Mechanical properties of Mo5Si3 intermetallics as a function of composition

Materials Characterization 55 (2005) 402 – 411

Mechanical properties of Mo5Si3 intermetallics as a function of composition Erik Stro¨m * Department of Materials Science and Engineering, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden Received 9 March 2004; received in revised form 9 September 2005; accepted 9 September 2005

Abstract A solubility range of 2.1 at.% Si at 1600 8C in Mo5Si3 was measured by energy dispersive spectrometry (EDS) in a SEM on samples containing the second phases Mo3Si and MoSi2, respectively. Of the three methods for elemental analysis used in this work, i.e. bulk chemical analysis, EDS, and Rietveld refinement of neutron powder diffraction data, it is concluded that EDS yields an underestimation of Si-content in Mo5Si3 compared to bulk chemical analysis. As a result, the homogeneity range of Mo5Si3 is displaced to lower Si-content compared to the Mo-Si binary phase diagram. Both hardness and toughness of Mo5Si3 displayed their highest values for the composition closest to stoichiometry. The single-phase sample contained Mo vacancies according to Rietveld refinement, which is in good agreement with the results of chemical analysis as these show a slight Mo-deficiency. D 2005 Elsevier Inc. All rights reserved. Keywords: EDXS; Transition metal silicides; Rietveld refinement; Hardness; Toughness

1. Introduction Transition metal silicides have received interest as future complements for Ni-based superalloys to be used for ultra-high temperature structural applications due to their high melting points and strength retention at high temperatures. Among the silicides, MoSi2 has received the most attention so far. Recently, Mo5Si3 has been considered for ultra-high temperature use due to its high melting temperature of 2180 8C [1] and reasonable density of 8.19 g/cm3 [2]. Moreover, its creep strength has been shown to be several orders of magnitude higher than for MoSi2 [3]. However, there are some key issues for the future development of Mo5Si3-based

* Tel.: +46 31 772 1252; fax: +46 31 772 1313. E-mail address: [email protected]. 1044-5803/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2005.09.001

materials. Firstly, it suffers from brittleness at ambient and elevated temperatures, probably as a result of the complex tetragonal D8m crystal structure (space group I4/mcm, W5Si3-structure [4]). Secondly, cast materials of Mo5Si3 show severe cracking on both a macro- and microscopic level. This is proposed to be a result of the large mismatch in the coefficient of thermal expansion (CTE) between the a- and c-directions in the tetragonal unit cell (CTEa = 5.2
10 6/8C and CTEc = 11.5
10 6/8C [5]). The brittle behaviour can be reduced by two different means: by alloying, or by microstructure control including in-situ composites. According to the Mo–Si phase diagram [1], shown in Fig. 1, Mo5Si3 exhibits a homogeneity range of ca. 2.5 at.% silicon. This implies that solid solubility of alloying elements is possible in Mo5Si3 as opposed to the line compound MoSi2, which exhibits a very limited solubility of most transition metals [6]. Alloying addi-

E. Stro¨m / Materials Characterization 55 (2005) 402–411

403

Fig. 1. The Mo–Si phase diagram [1].

tions may have positive effects on the physical properties of Mo5Si3, e.g. mechanical behaviour and thermal expansion. The solubility of alloying elements in Mo5Si3 was evaluated in a recent publication [7]. There, it was noticed that energy dispersive spectrometry (EDS) analysis, which uses internal standards from a database within the EDS software, gave unreasonable chemical compositions bearing in mind the assumed 5–3 stoichiometry of the studied compounds. According to the work by Huebsch et al. [8], the ZAF correction seemed to depend considerably on the standards used for quantification. In their work, the largest deviation from Mo5Si3 stoichiometry occurred when elemental Si and Mo standards were used. For some intermetallic compounds, off-stoichiometric composition has been shown to have positive effects on their mechanical behaviour. For instance, a ductility increase from 0 to 4% tensile elongation has been obtained using off-stoichiometric FeAl alloys (e.g. Fe – 40 at.% Al) instead of the stoichiometric composition [9]. Since the solubility range of Mo5Si3 extends on both sides of stoichiometry, this system offers an opportunity to study the effects of composition on mechanical properties. To the author’s knowledge, no

