Report on creep and LCF-Data on ODS-Materials for GEN IV-Reactors

Report on creep and LCF-Data on ODS-Materials for GEN IV-Reactors A Hobt 1, A. Klenk 1 M. Serrano 2, R. Hernandez 2 1 MPA University of Stuttgart 2 CIEMAT, Technoligy Department Abstract For the next generation of high temperature nuclear reactors new structural materials are necessary to meet the requirements of the loading and sufficient life time. Therefore several Oxide-Dispersion-Strengthened (ODS) ferritic-martensitic materials have been development. Within the GetMat-project there have been testes different materials, differing in chromium content, under creep, fatigue and creep-fatigue loading at temperatures between 600 C and 750 C. The materials were made from sheets and rods. The work was then continued in the MaTiSSE-project for tube materials. This contribution deals with the evaluation of the uniaxial creep tests for rod material with specimens extracted in different orientations and plate material. Especially for structural materials the properties and material behaviour in different orientations and different products are essential to know within the life-time assessment and the evaluation of the operational loadings in terms of stress and strain field analysis. This can be achieved by providing appropriate material laws for an optimized description of the material behaviour.

Introduction In order to keep up the competitiveness of the European nuclear power stations in the long run, innovative solutions must be found, guaranteeing a more efficient and at the same time safe operation of the power plants. The increase in efficiency normally goes along with an increase in the operating temperature and pressure. At the same time the power generation is on the step to a new demand on the operation due to a rising portion of renewable energies forcing a flexible operation of the power plants. To meet the demand, new materials must be developed with increased strength. Beside the material development, the behaviour of the materials under operating conditions must be described using adequate material models. For this purpose the GETMAT project was launched to address the cross-cutting aspects of classical ferritic-martensitic materials and the new Oxide-Dispersion-Strengthened(ODS) materials for core and primary circuit components. This envelopes the issues of qualification of the materials by mechanical testing, investigation of joining and fabrications techniques and the development of modelling techniques based on a fundamental understanding of the material. The present publication deals with the mechanical characterisation of ODS alloys with different chromium contents and the link to the microstructure which enables an interpretation of the material behaviour. Material characterisation and specimen fabrication Two ferritic ODS alloys and one martensitic ODS alloy are characterised. A 14Cr ODS extruded bar, supplied by CEA (J27 heat) and a 12Cr ODS and 9Cr ODS plates supplied by Kobelco in the framework of the GETMAT 7FWP project [2]. The 14Cr ODS bar was produced by CEA by mechanical alloying f a master alloy of composition (wt. %) 13.98Cr, 1.03W, 0.39Ti, 0.29Mn, 0.32Si, 0.17Ni with 0.3 wt% Y2O3 under hydrogen atmosphere in a vertical attritor. Subsequently, the material was hot extruded in the form of bar at 1100 ºC and annealed for 1.5 h at 1050 ºC [3]. By this process, there are generated two regions with different mechanical properties and also material properties, which can be clearly distinguished, see Figure 1a). The 12Cr ODS plate were produced with a pre alloyed metal powder of composition (wt. %) 11.59Cr, 1.87W, 0.22Ti and 0.1Si was prepared by argon gas atomisation method. The powders were mechanically alloyed in dry type attrition ball mill with 0.23 wt. % Y 2 O 3 and subsequently extruded at 1150 C, hot forged at 1150 C and annealed at 1150 C for 1h. Finally, the forged plates were cold rolled with 40 % reduction and annealed to re-crystallisation at 1200 C for 1 hour [4], see Figure 1b). The 9Cr ODS plate was produced my mechanical alloying T91 powders with 0.3 wt. % Y 2 O 3. These powders were extruded at 1160ºC, hot forging at 1150ºC and heat treatment at 780ºC for 1 hour and air cooled afterwards. Finally a cold working up to 40% reduction were performed and heat treated for normalization at 1050ºC for 1 hour and tempered at 750ºC for 1 hour, [5]. The fabrication process leads to inhomogeneous grain size distribution comparing the longitudinal and transversal direction. The microstructure of the 14Cr ODS bar show an elongated-grained structure parallel to the extrusion direction, see Figure 3a.

