Long-term ageing characteristics of Hastelloy alloy X

Abstract

Transmission electron microscopy and diffraction were used to characterize the microstructural changes which occur in Hastelloy alloy X after long-term ageing (up to 16 000 h) at various temperatures in the range 540 to 870° C. The corresponding effects on mechanical properties were determined. It was found that the alloy age hardens at temperatures in the range of 650 to 870° C. Overageing occurred at 760 and 870° C. Marked reductions in room-temperature tensile elongation were observed after ageing at all the temperatures investigated. However, in no case was the elongation reduced to less than 15 to 30%. Also, the tensile elongation at temperatures corresponding to the respective ageing temperatures was unaffected by 16 000 h ageing at a temperature. It was concluded that the above effects were associated with precipitation of various phases, such as carbides, sigma- and mu-phases, in the matrix and at the grain boundaries.