detailed study of the mechanical behaviour of off-stoichiometric Mo5Si3 have so far been conducted. However, because Mo5Si3 is brittle at room temperature, mechanical testing is severely limited. Cracks that are associated with the CTE-mismatch of the compound form during preparation, which makes testing of large samples difficult in terms of obtaining reliable and representative data. Microindentation probes only a small polished area and thereby eliminates many of the problems associated with bulk testing. Nørlund Christensen [4] refined the structure of Mo5Si3 in an early study. As arc-melting of Mo silicides generally results in Si loss due to volatilisation [6,10,11], designing the alloys for exact stoichiometry can easily result in hypostoichiometric, i.e. Sideficient, silicides. It would be of interest to refine the structural parameters of Mo5Si3 having composition as close as possible to the exact stoichiometry. Rietveld refinement is based on the calculation of a theoretical diffraction pattern and the fitting of the calculated diagram to the experimental one [12]. Using the innovative, two-step arc-melting technique, described in more detail in [7], allows for a more accurate composition control than just mixing the elemental powders followed by arc-melting.

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E. Stro¨m / Materials Characterization 55 (2005) 402–411

The aim of the present study is threefold: (1) to investigate the solubility range in Mo5Si3 by means of EDS measurements in a scanning electron microscope (SEM); (2) to investigate the mechanical properties of Mo5Si3 as a function of composition; (3) to refine the structural parameters of Mo5Si3 for the composition showing the best mechanical properties. A standard sample consisting of single-phase Mo5Si3 with chemical composition measured by bulk chemical analysis has been used as a reference during the EDS measurements of Mo3Si–Mo5Si3 and Mo5Si3–MoSi2 two-phase alloys, i.e. compositions on each side of the Mo5Si3 intermetallic compound in the Mo–Si binary system. The results are compared with those obtained using elemental Mo and Si standards during EDS. Also, comparisons are made between EDS data and bulk chemical analysis data. Furthermore, Vickers hardness and indentation toughness values from the three different samples have been measured. Finally, Rietveld refinement of neutron powder diffraction (NPD) data on the single-phase has been performed. Mechanical and physical properties of Mo5Si3 are discussed in relation to site occupancy results from the refinement. 2. Experimental The studied materials were prepared by arc-melting commercially pure Mo and Si using a two-step method, as described in Ref. [7]. One sample of each two-phase composition, i.e. Mo3Si–Mo5Si3 and Mo5Si3–MoSi2, respectively, was annealed for 48 h at 1600 8C in an Ar atmosphere. The sample intended for use as a standard during the quantification was annealed at 1600 8C for 96 h to ensure chemical homogeneity. The samples are listed in Table 1. Bulk chemical analysis was performed by inductive coupled plasma on a Perkin Elmer Optima 3000-OES to determine the alloy compositions as well as the C, O, and N content. O and N content was measured by combustion on a LECO TS 436. The results of the chemical analyses of the studied alloys are shown in Table 2. Table 1 Phase assemblage and heat treatment details of the samples analysed in this study Sample

Phases

Heat treatment

Mo5Si3 standard M1-1 MA1-1 M1-6 MA1-6

Mo5Si3 Mo5Si3, Mo5Si3, Mo5Si3, Mo5Si3,

96 h, 1600 8C, Ar As-cast 48 h, 1600 8C, Ar As-cast 48 h, 1600 8C, Ar

MoSi2 MoSi2 Mo3Si Mo3Si

Table 2 Chemical analysis of as-cast alloys (compositions given in wt.%) Sample

Mo

C

O

N

Others*

Mo5Si3 nominal Mo5Si3 standard M1-1 M1-6

85.06

–

14.94 –

–

–

–

84.0

0.04

15.4

0.037 0.013 0.0027 b0.01 each

b0.001 16.8 0.051 13.7

0.009 0.067 0.0057 b0.01 each 0.023 0.033 0.0033 b0.01 each

82.9 86.0

W

Si

* Cr, Ti, Nb, Co, Ni, Fe.

