Isothermal nature of martensite formation in Pt-modified β-NiAl alloys

Numerous experimental and modeling efforts to describe the cyclic behavior of β-NiAl-based coatings have recognized and considered the propensity of Ni-rich β compositions to exhibit a reversible martensitic transformation upon cooling. However, until now it has not been clear whether this martensite forms by an athermal process or if the transformation is isothermal in nature, exhibiting time-dependent kinetics. In this study model Ni–37Al–xPt (x = 5, 10 and 30) alloys (all at.%) were examined primarily using thermal analysis, which showed that the martensite transformation is isothermal in nature with a single activation energy. In addition, the temperature range where this reversible transformation occurs increases with higher Pt contents. The autocatalytic behavior of this transformation was further illustrated during slow cooling in the differential scanning calorimeter, whereupon a series of ‘bursts’ corresponding to a sequence of individual exothermic peaks were observed. The potential implications of the isothermal martensite transformation in the β-based coatings is discussed.