Figure 1: Product forms from which the specimens have been extracted a) 14Cr b) 14Cr c) 12Cr / 9Cr The mean gran size in the transverse orientation is 432 nm length and 415 nm width, while the size in the longitudinal orientation is 826 nm length and 496 nm width. A preferential crystallographic orientation of the grains along <110>// to the extrusion direction is observed. A detailed microstructure description of the 14Cr ODS bar can be found in [6]. Regarding the microstructure of the 12Cr ODS plate, in all the sections studied it is possible to distinguish a bimodal grain size distribution, existing small grains with sizes usually lower than ten micron and strange bent shapes and some big and elongated grains along the extrusion direction which sizes are bigger than tens of micron up to two hundred micron, see Figure 3b. This could be an indication of an incomplete recrystallization on the fabrication process. EBSD maps show that the smallest grains seem to have an orientation along <110> in the ED, while larger grains have not got a preferential orientation. No detailed microstructure of the 9Cr ODS were performed but information can be found in [7]. Figure 2: Microstructure in the as-received state b) 12Cr a) 14Cr

Results from conducted tests and first derivation of material laws Within the GETMAT project the 9-14 %Cr-ODS materials have been analysed by basic experiments, comprising, both in air atmosphere and Argon atmosphere at high temperatures: Tensile tests at different strain rates Creep tests Fatigue tests Based on these tests it is possible to describe the material behaviour with macroscopic material models which then can be used in the analyses of components under high temperature operation. For the design of the creep tests and the fatigue tests, plastic flow behaviour of the materials was derived by tensile tests. To quantify the influence of the loading speed, the strain rate of the tensile tests was changed for the 9Cr ODS and the 14Cr ODS materials. Due to the high temperature of 750 C, all tests at MPA Stuttgart on the 14Cr ODS material were conducted in Argon atmosphere at ambient pressure. The tests on the other two material were conducted in ambient air. The basic material characterisation was done by tensile tests. At MPA Stuttgart, the specimens were all extracted in longitudinal direction. The testing temperature was the main temperature also for the further investigations, see Table 1, together with the main results. At CIEMAT, also specimens in transversal direction have been analysed. At CIEMAT also tensile tests were performed, using with dog-bone specimens (gage section 15x3x2 mm) mechanised in longitudinal (L) and transverse (T) orientation for the 14Cr ODS and 12Cr ODS steel, defined as the direction of the loading during the test, for the bar and plate. Tensile tests were performed at 22 C, 400 C, 600 C and 700 C a displacement rate of 0.1 mm/min (corresponding to a strain rate of 1x10-4 /s) in a servo-hydraulic MTS testing machine. Total elongation and reduction in area measurements were performed on the broken specimens. A summary is given in Table 2. Table 1: Summary of conducted tensile tests, MPA Stuttgart Specimen Material Temp / C Strain rate / 1/s UTS / MPa Strain at failure /% 2AB4 10-4 315 1,8 2AB1 2 10-5 315 1,34 14Cr ODS 750 2AB2 2 10-5 306 1,31 2AB3 10-6 283 0,54 X2C/2D 12Cr ODS 650 10-5 402 13,8 9CB2 10-3 592 37 9CA2 10-4 488 35,5 9Cr ODS 600 9CB1 10-5 394 27,9 9CA1 10-6 354 16,4