The EDS analysis was performed in a LEO 1550 Gemini SEM equipped with an Oxford attachment including a Germanium detector and LINK Inca software for quantification. Inca uses the PAP procedure for quantification, where mass absorption coefficients are calculated using MAC 30 [13]. An accelerating voltage of 20 kV was used in order to improve the phase contrast of the samples using back-scattering mode. Accordingly, EDS measurements were carried out at 20 kV and a 60 Am condenser aperture, which equals a probe current of 1.25 nA yielding approximately 8500 X-ray counts per second (cps). Having a live-time of 100 s resulted in a total of ca. 850,000 counts per spectrum. The electron probe current was recorded once every 2 h using a Co specimen for gain calibration, and the variation was less than F1%. Setup of the SEM was arranged to be the same as those conditions used for alloyed Mo5Si3, where 20 kV is required for contrast reasons to detect e.g. chemical gradients. To confirm that the standard sample was single-phase, powder X-ray diffraction (XRD) experiments were carried out at room temperature using a Bruker axs D8 ADVANCE diffractometer (Cr-Ka radiation) operated at 35 kV and 40 mA. X-ray data was collected using a 0.058 (2h) step scan with a count time of 5 s. Hardness and fracture toughness measurements were performed at room temperature on a Leitz microhardness tester with a Vickers indenter. A load of 500 g and dwell time of 15 s were used. Twenty indentations were measured on each sample. Measurements of impression diagonals and crack lengths were made under high magnification with SEM. Hardness values (H) [14] and fracture toughness values (K) [15] were computed from the following equations: H ¼ 1:8544 P=d 2



K ¼ 0:016 ð E=H Þ1=2 P=c3=2



ð1Þ ð2Þ

where P is the load (N), d is the mean impression diagonal (m), and c is the average crack length (m).

E. Stro¨m / Materials Characterization 55 (2005) 402–411

A Young’s modulus value (E) of 323 GPa [6] was used in the calculations. Neutron diffraction experiments were conducted at the 50 MW R2 Research Reactor at Studsvik, Sweden. The double monochromator system, consisting of two parallel Cu crystals in the (220) mode, was aligned to ˚ . A powder sample of give a wavelength of 1.470(1) A about 5 g was ground using an agate mortar and pestle, and was contained in a vanadium can of 8 mm diameter during the experiment. Data were collected at 293 K using a step size of 0.088 (2h), and the collection angle covered a range of 48–139.928 (2h). The detector bank consisted of 35 3He detectors with Gd coated mylar film collimators. For the Rietveld refinements, the Full Profile (Fullprof) software was used [16]. 3. Results and discussion 3.1. Set-up of the Mo5Si3 standard First, a single point, standard EDS-spectrum was measured on the sample denoted Mo5Si3 standard in Table 1, confirmed to be single-phase according to XRD and SEM, under the conditions mentioned above. Process-time (PT) can be varied from 1 to 6 in Inca, and PT 5 resulted in ~44% dead-time, which equals an X-ray acquisition rate of approximately 4600 cps. This high dead-time was chosen according to Fiory and Swyt [17] and Statham [18], because a high dead-time can be used when the spectrum consists of roughly equal peak heights. Secondly, 30 EDS-spectra were obtained on the standard sample to allow for comparison between EDS data and chemical data. According to Fig. 2, the total yield is reasonable once

Fig. 2. Chemical composition of a Mo5Si3 sample using a Mo5Si3 standard, Mo and Si elemental standards, and from bulk chemical analysis, respectively.

