Dynamic strain aging & Creep Behavior of Alloy 690

Alloy 690, a solid solution superalloy, containing 27-31% Ni, 7-11% Cr, and Fe (wt.%) is used for fabricating steam generator tubing, vitrification equipment for nuclear waste management, etc., for which the alloy is exposed to temperatures as high as 1100 °C at which the alloy may undergo may changes. The present study examines the effects of high temperatures (up to 1100 °C) on the tensile and creep deformation behavior of Alloy 690. It also investigates how these conditions affect the alloy's mechanical properties and fracture behavior, employing advanced material characterization techniques.
Alloy 690 shows dynamic strain ageing (DSA) from ~ 200-600 ºC, with F, A, B, and C-type serrations. The serration types and behaviour (normal- and inverse-PLC) changes with strain, temperature, and strain rate. Normal-PLC activation energy ranges from ~107-120 kJ/mol (attributed to the formation of Cr-C complex), while inverse-PLC is about 250 kJ/mol (attributed to Cr atmosphere). It exhibits ductile fracture in the normal-PLC regime but experiences a mixed-mode failure involving shallow ductility and transgranular brittle cleavage of grains in the inverse-PLC regime.
The creep behavior of the alloy was investigated at temperatures ranging from 700-1100 °C and applied stresses between 7-150 MPa. Creep in the studied temperature and stress ranges is primarily governed by dislocation climb. The alloy exhibits a relatively high apparent activation energy, which can be attributed to the interaction between dislocations and carbide particles formed during creep. Creep fracture behavior is determined by two distinct mechanisms: necking (low-temperature and low-stress condition) and cavitation (high-temperature and high-stress). Extensive creep deformation during necking of the alloy causes dynamic recrystallization of grains. At higher temperatures, such as 1100 °C, extensive precipitation of needle shape Cr2N phase occurs due to internal nitridation of the alloy in the vicinity of cracks and cavities.

1. Dynamic strain aging & Creep Behavior of Alloy 690 Ph.D. Viva-Voce Seminar 18 th August, 2023 Kumar Sourabh Homi Bhabha National Institute Enrolment ID: ENGG01201604019 Guide: Prof. J. B. Singh , HBNI

2. Alloy 690 is a Ni-Cr-Fe solid solution strengthened superalloy Nominal composition (in wt.%) of Alloy 690 and the estimated composition of the alloy under study: Ni Cr Fe Mn S Cu C Nominal Composition (wt.%) Balance (58 min) 28-31 7-11 0.5 max 0.015 max 0.5 max 0.04 max Composition of Alloy 690 under study ( wt.%) 60.77 29.6 9.3 0.27 0.03 0.01 0.02 Alloy 690 may get strength from the secondary carbides (M 23 C 6 ) in the temperature range of 650-900 º C 1 Alloy 690 1 J.B. Singh Alloy 625: Microstructure, Properties and Performance, 2022

3. Applications Pic courtesy: (Tube and tube sheet ) González et al.  Materials . 2022; 15(1):261. https://doi.org/10.3390/ma15010261 Pic courtesy: (Baffles) : https://www.webbusterz.org/baffles-in-heat-exchangers/ Is a flow instability during deformation due to locking of dislocation by solute (First observed by Portevin and Le-Chatelier thus called “PLC effect” ) Tube Sheet (Alloy 690) Tubes (Alloy 690) Baffles (Alloy 690) Fabrication Metal Forming Dynamic strain ageing (DSA)

