A pseudo binary interdiffusion study in the β Ni(Pt)Al phase

Transcription

1 A pseudo bnary nterdffuson study n the β N(Pt)Al phase Perumalsamy Kruthka and Aloke Paul Dept. of Materals Engneerng, Indan Insttute of Scence, Bangalore, Inda *Correspondng author: E-mal: aloke@materals.sc.ernet.n, Tel.: , Fax: Source: P. Kruthka and A. Paul, A pseudo-bnary nterdffuson study n the β-n(pt)al phase, Phlosophcal Magazne Letters 95 (2015) Abstract Interdffuson study s conducted n the N-rch part of the β N(Pt)Al phase followng the pseudo-bnary approach. Interdffuson coeffcents over the whole composton range consdered n ths study ncreases wth the ncrease n Pt content, whch s n lne wth the theoretcal study predctng the decrease n vacancy formaton and mgraton energy because of Pt addton. The trend of change n dffuson coeffcent wth the ncrease n N and Pt content ndcates that Pt preferably replaces N antstes. Keywords: dffuson, ntermetallcs, defects, pseudo bnary approach 1

2 1. Introducton Bond coat s an ntegral part n jet engne applcatons for the protecton of the base materal.e. N-based superalloys from oxdaton. β N(Pt)Al s one of the bond coats used preferably on the N-based superalloys. Durng servce, an Al2O3 layer grows on top of t thereby protectng the base metal from oxdaton. Bond coat acts as a reservor for the contnuous supply of Al such that a protectve layer of alumna can grow mmedately after spallaton because of the thermal stress at the bond coat/alumna nterface. Addton of Pt n β NAl ncreased the servce lfe of turbne blades by manfold. Although the mechansm by whch Pt provdes benefcal effect s not well understood, t s found that Pt addton decreases the segregaton of S at the bond coat and Al2O3 nterface. In addton, t s beleved that the dffuson rate of components ncreases to facltate hgher growth rate of the oxde layer by supplyng Al at hgher rate. Ths fnds support n the theoretcal analyss by Marno and Carter [1]. However, to the best of our knowledge, no relevant expermental studes are avalable n the lterature. Mnamno et al. [2] estmated the dffuson rate of Pt n dfferent β-nal alloys. However, t should be noted here that the dffuson rates of N and Al because of presence of Pt s more mportant to study. Therefore, the am of ths present study s to conduct dffuson couple experments examnng the role of Pt on the dffuson rates of N and Al based on quanttatve dffuson analyss. A pseudo-bnary approach seems to be a sutable technque for ths purpose [3]. Added advantage of ths method s that t mmcs the composton profle of the bond coat n real applcaton whch develops by the nterdffuson between the bond coat and the superalloy [4 7]. 2

3 2. Expermental procedure N (99.9 wt.%), Al (99.9 wt.%) and Pt (99.99 wt.%) were used to prepare the alloys for makng dffuson couple. For the purpose of makng pseudo bnary dffuson couples two sets of alloys, N60-xPtxAl40 and N50-xPtxAl50, x = 5, 10 and 15 (all n atomc percentage), were melted n an arc meltng furnace under Ar atmosphere. Average devaton of the compostons from ntended ones was around ±1 at.%. These were homogenzed n a vacuum furnace (~10-4 Pa) at 1200 o C for 100 h and the average compostons were measured randomly at dfferent places of the blocks n an electron probe mcro analyzer (EPMA). Dffuson couples were prepared such that Pt remans constant n both sde of the end members varyng only N and Al. These experments were conducted at 1100 o C for 25 h. After the experments, the samples were cross sectoned and metallographcaly prepared for EPMA analyss. 3. Results and dscusson Tll date, several expermental nterdffuson studes are conducted n the bnary N Al system [8 14]. However, as mentoned already the role of Pt on nterdffuson of N and Al are not examned. For the sake of comparson both bnary and ternary (by pseudo-bnary approach) experments are conducted. In ths, N rch part of the β N(Pt)Al s studed because of relevance to the applcaton. Fgure 1, shows the composton profles developed n N60Al40/N50Al50 and N50Pt10Al40/N40Pt10Al50 dffuson couples durng annealng. To apply pseudo bnary approach t s mportant that Pt has almost constant concentraton over the whole nterdffuson zone, whch s the case n these experments. Reason for fndng ths behavor could be understood from Fgure 2, whch shows the varaton of actvty of components for 3

