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The effect of Cr addition on mechanical and chemical properties of Ni3Si alloys
Materials Science and Engineering A329– 331 (2002) 446– 454 www.elsevier.com/locate/msea
The effect of Cr addition on mechanical and chemical properties of Ni3Si alloys T. Takasugi *, H. Kawai, Y. Kaneno Department of Metallurgy and Materials Science, Graduate School of Engineering, Osaka Prefecture Uni6ersity, 1 -1 Gakuencho, Sakai, Osaka 599 -8531, Japan
Abstract The alloying behavior and microstructure of the Ni– Si– Cr ternary alloys were firstly characterized by optical microscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) with electron probe analysis. The microstructures of the Ni–Si–Cr ternary alloys consisted of largely dispersed Ni5Si2 phase and finely precipitated Ni3Si phase in Ni solid solution. Then, the high-temperature mechanical properties, three-point bend test, oxidation and corrosion properties were investigated. The Ni–Si–Cr ternary alloys showed significant strengthening over a wide range of temperatures, and also large compressive plastic deformation at high temperatures. The strength and fracture toughness at ambient temperatures were correlated with the volume fraction of the Ni5Si2 phase. The Ni–Si–Cr ternary alloys showed substantially improved oxidation resistance in air at 1173 K, comparing with the Ni3Si and Ni3(Si,Ti) alloys. Also, the Ni– Si – Cr ternary alloys showed corrosion resistance comparable to the Ni3Si and Ni3(Si,Ti) alloys. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Nickel silicide; Intermetallic compound; High-temperature strength; Oxidation resistance; Corrosion resistance
1. Introduction Ni3Si-based alloys have been considered to be a candidate compound, which could be used as the basis of high-temperature structural materials and chemical parts because Ni3Si displays an increasing strength with increasing temperatures [1] and also shows excellent oxidation and corrosion resistance over a wide range of temperatures. Ti addition to Ni3Si resulted in (1) increased strength as a result of the solid solution hardening [2]; (2) enhanced peak temperature in the strength versus temperature curve [2,3] through modifying the specific dislocation structure [3,4]; and also (3) high tensile ductility at ambient temperatures by reducing the propensity to intergranular fracture [2,5]. Thus, the Ni3(Si,Ti) alloys have been shown to have a number of useful mechanical properties [2 – 5]. Therefore, the subsequent improvement of mechanical and related properties has been performed on the Ni3(Si,Ti) alloys. The doping with interstitials such as boron, carbon and
* Corresponding author. Tel.: +81-722-54-9314; fax: + 81-722-549912. E-mail address: [email protected] (T. Takasugi).
beryllium has been conducted and shown to improve further the ambient temperature ductility via preventing intergranular fracture [5–7]. Alloying of some substitutional (transition) metals has also been performed mostly within their solubility limits, and shown to result in favorable mechanical properties [8]. On the other hand, a recent study showed that Nb-containing second-phase dispersions (such as D0a-type Ni3Nb) in the Ni3(Si,Ti) phase matrix resulted in a further strengthening over a wide range of temperatures, and also improved the moisture-induced embrittlement as well as the high-temperature tensile elongation [9–11], from which the Ni3Si and Ni3(Si,Ti) alloys have been suffering for a long time. In the present study, Cr, which was expected to improve particularly chemical property such as oxidation and corrosion resistance, was added to the Ni3Si alloys. The Cr was added to the Ni3Si beyond the solubility limits in their L12 phases. The microstructures of the Cr-added Ni3Si alloys are, therefore, expected to consist of multi-phase. The alloying behavior, microstructure, mechanical properties and chemical properties are investigated by optical microscopy (OM), X-ray diffraction (XRD), scanning microscopy with
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Table 1 Composition, microstructure and fracture strength (in three-point bend test) of the Ni–Si–Cr ternary alloys used in this study Alloy
Composition (at.%)
Microstructure
Volume fraction of Ni5Si2 phase (%)
Fracture strength in three-point bend test (MPa)
TA-alloy TB-alloy TC-alloy TD-alloy
Ni72Si23Cr5 Ni70Si20Cr10 Ni67Si18Cr15 Ni75Si20Cr5
L12,Ni5Si2,Niss L12,Ni5Si2,Niss L12,Ni5Si2,Niss L12,Ni5Si2,Niss
73 44 29 41
134 493 790 597
electron probe analysis, tensile test, three-point bend test, oxidation and corrosion measurements. It is shown that the alloys, with a multi-phase microstructure consisting of Ni3Si (L12) phase, Ni5Si2 phase and Ni solid solution, have excellent mechanical and chemical properties.
2. Experimental procedure Alloy compositions used in this study are shown in Table 1 and denoted with TA-, TB-, TC- and TD-alloys in the followings. These alloy compositions are also plotted on an Ni–Si – Cr ternary phase diagram at 1323 K (Fig. 1), and suggested to lie in the three-phase region consisting of the Ni3Si (L12), Ni5Si2 (hexagonal) and Ni solid solution (fcc). All the Ni– Si – Cr ternary alloys used in this study were doped with 0.005 mass% boron to improve the low temperature ductility. Also, for comparison, the Ni3Si binary and Ni3(Si,Ti) ternary alloys were prepared by the same procedure. Starting raw materials were 99.9 wt.% Ni, 99.999 wt.% Si, 99.99 wt.% Cr and 99.9 wt.% boron. Alloy button ingots with a dimension of 50× 25 × 15 mm3 were prepared by non-consumable arc melting in argon gas on copper hearth. All the button ingots were homogenized at 1323 K for 1 day in vacuum. The as-homogenized materials were used for all tests and measurements. Metallographic, chemical and structural observations of the Ni–Si –Cr ternary alloys were carried out by OM, XRD and scanning electron microscopy (SEM) with wave-length dispersive spectroscopy (WDS) or energy dispersive spectroscopy (EDS). X-ray diffraction profiles were obtained using CuKa radiation. The electron probe data for the alloy compositions in the constituent phases were collected from more than five areas and then averaged. The determination of the constituent phases and their crystal structures were conducted on the basis of XRD and SEM electronprobe analysis. Using a precision wheel cutter and an electrodischarge machine (EDM), compressive specimens with a dimension of 2.5×2.5 ×5 mm3 were cut from the homogenized button ingots. The faces of the com-
pressive specimens were abraded with a sufficiently fine SiC paper before the mechanical test. Using an Instrontype testing machine, the mechanical tests were conducted in vacuum in a temperature range between room temperature and 1073 K. Nominal strain rates used in the compressive tests were 3.3× 10 − 4 s − 1. To evaluate ductility (or fracture toughness) of the Ni – Si–Cr ternary alloys at room temperature, rectangular specimens with a dimension of 1× 5× 20 mm3 were used for three-point bend test at room temperature. All specimen surfaces were mechanically abraded with up to 1500-grit SiC paper. The bend tests were conducted in air at a displacement rate of 0.1 mm min − 1. A fully articulated three-point bending fixture (with a span of 10 mm and a pushing rod of 3-mm diameter) was used for fracture of the bend specimens. The bend strength |f was calculated from a load versus displacement curve, using the relationship of |f = 1.5Pl/wt 2 where P is the load, l is the span (i.e. 10 mm), and w and t are the width and the thickness of the specimen, respectively. At least two specimens were used for measurement of each material condition. Fracture surfaces of the bent Ni–Si–Cr ternary alloys were examined by SEM.
