Isothermal aging kinetics of NiAl precipitates in a secondary hardening steel

Secondary hardening steels additionally alloyed with Ni and Al exhibit the precipitation of secondary hardening carbides and an intermetallic NiAl-based phase. The hardening response of a Fe-1.4C-2.6Cr-1.4Mo-6.0 Ni-5.0Al (composition in at.%) experimental alloy has been studied by hardness measurements conducted on specimens, which were subjected to aging treatments in a temperature range typically used for heat treatment of conventional secondary hardening steels. The results revealed a rapid hardness increase within the first minutes of aging. Nevertheless, when exceeding a certain critical temperature, the intermetallic NiAl-based phase is dissolved in the matrix. By applying standard relationships between hardness and NiAl phase fraction, the activation energy and time exponent of the intermetallic precipitation reaction were estimated. The derived results are critically discussed and quantitatively compared with data as provided by literature.