4 N50Pt10Al40/N40Pt10Al50 across the nterdffuson zone extracted usng CALPHAD. Actvty for N and Al vares sgnfcantly, whereas, t s more or less constant for Pt. In a bnary system, the nterdffuson flux of a component at dfferent compostons can be estmated by [3, 15, 16] ( 1 Y ) * + x x ~ N Y J = ( 1 Y ) dx + Y dx (1) 2t * V x m V x m where, J ~ s the nterdffuson flux. Mnus sgn comes from the fact that the element dffuses from rght to left. x * s the poston of nterest. Y = N N + N N (where, N = N + N ) s the composton normalzed varable ntroduced by Wagner [15], N s the composton of the element n terms of atomc fracton or mol fracton. and + are the left and rght hand sde of the unaffected parts of the dffuson couple. Vm s the molar volume. Followng, the nterdffuson coeffcents at the pont of nterest can be estmated by ~ J ~ dc = D (2) dx where C = N / V s the concentraton. In a bnary system, one can consder any of the m components. The same estmaton procedure can be followed n the pseudo bnary dffuson couple. In ths case, as explaned n Ref. [3], the nterdffuson coeffcents can be calculated drectly from the Al composton profle. On the other hand, (N+Pt) composton profle can also be used for the estmaton of dffuson coeffcents, snce Pt shares the same sublattce as N [17]. Molar volume varaton at dfferent Pt content are estmated usng the lattce parameter data avalable n lterature [18], whch were, n fact, determned from the alloys used n ths work. 4

5 These are found to be smlar to the data avalable n another reference [19]. The varatons of the molar volumes for dfferent Pt contents are shown n Fgure 3. The estmated nterdffuson coeffcents are shown n Fgure 4a. For comparson, two data sets estmatng the nterdffuson coeffcents n the bnary N Al system are ncorporated [9, 13], n whch the molar volume varaton was used for the estmaton. It can be seen that the data estmated n the present study are very close to them. Another nterestng pont to be noted here s that, as shown n Fgure 4b, wth ncrease n Pt content, there s sgnfcant ncrease n the nterdffuson coeffcent. Interestngly, nterdffuson coeffcent ncreases at hgher rate as the composton moves towards the stochometrc 50(N+Pt):50 Al composton. Ths trend ndcates the change n concentraton of the defects assstng the dffuson process n a certan way. Frst of all, the nterdffuson coeffcent ncreases at all compostons wth the ncrease n Pt content. Theoretcal analyss by Marno and Carter [1] ndcates that the defect formaton energy and mgraton energy decreases wth the ncrease n Pt content leadng to ncrease n dffuson coeffcent. Secondly, t s a known fact that n the N rch sde of the β N(Pt)Al phase, dffuson of both the components s asssted by the presence of N antstes [1]. The concentraton of N antstes ncreases wth the ncrease n devaton towards N rch (Al lean) sde to compensate the devaton from the stochometrc composton. Hgher the devaton means hgher the concentraton of these defects leadng to hgher rate of dffuson of both the components, as t was found n nterdffuson studes [9, 14, 20]. The ncrease n N dffuson rate wth ncrease n N content n the N rch sde of the β NAl was also found based on N tracer dffuson studes [21, 22]. Therefore, nterdffuson coeffcent ncreases wth the ncrease n N content. Addtonally, as shown n Fgure 4b wth ncrease n Pt content the rate of 5

6 ncrease of the nterdffuson coeffcent s lesser on the N rch (Al lean) sde than the ones near the stochometrc composton. There could be two reasons behnd ths. It s well possble that the effect of Pt on ncreasng defect concentratons s hgher n the N rch sde. On the other hand, t s possble that Pt replaces some amount of N antstes snce Pt shares the same sublattce [23]. Effect of alloyng on defect concentraton and dffuson rate was establshed before n another system. Retveld analyss of the Nb S Mo alloy conducted by L et al. [24] shows that Mo has the ste preference for the S antstes n the Nb5S3 phase. On the other hand, S s the man dffusng speces for the growth of ths phase. Therefore, addton of Mo n ths phase leads to the decrease n growth rate and the nterdffuson coeffcent [25]. 4. Concluson The role of Pt addton on dffusng components of N and Al n β NAl s examned. Estmated nterdffuson coeffcents ndcate that the dffuson rate ncreases wth the ncrease n Pt content. Our results are n lne wth the theoretcal predctons of Ref. [1] where t was obtaned that the defect formaton energy as well as the mgraton energy decreases wth the ncrease n Pt content. The rate of ncrease n nterdffuson coeffcent because of Pt addton decreases wth the ncrease n N content (decrease n Al content), whch ndcates that Pt preferably replaces N antstes. Acknowledgement: We would lke to acknowledge the fnancal support from ARDB, Inda. 6

