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Microstructural evolution and mechanical properties of an Nb–16Si in-situ composite with Fe additions prepared by arc-melting
Intermetallics 34 (2013) 1e9
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Microstructural evolution and mechanical properties of an Nbe16Si in-situ composite with Fe additions prepared by arc-melting J.R. Zhou, J.B. Sha* School of Materials Science and Engineering, Beihang University, 37 Xueyuan Rd., Beijing 100191, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2012 Received in revised form 23 September 2012 Accepted 23 October 2012 Available online 27 November 2012
This paper deals with phase constitutions, microstructural evolutions, and mechanical properties of Nbe16SiexFe in-situ composites (where x ¼ 2, 4, 6 at.%, referred as to 2Fe, 4Fe and 6Fe alloys, hereafter) prepared by arc-melting. It is found that with additions of Fe, Nb4FeSi silicide arises and microstructures of as-cast samples are consisted of dendritic-like NbSS phase, Nb3Si block, and Nb4FeSi matrix in the 2Fe and 4Fe alloys, and of the dendritic-like NbSS phase and Nb4FeSi matrix in the 6Fe alloy. When heat-treated at 1350 C for 100 h, part of the Nb3Si phase decomposes in the 2Fe and 4Fe alloys, and the 6Fe alloy shows no change in microstructure as compared with the as-cast one. The Nb4FeSi silicide is found to be brittle, its fracture toughness and elastic modulus are first obtained, having values about 1.22 MPa m1/2, and 310 GPa, respectively. The fracture toughness of the bulk as-cast and heat-treated Nbe16SiexFe samples are changed slightly by the Fe additions, which is in a range of 9.03e10.19 MPa m1/2. It is interesting that at room temperature, strength is improved by the Fe additions, whereas at 1250 C and 1350 C the strength decreases. As the Fe content increased from 2 at.% to 6 at.%, for example, the 0.2% yield strength increases from 1410 MPa to 1580 MPa at room temperature, decreases from 479 MPa to 385 MPa at 1250 C. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Intermetallics A. Ternary alloy system B. Phase identification D. Microstructure E. Mechanical properties
1. Introduction It is well known that structural materials used in advanced aeronautics and astronautics propulsion systems must possess a balance combination of properties, including ductility and toughness at low temperatures, strength and creep resistance at elevated temperatures, as well as environmental stability. In order to enable Nb-based alloys to achieve such balanced properties, a concept of multi-phase alloys has been developed for design of the NbeSi based alloys [1e7]. It involves incorporation of ductile Nb solid solution (NbSS) with stiffening intermetallic, such as silicide Nb5Si3 or Nb3Si [1e10], with oxidation resistant phase Cr2Nb [11e13], to form a multi-phase NbSS/Nb5Si3/Cr2Nb microstructure. By adjusting proportion and morphology of the NbSS, Nb5Si3 and Cr2Nb phases in the NbeSi based alloys; the balanced properties may be obtained to meet the requirements of the ultra-high temperature structural materials. Besides the aforementioned microstructural design, alloying is also an effective way to optimize each property. W and Mo can increase the melting point of the NbeSi based alloys, they are found
* Corresponding author. E-mail address: [email protected] (J.B. Sha). 0966-9795/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.intermet.2012.10.017
to be the strongest solid solution strengthening elements for Nb [7,8,14]. Ti and Hf have been proved to decrease the brittle-ductile transfer temperature (BDTT) of Nb [15], they play a key role on improving the fracture toughness and ductility of the Nb solid solution [16]. In addition, thermomechanical processing, such as directional solidification [6e8] and hot extrusion [17e20], have been frequently used to improve the mechanical properties of the NbeSi based alloys. The additions of Al, Fe and Cr to the NbeSi insitu composites reduce oxygen diffusion and solubility in Nb and form an oxidation against phase, Cr2Nb intermetallic compound [11e13]. The short-term goal corresponding to a material thickness loss of only 200 microns in 10 h air exposure at 1370 C has been successfully achieved in the multi-components NbSS/Nb5Si3/Cr2Nb [11e13] alloys. Recently, some researchers have focused on the role of Fe on microstructure evolution of the NbeSi based alloys [19e22]. Fe has been reported to be almost equally as effective as Cr at concentrations up to 5 at.% to improve oxidation resistance [11]. Researches revealed that with 5 at.% Fe addition, Nbe18Sie24Tie5Cr and Nbe18Sie24Tie5Cre5Sn alloys had complex phase constitutions, in which a new silicide, Nb4FeSi was observed [21,22]. A tensile elongation of 2% at room temperature and superplasticity, of about 512% elongation at 1450 C, were amazedly obtained in the Nbe16Sie2Fe alloy prepared by hot
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pressing sintering [20], due to formation of Nb4Fe3Si5 phase with a low melting point of 1359 C. However, till to date, understanding of how Fe affects the phase and microstructural evolution, roomand high temperature properties of the cast NbeSi based alloys is still poor. The objective of this work is to understand basic changes in metallurgy and mechanical behavior of the Nbe16Si binary with Fe additions under as-cast and heat-treated states. 2. Experimental procedures Button ingots of Nbe16SiexFe alloys (x ¼ 2, 4, 6 at.%), each with a weight of 160 g, were prepared by arc-melting six times with electromagnetic stirring in an argon atmosphere under a pressure of w0.5 Pa. The ingots were then annealed at 1350 C in vacuum for 100 h and furnace-cooled to ensure quasi-equilibrium microstructures. The purity of all raw elements used was 99.9% (in mass) or higher. X-ray diffraction (XRD) analysis was performed on bulk samples to identify the phase constitutions using Cu Ka (l ¼ 0.15405 nm) radiation at 40 kV and 40 mA (Rigaku D/Max 2500PC). The backscattered electron (BSE) images were taken to investigate microstructures of the as-cast and heat-treated samples, and crack paths of the tested samples by using a SEM (JEOL JXA-8100) equipped with an energy-dispersion X-ray spectroscopy (EDS, Oxford Instruments, UK) to analyze phase compositions. Volume fractions of the phases in each as-cast and heat-treated sample were statistically determined by quantitative metallographic analysis on the back-scattered microstructures through a software. Vickers hardness (Hv) was measured under an applied load of 0.98N on a digital HXZ-1000 micro-sclerometer (Shanghai, China) with a loading-maintaining time of 15 s, and an average value was obtained by taking 10 Hv data for each sample. Fracture toughness KIC of the stiffening silicides were estimated by indentation method according to the formula (1) given by [23],
K1C ¼ CV
1=2 E P H c3=2
length, w: specimen width) was introduced by electrical discharge machining with a Cu wire 0.2 mm in diameter. The three-pointbending specimen is 30 mm in length, 6 mm in width and 3 mm in thickness, and tests were conducted on a SANS testing machine with a loading span of 24 mm and a crosshead displacement rate of 0.1 mm/min. Compressive tests at room temperature were conducted on a SANS testing machine at a strain rate of 3 104 s1 to measure the compressive strength while size of the rectangular compressive samples was 3 mm 3 mm 6 mm. High temperature compressive tests (at 1250 C and 1350 C) were conducted in an argon atmosphere at a strain rate of 1 103 s1 using a Gleeble 1500 testing machine. The compressive cylinders were 6 mm in diameter and 9 mm in length. All specimens for the three-pointbending and compressive tests were mechanically polished using SiC paper (1000-grit) with water before testing. 3. Results 3.1. Phase constitutions and microstructural characteristics of the as-cast and heat-treated alloys Typical X-ray diffraction patterns of the as-cast and heat-treated Nbe16SiexFe alloys are shown in Fig. 1. Under the as-cast condition, as shown in Fig. 1(a), the characteristic diffraction peaks of three phases are detected, Nb solid solution NbSS (b.c.c), silicides
(1)
where E is the Young’s modulus, H is the hardness, P is the indentation load, c is the crack length, and CV is a material-independent constant having value, 0.016 [23]. The Young’s modulus of the stiffening silicides was evaluated using loading and displacement sensing indentation method [24], which is the first time for the Nb4FeSi phase. The indentation loading-unloading vs displacement curves were measured using a nano-indentation tester (Nano Indenter II), to which a Berkovich-type diamond indenter was attached. With loading (unloading) versus indenter displacement curve of an indentation experiment, the Young’s modulus of the indented phase can be calculated by a series of formulae, while some parameters, such as the Young’s modulus and Poisson’s ratio of the indenter, as well as the Poisson’s ratio of the indented phase, etc., are needed for the Young’s modulus calculation. For the diamond indenter in the nano-indentation tester, the Young’s modulus and Poisson’s ratio are 1050 GPa and 0.104, respectively. The Poisson’s ratio of the silicide taken here is about 0.2 (this data is determined by referring to the Poisson’s ratio of metal-based silicides (Mo/Si silicide and Ir/Si silicide [26,27])). In the indenter loading and displacement measurements, the loading/unloading rate was 1 nN/s and the load at the peak was held constant for a period of 10 s, while 5 grains of each kind of phase were taken for indenting to obtain an accurate average. Single-notched three-point-bending specimens were used instead of single fatigue-cracked specimens for the toughness (KQ) measurements of the bulk alloys. A notch up to a/w ¼ 0.5 (a: notch
Fig. 1. X-ray diffraction patterns of (a) the as-cast and (b) the heat-treated Nbe16SiexFe alloys.
