High-Temperature Tensile Behaviors of Base Metal and Electron Beam-Welded Joints of Ni-20Cr-9Mo- 4Nb Superalloy R.K. GUPTA, V. ANIL KUMAR, ARJUN SUKUMARAN, and VINOD KUMAR Electron beam welding of Ni-20Cr-9Mo-4Nb alloy sheets was carried out, and high-temperature tensile behaviors of base metal and weldments were studied. Tensile properties were evaluated at ambient temperature, at elevated temperatures of 625 °C to 1025 °C, and at strain rates of 0.1 to 0.001 s 1 . Microstructure of the weld consisted of columnar dendritic structure and revealed epitaxial mode of solidification. Weld efficiency of ~ 90 pct in terms of strength (UTS) was observed at ambient temperature and up to an elevated temperature of 850 °C. Reduction in strength continued with further increase of test temperature (up to 1025 °C); however, a significant improvement in pct elongation is found up to 775 °C, which was sustained even at higher test temperatures. The tensile behaviors of base metal and weldments were similar at the elevated temperatures at the respective strain rates. Strain hardening exponent ‘n’ of the base metal and weldment was ~ 0.519. Activation energy ‘Q’ of base metal and EB weldments were 420 to 535 kJ mol 1 determined through isothermal tensile tests and 625 to 662 kJ mol 1 through jump-temperature tensile tests. Strain rate sensitivity ‘m’ was low ( < 0.119) for the base metal and ( < 0.164) for the weldment. The d phase was revealed in specimens annealed at 700 °C, whereas, twins and fully recrystallized grains were observed in specimens annealed at 1025 °C. Low-angle misorientation and strain localization in the welds and the HAZ during tensile testing at higher temperature and strain rates indicates subgrain formation and recrystallization. Higher elongation in the weldment (at Test temperature > 775 °C) is attributed to the presence of recrystallized grains. Up to 700 °C, the deformation is through slip, where strain hardening is predominant and effect of strain rate is minimal. Between 775 °C to 850 °C, strain hardening is counterbalanced by flow softening, where cavitation limits the deformation (predominantly at lower strain rate). Above 925 °C, flow softening is predominant resulting in a significant reduction in strength. Presence of precipitates/accumulated strain at high strain rate results in high strength, but when the precipitates were coarsened at lower strain rates or precipitates were dissolved at a higher temperature, the result was a reduction in strength. Further, the accumulated strain assisted in recrystallization, which also resulted in a reduction in strength. https://doi.org/10.1007/s11661-018-4652-x Ó The Minerals, Metals & Materials Society and ASM International 2018 I. INTRODUCTION NICKEL-BASED superalloy Ni-20Cr-9Mo-4Nb is widely used in aeronautical, aerospace, chemical, petro- chemical and marine applications due to its superior combination of high-temperature mechanical properties, weldability and resistance to high-temperature corrosion and oxidation on prolonged exposure to an aggressive environment. [1] The alloy is strengthened by addition of molybdenum and columbium. It contains a high level of chromium which along with molybdenum imparts excellent oxidation and corrosion resistance. It also exhibits high strength at cryogenic temperature and can be joined by welding and brazing. [1,2] Generally, the microstructures of superalloys are highly complex, where numerous dispersed inter- metallics and other phases are present, which govern the mechanical behavior of the alloy. It can be further described as fcc structure (c-phase matrix) containing number of secondary phases like face-centered cubic(FCC) carbides, ordered FCC c¢, ordered body-centered tetragonal(BCT) c¢¢, ordered intermetallic R.K. GUPTA and V. ANIL KUMAR are with the Materials and Mechanical Entity, Vikram Sarabhai Space Centre, Trivandrum, India. Contact e-mail: rohitkumar_gupta@vssc.gov.in ARJUN SUKUMARAN is with the National Institute of Technology, Calicut, India. VINOD KUMAR is with the Steel Authority of India Ltd., RDCIS, Ranchi, India. Manuscript submitted December 10, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS A