2) a B site in the Moe B layer (Fig. 3(b and f)), (3) a Mo-2 site in the Mo layer (Fig. 3(c and e)), or (4) a Si site in the Si layer (Fig. 3(d)). The formation energy for each Re site-substitution case was calculated (Fig. 4), and it was found that the Mo-1 and Mo-2 sites are more stable in the Readded model. The Mo-1 site appears to be a slightly more favorable than the Mo-2 site, but Re substitution at a Mo site results in a small increase of the formation enthalpy with respect to the Re-free ternary T2. The change in the equilibrium formation enthalpy of the possible constituent phases, Moss, Mo2B, Mo3Si, etc., resulting from Re substitution was also estimated using a super cell and a fixed Re concentration of about 3 at.%. Structural data, super cell and k-point mesh used in this study are summarized in Table 3 [16e19]. The atomic position of the Re substitution was determined so as to minimize the equilibrium formation enthalpy. Fig. 5 shows that the enthalpy increased slightly in all the constituent phases except for Moss, suggesting that Re decreases the phase stability of all the phases except Moss. This result is in good agreement with the
Table 1 Weighed compositions and constitution phases of the MoeSieBeRe and MoeSieB alloys. Sample No.
Q1 Q2 Q3 Q4 Q5 S1 S2 S3 S4 S5
Composition (at.%)
Phase constitution
Mo
Si
B
Re
58.9 61.6 64.6 64.6 62.6 62.5 63.0 66.0 66.0 64.0
9.8 8.0 9.0 13.0 19.0 12.5 8.0 9.0 13.0 19.0
30.1 29.0 25.0 21.0 17.0 25.0 29.0 25.0 21.0 17.0
1.2 1.4 1.4 1.4 1.4 e e e e e
*Chemically analyzed
T2 þ Mo3Si þ MoB T2 þ MoB þ Mo2B þ Re-rich phase T2 þ Mo þ Mo2B T2 þ Mo þ Mo3Si T2 þ Mo3Si þ Mo5Si3 T2 þ MoB T2 þ MoB þ Mo2B T2 þ Mo þ Mo2B T2 þ Mo þ Mo3Si T2 þ Mo3Si þ Mo5Si3
J. Nakamura et al. / Intermetallics 53 (2014) 85e91
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Fig. 2. (a) MoeSieB ternary phase diagram at 1800 C [10] and (b) weighed compositions of the MoeSieBeRe and MoeSieB alloys.
Fig. 3. Crystal structure of Mo5SiB2: the (a) unit cell, (b) MoeB layer (z ¼ 0), (c) Mo layer (z ¼ 1/8), (d) Si layer (z ¼ 1/4), (e) Mo0 layer (z ¼ 3/8), (f) MoeB0 layer (z ¼ 1/2), and (g) [001] projection.
preferable partitioning of Re into Moss in the MoeSieRe [20] and MoeSieBeRe systems [10,21]. Fig. 6(aee) show back-scattered electron (BSE) images of the microstructures of the Q1 to Q5 alloys after the heat treatment. In the image of the Q1 alloy (Fig. 6(a)), the T2 matrix is shown by the grey contrast, and there are small amounts of Mo5Si3 (T1) and MoB dispersions that have a darker contrast. The phase constitution is the same as that of the ternary alloy without Re. The composition of the Q2 alloy projected onto the ternary phase diagram lies in the T2Mo2BeMoB triangle (Fig. 2(b)), and the T2 matrix can be seen in
Fig. 6(b) along with Mo2B at a brighter contrast and MoB at a darker contrast as the secondary phases. Furthermore, there is a Re-rich phase with a brightest contrast. It was confirmed by the selected area electron diffraction (SAED) method in TEM that the Re-rich phase has a crystal structure equivalent to Mo2B. However, the phase with a higher Re concentration is not shown in phase diagrams. Two types of Mo2B with different Re concentrations might exist in the MoeReeSieB quaternary system, but it is unclear at present. For the Q3, Q4, and Q5 alloys, three-phase constitutions of T2, Moss, and Mo2B for the Q3 alloy, T2, Moss, and Mo3Si for the Q4
Table 2 Crystal structure data of Mo5SiB2 [13e15]. Structure type Cr5B3 No
Site notation
Atom
Pearson symbol
Space group
Cell parameters
tl32
I4/mcm
a ¼ 0.6013 nm, b ¼ 0.6013 nm, c ¼ 1.103 nm, a ¼ 90 , b ¼ 90 , g ¼ 90
Multiplicity
Atom coordinates for initial model 1 Mo-2 Mo 16 2 B B 8 3 Mo-1 Mo 4 4 Si Si 4 Refined parameters of the relaxed structure model 1 Mo-2 Mo 16 2 B B 8 3 Mo-1 Mo 4 4 Si Si 4
Wyckoff
Site symmetry
x
y
z
Occupancy
l h c a
..m m.2m 4/m.. 422
0.160 0.375 0.000 0.000
0.660 0.875 0.000 0.000
0.143 0.000 0.000 1/4
1.0 1.0 1.0 1.0
l h c a
..m m.2m 4/m.. 422
0.16441 0.37905 0.00000 0.00000
0.66441 0.87905 0.00000 0.00000
0.13908 0.00000 0.00000 0.25000
1.0 1.0 1.0 1.0
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J. Nakamura et al. / Intermetallics 53 (2014) 85e91
Fig. 4. Change in the formation enthalpy of Mo5SiB2 with substitutional Re at different sites calculated using a first-principles method.