Table 2: Summary of conducted tensile tests, CIEMAT Specimen Material Temp / C Strain rate / 1/s UTS / MPa Strain at failure /% 14B13-L1 24 1.00E-04 1112 15 14B13-L4 24 1.00E-04 1131 20 14B22-L2 27 1.00E-04 1123 22 14B22-L1 29 1.00E-04 1127 22 14B22-L6 400 1.00E-04 910 14 14B13-L23 400 1.00E-04 905 17 14B13-L5 400 1.00E-04 893 20 14B22-L3 600 1.00E-04 461 12 14B22-L4 600 1.00E-04 471 11 14B13-L24 600 1.00E-04 423 22 14B22-L7 14Cr ODS 700 1.00E-04 396 7 14B22-L8 700 1.00E-04 356 9 14B22-L8 700 1.00E-04 356 9 14B13-L30 700 1.00E-04 400 12 14B13-L2 700 1.00E-04 374 7 14B13-L3 700 1.00E-04 379 6 14B22-L10 800 1.00E-04 300 2 14B22-L11 800 1.00E-04 298 7 14B22-L12 800 1.00E-04 307 6 14B13-L6 800 1.00E-04 292 2 14B13-L7 800 1.00E-04 269 8 12L1 24 1.00E-04 1160 12 12L2 24 1.00E-04 1138 10 12L3 400 1.00E-04 861 7 12L4 400 1.00E-04 896 11 12L5 600 1.00E-04 482 20 12Cr ODS 12L6 600 1.00E-04 478 21 12L7 700 1.00E-04 317 9 12L8 700 1.00E-04 335 11 12L10 800 1.00E-04 258 7 12L9 850 1.00E-04 229 1 9L1 24 1.00E-04 1098 15 9L4 25 1.00E-04 1098 17 9L2 400 1.00E-04 864 6 9L3 400 1.00E-04 888 12 9L5 9Cr ODS 600 1.00E-04 397 22 9L6 600 1.00E-04 393 21 9L7 700 1.00E-04 229 9 9L8 700 1.00E-04 237 9 9L9 800 1.00E-04 170 10 In Table 3 and Table 4 the parameters and the main results of the conducted creep tests are summarised. Again, the creep test specimens at MPA Stuttgart were all tested in longitudinal material direction. Some of the specimens have been dismounted at the end of the GETMAT project. The tests were stopped because the results were sufficient for the characterisation in correlation

with the commonly used ferritic-martensitic steels such as the P92. And also the remaining testing time could not be estimated precisely. Table 3: Summary of conducted creep tests, MPA Stuttgart Specimen Material Temperature / C Stress / MPa Time to rupture / h Strain at failure /% 3A1/B1 250 10.3 2.89 3A2/B2 230 15.6 1.36 4A3/B3 14Cr ODS 750 170 227.3 1.68 4A2/B2 150 4,859.4 0.734 4A1/B1 130 dism. after 6.190 - X3A/X3B 12Cr ODS 650 180 dism. After 4369-9CB3 170 2301 1,7 9Cr ODS 600 9CA4 250 199 4,2 Table 4: Summary of conducted creep tests, CIEMAT Sepecimen Material Temperature T / C Stress / MPa Time to rupture / h 600 205 7416 CIEMAT-J27 Longitud. 600 600 281 326 8088 216 600 387 528 CIEMAT-J27 Tranverse CIEMAT-J27 Longitud. 14Cr ODS 600 382 48 600 278 72 600 205 216 650 182 4248 650 184 2856 650 204 2712 650 225 888 650 227 2856 650 253 192 650 255 216 650 255 48 650 281 48 650 281 96 650 281 24 650 324 24 650 324 48 After the tensile tests, there have been also conducted fatigue tests at MPA Stuttgart according to the summary in Table 5. For the 9Cr ODS and the 12Cr ODS there have been conducted creep-fatigue tests with a hold time of 10 minutes in the tension section. The creep fatigue tests for the 14Cr ODS could not be finished due to buckling of the specimens.

Table 5: Summary of conducted fatigue tests Specimen Material Temperature / C Total strain range / % Cycles to failure / - A1/B1 0,5 9101 A3/B3 0,8 397 14Cr ODS 750 C1/D1 0,6 1458 E3/F3 1,5 9101 X1A/X1B 0,8 778 12Cr ODS 650 X1C/X1D 0,8 (10 Min HT in tension) 379 9CB6 0,5 17108 9CA6 0,6 14692 9CB5 0,8 1953 9Cr ODS 600 9CA5 1 1040 9CB7 0.5 (10 Min HT in tension) 3097 9CA7 0.8 (10 Min HT in tension) 246 In the following section, the results from the described tests above are shown in more detail. In Figure 3 the results on the 14Cr ODS material are shown. It can be seen, that at different strain rates at the highest temperature the strength is on almost the same level, but the elongation at failure is decreasing with decreasing testing speed, Figure 3a). When the transverse and the longitudinal direction is compared, it can be seen, that the longitudinal direction has a higher strength, tested for different temperatures, Figure 3b). Stress σ / MPa 350 300 250 200 150 100 50 0 2AB3 Figure 3: Tensile test results on 14Cr ODS a) longitudinal b) longitudinal and transverse 2AB1 2AB2 2AB4 2AB4 [dε/dt = 10-4 /s] 2AB1 [dε/dt = 10-5 /s] 2AB2 [dε/dt = 10-5 /s] 2AB3 [dε/dt = 10-6 /s] 14Cr-ODS, T = 750 C, Argon-Athmosphere 0,0 0,5 1,0 1,5 2,0 2,5 Strain ε / % The tensile test results for the 12Cr ODS are shown in Figure 4a). It was conducted one tensile test only, to have a comparison of the strength with the 9Cr- and 14Cr material to be able to design the loads for the fatigue tests and the creep tests. The results for the conducted tests on the 9Cr ODS