4. Joule-heated ceramic Melter Firebricks Canister Molten Glass Off-gas High level nuclear waste Thermowell (Alloy 690) Electrode (Alloy 690) Process pot (Alloy 690) Pic courtesy: (Tube and tube sheet ) González et al. Analysis of Tube-to- Tubesheet Welding in Carbon Steel Heat Exchangers of a Double Plate Header Box.  Materials . 2022; 15(1):261. https://doi.org/10.3390/ma15010261 Pic courtesy: (Baffles) : https://www.webbusterz.org/baffles-in-heat-exchangers/ The temperature may reach 1100 °C Creep Applications Time-dependent plastic deformation of material at stresses below the yield strength at elevated temperatures (> 0.5 T m ) (~ i.e. T > 600 °C for Alloy 690 ) . Is a flow instability during deformation due to locking of dislocation by solute (First observed by Poitevin and Le- Chatelier thus called “PLC effect” ) Tube Sheet (Alloy 690) Tubes (Alloy 690) Baffles (Alloy 690) Metal Forming Dynamic strain ageing (DSA) Fabrication

5. Literature GAP Bibliometric details of the studies over the past 30 Years on Alloy 690 for engineering applications Over the past 30 years, there is an increasing interest in the research of Alloy 690

6. Literature GAP Bibliometric details of the studies over the past 30 Years on Alloy 690 for engineering applications Less report on high temperature tensile and fracture behaviour Over the past 30 years, there is an increasing interest in the research of Alloy 690

7. RT T Solidus 100 °C 200 °C 300 °C 400 °C 500 °C 600 °C 700 °C 800 °C 900 °C 1000 °C 1100 °C Tensile test Preliminary investigation

8. RT T Solidus 100 °C 200 °C 300 °C 400 °C 500 °C 600 °C 700 °C 800 °C 900 °C 1000 °C 1100 °C Tensile test Stress-Strain Curve of Alloy 690 from Room temperature to 1100 º C Serrations appeared in the stress-strain curve in the temperature range of 200-600 º C Preliminary investigation

9. RT T Solidus 100 °C 200 °C 300 °C 400 °C 500 °C 600 °C 700 °C 800 °C 900 °C 1000 °C 1100 °C Tensile test Preliminary investigation

10. RT T Solidus 100 °C 200 °C 300 °C 400 °C 500 °C 600 °C 700 °C 800 °C 900 °C 1000 °C 1100 °C Tensile test Preliminary investigation

11. Dynamic Strain ageing (DSA) To study the DSA and its effect on the mechanical properties and fracture behaviour. Mechanism of DSA. Creep Objective: 1 To study the Parametric relationship and mechanism of creep deformation. Objective: 2 Specific Objectives RT T Solidus 100 °C 200 °C 300 °C 400 °C 500 °C 600 °C 700 °C 800 °C 900 °C 1000 °C 1100 °C Tensile test To study the Microstructural changes during creep. Objective: 3

12. DSA behaviour in Alloy 690 ( Objective-1 ) Serrated flow behaviour in Alloy 690 at different temperatures and strain rate Similar kind of serration can be observed at low temperature-low strain rate and high temperature-high strain rate

13. DSA behaviour in Alloy 690 ( Objective-1 ) Serrated flow behaviour in Alloy 690 at different temperatures and strain rate The general sequence is F>A>B>C increase in temperature or decrease in strain rate.

14. Tensile behaviour in DSA temperature regime of Alloy 690 at 10 -4 s -1 strain rate DSA behaviour in Alloy 690 ( Objective-1 )  T =  + K   The alloy shows a continuous decrease in the YS except in the temperature range of 300-500 °C A continuous decline in the UTS with a near plateau in the temperature range of 300- 500 °C Ductility is near plateau till 500 °C with a minima at 600 °C The alloy exhibited a high value of n (0.65) up to nearly about 550 °C , followed by a drastic drop above it Shaded region indicates the presence of DSA

15. Tensile behaviour in DSA temperature regime of Alloy 690 at 10 -4 s -1 strain rate DSA behaviour in Alloy 690 ( Objective-1 )  T =  + K   The alloy shows a continuous decrease in the YS except in the temperature range of 300-500 °C A continuous decline in the UTS with a near plateau in the temperature range of 300- 500 °C Ductility is near plateau till 500 °C with a minima at 600 °C The alloy exhibited a high value of n (0.65) up to nearly about 500 °C , followed by a drastic drop above it Shaded region indicates the presence of DSA Is this a generalised manifestation ??