alloy, and T2, Mo3Si, and T1 for the Q5 alloy were observed (Fig. 6(c, d, and e), respectively). The obtained phase constitutions are also listed in Table 1, and the results are in good agreement with the MoeSieB ternary phase diagram at 1800 C shown in Fig. 1, although the Re-rich phase was equilibrated in the Q2 alloy for a 1.4 at.% Re addition. Table 4 lists the Si and Re concentrations in the Moss, T2, Mo3Si, T1, Mo2B, MoB, and Re-rich phase as measured by SEM-EDS. The Re concentration is much higher in the Moss and Re-rich phase. However, these concentrations were somewhat overestimated since the estimation was performed only for Mo, Si, and Re but not B because of the low quantitative capability against B. Fig. 7 summarizes the Si and Re concentrations in the T2 phase of the Q1 to Q5 alloys as measured by SEM-EDS. The Si concentration ranges from 13.5 to 17 at.%. It is difficult to ascertain the true Si concentration in the T2 phases, but there is no doubt that the T2 phases had an offstoichiometric Mo5SiB2 composition. The Re concentration detected in this analysis was about 1.9e2.9 at.%, indicating that there is less than one Re atom substitution per unit cell, but the true Re concentration will be lower because B was neglected in the estimation. The Re concentration appears to correlate with the Si concentration except for Q5, which has the highest Si concentration but a slightly lower Re concentration. This would be an offstoichiometric effect of the T2 phase. HAADF-STEM observations were conducted for the S1 (ternary) and Q1 (quaternary) alloys to examine the site-occupation behavior of Re in the T2 phase. Zone axes of [001] and [110] were chosen so as to ensure that the Mo and Re positions could be distinguished in the Mo layers (Fig. 3(c and e)) and MoeB layers (Fig. 3(b and f)). Fig. 8(a) shows a HAADF image obtained along the [001] zone axis
Fig. 5. Change in the formation enthalpy of Moss, T2, MoB, Mo2B, Mo5Si3, and Mo3Si with Re addition.
of the T2 phase in the Re-free S1 alloy. The contrast brightness in these images is correlated to the atomic number density of the atomic columns along the viewing direction (for reference, the atomic numbers of B, Si, Mo, and Re are 5, 14, 42, and 75, respectively). Along the [001] direction, there are two Mo atoms at Mo-1 sites in the MoeB layers and two Si atoms in the Si layers that completely overlap in the MoeSi columns, and two Mo atoms at Mo-2 sites in the Mo layers and two B atoms in the MoeB layers that completely overlap in the MoeB columns. Thus, the density along the [001] direction is [(2 42) þ (2 14)]/ 0.6013 ¼ 186.3 nm1 for the MoeSi columns and [(2 42) þ (2 5)]/0.6013 ¼ 156.3 nm1 for the MoeB columns. Consequently, the MoeSi columns have a slightly stronger contrast than the MoeB columns in the ternary alloy. Fig. 8(b) shows a HAADF image of a relatively wide area of the T2 phase in the Q1 alloy obtained along the [001] axis. Some of the MoeSi columns (e.g. indicated by white triangle pairs along <110>) in this image have a much higher contrast than the other MoeSi columns, which suggests that Re preferentially occupies the Mo-1 sites in the MoeB layers. Fig. 8(c) shows these brighter MoeSi columns in more detail. If the Re atoms are homogeneously distributed in the T2 phase, then all of the MoeSi columns in Fig. 8(c) should have a higher contrast than the MoeB columns. These results combined with the result that the Re concentration as determined by SEM-EDS was less than 3.0 at.% (Fig. 7), which corresponds to less than one Re atom per T2 unit cell, suggest that Re atoms tend to segregate in local regions of the T2 phase. In the HAADF image of the T2 phase in the Q1 alloy obtained along the [110] axis (Fig. 8(d)), periodic bright spots corresponding to Mo-atom arrangements are observed in the MoeB and Mo layers. The Si layers are apparent from their low contrast owing to the low atomic number. If Re preferentially occupies Si
Table 3 Crystal structure data for first principle calculation of Mo, MoB, Mo2B, M5Si3, Mo3Si [16e19]. Phase
Space group
Cell parameters
Super cell
k-point
Mo
Im-3m
333
555
MoB
Cmcm
212
12 8 12
Mo2B
I4/mcm
221
5 5 10
Mo5Si3
I4/mcm
111
6 6 12
Mo3Si
P-3mn
a ¼ 0.3147 nm, b ¼ 0.3147 nm, c ¼ 0.3147 nm, a ¼ 90 , b ¼ 90 , g ¼ 90 a ¼ 0.316 nm, b ¼ 0.316 nm, c ¼ 0.316 nm, a ¼ 90 , b ¼ 90 , g ¼ 90 a ¼ 0.5543 nm, b ¼ 0.5543 nm, c ¼ 0.4734 nm, a ¼ 90 , b ¼ 90 , g ¼ 90 a ¼ 0.9643 nm, b ¼ 0.9643 nm, c ¼ 0.4910 nm, a ¼ 90 , b ¼ 90 , g ¼ 90 a ¼ 0.4893 nm, b ¼ 0.4893 nm, c ¼ 0.4893 nm, a ¼ 90 , b ¼ 90 , g ¼ 90
221
6 6 12
J. Nakamura et al. / Intermetallics 53 (2014) 85e91
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Fig. 6. Microstructure of the MoeSieBeRe quaternary alloys: (a) Q1, (b) Q2, (c) Q3, (d) Q4, and (e) Q5.