material are shown in Figure 4b). For the 9Cr material a strong influence of the strain rate can be seen, not only on the strength properties but also on the ductility. The 9Cr ODS and 12Cr ODS exhibit larger ductility than the 14Cr ODS, although the temperatures are lower. This behaviour is not known from the classical 9-12Cr ODS martensitic materials. Figure 4: Tensile test results on 12Cr and 9Cr ODS a) 12Cr b) 9Cr Stress σ / MPa 500 450 400 350 300 250 200 150 100 50 0 12CrODS T = 650 C ε& = 10-5 /s End of clip gauge control 0 2 4 6 8 10 12 14 Strain ε / % Stress σ / MPa 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0 9CB2 [dε/dt = 10-3 /s] 9CA2 [dε/dt = 10-4 /s] 9CB1 [dε/dt = 10-5 /s] 9CA1 [dε/dt = 10-6 /s] 9Cr-ODS, T = 600 C 0 10 20 30 40 Strain ε / % For the analysis of the long-term behaviour of the materials, creep tests were conducted. For the tests in Argon atmosphere, the specimen was put in a elastic, soft bellow, which was filled with Argon at a slight overpressure against ambient conditions to ensure that the specimen is not getting in contact with air, see Figure 5. Figure 5: Test setup for the creep tests in Argon atmosphere Digester with soft bellows The tests at MPA Stuttgart where conducted on specimens out of the longitudinal direction. At CIEMAT there have been conducted several specimens in transverse direction, see Figure 6. It can be seen, that the ODS alloys have a higher creep strength than the classical P92 martensitic material, in case of the 14Cr ODS even at a higher temperatures. Also shown are results obtained at JRC at 650 C in air, [2].The 9Cr ODS and the 12Cr ODS alloys are on the level of the 14Cr ODS at 650 C. It has to be mentioned, that the 9Cr ODS alloy was tested at 600 C. Comparing only different testing temperatures of the 14Cr ODS experiments, the expected increase in creep strength with decreasing temperature is evident. What also can be seen, that beside the high scatter of the material itself, the specimens from transverse direction exhibiting a lower creep strength than the specimens form longitudinal direction. This will be discussed in connection with the microstructural investigations. Beside the high creep strength also the ductility must be considered. Looking at the creep strain