16. DSA behaviour in Alloy 690 ( Objective-1 ) Tensile behaviour in DSA temperature regime of Ni Superalloys Present Study Published literature YS UTS YS UTS YS UTS UTS YS UTS

17.   DSA behaviour in Alloy 690 ( Objective-1 ) Negative strain rate sensitivity is a necessary condition for serrated flow ( P. Penning, 1972 ) Strain Rate sensitivity of Alloy 690

18. Arrhenius method   DSA behaviour in Alloy 690 ( Objective-1 ) Determination of activation energy for the onset of serration to identify mechanism : Method 1

19. Stress-Drop method DSA behaviour in Alloy 690 ( Objective-1 ) Determination of activation energy to identify mechanism : Method 2

20. Critical strain ( ) method   DSA behaviour in Alloy 690 ( Objective-1 ) Determination of activation energy to identify mechanism : Method 3

21. Critical strain ( ) method   DSA behaviour in Alloy 690 ( Objective-1 ) Determination of activation energy to identify mechanism : Method 3 Normal PLC Inverse PLC Normal PLC Inverse PLC

22. Method Activation Energy (kJ/mol) Arrhenius method Normal PLC Inverse PLC F A B C 107 110 120 250 Critical Strain Method 37.1-56.3 (m + β = 1.8) 104.8-182 (m + β = 2.6) Stress drop method 87 - Possible solute Activation Energy in Ni Remark Cr and Fe  280 kJ/mol 1 Cr content is three times that of the Fe, Therefore Cr solute is responsible for the inverse PLC at high temperatures. C,N  105-120 kJ/mol 1 the kinetics of C atoms is very fast, and it seems unreasonable to associate the locking of dislocations to C atoms DSA behaviour in Alloy 690 ( Objective-1 ) Activation Energy and Species responsible for DSA in Alloy 690 1. J. Askill , Tracer diffusion data for metals, alloys, and simple oxides (1970 )

23. Method Activation Energy (kJ/mol) Arrhenius method Normal PLC Inverse PLC F A B C 107 110 120 250 Critical Strain Method 37.1-56.3 (m + β = 1.8) 104.8-182 (m + β = 2.6) Stress drop method 87 - DSA behaviour in Alloy 690 ( Objective-1 ) Activation Energy and Species responsible for DSA in Alloy 690 Weaver and Hale 1 have suggested the possibility that such activation energy values can come from the diffusion of substitutional–interstitial pairs. Since Cr and C have a good affinity towards each other thus Cr-C pair could be the species responsible for DSA in the normal PLC domain. 1. M.L. Weaver and C.S. Hale, Effects of Precipitation on Serrated Yielding in Inconel 718, in: Proc. Int. Symp. Superalloys Var. Deriv., TMS, 2001, p 421-432 Doi:  https://doi.org/10.7449/2001/superalloys_2001_421_432

24. The alloy exhibits ductile fracture in the normal PLC regime, while it shows a mixed-mode fracture involving shallow ductility transgranular brittle cleavage of grains in the inverse PLC regime . Effect of DSA on fracture behaviour of Alloy 690 normal PLC regime Inverse PLC regime DSA behaviour in Alloy 690 ( Objective-1 )

25. Objective 2 : Creep Study of Alloy 690

26. * Test conducted for 100 h Creep Study of Alloy 690 (Objective-2) Creep Test Matrix Creep properties such as rupture time and minimum creep rate were obtained Temperature (°C) Stress (MPa)  Stress level w.r.t. Yield strength (YS) 0.3YS 0.4YS 0.5YS 0.6YS 0.7YS 700 °C 100 120 150 170 800 °C - - 75 90 105 900 °C - 32 40 48 56 1000 °C - - 25 30 35 1100 °C 7* - 12* 15 18 Creep curves

