gth Liege Conference : Mat neering 2010 edited by J. Lecomte-Becl :i B. Kuhn.
ANISOTROPY OF MECHANICAL PROPERTIES OF SINGLE CRYSTALS IN FOURTH GENERATION NI-BASED SUPERALLOY
I.L. Svetlov, N. V. Petrushin, D. V. Shchegolev, K.K. Khvatskiy
All-Russian Scientific-Research Institute of Aviation Materials (VIAM), Moscow, Russia
Abstract
Single crystals of Ni-based superalloys have significant anisotropy of mechanical properties .. Data about anisotropy of Young modulus, ultimate strength and yield strength, deformation diagrams and rupture strength of Russian single crystal Ni-based superalloys of fourth generation with <001>, <011>, <111> crystallographic orientation are presented in this article.
Keywords: anisotropy of mechanical properties, Ni-based superalloys, single crystal.
1. Introduction
In contrast to polycrystalline isotropic materials mechanical properties of single crystals of Ni-based superalloys with face-centered cubic structure are significantly anisotropic and depend on crystallographic directions, along which these properties are defined [1]. For structural strength analysis of single crystal blades and life-time projecting of GTE it's necessary to have data about temperature and orientation dependence of physical-mechanical properties in superalloys. Vital information, mentioned in this article, shows fundamental mechanisms of temperature and orientation dependence of Young modulus and characteristics of short-time strength (ultimate strength and yield strength, elongation and reduction area), strength rupture of Russian single crystal Ni-based superalloys of fourth generation with <001>, <011>, <111> crystallographic orientations.
2. Materials and data processing technique
The chemical composition of the investigated single crystal superalloy is given in table I [2].
Specimens for mechanical tests (gage length - 25 mm, diameter - 5 mm) were manufactured from round single crystal bars, which axes coincide within 10 degrees of the crystallographic directions <001>, <011> or <111>. Single crystal bars were produced by DS method with cooling in alurninum melt (LMC method) in full-scale plant. Thermal treatment, including step-by-step homogenization in 1285-1320 °C temperature range for 26 hours and two-step aging at 1130 and 870 °C was performed in order to form optimal microstructure. Characteristics of short-time strength, elasticity and Young modulus were measured using MTS-810 machine at a strain rate of 1 mm/min at temperature range of20-1150 °C.
660 gth Liege Conference : Materials for Advanced Power Engineering 2010 edited by J. Lecomte-Beckers, Q. ContrepoiB, T. Beck and B. Kuhn.
Long-term strength tests were conducted at 950--1150 °C temperature range and rupture times > 1000 hours in air without protective coats. Experimental data treatment was made using equation of temperature - strength dependence of creep-rupture lifetime 'f [3]:
'f =$'"'u-•exp(Uo;;u) (1)
where T- temperature; u- stress; .; m, n, U0, 71 are coefficients, defmed by test results; R universal gas constant.
3. Experimental results and discussion
3.1 Young modulus anisotropy
Fig. l shows diagrams of Young modulus vs temperature in single crystals with <001>, <011> and <111> crystallographic orientations. While increasing test temperature from 20 up to 1150 oc, Young modulus monotonically decreases. The single crystals with <111> crystallographic orientation have maximum value of Young modulus and <001> the single crystals - the minimum value. Intermediate values of Young modulus are typical for <0 11 > single crystals. It should be mentioned, that the significant anisotropy of Young modulus remains in all investigated temperature interval. E,GPa 400 .----.----,----.---.----.----,
3
0 +---~----~----+----+----4---~ 0 200 400 600 BOO 1000 1200 T, °C
Fig. 1 Temperature dependence ofYoung modulus in single crystals of 1- <001>, 2- <011>, 3- <111> orientations
3.2 Anisotropy ofshort-time strength properties
The anisotropy of short-time strength properties in single crystals is defmed by the specific slip systems in the existing experimental conditions. Since plastic deformation behavior is defined as interaction of slip systems, tensile test diagrams will be different for different orientation of the single crystals [4-6].
661 1 9 " Liege Conference: Materials for Advanced Power Engineering 2010 edited by J. Lecomte-Beckers, Q. Contrepois, T. Beck and B. Kuhn.
3.2.1 Stress- Strain diagrams
Fig. 2 shows stress-strain diagrams at 20, 700 and 1000 °C of single crystals with <001>, <011> and
1760 1760 ~r" '3 3 1600 .,...,.,., 1600 ,- ~ I 1 1250 !..- 1250 / rl/l,f V' _.,.... 1000 1 I~ - 2 "' ll~ 2 • 760 ~ 760 ". 500 fl 500
2&0 'I r; 0 - 0 , 0 O.(il.& 210" lS 10:1530 4050 0 " s,% a .,.. -
,{(._ / ·''"-~ ~~ ',/. - ...... 1 ~ ...... J:L z -If " ... I
c Fig. 2 Deformation diagrams ofthe investigated alloy with crystal/agraphic orientations, close to <001>, <011> and <111> at temperatures: 20 oc (a), 700 °C(b) and 1000 oc (c)
662 gth Liege Conference : Materials for Advanced Power Engineering 2010 edited by J. Lecomte-Beckers, Q. Contrepois, T. Beck and B. Kuhn.