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a standard sample has been used. The largest deviation from chemical analysis can be seen for the Si concentration, i.e. 14.65 F 0.15 wt.% for EDS and 15.4 wt.% for chemical analysis, respectively. The total is close to 100 wt.% when the Mo5Si3 standard is used while it is in the order of 75 wt.% when elemental standards are used. In this case, the elemental standards denote data stored in the EDS software, rather than standard data obtained during the actual experiment. However, the data are obtained from real experiments, and are delivered within the software from the manufacturer. 3.2. Accuracy; measurements on two-phase alloys At least 25 spectra each were obtained on the Mo5Si3 phase in the samples corresponding to the Si-rich and the Si-deficient sides of Mo5Si3 in the Mo–Si system. The microstructures of the first solidified regions of the as-cast alloys, i.e. samples M1-1 and M1-6 in Table 1, are shown in Figs. 3 and 4, respectively. Generally, arccast Mo5Si3 has a grain size in the order of 10 Am close to the mould while grains away from the mould surface typically have a size in the mm-range. Based on the recommendations by Newbury [19], only spectra falling into the 98-102 wt.% interval should be used for quantification. This advice was followed throughout this work with one exception; for sample M1-1 the total yield was 102.41 F 0.44 wt.% for reasons that are not identified. The standard deviation of each data set varied between 0.44–1.21 wt.%. This corresponds to a precision that varies between 0.10– 0.21 wt.% for each point, which equals 0.03–0.07 at.%. It is noted that the composition in Table 3 is reasonable when the Mo5Si3 standard is used, i.e. close to the 37.5 at.% Si expected from stoichiometry, while the data are consistently Si-deficient when elemental standards are used. The work by Huebsch et al., on the contrary, shows a systematic overestimation in Si concentration when elemental standards are used, as seen from Table 3. When comparing the data in Fig. 2, it is noted that the relative underestimation is larger for Si than for Mo in this work; the ratio between elemental standards and Mo5Si3 standard is 0.70 for Si and 0.74 for Mo. This may explain why the Si concentration in Table 3 is lower in this work than in that by Huebsch et al. [8], who report an underestimation especially in Mo concentration when elemental standards are used. It should be pointed out here that the latter use EPMA data that is based on wavelength dispersive spectrometry (WDS), which is considered a more accurate method than EDS due to its better spectral resolution. EDS, on the other hand, is a less time-consuming technique

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E. Stro¨m / Materials Characterization 55 (2005) 402–411 Table 3 Si-content [at.%] in the Mo5Si3 phase using different standards Sample

Standards used Mo5Si3 Elemental Mo3Si [8]

Mo5Si3–Mo3Si 36.74 Mo5Si3 37.22 Mo5Si3–MoSi2 38.84

34.79 35.86 37.33

MoSi2 [8]

Elemental [8]

37.1

37.7

39.9

38.5

39.1

41.4

The samples in this study have been annealed at 1600 8C.

Fig. 3. Backscattered electron image (BSI) of the first solidified parts of Si-rich sample M1-1 consisting of the phases Mo5Si3 (bright contrast) and MoSi2.

and has become more and more popular over the past 30 years [19]. Nevertheless, systematic deviations are noticed for both WDS and EDS. The error in concentration of the quantitative data using elemental Mo- and Si-standards probably originates from the difference in relative absorption between, on the one hand, an element in solid solution, and on the other the same element in an intermetallic compound such as Mo5Si3, which has a high degree of covalent bonding. Therefore, systematic deviations will occur if elemental standards are used for a compound. It can thus be concluded that the internal standards used for quantitative EDS should be used with caution in the Mo–Si system. 3.3. Solubility range in Mo5Si3 In this study, the Mo5Si3 phase has a homogeneity range of 36.1 to 39.3 at.% Si in the quenched parts of

Fig. 4. BSI of the first solidified parts of Si-deficient sample M1-6 consisting of matrix phase Mo5Si3 and Mo3Si (bright contrast).