Fig. 1. The Ni– Si– Cr ternary [14] phase diagram at 1323 K, and alloy compositions used in this study. Note that the plotted open circles mean the average compositions (as the two phases of the Ni3Si and Ni solid solution) determined in this study.
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Fig. 2. OM microstructures of the (a) TA-; (b) TB-; (c) TC- and (d) TD-alloys.
Oxidation property was measured using specimens with a dimension of 10×10 ×1 mm3. Before the measurement, all major surfaces were abraded with 1500grid SiC paper, then ultrasonically cleaned with ethanol. Isothermal oxidation tests based on the cyclic test were performed in static laboratory air at 1173 K. The specimens placed on alumina boat were inserted into and removed from the hot zone of a furnace. Each cycle consisted of a period of desired hour at 1173 K and at room temperature. Mass gain of the specimens at each cycle was measured at room temperature, together with alumina boat. Therefore, the measured mass gain in this study includes the spalled oxide scales. The oxidized surfaces were observed by an eye view and SEM. Corrosion property was measured in a sulfuric acid at 363 K using specimens with a dimension of 10× 10×1 mm3. Measurement was performed using solutions of 40, 60, 80 and 98% sulfuric acid. Mass loss was measured after immersion in sulfuric acid for 24 h.
3. Results
3.1. Alloying beha6ior and microstructure Optical microstructures (OM) were observed on a longitudinal section of the button ingot along which the alloys were solidified. The OM microstructures of the Ni –Si–Cr ternary alloys are shown in Fig. 2. The microstructure of the TA-alloy showed large dendritic Ni5Si2 phase and two-phase regions consisting of the Ni3Si (L12) phase and Ni solid solution. The Ni3Si phase and Ni solid solution were resolved by a SEM image as shown in Fig. 3. Here, the dot-like images in the two-phase regions are the Ni3Si precipitates. The microstructures of the other ternary alloys were similar one another but somehow different from that of the TA-alloy. Isolated round-shaped Ni5Si2 phases were surrounded by two-phase regions of the Ni3Si (L12) phase and Ni solid solution. The volume fraction of the Ni5Si2 phase depended on the alloy, as shown in Table
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Fig. 3. BS-SEM microstructure of the TD-alloy. Note that the dot-like images in the two-phase regions are Ni3Si precipitates.
1; the TA-alloy had the largest volume fraction of the Ni5Si2 phase (73%) while the TC-alloy had the smallest volume fraction of the Ni5Si2 phase (29%). Based on this observation, it is suggested that the alloys solidify as (the TA-alloy) primary allotriomorphs of Ni5Si2 is to be followed by a eutectic solidification of Ni solid solution and Ni5Si2 (on rapid solidification in the arc furnace the equilibrium Ni3Si phase may not form). On heat treatment the eutectic would coarsen to some extent to give the large rounded areas of Ni5Si2 seen in Fig. 2a, b and d. On cooling from the heat treatment temperature a fine dispersions of Ni3Si would be precipitated from the Ni solid solution by a solid state transformation. The alloys containing lesser amounts of Si (the TB- and TD-alloys) do not contain primary Ni5Si2 (Fig. 2b and d) but only coarsened eutectic. The alloy with lowest Si content (the TC-alloy) consists of Ni solid solution and some coarsened eutectic, with precipitated Ni3Si in the Ni solid solution phase. The alloy compositions of the constituent phases were determined by an electron-probe analysis and shown in Table 2. The alloy composition of the Ni5Si2 phase was 69.6 at.% Ni, 28.4 at.% Si and 2.0 at.% Cr as an average value of the alloys studied. Thus, the solubility limit of Cr in the Ni5Si2 phase is small consistent with the previous observation [12– 14]. On the other
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hand, the alloy compositions of the Ni3Si phase and Ni solid solution were not determined because of too fine precipitation. Alternatively, an average composition as the two-phases of the Ni3Si and Ni solid solution was determined for each Ni–Si–Cr ternary alloy (Table 2), and also plotted as open circles in Fig. 1a. The broken line joining the open circles in Fig. 1a would represent the composition of the Ni solid solution at 1323 K, i.e. before the solid state precipitation of Ni3Si. This result would agree approximately with the Ni3Si/Ni solid solution solvus line in the literature [13,14]. Fig. 4 shows XRD profiles of the Ni–Si –Cr ternary alloys. Corresponding to OM and SEM microstructures, and the electron-probe analysis, all the Ni–Si–Cr ternary alloys showed diffraction peaks from the Ni3Si (L12) (or Ni solid solution) and the Ni5Si2 although their relative magnitudes were very much dependent on the studied alloys. This fluctuation may be due to strong anisotropy of the grain alignment of the cast materials. The Ni3Si and Ni solid solution phases were not resolved because of their superposed diffraction peaks. However, the observed ordered diffraction peaks indicate the presence of the Ni3Si (L12) phase.
