Alloying elements partitioning in TiAl-Ru intermetallic alloys

Intermetallics 7 (1999) 1283±1290

Alloying elements partitioning in TiAl-Ru intermetallic alloys S. Kim1, D.J. Larson2, G.D.W. Smith* Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK Received 11 March 1999; accepted 22 April 1999

Abstract Ruthenium additions are of interest for increasing low-pH resistance as well as improving mechanical properties of TiAl. Two phase Ti-48 at% Al-0.5 at% Ru was investigated. The partitioning behaviour of alloying elements in this condition was investigated by atom-probe ®eld-ion microscopy (AP-FIM). Ruthenium was found to be depleted in 2 and interstitial elements were localised in the 2 phase. The degree of localisation for interstitials is much more signi®cant than that for substitutional elements. The ruthenium partitioning behaviour resembles that of niobium. No evidence was found for the segregation of ruthenium, oxygen or carbon to 2/ interfaces. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Titanium aluminides, based on TiAl; A. Ternary alloy systems; D. Site occupancy

1. Introduction The recent emphasis on development of light-weight and high-temperature alloys has generated renewed interest in titanium aluminides based on the phase because they exhibit excellent high-temperature properties such as high strength/density ratio, high modulus, good oxidation resistance and good creep/stress rupture properties [1,2]. The hurdle to overcome for commercial use is poor ductility at ambient temperatures. It is well known that interstitial impurities play an important role in determining the ductility of TiAl alloys [3]. Extensive research has been performed to overcome this low ductility without deterioration of high temperature properties, by microstructure and composition modi®cation [3]. Two-phase TiAl alloys containing around 48 at% aluminium have been found to exhibit higher ductility than single-phase alloys. However, TiAl alloys are also known to be susceptible to cracking in low pH environments [4±6]. The response of these materials in environments containing hydrogen is of interest, * Corresponding author Tel.: +44-(0)1865 273762; fax: +44(0)1865-273789. E-mail address: [email protected] (G.D.W. Smith) 1 Current address: Department of Materials Engineering, Monash University, Clayton, 3168, Victoria, Australia. 2 Current address: Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.

partly because of the possibility of future hydrogenfuelled aircraft, and a number of studies have been performed on the e€ects of hydrogen during the past few years [7±9]. In order to minimise these drawbacks, b-stabilising elements such as Mo, Nb, V, Zr, Mn, and Cr have been explored as alloying elements [10]. Platinum-group metals (PGMs), however, have not been used until recently as alloy additions in TiAl intermetallic alloys for improving environmental and mechanical properties [11±13]. These have been used in titanium metallurgy for the improvement of corrosion resistance and as oxidation resistant coatings. In addition, PGMs have also been used as alloying additions in steels and nickel aluminides in order to enhance environmental and mechanical properties [14±16] Among PGMs, Pd has long been used in titanium alloys, since Pd additions result in outstanding resistance in low pH environments. Ru is, however, also attractive because of its relatively low cost compared to Pd [17], while providing titanium alloys with improved strength and hardness [18]. Recent research on crystalline parameters and site occupancy of TiAl alloyed with Ru also suggested ductility improvement [19,20]. Understanding of the distribution of alloying elements between phases is essential as a ®rst step toward the systematic knowledge needed for identifying various metallurgical mechanisms. The atom-probe ®eld-ion microscope is a unique tool for accurate measurement

0966-9795/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00048-5

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of phase chemistry at the atomic scale [21] Some atom probe studies have recently been made on titanium aluminides, in particular, examining the distribution of Mn or Cr and of interstitial elements [22±24]. Research performed on titanium aluminides using APFIM has been reviewed by Larson and Miller [25]. In this paper, the result of atom probe ®eld ion microscopy characterisation of two phase TiAl alloys with ruthenium addition is discussed, focussing on the solute partitioning e€ects of alloying elements and impurities between phases.

for the second stage. Both electropolishing stages were performed at room temperature. APFIM investigations were carried out using the Energy Compensated Optical Position Sensitive Atom Probe (ECOPoSAP) instrument developed at Oxford University [27]. The experimental conditions were as follows; the imaging gas for FIM observation was neon with a pressure of 3  10ÿ3 Pa, reduced to a vacuum of a few 10ÿ9 Pa during atom probe analysis, with a tip temperature of 60 K and a pulse fraction of 18%. 3. Results