27. Such as: Monkman and Modified Monkman Grant Iso -thermal Plot Parametric relationships Larson Miller Manson-Scoop Manson- Haffered Orr- Sherby -Dorn Data from the literature and the present study were used to generate master curves for the prediction of creep properties using different parametric and empirical relationships Creep Study of Alloy 690 (Objective-2)

28. Creep Study of Alloy 690 (Objective-2) . Kim, W. Kim, M. Kim, J. Kwon, Creep Life Assessment of Alloy 690 Steam Generator Tube using Larson-Miller Parameter, in: Trans. Korean Nucl . Soc. Virtual Autumn Meet., 2020: pp. 13–14. Modified Monkman-Grant Monkman and modified Monkman- Grant Monkman-Grant    

29. Creep Study of Alloy 690 (Objective-2) J. Kim, W. Kim, M. Kim, J. Kwon, Creep Life Assessment of Alloy 690 Steam Generator Tube using Larson-Miller Parameter, in: Trans. Korean Nucl . Soc. Virtual Autumn Meet., 2020: pp. 13–14. J. Kim, W. Kim, M. Kim, Evaluation of creep properties of alloy 690 steam generator tubes at high temperature using tube specimen, Am. Soc. Mech. Eng. Press. Vessel. Pip. Div. PVP. 6B-2019 (2019). https://doi.org/10.1115/PVP2019-93498. Open symbol: Literature 1,2 Closed Symbol: present study Isothermal Plot 650 -1100 C

30. Creep Study of Alloy 690 (Objective-2) Parametric relationship Manson-Scoop Manson- Haffered Orr- Sherby -Dorn Larson-Miller        

31. Apparent stress exponent 5.5 - 7 The activation energy for Ni self-diffusion is 284 kJ/mol and the obtained value is 363-498 kJ/mol. Creep Study of Alloy 690 (Objective-2) R.W. Hayes, F. Azzarto , E.A. Klopfer , M.A. Crimp, Characterization of creep deformation of Ni-Cr solid solution alloy Nimonic 75 Mater Sci Eng , 690 (2017) T. Yamane, Y. Takahashi, K. Nakagawa, Effect of carbide precipitates on high temperature creep of a 20Cr-25Ni austenitic stainless steel J Mater Sci Lett, 3 (1984) A.S. Alomari , N. Kumar, K.L. Murty , Investigation on creep mechanisms of alloy 709, Int Conf Nucl Eng , ASME, North Carolina, USA (2017) Apparent activation energy and stress exponent BMD equation   Dislocation creep Researchers 1,2,3 have attributed high values of activation energy to the threshold effect of carbides in solid solution strengthened alloy containing carbides. Apparent activation energy 363 – 498 kJ/ mol

32. Time-temperature-carbide precipitation curve of Alloy 690 1 1. Lee et al., Effect of a heat treatment on the precipitation behavior and tensile properties of alloy 690 steam generator tubes J Nucl Mater, 479 (2016) Creep Study of Alloy 690 (Objective-2) Creep tested temperature range Determination of threshold stress by extrapolation technique 1

33. Time-temperature-carbide precipitation curve of Alloy 690 1 1. Lee et al., Effect of a heat treatment on the precipitation behavior and tensile properties of alloy 690 steam generator tubes J Nucl Mater, 479 (2016) Creep Study of Alloy 690 (Objective-2) Creep tested temperature range Determination of threshold stress by extrapolation technique 1 Is there any evidence ??