Table 4 Si and Re concentrations in the constituent phases of the MoeSieBeRe alloys as measured by SEM-EDS (at.%). Alloys
Moss Si
Q1 Q2 Q3 Q4 Q5
6.04 8.90
Mo5SiB2 Re
8.99 9.93
Mo3Si
Si
Re
16.23 13.70 14.55 16.70 17.06
2.45 1.94 2.03 2.89 2.36
Si
23.32 24.02
Mo5Si3 Re
3.90 4.24
sites, then the brightness of the Si layers in an image obtained along the [110] direction should be much stronger than that observed in Fig. 8(d), which further supports the conclusion that Re preferentially occupies the Mo-1 sites in the MoeB layers. To evaluate the effects of the off-stoichiometry and Re substitution on the room-temperature hardness of the T2 phase, the nanoindentation hardness of each sample was measured. The hardness values were obtained by averaging the values of ten testpoints and excluding data that showed pop-in behavior in the loadedisplacement curves. The nanoindentation hardness results are shown in Fig. 9 for both the ternary (S1 to S5) and quaternary
Mo2B
Si
Re
36.47
7.19
35.96
MoB
Si
Re
2.33 3.05
4.59 4.60
Re-rich phase
Si
Re
Si
Re
0.84 0.82
1.19 1.39
5.55
12.53
3.18
(Q1 to Q5) alloys. For the ternary alloys, the S3 alloy, which consists of T2þMoss þ Mo2B, i.e., a Mo- and B-rich composition, exhibited the highest hardness. The second highest was that of S4, T2þMoss þ Mo3Si (a Mo-rich composition), followed by S5, T2þMo3Si þ T1 (a Mo- and Si-rich composition). The S1 alloy, T2þT1þMoB (a Si-rich composition), showed the lowest hardness, and the S2 alloy, T2þMo2B þ MoB (a B-rich composition), had the second lowest hardness. Therefore, an off-stoichiometry composition toward a Mo- and B-rich composition appears to induce hardening of the T2 phase. For the quaternary alloys, a similar offstoichiometric effect on the nanoindentation hardness was observed. However, Q4, consisting of T2þMoss þ Mo3Si, exhibited a higher hardness than Q3 (T2þMoss þ Mo2B). This difference in nanoindentation hardness trend would appear to be caused by solid-solution hardening by Re, and in fact, the Q4 alloy had the highest Re concentration in the T2 phase, as shown in Fig. 7. Moreover, the hardness of Q1, which also had a relatively high Re concentration in the T2 phase, increased remarkably in comparison with the other 4 alloys. Thus, the solid-solution hardening effect of Re in T2 is quite reasonable. Therefore, the T2 phase is hardened by both the solid-solution hardening effect of Re and the offstoichiometric effect.
4. Conclusions
Fig. 7. Si and Re concentrations in the Mo5SiB2 phase as measured by SEM-EDS.
The site-occupation behavior of Re in the T2 phase has been studied both theoretically and experimentally, and the effect of Re on the solid-solution hardening of T2 was investigated by taking into account the off-stoichiometry of the T2 phase. The following conclusions were obtained:
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J. Nakamura et al. / Intermetallics 53 (2014) 85e91
Fig. 8. (a) Average image obtained from high-resolution HAADF-STEM image of Mo5SiB2 in the Re-free S1 alloy taken along the [001] zone axis. (b) High-resolution HAADF-STEM image of Mo5SiB2 in the Re-added Q1 alloy taken along the [001] zone axis. (c) Average image of Mo5SiB2 in the Q1 alloy taken along the [001] zone axis. (d) Average image of Mo5SiB2 in the Q1 alloy taken along the [110] zone axis.
(1) The addition of Re to MoeSieB alloys slightly decreases the phase stability of the constituent phases including the T2 phase but not the Moss phase. (2) An addition of 1.4 at.% of Re to MoeSieB alloys with compositions near T2 does not result in a considerable change to the phase equilibria. Only in the T2þMo2B þ MoB alloy was the Re-rich phase found to be equilibrated in the quaternary alloys. (3) Re preferentially occupies the Mo-1 sites in the MoeB layers.
(4) Off-stoichiometry toward a Mo- and B-rich composition hardens the ternary T2 phase. For the quaternary T2 containing Re, the T2 phase is hardened by the solid-solution hardening effect of Re and the off-stoichiometric effect. Acknowledgments This work was supported by the funding program for Next Generation World-Leading Researchers (NEXT Program) (No. GR017) administered by the Japan Society for the Promotion of Science (JSPS). References
Fig. 9. Nanoindentation hardness of Mo5SiB2 phase in the Q1 to Q5 and S1 to S5 alloys.
