Single-phase interdiffusion in B2-RuAl intermetallic compound

Scripta Materialia 57 (2007) 1–4 www.elsevier.com/locate/scriptamat

Single-phase interdiffusion in B2-RuAl intermetallic compound K. Woll,a C. Holzapfel,b H.A. Gobrana and F. Mu¨cklicha,* a

Functional Materials, Department of Materials Science, Saarland University, Saarbru¨cken D-66041,Germany b Schleifring und Apparatebau GmbH, 82256 Fu¨rstenfeldbruck, Germany Received 26 January 2007; revised 15 March 2007; accepted 16 March 2007 Available online 16 April 2007

The interdiffusion behaviour of single-phase B2-type RuAl was investigated in the temperature range between 1473 and 1773 K. An Arrhenius behaviour is observed with an activation energy of Q = (236 ± 18) kJ mol1. Variations in DRu–Al due to composition effects were found to be small.  2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Diffusion; Intermetallic compound; RuAl

The intermetallic compound RuAl appears as a new material for high temperature applications due to its enhanced room temperature ductility [1–3], high expected creep resistance by reason of its high melting point (Tm  2333 K) and extraordinary oxidation resistance [4,5]. In addition, its thermal expansion coefficient is nearly equal to that of Al2O3 [6], which predestines RuAl for applications that demand protection from oxidation. To understand creep and oxidation, a detailed knowledge of the interdiffusion behaviour is essential as both of these processes are controlled by diffusion. Moreover, interdiffusion data are the basis for the description of phase transformations, grain growth and recrystallization. Thus, the determination of the diffusion coefficients is motivated by both a fundamental and a practical point of view. Due to the ordered structure, diffusion of intermetallic compounds is much more complex and has to be distinguished from that of pure metals or alloys. Within the B2-aluminides many investigations have been performed on NiAl to elucidate both the microscopic diffusion mechanisms [7–11] and the macroscopic interdiffusion behaviour [12–15]. It can be concluded that highly correlated diffusion mechanisms, e.g. cyclic mechanisms, are responsible for the transport of atoms through the lattice on the microscopic scale. On macroscopic scale, the influences of temperature as well as composition become of interest. Interdiffusion experiments on NiAl showed an Arrhenius-like temperature dependence that is characterized * Corresponding author. Tel.: +49 681 302 2048; fax: +49 681 302 4876; e-mail: [email protected]

by an activation energy of about 358 kJ mol1 [14]. The correlation of the interdiffusion coefficient with composition shows a minimum value at the stoichiometry [12]. In contrast to the investigations concerning NiAl, diffusion experiments on single-phase RuAl have never been performed before, neither on a microscopic nor a macroscopic scale. To determine the interdiffusion coefficient, we use the established diffusion couple technique. The purpose of this paper is the exact determination of the interdiffusion coefficient and the analysis of the influence of temperature on interdiffusivity. RuAl powder mixtures with nominal compositions of 75, 76 and 78 wt.% Ru (44.47, 45.81 and 48.63 at.% Ru) were cold pressed at 750 MPa and then reactive sintered in a uniaxial hot press applying a pressure of 264 MPa. Samples were then annealed for homogenization at 1823 K for 36 h to obtain single-phase RuAl. Further details can be found in Ref. [16]. The samples were cylindrical, with a diameter of 10 mm. Their length was at least 4 mm. The top surfaces of each specimen were mechanically ground and polished down to 1 lm. Before clamping together to obtain a diffusion couple, all specimens were analysed with regard to homgeneity of concentration, grain size and porosity to verify their validity as end members. The homogeneity was proofed by energy-dispersive spectroscopy (EDS) measurements. To obtain information concerning the grain size of RuAl, electron backscattered diffraction analysis was performed for the first time. Porosity measurements were conducted using quantitative image analysis. Diffusion experiments were then carried out in vacuum (<105 mbar) between 1473 and 1773 K. The temperatures and times for each experiment are given in Table 1.