evolution, see Figure 7, it can be seen, that with increasing testing time, the creep ductility decreases and is generally on a level between 2-4% which is classically defined as creep brittle. This must be taken into consideration when assessing these material parameters in a structural component design. Figure 6: Creep strength diagram, ODS materials a) Comparison different ODS alloys b) 14Cr ODS, diff. temperatures / directions Creep rupture stress R u/t/t / MPa 450 400 350 300 250 200 150 100 50 0 running at the End of GETMAT * Stress increased from 120 Mpa to 180 MPa 14CrODS [750 C, Argon] 12CrODS [650 C] 9CrODS [600 C] 14CrODS [650 C, Data from JRC, air] all longitudinal direction P92 [650 C] Mean Data dism.( ) dism.( ) 0,1 1 10 100 1000 10000 100000 Time to rupture t U / h * Creep rupture stress R u/t/t / MPa 450 400 350 300 250 200 150 100 50 running at the End of GETMAT * Stress increased from 120 Mpa to 180 MPa * 14CrODS [750 C, Argon] dism.( ) 14CrODS [650 C, Data from JRC, air] 14CrODS [650 C, Data from CIEMAT, air] 14CrODS [600 C, Data from CIEMAT, air] 14CrODS [600 C, Data from CIEMAT, air, Trans.]* *rest in longitudinal direction P92 [650 C] Mean data 0 0,1 1 10 100 1000 10000 100000 Time to rupture t U / h Figure 7: Creep deformation diagrams a) 14Cr ODS b) 9Cr ODS Creep strain ε c / % 10 1 0,1 0,01 14CrODS T = 750 C 3A1/3B1 [σ n = 250 MPa] 3A2/3B2 [σ n = 230 MPa] 4A3/4B3 [σ n = 170 MPa] 4A2/4B2 [σ n = 170 MPa] 3A3/3B3 [σ n = 150 MPa] 4A1/4B1 [σ n = 130 MPa] B B 0,1 1 10 100 1000 10000 Time t / h B B test dur. 4502 hrs. Creep strain ε c / % 10 1 0,1 0,01 9CrODS T = 600 C 9CB3 [σ n = 170 MPa] 9CA4 [σ n = 250 MPa] 0,1 1 10 100 1000 10000 Time t / h The fracture anisotropy of the 14Cr ODS and 12Cr ODS steels has been characterised by tensile, impact and small punch tests, see [9]. The longitudinal orientation, parallel to the extrusion/rolling direction, shows in general better mechanical properties than the transverse orientation. This anisotropy found in the fracture behaviour could be attributed to the elongated grained structure and to the grain boundary weakness due to the presence of micron size oxide particles. In this paper different behaviour is found for the creep properties of the 14Cr ODS and 12Cr ODS steels. For the 14Cr ODS the anisotropy on the creep properties is clear, being the transversal specimens less resistant to creep strength. Whereas the 12Cr ODS do not show an appreciable effect of specimen orientation. This different behaviour can be due a very different grain morphology and a different grain aspect ratio, being higher for the 14Cr ODS than the one for the 12Cr ODS, were the grains are

larger but equiaxed and thus can enhance creep strength because grain boundary sliding is inhibited, see [10]. Figure 8: Determination of anisotropy a) b) c) d) The conducted low-cycle-fatigue tests performed at MPA are shown in Figure 9, together with the results obtained at JRC in air medium and from ENEA in GFR atmosphere, [2].

Figure 9: Low-cycle-fatigue GETMAT results Total strain range ε A / % 1 0,1 * not evaluated, thread failure 14Cr [650 C, data from JRC, Air] 14Cr [data WP2 (750 C, GFR env.)] 14Cr [750 C, data from JRC, Air] 14Cr [750 C, Argon] 12Cr [650 C, Air] 12Cr [650 C, Air, t H = 10min] 9Cr [600 C, Air] 9Cr [600 C, Air, t H = 10min] Manson-Coffin 14Cr Manson-Coffin 9Cr 10 100 1000 10000 Cycles to failure N A / - *( ) running at the End of GETMAT It can be seen, that the results in Argon are creating a common scatter band with the results in GFR at lower strain ranges. Compared the results in air from JRC, there can be seen no influence of the temperature. What is worth mentioning is that the results from the 12Cr ODS, test in air, matching the results from the 14Cr ODS in Argon but at a lower testing temperature and the 9Cr ODS results come together with the results in air of the 14Cr ODS alloy, but the temperature is also markedly lower. The effect of a hold time seems to be as expected based on the knowledge, see for example [3], [4]. First modelling approach Based on the results obtained from the basic material tests, preliminary modelling approaches could be derived. Based on this modelling it is possible to adapt assessment procedures, e.g. according to the standards, for the design of structural components in high temperatures power plants. For the numerical analysis of transient loading conditions, the inelastic strains must be calculated to evaluate stress and strain ranges during cyclic operation. Therefore the dependence on the strain rate is necessary to describe the materials behaviour correctly. A very simple approach was chosen, based on the tensile tests, conducted at different strain rates. The modelled stress-strain curves for the 9Cr- ODS material are shown in Figure 10. To model the influence of the strain rate, a visco-plastic model based on a formulation from Chaboche was used, [3], [4], see Eq. (1)-(3). Figure 10: Modelling approach to describe strain rate dependency = = (1) (2) = = (3) Here ε& is the total inelastic strain rate tensor and in p & is the derivation in time of the accumulated inelastic strain