[1] Sakidja R, Myers J, Kim S, Perepezko JH. The effect of refractory metal substitution on the stability of Mo(ss) þ T2 two-phase field in the MoeSieB system. Int J Refract Met Hard Mater 2000;18:193e204. [2] Dimiduk DM, Perepezko JH. MoeSieB alloys: developing a revolutionary turbine-engine material. MRS Bull 2003;28:639e45. [3] Schneibel JH. High temperature strength of MoeMo 3SieMo 5SiB2 molybdenum silicides. Intermetallics 2003;11:625e32. [4] Yang Y, Chang YA, Tan L, Cao W. Multiphase equilibria in the metal-rich region of the MoeTieSieB system: thermodynamic prediction and experimental validation. Acta Mater 2005;53:1711e20. [5] Krüger M, Schliephake D, Jain P, Kumar S, Schumacher G, Heilmaier M. Effects of Zr additions on the microstructure and the mechanical behavior of PM Moe SieB alloys. JOM 2013;65:301e6. [6] Miyamoto S, Yoshimi K, Ha SH, Kaneko T, Nakamura J, Sato T, et al. Phase equilibria, microstructure, and high-temperature strength of TiC-added Moe SieB alloys. Metall Mater Trans A 2014;45:1112e23. [7] Northcott L. Some features of the refractory metals. In: Quarrell AG, editor. Niobium, tantalum, molybdenum and tungsten. New York: Elsevier; 1961. p. 3. [8] Agnew SR, Leonhardt T. The low-temperature mechanical behavior of molybdenumerhenium. JOM 2003;55:25e9.
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[15] ICDS database code: 96643. [16] Haschke H, Nowotny H, Benesovsky F. Untersuchungen in den Dreistoffen: {Mo, W}{Fe, Co, Ni}B. Monatsh Chem 1966;97:1459e68. [17] Zakharov AM, Golubev MY. The polythermal cross-section MoeHfB2 of the MoeHfeB system. Inorg Mater 1980;16:579e81. [18] Naidu SVN, Mays F, Houska CR. Integrated intensities and lattice parameters for Mo5Si3. J Am Ceram Soc 1982;65:C58e9. [19] Flükiger R, Paoli A, Muller J. Electronically ‘atypical’ a 15-type compounds based on chromium and molybdenum. Solid State Commun 1974;14:443e7. [20] Bei H, Yang Y, Viswanathan GB, Rawn CJ, George EP, Tiley J, et al. Formation, stability and crystal structure of the s phase in MoeReeSi alloys. Acta Mater 2010;58:6027e34. [21] Sakidja R, Perepezko JH, Kim S, Sekido N. Phase stability and structural defects in high-temperature MoeSieB alloys. Acta Mater 2008;56:5223e44.
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Site-occupation behavior and solid-solution hardening effect of rhenium in Mo5SiB2 Junya Nakamura a, *, Takahiro Kaneko b, Takashi Hara a, Kyosuke Yoshimi a, Kouichi Maruyama a, Hirokazu Katsui c, Takashi Goto c a b c
Department of Materials Science and Engineering, Tohoku University, 6-6-04 Aramaki Aza Aoba, Aoba-ku, Sendai 980-8579, Japan Department of Environmental Studies, Tohoku University, 6-6-20 Aramaki Aza Aoba, Aoba-ku, Sendai 980-8579, Japan Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 February 2014 Received in revised form 17 April 2014 Accepted 20 April 2014 Available online 15 May 2014
The site-occupation behavior of Re in Mo5SiB2 (T2) was studied both theoretically and experimentally, and the effect of Re on the solid-solution hardening of T2 was investigated by taking into account the offstoichiometry of the T2 phase. MoeSieB quaternary alloys containing 1.4 at.% Re were produced using a conventional Ar arc-melting technique, and the cast samples were homogenized at 1800 C for 24 h in an Ar atmosphere. High-resolution high-angle annular dark-field scanning transmission electron microscopy observations strongly suggest that Re preferentially occupies the Mo sites in the MoeB layers of the T2 unit cell, which was confirmed by the site-occupation behavior predicted by first-principles calculations. Nanoindentation measurements indicate that the hardness of the T2 phase is affected by both the off-stoichiometry and Re addition. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Intermetallics B. Solid-solution hardening D. Site occupancy E. Ab-initio calculations F. Nanoindentation
1. Introduction MoeSieB-based alloys are considered to be promising ultrahigh temperature materials capable of operating at temperatures higher than those of Ni-based SX superalloys. However, the high density and low room-temperature fracture toughness of these MoeSieB alloys hinder their practical application. To solve this problem, the inclusion of additional elements has been investigated as a way to improve the material properties of MoeSieB alloys [1e 6]. One elemental candidate, Re, has been shown to improve the yield strength, ultimate tensile strength, and elongation at room temperature of Mo with an Re addition of higher than 20 at.% [7]; the effects on the mechanical properties of Mo are shown in Fig. 1 and are collectively known as the “rhenium effect”. For Mo and W alloys, the “rhenium effect” is also effective in improving the hightemperature strength and room-temperature ductility, decreasing the ductileebrittle transition temperature (DBTT), and reducing recrystallization embrittlement [8,9]. Re is therefore an element capable of improving the mechanical properties of MoeSieB alloys.