34. Optical micrograph indicating formation of carbides Creep Study of Alloy 690 (Objective-2) Gauge region Deformation Increased precipitation kinetics and is reflected in the increased amount of etch pit density compared to the shoulder region

35. Proposed deformation mechanism map for Ni-32 at.% Cr (similar to Alloy 690) Creep Study of Alloy 690 (Objective-2)

36. Fracture behavior of alloy 690 transgranular ductile fracture characterised by dimples Combined transgranular and intergranular modes Necking Cavitation Increasing temperature Necking Cavitation Increasing stress Creep Study of Alloy 690 (Objective-2)

37. Objective 3: Microstructural changes during creep

38. Microstructural changes during creep (Objective-3) Dynamic recrystallization of grains during creep deformation Necking Cavitation Increasing temperature Cavitation Increasing stress Cavitation Increasing temperature Necking 800 °C 900 °C GOS Map

39. Microstructural changes during creep (Objective-3) Variation of Dynamic recrystallization fraction Sample preparation for EBSD 800 °C 900 °C Cavitation Increasing temperature Necking 800 °C 1000 °C 0 mm 2 mm 4mm Decreasing Strain (Near fracture)

40. geometric dynamic recrystallization ( gDRX ) continuous dynamic recrystallization ( cDRX ) discontinuous dynamic recrystallization ( dDRX ) Wang and Zhao et al. (2020) Microstructural changes during creep (Objective-3) Dynamic recrystallization mechanism

41. Microstructural changes during creep (Objective-3) Dynamic recrystallization mechanism in Creep deformed Alloy 690 no evidence of gDRX

42. no evidence of cDRX Microstructural changes during creep (Objective-3) Dynamic recrystallization mechanism in Creep deformed Alloy 690 no evidence of gDRX 0 mm 2 mm 4 mm 800 °C_ 105 MPa

43. no evidence of cDRX Strong evidence of dDRX Microstructural changes during creep (Objective-3) Dynamic recrystallization mechanism in Creep deformed Alloy 690 no evidence of gDRX 0 mm 2 mm 4 mm

44. Needle shape precipitate formation near the crack 900-1000 °C Microstructural changes during creep (Objective-3) Precipitation during Creep deformation in Alloy 690 900 C_ 25 MPa 900 C_ 30 MPa 1000 C_ 25 MPa 1000 C_ 30 MPa Temp. (°C) Stress (MPa) 800 75 90 105 900 25 30 40 48 56 1000 25 30 35 1100 7 100h (interrupted) 12 100h (interrupted) 15 18 No precipitation observed precipitation

45. Temp. (°C) Stress (MPa) 800 75 90 105 900 25 30 40 48 56 1000 25 30 35 1100 7 100h (interrupted) 12 100h (interrupted) 15 18 1100 °C Needle shape precipitate Microstructural changes during creep (Objective-3) Precipitation during Creep deformation in Alloy 690 at 1100 °C 1 00 μ m Incremental Load of 2.5N/h after 100 h of creep test ruptured in 3.5 h 7 MPa No precipitation observed precipitation 100 h interrupted

46. Temp. (°C) Stress (MPa) 800 75 90 105 900 25 30 40 48 56 1000 25 30 35 1100 7 100h (interrupted) 12 100h (interrupted) 15 18 1100 °C Needle shape precipitate Microstructural changes during creep (Objective-3) Precipitation during Creep deformation in Alloy 690 at 1100 °C 1 00 μ m Incremental Load of 2.5N/h after 100 h of creep test ruptured in 3.5 h 7 MPa 100 h interrupted No precipitation observed precipitation Needle shaped Cr 2 N precipitates formed at the end of tertiary stage (close to rupture) Thus no noticeable effect on the alloy’s creep life was observed

47. EDS Map Composition is consistent with the Cr 2 N phase stoichiometry Elements Cr N Ni Fe at.% 80-84 10 4-7 1 Microstructural changes during creep (Objective-3) Identification of needle shaped precipitate 1100 °C Needle shape precipitate 1 00 μ m 7 MPa 100 h interrupted Incremental Load of 2.5N/h after 100 h of creep test ruptured in 3.5 h

48. 1100 °C_15 MPa Characteristic feature of internal nitridation isothermal section of the phase stability map Ni-Cr-N (bold) and Ni-Cr-10 Fe (broken lines) Critical partial pressure Cr concentration corresponding to Alloy 690 Microstructural changes during creep (Objective-3) N 2 partial pressure in air