J.R. Zhou, J.B. Sha / Intermetallics 34 (2013) 1e9
Nb3Si (Ti3P-type) and Nb4FeSi (b.c.t) in the 2Fe and 4Fe alloys, where some peaks of the Nb3Si and Nb4FeSi phases overlap with each other. As the Fe content increases to 6 at.%, the independent characteristic peaks from the Nb3Si phase disappear and those from the NbSS and Nb4FeSi phases are still remained. In comparison with the as-cast Nbe16Si binary composed of the NbSS and Nb3Si phases [25], it is known that the addition of Fe promotes the generation of the new silicide Nb4FeSi, while suppresses the formation of the Nb3Si phase. After heat treatment at 1350 C for 100 h, as shown in Fig. 1(b), the 2Fe alloy still contains the independent characteristic peaks of the NbSS, Nb3Si and Nb4FeSi phases, but the intensity and the quantity of the Nb3Si peaks both decrease, as compared with those in the as-cast sample shown in Fig. 1(a). In the heat-treated 4Fe and 6Fe alloy, the independent peaks of the Nb3Si phase disappear and the remaining, are from the NbSS and Nb4FeSi phases, while the overlapping peaks of the Nb4FeSi and Nb3Si phases have also been marked for comparison (see Fig. 1(b)). Typical BSE images of the as-cast and heat-treated three alloys are shown in Figs. 2 and 3, respectively. The microstructural analysis, together with the X-ray diffraction patterns (Fig. 1) and the EDS data of phase compositions (Table 1), shows that in the as-cast
3
samples the white block is the NbSS phase, the light grey phase is the Nb3Si silicide, and the dark grey phase is the Nb4FeSi silicide. For the as-cast 2Fe and 4Fe alloys, the microstructures are composed of the dendritic-like NbSS arms, Nb3Si blocks, Nb4FeSi matrix and small amount of the NbSS þ Nb4FeSi eutectic (marked with rectangles in Fig. 2(a’) and (b’)) distributed on the edge of the Nb3Si blocks. Inside the Nb3Si blocks, the NbSS islands are found (Fig. 2(a’) and (b)), while the fine Fe-rich NbSS þ silicide eutectic (marked with circles in Fig. 2(b’)) appears in the 4Fe alloy. The morphology in Fig. 2(c) and (c’) reveals that the as-cast 6Fe alloy has different microstructures in comparison with the 2Fe and 4Fe alloys, it contains the primary NbSS, the Nb4FeSi matrix, the eutectic (marked with rectangle in Fig. 2(c’)) of them, as well as the Fe-rich eutectic (marked with circles in Fig. 2(c’)). The Nb4FeSi matrix is also found to show different contrast in correspondence with the different Fe contents. EDS results (Table 1) indicate that the dark Nb4FeSi phase, i.e. Fe-rich Nb4FeSi, has a Fe concentration of about 21.28 at.%, higher than that of about 4.63 at.% of the grey Nb4FeSi phase. For the Fe-rich eutectic in the 4Fe and 6Fe alloys, the Fe concentration is about 32 at.%, also far higher than that of other phases.
Fig. 2. BSE images of the as-cast Nbe16SiexFe alloys.
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Fig. 3. BSE images of the heat-treated Nbe16SiexFe alloys.
The heat-treated microstructures of the 2Fe and 4Fe alloys are still composed of the NbSS, Nb3Si and Nb4FeSi phases (see Fig. 3(a) and (b)). However, part of the Nb3Si phase transforms into fine eutectoid of NbSS þ Nb4FeSi (see Fig. 3(a’) and (b’)), which means that incomplete eutectoid decomposition of the Nb3Si phase takes place during annealing. For the heat-treated 6Fe alloy, the microstructure is consisted of the NbSS phase and the Nb4FeSi matrix, the Fe-rich eutectic in the as-cast sample vanishes. Incidentally, black particles precipitate on the NbSS phase of all three samples during annealing, as seen in Fig. 3(a’)e(c’). Table 1 shows the phase compositions of the as-cast and heattreated Nbe16SiexFe alloys. The data in Table 1 displays that in the as-cast sample Fe atoms dissolve into the Nb4FeSi and NbSS phases rather than into the Nb3Si phase. The concentration of Fe in the NbSS phase shows an increasing tendency, from 1.14 at.% to 2.30 at.%, as the nominal Fe content increases from 2 at.% to 6 at.%. The average composition of the Nb3Si phase is about Nbe23.4Sie 0.22Fe, in which the atomic ratio of Nb to Si is close to 3:1, the stoichiometric ratio in the Nb3Si binary. For the Nb4FeSi phase, the concentration of Nb is nearly 69 at.% and that of Fe varies from 4.15 to 4.69 at. %. However, the atomic ratio of Nb to (Si þ Fe) is found to be constant, close to 6.9:3.1, which is even as same as that for the
Fe-rich Nb4FeSi phase in the 6Fe alloy. This constant ratio is consistent with the Nb to (Si þ Fe) ratio of the Nb4FeSi phase found in the multi-component Nbe24Tie18Sie5Fee5Sn and Nbe45Tie 15Sie5Fee5Sn alloys [21,22]. Combining Table 1 and results in references [21,22] indicates that the Fe and Si concentrations in the Nb4FeSi phase have wide range, but sum is about 31 at.%. As for the fine Fe-rich regions, looked like eutectic, their average Nb and Si concentrations are far lower than those of the Fe-rich Nb4FeSi phase, whereas the Fe concentration is contrary (see Table 1). After heat treatment, the silicide precipitates in the NbSS phase (see Fig. 3), which is, naturally, resulted from a decrease in the concentrations of both Fe and Si solidified in the NbSS phase. For the Nb3Si and Nb4FeSi phases in the 2Fe and 4Fe alloys, as well as, for the Nb4FeSi and Fe-rich Nb4FeSi phases in the 6Fe alloy, the heat treatment seems not to bring any considerable change in their average compositions. Fig. 4 gives the phase’s volume fractions of the as-cast and heat-treated samples as a function of the nominal Fe contents. The NbSS and silicides (Nb3Si þ Nb4FeSi) fractions seem to be independent of the nominal Fe content, while the NbSS fraction is in a range of 42e44% for the three as-cast samples. For the silicides, their fractions show a reversed variation with Fe. The Nb3Si fraction
J.R. Zhou, J.B. Sha / Intermetallics 34 (2013) 1e9
5
Table 1 Phase compositions of the as-cast and heat-treated Nbe16SiexFe alloys. Heat-treated (1350 C, 100 h)
Alloy
Phase
As-cast Nb(at.%)
Si(at.%)
Fe(at.%)
Nb(at.%)
Si(at.%)
Fe(at.%)
2Fe
NbSS
96.45 95.83e97.70 76.82 76.65e76.84 69.08 68.91e69.16 95.81 95.53e96.04 75.94 75.86e76.04 68.54 68.29e68.79 61.04 54.86e66.22 95.45 95.16e95.74 68.86 68.69e69.06 68.68 68.45e68.94 60.46 54.49e63.07
2.41 1.56e2.68 22.95 22.88e23.06 26.77 25.49e28.01 2.27 2.13e2.45 23.85 23.74e23.96 26.77 24.46e28.07 7.22 5.82e8.50 2.25 2.08e2.35 26.51 25.48e26.94 10.04 9.98e10.09 7.02 6.06e8.54
1.14 0.74e1.62 0.23 0.17e0.29 4.15 2.85e5.32 1.92 1.69e2.32 0.21 0.17e0.24 4.69 3.46e6.99 31.74 27.96e36.84 2.30 1.93e2.56 4.63 4.14e5.76 21.28 21.09e21.46 32.52 28.70e36.97
97.75 97.42e98.04 76.62 76.53e76.72 69.01 68.98e69.19 98.17 98.07e98.58 76.18 75.85e76.62 68.72 67.85e69.29
1.63 1.14e2.32 23.27 23.19e23.34 26.53 24.28e28.40 0.74 0.49e1.07 23.62 23.18e23.95 26.9 23.66e28.58
0.62 0.27e0.81 0.11 0.05e0.14 4.46 2.76e6.43 1.09 0.70e1.32 0.20 0.19e0.20 4.38 3.34e7.22
97.22 96.40e97.56 68.68 68.39e69.08 68.61 68.14e69.32
0.69 0.53e1.41 25.89 22.85e27.98 10.61 10.07e11.60
2.09 1.91e2.19 5.43 3.07e8.08 20.78 20.61e21.07
Nb3Si Nb4FeSi 4Fe
NbSS Nb3Si Nb4FeSi Fe-rich eutectic
6Fe
NbSS Nb4FeSi Fe-rich Nb4FeSi Fe-rich eutectic
For an element content, the first line is the average value and the second line is the value range.
decreases from 31% for the 2Fe alloy to 10% for the 4Fe alloy, and finally, to zero for the 6Fe alloy, whereas the Nb4FeSi fraction increases from 26 at.% to 58 at.%. For the heat-treated samples, the Nb3Si fractions of the 2Fe and 4Fe alloys are lower than those of the corresponding as-cast alloys due to the eutectoid decomposing of the Nb3Si phase, in return for this, the fractions of the NbSS and Nb4FeSi phases both increase. The NbSS and Nb4FeSi fractions in the 6Fe alloy are almost not changed by the heat treatment (see Fig. 4). 3.2. Mechanical response at room temperature
VOLUME FRACTION OF PHASES
3.2.1. Vickers hardness (Hv) and fracture toughness (K1C) of constituent phases Fig. 5 shows Hv of the as-cast and heat-treated NbSS phase as a function of the nominal Fe content. Both the as-cast and heattreated NbSS phase show an increasing Hv with Fe, which may be
NbSS
attributed to increase of the Fe concentration in the NbSS phase (see Table 1). In each sample, the heat-treated NbSS phase has a lower Hv than the as-cast one by about 27e33%. The precipitation takes place in the NbSS phase during heat treatment is deduced to weak the solid solution strengthening of the NbSS phase. Table 2 lists the mechanical properties of the Nb3Si and Nb4FeSi silicides in the heat-treated samples. It is clearly seen that hardness of the Nb4FeSi phase measured by using both the microsclerometer and nano-indentation tester is higher than that of the Nb3Si phase. The hardness of the Nb4FeSi phase is about 1110 in Hv and 17.9 GPa in nano-hardness, while that of the Nb3Si phase is 975 in Hv and 16.5 GPa in nano-hardness. Typical loading and unloading versus nano indenter displacement curves for the highest load experiment performed on the silicide grains are exhibited in Fig. 6. Under a load of 20 nN, the maximum penetration depth, hmax, of the indenter is about 258 nm in the Nb3Si phase and 238 nm in the Nb4FeSi phase. The final depth of the constant impression after unloading, hf, is about 166 nm and
Nb4FeSi
Nb3Si
26% 37% 46%
51%
58%
31%
59%
15% 10% 3%
43%
44%
42%
48%
46%
AC 2Fe
AC 4Fe
AC 6Fe
HT 2Fe
HT 4Fe
41%
HT 6Fe
Fig. 4. Volume fractions of the phases in the as-cast and heat-treated Nbe16SiexFe alloys.
Fig. 5. Vickers hardness of the NbSS phase in the as-cast and heat-treated samples as a function of the nominal Fe content.
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Table 2 Hardness, Young’s modulus and KIC of the Nb3Si and Nb4FeSi phases. Phase
Hardness/HVa
Hardness/GPab
Young’s modulus/GPa
K1C/MPa m1/2
Nb3Si Nb4FeSi
975 48 1110 44
16.5 1.02 17.9 1.33
240 18 310 22
1.01 0.061 1.22 0.053
a b
Determined by Vickers hardness tester. Determined by Nano-indentation tester.
155 nm, in the Nb3Si and Nb4FeSi phases respectively. The calculated Young’s modulus by equations [24] in Table 2 reveals that the Nb4FeSi phase has the Young’s modulus of about 310 GPa, higher than that of 240 GPa of the Nb3Si phase. Room temperature plasticity of the NbSS phase, as well as that of the silicides, can be characterized with indentation. Indentation feature of the phases obtained by the Hv tests is displayed in Fig. 7. As can be seen that the NbSS phase and silicides show different behaviors during penetration of the hardness indenter. In the silicides, for either of the Nb4FeSi or Nb3Si phases, cracks visibly initiate at both the tip and bottom of the indentation, as seen in Fig. 7(b) and (c). However, no crack appears around the indentation within the NbSS phase (see Fig. 7(a)). It is most likely that the Nb3Si and Nb4FeSi phases have brittle characteristics, while the NbSS phase deforms plastically at room temperature. By substituting the crack length and relative parameters: P of 0.98N, Hv of 975, Cv of 0.16, as well as E of 240 GPa, into formula (1), the fracture toughness, KIC, of the Nb3Si phase can be obtained, which is also listed in Table 2 with that of the Nb4FeSi phase. The KIC value of the Nb4FeSi phase is 1.22 MPa m1/2, almost at the same level as that of the Nb3Si phase. With such a low toughness, the Nb4FeSi silicide is thought to be an intrinsically brittle phase. 3.2.2. Fracture toughness (KQ) and compressive behavior of bulk samples The fracture toughness, KQ, of the bulk as-cast and heat-treated samples measured by notched three-point-bending tests is given in Table 3. An increase in the nominal Fe content does not result in visible influence on the KQ of the bulk as-cast samples, the KQ values of the three as-cast alloys are in a range of 9.03e9.66 MPa m1/2. By heat-treating at 1350 C for 100 h, the 2Fe and 4Fe alloys show a small increment in the KQ by about 0.6e0.8 MPa m1/2, when
hmax
LOADING, /nN
20 Nb3Si
15
S
Nb4FeSi Loading
10
Unloading
5 hf 0 0
150 250 100 200 50 DISPLACEMENT INTO SURFACE, /nm
300
Fig. 6. Load-indenter displacement curves of the silicides Nb3Si and Nb4FeSi, S is the initial unloading stiffness.
compared with the corresponding as-cast samples. The KQ value of the 6Fe alloy before and after heat-treating is at the same level. This small improvement in the 2Fe and 4Fe alloys is believed to be resulted from the formation of the eutectoid NbSS phase in the heat-treated samples. It should be noticed that as the nominal Fe contents increase from 2 at.% to 6 at.%, the ductile NbSS fraction of the heat-treated samples decreases from 48% to 41% (see Fig. 4), consequently, the KQ value decreases by about 6.7%, from 10.19 MPa m1/2 to 9.51 MPa m1/2. The true compressive stressestrain curves of the three heat-treated samples at room temperature are shown in Fig. 8. It is clear that the stress level at room temperature increases with the nominal Fe content. 3.3. Compressive behavior of the bulk heat-treated samples at high temperature Fig. 9 shows the true compressive stressestrain curves of the three heat-treated samples at 1250 C and 1350 C. At 1250 C and 1350 C, the curves show a very short deformation-hardening stage. After the stress is beyond the peak, it remains at high level, larger than 60e85% of the peak value until the total strain is greater than 25%. Among the three alloys, the 2Fe alloy shows the highest stress level at both 1250 C and 1350 C. Table 4 summaries the average 0.2% yield compressive strength (s0.2) and maximum compressive strength (smax) of the heattreated alloys at high temperatures, those at room temperature taken from Fig. 8 are also list for comparison. At high temperatures Fe seems to play a weakening influence on strength of the NbeSi based alloys. The 2Fe alloy has the highest s0.2, of 479 MPa at 1250 C, and of 295 MPa at 1350 C. The s0.2 value of the 4Fe and 6Fe alloys is almost at the same level, but lower than that of the 2Fe alloy by about 15e20%, as seen in Table 4. 4. Discussion The as-cast microstructure of the Nbe16Si binary is usually composed of the primary dendritic-like NbSS phase and the Nb3Si þ NbSS eutectic [10]. With replacements of 2 at.% to 6 at.% Nb by Fe, however, the solidification path and the phase constitution are changed, i.e., the Nb4FeSi phase appears, while the Nb3Si phase reduces gradually (in the 2Fe and 4Fe alloys) and finally disappears (in the 6Fe alloy), as can be seen in Fig. 2. This suggests that Fe is an element that restrains formation of the Nb3Si phase. According to the NbeSi binary diagram [25] and Fig. 2, the solidification path of the Nbe16Sie(2, 4)Fe alloys is primarily recommended as L / L þ NbSS / L þ NbSS þ Nb3Si / L þ NbSS þ Nb3Si þ (Nb4FeSi þ NbSS) eutectic / NbSS þ Nb3Si þ (Nb4FeSi þ NbSS) (with additional Fe-rich eutectic in this step for the 4Fe alloy), and that recommended for the Nbe16Sie6Fe alloy is L / L þ NbSS / L þ NbSS þ (Nb4FeSi þ NbSS) eutectic / NbSS þ (Nb4FeSi þ NbSS) eutectic þ Fe-rich eutectic. Comparing the microstructures of the heat-treated samples with those of the as-cast ones at the corresponding Fe contents finds that the Nb4FeSi phase is a stable phase as no decomposition takes place during heat treatment. In general, an increasing Fe content results in a considerable change in the phase constitution, from NbSS þ Nb3Si þ Nb4FeSi to NbSS þ Nb4FeSi, but small variations in the phase fraction (see that of the NbSS and silicides in Fig. 4), and the Fe concentration in the NbSS phase (see Table 1). These metallurgical changes affect the mechanical properties of the NbeSi based alloys to a certain extent. As a result, both the Fe concentration in the NbSS phase and the fraction of the stiffening silicides increase with the nominal Fe contents. The above two factors are assuredly negative for the bulk toughness of the sample because Fe raises the BDTT of Nb and embrittles Nb [18], while the silicides are brittle phases. However,
J.R. Zhou, J.B. Sha / Intermetallics 34 (2013) 1e9
7
Fig. 7. The indentation morphologies in NbSS (a), silicides Nb3Si (b) and Nb4FeSi (c).
work found that the NbeSieFe alloys began to melt at 1450 C [28], while the Nbe16Si binary containing the NbSS and Nb3Si phases, however, was still at the solid state at 1880 C [25]. The same is true even for the Nb4Fe3Si5 silicide found in the Nbe16Sie2Fe alloy prepared by hot pressing sintering, whose melting point is as low as 1359 C [20]. This means that the Fe additions decrease the melting points of the Nb/Si silicides dramatically. In general, the strengthening and weakening mechanisms co-exist in the NbeSi based alloys with the Fe additions. The one which may be the dominant factor for the bulk strength depends on the testing temperature. It is believed that the higher testing
1600
TRUE STRESS, MPa
as the concentration of Fe atoms in the NbSS phase and the changes in the silicides fraction with Fe, are very small; these small increases cannot cause a visible degradation in the bulk toughness as seen in Table 3. At room temperature, the s0.2 is sensitive to the Fe additions, by about a 170 MPa increment when the nominal Fe contents increase from 2 at% to 6 at.% (see Table 4). Metallurgically, both the solid solution hardened NbSS phase (Fig. 4) and the increasing stiffening Nb4FeSi fraction from 37% to 59% (Table 2 and Fig. 4) with Fe may contribute to the strength of the NbeSieFe ternary according to the rule of the mixture [15]. The reported Hv of pure Nb is about 82 [24], thus, increment of the Hv from 82 of pure Nb to 241 of the heat-treated NbSS phase comes from the solute concentrations of 1.63Siþ0.62Fe (see compositions of the NbSS phase in the 2Fe alloy in Table 1). A further increment in the Hv of the NbSS phase, from 241 up to 319, is absolutely attributed to an increase of the Fe concentration in the NbSS phase from 0.62 at.% to 2.09 at.%, whereas the solid solution strengthening by Si becomes weak as the Si concentration in the NbSS decreases. These strongly confirm the significant effect of Fe on the solid solution strengthening of the NbSS phase, as well as on the bulk strength of the NbeSi based alloys at room temperature. Unfortunately, the strength of the Nbe16Si alloy at high temperatures is decreased by the Fe additions, even the silicides fraction increases with Fe, as seen in Fig. 9(a) and (b). Our early
Room temperature
4Fe
6Fe
2Fe
1200
800
400
Table 3 Fracture toughness KQ of the Nbe16SiexFe samples at room temperature. Alloys
2Fe 4Fe 6Fe
KQ/MPa m1/2
2
As-cast
Heat-treated
9.37 0.59 9.03 0.47 9.66 0.39
10.19 0.36 9.63 0.51 9.51 0.38
4
6
8
TRUE STRAIN, % Fig. 8. Compressive stressestrain curves of the heat-treated Nbe16SiexFe alloys at room temperature.
8
J.R. Zhou, J.B. Sha / Intermetallics 34 (2013) 1e9
700
5. Conclusions
a
1250ºC
TRUE STRESS, MPa
600
2Fe
500
4Fe
400 300
6Fe
200 100 0
0 0
400
10 0.1
15 0.15
20 0.2
25 0.25
TRUE STRAIN, %
b
350
TRUE STRESS, MPa
5 0.05
30 0.3
35 0.35
1350ºC
(1) The microstructure consists of the NbSS, Nb3Si and Nb4FeSi phases at Fe contents of 2 at.% and 4 at.%, but of the NbSS and Nb4FeSi phases at a Fe content of 6 at.%. Heat treatment can not change the phase constitution of each alloy, however partly decomposition of the Nb3Si phase in the 2Fe and 4Fe alloys takes place. (2) The Nb4FeSi silicide is a brittle phase, it has a low toughness of 1.22 MPa m1/2, its Young’s modulus and Hv are 310 GPa and 1110, respectively. (3) The Fe additions from 2 to 6 at.% show a small effect on the fracture toughness KQ of the bulk samples, the KQ of all the as-cast and heat-treated samples are in a range of 9.03e10.19 MPa m1/2. (4) The room temperature compressive strength of the Nbe16Si alloy is improved by the Fe additions, whereas at 1250 C and 1350 C, the compressive strength decreases. As the Fe content increased from 2 to 6 at.%, the 0.2% yield strength increases from 1410 to 1580 MPa at room temperature, while it decreases from 479 to 385 MPa at 1250 C and 295 to 251 MPa at 1350 C. Acknowledgments
300
2Fe The authors are grateful to the support from the Program for New Century Excellent Talents in Universities (NCET-06-0173) and the National Natural Sciences Foundation of PR China (51071009 and 5071111) and Aerospace Funding (2010ZF51071).
4Fe
250
6Fe 200 150
References
100 50 0
0 0
5 0.05
10 0.1
15 0.15
20 0.2
25 0.25
30 0.3
35 0.35
TRUE STRAIN, % Fig. 9. Compressive stressestrain curves of the heat-treated Nbe16SiexFe alloys at high temperatures, (a) 1250 C, (b) 1350 C.
temperature (close to the melting point of the Nb4FeSi phase) may be favorable for the weakening mechanism. At room temperature, the solid solution hardening of the NbSS phase and the stiffening phase hardening of the silicides are dominant; therefore, the bulk strength shows an increasing tendency with Fe. When the testing performs at 1250 C and 1350 C, the testing temperatures are below the melting point of the Nb4FeSi phase by 100e200 C (referred to 1450 C [28], which is the temperature when the NbeSieFe alloys began to melt); thus, the Nb4FeSi phase probably becomes a soft zone and shows the weakening behavior for the bulk strength at this temperature range. This is the reason why the 4Fe and 6Fe alloys with higher Nb4FeSi fraction than the 2Fe alloy show lower bulk strength at high temperatures. Further works in determination of the accurate transformation temperature between room temperature and 1250 C, and mechanism from strengthening to weakening of the NbeSi alloys with the Fe additions are still needed.
[1] Bewlay BP, Lewandowksi JJ, Jackson MR. Refractory metal-intermetallic in-situ composites for aircraft engines. JOM 1997;49:44e5. [2] Subramanian PR, Mendiratta MG, Dimiduk DM. The development of Nb-based advanced intermetallic alloys for structural applications. JOM 1996;48(1):33e8. [3] Rigney JD, Lewandowksi JJ. Loading rate and test temperature effects on fracture of in Situ niobium silicide-niobium composites. Metall Mater Trans A 1996;27A:3292e306. [4] Bewlay BP, Jackson MR, Lipsitt HA. The balance of mechanical and environmental properties of a multielement niobium-niobium silicide-based in Situ composite. Metall Mater Trans A 1996;27A:3801e8. [5] Bewlay BP, Jackson MR, Subramanian PR, Zhao JC. A review of very-hightemperature Nb-silicide-based composites. Metall Mater Trans A 2003;34A: 2043e52. [6] Bewlay BP, Jackson MR, Subramanian PR. Processing high-temperature refractory-metal silicide in-situ composites. JOM 1999;51:32e6. [7] Fujikara M, Kasama A, Tannaka R, Hanada S. Effect of alloy chemistry on the high temperature strengths and room temperature fracture toughness of advanced Nb-based alloys. Mater Trans JIM 2004;45(2):493e501. [8] Sha JB, Hirai H, Tabaru T, Ueno H, Kitahara A, Hanada S. Mechanical properties of As-cast and directionally solidified Nb-Mo-W-Ti-Si in-situ composites at high temperatures. Metall Mater Trans A 2003;34A:85e94. [9] Kim J, Tabaru T, Sakamoto M, Hanada S. Mechanical properties and fracture behavior of an NbSS/Nb5Si3 in-situ composite modified by Mo and Hf alloying. Mater Sci Eng A 2004;372:137e44. [10] Bewlay BP, Lipsitt A, Jackson MR, Reeder WJ. Solidification processing of high temperature intermetallic eutectic-based alloys. Mater Sci Eng A 1995;192/3: 534e43. [11] Jackson MR, Bewlay BP and Zhao JC. American Patent: US 0066578; 2003. [12] Zhao JC, Westbrook JH. Ultrahigh-temperature materials for jet engines. MRS Bulletin/September 2003:622e30. [13] Bewlay BP, Jackson MR, Zhao JC, Subramanian PR, Mendiratta MG, Lewandowski JJ. Ultrahigh-temperature Nb-Silicide-based composites. MRS Bulletin/September 2003:646e53. [14] Ma CL, Tanaka H, Kasama A, Tanaka R, Mishima Y, Hanada S. Proc High Temp Ordered Intermetallic Alloys IX 2001;646. N5.39.1eN5.39.6.
Table 4 Compressive strength of the heat-treated Nbe16SiexFe alloys at room- and high temperatures. Alloys
smax/MPa s0.2/MPa
1250 C
RT
1350 C
2Fe
4Fe
6Fe
2Fe
4Fe
6Fe
2Fe
4Fe
6Fe
1505 74 1410 86
1558 82 1460 68
1650 53 1580 62
543 34 479 38
452 46 393 32
445 19 385 20
322 35 295 22
293 25 258 17
280 36 251 29
J.R. Zhou, J.B. Sha / Intermetallics 34 (2013) 1e9 [15] Chan KS. Alloying effects on fracture mechanisms in Nb-based intermetallic in-situ composites. Mater Sci Eng A 2002;329:513e22. [16] Jackson MR, Bewlay BP, Rowe RG. High-temperature refractory metal-intermetallic composites. JOM 1996;48:39e44. [17] Mendiratta MG, Dimiduk DM. Strength and toughness of an Nb/Nb5Si3 composite. Metall Mater Trans A 1993;24A:501e4. [18] Mendiratta MG, Lewandowski JJ, Dimiduk DM. Strength and ductile-phase toughening in the two-phase Nb/Nb5Si3 alloys. Metall Mater Trans A 1991; 22A:1573e83. [19] Yu JL, Zhang KF, Li ZK, Zheng X, Wang GF, Bai R. Fracture toughness of a hot-extruded multiphase Nb-10Si-2Fe in situ composite. Scripta Materialia 2009;61:620e3. [20] Yu JL, Zhang KF. Tensile properties of multiphase refractory Nb-16Si-2Fe in situ composite. Scripta Materiala 2008;59:714e7. [21] Vellios N, Tsakiropoulos P. Study of the role of Fe and Sn additions in the microstructure of Nb-24Ti-18Si-5Cr silicide based alloys. Intermetallics 2010; 18:1729e36.