* Corresponding author. Tel.: þ81 22 795 7326; fax: þ81 22 795 7325. E-mail address: [email protected] (J. Nakamura). http://dx.doi.org/10.1016/j.intermet.2014.04.009 0966-9795/Ó 2014 Elsevier Ltd. All rights reserved.
Recently, the partitioning behavior of Re in Mo solid solution (Moss) þ Mo3Si þ Mo5SiB2 (T2) three-phase alloys were studied by Yang et al. [10], who reported that Re partitioned mainly into the Moss phase and indicated that the addition of Re also enhanced the solubility of Si in Moss. Furthermore, they clarified that Re had a low solubility in the Mo3Si, Mo2B, and T2 phases at 1600 C [10]. While the effect of Re on the mechanical properties of Moss was discussed, the effects were unclear for the other phases. The role and behavior of Re in the T2 phase is thought to be one key factor in controlling the mechanical properties and thus should be studied in detail. Therefore, the purpose of this work was to assess the Re effect in the T2 phase of MoeSieB multiphase alloys, and to do this, the siteoccupation behavior of Re in T2 was studied both theoretically and experimentally. The effect of Re on the solid-solution hardening of T2 was investigated by considering the off-stoichiometry of the T2 phase. 2. Experimental procedure Five quaternary Re-added MoeSieB alloys (Q1 to Q5) were prepared with 99.99 wt.% Mo, 99.999 wt.% Si, 99 wt.% B, and 99.9 wt.% Re using a conventional arc-melting technique in an Ar atmosphere. The added amount of Re was about 1.4 at.% in all the quaternary alloys. In addition, five ternary MoeSieB alloys (S1 to
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where E (ApBq), Eeq (A), and Eeq (B) are the total energies per atom of the compound ApBq and the constituents A and B in their equilibrium (zero-pressure) geometries, respectively, and xA and xB are the concentrations of A and B, respectively, where xA ¼ p/(p þ q) and xB ¼ q/(p þ q). Microstructure observations and phase identification were performed using scanning electron microscopy (SEM) (Philips XL30FEG) with energy-dispersive X-ray spectroscopy (EDS). Highangle annular dark-field (HAADF) images were obtained using a field-emission-gun scanning transmission electron microscope (STEM) (Jeol ARM200F) operated at 200 keV. HAADF images were simulated with the MacTempas X software (Total Resolution, USA). The nanoindentation hardness of the T2 phase in each alloy was measured at room temperature with a maximum load of 100 mN.
3. Results and discussion
Fig. 1. Effect of Re on the mechanical properties of MoeRe alloys at room temperature.
S5) were prepared using the same technique for comparison. The prepared ingots were then subject to a heat treatment at 1800 C for 24 h in an Ar atmosphere, and subsequently furnace-cooled. Temperature rapidly decreased to 1450 C for 2 min, and reached room temperature for about 3 h. The weighed compositions of these alloys are listed in Table 1 and shown on the MoeSieB ternary phase diagram in Fig. 2 [11]. As shown in the figure, the composition of each alloy was chosen around the T2 composition; the composition of T2 in a sample corresponds to that at one of the corners of the T2 single-phase region. First-principles calculations were performed to assess the phase stability and site-occupation behavior of the Re-added T2 using the “Advance/PHASE” software package (Advance Soft Corp., Japan). All the calculations were conducted using projector augmented wave (PAW) pseudopotentials within the generalized gradient approximation (GGA) of density functional theory, and all the examined crystal structures were fully relaxed with respect to the cellinternal coordinates on periodic boundary condition. In the case of the T2 phase, a 13 13 5 MonkhorstePack mesh was used for the k-point sampling, and the cutoff energy of 800 eV was used. The equilibrium formation enthalpy DHeq (ApBq) is one key indicator of the stability of the constituent phases and that of an ApBq phase is given by the energy difference between the ApBq phase and the composition-weighted average of the constituent elements A and B in their equilibrium crystal structures [12]:
DH eq Ap Bq ¼ E Ap Bq ½xA Eeq ðAÞ þ xB Eeq ðBÞ
(1)
The atomic arrangement of the unit cell in the T2 phase is shown in Fig. 3. This phase has a D8l-type structure with lattice parameters of a ¼ 0.60 nm and c ¼ 1.10 nm [13e15], and it consists of 2 MoeB layers, 4 Mo layers, and 2 Si layers, as shown in Fig. 3(bef), stacked in the [001] direction. The crystal structure data used in this study are listed in Table 2. Also, initial atomic coordinates and refined parameters due to atomic structure relaxation by the first principle calculation are shown in Table 2. The site-occupation behavior of Re in T2 was predicted from first-principles using the atomic structure shown in Fig. 3(a) with a composition of Mo20Si4B8 and by assuming periodic boundary conditions. Replacing one atom in the T2 unit cell with Re corresponds to an Re concentration of approximately 3.1 at.%. There are four possible atomic positions for Re site-substitution, namely (1) a Mo-1 site in the MoeB layer (Fig. 3(b and f)), (2) a B site in the Moe B layer (Fig. 3(b and f)), (3) a Mo-2 site in the Mo layer (Fig. 3(c and e)), or (4) a Si site in the Si layer (Fig. 3(d)). The formation energy for each Re site-substitution case was calculated (Fig. 4), and it was found that the Mo-1 and Mo-2 sites are more stable in the Readded model. The Mo-1 site appears to be a slightly more favorable than the Mo-2 site, but Re substitution at a Mo site results in a small increase of the formation enthalpy with respect to the Re-free ternary T2. The change in the equilibrium formation enthalpy of the possible constituent phases, Moss, Mo2B, Mo3Si, etc., resulting from Re substitution was also estimated using a super cell and a fixed Re concentration of about 3 at.%. Structural data, super cell and k-point mesh used in this study are summarized in Table 3 [16e19]. The atomic position of the Re substitution was determined so as to minimize the equilibrium formation enthalpy. Fig. 5 shows that the enthalpy increased slightly in all the constituent phases except for Moss, suggesting that Re decreases the phase stability of all the phases except Moss. This result is in good agreement with the
Table 1 Weighed compositions and constitution phases of the MoeSieBeRe and MoeSieB alloys. Sample No.