3.2.2 Strength and plasticity
The temperature dependences of ultimate and yield strengths of single crystals with the three tested crystallographic orientations are shown on fig. 3. In 20-700 oc temperature range, the single crystals with <001> and
proceeded at more elevated temperatures. Such changing of 0"8 and Uo.2 in two-phase single crystal Ni-based superalloys at elevated temperatures can be explained using the abnormal temperature dependence of yield stress in particles of intermetallic y'-phase [ l]. With respect to single crystals with
- t\ / -1000 \ - __ _, ------1\ io------.... 100 • \ ., "' ' ~ ,lOO ti' '~ \ ·~ . -100 -100
0 0 0 100 - - loo 1000 1200 0 100 - 100 lOo IQOO uoo T, "C T, "C a b ......
11600 r-. -- '\ -...... - "' -- -... \ ~ - ·~ ·-loo ti' i\ 6oo "\. -100 0 0 lOO - 6oo 1100 1000 1100 T,"C c
Fig. 3. Temperature and orientation dependence ofyield and tensile strengths in the single crystals ofinvestigated alloy. a- <001> orientation, b- <011> orientation, c- <111> orientation
663 gth Liege Conference : Materials for Advanced Power Engineering 2010 edited by J. Lecomte-Beckers, Q. Contrepois, T. Beck and B. Kuhn.
It should be mentioned, that yield strength of the Re-Ru containing single crystals superalloys N generation has lower values, than the Re-containing single crystals superalloys Ill generation. Similar ratio of ultimate strength and yield strength is observed for single crystals of Re-containing superalloy PWA 1484 and Ru-containing superalloy EPM 102 [7] The reduced yield strength is typical characteristic of single crystals in Re-Ru containing superalloys. It can be associated with the influence of Ru on the dislocation mechanism of plastic deformation. The temperature dependence of the elongation and reduction area in the single crystals with the three crystallographic orientations are shown on fig. 4. On increasing the temperature, both of these properties decrease to a minimum at peak temperature. At temperatures, higher than the peak, significant increasing of elongation and reduction of area was observed.
15,% "10
60 60 ~0 / / 40 I 2 I A -...::...... 10 r--... L ~ ll ~ ...... __ 30 .._,. V 1 -- 1// 1 20 ...... 20 ./1 I-- W" --- ,.,. 10 .. 10 I -- j-e---< -- -.....- 0 0 - 0 200 400 600 800 1000 1200 T, DC 0 200 400 600 800 1000 1200T, DC
a b
Fig 4 Temperature-oriented dependence of elongation ~ (a) and reduction area 1{1 (b) of <001> (1), <011> (2) and <111> (3) single crystals
3.2.3 Creep rupture strength
Table 2 shows rupture strength for lOO and 1000 hours, calculated using experimental data, for three crystallographic orientations in 750-1150 oc temperature range. For estimation of anisotropy of stress rupture it's suitable to use coefficient [8]: lUll ...,, - a~ K; - a.,•t• where a~u' - stress rupture of single crystal with
664 gth Liege Conference : Materials for Advanced Power Engineering 2010 edited by J. Lecomte-Beckers, Q. Contrepois, T. Beck and B. Kuhn.
Table 2. Stress rupture for 100 and 1000 hours of<001>, <011> and <111> single crystals Rupture strength for 100 hours, Rupture strength for 1000 hours, Temperature, O'too. MPa, and Ktoo 0' tooch MP a, and Kwoo T°C K!OO Km Kon Kilt <001> <011> <111> 100 <001> <011> 1000 <111> 1000 7SO 101S 690 0.67 - - 83S 616 0.73 - - 8SO 72S 48S 0.67 800 1.1 S6S 39S 0.78 640 1.13 9SO 40S 340 0.83 S40 1.33 26S 240 0.90 360 1.3S llSO 12S 102 0.82 11S 0.92 74 77 1.04 6S 0.87
Irreversible changes of alloy phase structure and morphology of strengthening y' phase particles is proceeded while high-temperature creep in the structure of single crystals. Such changes are visually illustrated by the evolution of voluentary unit microstructure, enc1osured with <001> planes, in single crystal specimen <001> of initial state and after high - temperature creep at temperatures of 1000 and 11 00 °C (fig. S).