the ingot close to the water-cooled Cu-mould, while the range for the samples annealed at 1600 8C is 36.7 to 38.8 at.% Si. The Si-concentrations in the investigated samples are shown in Fig. 5, which equals a solubility interval of 3.2 at.% and 2.1 at.% at ambient and 1600 8C, respectively. The latter is in good agreement with the value of 1.4 at.% at 1800 8C reported by Huebsch et al. [8], bearing in mind that the annealing temperature is slightly lower in this work. According to the binary Mo–Si phase diagram in Fig. 1, Mo5Si3 has a homogeneity range of approximately 37.2–40 at.% Si at 1600 8C. So, the solubility range of Mo5Si3 in this work is in good agreement with the phase diagram, although there is a deviation in the absolute concentration. As seen in Fig. 2, EDS data yielded lower Si content compared to data obtained from conventional bulk chemical analysis in this work. This may explain the deviation in absolute Si-concentration of the EDS-analyses compared to the phase diagram. 3.4. Mechanical properties The microindentation hardness values obtained using a 500 g load, are plotted against alloy composition, as determined by EDS, in Fig. 6. It can be seen that both the hypostoichiometric sample MA1-6 ( Fig. 7) and the hyperstoichiometric sample MA1-1 (Fig. 8)

Fig. 5. Concentration of Si in the Mo5Si3 phase as a function of alloy composition and annealing temperature. Data for 0 8C denote the quenched parts of respective ingot. The samples M1-6 through M1-1 are listed in Table 1.

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Fig. 6. Vickers hardness of Mo5Si3 as a function of composition. Compositions were obtained from EDS measurements. The samples MA1-6 through MA1-1 are listed in Table 1.

display lower hardness than the Mo 5 Si 3 standard sample. Although the indents are deliberately directed in the Mo5Si3 areas of the two-phase alloys MA1-1 and MA1-6, second phase particles beneath the surface are presumably included in the measured values. In the case for MA1-1, where MoSi2 is second phase, the slight drop in hardness can be attributed to the fact that MoSi2 has lower hardness (~900 kg/mm2 or ~8.8 GPa [11]) than Mo5Si3 (11.7–13.0 GPa [6]). On the contrary, Mo3Si has a hardness (~1320 kg/mm2 or ~12.9 GPa [20]) similar to Mo5Si3. Thus, the rather large drop in hardness of MA1-6 cannot be attributed to the presence of Mo3Si second phase particles, but rather to the considerable amount of microcracks present, as

seen in Fig. 7. The majority of these cracks seem to be oriented perpendicular to the direction of the Mo3Si particles in Fig. 7, and are believed to result from the CTE-anisotropy as mentioned previously. Hyperstoichiometric Mo5Si3, on the other hand, seems to exhibit less CTE-associated cracking, as seen from Fig. 8. Also, its hardness is closer to that of the slightly hyperstoichiometric Mo5Si3 standard sample in this study, see Fig. 6. The toughness values obtained from Vickers indentations are plotted against composition in Fig. 9. According to this figure, Mo5Si3 has its maximum in toughness near the stoichiometric composition. The hyperstoichiometric Mo5Si3 sample (MA1-1) displays

Fig. 7. Hypostoichiometric Mo5Si3 (MA1-6) consisting of matrix phase Mo5Si3 and Mo3Si (bright contrast).

Fig. 8. Hyperstoichiometric Mo5Si3 (MA1-1) consisting of the phases Mo5Si3 (bright contrast) and MoSi2.

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Fig. 9. Indentation toughness of Mo5Si3 as a function of composition. Compositions were obtained from EDS. The samples MA1-6 through MA1-1 are listed in Table 1.

the lowest toughness in this study. As can be noted in Fig. 8, two of the corner cracks are much longer and seem to have grown perpendicular to the direction of the second phase, which is similar to the aforementioned cracks in MA1-6. The same behaviour is noted for MA1-6; the longest indent cracks are perpendicular to the second phase, see Fig. 7. These cracks also hinder propagation of indent cracks, which may explain the higher toughness value compared to hyperstoichiometric Mo5Si3. Chu et al. report on CTE-mismatch, hardness, and toughness, respectively, of 2.21, ~1150–1280 kg/mm2 (i.e. ~11.7–13.0 GPa) and 2–2.5 MPa m1/2, respectively, for their single crystal sample of measured composition 62.92 Mo–37.08 Si (at.%) [6]. The reported ˚ and lattice parameters in that study were a = 9.59 A ˚ for a sample having composition 62.75 Mo– c = 4.87 A