Q1 Q2 Q3 Q4 Q5 S1 S2 S3 S4 S5
Composition (at.%)
Phase constitution
Mo
Si
B
Re
58.9 61.6 64.6 64.6 62.6 62.5 63.0 66.0 66.0 64.0
9.8 8.0 9.0 13.0 19.0 12.5 8.0 9.0 13.0 19.0
30.1 29.0 25.0 21.0 17.0 25.0 29.0 25.0 21.0 17.0
1.2 1.4 1.4 1.4 1.4 e e e e e
*Chemically analyzed
T2 þ Mo3Si þ MoB T2 þ MoB þ Mo2B þ Re-rich phase T2 þ Mo þ Mo2B T2 þ Mo þ Mo3Si T2 þ Mo3Si þ Mo5Si3 T2 þ MoB T2 þ MoB þ Mo2B T2 þ Mo þ Mo2B T2 þ Mo þ Mo3Si T2 þ Mo3Si þ Mo5Si3
J. Nakamura et al. / Intermetallics 53 (2014) 85e91
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Fig. 2. (a) MoeSieB ternary phase diagram at 1800 C [10] and (b) weighed compositions of the MoeSieBeRe and MoeSieB alloys.
Fig. 3. Crystal structure of Mo5SiB2: the (a) unit cell, (b) MoeB layer (z ¼ 0), (c) Mo layer (z ¼ 1/8), (d) Si layer (z ¼ 1/4), (e) Mo0 layer (z ¼ 3/8), (f) MoeB0 layer (z ¼ 1/2), and (g) [001] projection.
preferable partitioning of Re into Moss in the MoeSieRe [20] and MoeSieBeRe systems [10,21]. Fig. 6(aee) show back-scattered electron (BSE) images of the microstructures of the Q1 to Q5 alloys after the heat treatment. In the image of the Q1 alloy (Fig. 6(a)), the T2 matrix is shown by the grey contrast, and there are small amounts of Mo5Si3 (T1) and MoB dispersions that have a darker contrast. The phase constitution is the same as that of the ternary alloy without Re. The composition of the Q2 alloy projected onto the ternary phase diagram lies in the T2Mo2BeMoB triangle (Fig. 2(b)), and the T2 matrix can be seen in
Fig. 6(b) along with Mo2B at a brighter contrast and MoB at a darker contrast as the secondary phases. Furthermore, there is a Re-rich phase with a brightest contrast. It was confirmed by the selected area electron diffraction (SAED) method in TEM that the Re-rich phase has a crystal structure equivalent to Mo2B. However, the phase with a higher Re concentration is not shown in phase diagrams. Two types of Mo2B with different Re concentrations might exist in the MoeReeSieB quaternary system, but it is unclear at present. For the Q3, Q4, and Q5 alloys, three-phase constitutions of T2, Moss, and Mo2B for the Q3 alloy, T2, Moss, and Mo3Si for the Q4
Table 2 Crystal structure data of Mo5SiB2 [13e15]. Structure type Cr5B3 No
Site notation
Atom
Pearson symbol
Space group
Cell parameters
tl32
I4/mcm
a ¼ 0.6013 nm, b ¼ 0.6013 nm, c ¼ 1.103 nm, a ¼ 90 , b ¼ 90 , g ¼ 90
Multiplicity
Atom coordinates for initial model 1 Mo-2 Mo 16 2 B B 8 3 Mo-1 Mo 4 4 Si Si 4 Refined parameters of the relaxed structure model 1 Mo-2 Mo 16 2 B B 8 3 Mo-1 Mo 4 4 Si Si 4
Wyckoff
Site symmetry
x
y
z
Occupancy
l h c a
..m m.2m 4/m.. 422
0.160 0.375 0.000 0.000
0.660 0.875 0.000 0.000
0.143 0.000 0.000 1/4
1.0 1.0 1.0 1.0
l h c a
..m m.2m 4/m.. 422
0.16441 0.37905 0.00000 0.00000
0.66441 0.87905 0.00000 0.00000
0.13908 0.00000 0.00000 0.25000
1.0 1.0 1.0 1.0
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Fig. 4. Change in the formation enthalpy of Mo5SiB2 with substitutional Re at different sites calculated using a first-principles method.