1359-6462/$ - see front matter  2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.03.028

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K. Woll et al. / Scripta Materialia 57 (2007) 1–4

Table 1. Experimental conditions of the diffusion experiments, length of the diffusion zone X = 2(Dt)1/2 and interdiffusion coefficients D of RuAl Temperature (K)

Time t (h)

X (lm)

log(D) (m2/s)

1473

96

175.67 ± 35.78 143.99 ± 41.86 142.35 ± 36.70

13.65 ± 0.16 13.83 ± 0.14 13.83 ± 0.19

1573

30

114.47 ± 6.97 113.32 ± 4.85

13.52 ± 0.06 13.52 ± 0.04

1673

10 48

129.53 ± 22.09 255.57 ± 14.30

13.02 ± 0.04 12.93 ± 0.13

1773

4

115.37 ± 6.18 129.49 ± 10.38

12.64 ± 0.05 12.53 ± 0.06

After diffusion annealing, the diffusion couples were cut perpendicular to the welded interface and were subsequently prepared for diffusion profile measurements. The variation of chemical composition perpendicular to the interface between the end members of each diffusion couple was examined using a Dual Beam Workstation (Strata DB 235, FEI) with an acceleration voltage of 15 kV. The concentration vs. distance profiles were obtained performing standardless EDS by point-bypoint analysis with a constant point distance, which ranged between 5 and 15 lm. In order to improve statistics, the live time at every point was chosen to be 150 s [17]. The intensities of Ru La and Al Ka lines were quantified and converted to concentrations with appropriate ZAF corrections [17]. At least two profiles per diffusion couple were determined. Application of the powder metallurgy technique is the only way of synthesizing RuAl with a single-phase microstructure. Therefore, porosity and a polycrystalline microstructure have to be accepted in favour of single-phase samples. However, the chosen process parameters lead to microstructures with a maximal degree of homogeneity, grain size and density. Nonetheless, estimations concerning these microstructural parameters are necessary. The homogeneity of every sample was revealed using the constraints according to Ref. [17]. The average grain size was determined to be 6 lm. Long time tests at 2073 K result in no significant increase of the grain size. Hence, this grain size has to be accepted. Nevertheless, studies concerning the maximal effect of grain boundary diffusion must be carried out. Using the optimal parameters to obtain samples with maximal density [16], the porosity was verified to be 5% at maximum. Previous diffusion experiments on samples that were synthesized by powder metallurgy [18–20] accepted a porosity of 5%. Thus we assume this value to be adequate for the present study. Figure 1 depicts a typical diffusion profile, normalized to the initial end member compositions of 50.3 and 53.2 at.% Al. The diffusion couple was annealed for 4 h at 1773 K. Comparing the end member concentrations of the diffusion couples with those of the phase boundaries, it is clear that the diffusion profile spans nearly the complete composition range of RuAl. The diffusion length was always about one order of magnitude higher compared with values where convolution

Figure 1. Diffusion profile developed at 1773 K after 4 h, normalized to the initial end member compositions (50.3 and 53.2 at.% Al, respectively). The symmetrical fit is plotted. A slight asymmetry is visible. The inset shows the composition dependence of D on a semilogarithmical scale.