a b c
Fig. 5. Voluentary unit microstructure of <001> single crystal after heat treatment (a), creep tests at 1000 'C, a=200 MPa test period -1274 h (b) and 1100 'C, a=120 MPa test period- 1303 h (c)
As it can be seen from fig. Sa, alloy voluentary unit microstructure in initial conditions consists of y-solid solution matrix, where cuboidal particles of y' phase have tendency to ordered arrangement, forming quasiperiodic three-dimensional macro lattice. Such microstructure is morphologically unstable at high temperature creep tests. In transition phase of high temperature creep initial cuboidal particles of y'-phase are specifically coagulated at the expense of plate splicing. Broad facets of plates are perpendicular to load axis (raft structure) (fig.Sb). Regular raft-structure of single crystals make active reaction on to plastic creep deformation at high temperatures, blocking y' -phase plates intersection. Dislocations migrate in interlayers ofy-solid solution. In this connection any disturbance of raft-structure promotes softening and decreasing of stress rupture. Phase instability in single crystals of analyzed alloy is discovered in creep process in 1000-1100 °C temperature range. Phase instability is visualized in initiation of lamellate particles, Re inreached (TCP phases). TCP phases are isolated in first order dendrite axes in <111> plates. These plates transverse with voluentary unit cube face in <110> crystallographic direction (fig. Se). Volume concentration and geometry (thickness and length) of TCP phase plates increase while increasing test period at 1100 °C. Lamellate
665 gth Liege Conference : Materials for Advanced Power Engineering 2010 edited by J. Lecomte-Beckers, Q. Contrepois, T. Beck and B. Kuhn.
isolations in single crystal structure are not visualized after creep tests at 1150 °C during 3000 hours test period.
3.3 Deformation porosity
One of mechanisms of Ni-based superalloys stationary creep at elevated temperatures and low tension provides for nonconservative motion of dislocations along interface of y'/y plates in raft-structure. Supernumerary vacancies, condensed in pores of deformation origin, were generated [9] These pores were visualized in single crystals at 1000, llOO and 1150 °C and low tension, i.e. at 1000 hours test period. Pores are located in interaxial areas at lineages of blocks (fig. 6a). Unique feature of such pores is contained in cuboidal morphology and enclosing <110> plates. Such enclosure is stipulated because of significant anisotropy of surface tension at I 000-1100 °C test temperatures. Pores in the single crystals are stress raisers, and micro cracks are initiated while tertiary creep. Micro crack initiation originates either on coarse homogenized spherical form pores, or fine deformation pores, generated while high temperature creep in wide test period. It depends on temperature and test period. At temperatures, more than 1000 °C and low stress i.e. long rupture time micro crack initiation take place by deformation pores (fig. 6b).
a 6 Fig. 6. Morphology ofdeformation pores at lineages ofblocks in specimen volume (a); micro crack nucleation on deformation pores (b)
Micro cracks initiate in interdendritic areas. Microcrack coalescence in most damaged area reduces to a main crack propagation and final specimen destruction.
4 Conclusions
l. The Youngs modulus anisotropy remains in all temperature range 20-1150 °C, <111> single crystals have the maximum value of Young modulus, <00 1> single crystals - minimum. 2. Significant anisotropy of ultimate strength and yield strength is observed only down to 800 °C. The anisotropy almost degenerates up to this temperature, i.e. crystals begin to soften with the same speed. At 800 oc strength properties of <001> and <011> single crystals are at peak values, plasticity properties - minimum. Monotonic decreasing of strength is typical for
666 gth Liege Conference : Materials for Advanced Power Engineering 2010 edited by J. Lecomte-Beckers, Q. Contrepois, T. Beck and B. Kuhn.
3. Significant stress rupture anisotropy for single crystal of three orientations is observed,
5 References
l. Shalin R.E., Svetlov I.L. Kachanov E.B.,Toloraia V.N., Gavrilin O.S. Single crystals of nickel-base superalloys. Mashinostroenie (Moscow) 1997, 336 p. (in Russian). 2. Svetlov I.L., Petrushin N. V., Golubovskiy E.R. ., Khvatskiy K.K., Shchegolev D. V. Mechanical properties of single crystals from high temperature alloy containing the rhenium and ruthenium //Deformation and Fracture of Materials, 2008, N!!ll, pp.26-35. 3. Kablov E. N, Golubovskiy E.R, High temperature strength of nickel alloys, Mashinostroenie (Moscow, Russia) 1998,464 p. (in Russia). 4. Koji Kakehi. Influence of precipitate size and crystallographic orientation on strength of a single crystal Ni-base superalloy //Materials Transactions, JIM, 1999, v.40, N!!2, pp.l59-l67. 5. Miner R.V., Voigt R.C., Gayda J., Cabb T.P. Orientation and temperature dependence of some mechanical properties of the single crystal nickel-base superalloy Rene N4: Pt.l. Tensile behavior I /Metallurgical Transaction. 1986, v.17 A, N!!3, pp.491-496. 6. Kear B.H., Piearcy B.J. Tensile and creep properties of single crystals of the nickel base superalloy Mar-M200 //Transaction Met. Sos. AIME, 1967, v.239, pp.l209-l215. 7. Walston S., Cetel A., MacKay, Hara K.O., Duhl D. Dreshfield R. Joint development of a fourth generation single crystal superalloy //In: Superalloys 2004, TMS, 2004, pp.l5-24. 8 Golubovskiy E.R., Nozhnitsky Y.A., Svetlov I.L., Relationship of Stress Rupture and Crystallographic Orientation for Ni-base Superalloys Single Crystal /!Proc. of the European Conference for Aerospace Sciences (EUCASS), 2005, Moscow, Russia, N. 4. 9. Epishin A., Link T. Mechanisms of high temperature creep of nickel-base superalloys under low applied stresses //Philosophical Magazine, 2004, v.84, N!!l9, p.p.l979- 2000.
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