3.2. Mechanical properties Fig. 5 plots compressive yield stress (defined at 0.2% offset strain) of the Ni–Si–Cr ternary alloys as a function of test temperature. Here, the data for the Ni3Si binary alloy [15] are plotted for reference. It has been observed that as the temperature increased the yield stress of the Ni3Si binary alloy increased, and then showed a peak, followed by a rapid decrease. All the Ni –Si–Cr ternary alloys did not display a positive temperature dependence of the yield stress in contrast to the Ni3Si binary alloy [1,15]. As the test temperature increases, the yield stress of the TA-alloy rapidly decreases from room temperature, those of the TB- and TD-alloys from 473 K and that of the TC-alloy from 873 K. On the other hand, the yield stress level of the Ni –Si–Cr ternary alloys was substantially high particularly at low temperatures, in comparison to that of the Ni3Si binary alloy. It is suggested that the yield stress at low temperatures is closely related to the volume fraction of the Ni5Si2 phase. It is evident from Table 1 and
Table 2 Alloy compositions of the constituent phases in the Ni–Si–Cr ternary alloys Alloy
TA-alloy TB-alloy TC-alloy TD-alloy
Ni5Si2 (at.%)
Ni3Si (at.%)
Ni3Si+Ni s.s. (at.%)
Ni
Si
Cr
Ni
Si
Cr
Ni
Si
Cr
69.3 70.0 68.6 70.5
26.7 28.6 29.2 28.9
4.0 1.4 2.2 0.6
– – – –
– – – –
– – – –
72.7 69.8 65.1 77.9
13.7 12.6 12.8 12.7
13.6 17.6 22.1 9.4
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Fig. 4. X-ray diffraction profiles of the (a) TA- (b) TB- (c) TC- and (d) TD-alloys.
Fig. 5 that the TA-alloy among the studied Ni– Si– Cr ternary alloys shows the highest strengthening at room temperature because of the high volume fraction of the Ni5Si2 phase. Fig. 6 shows changes of nominal stress versus nominal strain curve with temperature for the Ni– Si–Cr ternary alloys measured by compression test. Depending on the alloy composition and temperature, largely different stress versus strain curves were observed. Generally describing, at low temperatures (region I) the Ni – Si –Cr ternary alloys drew very limited plastic deformation with a large strain-hardening rate after yielding, and then quickly ruptured. At intermediate temperatures (region II), the Ni– Si – Cr ternary alloys drew moderate plastic deformation with a moderate strain-hardening rate, and then ruptured. At high temperatures (region III), the Ni– Si – Cr ternary alloys drew steady-state flow after yielding. For examples, in the TA-alloy with the largest volume fraction of the Ni5Si2 phase (73%), the region I operated up to 673 K, the region II was not observed, and the region III began to operate from 873 K. In the TC-alloy with the smallest volume fraction of the Ni5Si2 phase (29%), the region I operated up to 473 K, the region II operated at 673 K, and the region III began to operate from 873 K. To evaluate ductility (or fracture toughness) of the Ni –Si –Cr ternary alloys, three-point bend test was performed at room temperature. Their fracture strength
is shown in Table 1. The result clearly indicates that the fracture strength is inversely proportional to the volume fraction of the brittle Ni5Si2 phase; the TA-alloy with the largest volume fraction of the Ni3Si2 phase showed the lowest fracture strength (134 MPa), while the TC-alloy with the smallest volume fraction of the Ni5Si2 phase showed the highest fracture strength ( 790 MPa). The fracture surfaces of the bent speci-
Fig. 5. Compressive yield stress of the Ni – Si– Cr ternary alloys as a function of test temperature. Data of the Ni3Si binary alloy [15] were also included for reference.
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Fig. 6. Changes of nominal stress vs. nominal strain curve with temperature for the (a) TA-; (b) TB-; (c) TC- and (d) TD-alloys. The deformation was conducted by compression test. Note that the cross marks mean rupturing.
mens are shown in Fig. 7. The fractography of the TA-alloy, which showed the lowest fracture strength, consisted of brittle fracture patterns (with river-like patterns) associated with the Ni5Si2 phase or the interface between the Ni5Si2 and the eutectic domains. On the other hand, the fractography of the TC-alloy, which showed the highest fracture strength, consisted of a mixture of ductile fracture patterns with brittle fracture patterns. Thus, SEM fractography is consistent with the result of the three-point bend test.
3.3. Chemical properties 3.3.1. Oxidation property Mass gain data of the Ni– Si – Cr ternary alloys during an isothermal oxidation at 1173 K are plotted as a logarithmic scale of time in Fig. 8. In this figure, data for the Ni3Si binary and Ni3(Si,Ti) alloys were included for reference. Concerning the Ni– Si – Cr ternary alloys, the mass gain was increasing in order, TD-alloyBTAalloy BTB-alloy TC-alloy. Also, the mass gain of all the Ni–Si –Cr ternary alloys was smaller than that of the Ni3Si binary alloy. Here, it is interesting to note that the TC-alloy with the largest Cr concentration showed the most inferior oxidation resistance. The Cr addition is thus mostly effective in reducing the mass gain in an isothermal oxidation. However, scale spallation was observed to occur at 70 h for the TC-alloy, which had the largest (o6erall) Cr concentration for the measured alloys. Rapidly formed oxide scale may result in a premature spallation.
A quasiparabolic oxidation rate was found for all the measured alloys; the growth rate of the oxidation scale mostly ranged between 0.4 and 0.5 except for an initial exposure time. This result suggests that the oxidation is basically controlled by the bulk diffusion. However, the initial oxidation rate for the TA-alloy, the TC-alloy and the Ni3Si binary alloy was rather high, while that for the TB-alloy and the TD-alloy was rather low.
3.3.2. Corrosion property Corrosion property of some alloys in sulfuric acid is shown in Table 3. The lowest mass loss was observed for the Ni3Si binary alloy. The Cr addition to the Ni3Si binary and Ni3(Si,Ti) alloys more or less resulted in an increase in the mass loss, irrespective of the tested sulfuric acid. However, the mass loss for the Cr-added alloys was not so much severe particularly in high content of sulfuric acid, comparing with those of the Ni3Si binary and Ni3(Si,Ti) alloys.