2. Experimental The present investigation was carried out on Ti-48 at%Al-0.5 at%Ru (Percentages are at% unless otherwise mentioned) which consists of two phases, 2 and , with total impurity levels of 1500±2000 at ppm O and 500±600 at ppm C. The alloys were made by plasma arc melting. Titanium sponge (99.8% in purity) and aluminium were melted in a water cooled copper crucible under an argon atmosphere, and remelted at least ®ve times to guarantee homogenisation of the alloys. The material was analysed by X-ray di€raction (XRD) of heat-treated specimens, which con®rmed that there were no phases detectable other than 2 and . The nominal and actual compositions of the alloy investigated are shown in Table 1. In order to obtain the microstructure of interest, the ingots were homogenised at 1273 K for 12 h, solution treated at 1673 K for 2 h, then directly cooled to 1173 K and aged for 6 h followed by furnace cooling. All the samples were encapsulated in quartz tubes that were evacuated to 110ÿ3 Pa before being inserted into the heat treatment furnace. After this heat treatment, the so-called fully lamellar (FL) scheme [26], the resultant microstructure would be expected to consist of 2+ laths. After heat treatment, the outer few millimeters of the samples were ground away from all surfaces in order to remove any contamination layers. In order to perform APFIM experiments, heat treated samples were cut into 0.50.512 mm bars, with a diamond saw. Atom-probe specimens were fabricated by the standard two-stage electropolishing procedure, using 5% perchloric acid and 95% acetic acid solution at 30 V d.c. during the ®rst stage and ®nally in 1% perchloric acid and 99% acetic acid solution at 10 V d.c.

Examination of the macrostructure of the ingots con®rmed that there was no severe segregation in the ascast structure. The heat treated materials exhibit a fully transformed lamellar microstructure as shown in Fig. 1. Field ion images of 2/ (phase boundaries are shown in Fig. 2(a) and (b). This interface is planar and very smooth on an atomic scale as observed previously in Crcontaining alloys [22]. In some of the samples in the FIM investigation, only or 2 phase was observed due to the relatively large thickness of the lamellae in relation to the area of analysis in the FIM. Selected volume analyses were performed on several samples which contained 2/ interfaces. An example of an atom probe analysis from a selected area is shown in Figs. 3±6. Fig. 3 shows ®eld-ion micrographs before and after the analysis. The apparently constant position of the boundary indicates that its plane was aligned approximately along the specimen axis. This was con®rmed by the isosurface construction [21] based upon the isoconcentration level of 40 at%Al, clearly indicating that the 2/ interface is nearly parallel to the specimen axis as shown in Fig. 4. ECOPoSAP mass spectra for both and 2 phases are shown in Fig. 5(a) and (b) respectively. Each mass spectrum is constructed based upon approximately the same number of ions detected (150 000‹5000 ions).

Table 1 Composition of TiAl-Ru alloy studied Composition

Ti

Al

Ru

Oa

Ca

at% wt%

51.5 64.7

47.9 33.9

0.5 1.3

1310 550

410 130

a

ppm.

Fig. 1. Microstructure of Ti-48at%Al-0.5at%Ru after heat treatment.

S. Kim et al. / Intermetallics 7 (1999) 1283±1290

Fig. 2. Field ion micrographs of interphase lamellar boundaries in TiAl-0.5Ru showing various lamellar thickness. All images taken at the temperature of 60 K with voltages of (a) 17 kV and (b) 15 kV.

Titanium and aluminium were mainly detected as doubly charged ions and ruthenium was also detected doubly charged. No convolution problem arose between detected substitutional alloying species as had been experienced previously in the analysis of TiAl-2Cr [22]. Since no Ti+1 peaks are detected in TiAl alloys, possible convolution of mass peaks of 96Ru2+, 98Ru2+ and 100 Ru2+ with mass peaks of 48Ti+, 49Ti+ and 50Ti+, respectively, is avoided. As for the case of other TiAlbased systems, analysis of interstitial elements in TiAl0.5Ru presents a minor diculty in the determination of oxygen content. The oxygen atom peak 16O+ at 16 amu has the same mass-to-charge value as the titanium atoms detected as 48Ti3+. Experimentally, it is found that the proportion of 48Ti3+ ions is in good agreement with the natural abundance of this titanium isotope.

