High Temperature Tensile Behavior of a Nickel-Based Superalloy 55Ni-17Cr-12Fe-9Mo-2Nb-1.5Al Used in Launch Vehicle Applications

Abstract

55Ni-17Cr-12Fe-9Mo-2Nb-1.5Al alloy is a nickel-based superalloy (Russian designation is XH55MбЮ or KhN55MBYu, XH55) without any equivalent in American/European alloy designation. It is used in cryogenic engine of satellite launch vehicles application in two different heat-treated conditions: (1) standard aged (STA) at 730 °C/15 h + 650 °C/10 h and (2) STA + BC (brazing cycle) treatment carried out in vacuum at 1030 °C with holding time of 30 min. Due to the braze cycle adopted for manufacturing, it is essential to study the deterioration in mechanical properties, if any. Hence, the present work is carried out to understand the material behavior in tensile mode (25, 425, 575, 700 and 900 °C) for XH55 alloy in STA condition and STA + BC conditions, compared with corresponding microstructural analysis, morphology and composition using microscopy at various length scales. The tensile stress–strain curve shows characteristic sudden drops in stress with respect to strain, attributed to dynamic strain aging at different temperatures for both STA and STA + BC conditions. In STA condition, the yield strength of the material decreased with increase in temperature. In STA + BC condition, the yield strength decreased up to 425 °C, increased up to 700 °C as the material was subjected to artificial aging during testing and finally decreased at 900 °C. Marginal deterioration in mechanical properties have been observed due to the braze cycle adopted against STA condition.