4. Discussion By the Cr addition to the Ni3Si binary alloy, various types of microstructures can be prepared—three-phase (consisting of the Ni3Si, Ni5Si2 and Ni solid solution) microstructure and two types of two-phase (the Ni3Si and Ni5Si2, and also the Ni3Si and Ni solid solution) microstructures. In this study, the three-phase microstructure was studied. In all the Ni–Si–Cr ternary alloys observed in this study, the Ni5Si2 phase existed as
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largely rounded (or dendritic) dispersions while the Ni3Si and Ni solid solution existed as finely distributed precipitates (i.e. in the form of eutectic microstructures). To obtain more favorable mechanical properties of these alloys, finely distributed Ni5Si2 is needed. Therefore, further experiments will be required for a selection of the alloy compositions and at the same time an adequate heat treatment procedure. As an another way to prepare such a microstructure, hot working and the subsequent heat treatment, by which the cast microstructure is modified, is suggested. The experimental result that the compressive stress– strain curves of the Ni – Si –Cr ternary alloys displayed steady-state flow under low flow stress level indicates that hot working and the subsequent annealing are possible. The Ni3Si alloys were substantially strengthened at low temperatures by the Cr addition, i.e. by the introduction of the second-phase dispersions. It is suggested that the prime strengthener is the Ni5Si2 phase as understood from Table 1 and Fig. 5. However, their
morphology and size appear to be not always favorable for enhancing the flow strength. More attempts are needed to improve further the flow strength at low temperatures. The second strengthener is the Ni solid solution but less effective in enhancing the flow strength. On the other hand, the introduction of the Ni5Si2 phase to the Ni3Si binary alloy reduced the ductility at low temperatures. It is likely that the Ni5Si2 phase dispersions act as a stress enhancer, resulting in premature fracture by facilitating transgranular or interfacial cracking. Therefore, finely distributed Ni5Si2 phase dispersions with coherency with the Ni3Si matrix may again be required to improve the low temperature ductility as well as the low temperature strength. At low to intermediate temperatures, the multiple phase microstructures of the Ni–Si–Cr ternary alloys were shown to be still largely effective in enhancing the yield strength. However, a positive temperature dependence of the yield strength took place no longer in all the Ni –Si –Cr ternary alloys. Thus, it is assumed that
Fig. 7. SEM fractography of the bent (a) TA-; (b) TB-; (c) TC- and (d) TD-alloys.
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lowest volume fraction of the Ni5Si2 phase, shows the most inferior oxidation resistance. It was thus shown in this study that the Cr addition to the Ni3Si binary alloys reduced significantly the oxidation thickness at 1173 K in the isothermal condition. Since the structure, chemistry and microstructure of the oxidized layers have not been analyzed in details, the realistic mechanism for the effect of the Cr addition on the oxidation of the studied alloys is not presented at the moment. An extending study is now undertaken and will be published elsewhere.
5. Conclusions
Fig. 8. Mass gain in air as a function of exposure time at 1173 K. Table 3 Mass loss of some alloys at 363 K for 24 h in various sulfuric acid (g m2 h−1) Alloy
40% H2SO4 60% H2SO4 80% H2SO4 98% H2SO4
Ni3Si 0.07 Ni79.5Si11Ti9.5 1.84 TB-alloy 9.46
1.04 1.44 4.50
0.03 0.11 0.64
0.02 0.15 0.12
the Ni3Si phase as well as the Ni5Si2 phase cannot be effective strengthener at temperatures above 900 K, as understood from Fig. 5. Alternatively, (compressive) plastic deformation was found to take place for the Ni –Si –Cr ternary alloys at high temperatures, as shown in Fig. 6. Probably, such plastic deformation appears to be guaranteed by activation of the glide and/or climb motions of the dislocations in the Ni5Si2 phase. Among the studied Ni– Si – Cr ternary alloys, the TC-alloy with the largest Cr concentration showed most inferior oxidation resistance. This result indicates that microstructural factor as well as (o6erall) chemical composition plays an important role in the oxidation behavior. The volume fraction of the Ni5Si2 phase was the smallest in the TC-alloy among the observed Ni– Si – Cr ternary alloys. Comparing with the Ni3Si and Ni solid solution, the Ni5Si2 phase appears to be highly resistive to the oxidation because of high Si concentration. Consequently, it is possible that the TC-alloy, which has the largest (o6erall) Cr concentration but the
The alloying behavior, microstructure, high-temperature mechanical properties, oxidation and corrosion properties of the Ni–Si –Cr ternary alloys consisting of a multiple phase were investigated by OM, XRD, scanning microscopy with electron probe analysis, mechanical test, three-point bend test, oxidation and corrosion measurement. The following results were obtained from the present study. The microstructures of the Ni–Si –Cr ternary alloys consisted of largely dispersed Ni5Si2 phase and finely precipitated Ni3Si phase in Ni solid solution. The Ni –Si –Cr ternary alloys showed significant strengthening over a wide range of temperatures, and also large compressive plastic deformation (deformability) at temperatures above 900 K. The strength and fracture toughness at ambient temperatures were correlated with the volume fraction of the Ni5Si2 phase. The Ni –Si –Cr ternary alloys showed substantially improved oxidation resistance in air at 1173 K, comparing with the Ni3Si binary and the Ni3(Si,Ti) alloys. Also, the Ni–Si–Cr ternary alloys showed corrosion resistance in sulfuric acid comparable to the Ni3Si binary and the Ni3(Si,Ti) alloys.
Acknowledgements This work was supported in part by Laboratory for Developmental Research of Advanced Materials, Institute for Materials Research, Tohoku University.
References [1] P.H. Thornton, R.G. Davies, Metall. Trans. 1 (1970) 549. [2] T. Takasugi, M. Nagashima, O. Izumi, Acta Metall. Mater. 38 (1990) 747. [3] T. Takasugi, S. Watanabe, O. Izumi, N.K. Fat-Halla, Acta Metall. Mater. 37 (1989) 3425. [4] M. Yoshida, T. Takasugi, Phil. Mag. 65 (1992) 41. [5] T. Takasugi, O. Izumi, M. Yoshida, J. Mater. Sci. 26 (1991) 1173.
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T. Takasugi, H. Suenaga, O. Izumi, J. Mater. Sci. 26 (1991) 1179. T. Takasugi, M. Yoshida, J. Mater. Sci. 26 (1991) 3032. T. Takasugi, M. Yoshida, J. Mater. Sci. 26 (1991) 3517. T. Takasugi, S. Hanada, in: E.P. George, M.J. Mills, M. Yamaguchi (Eds.), High-Temperature Ordered Intermetallic Alloys VIII, vol. 552, MRS Proceedings Publication, 1999, p. 399. [10] T. Takasugi, Mater. Sci. Technol. 16 (2000) 73.
[11] T. Takasugi, M. Yoshida, J. Mater. Sci., 36 (2001) 643. [12] S. Ochiai, Y. Oya, T. Suzuki, Acta Metall. 32 (1984) 289. [13] E.I. Gladyshevsky, L.K. Borusevich, Zhur. Neorg. Khim. 8 (1963) 1915. [14] R.W. Guard, E.A. Smith, J. Inst. Met. 88 (1959) 373. [15] D.-W. Wee, O. Noguchi, Y. Oya, T. Suzuki, Trans. JIM 21 (1980) 237.
The effect of Cr addition on mechanical and chemical properties of Ni3Si alloys T. Takasugi *, H. Kawai, Y. Kaneno Department of Metallurgy and Materials Science, Graduate School of Engineering, Osaka Prefecture Uni6ersity, 1 -1 Gakuencho, Sakai, Osaka 599 -8531, Japan
Abstract The alloying behavior and microstructure of the Ni– Si– Cr ternary alloys were firstly characterized by optical microscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) with electron probe analysis. The microstructures of the Ni–Si–Cr ternary alloys consisted of largely dispersed Ni5Si2 phase and finely precipitated Ni3Si phase in Ni solid solution. Then, the high-temperature mechanical properties, three-point bend test, oxidation and corrosion properties were investigated. The Ni–Si–Cr ternary alloys showed significant strengthening over a wide range of temperatures, and also large compressive plastic deformation at high temperatures. The strength and fracture toughness at ambient temperatures were correlated with the volume fraction of the Ni5Si2 phase. The Ni–Si–Cr ternary alloys showed substantially improved oxidation resistance in air at 1173 K, comparing with the Ni3Si and Ni3(Si,Ti) alloys. Also, the Ni– Si – Cr ternary alloys showed corrosion resistance comparable to the Ni3Si and Ni3(Si,Ti) alloys. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Nickel silicide; Intermetallic compound; High-temperature strength; Oxidation resistance; Corrosion resistance
1. Introduction Ni3Si-based alloys have been considered to be a candidate compound, which could be used as the basis of high-temperature structural materials and chemical parts because Ni3Si displays an increasing strength with increasing temperatures [1] and also shows excellent oxidation and corrosion resistance over a wide range of temperatures. Ti addition to Ni3Si resulted in (1) increased strength as a result of the solid solution hardening [2]; (2) enhanced peak temperature in the strength versus temperature curve [2,3] through modifying the specific dislocation structure [3,4]; and also (3) high tensile ductility at ambient temperatures by reducing the propensity to intergranular fracture [2,5]. Thus, the Ni3(Si,Ti) alloys have been shown to have a number of useful mechanical properties [2 – 5]. Therefore, the subsequent improvement of mechanical and related properties has been performed on the Ni3(Si,Ti) alloys. The doping with interstitials such as boron, carbon and
* Corresponding author. Tel.: +81-722-54-9314; fax: + 81-722-549912. E-mail address: [email protected] (T. Takasugi).
beryllium has been conducted and shown to improve further the ambient temperature ductility via preventing intergranular fracture [5–7]. Alloying of some substitutional (transition) metals has also been performed mostly within their solubility limits, and shown to result in favorable mechanical properties [8]. On the other hand, a recent study showed that Nb-containing second-phase dispersions (such as D0a-type Ni3Nb) in the Ni3(Si,Ti) phase matrix resulted in a further strengthening over a wide range of temperatures, and also improved the moisture-induced embrittlement as well as the high-temperature tensile elongation [9–11], from which the Ni3Si and Ni3(Si,Ti) alloys have been suffering for a long time. In the present study, Cr, which was expected to improve particularly chemical property such as oxidation and corrosion resistance, was added to the Ni3Si alloys. The Cr was added to the Ni3Si beyond the solubility limits in their L12 phases. The microstructures of the Cr-added Ni3Si alloys are, therefore, expected to consist of multi-phase. The alloying behavior, microstructure, mechanical properties and chemical properties are investigated by optical microscopy (OM), X-ray diffraction (XRD), scanning microscopy with
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 6 1 9 - 7
T. Takasugi et al. / Materials Science and Engineering A329–331 (2002) 446–454
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Table 1 Composition, microstructure and fracture strength (in three-point bend test) of the Ni–Si–Cr ternary alloys used in this study Alloy
Composition (at.%)
Microstructure
Volume fraction of Ni5Si2 phase (%)
Fracture strength in three-point bend test (MPa)
TA-alloy TB-alloy TC-alloy TD-alloy
Ni72Si23Cr5 Ni70Si20Cr10 Ni67Si18Cr15 Ni75Si20Cr5
L12,Ni5Si2,Niss L12,Ni5Si2,Niss L12,Ni5Si2,Niss L12,Ni5Si2,Niss
73 44 29 41
134 493 790 597
electron probe analysis, tensile test, three-point bend test, oxidation and corrosion measurements. It is shown that the alloys, with a multi-phase microstructure consisting of Ni3Si (L12) phase, Ni5Si2 phase and Ni solid solution, have excellent mechanical and chemical properties.