effects between the interaction volume of the electron beam and the profile curvature become significant [21]. The ordering energy and the associated point defect structure that compensates deviations from stoichiometry strongly dictate which mechanism dominates the atomic transport [22]. Hence, an influence of composition on the macroscopic diffusion coefficient is commonly found. For example, the interdiffusion coefficient of NiAl varies with composition within several orders of magnitude (in the composition range of nearly 10 at.% Al) [12–14]. Generally, composition effects are indicated by asymmetric diffusion profiles [13,14,23,24]. This behaviour is characterized in Figure 1 by a slightly steeper slope of the profile at the right end of the interdiffusion zone than that at the left end. Boltzmann–Matano analysis (BMA) [23,24] was performed to study this effect. Due to the small difference between the extreme concentrations of each couple (3 at.% Al), the exact investigation is difficult and produces uncertain results. This uncertainty was estimated (see the inset in Fig. 1). To check the magnitude of the diffusivity, symmetrical fitting was also performed using an analytical solution of Fick’s second law for infinite boundary conditions [23,24]. The symmetrical solution implies that the interdiffusion coefficient D is independent from concentration [23,24]. Thus, D values determined in this manner represent the interdiffusivity at the average composition of the diffusion couple. Figure 1 demonstrates that the symmetrical solution fits the measured points very well with respect to the data scatter. Comparing the results of the BMA with those of the symmetrical fit (D = const.), it is obvious that D at the average composition (Dav) is nearly equal for both of the approximations, keeping the errors in mind (see Fig. 1, inset). Thus, the two independent profile analyses describe the interdiffusion behaviour correctly. The coefficients that are determined under the assumption of a constant D were proven to be valid for the average composition of the diffusion couple. This strongly corroborates that the order of magnitude for D was correctly defined by the present study. The investigations also show a variation of D from Dav to D at

K. Woll et al. / Scripta Materialia 57 (2007) 1–4

Figure 2. Logarithm of the interdiffusion coefficient as a function of inverse temperature. Individual data points and respective errors are values averaged from Table 1. The dashed lines denote the 95% confidence interval.

the extreme concentrations by a factor of 2.5. However, the relatively large errors due to the small composition variation prevent the development of a more detailed model. Hence, Table 1 summarizes the results of symmetrical fitting (D = const.). It should be emphasized that these diffusion coefficients are valid for average compositions. The time row experiment at 1673 K revealed the time independence of D. This clearly identifies diffusion controlled processes [24,25]. Also, heating rate effects on D could be excluded. The influence of temperature can be studied by plotting the interdiffusion coefficients in an Arrhenius diagram (see Fig. 2). Each data point and its error are averaged values from those summarized in Table 1 at each individual temperature. Figure 2 clearly shows that all the data with the respective error fall within the 95% confidence interval. Consequently, Arrhenius behaviour is demonstrated. From the linear fit the activation energy Q and the frequency factor D0 were determined to be 236(±18) kJ mol1 and 2.2(±1.6) · 106 m2 s1, respectively. Hence, the Ru–Al interdiffusion coefficient of the RuAl phase can be described by the following Arrhenius equation (for average compositions): D ¼ 2:2ð1:6Þ  106

236; 000ð18; 000Þ m2  exp  s 8:314  T ðKÞ

3

Figure 3. Arrhenius plot of interdiffusion coefficients for stoichiometric composition of CoAl [14], FeAl [14], NiAl [14] and RuAl [this work], respectively.

Taking the melting point of FeAl (Tm = 1583 K) into account, temperatures above 1473 K correspond to very high homologous temperatures (Thom = T/Tm > 0.93), whereas for NiAl, CoAl and RuAl Thom is >0.77, >0.77 and >0.63, respectively. Diffusivity in general is affected by the distance to Tm [23]. Consequently, when investigating the relative diffusivity of different intermetallic compounds, we suggest the comparison of the interdiffusion coefficients on the homologous temperature scale. Figure 4 compares D of the four intermetallics mentioned above at stoichiometric composition in the homologous temperature range between 0.5 and 1. The interdiffusion in RuAl is fastest and exceeds that of NiAl and FeAl, whereas in the absolute temperature scale the situation is reversed (Fig. 3). In addition, at Thom = 1, the diffusion coefficients only vary between 3 · 1011 and 6 · 1013 m2 s1 considering FeAl and CoAl for the upper and lower limits, respectively (see Fig. 4). Hence, the values of interdiffusion coefficients of B2-aluminides cluster in a very narrow range at Tm

