Microstructural Investigation of Alloy 617 Creep-ruptured at High Temperature in a Helium Environment

Abstract

A very high temperature reactor (VHTR) is one of the next-generation nuclear reactors chosen by the Generation IV International Forum. A Ni-base superalloy, Alloy 617, is considered as a primary candidate material for an intermediate heat exchanger (IHX) and hot gas duct (HGD) of the VHTR because of the superior creep resistance at a high temperature above 850 °C. In this study, the microstructures of the specimens creep-ruptured at high temperatures in a helium environment were investigated. The decrease in rupture time was more pronounced with increasing creep temperature. In the specimens crept at 950 °C, the external Cr-oxide layer of the specimens increased in thickness with increasing creep rupture time, and delaminated after a long-time creep. The high creep stress induced a deep penetration of carbide depletion along the grain boundaries at the early stage of the creep test. The creep temperature enhanced the growth rate of the decarburized zone depth clearly, but the temperature effect on the growth of the external oxide and internal oxide was not well understood as the surface reaction and creep stress affected the microstructures complexly.