37.25 Si (at.%) [6]. Schneibel et al. report on a CTEmismatch of 2.0 for their sample, but do not report on measured composition [21]. However, the lattice para˚ and c = 4.9082 A ˚, meters are given as a = 9.6429 A respectively [21], which are close to the values measured on the single-phase sample in this study, see Table 4. Nørlund Christensen reported on slightly larger ˚ and c = 4.911(1) A ˚ lattice parameters of a = 9.650(2) A [4]. Based on these data, it is assumed that the singlephase sample in this work has CTE-mismatch close to the value reported by Schneibel et al. [21]. Furthermore, this study indicates that the CTE-anisotropy in Mo5Si3 is dependent on composition. When comparing Figs. 7 and 8, it can be seen that the latter contains fewer cracks than the former. Hence, the sample containing small amounts of MoSi2 second phase could potentially have a CTE-mismatch below 2.0. Similar

Table 4 Refined parameters of Mo5Si3 standard ˚] a [A

˚] c [A

Space group

R B [%]

R exp [%]

R wp [%]

v2

9.6422(3)

4.9059(2)

I4/mcm (140)

5.29

4.50

7.38

2.70

Atom

Wyckoff position

X

Y

Z

˚ 2] B iso [A

Occupancy

Mo Mo Si Si

16k 4b 8h 4a

0.0769(2) 0 0.01664(3) 0

0.2238(2) 1/2 0.6664(3) 0

0 1/4 0 1/4

0.15(4) 0.00(6) 0.22(6) 0.38(8)

16 3.87 8 4

X, Y, and Z are fractional coordinates; v 2 , R B, and R wp are reliability factors; B iso are the isotropic temperature factors. Wyckoff positions are illustrated in Fig. 11. Estimated standard deviations are written in parentheses. R B = A |I obs  I calc| / A I obs; R wp = {A w( y obs  y calc)2 / A w( y obs)2 ;}1/2 ; R exp = {(N  P) / A w( y obs)2 }1/2 ; v 2 = (R wp / R exp)2 ; N= number of points in the pattern; P= number of refined parameters.

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Fig. 10. NPD data and best-fit Rietveld refinement of Mo5Si3 standard at 293 K. Open circles correspond to the observed data and the solid line corresponds to the calculated pattern. Tick marks below the pattern indicate the positions of the allowed Bragg reflections. At the very bottom the difference profile between observed and calculated data is shown.

observations are confirmed in a recent study by Zhao et al. [22]. In their work, the Mo5Si3 phase displayed a CTE-mismatch of 2.15 for a sample having composition 65.4 Mo–34.6 Si (at.%), i.e. close to the composition of the Mo-rich sample MA1-6 in this study. For their Mo-deficient sample, 51.0 Mo–49.0 Si (at.%), Zhao et al. measured a CTE-mismatch in the Mo5Si3 –phase of 1.83, i.e. considerably lower than previously reported values of 2.0–2.2 [5,21].

3.5. Refinement of neutron powder diffraction data The sample closest to stoichiometry in this study, denoted Mo5Si3 standard in Table 1, displayed the highest hardness and toughness. This sample was refined using the Rietveld method on data obtained from NPD experiments. Fig. 10 shows the observed and calculated data together with difference profile from the refinement. Table 4 lists lattice parameters, frac-

Fig. 11. The D8m unit cell of Mo5Si3, showing the 16k and 4b transition metal sites and 8 h and 4a Si sites, respectively.

