Size and aspect ratio of martensite in FeNiC alloys formed under applied magnetic field and tensile elastic stress at 4.2 K

The dimensions of fully grown martensite plates in Fe27Ni0.8C and Fe31Ni0.4C alloys formed at 4.2 K under applied magnetic field (10 Tesla) and tensile elastic stress (220 MPa) are reported. The plates formed in a cluster are classified into two categories during stereological measurements: the plates whose radial growth is limited by the austenitic grain boundaries only (Type I), and the plates whose radial growth is limited by the pre-existing martensitic plates only (Type II). It is demonstrated that the formation sequence of the plates has a strong effect on the radial growth and aspect ratio. The measured aspect ratio of type I plates in the Fe27Ni0.8C alloy agree very well with an earlier prediction based on the aspect ratio measurements in the temperature range of 193 to 77K in an Fe23.2Ni2.9Mn alloy. Both these alloys are non-invar. However, in the case of Fe31Ni0.4C alloy, which is expected to be invar, the measured aspect ratio is less than the predicted value at 4.2 K.