The effects of long-term ageing on Nimonic PE16

Following long-term ageing, at 450 and 550°C, the microstructure of Nimonic PE16 was observed to remain highly stable: the size and distribution of the principal strengthening phase γ' hardly altered at all, while only limited M23C6 carbide precipitation was observed. However, an investigation of the grain boundary composition of the alloy by Auger and STEM-EDS analysis did reveal that the grain boundaries of the “as-received” materials were enriched in Mo, Cr and P relative to the grain interiors. The presence of these elements at the grain boundaries has been attributed to segregation occurring during the alloy's standard pre-service heat treatment. Following ageing the grain boundary concentration of all three elements was observed to increase, but subsequent mechanical testing indicated that the enhancement of these elements had no effect on the strength of the alloy.