alloy, and T2, Mo3Si, and T1 for the Q5 alloy were observed (Fig. 6(c, d, and e), respectively). The obtained phase constitutions are also listed in Table 1, and the results are in good agreement with the MoeSieB ternary phase diagram at 1800 C shown in Fig. 1, although the Re-rich phase was equilibrated in the Q2 alloy for a 1.4 at.% Re addition. Table 4 lists the Si and Re concentrations in the Moss, T2, Mo3Si, T1, Mo2B, MoB, and Re-rich phase as measured by SEM-EDS. The Re concentration is much higher in the Moss and Re-rich phase. However, these concentrations were somewhat overestimated since the estimation was performed only for Mo, Si, and Re but not B because of the low quantitative capability against B. Fig. 7 summarizes the Si and Re concentrations in the T2 phase of the Q1 to Q5 alloys as measured by SEM-EDS. The Si concentration ranges from 13.5 to 17 at.%. It is difficult to ascertain the true Si concentration in the T2 phases, but there is no doubt that the T2 phases had an offstoichiometric Mo5SiB2 composition. The Re concentration detected in this analysis was about 1.9e2.9 at.%, indicating that there is less than one Re atom substitution per unit cell, but the true Re concentration will be lower because B was neglected in the estimation. The Re concentration appears to correlate with the Si concentration except for Q5, which has the highest Si concentration but a slightly lower Re concentration. This would be an offstoichiometric effect of the T2 phase. HAADF-STEM observations were conducted for the S1 (ternary) and Q1 (quaternary) alloys to examine the site-occupation behavior of Re in the T2 phase. Zone axes of [001] and [110] were chosen so as to ensure that the Mo and Re positions could be distinguished in the Mo layers (Fig. 3(c and e)) and MoeB layers (Fig. 3(b and f)). Fig. 8(a) shows a HAADF image obtained along the [001] zone axis
Fig. 5. Change in the formation enthalpy of Moss, T2, MoB, Mo2B, Mo5Si3, and Mo3Si with Re addition.
of the T2 phase in the Re-free S1 alloy. The contrast brightness in these images is correlated to the atomic number density of the atomic columns along the viewing direction (for reference, the atomic numbers of B, Si, Mo, and Re are 5, 14, 42, and 75, respectively). Along the [001] direction, there are two Mo atoms at Mo-1 sites in the MoeB layers and two Si atoms in the Si layers that completely overlap in the MoeSi columns, and two Mo atoms at Mo-2 sites in the Mo layers and two B atoms in the MoeB layers that completely overlap in the MoeB columns. Thus, the density along the [001] direction is [(2 42) þ (2 14)]/ 0.6013 ¼ 186.3 nm1 for the MoeSi columns and [(2 42) þ (2 5)]/0.6013 ¼ 156.3 nm1 for the MoeB columns. Consequently, the MoeSi columns have a slightly stronger contrast than the MoeB columns in the ternary alloy. Fig. 8(b) shows a HAADF image of a relatively wide area of the T2 phase in the Q1 alloy obtained along the [001] axis. Some of the MoeSi columns (e.g. indicated by white triangle pairs along <110>) in this image have a much higher contrast than the other MoeSi columns, which suggests that Re preferentially occupies the Mo-1 sites in the MoeB layers. Fig. 8(c) shows these brighter MoeSi columns in more detail. If the Re atoms are homogeneously distributed in the T2 phase, then all of the MoeSi columns in Fig. 8(c) should have a higher contrast than the MoeB columns. These results combined with the result that the Re concentration as determined by SEM-EDS was less than 3.0 at.% (Fig. 7), which corresponds to less than one Re atom per T2 unit cell, suggest that Re atoms tend to segregate in local regions of the T2 phase. In the HAADF image of the T2 phase in the Q1 alloy obtained along the [110] axis (Fig. 8(d)), periodic bright spots corresponding to Mo-atom arrangements are observed in the MoeB and Mo layers. The Si layers are apparent from their low contrast owing to the low atomic number. If Re preferentially occupies Si
Table 3 Crystal structure data for first principle calculation of Mo, MoB, Mo2B, M5Si3, Mo3Si [16e19]. Phase
Space group
Cell parameters
Super cell
k-point
Mo
Im-3m
333
555
MoB
Cmcm
212
12 8 12
Mo2B
I4/mcm
221
5 5 10
Mo5Si3
I4/mcm
111
6 6 12
Mo3Si
P-3mn
a ¼ 0.3147 nm, b ¼ 0.3147 nm, c ¼ 0.3147 nm, a ¼ 90 , b ¼ 90 , g ¼ 90 a ¼ 0.316 nm, b ¼ 0.316 nm, c ¼ 0.316 nm, a ¼ 90 , b ¼ 90 , g ¼ 90 a ¼ 0.5543 nm, b ¼ 0.5543 nm, c ¼ 0.4734 nm, a ¼ 90 , b ¼ 90 , g ¼ 90 a ¼ 0.9643 nm, b ¼ 0.9643 nm, c ¼ 0.4910 nm, a ¼ 90 , b ¼ 90 , g ¼ 90 a ¼ 0.4893 nm, b ¼ 0.4893 nm, c ¼ 0.4893 nm, a ¼ 90 , b ¼ 90 , g ¼ 90
221
6 6 12
J. Nakamura et al. / Intermetallics 53 (2014) 85e91
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Fig. 6. Microstructure of the MoeSieBeRe quaternary alloys: (a) Q1, (b) Q2, (c) Q3, (d) Q4, and (e) Q5.
