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Journal of Alloys and Compounds 671 (2016) 458e464

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Journal of Alloys and Compounds

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Effects of platinum group metals addition on the precipitation oftopologically close-packed phase in Ni-base single crystal superalloys

Shuang Gao, Yizhou Zhou**, Cai-Fu Li, Zhi-Quan Liu*, Tao JinInstitute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

a r t i c l e i n f o

Article history:Received 29 December 2015Received in revised form7 February 2016Accepted 14 February 2016Available online 16 February 2016

Keywords:Platinum group metalsNi-base single crystal superalloyTopologically close-packed phasePrecipitationIn-situ TEM

* Corresponding author.** Corresponding author.

E-mail addresses: yzzhou@imr.ac.cn (Y. Zhou), zql

http://dx.doi.org/10.1016/j.jallcom.2016.02.1220925-8388/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

Precipitation of topologically close-packed (TCP) phase in Ni-base single crystal superalloys with addi-tions of platinum group metals (PGMs) was investigated by in situ transmission electron microscopy(TEM). Pt, Ir and Ru additions are all helpful to suppress the precipitation of TCP phase, among which Ruhas the largest efficiency while Pt has the least one. Comparing with the TCP precipitation in non-PGMsalloys, addition of Ru mainly retards the nucleation rate of TCP phase, but Pt and Ir mainly reduces thegrowth rate of TCP precipitates. Elemental partitioning investigation reveals that Pt and Ir prefer par-titioning to the surrounding g matrix rather than in the precipitated m phase, and Ru distributeshomogenously in both g matrix and m phase. Cr distribution in precipitated m phase is higher in Pt/Ir-containing alloys than in Ru-containing alloy, which is supposed to be related to the different metallicbonding energy of CrePt, CreIr and CreRu.

© 2016 Elsevier B.V. All rights reserved.

1. Introduction

Ni-base single crystal superalloys have been widely used for thekey part of aero-gas turbine blades. In order to achieve an excellentcreep resistance during long-time high temperature exposure, highcontents of refractory elements are added to single crystal super-alloys [1,2]. However, when the concentration of refractory ele-ments exceeds the solid solution concentration limit of gammamatrix, excess amounts of W, Mo and Re promote the occurrence ofTCP phase heavily [3,4]. It is widely accepted that the precipitationof TCP phases degrade the high temperature mechanical perfor-mance extremely by decreasing both solid solution strengtheningbehavior and creep resistance [5,6].

Recently, Ru has been revealed as a crucial addition for singlecrystal superalloy because of its positive suppressing effect on TCPprecipitation [7e9]. Besides, other PGMs such as Ir and Pt also drawmany attentions on their abilities of stabilizing microstructure forsuperalloys. Favorable data has shown that the additions of Ir and Pthelp to increase the oxidation and hot corrosion resistance of thesuperalloys [10,11]. Yokokawa et al. [12] investigated the distribu-tion of PGMs in superalloys previously. It was found that Ru

iu@imr.ac.cn (Z.-Q. Liu).

segregates weakly to g matrix, Pt segregates weakly to g0, while Irdistributes homogenously. Moreover, PGMs addition also affectsthe segregation of non-PGMs elements. Carroll et al. [13] reportedthat Ru and Cr alter Re segregation together by changing systemthermodynamics, but Reed et al. [14] held the opposite viewpoint.Sluytman et al. [15] found that element Ir affects the partitionbehavior of both PGMs and non-PGMs elements in spite of its equalpartition between g and g0 phases.

Although some literatures have reported the effects of PGMsaddition on the phase stability of Ni-base single crystal superalloys,their investigations are still in the preliminary stage now. Most ofthem focused on the g and g0 phases, but few of them concentratedon the detrimental TCP phases. The suppression mechanism ofPGMs elements and their partition behavior during TCP phaseprecipitation were seldom studied. In this work, the precipitationbehaviors of TCP phase in PGMs-containing superalloys wereinvestigated by in situ TEM, which gives the direct evidence forsuppression effects of PGMs additions on TCP precipitation.Combining the elemental mapping of TCP precipitates, the distri-butions and partitions of PGMs as well as other TCP componentelements were revealed. These results are of great significance todiscuss the effects of PGMs addition on the precipitation of TCPphase quantitatively.