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Fig. 3. Field ion image of interphase boundary (a) before and (b) after atom probe analysis.

Thus the extent of 16O+ formation is assumed to be very small. Most of the detected oxygen atoms are in the state of TiO2+ ions [22,24]. This may be interpreted as due to the tight binding between titanium and oxygen. These TiO2+ peaks are expected to partly overlap with Ru3+, if this species is present. The mass-to-charge ratio (AMU) range expected for TiO2+ is 31±33 while that for Ru3+ is 32±34.66. However, the most abundant Ru3+ isotope peaks (100Ru3+, 101Ru3+, 102Ru3+, and 103 Ru3+) would not overlap with TiO2+ peaks, and these Ru3+ species were not detected. Thus essentially all of the ruthenium atoms are ®eld evaporated as Ru2+ ions, and are detected unambiguously. Since the mass spectra shown in Fig. 5 are based on roughly equal numbers of ions, it is possible to demonstrate the localisation of elements in each phase. The number of oxygen ions coming from the 2 phase is

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Fig. 4. Isosurface construction based on the concentration level of 40at%Al, indicating boundary nearly parallel to the specimen axis.

Fig. 5. Atom probe mass spectra from the Ti-48at%Al-0.5at%Ru alloy studied for (a) phase and (b) 2 phase.

much higher than from the phase. Conversely, the number of ruthenium ions detected in the phase is higher than in the 2 phase. Enlarged sections of the TiO2+ and Ru2+ peaks are also illustrated in Fig. 5, showing clear evidence of higher peaks of oxygen ions in the 2 phase and the higher peaks of ruthenium in the

phase. The ratio of peak intensities between the phases is much higher for oxygen than for ruthenium. Nitrogen atom distributions could not be determined, because of the peak overlap between N+ and AlH2+ ions at 14 amu. Carbon atoms were mainly detected as C+. Table 2 summarises the data on element concentrations in the and 2 phases, and atom-map reconstructions concerning solution partitioning for alloying element are illustrated in Fig. 6. The substitutional (Ru) element analysed in the present investigation was found to localise in the phase while the interstitial elements (C, O) were found to localise in the 2 phase. It is worthwhile to note that no evidence was found of segregation of either substitutional or interstitial elements to the 2/ interface. This is consistent with a previous report on Cr-alloyed TiAl [22]. Concentration pro®les of alloying elements across the interphase boundary for the selected volume indicated in Fig. 6(a) are shown in Fig. 7. Concentrations are shown for successive 0.1 nm slices of material, over a total distance of 20 nm. Both Figs. 6 and 7 indicate no obvious segregation or depletion at the boundary. The volume fraction, F2i of the 2 phase can also be calculated from the atom probe results in conjunction with the overall bulk concentration values for each alloying element. The results of this calculation are presented in Table 3. The volume fractions of 2 phase estimated from atom probe analysis for the di€erent elements in Ti-48Al-0.5Ru alloy are broadly consistent with each other. This suggests that the alloy studied in the present work is largely devoid of any phases other than 2 and .

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Fig. 6. Reconstructed atom maps of solute partitioning for selected area for analysis: (a) FIM image, (b) ruthenium, (c) oxygen and (d) carbon atoms in Ti-48at%Al-0.5at%Ru alloy studied.

4. Discussion 4.1. Ruthenium distribution The partitioning coecients for the alloying elements in TiAl alloys determined from the present investigation are shown in Table 4. The partitioning coecient can be de®ned as k2 ˆ C2 =C for the 2 phase partitioned elements, and as k ˆ C =C2 for the phase partitioned elements, respectively. Unlike the interstitial elements, ruthenium was observed to localise in the phase. The partitioning ratio of about 1.75 is, however, smaller than for the interstitial elements. The partitioning e€ect of the ruthenium is di€erent from that of chromium, which was found to be partitioned mainly into the 2 phase after certain heat treatments [24,28]. The localisation behaviour of the

ruthenium in the phase is similar to the niobium enrichment in the phase [29]. Niobium is considered to shift the (2+ )/ boundary to the Al-rich side, thus resulting in increased 2 phase formation, whereas chromium and manganese are known to shift the phase boundary to the Al-lean side, favouring the phase. Ruthenium, however, was found to follow chromium in terms of the (2+ )/ boundary shift [30], but follows niobium with regard to partitioning behaviour. Therefore, the similar behaviour of partitioning into the phase provided by the addition of ruthenium may not be explained in the same manner as for niobium. Two aspects need to be mentioned to explain the partitioning behaviour of ruthenium. One is the degree of  phase stabilisation o€ered by the third element at higher temperatures. Ruthenium appears to exhibit similar trends to chromium in terms of phase relations

