The stabilization effect of retained austenite has been studied using FeNiC alloys with Ms temperatures below 0°C via a two-step cooling procedure, i.e. the samples were first cooled to a temperature (Ta) below Ms temperature and then heated to room temperature (RT), after being held at RT for a while, the samples were recooled to low temperatures (23 or 82 K) and then heated to RT. It was found that, during the second step of cooling, the martensitic transformation occurred at a temperature of Ms′ which was lower than Ta. With increasing the amount of martensite formed during the first cooling, the difference in the martensitic transformation starting temperatures, ΔMs = Ms - Ms′, increased. The mechanism of the stabilization of retained austenite during the second step of cooling is proposed to be mainly due to the inhibition effect produced by the previously formed martensite. The aging processes, which retard the growth of the previously formed martensite plates and reduce the number of the available nucleation sites, are the necessary conditions for the above mechanism to operate. By simplifying the internal resisting stress acting on the retained austenite due to the existence of martensite phase as a hydrostatic compressive stress, which increases with increasing the amount of martensite, the change in ΔMs is discussed from a thermodynamic point of view.
- martensitic transformations
- retained austenite