J mol

Figure 3 summarizes the interdiffusion coefficients above 1450 K for stoichiometric compositions of different B2aluminides in an Arrhenius diagram. Data for the activation energies and frequency factors are taken from Ref. [14]. According to the diagram, the interdiffusivity of RuAl is in the same order of magnitude as that of NiAl and about one order of magnitude higher than that of CoAl. However, RuAl is characterized by an interdiffusion coefficient that is three orders of magnitude lower than that of FeAl. Morevoer, the temperature dependence of the interdiffusion behaviour is lower for RuAl due to the decreased slope of the RuAl line compared with that of NiAl, CoAl and FeAl. D of RuAl varies within one order of magnitude in the considered temperature range, whereas that of CoAl and NiAl increases by five and three orders of magnitude, respectively.

Figure 4. Logarithm of the interdiffusion coefficients at the stoichiometric composition of CoAl [14], NiAl [14], FeAl [14] and RuAl [this work] as a function of the inverse homologous temperature Thom. The experimentally investigated temperature range is also shown. According to Ref. [26], the interval at Tm where the values of self-diffusion coefficients for bcc transition metals cluster is denoted.

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(the difference is in the order of a factor of 50). This observation is consistent with those of other studies [23,26] which determined that the self-diffusion coefficient at Tm should be nearly constant for a given crystal structure and type of bond. According to Brown and Ashby [26], the self-diffusion coefficient of transition metals with a body-centred cubic (bcc) structure at Tm ranges between 1011 and 5 · 1012 m2 s1. These values are very similar to the range mentioned above. Consequently, the self-diffusional behaviour of B2-intermetallic aluminides is similar to that of transition metals with a bcc structure [27]. Regarding the extrapolation of the Arrhenius plot for RuAl to the melting point, the value of D clearly falls well within this range. This strongly corroborates our experimental findings. Using Girifalco’s theory, which correlates the activation energy in an ordered structure QO to the long-range order parameter S (QO  S2) [28,29], it is generally possible to estimate the interdiffusion coefficient of a B2intermetallic aluminide knowing only its state of order. Thus, increasing the state of order results in an increase in the activation energies (i.e. a steeper slope in the Arrhenius plot). Classifying intermetallics in strongly, intermediately and weakly ordered phases (with respect to their ordering energy), it can be qualitatively concluded that intermetallics that originate in the group of strongly ordered phases (e.g. NiAl [30,31] and CoAl [30]) generally show a steeper slope in the Arrhenius plot compared with those that are grouped within the intermediately ordered phases (e.g. FeAl [30,32] and RuAl [33]). The values of the intermediately ordered compound RuAl are consistent with this trend (see Fig. 4). Also, the maximum effect of grain boundary diffusion on the measured interdiffusion coefficients was estimated using the following assumptions: grain boundary diffusion takes place in the A-regime according to Harrison [34]; the grain boundary diffusion coefficients are greater than those for pure volume diffusion by four orders of magnitude [22] (constant over the investigated temperature range); and the average grain size is 6 lm. Using Hart’s relation [22,35], the volume diffusion coefficient DV can be estimated. Then, the relation (DV · t)1/2 > d can be verified, where t denotes the diffusion time and d the grain size. This justifies the assumption of grain boundary diffusion corresponding to A-regime kinetics [22,34]. Finally, it can be shown that the measured diffusion coefficients in this study overestimate the real volume interdiffusion coefficients by a factor of 2 at most. In conclusion, in this study we were able to measure for the first time Ru–Al interdiffusion coefficients in B2-structured intermetallic RuAl. Over the temperature range investigated only a single diffusion mechanism occurred, and an Arrhenius behaviour was found with D0 = 2.2(±1.6) · 106 m2 s1 and Q = 236(±18) kJ mol1. Due to the small composition range, the effect of composition is small and average Ds were used in further evaluations. Although the grain size is about 6 lm in the polycrystalline diffusion couples, only insignificant contributions of grain boundary diffusion are present. The present investigations were supported by funds of the Deutsche Forschungsgemeinschaft (Project: Mu

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