2. Experimental procedure Alloy compositions used in this study are shown in Table 1 and denoted with TA-, TB-, TC- and TD-alloys in the followings. These alloy compositions are also plotted on an Ni–Si – Cr ternary phase diagram at 1323 K (Fig. 1), and suggested to lie in the three-phase region consisting of the Ni3Si (L12), Ni5Si2 (hexagonal) and Ni solid solution (fcc). All the Ni– Si – Cr ternary alloys used in this study were doped with 0.005 mass% boron to improve the low temperature ductility. Also, for comparison, the Ni3Si binary and Ni3(Si,Ti) ternary alloys were prepared by the same procedure. Starting raw materials were 99.9 wt.% Ni, 99.999 wt.% Si, 99.99 wt.% Cr and 99.9 wt.% boron. Alloy button ingots with a dimension of 50× 25 × 15 mm3 were prepared by non-consumable arc melting in argon gas on copper hearth. All the button ingots were homogenized at 1323 K for 1 day in vacuum. The as-homogenized materials were used for all tests and measurements. Metallographic, chemical and structural observations of the Ni–Si –Cr ternary alloys were carried out by OM, XRD and scanning electron microscopy (SEM) with wave-length dispersive spectroscopy (WDS) or energy dispersive spectroscopy (EDS). X-ray diffraction profiles were obtained using CuKa radiation. The electron probe data for the alloy compositions in the constituent phases were collected from more than five areas and then averaged. The determination of the constituent phases and their crystal structures were conducted on the basis of XRD and SEM electronprobe analysis. Using a precision wheel cutter and an electrodischarge machine (EDM), compressive specimens with a dimension of 2.5×2.5 ×5 mm3 were cut from the homogenized button ingots. The faces of the com-
pressive specimens were abraded with a sufficiently fine SiC paper before the mechanical test. Using an Instrontype testing machine, the mechanical tests were conducted in vacuum in a temperature range between room temperature and 1073 K. Nominal strain rates used in the compressive tests were 3.3× 10 − 4 s − 1. To evaluate ductility (or fracture toughness) of the Ni – Si–Cr ternary alloys at room temperature, rectangular specimens with a dimension of 1× 5× 20 mm3 were used for three-point bend test at room temperature. All specimen surfaces were mechanically abraded with up to 1500-grit SiC paper. The bend tests were conducted in air at a displacement rate of 0.1 mm min − 1. A fully articulated three-point bending fixture (with a span of 10 mm and a pushing rod of 3-mm diameter) was used for fracture of the bend specimens. The bend strength |f was calculated from a load versus displacement curve, using the relationship of |f = 1.5Pl/wt 2 where P is the load, l is the span (i.e. 10 mm), and w and t are the width and the thickness of the specimen, respectively. At least two specimens were used for measurement of each material condition. Fracture surfaces of the bent Ni–Si–Cr ternary alloys were examined by SEM.
Fig. 1. The Ni– Si– Cr ternary [14] phase diagram at 1323 K, and alloy compositions used in this study. Note that the plotted open circles mean the average compositions (as the two phases of the Ni3Si and Ni solid solution) determined in this study.
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Fig. 2. OM microstructures of the (a) TA-; (b) TB-; (c) TC- and (d) TD-alloys.
Oxidation property was measured using specimens with a dimension of 10×10 ×1 mm3. Before the measurement, all major surfaces were abraded with 1500grid SiC paper, then ultrasonically cleaned with ethanol. Isothermal oxidation tests based on the cyclic test were performed in static laboratory air at 1173 K. The specimens placed on alumina boat were inserted into and removed from the hot zone of a furnace. Each cycle consisted of a period of desired hour at 1173 K and at room temperature. Mass gain of the specimens at each cycle was measured at room temperature, together with alumina boat. Therefore, the measured mass gain in this study includes the spalled oxide scales. The oxidized surfaces were observed by an eye view and SEM. Corrosion property was measured in a sulfuric acid at 363 K using specimens with a dimension of 10× 10×1 mm3. Measurement was performed using solutions of 40, 60, 80 and 98% sulfuric acid. Mass loss was measured after immersion in sulfuric acid for 24 h.
3. Results
3.1. Alloying beha6ior and microstructure Optical microstructures (OM) were observed on a longitudinal section of the button ingot along which the alloys were solidified. The OM microstructures of the Ni –Si–Cr ternary alloys are shown in Fig. 2. The microstructure of the TA-alloy showed large dendritic Ni5Si2 phase and two-phase regions consisting of the Ni3Si (L12) phase and Ni solid solution. The Ni3Si phase and Ni solid solution were resolved by a SEM image as shown in Fig. 3. Here, the dot-like images in the two-phase regions are the Ni3Si precipitates. The microstructures of the other ternary alloys were similar one another but somehow different from that of the TA-alloy. Isolated round-shaped Ni5Si2 phases were surrounded by two-phase regions of the Ni3Si (L12) phase and Ni solid solution. The volume fraction of the Ni5Si2 phase depended on the alloy, as shown in Table
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Fig. 3. BS-SEM microstructure of the TD-alloy. Note that the dot-like images in the two-phase regions are Ni3Si precipitates.
1; the TA-alloy had the largest volume fraction of the Ni5Si2 phase (73%) while the TC-alloy had the smallest volume fraction of the Ni5Si2 phase (29%). Based on this observation, it is suggested that the alloys solidify as (the TA-alloy) primary allotriomorphs of Ni5Si2 is to be followed by a eutectic solidification of Ni solid solution and Ni5Si2 (on rapid solidification in the arc furnace the equilibrium Ni3Si phase may not form). On heat treatment the eutectic would coarsen to some extent to give the large rounded areas of Ni5Si2 seen in Fig. 2a, b and d. On cooling from the heat treatment temperature a fine dispersions of Ni3Si would be precipitated from the Ni solid solution by a solid state transformation. The alloys containing lesser amounts of Si (the TB- and TD-alloys) do not contain primary Ni5Si2 (Fig. 2b and d) but only coarsened eutectic. The alloy with lowest Si content (the TC-alloy) consists of Ni solid solution and some coarsened eutectic, with precipitated Ni3Si in the Ni solid solution phase. The alloy compositions of the constituent phases were determined by an electron-probe analysis and shown in Table 2. The alloy composition of the Ni5Si2 phase was 69.6 at.% Ni, 28.4 at.% Si and 2.0 at.% Cr as an average value of the alloys studied. Thus, the solubility limit of Cr in the Ni5Si2 phase is small consistent with the previous observation [12– 14]. On the other
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hand, the alloy compositions of the Ni3Si phase and Ni solid solution were not determined because of too fine precipitation. Alternatively, an average composition as the two-phases of the Ni3Si and Ni solid solution was determined for each Ni–Si–Cr ternary alloy (Table 2), and also plotted as open circles in Fig. 1a. The broken line joining the open circles in Fig. 1a would represent the composition of the Ni solid solution at 1323 K, i.e. before the solid state precipitation of Ni3Si. This result would agree approximately with the Ni3Si/Ni solid solution solvus line in the literature [13,14]. Fig. 4 shows XRD profiles of the Ni–Si –Cr ternary alloys. Corresponding to OM and SEM microstructures, and the electron-probe analysis, all the Ni–Si–Cr ternary alloys showed diffraction peaks from the Ni3Si (L12) (or Ni solid solution) and the Ni5Si2 although their relative magnitudes were very much dependent on the studied alloys. This fluctuation may be due to strong anisotropy of the grain alignment of the cast materials. The Ni3Si and Ni solid solution phases were not resolved because of their superposed diffraction peaks. However, the observed ordered diffraction peaks indicate the presence of the Ni3Si (L12) phase.