The next step in describing the material behaviour was to describe cyclic loading by modelling the S- N curves to gain the relevant number of cycles to crack initiation. This can be done by using a Manson-Coffin equation, see Eq. (4)-(5). ' σ f b ε = ε + ε = ( 2N) + ε ( 2N) c (4) a a,el a,pl n ' = b c E K' ' f ( b ) ' c σ f ' = (5) ε f The parameters have been fitted based on the stable hysteresis loop at mid-life. The strain amplitude was splitted in the elastic part ε a,el and plastic part ε a,el. Within the equation the yield strength σ f and the Young s Modulus E, gained from the tensile tests was adopted. The results for the 14Cr ODS and 9Cr ODS alloy is shown in Figure 11. Since for the 12Cr ODS only one specimen was tested, it was not possible to gain the equations for this alloy. Figure 11: Possible modelling of the fatigue behaviour Total strain range ε A / % 1 14Cr [650 C, data from JRC, air] 14Cr [data WP2 (750 C, GFR env.)] 14Cr [750 C, data from JRC, air] 14Cr [750 C, Argon] Manson-Coffin-Fit ε el Total strain range ε A / % 1 ( )* not evaluated, thread failure 14Cr [650 C, Data from JRC, air] 14Cr [750 C, Data from JRC, air] 9Cr [600 C, air] 9Cr [600 C, t h = 10 min, air] Manson-Coffin-Fit ε el ε pl ( )* 0,1 ε pl 10 100 1000 10000 Cycles to failure N f / - 0,1 10 100 1000 10000 Cycles to failure N f / - In the last step, the creep behaviour was modelled using a modified creep law based on a Graham and Walles formulation, [5], see Eq (6) and Eq. (7). This formulation was developed at MPA Stuttgart, [6], the creep strain rateε& is dependent on the creep strain e and the von Mises stress σ vm. The tertiary creep stage is described using a damage parameter D. To account for the dependency of the damage evolution on the multiaxiality of the stress state, the parameter q according to [7] was introduced. n n2 1 σ σ A 1 m1 A2 m2 ε& 10 vm ε vm v 10 ε (6) = + v ( 1 D) ( 1 D) nd2 nd2 α α AD 3 1 ~ md 3 1 AD 2 ~ md2 = 10 + 10 D & σ ε σ ε (7) q vm q vm This was applied on the creep strain curves gained from the 14Cr ODS material, since there were available the most creep data. The results is shown in Figure 12. As it can be seen, there is a strong sensitivity to the stress. Therefore the material law is not capable of describing the full range of test stresses. The material parameters, the exponents of the creep equations, have been found to fit best the creep strain curves in the long term regime.

Figure 12: Modelling of the creep deformation 10 14CrODS T = 750 C Creep strain ε c / % 1 0,1 3A1/3B1 [σ n = 250 MPa] 3A2/3B2 [σ n = 230 MPa] 4A3/4B3 [σ n = 170 MPa] 4A2/4B2 [σ n = 170 MPa] 3A3/3B3 [σ n = 150 MPa] 4A1/4B1 [σ n = 130 MPa] FIT G&W 0,01 0,01 0,1 1 10 100 1000 10000 Time t / h Microstructural analysis and link to experimental results Based on the microstructural investigations, the behaviour in the experimental results shown above can be explained. This will comprise the creep behaviour and the material behaviour under cyclic loading and it is necessary for a safe design for these materials. In Figure 13 the specimen 4A2/B2 is shown after the rupture. As it can be seen, the fracture was brittle- there is less deformation in the ruptured cross-section. On the microsection cavities are visible and secondary cracking along the grain boundaries in longitudinal direction. This is linked with the low creep ductility after rupture. What must be mentioned are the damage phenomena which might be influenced by the Argon atmosphere. Figure 13: Example of microstructure of 14cr ODS after creep loading As it can be seen from TEM analysis, Figure 14, the microstructure does show relevant changes during creep loading. The example is taken from the creep test specimen on 14Cr ODS alloy, tested at 750 C. The pictures are showing the longitudinal direction exemplarily, since the findings in the transverse direction are similar. When evaluating the dislocation density and the subgrain size there can be measured only small changes. Again this validates the brittle behaviour without the softening

