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.