S. Gao et al. / Journal of Alloys and Compounds 671 (2016) 458e464 459

1.1. Experimental procedure

Chemical compositions of experimental alloys are listed inTable 1. Three percent in weight percentage of Pt, Ir and Ru wereadded in original alloy, which were named as alloy 3Pt, 3Ir and 3Rurespectively. Single crystal bars were casted along [001] direction,and the whole heat treatment procedures were listed in Table 2.Long-term aging was conducted to shorten the inducing time ofTCP precipitation as well as to eliminate elemental segregationbetween dendritic and interdendritic areas. To prepare TEM sam-ples, experimental alloy bars were cut perpendicular to [001] di-rection into pieces with 800 um thick firstly. Then these pieceswere mechanically ground into 20 um thick. Subsequently, 3 mmdiameter discs were punched out and thinned by argon guns intoelectron transparent films.

In situ heating experiment was conducted in a JEM-2100transmission electron microscope with a single tilt heating holderat an elevated temperatures of 950 and 1050 �C, respectively.Although an amount of TCP phases had already precipitated in al-loys before heating, in situ observations were conducted in primaryg matrix far from any pre-existed TCP phase. Elemental EDS anal-ysis was also conducted on the selected primary g matrix before insitu experiments to ensure a similar composition for differentsamples of PGMs addition. Therefore, the impacts of chemicalcomposition of g matrix on TCP precipitation can be appropriatelyignored. After cooling down to room temperature from in-situheating, the precipitated TCP phase was further characterized us-ing a double-tilt holder. Scanning transmission electronmicroscopy(STEM) and high angle annular dark field (HAADF) observationswere conducted in a Tecnai F20 transmission electron microscopewith an EDS spectrometer. An image processing software was usedto analyze the precipitated TCP area fraction and length overrepresentative areas for each sample.

2. Results and discussions

2.1. In-situ precipitation of TCP phase in different PGMs-containingalloys

Alloy samples were heated up to 950 �C and kept for differentminutes to observe the in situ precipitation behaviors of TCP phaseas shown in Fig. 1. Without any addition of PGMs element, TCPphases nucleated and grew up quickly from gmatrix in the originalalloy sample (Fig. 1(a)e(d)). By addition of Pt (Fig. 1(e)e(h)) and Ir(Fig. 1(i)e(l)), the size of precipitated TCP phase reduced muchshowing an effect of suppression. For example, the average sizes ofTCP precipitates are decreased from over 200 nm in original alloy(Fig. 1(d)) to less 50 nm in alloy 3Ir (Fig. 1(l)). TCP precipitation isseverely restrained by Ru addition as shown in Fig.1(m)e(p), wherefew nucleus and precipitate was observed in primary gmatrix evenafter 45 min exposure (Fig. 1(p)). In order to study the suppressioneffects of PMGs addition further, in situ heating experiments at ahigher temperature of 1050 �C were also conducted on differentalloy samples. As shown in Fig. 2, TCP precipitationwas accelerated,and a larger quantity of TCP phases formed in the original and Pt/Ir-containing alloys after 45min heating (see Fig. 2(d), (h) and (l)). It is

Table 1Nominal chemical compositions of experimental alloys (wt.%).

Alloy Co Cr þ Mo þ W Re Pt/Ir/Ru Al Ta Ni

Original 12.0 12.0 5.0 e 6.0 8.0 bal3Pt 12.0 12.0 5.0 3.0Pt 6.0 8.0 bal3Ir 12.0 12.0 5.0 3.0Ir 6.0 8.0 bal3Ru 12.0 12.0 5.0 3.0Ru 6.0 8.0 bal

found that additions of element Pt and Ir have fewer suppressioneffects on TCP precipitation at 1050 �C, but element Ru still keeps anoutstanding microstructure stabilizing ability (see Fig. 2(p)).