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Table 2 Chemical composition by atom probe analysis for the 2 and the phases in Ti-48Al-0.5Ru (at% or at ppm.) Elements

Ti Al Ru O (at. ppm) C (at. ppm) Total

Table 4 Partitioning coecients of alloying elements between 2 and phases in TiAl-0.5Ru alloys

Composition 2

63.2‹0.12 35.1‹0.12 0.31‹0.02 12190‹280 1710‹110 100.0%

50.6‹0.13 48.8‹0.13 0.54‹0.02 370‹50 230‹40 100.0%

Fig. 7. Composition pro®les of alloying elements in TiAl-0.5Ru for the selected volume indicated by the rectangle in Fig. 6(a).

in Ti-Al based phase diagrams [30,31]. It was reported that the addition of chromium to TiAl alloys had the e€ect of stabilising the  phase relative to at high temperatures [32,33]. However, after the ÿ2 disorder±order transformation, this e€ect appeared to

k2 k

Ru

O

C

± 1.74‹0.18

32.95‹5.21 ±

7.43‹1.77 ±

change by destabilising the 2 phase at low temperature. A similar e€ect was also observed in the case of vanadium additions [34]. Therefore, the partitioning of the chromium to the 2 phase which is observed in many cases seems to be attributable to  phase stabilisation by the chromium, which is then locked in the 2 phase after the transformation. This was con®rmed by a recent investigation on partitioning of the chromium in Ti-(47± 49)at%Al-(1.6±1.8)at%Cr-(1.5±1.9)at%Nb alloys [35]. It was found that a signi®cant decrease in chromium content in the 2 phase and a reversal in the chromium partitioning occurred after extensive heat treatment of more than 720 h at 800 C. Consequently, the di€erences in partitioning behaviour between chromium and ruthenium may be more related to di€erences in the kinetics of redistribution of these elements than to differences in phase equilibria. The other aspect of Ru partitioning may be attributable to the solubility limit of Ru in the 2 phase being less than the bulk alloy content of Ru at lower temperatures. The approximate maximum ruthenium solubility at 1223 K in the 2 and phases was previously suggested to be 0.5at% and 1.0at%, respectively [11]. Therefore, it is possible that the excess ruthenium atoms beyond the solubility limit in the 2 phase after the ÿ2 transformation tends to be repelled into the phase. As a result, the observed concentration of the ruthenium in the 2 phase is expected to indicate a nearmaximum solubility limit in the 2 phase. The solubility in the 2 phase observed by atom probe was 0.3 at% following heat treatment at 1173 K, slightly smaller than the previously reported maximum solubility at 1223 K. No evidence was found for the segregation of the ruthenium to the 2/ interphase boundary. The reasonably consistent volume fraction of the 2 phase calculated from the ruthenium with those calculated from the other elements also indicates absence of any localised depletion or enrichment of the element.

Table 3 Volume fractions of 2 phase estimated from atom-probe analysis of di€erent elements Ti-48Al-0.3Ru

Ti

Al

Ru

O

C

Fi2

8.0‹9.0%

6.8‹0.9%

8.7‹1.5%

7.8‹0.7%

11.4‹3.2%

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Table 5 Comparison of oxygen and carbon concentrations of the phase in di€erent TiAl base alloys Interstitial contents in the phase Material (at%)

Microstructure

Oxygen (at. ppm)

Carbon (at. ppm)

Ref.

