EFFECT OF MAGNETIC-FIELD ON SUCCESSIVE GAMMA-]EPSILON'-]ALPHA' MARTENSITIC TRANSFORMATIONS IN AN FE-MN-C ALLOY

The effect of magnetic fields on successive gamma-->epsilon'-->alpha' martensitic transformations in an Fe-13.0Mn-2.1C (at%) alloy, whose transformation start temperatures for the gamma-->epsilon' (M(s)epsilon') and epsilon'-->alpha' (M(s)alpha') transformations are 223 and 213 K, respectively, has been examined by measuring magnetic field susceptibility and magnetization and by observing optical microstructure, applying a static and/or a pulsed ultra high magnetic field up to about 31 MA/m. As a result, the epsilon'--> alpha' transformation was found to be induced under magnetic fields higher than a certain critical one even at extremely higher temperatures than M(s)alpha', while the gamma--> epsilon' and direct gamma-->alpha' transformations were not at any temperatures higher than M(s)epsilon' and M(s)alpha'. Morphology of the magnetic field-induced-alpha' martensites in pre-existed-epsilon' martensite plates was the same as that of thermally-induced ones in the alloy. A thermodynamic analysis suggested that the transformation behavior for the above three different types of transformations under magnetic fields could be systematically explained by the equation previously proposed to estimate the critical field vs transformation temperature, considering that the gamma-phase of this alloy system near M(s)alpha' temperature exhibits a paramagnetic to antiferromagnetic transition.