Transformation of Η-Phase into Γ’-Phase, Γ-Matrix and its Effect on the High Temperature Hardness and Compression of Fe-Ni-Based A286 Heat-Resistant Superalloy

Precipitation-hardening is achieved in an Fe-Ni-based A286 austenitic superalloy by means of aging process. During the aging process gamma prime phases (γ’), eta (η) phases and TiC phases nucleate in an austenitic matrix of this alloy. In this research, the behavior of η-phases’ precipitation, dissolution and transformation into γ’-phase and γ-matrix, and their consequent effect on high temperature hardness testing and compression is investigated in an Fe-Ni-based alloy. The behavior of phase nucleation is examined after aging at 710 ºC for different lengths of time (1h, 2h, 4h and 8h). Scanning electron microscopy, adjacent with energy dispersive spectroscopy (EDS) and transmission electron microscopy is used to identify the phases ((γ’) Ni3(Al,Ti), eta (η) Ni3Ti and titanium carbides). The nucleation trend is declined with the increase in time of aging and the 8h-aged sample is found to contain the lowest quantity of phases. Then, the 8h-aged sample is investigated for η-phase’ dissolution and transformation behavior. Most of the (η) Ni3Ti phases are transformed into gamma prime phases (γ’) Ni3(Al,Ti) because aluminum is identified in dissolution areas. In (η) Ni3Ti phase dissolution areas, aluminum enhanced up-to 0.37wt.% which is more than double the amount used in composition and titanium increased up-to 4.78wt.%, and nickel increases to more than 27.46 wt.%. Hardness range of the 8h-aged sample is remained from 341 to 265 HV at each temperature from 50 ºC to 700 ºC respectively. Whereas hardness range of 1h-aged sample was between 281 to 215 HV, which is almost 60HV lower than the 8h-aged sample. During compression tests of the aged samples, it is found that the 8h-aged sample showed almost negligible cracks in eta and TiC phases with deformation bands in austenitic matrix. Here, synergic effect of deformation bands and γ’-precipitation enhanced the strengthening of the 8h-aged sample.