Abstract
This thesis investigates and explains the effect of strong magnetic fields on two types of phase transformation. The first of these is a diffusionless, martensitic transformation, occurring isothermally in a Fe - 26% Ni - 2% Mn alloy. The experimental results have been used to deduce information about the nature of martensite embryos, and tend to support the Kaufman-Cohen model, rather than the more recent Ragharan Cohen proposals. The second transformation studied was the bcc → fcc transformation in Fe-Co alloys containing 30 - 50 wt. % Co. This is the first time that systematic information has been established for magnetic field effects on diffusion-controlled, high temperature transformations. The experimental results have been used to check the accuracy of empirical phase-stability data, and to assess the general validity of current models describing nucleation kinetics. Generally, similar explanations of the magnetic effect apply to both transformations, but certain of the nucleation parameters can be differentiated. The size of critical embryos, deduced from the experimental data, is much larger (r[c] ~ 200 A) for martensite nucleation than for diffusion-controlled nucleation at high temperatures (r[c] ~ 70 A) Also, the calculated nucleus / matrix interfacial energy for martensite embryos is about 110 erg / cm[2] (i.e. appreciable incoherency) whereas for diffusion-controlled nucleation this parameter has the much lower value of ~ 25 - 30 erg/cm[2], indicating that nuclei are fully coherent.