To compare the suppression effects quantitatively, the areafractions and lengths of TCP phases in different alloys were calcu-lated by statistical analysis. Considering the approximately equalthickness of the observed area, the measured TCP area fraction canbe considered as the actual TCP volume fraction namely. Based onthe observed in situ TCP precipitation images including those inFigs.1 and 2, the combined area fractions of precipitated TCP phaseswere measured and presented in Fig. 3 as a function of exposuretime. At both 950 �C (Fig. 3(a)) and 1050 �C (Fig. 3(b)), the TCP areafraction curves show the same trend from highest to lowest as:original alloy, alloy 3Pt, alloy 3Ir and alloy 3Ru. Therefore, it isdemonstrated that additions of Pt, Ir and Ru all lead to the decreaseof TCP precipitation. Element Ru has the most efficient effect andelement Pt has the least effect. For example in Fig. 3(a), TCP areafraction after 45 min for original alloy, alloy 3Pt, alloy 3Ir and alloy3Ru is about 21%, 12%, 2% and 1% respectively. Thus, it can becalculated that TCP suppression rate with Pt, Ir and Ru addition isapproximately 2 (21/12),10 (21/2) and 21 times.When temperaturegoes up to 1050 �C in Fig. 3(b), TCP area fractions after 45 minexposure are more than that of 950 �C in PGMs-containing alloys,especially for alloy 3Ir of about 14% which is far beyond the originaldata of 2%. This should be related to the accelerated atomic diffu-sion rate at a higher temperature especially for Ir element. How-ever, the TCP area fraction of the original alloy only increased littlefrom 21% of 950 �C to about 22% of 1050 �C, which indicates thatalmost all the refractory elements in original alloy are depleted todevote into TCP precipitation at both 950 and 1050 �C.

After 45 min exposure at high temperature, the length of about100 TCP phases in different alloys was measured and statisticallyplotted in Fig. 4. At 950 �C of Fig. 4(a), the peak of TCP size distri-bution is in the range of 50e99 nm for alloy 3Ir, 100e149 nm foralloys 3Pt and 3Ru, and 200e249 nm for original alloy. When thetemperature was increased to 1050 �C (Fig. 4(b)), the size peakmoves to 150e199 nm for alloys 3Ir and 3Pt, 200e249 nm for alloy3Ru, and 250e299 nm for original alloy. Of course, the TCP length at1050 �C is larger than that of 950 �C due to the accelerated diffusionrate of atoms during precipitation. There is even no TCP phasesmaller than 100 nm at 1050 �C.

The TCP area fraction is controlled by TCP quantity, individualTCP area/size and gmatrix area. Although gmatrix area might varya little bit for four alloys, it only affects the total TCP area fractionslightly. The quantity of TCP precipitates in alloys 3Pt and 3Ir issimilar to that in the original alloy as shown in Figs. 1 and 2.However, with the addition of element Pt and Ir, TCP length isobviously smaller than that in original alloy. Even at 1050 �C themajority of TCP length with Pt or Ir addition is still under 200 nm,while in original alloy it is far over 200 nm (see Fig. 4). Therefore, itis believed that the addition of Pt and Ir elements can slow downthe growth rate of TCP phases, hence suppress the TCP precipitationin Fig. 3. On the other hand, the average TCP length/size in alloy 3Ruis larger than those in alloys 3Pt and 3Ir but smaller than those inthe original alloy. For example, most of TCP lengths are larger than200 nm at 1050 �C in alloy 3Ru, while those in alloys 3Pt and 3Ir areless than 200 nm. However, alloy 3Ru has the smallest TCP areafraction among four alloys, implying the strongest suppression ef-fect. Its final TCP area fraction after 45 min exposure at 1050 �C isonly about 4%, far below 22% of original alloy (See Fig. 3(b)).Combining its large TCP size but small area fraction, it can beconcluded that addition of element Ru can inhibit the nucleation ofTCP phase efficiently.

In advanced single crystal superalloy, formation of TCP phase isa solid-phase transformation process. The volume free energy

Table 2Heat treatment procedures of experimental alloys.

Alloy Solution treatment Primary aging Secondary aging Long-term aging

Original 1325 �C/16 h þ 1335 �C/16 h A.C. 1150 �C/4 h A.C. 870 �C/24 h A.C. 1100 �C/270 h A.C.3Pt 1320 �C/4 h þ 1330 �C/16 h A.C.3Ir 1315 �C/4 h þ 1325 �C/16 h A.C.3Ru 1290 �C/4 h þ 1300 �C/16 h A.C.

Fig. 1. In situ observation of TCP precipitation at 950 �C in original alloy for (a) 0 min, (b) 15 min, (c) 30 min, (d) 45 min, in alloy 3Pt for (e) 0 min, (f) 15 min, (g) 30 min, (h) 45 min,in alloy 3Ir for (i) 0 min, (j) 15 min, (k) 30 min, (l) 45 min and in alloy 3Ru for (m) 0 min, (n) 15 min, (o) 30 min, (p) 45 min.