7 5. References 1. K.A. Marno and E.A. Carter, Intermetallcs 18 (2010) p Y. Mnamno,Y. Kozum,N. Tsuj,M. Moroka,K. Hraoand Y Shra, Sc. Technol. Adv. Mater. 1 (2000) p A. Paul, Phl. Mag. 93 (2013) p D.K. Das, J.P. McDonald, S.M. Yalsove and T.M. Pollock, Spectrochmca Acta Part B: Atomc Spectroscopy 63 ( 2008) p N. Valas and D. Monceau, Surface and Coatngs Technology 201 (2006) p J. Angenete and K. Stller, Materals Scence and Engneerng A316 (2001) p H.M. Tawancy and N.M. Abbas, Surface and Coatngs Technology, 49 (1991) p Jolanta Romanowska, Calphad 44 (2014) p A. Paul, A.A. Kodentsov and F.J.J. van Loo, Journal of Alloys and Compounds 10 (2005) p H. We, X. Sun, Q. Zhenk, H. Guan and Z. Hu, Acta Mater. 52 (2004) p R. Nakamura, K. Takasawa, Y. Yamazak and Y. Ijma, Intermetallcs 10 (2002) p S. Km and Y.A. Chang, Metall. Mater. Trans. A 31A (2000) p M. Watanabe, Z. Horta and M. Nemoto,Defect Dffus. Forum, (1997) p S. Shankar and L.L. Segle, Met. Trans. A9A (1978) p C. Wagner, Acta Metallurgca 17(1969) p A. Paul, T. Laurla, V. Vuornen and S. Dvnsk, A text book on Thermodynamcs, Dffuson and the Krkendall effect n Solds,Sprnger, Hedelberg, Germany, 2014, p C. Jang, Acta Materala 55 (2007) p N. Jaya PhD Thess, Mcro-scale fracture testng of graded (Pt,N)Al bond coats, Indan Insttute of Scence, Bangalore, Inda, R. Hu, H.N. Su and P. Nash. Pure Appl. Chem., 79 (2007) p A. Paul, A.A. Kodentsov and F.J.J. van Loo, Acta Materala 52 (2004) p

8 21. G.F. Hancock and B.R. McDonnel, Physca Status Sold A 4 (1971) p St. Frank, S.V. Dvnsk, U. Södervall and Chr. Herzg, Acta Materala 49 (2001) p Y. Terada, K. Ohkubo, T. Mohr and T. Suzuk, Materals Scence and Engneerng A (2002) p W. L, H. Yang, A. Shan, L. Zhang, and J. Wu, Intermetallcs14 (2006) p S. Prasad and A. Paul, Intermetallcs, 22 (2012) p

9 70 60 Composton (at.%) Al N Dstance (mcron) (a) Composton (at.%) Al N 10 Pt Dstance (mcron) (b) Fg 1: Compostonal profles developed n (a) N60Al40/N50Al50 (b) N50Pt10Al40/N40Pt10Al50 dffuson couples. 9

10 a(al) a(n) a(pt) Actvty Al (at.%) Fgure 2 Varaton of actvtes n β-n(10p)al/n(10pt)50al dffuson couple extracted usng CALPHAD. 10

11 V m (x10-6, m 3 /mol) Pt 10 Pt 5 Pt 0 Pt Al (at.%) Fgure 3: Varaton of molar volume wth Pt n β-nptal. 11

12 NAl Bnary [M. Watanabe et al.] NAl Bnary [A. Paul et al.] NAl Bnary [Present Study] 5 Pt [Present Study] 10 Pt [Present Study] 15 Pt [Present Study] ~ D (m 2 /s) (a) Al (At.%) at.% Al 45 at.% Al 48 at.% Al ~ D (m 2 /s) (b) Pt (at.%) Fgure 4: a) Varaton of nterdffuson coeffcent wth Pt n β-n(pt)al as a functon of Al content. b) Varaton of nterdffuson coeffcent wth Pt n β-n(pt)al for 42, 45 and 48 at.% Al content. 12