3.2. Mechanical properties Fig. 5 plots compressive yield stress (defined at 0.2% offset strain) of the Ni–Si–Cr ternary alloys as a function of test temperature. Here, the data for the Ni3Si binary alloy [15] are plotted for reference. It has been observed that as the temperature increased the yield stress of the Ni3Si binary alloy increased, and then showed a peak, followed by a rapid decrease. All the Ni –Si–Cr ternary alloys did not display a positive temperature dependence of the yield stress in contrast to the Ni3Si binary alloy [1,15]. As the test temperature increases, the yield stress of the TA-alloy rapidly decreases from room temperature, those of the TB- and TD-alloys from 473 K and that of the TC-alloy from 873 K. On the other hand, the yield stress level of the Ni –Si–Cr ternary alloys was substantially high particularly at low temperatures, in comparison to that of the Ni3Si binary alloy. It is suggested that the yield stress at low temperatures is closely related to the volume fraction of the Ni5Si2 phase. It is evident from Table 1 and
Table 2 Alloy compositions of the constituent phases in the Ni–Si–Cr ternary alloys Alloy
TA-alloy TB-alloy TC-alloy TD-alloy
Ni5Si2 (at.%)
Ni3Si (at.%)
Ni3Si+Ni s.s. (at.%)
Ni
Si
Cr
Ni
Si
Cr
Ni
Si
Cr
69.3 70.0 68.6 70.5
26.7 28.6 29.2 28.9
4.0 1.4 2.2 0.6
– – – –
– – – –
– – – –
72.7 69.8 65.1 77.9
13.7 12.6 12.8 12.7
13.6 17.6 22.1 9.4
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Fig. 4. X-ray diffraction profiles of the (a) TA- (b) TB- (c) TC- and (d) TD-alloys.
Fig. 5 that the TA-alloy among the studied Ni– Si– Cr ternary alloys shows the highest strengthening at room temperature because of the high volume fraction of the Ni5Si2 phase. Fig. 6 shows changes of nominal stress versus nominal strain curve with temperature for the Ni– Si–Cr ternary alloys measured by compression test. Depending on the alloy composition and temperature, largely different stress versus strain curves were observed. Generally describing, at low temperatures (region I) the Ni – Si –Cr ternary alloys drew very limited plastic deformation with a large strain-hardening rate after yielding, and then quickly ruptured. At intermediate temperatures (region II), the Ni– Si – Cr ternary alloys drew moderate plastic deformation with a moderate strain-hardening rate, and then ruptured. At high temperatures (region III), the Ni– Si – Cr ternary alloys drew steady-state flow after yielding. For examples, in the TA-alloy with the largest volume fraction of the Ni5Si2 phase (73%), the region I operated up to 673 K, the region II was not observed, and the region III began to operate from 873 K. In the TC-alloy with the smallest volume fraction of the Ni5Si2 phase (29%), the region I operated up to 473 K, the region II operated at 673 K, and the region III began to operate from 873 K. To evaluate ductility (or fracture toughness) of the Ni –Si –Cr ternary alloys, three-point bend test was performed at room temperature. Their fracture strength
is shown in Table 1. The result clearly indicates that the fracture strength is inversely proportional to the volume fraction of the brittle Ni5Si2 phase; the TA-alloy with the largest volume fraction of the Ni3Si2 phase showed the lowest fracture strength (134 MPa), while the TC-alloy with the smallest volume fraction of the Ni5Si2 phase showed the highest fracture strength ( 790 MPa). The fracture surfaces of the bent speci-
Fig. 5. Compressive yield stress of the Ni – Si– Cr ternary alloys as a function of test temperature. Data of the Ni3Si binary alloy [15] were also included for reference.
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Fig. 6. Changes of nominal stress vs. nominal strain curve with temperature for the (a) TA-; (b) TB-; (c) TC- and (d) TD-alloys. The deformation was conducted by compression test. Note that the cross marks mean rupturing.
mens are shown in Fig. 7. The fractography of the TA-alloy, which showed the lowest fracture strength, consisted of brittle fracture patterns (with river-like patterns) associated with the Ni5Si2 phase or the interface between the Ni5Si2 and the eutectic domains. On the other hand, the fractography of the TC-alloy, which showed the highest fracture strength, consisted of a mixture of ductile fracture patterns with brittle fracture patterns. Thus, SEM fractography is consistent with the result of the three-point bend test.
3.3. Chemical properties 3.3.1. Oxidation property Mass gain data of the Ni– Si – Cr ternary alloys during an isothermal oxidation at 1173 K are plotted as a logarithmic scale of time in Fig. 8. In this figure, data for the Ni3Si binary and Ni3(Si,Ti) alloys were included for reference. Concerning the Ni– Si – Cr ternary alloys, the mass gain was increasing in order, TD-alloyBTAalloy BTB-alloy TC-alloy. Also, the mass gain of all the Ni–Si –Cr ternary alloys was smaller than that of the Ni3Si binary alloy. Here, it is interesting to note that the TC-alloy with the largest Cr concentration showed the most inferior oxidation resistance. The Cr addition is thus mostly effective in reducing the mass gain in an isothermal oxidation. However, scale spallation was observed to occur at 70 h for the TC-alloy, which had the largest (o6erall) Cr concentration for the measured alloys. Rapidly formed oxide scale may result in a premature spallation.
A quasiparabolic oxidation rate was found for all the measured alloys; the growth rate of the oxidation scale mostly ranged between 0.4 and 0.5 except for an initial exposure time. This result suggests that the oxidation is basically controlled by the bulk diffusion. However, the initial oxidation rate for the TA-alloy, the TC-alloy and the Ni3Si binary alloy was rather high, while that for the TB-alloy and the TD-alloy was rather low.