9
[22] Vellios N, Tsakiropoulos P. The role of Fe and Ti additions in the microstructure of Nb-18Si-5Sn silicide based alloys. Intermetallics 2007;15: 1529e37. [23] Lankford J. Indentation microstructure in the palmqvist crack regime: implication for fracture toughness evaluation by the indentation method. J Mater Sci Lett 1982;1(8):493e8. [24] Oliver WC, Pharr GM. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res 1992;7(6):1564e83. [25] Massalski TB, editor. Binary alloy phase diagrams, vol. 1. Materials Park, OH: ASM; 1990. p. 1511. [26] Sha JB, Yamabe-mitarai Y. Phase and microstructural evolution of Ir-Si binary alloys with fcc/silicide structure. Intermetallics 2006;14:672e84. [27] Chu F, Tomas DJ, McClellan KJ, Peralta P, Li FX, Fodra E. MRS symposium proceeding, high-temperature ordered intermetallic alloys VIII, vol. 522. Boston, USA; 1998. p. KK6.7.1-11. [28] Sha JB. unpublished work.
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Microstructural evolution and mechanical properties of an Nbe16Si in-situ composite with Fe additions prepared by arc-melting J.R. Zhou, J.B. Sha* School of Materials Science and Engineering, Beihang University, 37 Xueyuan Rd., Beijing 100191, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2012 Received in revised form 23 September 2012 Accepted 23 October 2012 Available online 27 November 2012
This paper deals with phase constitutions, microstructural evolutions, and mechanical properties of Nbe16SiexFe in-situ composites (where x ¼ 2, 4, 6 at.%, referred as to 2Fe, 4Fe and 6Fe alloys, hereafter) prepared by arc-melting. It is found that with additions of Fe, Nb4FeSi silicide arises and microstructures of as-cast samples are consisted of dendritic-like NbSS phase, Nb3Si block, and Nb4FeSi matrix in the 2Fe and 4Fe alloys, and of the dendritic-like NbSS phase and Nb4FeSi matrix in the 6Fe alloy. When heat-treated at 1350 C for 100 h, part of the Nb3Si phase decomposes in the 2Fe and 4Fe alloys, and the 6Fe alloy shows no change in microstructure as compared with the as-cast one. The Nb4FeSi silicide is found to be brittle, its fracture toughness and elastic modulus are first obtained, having values about 1.22 MPa m1/2, and 310 GPa, respectively. The fracture toughness of the bulk as-cast and heat-treated Nbe16SiexFe samples are changed slightly by the Fe additions, which is in a range of 9.03e10.19 MPa m1/2. It is interesting that at room temperature, strength is improved by the Fe additions, whereas at 1250 C and 1350 C the strength decreases. As the Fe content increased from 2 at.% to 6 at.%, for example, the 0.2% yield strength increases from 1410 MPa to 1580 MPa at room temperature, decreases from 479 MPa to 385 MPa at 1250 C. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Intermetallics A. Ternary alloy system B. Phase identification D. Microstructure E. Mechanical properties
1. Introduction It is well known that structural materials used in advanced aeronautics and astronautics propulsion systems must possess a balance combination of properties, including ductility and toughness at low temperatures, strength and creep resistance at elevated temperatures, as well as environmental stability. In order to enable Nb-based alloys to achieve such balanced properties, a concept of multi-phase alloys has been developed for design of the NbeSi based alloys [1e7]. It involves incorporation of ductile Nb solid solution (NbSS) with stiffening intermetallic, such as silicide Nb5Si3 or Nb3Si [1e10], with oxidation resistant phase Cr2Nb [11e13], to form a multi-phase NbSS/Nb5Si3/Cr2Nb microstructure. By adjusting proportion and morphology of the NbSS, Nb5Si3 and Cr2Nb phases in the NbeSi based alloys; the balanced properties may be obtained to meet the requirements of the ultra-high temperature structural materials. Besides the aforementioned microstructural design, alloying is also an effective way to optimize each property. W and Mo can increase the melting point of the NbeSi based alloys, they are found
* Corresponding author. E-mail address: [email protected] (J.B. Sha). 0966-9795/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.intermet.2012.10.017
to be the strongest solid solution strengthening elements for Nb [7,8,14]. Ti and Hf have been proved to decrease the brittle-ductile transfer temperature (BDTT) of Nb [15], they play a key role on improving the fracture toughness and ductility of the Nb solid solution [16]. In addition, thermomechanical processing, such as directional solidification [6e8] and hot extrusion [17e20], have been frequently used to improve the mechanical properties of the NbeSi based alloys. The additions of Al, Fe and Cr to the NbeSi insitu composites reduce oxygen diffusion and solubility in Nb and form an oxidation against phase, Cr2Nb intermetallic compound [11e13]. The short-term goal corresponding to a material thickness loss of only 200 microns in 10 h air exposure at 1370 C has been successfully achieved in the multi-components NbSS/Nb5Si3/Cr2Nb [11e13] alloys. Recently, some researchers have focused on the role of Fe on microstructure evolution of the NbeSi based alloys [19e22]. Fe has been reported to be almost equally as effective as Cr at concentrations up to 5 at.% to improve oxidation resistance [11]. Researches revealed that with 5 at.% Fe addition, Nbe18Sie24Tie5Cr and Nbe18Sie24Tie5Cre5Sn alloys had complex phase constitutions, in which a new silicide, Nb4FeSi was observed [21,22]. A tensile elongation of 2% at room temperature and superplasticity, of about 512% elongation at 1450 C, were amazedly obtained in the Nbe16Sie2Fe alloy prepared by hot
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J.R. Zhou, J.B. Sha / Intermetallics 34 (2013) 1e9
pressing sintering [20], due to formation of Nb4Fe3Si5 phase with a low melting point of 1359 C. However, till to date, understanding of how Fe affects the phase and microstructural evolution, roomand high temperature properties of the cast NbeSi based alloys is still poor. The objective of this work is to understand basic changes in metallurgy and mechanical behavior of the Nbe16Si binary with Fe additions under as-cast and heat-treated states. 2. Experimental procedures Button ingots of Nbe16SiexFe alloys (x ¼ 2, 4, 6 at.%), each with a weight of 160 g, were prepared by arc-melting six times with electromagnetic stirring in an argon atmosphere under a pressure of w0.5 Pa. The ingots were then annealed at 1350 C in vacuum for 100 h and furnace-cooled to ensure quasi-equilibrium microstructures. The purity of all raw elements used was 99.9% (in mass) or higher. X-ray diffraction (XRD) analysis was performed on bulk samples to identify the phase constitutions using Cu Ka (l ¼ 0.15405 nm) radiation at 40 kV and 40 mA (Rigaku D/Max 2500PC). The backscattered electron (BSE) images were taken to investigate microstructures of the as-cast and heat-treated samples, and crack paths of the tested samples by using a SEM (JEOL JXA-8100) equipped with an energy-dispersion X-ray spectroscopy (EDS, Oxford Instruments, UK) to analyze phase compositions. Volume fractions of the phases in each as-cast and heat-treated sample were statistically determined by quantitative metallographic analysis on the back-scattered microstructures through a software. Vickers hardness (Hv) was measured under an applied load of 0.98N on a digital HXZ-1000 micro-sclerometer (Shanghai, China) with a loading-maintaining time of 15 s, and an average value was obtained by taking 10 Hv data for each sample. Fracture toughness KIC of the stiffening silicides were estimated by indentation method according to the formula (1) given by [23],
K1C ¼ CV
1=2 E P H c3=2
length, w: specimen width) was introduced by electrical discharge machining with a Cu wire 0.2 mm in diameter. The three-pointbending specimen is 30 mm in length, 6 mm in width and 3 mm in thickness, and tests were conducted on a SANS testing machine with a loading span of 24 mm and a crosshead displacement rate of 0.1 mm/min. Compressive tests at room temperature were conducted on a SANS testing machine at a strain rate of 3 104 s1 to measure the compressive strength while size of the rectangular compressive samples was 3 mm 3 mm 6 mm. High temperature compressive tests (at 1250 C and 1350 C) were conducted in an argon atmosphere at a strain rate of 1 103 s1 using a Gleeble 1500 testing machine. The compressive cylinders were 6 mm in diameter and 9 mm in length. All specimens for the three-pointbending and compressive tests were mechanically polished using SiC paper (1000-grit) with water before testing. 3. Results 3.1. Phase constitutions and microstructural characteristics of the as-cast and heat-treated alloys Typical X-ray diffraction patterns of the as-cast and heat-treated Nbe16SiexFe alloys are shown in Fig. 1. Under the as-cast condition, as shown in Fig. 1(a), the characteristic diffraction peaks of three phases are detected, Nb solid solution NbSS (b.c.c), silicides
(1)
where E is the Young’s modulus, H is the hardness, P is the indentation load, c is the crack length, and CV is a material-independent constant having value, 0.016 [23]. The Young’s modulus of the stiffening silicides was evaluated using loading and displacement sensing indentation method [24], which is the first time for the Nb4FeSi phase. The indentation loading-unloading vs displacement curves were measured using a nano-indentation tester (Nano Indenter II), to which a Berkovich-type diamond indenter was attached. With loading (unloading) versus indenter displacement curve of an indentation experiment, the Young’s modulus of the indented phase can be calculated by a series of formulae, while some parameters, such as the Young’s modulus and Poisson’s ratio of the indenter, as well as the Poisson’s ratio of the indented phase, etc., are needed for the Young’s modulus calculation. For the diamond indenter in the nano-indentation tester, the Young’s modulus and Poisson’s ratio are 1050 GPa and 0.104, respectively. The Poisson’s ratio of the silicide taken here is about 0.2 (this data is determined by referring to the Poisson’s ratio of metal-based silicides (Mo/Si silicide and Ir/Si silicide [26,27])). In the indenter loading and displacement measurements, the loading/unloading rate was 1 nN/s and the load at the peak was held constant for a period of 10 s, while 5 grains of each kind of phase were taken for indenting to obtain an accurate average. Single-notched three-point-bending specimens were used instead of single fatigue-cracked specimens for the toughness (KQ) measurements of the bulk alloys. A notch up to a/w ¼ 0.5 (a: notch
Fig. 1. X-ray diffraction patterns of (a) the as-cast and (b) the heat-treated Nbe16SiexFe alloys.
