TRANSFORMATIONS BETWEEN MN-LIKE PHASES IN THE FE–MO–CR–C SYSTEM

Abstract

Transmission electron microscopy was employed to establish the nature of transformations between Mn-like phases in the Fe–Mo–Cr–C system illustrated by the example of structural and phase states of the rapidly quenched Fe_51.9Mo_11.1Cr_26.3C_10.7 (at.%) alloy. The alloy was produced as ribbons by melt spinning at a cooling rate of 5 × 10⁶ K/sec using the planar flow casting technique. The phase constituents of the alloy were π_Fe,Mo,Cr,C and  χ_Fe,Mo,Cr,C phases, corresponding to the β- and α-polymorphs of Mn, respectively. The direct π_Fe,Mo,Cr,C → χ_Fe,Mo,Cr,C transformation was established to occur in the alloy. This transition proceeded completely, without changes in the composition or in the specific volume of the unit cell, which is ~0.012 nm³/atom for both phases. In electron microscopy images, the π_Fe,Mo,Cr,C → χ_Fe,Mo,Cr,C transformation appears as a two-level nanoscale deformation contrast within the grains of the polygonal π_Fe,Mo,Cr,C phase. Specifically, it is manifested as a conglomerate of lamellar packets that are perpendicular to the spinning ribbon surface (with lamellae 10–20 nm wide), containing a substructure of transverse fragments (~ 10 nm in width). It was concluded that the π_Fe,Mo,Cr,C → χ_Fe,Mo,Cr,C phase transition displayed the features of a diffusionless martensitic-type transformation between coherent phases. The transformation occurs within 700–727 °C, the same temperature range as in the decomposition of the π phase in rapidly quenched Fe–Mo–C alloys and as in the β-Mn α-Mn polymorphic transformation in elemental manganese. The quaternary π_Fe,Mo,Cr,C phase is manifested morphologically in two microstructural types. One corresponds to the metastable state of the π_Fe,Mo,Cr,C phase: elongated crystallites without signs of solid-state transformation, ranging in size from 200 nm to several microns. This type of microstructure is preserved from the crystallization temperature of the π_Fe,Mo,Cr,C phase to the observation temperature in regions adjacent to the spinning ribbon surface. The other microstructural type consists of polyhedral crystallites shaped as regular pentagons and convex deltoids with an average linear size of 50–200 nm. This polyhedral microstructure of the π_Fe,Mo,Cr,C phase, characteristic of the annealed state, forms in the ‘thermal bath’ within the ribbon under near-equilibrium conditions. It persists from the π_Fe,Mo,Cr,C phase crystallization temperature to the π_Fe,Mo,Cr,C → χ_Fe,Mo,Cr,C transformation temperature. The χ_Fe,Mo,Cr,C solid solutions appear morphologically as nanoscale deformation substructures within the grains of the initial polygonal π_Fe,Mo,Cr,C phase and are observed from the π_Fe,Mo,Cr,C → χ_Fe,Mo,Cr,C transformation temperature to room temperature. Similar to the metastable π_Fe,Mo,Cr,C phase, the metastable χ_Fe,Mo,Cr,C solid solutions exhibit high kinetic stability at room temperature.