Ti-48Al-0.5Ru Ti-48Al-2Cr Ti-48Al Ti-47Al-2Cr-1.8Nb-0.15B-0.2W Ti-52Al-3Cr Ti-52Al-3Nb Ti-52Al-3Mn

2 ‡ 2 ‡ 2 ‡ 2 ‡ Fully Fully Fully

370‹50 380‹120 230‹70 410‹100 275‹125 210‹110 185‹110

230‹40 340‹120 330‹70 ± 70‹60 130‹90 120‹60

[This study] [22] [24] [33] [41] [41] [41]

4.2. Oxygen Two-phase (2+ ) TiAl alloys are known to exhibit higher ductility compared to single phase alloys [36]. A microstructural e€ect [37] and/or a puri®cation e€ect of phase from interstitial elements [38,39] have been suggested to explain the e€ect on the improved ductility. Initially, it was considered that the scavenging of interstitial elements provided by the 2 phase decreased interstitial levels in the phase, leading to a change in deformation mechanism and thus resulting in improved ductility [40,41]. Contrary to this initial simple scavenging model, it was recently reported that the 2 phase only acts as an alternative sink for oxygen when the oxygen concentration in the alloy is above the solubility limit in the phase [24]. In addition to the suggestion that oxygen level remains constant regardless of 2 phase volume fraction, it was also reported that the maximum solubility of oxygen in the phase was not in¯uenced by the addition of a third element such as Cr, Mn or Nb [42]. The present atom-probe investigations on TiAl alloys showed an oxygen level in the phase which is consistent with those for other TiAl alloys reported elsewhere, within the experimental error, as shown in Table 5. The preferential location of interstitial elements in the 2 phase in the TiAl alloys has been interpreted as a chemical e€ect [43]. The octahedral cavities surrounded by six titanium atoms are suggested to be preferred by the interstitial elements, and those cavities exist in DO19 structure of the 2 phase whereas they are not available in L10 structure of the phase. 4.3. Carbon In the present investigation, carbon has also been found to localise in the 2 phase as in the case of oxygen, but in a much less pronounced manner. The ratio of carbon partitioning is about 7, whereas that for oxygen is approximately 33. This partitioning behaviour is very similar to the carbon behaviour found in TiAl-2Cr [22].

The slight deviation of volume fraction of 2 phase calculated from carbon concentration from those of the other elements suggests that the concentration of carbon may be slightly underestimated in the atom-probe analysis. Two possibilities exist to explain this discrepancy as suggested by Denquin et al. [41]. One is due to the carbon ®eld evaporating in the state of molecular ions that could be obscured by other peaks, 48 2+ Ti peaks. The other is due to such as 12(C)+ 2 in inhomogeneous distribution of carbon atoms, arising from the segregation. Since no evidence was found for carbon enrichment or depletion at interfaces in the present study, the ®rst explanation seems to be more plausible. The carbon concentrations in the phase measured in this study and in previous reports are similar within the experimental error regardless of the third alloying elements [22,24,39]. Also, considering that the bulk compositions of carbon were found to be di€erent for each alloy, it is assumed that carbon concentration in the phase is mainly a€ected by the carbon solubility limit in the phase, which is analogous to the partitioning behaviour of the oxygen. The carbon concentrations in the phase for di€erent TiAl alloys are also summarised in Table 5. 5. Conclusions The solute partitioning behaviour in two phase 2+ Tiÿ48at%Al-0.5at%Ru alloys was investigated using atom-probe ®eld-ion microscopy. Ruthenium is found to be localised preferentially in the phase, with a partition coecient of approximately 1.75. Oxygen and carbon atoms are found to be very strongly localised in the 2 phase, with a partition coecient of about 33 and 7.5, respectively. The partitioning behaviour of interstitial elements in this investigation is in good agreement with previous studies performed on the TiAl alloy systems alloyed with di€erent substitutional elements such as Cr, Mn and/or Nb. The calculations of volume fraction of 2 phase using various elements in the present study indicate reasonable consistency with each other.

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No evidence was found for the segregation of ruthenium, oxygen or carbon to 2/ interfaces. Acknowledgements SK would like thank Professor C.-P. Hong at the Yonsei University for the provision of a plasma arcmelting facility, and Mr. D.S. Kim and Drs. J.C. Kim and J.M. Oh at the Korea Electric Power Research Institute for research support. The atom-probe research facility at Oxford University is funded by EPSRC and by the R.W. Paul Instrument Fund.

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