J.R. Zhou, J.B. Sha / Intermetallics 34 (2013) 1e9
Nb3Si (Ti3P-type) and Nb4FeSi (b.c.t) in the 2Fe and 4Fe alloys, where some peaks of the Nb3Si and Nb4FeSi phases overlap with each other. As the Fe content increases to 6 at.%, the independent characteristic peaks from the Nb3Si phase disappear and those from the NbSS and Nb4FeSi phases are still remained. In comparison with the as-cast Nbe16Si binary composed of the NbSS and Nb3Si phases [25], it is known that the addition of Fe promotes the generation of the new silicide Nb4FeSi, while suppresses the formation of the Nb3Si phase. After heat treatment at 1350 C for 100 h, as shown in Fig. 1(b), the 2Fe alloy still contains the independent characteristic peaks of the NbSS, Nb3Si and Nb4FeSi phases, but the intensity and the quantity of the Nb3Si peaks both decrease, as compared with those in the as-cast sample shown in Fig. 1(a). In the heat-treated 4Fe and 6Fe alloy, the independent peaks of the Nb3Si phase disappear and the remaining, are from the NbSS and Nb4FeSi phases, while the overlapping peaks of the Nb4FeSi and Nb3Si phases have also been marked for comparison (see Fig. 1(b)). Typical BSE images of the as-cast and heat-treated three alloys are shown in Figs. 2 and 3, respectively. The microstructural analysis, together with the X-ray diffraction patterns (Fig. 1) and the EDS data of phase compositions (Table 1), shows that in the as-cast
3
samples the white block is the NbSS phase, the light grey phase is the Nb3Si silicide, and the dark grey phase is the Nb4FeSi silicide. For the as-cast 2Fe and 4Fe alloys, the microstructures are composed of the dendritic-like NbSS arms, Nb3Si blocks, Nb4FeSi matrix and small amount of the NbSS þ Nb4FeSi eutectic (marked with rectangles in Fig. 2(a’) and (b’)) distributed on the edge of the Nb3Si blocks. Inside the Nb3Si blocks, the NbSS islands are found (Fig. 2(a’) and (b)), while the fine Fe-rich NbSS þ silicide eutectic (marked with circles in Fig. 2(b’)) appears in the 4Fe alloy. The morphology in Fig. 2(c) and (c’) reveals that the as-cast 6Fe alloy has different microstructures in comparison with the 2Fe and 4Fe alloys, it contains the primary NbSS, the Nb4FeSi matrix, the eutectic (marked with rectangle in Fig. 2(c’)) of them, as well as the Fe-rich eutectic (marked with circles in Fig. 2(c’)). The Nb4FeSi matrix is also found to show different contrast in correspondence with the different Fe contents. EDS results (Table 1) indicate that the dark Nb4FeSi phase, i.e. Fe-rich Nb4FeSi, has a Fe concentration of about 21.28 at.%, higher than that of about 4.63 at.% of the grey Nb4FeSi phase. For the Fe-rich eutectic in the 4Fe and 6Fe alloys, the Fe concentration is about 32 at.%, also far higher than that of other phases.
Fig. 2. BSE images of the as-cast Nbe16SiexFe alloys.
4
J.R. Zhou, J.B. Sha / Intermetallics 34 (2013) 1e9
Fig. 3. BSE images of the heat-treated Nbe16SiexFe alloys.
The heat-treated microstructures of the 2Fe and 4Fe alloys are still composed of the NbSS, Nb3Si and Nb4FeSi phases (see Fig. 3(a) and (b)). However, part of the Nb3Si phase transforms into fine eutectoid of NbSS þ Nb4FeSi (see Fig. 3(a’) and (b’)), which means that incomplete eutectoid decomposition of the Nb3Si phase takes place during annealing. For the heat-treated 6Fe alloy, the microstructure is consisted of the NbSS phase and the Nb4FeSi matrix, the Fe-rich eutectic in the as-cast sample vanishes. Incidentally, black particles precipitate on the NbSS phase of all three samples during annealing, as seen in Fig. 3(a’)e(c’). Table 1 shows the phase compositions of the as-cast and heattreated Nbe16SiexFe alloys. The data in Table 1 displays that in the as-cast sample Fe atoms dissolve into the Nb4FeSi and NbSS phases rather than into the Nb3Si phase. The concentration of Fe in the NbSS phase shows an increasing tendency, from 1.14 at.% to 2.30 at.%, as the nominal Fe content increases from 2 at.% to 6 at.%. The average composition of the Nb3Si phase is about Nbe23.4Sie 0.22Fe, in which the atomic ratio of Nb to Si is close to 3:1, the stoichiometric ratio in the Nb3Si binary. For the Nb4FeSi phase, the concentration of Nb is nearly 69 at.% and that of Fe varies from 4.15 to 4.69 at. %. However, the atomic ratio of Nb to (Si þ Fe) is found to be constant, close to 6.9:3.1, which is even as same as that for the
Fe-rich Nb4FeSi phase in the 6Fe alloy. This constant ratio is consistent with the Nb to (Si þ Fe) ratio of the Nb4FeSi phase found in the multi-component Nbe24Tie18Sie5Fee5Sn and Nbe45Tie 15Sie5Fee5Sn alloys [21,22]. Combining Table 1 and results in references [21,22] indicates that the Fe and Si concentrations in the Nb4FeSi phase have wide range, but sum is about 31 at.%. As for the fine Fe-rich regions, looked like eutectic, their average Nb and Si concentrations are far lower than those of the Fe-rich Nb4FeSi phase, whereas the Fe concentration is contrary (see Table 1). After heat treatment, the silicide precipitates in the NbSS phase (see Fig. 3), which is, naturally, resulted from a decrease in the concentrations of both Fe and Si solidified in the NbSS phase. For the Nb3Si and Nb4FeSi phases in the 2Fe and 4Fe alloys, as well as, for the Nb4FeSi and Fe-rich Nb4FeSi phases in the 6Fe alloy, the heat treatment seems not to bring any considerable change in their average compositions. Fig. 4 gives the phase’s volume fractions of the as-cast and heat-treated samples as a function of the nominal Fe contents. The NbSS and silicides (Nb3Si þ Nb4FeSi) fractions seem to be independent of the nominal Fe content, while the NbSS fraction is in a range of 42e44% for the three as-cast samples. For the silicides, their fractions show a reversed variation with Fe. The Nb3Si fraction
J.R. Zhou, J.B. Sha / Intermetallics 34 (2013) 1e9
5
Table 1 Phase compositions of the as-cast and heat-treated Nbe16SiexFe alloys. Heat-treated (1350 C, 100 h)
Alloy
Phase
As-cast Nb(at.%)
Si(at.%)
Fe(at.%)
Nb(at.%)
Si(at.%)
Fe(at.%)
2Fe
NbSS
96.45 95.83e97.70 76.82 76.65e76.84 69.08 68.91e69.16 95.81 95.53e96.04 75.94 75.86e76.04 68.54 68.29e68.79 61.04 54.86e66.22 95.45 95.16e95.74 68.86 68.69e69.06 68.68 68.45e68.94 60.46 54.49e63.07
2.41 1.56e2.68 22.95 22.88e23.06 26.77 25.49e28.01 2.27 2.13e2.45 23.85 23.74e23.96 26.77 24.46e28.07 7.22 5.82e8.50 2.25 2.08e2.35 26.51 25.48e26.94 10.04 9.98e10.09 7.02 6.06e8.54
1.14 0.74e1.62 0.23 0.17e0.29 4.15 2.85e5.32 1.92 1.69e2.32 0.21 0.17e0.24 4.69 3.46e6.99 31.74 27.96e36.84 2.30 1.93e2.56 4.63 4.14e5.76 21.28 21.09e21.46 32.52 28.70e36.97
97.75 97.42e98.04 76.62 76.53e76.72 69.01 68.98e69.19 98.17 98.07e98.58 76.18 75.85e76.62 68.72 67.85e69.29
1.63 1.14e2.32 23.27 23.19e23.34 26.53 24.28e28.40 0.74 0.49e1.07 23.62 23.18e23.95 26.9 23.66e28.58
0.62 0.27e0.81 0.11 0.05e0.14 4.46 2.76e6.43 1.09 0.70e1.32 0.20 0.19e0.20 4.38 3.34e7.22
97.22 96.40e97.56 68.68 68.39e69.08 68.61 68.14e69.32
0.69 0.53e1.41 25.89 22.85e27.98 10.61 10.07e11.60
2.09 1.91e2.19 5.43 3.07e8.08 20.78 20.61e21.07
Nb3Si Nb4FeSi 4Fe
NbSS Nb3Si Nb4FeSi Fe-rich eutectic
6Fe
NbSS Nb4FeSi Fe-rich Nb4FeSi Fe-rich eutectic
For an element content, the first line is the average value and the second line is the value range.