of the material through classical damage mechanisms such as precipitation coarsening or other changes of the microstructure. Figure 14: TEM analysis of microstructure, 14Cr ODS, longitudinal direction, After creep loading tu = 4859 h, T = 750 C As received Regarding the cyclic behaviour of the ODS materials, the high strength of the material will cause a very strong dependency of the microstructure. At a strain range of 1.5 % which includes plastic portions, there can be seen, that the fracture is not homogeneously perpendicular to the loading direction, but along the grain microstructures of the material, see Figure 15. On the fracture surface the striations can be seen clearly. But there are also secondary cracking visible which might be due to the stress concentrations at the grain boundaries due to their small amount of deformability. Figure 15: Microstructural analysis of 14Cr ODS LCF specimen, T = 750 C, ε = 1.5 %, The effect of the fracture along the laths of the grain structure is more evident for the 9Cr ODS and the 12Cr ODS material, although their deformability in the first glance at the tensile tests is larger

than for the 14Cr ODS. In Figure 16, the micrographs are showing a LCF specimen of 9Cr ODS alloy showing a crack path where the angle between the fracture surface and the loading direction is changing. On the fracture surface there can be seen also secondary cracking. In Figure 17 micrographs of a specimen, also from 9Cr ODS, with the same strain range but with a hold time in tension section is shown. Here the secondary cracking along the grain structure in longitudinal direction is more evident. Figure 16: Microstructural analysis of 9Cr ODS LCF specimen, T = 600 C, ε = 0.8 % Figure 17: Microstructural analysis of 9Cr ODS LCF specimen, T = 600 C, ε = 0.8 %, t h = 10 min Conclusions On different ODS alloys at their probable operating temperature different mechanical characterisations have been conducted from which the fundamental yield properties, fatigue behaviour and the creep properties could be derived. Based on these results component design

according to the standards became possible. However, the results are showing that the material behaviour exhibits higher strength at high temperatures than the classical martensitic steels. This was the aim when producing the ODS alloys. But there are some issues that must be considered within the component design. The first point is the brittle material behaviour which influences safety factors and the design of the component. The second point is due to the anisotropic material structure. It must be considered, that the strength of the material is not isotropic. Therefore especially for components multiaxial loading must be closely analysed. Based on this the behaviour under multiaxial loading in creep and creep fatigue regime must be analysed and the material behaviour must be characterised. Outlook to the next phase The next steps is the mechanical characterisation of cladding tubes for Generation IV nuclear reactors made from ODS materials with the focus on multiaxial loading. This will be the work within the MATISSE project which is ongoing. The testing of the cladding tubes is done under internal pressure generating a two dimensional state of stress at the minimum. Since joining by welding is difficult for those materials and the thin walled tubes in particular, first a mounting device must be constructed. A first design was provided by VTT, which is the leader of the workpackage for the mechanical characterisation. Acknowledgements The presented work was funded by the EU within the GETMAT-Project, GA No. FP7-212175. References [1] Y. d. C. R. L. G. M. M. B. R. C. F. D. M. Serrano, ODS Fabrication, Basic Characterization And Thermal Stability, in GETMAT workshop, Berlin, September 17-19, 2013. [2] Y. De Carlan, Production and characterisation of 14Cr1WYTi Ferritic ODS, 2005. [3] G. Muller, Supply and characterization of 12CrODS, 2005. [4] A. e. Heinzel, Production and characterization of a 9CrMoVNb ODS steel, 2012. [5] A. e. a. García-Junceda, Influence of the microstructure on the tensile and impact properties of a14 Cr ODS steel bar, Materials Science & Engineering A556, pp. 696-703, 2012. [6] L. D. L. J. A. Straßberger, High temperature tensile properties of oxide dispersion strengthened T91 and their correlation with microstructural evolution, DOI 10.1179/1743284714Y.0000000562, 2014.

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