3.3.2. Corrosion property Corrosion property of some alloys in sulfuric acid is shown in Table 3. The lowest mass loss was observed for the Ni3Si binary alloy. The Cr addition to the Ni3Si binary and Ni3(Si,Ti) alloys more or less resulted in an increase in the mass loss, irrespective of the tested sulfuric acid. However, the mass loss for the Cr-added alloys was not so much severe particularly in high content of sulfuric acid, comparing with those of the Ni3Si binary and Ni3(Si,Ti) alloys.
4. Discussion By the Cr addition to the Ni3Si binary alloy, various types of microstructures can be prepared—three-phase (consisting of the Ni3Si, Ni5Si2 and Ni solid solution) microstructure and two types of two-phase (the Ni3Si and Ni5Si2, and also the Ni3Si and Ni solid solution) microstructures. In this study, the three-phase microstructure was studied. In all the Ni–Si–Cr ternary alloys observed in this study, the Ni5Si2 phase existed as
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largely rounded (or dendritic) dispersions while the Ni3Si and Ni solid solution existed as finely distributed precipitates (i.e. in the form of eutectic microstructures). To obtain more favorable mechanical properties of these alloys, finely distributed Ni5Si2 is needed. Therefore, further experiments will be required for a selection of the alloy compositions and at the same time an adequate heat treatment procedure. As an another way to prepare such a microstructure, hot working and the subsequent heat treatment, by which the cast microstructure is modified, is suggested. The experimental result that the compressive stress– strain curves of the Ni – Si –Cr ternary alloys displayed steady-state flow under low flow stress level indicates that hot working and the subsequent annealing are possible. The Ni3Si alloys were substantially strengthened at low temperatures by the Cr addition, i.e. by the introduction of the second-phase dispersions. It is suggested that the prime strengthener is the Ni5Si2 phase as understood from Table 1 and Fig. 5. However, their
morphology and size appear to be not always favorable for enhancing the flow strength. More attempts are needed to improve further the flow strength at low temperatures. The second strengthener is the Ni solid solution but less effective in enhancing the flow strength. On the other hand, the introduction of the Ni5Si2 phase to the Ni3Si binary alloy reduced the ductility at low temperatures. It is likely that the Ni5Si2 phase dispersions act as a stress enhancer, resulting in premature fracture by facilitating transgranular or interfacial cracking. Therefore, finely distributed Ni5Si2 phase dispersions with coherency with the Ni3Si matrix may again be required to improve the low temperature ductility as well as the low temperature strength. At low to intermediate temperatures, the multiple phase microstructures of the Ni–Si–Cr ternary alloys were shown to be still largely effective in enhancing the yield strength. However, a positive temperature dependence of the yield strength took place no longer in all the Ni –Si –Cr ternary alloys. Thus, it is assumed that
Fig. 7. SEM fractography of the bent (a) TA-; (b) TB-; (c) TC- and (d) TD-alloys.
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lowest volume fraction of the Ni5Si2 phase, shows the most inferior oxidation resistance. It was thus shown in this study that the Cr addition to the Ni3Si binary alloys reduced significantly the oxidation thickness at 1173 K in the isothermal condition. Since the structure, chemistry and microstructure of the oxidized layers have not been analyzed in details, the realistic mechanism for the effect of the Cr addition on the oxidation of the studied alloys is not presented at the moment. An extending study is now undertaken and will be published elsewhere.
5. Conclusions
Fig. 8. Mass gain in air as a function of exposure time at 1173 K. Table 3 Mass loss of some alloys at 363 K for 24 h in various sulfuric acid (g m2 h−1) Alloy
40% H2SO4 60% H2SO4 80% H2SO4 98% H2SO4
Ni3Si 0.07 Ni79.5Si11Ti9.5 1.84 TB-alloy 9.46
1.04 1.44 4.50
0.03 0.11 0.64
0.02 0.15 0.12
the Ni3Si phase as well as the Ni5Si2 phase cannot be effective strengthener at temperatures above 900 K, as understood from Fig. 5. Alternatively, (compressive) plastic deformation was found to take place for the Ni –Si –Cr ternary alloys at high temperatures, as shown in Fig. 6. Probably, such plastic deformation appears to be guaranteed by activation of the glide and/or climb motions of the dislocations in the Ni5Si2 phase. Among the studied Ni– Si – Cr ternary alloys, the TC-alloy with the largest Cr concentration showed most inferior oxidation resistance. This result indicates that microstructural factor as well as (o6erall) chemical composition plays an important role in the oxidation behavior. The volume fraction of the Ni5Si2 phase was the smallest in the TC-alloy among the observed Ni– Si – Cr ternary alloys. Comparing with the Ni3Si and Ni solid solution, the Ni5Si2 phase appears to be highly resistive to the oxidation because of high Si concentration. Consequently, it is possible that the TC-alloy, which has the largest (o6erall) Cr concentration but the
The alloying behavior, microstructure, high-temperature mechanical properties, oxidation and corrosion properties of the Ni–Si –Cr ternary alloys consisting of a multiple phase were investigated by OM, XRD, scanning microscopy with electron probe analysis, mechanical test, three-point bend test, oxidation and corrosion measurement. The following results were obtained from the present study. The microstructures of the Ni–Si –Cr ternary alloys consisted of largely dispersed Ni5Si2 phase and finely precipitated Ni3Si phase in Ni solid solution. The Ni –Si –Cr ternary alloys showed significant strengthening over a wide range of temperatures, and also large compressive plastic deformation (deformability) at temperatures above 900 K. The strength and fracture toughness at ambient temperatures were correlated with the volume fraction of the Ni5Si2 phase. The Ni –Si –Cr ternary alloys showed substantially improved oxidation resistance in air at 1173 K, comparing with the Ni3Si binary and the Ni3(Si,Ti) alloys. Also, the Ni–Si–Cr ternary alloys showed corrosion resistance in sulfuric acid comparable to the Ni3Si binary and the Ni3(Si,Ti) alloys.
Acknowledgements This work was supported in part by Laboratory for Developmental Research of Advanced Materials, Institute for Materials Research, Tohoku University.
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