decreases from 31% for the 2Fe alloy to 10% for the 4Fe alloy, and finally, to zero for the 6Fe alloy, whereas the Nb4FeSi fraction increases from 26 at.% to 58 at.%. For the heat-treated samples, the Nb3Si fractions of the 2Fe and 4Fe alloys are lower than those of the corresponding as-cast alloys due to the eutectoid decomposing of the Nb3Si phase, in return for this, the fractions of the NbSS and Nb4FeSi phases both increase. The NbSS and Nb4FeSi fractions in the 6Fe alloy are almost not changed by the heat treatment (see Fig. 4). 3.2. Mechanical response at room temperature
VOLUME FRACTION OF PHASES
3.2.1. Vickers hardness (Hv) and fracture toughness (K1C) of constituent phases Fig. 5 shows Hv of the as-cast and heat-treated NbSS phase as a function of the nominal Fe content. Both the as-cast and heattreated NbSS phase show an increasing Hv with Fe, which may be
NbSS
attributed to increase of the Fe concentration in the NbSS phase (see Table 1). In each sample, the heat-treated NbSS phase has a lower Hv than the as-cast one by about 27e33%. The precipitation takes place in the NbSS phase during heat treatment is deduced to weak the solid solution strengthening of the NbSS phase. Table 2 lists the mechanical properties of the Nb3Si and Nb4FeSi silicides in the heat-treated samples. It is clearly seen that hardness of the Nb4FeSi phase measured by using both the microsclerometer and nano-indentation tester is higher than that of the Nb3Si phase. The hardness of the Nb4FeSi phase is about 1110 in Hv and 17.9 GPa in nano-hardness, while that of the Nb3Si phase is 975 in Hv and 16.5 GPa in nano-hardness. Typical loading and unloading versus nano indenter displacement curves for the highest load experiment performed on the silicide grains are exhibited in Fig. 6. Under a load of 20 nN, the maximum penetration depth, hmax, of the indenter is about 258 nm in the Nb3Si phase and 238 nm in the Nb4FeSi phase. The final depth of the constant impression after unloading, hf, is about 166 nm and
Nb4FeSi
Nb3Si
26% 37% 46%
51%
58%
31%
59%
15% 10% 3%
43%
44%
42%
48%
46%
AC 2Fe
AC 4Fe
AC 6Fe
HT 2Fe
HT 4Fe
41%
HT 6Fe
Fig. 4. Volume fractions of the phases in the as-cast and heat-treated Nbe16SiexFe alloys.
Fig. 5. Vickers hardness of the NbSS phase in the as-cast and heat-treated samples as a function of the nominal Fe content.
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Table 2 Hardness, Young’s modulus and KIC of the Nb3Si and Nb4FeSi phases. Phase
Hardness/HVa
Hardness/GPab
Young’s modulus/GPa
K1C/MPa m1/2
Nb3Si Nb4FeSi
975 48 1110 44
16.5 1.02 17.9 1.33
240 18 310 22
1.01 0.061 1.22 0.053
a b
Determined by Vickers hardness tester. Determined by Nano-indentation tester.
155 nm, in the Nb3Si and Nb4FeSi phases respectively. The calculated Young’s modulus by equations [24] in Table 2 reveals that the Nb4FeSi phase has the Young’s modulus of about 310 GPa, higher than that of 240 GPa of the Nb3Si phase. Room temperature plasticity of the NbSS phase, as well as that of the silicides, can be characterized with indentation. Indentation feature of the phases obtained by the Hv tests is displayed in Fig. 7. As can be seen that the NbSS phase and silicides show different behaviors during penetration of the hardness indenter. In the silicides, for either of the Nb4FeSi or Nb3Si phases, cracks visibly initiate at both the tip and bottom of the indentation, as seen in Fig. 7(b) and (c). However, no crack appears around the indentation within the NbSS phase (see Fig. 7(a)). It is most likely that the Nb3Si and Nb4FeSi phases have brittle characteristics, while the NbSS phase deforms plastically at room temperature. By substituting the crack length and relative parameters: P of 0.98N, Hv of 975, Cv of 0.16, as well as E of 240 GPa, into formula (1), the fracture toughness, KIC, of the Nb3Si phase can be obtained, which is also listed in Table 2 with that of the Nb4FeSi phase. The KIC value of the Nb4FeSi phase is 1.22 MPa m1/2, almost at the same level as that of the Nb3Si phase. With such a low toughness, the Nb4FeSi silicide is thought to be an intrinsically brittle phase. 3.2.2. Fracture toughness (KQ) and compressive behavior of bulk samples The fracture toughness, KQ, of the bulk as-cast and heat-treated samples measured by notched three-point-bending tests is given in Table 3. An increase in the nominal Fe content does not result in visible influence on the KQ of the bulk as-cast samples, the KQ values of the three as-cast alloys are in a range of 9.03e9.66 MPa m1/2. By heat-treating at 1350 C for 100 h, the 2Fe and 4Fe alloys show a small increment in the KQ by about 0.6e0.8 MPa m1/2, when
hmax
LOADING, /nN
20 Nb3Si
15
S
Nb4FeSi Loading
10
Unloading
5 hf 0 0
150 250 100 200 50 DISPLACEMENT INTO SURFACE, /nm
300
Fig. 6. Load-indenter displacement curves of the silicides Nb3Si and Nb4FeSi, S is the initial unloading stiffness.
compared with the corresponding as-cast samples. The KQ value of the 6Fe alloy before and after heat-treating is at the same level. This small improvement in the 2Fe and 4Fe alloys is believed to be resulted from the formation of the eutectoid NbSS phase in the heat-treated samples. It should be noticed that as the nominal Fe contents increase from 2 at.% to 6 at.%, the ductile NbSS fraction of the heat-treated samples decreases from 48% to 41% (see Fig. 4), consequently, the KQ value decreases by about 6.7%, from 10.19 MPa m1/2 to 9.51 MPa m1/2. The true compressive stressestrain curves of the three heat-treated samples at room temperature are shown in Fig. 8. It is clear that the stress level at room temperature increases with the nominal Fe content. 3.3. Compressive behavior of the bulk heat-treated samples at high temperature Fig. 9 shows the true compressive stressestrain curves of the three heat-treated samples at 1250 C and 1350 C. At 1250 C and 1350 C, the curves show a very short deformation-hardening stage. After the stress is beyond the peak, it remains at high level, larger than 60e85% of the peak value until the total strain is greater than 25%. Among the three alloys, the 2Fe alloy shows the highest stress level at both 1250 C and 1350 C. Table 4 summaries the average 0.2% yield compressive strength (s0.2) and maximum compressive strength (smax) of the heattreated alloys at high temperatures, those at room temperature taken from Fig. 8 are also list for comparison. At high temperatures Fe seems to play a weakening influence on strength of the NbeSi based alloys. The 2Fe alloy has the highest s0.2, of 479 MPa at 1250 C, and of 295 MPa at 1350 C. The s0.2 value of the 4Fe and 6Fe alloys is almost at the same level, but lower than that of the 2Fe alloy by about 15e20%, as seen in Table 4. 4. Discussion The as-cast microstructure of the Nbe16Si binary is usually composed of the primary dendritic-like NbSS phase and the Nb3Si þ NbSS eutectic [10]. With replacements of 2 at.% to 6 at.% Nb by Fe, however, the solidification path and the phase constitution are changed, i.e., the Nb4FeSi phase appears, while the Nb3Si phase reduces gradually (in the 2Fe and 4Fe alloys) and finally disappears (in the 6Fe alloy), as can be seen in Fig. 2. This suggests that Fe is an element that restrains formation of the Nb3Si phase. According to the NbeSi binary diagram [25] and Fig. 2, the solidification path of the Nbe16Sie(2, 4)Fe alloys is primarily recommended as L / L þ NbSS / L þ NbSS þ Nb3Si / L þ NbSS þ Nb3Si þ (Nb4FeSi þ NbSS) eutectic / NbSS þ Nb3Si þ (Nb4FeSi þ NbSS) (with additional Fe-rich eutectic in this step for the 4Fe alloy), and that recommended for the Nbe16Sie6Fe alloy is L / L þ NbSS / L þ NbSS þ (Nb4FeSi þ NbSS) eutectic / NbSS þ (Nb4FeSi þ NbSS) eutectic þ Fe-rich eutectic. Comparing the microstructures of the heat-treated samples with those of the as-cast ones at the corresponding Fe contents finds that the Nb4FeSi phase is a stable phase as no decomposition takes place during heat treatment. In general, an increasing Fe content results in a considerable change in the phase constitution, from NbSS þ Nb3Si þ Nb4FeSi to NbSS þ Nb4FeSi, but small variations in the phase fraction (see that of the NbSS and silicides in Fig. 4), and the Fe concentration in the NbSS phase (see Table 1). These metallurgical changes affect the mechanical properties of the NbeSi based alloys to a certain extent. As a result, both the Fe concentration in the NbSS phase and the fraction of the stiffening silicides increase with the nominal Fe contents. The above two factors are assuredly negative for the bulk toughness of the sample because Fe raises the BDTT of Nb and embrittles Nb [18], while the silicides are brittle phases. However,
J.R. Zhou, J.B. Sha / Intermetallics 34 (2013) 1e9
7
Fig. 7. The indentation morphologies in NbSS (a), silicides Nb3Si (b) and Nb4FeSi (c).