49. Summary Alloy 690 exhibits the DSA behaviour over a temperatures ranging from 200-600 °C Serrations of type F , A, B, and C were observed in the present study Serration Type changes with the temperature, strain rate and strains Different serrations can be classified as Normal and Inverse PLC behavior Normal PLC Inverse PLC Serration Type F , A, and B C Responsible element (Mechanism) Cr-C complexes Cr Fracture behaviour Ductile fracture Transgranular brittle cleavage Creep study of Alloy 690 was studied at temperatures ranging from 700 to 1100  C and stress ranging from 7-150 MPa Dominant Creep mechanism was Dislocation Creep High value of creep activation energy Threshold stress exerted by carbides Necking and cavitation are the two dominant fracture mechanism.

50. Summary Low temperature Low Stress   High temperature High Stress   Cavitation Necking Extensive Dynamic Recrystallisation of grain Predominantly Continuous Dynamic recrystallization Mechanism Fracture behaviour of creep deformed samples

51. Summary Low temperature Low Stress   High temperature High Stress   Cavitation Necking Extensive Dynamic Recrystallisation of grain Predominantly Continuous Dynamic recrystallization Mechanism High temperature Low Stress   Promotes formation of Cr 2 N needles towards the end of tertiary regime Extensive Precipitation observed at 1100 ºC Not affecting Rupture life Fracture behaviour of creep deformed samples Precipitation during creep

52. Sr. No. Title Journal details and Status of the Paper 1 Creep behaviour of alloy 690 in the temperature range 800–1000 °C Kumar Sourabh , J.B. Singh Journal of Materials Research and Technology DOI: https://doi.org/10.1016/j.jmrt.2022.01.060 Status: Published 2 Tensile Behaviour of Alloy 690 in the Dynamic Strain Ageing Regime Kumar Sourabh , J.B. Singh Journal of materials engineering and performance DOI: https://doi.org/10.1007/s11665-022-07314-1 Status: Published 3 Dynamic Recrystallization of Grains during Creep Deformation of Alloy 690 at 800 –1000 °C Kumar Sourabh , J.B. Singh Materials Characterization DOI: https://doi.org/10.1016/j.matchar.2022.112429 Status: Published 4 Internal Nitridation of Alloy 690 during Creep Deformation at 1100 °C Kumar Sourabh , J.B. Singh, K.V. Ravikanth , A. Verma Materials Science and Engineering: A DOI: https://doi.org/10.1016/j.msea.2023.144719 Status: Published Publications

53. Conferences Sr. No. Conference Title Conference detail 1 Effect of temperature and strain rate on plastic flow instability in Alloy 690 NMD-ATM November-2019 2 Meso - Scale Characterization of Creep ruptured Alloy 690 in the temperature range of 800-1000 °C Asia pacific Microscopy Conference (APMC) February-2020 3 Microstructural instability during tertiary creep of alloy 690 Creep, Fatigue and creep-fatigue interaction (CF-8), August-2021

54. Acknowledgements I sincerely thank Dr. J.B. Singh for their invaluable guidance and unwavering attention to details of the work My doctoral committee – Dr. Tewari , Dr. Arya, Dr. Majumdar and Dr. R. Kapoor – for constructive remarks Dr R. N. Singh and Dr. R. Kapoor for providing experimental facility My thesis reviewers Vivek Patel, Dr. Annesha Das, Dr. A. Verma , Dr. Arpan Das, Dr. Naveen Kumar, Bharat Gyan Reddy, Saurav Sunil, Ravi Hankare , Subham Chakraborty for their help and assistance and constant support in the journey Analytical Chemistry Division BARC, for chemical analysis and Material Science Division BARC for providing the facility for TEM and EBSD sample preparation Akash Chahal for being with me in the toughest of time

55. Thank You