410

E. Stro¨m / Materials Characterization 55 (2005) 402–411

tional coordinates and site occupancies together with residual value of the refinement. The occupancies of Si were set as 8 and 4 for the 8h and 4a sites, respectively, during the refinement as this resulted in the best Rietveld fit. In the case of Mo, the best fit of calculated data was obtained for the model where Mo partial vacancies on the 4b chain site of the D8m-structured unit cell of Mo5Si3, displayed in Fig. 11, are present. This is inconsistent if compared to the composition as measured by EDS, c.f. Table 3. However, according to the results from bulk chemical analysis in Table 2, the Mo5Si3 standard sample has normalised composition 61.5 Mo–38.5 Si (at.%). Based on the refined data in Table 4, the composition of Mo5Si3 standard would be 62.1 Mo–37.9 Si (at.%), which is in good agreement with the data obtained from bulk chemical analysis. This result, together with the observed density of microcracks for the three samples used in this study, indicates that absence of 4b Mo atoms has a beneficial effect on the thermal anisotropy of Mo5Si3. Zhao et al. [22] confirm the present results as they suggest that Mo vacancies on the 4b site result in stretching of the Mo– Mo bonds along the c-axis, thereby decreasing the thermal expansion in the c-direction of Mo-deficient Mo5Si3. 4. Summary The solubility range in Mo5Si3 has been investigated by means of EDS measurements in a SEM. A standard sample consisting of single-phase Mo5Si3 has been used as a reference during the EDS measurements of Mo3Si– Mo5Si3 and Mo5Si3–MoSi2 two-phase alloys, i.e. compositions on each side of Mo5Si3 in the Mo–Si binary system. The results have been compared with those obtained using elemental standards in EDS as well as from bulk chemical analysis data. Hardness and indentation toughness of Mo5Si3 as a function of composition have been measured by Vickers indentation on the three samples. Finally, the single phase sample has been refined using the Rietveld method on NPD data. The important findings in our study may be summarized as follows: ! The total yield in the EDS measurements was close to 100 wt.% when utilising a Mo5Si3 standard sample of composition measured from bulk chemical analysis. The precision in the measurements was 0.07 at.% or better. ! Using Mo and Si elemental standards resulted in a total yield of less than 75 wt.%. The underestimation in Si content was more pronounced than the Mo

content, resulting in Si contents lower than the 37.5 at.% expected from stoichiometry. ! A solubility interval of 2.1 at.% Si in Mo5Si3 was measured by EDS on two-phase samples heat-treated at 1600 8C, which is in good agreement with the Mo– Si phase diagram. For the rapidly cooled parts of ascast ingots, the interval was 3.2 at.% Si in Mo5Si3. ! Based on the three methods for elemental analysis used in this work, i.e. bulk chemical analysis, EDS, and Rietveld refinement of NPD data, it is concluded that EDS yields an underestimation of Si-content in Mo5Si3 compared to bulk chemical analysis. As a result, the homogeneity range of Mo5Si3 is displaced to lower Si-content compared to the Mo–Si binary phase diagram. Furthermore, the composition resulting from Rietveld refinement of NPD data is in better agreement with bulk chemical data, and thus with the Mo–Si phase diagram, than with data obtained from EDS. ! Both hardness and toughness of Mo5Si3 displayed their highest values for the composition closest to stoichiometry. The hypostoichiometric sample, containing Mo3Si, had the lowest hardness among the tested samples. This is probably due to the high density of microcracks in that sample. The hyperstoichiometric sample, having some MoSi2 as second phase, was almost as hard as the near-stoichiometric sample, but displayed much lower toughness. Also, the latter sample contained the lowest amount of microcracks, while the most Si-deficient sample contained the highest amount of microcracks. It is thus indicated that the CTE-anisotropy in Mo5Si3 is dependent on composition and can be reduced by increasing the Si content. Based on refinement results, it is further indicated that the Mo 4b chain sites play an important role in the thermal behaviour of Mo5Si3. Acknowledgements This work was supported by the Ministry of Science and Technology of China and Swedish Vinnova. International Secretariat at Chalmers, Sweden, and the National Nature Science Foundation of China under grant 59881004 are also thanked for their support. Discussions with Sten Eriksson, Uta Klement and Maria Knutson-Wedel are appreciated. References [1] Massalski TB, editor. Binary alloy phase diagrams. Metals Park, OH, USA7 American Society for Metals; 1990. p. 2664. [2] JCPDS data 34-0371.

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