work found that the NbeSieFe alloys began to melt at 1450 C [28], while the Nbe16Si binary containing the NbSS and Nb3Si phases, however, was still at the solid state at 1880 C [25]. The same is true even for the Nb4Fe3Si5 silicide found in the Nbe16Sie2Fe alloy prepared by hot pressing sintering, whose melting point is as low as 1359 C [20]. This means that the Fe additions decrease the melting points of the Nb/Si silicides dramatically. In general, the strengthening and weakening mechanisms co-exist in the NbeSi based alloys with the Fe additions. The one which may be the dominant factor for the bulk strength depends on the testing temperature. It is believed that the higher testing
1600
TRUE STRESS, MPa
as the concentration of Fe atoms in the NbSS phase and the changes in the silicides fraction with Fe, are very small; these small increases cannot cause a visible degradation in the bulk toughness as seen in Table 3. At room temperature, the s0.2 is sensitive to the Fe additions, by about a 170 MPa increment when the nominal Fe contents increase from 2 at% to 6 at.% (see Table 4). Metallurgically, both the solid solution hardened NbSS phase (Fig. 4) and the increasing stiffening Nb4FeSi fraction from 37% to 59% (Table 2 and Fig. 4) with Fe may contribute to the strength of the NbeSieFe ternary according to the rule of the mixture [15]. The reported Hv of pure Nb is about 82 [24], thus, increment of the Hv from 82 of pure Nb to 241 of the heat-treated NbSS phase comes from the solute concentrations of 1.63Siþ0.62Fe (see compositions of the NbSS phase in the 2Fe alloy in Table 1). A further increment in the Hv of the NbSS phase, from 241 up to 319, is absolutely attributed to an increase of the Fe concentration in the NbSS phase from 0.62 at.% to 2.09 at.%, whereas the solid solution strengthening by Si becomes weak as the Si concentration in the NbSS decreases. These strongly confirm the significant effect of Fe on the solid solution strengthening of the NbSS phase, as well as on the bulk strength of the NbeSi based alloys at room temperature. Unfortunately, the strength of the Nbe16Si alloy at high temperatures is decreased by the Fe additions, even the silicides fraction increases with Fe, as seen in Fig. 9(a) and (b). Our early
Room temperature
4Fe
6Fe
2Fe
1200
800
400
Table 3 Fracture toughness KQ of the Nbe16SiexFe samples at room temperature. Alloys
2Fe 4Fe 6Fe
KQ/MPa m1/2
2
As-cast
Heat-treated
9.37 0.59 9.03 0.47 9.66 0.39
10.19 0.36 9.63 0.51 9.51 0.38
4
6
8
TRUE STRAIN, % Fig. 8. Compressive stressestrain curves of the heat-treated Nbe16SiexFe alloys at room temperature.
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J.R. Zhou, J.B. Sha / Intermetallics 34 (2013) 1e9
700
5. Conclusions
a
1250ºC
TRUE STRESS, MPa
600
2Fe
500
4Fe
400 300
6Fe
200 100 0
0 0
400
10 0.1
15 0.15
20 0.2
25 0.25
TRUE STRAIN, %
b
350
TRUE STRESS, MPa
5 0.05
30 0.3
35 0.35
1350ºC
(1) The microstructure consists of the NbSS, Nb3Si and Nb4FeSi phases at Fe contents of 2 at.% and 4 at.%, but of the NbSS and Nb4FeSi phases at a Fe content of 6 at.%. Heat treatment can not change the phase constitution of each alloy, however partly decomposition of the Nb3Si phase in the 2Fe and 4Fe alloys takes place. (2) The Nb4FeSi silicide is a brittle phase, it has a low toughness of 1.22 MPa m1/2, its Young’s modulus and Hv are 310 GPa and 1110, respectively. (3) The Fe additions from 2 to 6 at.% show a small effect on the fracture toughness KQ of the bulk samples, the KQ of all the as-cast and heat-treated samples are in a range of 9.03e10.19 MPa m1/2. (4) The room temperature compressive strength of the Nbe16Si alloy is improved by the Fe additions, whereas at 1250 C and 1350 C, the compressive strength decreases. As the Fe content increased from 2 to 6 at.%, the 0.2% yield strength increases from 1410 to 1580 MPa at room temperature, while it decreases from 479 to 385 MPa at 1250 C and 295 to 251 MPa at 1350 C. Acknowledgments
300
2Fe The authors are grateful to the support from the Program for New Century Excellent Talents in Universities (NCET-06-0173) and the National Natural Sciences Foundation of PR China (51071009 and 5071111) and Aerospace Funding (2010ZF51071).
4Fe
250
6Fe 200 150
References
100 50 0
0 0
5 0.05
10 0.1
15 0.15
20 0.2
25 0.25
30 0.3
35 0.35
TRUE STRAIN, % Fig. 9. Compressive stressestrain curves of the heat-treated Nbe16SiexFe alloys at high temperatures, (a) 1250 C, (b) 1350 C.
temperature (close to the melting point of the Nb4FeSi phase) may be favorable for the weakening mechanism. At room temperature, the solid solution hardening of the NbSS phase and the stiffening phase hardening of the silicides are dominant; therefore, the bulk strength shows an increasing tendency with Fe. When the testing performs at 1250 C and 1350 C, the testing temperatures are below the melting point of the Nb4FeSi phase by 100e200 C (referred to 1450 C [28], which is the temperature when the NbeSieFe alloys began to melt); thus, the Nb4FeSi phase probably becomes a soft zone and shows the weakening behavior for the bulk strength at this temperature range. This is the reason why the 4Fe and 6Fe alloys with higher Nb4FeSi fraction than the 2Fe alloy show lower bulk strength at high temperatures. Further works in determination of the accurate transformation temperature between room temperature and 1250 C, and mechanism from strengthening to weakening of the NbeSi alloys with the Fe additions are still needed.
[1] Bewlay BP, Lewandowksi JJ, Jackson MR. Refractory metal-intermetallic in-situ composites for aircraft engines. JOM 1997;49:44e5. [2] Subramanian PR, Mendiratta MG, Dimiduk DM. The development of Nb-based advanced intermetallic alloys for structural applications. JOM 1996;48(1):33e8. [3] Rigney JD, Lewandowksi JJ. Loading rate and test temperature effects on fracture of in Situ niobium silicide-niobium composites. Metall Mater Trans A 1996;27A:3292e306. [4] Bewlay BP, Jackson MR, Lipsitt HA. The balance of mechanical and environmental properties of a multielement niobium-niobium silicide-based in Situ composite. Metall Mater Trans A 1996;27A:3801e8. [5] Bewlay BP, Jackson MR, Subramanian PR, Zhao JC. A review of very-hightemperature Nb-silicide-based composites. Metall Mater Trans A 2003;34A: 2043e52. [6] Bewlay BP, Jackson MR, Subramanian PR. Processing high-temperature refractory-metal silicide in-situ composites. JOM 1999;51:32e6. [7] Fujikara M, Kasama A, Tannaka R, Hanada S. Effect of alloy chemistry on the high temperature strengths and room temperature fracture toughness of advanced Nb-based alloys. Mater Trans JIM 2004;45(2):493e501. [8] Sha JB, Hirai H, Tabaru T, Ueno H, Kitahara A, Hanada S. Mechanical properties of As-cast and directionally solidified Nb-Mo-W-Ti-Si in-situ composites at high temperatures. Metall Mater Trans A 2003;34A:85e94. [9] Kim J, Tabaru T, Sakamoto M, Hanada S. Mechanical properties and fracture behavior of an NbSS/Nb5Si3 in-situ composite modified by Mo and Hf alloying. Mater Sci Eng A 2004;372:137e44. [10] Bewlay BP, Lipsitt A, Jackson MR, Reeder WJ. Solidification processing of high temperature intermetallic eutectic-based alloys. Mater Sci Eng A 1995;192/3: 534e43. [11] Jackson MR, Bewlay BP and Zhao JC. American Patent: US 0066578; 2003. [12] Zhao JC, Westbrook JH. Ultrahigh-temperature materials for jet engines. MRS Bulletin/September 2003:622e30. [13] Bewlay BP, Jackson MR, Zhao JC, Subramanian PR, Mendiratta MG, Lewandowski JJ. Ultrahigh-temperature Nb-Silicide-based composites. MRS Bulletin/September 2003:646e53. [14] Ma CL, Tanaka H, Kasama A, Tanaka R, Mishima Y, Hanada S. Proc High Temp Ordered Intermetallic Alloys IX 2001;646. N5.39.1eN5.39.6.
Table 4 Compressive strength of the heat-treated Nbe16SiexFe alloys at room- and high temperatures. Alloys
smax/MPa s0.2/MPa
1250 C
RT
1350 C
2Fe
4Fe
6Fe
2Fe
4Fe
6Fe
2Fe
4Fe
6Fe
1505 74 1410 86
1558 82 1460 68
1650 53 1580 62
543 34 479 38
452 46 393 32
445 19 385 20
322 35 295 22
293 25 258 17
280 36 251 29
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[22] Vellios N, Tsakiropoulos P. The role of Fe and Ti additions in the microstructure of Nb-18Si-5Sn silicide based alloys. Intermetallics 2007;15: 1529e37. [23] Lankford J. Indentation microstructure in the palmqvist crack regime: implication for fracture toughness evaluation by the indentation method. J Mater Sci Lett 1982;1(8):493e8. [24] Oliver WC, Pharr GM. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res 1992;7(6):1564e83. [25] Massalski TB, editor. Binary alloy phase diagrams, vol. 1. Materials Park, OH: ASM; 1990. p. 1511. [26] Sha JB, Yamabe-mitarai Y. Phase and microstructural evolution of Ir-Si binary alloys with fcc/silicide structure. Intermetallics 2006;14:672e84. [27] Chu F, Tomas DJ, McClellan KJ, Peralta P, Li FX, Fodra E. MRS symposium proceeding, high-temperature ordered intermetallic alloys VIII, vol. 522. Boston, USA; 1998. p. KK6.7.1-11. [28] Sha JB. unpublished work.