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(DGv) is the driving force of this process. In general, DGv is pro-portional to the super saturations (DC) of the TCP formation ele-ments in g matrix, such as Re, W and Cr. According to our in situinvestigation in Figs. 1e4, Ru can effectively suppress the precipi-tation of TCP phases at high temperatures, which is in agreementwith previous reports [8,9,16]. The role of Ru in suppressing TCPprecipitation is attributed to the so-called reverse partitioning ef-fect. Many works support the opinion that Ru addition coulddecrease the partitioning of refractory elements, such as Re, W andCr [7,17]. Tan et al. [16] reported that Ru can not only decrease thesupersaturation of g phase but also increase the elastic strain

energy of nucleation of TCP phase. So Ru addition can suppress thenucleation/formation of TCP phase effectively and stabilize themicrostructure of single crystal superalloy. For the role of Ir addi-tion on TCP precipitation, Yokokawa et al. [12] reported that Iralmost equally partitions into both g and g0 phase. In addition, Iralso has the preference to lower the volume fraction of g0 phase andto reduce the partitioning of Re, W and Ru [15]. The lower thevolume fraction of g0 phase and refractory elements partitioning,the less of supersaturation of TCP elements in the g matrix. So Iralso has the tendency to suppress the formation of TCP phase as Ru.Pt is considered to partition slightly into g0 phase and has no

Fig. 2. In situ observation of TCP precipitation at 1050 �C in original alloy for (a) 0 min, (b) 15 min, (c) 30 min, (d) 45 min, in alloy 3Pt for (e) 0 min, (f) 15 min, (g) 30 min, (h) 45 min,in alloy 3Ir for (i) 0 min, (j) 15 min, (k) 30 min, (l) 45 min and in alloy 3Ru for (m) 0 min, (n) 15 min, (o) 30 min, (p) 45 min.

S. Gao et al. / Journal of Alloys and Compounds 671 (2016) 458e464 461

obvious effects on elements segregation [12]. So it can be seen thatPt has the least effect on the suppression of TCP precipitation in thiswork.

2.2. Element partitioning behaviors during TCP phase precipitation

Many literatures have reported the partitioning characteristicsof PGMs elements on the g and g0 phases, but less of them studiedthe element partitioning behaviors in TCP precipitates. According tothe selected area electron diffraction (SAED) analyses with TEM,precipitated TCP phases in four alloys were all identified as m phase,as reported previously [18]. The main elements in m phase werelisted in Table 3, together with their characteristic X-ray energyused for elemental EDS mapping. Theoretically, Ka peaks are suit-able to map Cr and Ni, La peaks are used to map Mo and Ru, whileMa peaks should be selected to map W, Re, Ir and Pt. However, theenergy difference between Ma peaks of W, Re, Ir and Pt is too small(<0.13 keV), which are inseparable from each other with respect tothe resolution limit of 0.15e0.2 keV for EDS probe. Therefore, Lapeaks of W, Re, Ir and Pt were used during elemental mapping,which have negligible energy overlaps within the EDS resolutionlimit. In order to insure the collection of La peaks, EDS acquisitiontimewas prolonged to about 2000e2500 s. Besides, position of EDSprobe was monitored and corrected periodically to reduce thepossible specimen drift.

Fig. 5 details the distributions of major elements in m phase after

in situ precipitation, in which row (a) is from original alloy whilerows (b)e(d) come from alloy 3Pt, 3Ir and 3Ru, respectively. It wasfound that Ni is obviously poor in m phase in all four alloys, whileMo, W are slightly rich and Re is evidently rich in precipitated mphase. PGMs addition has little effect on the distributions of Ni, Mo,W, and Re elements. As well known, m phase is based on the A7B6composition where A is transition metals (Fe, Co, Ni) with smallatomic radius and B is refractory metals (W, Mo, Re) with largeatomic radius. Compared with the surrounding region of g (Ni) andg0 (Ni3Al), atomic percent of Ni in m phase (A7B6) is much less butW,Mo and Re are much more than the surrounding region, whichexplains the distribution tendencies of Ni, W, Mo and Re in Fig. 5.Considering the distribution of PGMs themselves, Pt and Ir werefound prefer partitioning to the surrounding region rather than mphase, while Ru distributes homogenously. As far as known, thesurrounding region of m phase resembles g0 envelope very much,due to the depletion of refractory elements during m phase pre-cipitation. Thus, the distributions of Ru, Pt and Ir in the surroundingregion can be considered as distribution in g0 phase of single crystalsuperalloy. The preferential segregation of Pt to the surroundingregion observed in Fig. 5(b), accords well with the early report of Ptpartitioning to g0 in single crystal superalloy [12].

