March 5, In-situ observation of morphological changes of γ' precipitates in pre-deformed single-crystal Ni-base superalloy This research project has been supported by the European Commission under the 6th Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures. Contract n°: RII3-CT (NMI3). 1 Nuclear Physics Institute Řež near Prague, Czech Republic 2 Helmholtz Centre Berlin for Materials and Energy GmbH, Glienickerstr. 100, D Berlin, Germany 3 Federal Institute for Materials Research and Testing (BAM), Unter den Eichen 87, Berlin, Germany 4 ILL Grenoble, France 5 Research Center Řež, CZ Řež near Prague, Czech Republic P. Strunz 1,5, G. Schumacher 2, H. Klingelhöffer 3, A. Wiedenmann 2,4, J. Šaroun 1,5, U. Keiderling 2,

March 5, Nuclear Physics Institute Řež near Prague, Czech Republic ( ) 2 Helmholtz Centre Berlin for Materials and Energy GmbH, Glienickerstr. 100, D Berlin, Germany 3 Federal Institute for Materials Research and Testing (BAM), Unter den Eichen 87, Berlin, Germany 4 ILL Grenoble, France 5 Research Center Řež, CZ Řež near Prague, Czech Republic P. Strunz 1,5, G. Schumacher 2, H. Klingelhöffer 3, A. Wiedenmann 2,4, J. Šaroun 1,5, U. Keiderling 2

March 5, In-situ observation of morphological changes of γ' precipitates in pre- deformed single-crystal Ni-base superalloy This research project has been supported by the European Commission under the 6th Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures. Contract n°: RII3-CT (NMI3).

March 5,  Coherent cuboidal γ’-precipitates (L 12 lattice) in γ fcc solid solution => excellent strength of single crystal superalloys  Exposure to an external creep load: superposition of internal and external stress fields => anisotropy of the stress fields at differently aligned γ/γ’ interfaces  At sufficiently high temperatures, this anisotropy causes directed diffusion of alloying elements which results in anisotropic coarsening of the γ’-precipitates, so-called rafting.  rafting can also occur at thermal treatment without simultaneous external load [1-3] in case the specimen was pre- deformed either plastically [1] or by creep [2-3].  It was found (also using in-situ SANS [4, 5] ) that rafting can occur if a certain threshold for creep-strain is exceeded during the pre-deformation.  The results point to an anisotropic arrangement of dislocations in the pre-deformed specimen which leads to the observed formation of γ’-rafts at high temperatures.  Coherent cuboidal γ’-precipitates (L 12 lattice) in γ fcc solid solution => excellent strength of single crystal superalloys  Exposure to an external creep load: superposition of internal and external stress fields => anisotropy of the stress fields at differently aligned γ/γ’ interfaces  At sufficiently high temperatures, this anisotropy causes directed diffusion of alloying elements which results in anisotropic coarsening of the γ’-precipitates, so-called rafting.  rafting can also occur at thermal treatment without simultaneous external load [1-3] in case the specimen was pre- deformed either plastically [1] or by creep [2-3].  It was found (also using in-situ SANS [4, 5] ) that rafting can occur if a certain threshold for creep-strain is exceeded during the pre-deformation.  The results point to an anisotropic arrangement of dislocations in the pre-deformed specimen which leads to the observed formation of γ’-rafts at high temperatures. Ni-superalloys - rafting

March 5,  in situ SANS study of evolution of γ’-morphology in pre-deformed specimens of SCA425 superalloy  SANS: V4, BENSC, HZ Berlin  in situ SANS study of evolution of γ’-morphology in pre-deformed specimens of SCA425 superalloy  SANS: V4, BENSC, HZ Berlin Method Aim of the SANS experiment  to obtain a better understanding of the process of raft-formation after pre-deformation

March 5, Samples and thermomechanical treatment strainTemperaturein-situ during SANS exp.Result 0.1 %RTT= 1050°C, t=20 hNo rafting 0.5 %RTT= 940°C/14h /1050°C /8h /1100°C /9h subsequently, without cooling No rafting 2.0%RTT= 950°C, t=21 hNo rafting 2.0%700°CT= 1050°C, t=11.5 hIndication of rafting 2.0%950°CT= 1050°C, t=6.5 hRafting  SCA 425 experimental Ni-base superalloy  wt%: Co: 5, Cr: 16, Mo: 1, W: 4, Al: 4, Ti: 2, Ta: 5  originally: large volume fraction of cubic γ’-precipitates  uniaxial compressive load along [001] (έ=1×10 -4 s -1 )  could result in needle-like rafts along [001] direction  SCA 425 experimental Ni-base superalloy  wt%: Co: 5, Cr: 16, Mo: 1, W: 4, Al: 4, Ti: 2, Ta: 5  originally: large volume fraction of cubic γ’-precipitates  uniaxial compressive load along [001] (έ=1×10 -4 s -1 )  could result in needle-like rafts along [001] direction