Table 4 Si and Re concentrations in the constituent phases of the MoeSieBeRe alloys as measured by SEM-EDS (at.%). Alloys
Moss Si
Q1 Q2 Q3 Q4 Q5
6.04 8.90
Mo5SiB2 Re
8.99 9.93
Mo3Si
Si
Re
16.23 13.70 14.55 16.70 17.06
2.45 1.94 2.03 2.89 2.36
Si
23.32 24.02
Mo5Si3 Re
3.90 4.24
sites, then the brightness of the Si layers in an image obtained along the [110] direction should be much stronger than that observed in Fig. 8(d), which further supports the conclusion that Re preferentially occupies the Mo-1 sites in the MoeB layers. To evaluate the effects of the off-stoichiometry and Re substitution on the room-temperature hardness of the T2 phase, the nanoindentation hardness of each sample was measured. The hardness values were obtained by averaging the values of ten testpoints and excluding data that showed pop-in behavior in the loadedisplacement curves. The nanoindentation hardness results are shown in Fig. 9 for both the ternary (S1 to S5) and quaternary
Mo2B
Si
Re
36.47
7.19
35.96
MoB
Si
Re
2.33 3.05
4.59 4.60
Re-rich phase
Si
Re
Si
Re
0.84 0.82
1.19 1.39
5.55
12.53
3.18
(Q1 to Q5) alloys. For the ternary alloys, the S3 alloy, which consists of T2þMoss þ Mo2B, i.e., a Mo- and B-rich composition, exhibited the highest hardness. The second highest was that of S4, T2þMoss þ Mo3Si (a Mo-rich composition), followed by S5, T2þMo3Si þ T1 (a Mo- and Si-rich composition). The S1 alloy, T2þT1þMoB (a Si-rich composition), showed the lowest hardness, and the S2 alloy, T2þMo2B þ MoB (a B-rich composition), had the second lowest hardness. Therefore, an off-stoichiometry composition toward a Mo- and B-rich composition appears to induce hardening of the T2 phase. For the quaternary alloys, a similar offstoichiometric effect on the nanoindentation hardness was observed. However, Q4, consisting of T2þMoss þ Mo3Si, exhibited a higher hardness than Q3 (T2þMoss þ Mo2B). This difference in nanoindentation hardness trend would appear to be caused by solid-solution hardening by Re, and in fact, the Q4 alloy had the highest Re concentration in the T2 phase, as shown in Fig. 7. Moreover, the hardness of Q1, which also had a relatively high Re concentration in the T2 phase, increased remarkably in comparison with the other 4 alloys. Thus, the solid-solution hardening effect of Re in T2 is quite reasonable. Therefore, the T2 phase is hardened by both the solid-solution hardening effect of Re and the offstoichiometric effect.
4. Conclusions
Fig. 7. Si and Re concentrations in the Mo5SiB2 phase as measured by SEM-EDS.
The site-occupation behavior of Re in the T2 phase has been studied both theoretically and experimentally, and the effect of Re on the solid-solution hardening of T2 was investigated by taking into account the off-stoichiometry of the T2 phase. The following conclusions were obtained:
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Fig. 8. (a) Average image obtained from high-resolution HAADF-STEM image of Mo5SiB2 in the Re-free S1 alloy taken along the [001] zone axis. (b) High-resolution HAADF-STEM image of Mo5SiB2 in the Re-added Q1 alloy taken along the [001] zone axis. (c) Average image of Mo5SiB2 in the Q1 alloy taken along the [001] zone axis. (d) Average image of Mo5SiB2 in the Q1 alloy taken along the [110] zone axis.
(1) The addition of Re to MoeSieB alloys slightly decreases the phase stability of the constituent phases including the T2 phase but not the Moss phase. (2) An addition of 1.4 at.% of Re to MoeSieB alloys with compositions near T2 does not result in a considerable change to the phase equilibria. Only in the T2þMo2B þ MoB alloy was the Re-rich phase found to be equilibrated in the quaternary alloys. (3) Re preferentially occupies the Mo-1 sites in the MoeB layers.
(4) Off-stoichiometry toward a Mo- and B-rich composition hardens the ternary T2 phase. For the quaternary T2 containing Re, the T2 phase is hardened by the solid-solution hardening effect of Re and the off-stoichiometric effect. Acknowledgments This work was supported by the funding program for Next Generation World-Leading Researchers (NEXT Program) (No. GR017) administered by the Japan Society for the Promotion of Science (JSPS). References
Fig. 9. Nanoindentation hardness of Mo5SiB2 phase in the Q1 to Q5 and S1 to S5 alloys.
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