With the addition of PGMs, little effect was observed on thedistributions of non-PGMs elements except for Cr. Compared withthe slight absence of Cr in m phase in original alloy (see Fig. 5(a)), Crbecomes rich in Pt/Ir-containing alloys (Fig. 5(b),(c)) and distributes

Fig. 3. The variation of combined TCP area fraction as a function of holding time at (a)950 �C and (b) 1050 �C.

Fig. 4. Statistic chart showing TCP length distribution and the corresponding per-centage obtained from about one hundred TCP phases precipitated after 45 minexposure at (a) 950 �C and (b) 1050 �C.

Table 3Characteristic X-ray Energy of Main Elements in m Phase (keV).

Cr Ni Mo Ru W Re Ir Pt

Ka 5.412 7.473 17.445 19.237 e e e e

La 0.573 0.852 2.293 2.559 8.398 8.653 9.175 9.442Ma e e e e 1.774 1.843 1.978 2.049

S. Gao et al. / Journal of Alloys and Compounds 671 (2016) 458e464462

homogenously in Ru-containing alloy (Fig. 5(d)). The distributiontendency of Cr is consistent with Ru distribution but contrary to Ptand Ir distribution. Since m phase belongs to intermetallic com-pounds, its chemical bond is substantially metallic bond, whosebonding energy is directly proportional to electronegativity dif-ference and inversely proportional to atomic radius of bondingatoms. Electronegativity was firstly defined in 1932 by Pauling [19]as “the power of an atom to attract electrons.” Due to the lack ofavailable data for calculation, further extension of Pauling's scale toelectronegativity values of all elements was limited. Later on in1958, Allred and Rochow [20] redefined electronegativity andcalculated electronegativity values with data that was easily esti-mated. According to Allred's scale, relative electronegativity valuesfor Cr, Ir, Pt and Ru are 1.56, 1.55, 1.44 and 1.42 respectively [20,21].Therefore, electronegativity difference between Cr and Ru is largerthan that of CrePt and CreIr, which indicates CreRu bonding is thestrongest one among them. Besides, atomic radius of these fourelements is sequenced as Cr < Ru < Ir < Pt, which also indicates aweaker metallic bond of CreIr and CrePt than CreRu. Therefore, Cris being favor of coexisting with Ru but not with Ir and Pt as shownin Fig. 5. It should be pointed out that with only 45 min exposure at950 or 1050 �C, diffusions of Ru, Pt and Ir are probably insufficientcompared with the fully heat treatment process. This may

introduce deviations of elements distribution in TCP phases andmatrix, which should be pay attention to in the further researchworks.

3. Conclusion

Effects of PGMs addition on TCP precipitation in Ni-base singlecrystal superalloys were studied by in situ TEM observation, andthe following conclusions were drawn.

1) By adding element Pt, Ir and Ru, TCP precipitations are allinhibited at both 950 and 1050 �C. Higher exposure temperaturereduces TCP suppression efficiency because of the acceleratedatomic diffusion rate. Element Ru has the highest TCP

Fig. 5. HAADF image of precipitated m phase after in situ experiment and its corresponding elemental mapping in (a) original alloy, (b) alloy 3Pt, (c) alloy 3Ir and (d) alloy 3Ru. (Scalebar e 100 nm).

S. Gao et al. / Journal of Alloys and Compounds 671 (2016) 458e464 463

suppression rate while Pt has the lowest effect. Element Ir has aconsiderable effect on diminishing TCP precipitation especiallyat 950 �C.

2) Statistical analyses on TCP area fraction and length revealdifferent suppression mechanism for PGMs-containing alloys.With Pt and Ir addition, the precipitated TCP length is decreased,which indicates a lower growth rate of TCP precipitation. For Ru-containing alloy, TCP quantity is obviously reduced, indicating asuppressed TCP nucleation rate.

3) Pt and Ir prefer partitioning to the surrounding region ratherthan the precipitated m phase, while Ru distributes homoge-nously. The composition of m phase is lack of Ni and rich in Mo,W and Re, and additions of PGMs have little effect on theirdistributions. However, the distribution of Cr is influenced byPGMs addition, which is contrary to Pt/Ir distribution butconsistent with Ru distribution. The metallic bonding of CrePt,CreIr and CreRu are discussed in terms of electronegativitydifference and atomic radius of the bonding atoms, whichrevealed that CreRu is stronger than CrePt and CreIr bonding.

Acknowledgments

This work was financially supported by the Natural ScienceFoundation of China (Grant No. 51271186).

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