March 5, [1] M. Veron, Y. Brechet und F. Louchet, Scr. Mater. 34 (1996) 1883 [2] N. Matan, D.C. Cox, C.M.F. Rae und R.C. Reed, Acta Mater. 47 (1999) 2031 [3] P. Henderson, L. Berglin und C. Jansson, Scr. Mater. 40 (1999) 229 [4] M. Veron, P. Bastie, Acta Mater. 45 (1997), [5] N. Ratel, B. Demé, P. Bastie and P. Caron: Scripta Materialia 59 (2008) [1] M. Veron, Y. Brechet und F. Louchet, Scr. Mater. 34 (1996) 1883 [2] N. Matan, D.C. Cox, C.M.F. Rae und R.C. Reed, Acta Mater. 47 (1999) 2031 [3] P. Henderson, L. Berglin und C. Jansson, Scr. Mater. 40 (1999) 229 [4] M. Veron, P. Bastie, Acta Mater. 45 (1997), [5] N. Ratel, B. Demé, P. Bastie and P. Caron: Scripta Materialia 59 (2008) References Subsequent pure thermal treatment (1050°C) => rafts Thermo- mechanical load (creep) at 950°C => no rafts

March 5, SCA, prestrained at 950°C hour at 1050°C [the grey scale map shows measured 2D data and the white equi-intensity lines depict the fitted curve]

March 5, prestrained at 950°C, in situ at 1050°C Modelled in 3D fitted in 2D after various hold at HT sections through the models (optimum fit) Modelled in 3D fitted in 2D after various hold at HT sections through the models (optimum fit) plane after 20 min after 20 min plane after 6.5 h after 6.5 h RT after in-situ 2 nd population (on cooling)

March 5, SANS results - prestrained at 950°C, in situ at 1050°C Average size in compressive load direction increases (over average distance of the original precipitates): Rafting Volume fraction decrease on heating, then constant Rounding of cuboids towards ellipsoidal shape Average size in compressive load direction increases (over average distance of the original precipitates): Rafting Volume fraction decrease on heating, then constant Rounding of cuboids towards ellipsoidal shape

March 5, SCA, prestrained at 700°C, in situ at 1050°C RT before in situ hour at 1050°C RT after in situ RT before in situ hour at 1050°C RT after in situ

March 5, prestrained at 700°C, in situ at 1050°C SANS sensitivity: detailed evaluation brings indication of rafting even though invisible in the raw data and EM See the evolution of the numerical model parameters with exposure time SANS sensitivity: detailed evaluation brings indication of rafting even though invisible in the raw data and EM See the evolution of the numerical model parameters with exposure time plane after 20 min after 20 min plane after 11.5 h after 11.5 h RT after in-situ RT before in-situ

March 5, SANS results - prestrained at 700°C, in situ at 1050°C Average size in load direction increases (approaches average distance of the original precipitates): onset of rafting Rounding of cuboids towards ellipsoidal shape on heating On cooling, the shape partially returns from the elipsoidical to the block-like form Average size in load direction increases (approaches average distance of the original precipitates): onset of rafting Rounding of cuboids towards ellipsoidal shape on heating On cooling, the shape partially returns from the elipsoidical to the block-like form

March 5, Electron microscopy after in situ  rafting clearly visible in the sample pre-deformed at 950°C  not sure otherwise  rafting clearly visible in the sample pre-deformed at 950°C  not sure otherwise  Pre-deformed at 700°C  Pre-deformed at RT  Pre-deformed at 950°C

March 5,  Weak pre-straining (0.1%, 0.5%) does not cause rafting on subsequent heating  Severe pre-straining (2%) performed at elevated temperature causes rafting during subsequent heating  Rafting occurs clearly (hold 6.5 hours at 1050°C) after 2% pre-straining at 950°C  Onset of rafting visible (hold 11.5 hours at 1050°C) after 2% pre-straining at 950°C  No rafting visible (hold at 950°C for 22 hours) after 2% pre- straining at RT  In all cases (strong or weak pre-straining): rounding of cuboids towards ellipsoidal shape on heating, and, on cooling, the shape partially returns to the block-like form  Volume fraction behaves as expected  Weak pre-straining (0.1%, 0.5%) does not cause rafting on subsequent heating  Severe pre-straining (2%) performed at elevated temperature causes rafting during subsequent heating  Rafting occurs clearly (hold 6.5 hours at 1050°C) after 2% pre-straining at 950°C  Onset of rafting visible (hold 11.5 hours at 1050°C) after 2% pre-straining at 950°C  No rafting visible (hold at 950°C for 22 hours) after 2% pre- straining at RT  In all cases (strong or weak pre-straining): rounding of cuboids towards ellipsoidal shape on heating, and, on cooling, the shape partially returns to the block-like form  Volume fraction behaves as expected Summary

March 5, SANS results - prestrained at RT, in situ at 950°C Average size in all directions has the same evolution (regardless the load axis): no rafting Small rounding of cuboids towards spherical shape on heating Average size in all directions has the same evolution (regardless the load axis): no rafting Small rounding of cuboids towards spherical shape on heating