A286 alloy is an iron-base superalloy and composition of this alloy was based on 51%Fe-29%Ni-14%Cr-2.1%Ti-0.2%Al(wt%) and contains other minor alloying additives. It is mainly strengthened by an aging treatment which precipitates the ordered FCC γ'. The specipic compositional modification of the Fe-base superalloy containing higher Ti and lower Al, investigated for this study was designed to increase the amount and stability of γ' by adding Nb element. Based on this new design, `Nb-modified A286 alloy` was developed to improve the strength, fatigue and creep-rupture resistances compared with the commercial A286 alloy.
Recently, It has been examined the damage mechanism under the low cycle fatigue with temperature and strain in the Nb-A286 alloy. By the theory, the fracture mode is changed from transgranular fracture mode to intergranular fracture mode during the low cycle fatigue by the change of microstructure. It is known that this change is the main reason of decreasing of fatigue life. By the theory, the precipitation of the η phase at the grain boundary during the low cycle fatigue is provided for the site for the grain boundary cavitation which promotes the intergranular fracture at high temperature. The planar slip bands which appeared during low cycle fatigue impinge on the grain boundary so that stress concentration increases at this grain boundary. Further, this is contributed to the change of fracture mode from transgranular fracture to intergranular fracture mode. The control of η phase and planar slip is an important factor to improve the fatigue properties.
In this study, the change of precipitates morphology, amount of η phase and control of γ' size are conducted by aging treatment. In the continuous low cycle fatigue, fatigue life is increased with increasing of aging time. In the investigation of fracture surface after low cycle fatigue, fracture mode is changed from intergranular fracture mode to transgranular fracture mode with increasing of aging time. It is changed from planar slip to bowing dislocation with increasing of γ' size. Slip induced cavitation by local stress concentration due to planar slip impinged on the grain boundary is retarded by larger γ'precipitation than none-aging treatment. Therefore, fatigue life is increased with increasing of aging time. The cellular η phase which grows in the grain prohibits crack propagation. Fatigue life is thought to be affected by cellular η phase. These results will be the basis of design new alloy for improvement of fatigue life.
