Influence of magnetic field on ferrite transformation in a Fe–C–Mn alloy

Abstract

The kinetics of ferrite transformation in a Fe–0.10mass%C–2.94mass%Mn alloy in a strong magnetic field of 8 T were studied with regard to alloying element-partitioned and partitionless growth. According to the theory of diffusion-controlled growth, the slow Mn diffusion dictates partitioned growth that occurs at a low undercooling, whereas partitionless growth at a larger undercooling is rate-controlled by fast carbon diffusion. The alloy was austenitized and isothermally reacted at temperatures that encompass the two growth modes. The nucleation and growth rates of ferrite increased at all temperatures in the magnetic field, whereas the amount of increase was somewhat greater at lower temperatures. In the region of slow growth, besides its sluggish diffusion Mn possibly destabilizes the ferrite phase due to the influence on the magnetic moment and the Curie temperature of bcc Fe solid solution, and partially offsets the accelerating effect of transformation. The temperature of transition from the slow to the fast growth is predicted to increase, due to the shift in the ferrite/austenite phase boundaries in the presence of magnetic field.

Introduction

Magnetic field effects on microstructure formation in metallic materials were first reported in the middle of the last century by Smoluchowski and Turner [1], who noted an increase in the 〈1 1 0〉 texture in the rolling direction in a Fe–Co alloy by magnetic annealing. Since then, a number of authors studied magnetic field effects on recrystallization [2], grain boundary mobility [3], spinodal decomposition [4], [5], martensitic transformation [6], [7], [8], [9], [10], precipitation [11], [12], [13] and growth of ordered domains [14]. In these studies the field strength was usually less than a few tesla (with an exception of the study by Chen [12]). With the advent of new superconducting materials and the progress of cryocooling technique, magnetic fields of ~10 T can be applied in heat treatment in laboratories. A magnetic field of such a strength can affect the transformation temperature and microstructure to a readily visible extent if the transformed phase is either ferromagnetic or has a substantially different susceptibility from the parent phase. Under this circumstance studies of magnetic field effects are expanded to other alloys and phase transformations [15], [16], [17], [18], [19], [20]. In particular, the effects on diffusional transformations in ferrous alloys have attracted considerable attention for a few decades. These include bainitic [21], pearlite [22], [23], [24] and proeutectoid ferrite transformations [25], [26], [27], [28], [29], [30], [31] in steel.

According to the theory of diffusion-controlled growth in Fe–C–X ternary alloys, where X is a substitutional alloying element, the transformation is controlled by slow diffusion of X at lower undercoolings, accompanying X partition between the parent and the product phases. On the other hand, it is controlled by carbon diffusion at larger undercoolings without X partition. The partitioning of substitutional alloying element was indeed verified experimentally in a number of Fe–C–X alloys by Aaronson and Domian [32], although the local condition at ferrite/austenite boundaries at larger undercoolings, i.e. paraequilibrium or no-partition local equilibrium, is the subject of keen interest for more than half a century [33]. In this study, in order to promote the understanding of the magnetic field effects on ferrite transformation, a high purity Fe–0.1mass%C–2.94mass%Mn alloy was austenitized and isothermally reacted in the magnetic field of 8 T. Nucleation and growth behaviors of proeutectoid ferrite were observed and are discussed with regard to Mn-partitioning and transition between the partitioned and partitionless growth.