Coherent-Incoherent Transition of ε-Carbide in Steels Found with Mechanical Spectroscopy

Although a coherent-incoherent transition in the epsilon-carbide precipitated in steels is supposedly linked to hardening and microstructural changes, the existence of this transition has not yet been confirmed. In this paper, we investigate this subject using mechanical spectroscopy. By measuring mechanical loss spectra below room temperature of quench-aged Fe-C alloys, mild steel, and pearlitic steel, we reveal a new broad peak (NBP). This peak is related to thermal activation, and its line shape obeys the equation of the Debye peak with a distribution in relaxation time. The Arrhenius plot yielded a large activation energy and gigantic pre-exponential factor. Its intensity grew by aging at temperatures where precipitation of e-carbide has been reported. However, it starts to decay at duration far too early for e-carbide to transform to cementite. For isothermal aging at 393 K (120 degrees C), the intensity sharply decreased at durations over 3 hours. This decay was accompanied by appearance of another similar peak (NBP'), which had a peak frequency two orders higher than that of NBP. These peaks had comparable intensity. We attribute NBP and NBP' to coherent and incoherent epsilon-carbides, respectively. We produced a model that attributes the relaxation peaks to reorientations of extra carbon pairs in the e-carbide. The extraordinary values of the Arrhenius parameters may be interpreted by using this model. Based on these results, we assert that mechanical spectroscopy can detect the coherent-incoherent transition in carbon steels. This method will be powerful in studying problems related to the coherency in carbon steels. (C) The Minerals, Metals & Materials Society and ASM International 2016