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 solidified Fe51.9Mo11.1Cr26.3C10.7 (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 alloy consisted of the πFe,Mo,Cr,C and χFe,Mo,Cr,C phases, corresponding to the β- and α-Mn-type polymorphs, respectively. The direct πFe,Mo,Cr,C → χFe,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 πFe,Mo,Cr,C → χFe,Mo,Cr,C transformation appears as a two-level nanoscale deformation contrast within the grains of the polygonal π F e,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 πFe,Mo,Cr,C → χFe,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 solidified Fe–Mo–C alloys and in the β-Mn ⇄ α-Mn polymorphic transformation in elemental manganese. The quaternary πFe,Mo,Cr,C phase is manifested morphologically in two microstructural forms. One corresponds to the metastable state of the πFe,Mo,Cr,C phase: elongated crystallites without signs of solid-state transformation, ranging in size from 200 nm to several microns. This microstructure is preserved from the crystallization temperature of the πFe,Mo,Cr,C phase to the observation temperature in regions adjacent to the melt-spun ribbon surface. The other microstructural form 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 πFe,Mo,Cr,C phase, characteristic of the annealed state, forms in the ‘thermal bath’ within the ribbon under nearequilibrium conditions. It persists from the πFe,Mo,Cr,C phase crystallization temperature to the πFe,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 original polygonal πFe,Mo,Cr,C phase and are observed from the πFe,Mo,Cr,C → χ Fe,Mo,Cr,C transformation temperature to room temperature. Like the metastable πFe,Mo,Cr,C phase, the metastable χFe,Mo,Cr,C solid solutions exhibit high kinetic stability at room temperature.

Transformations Between Mn-Like Phases in the Fe–Mo–Cr–C System

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发表日期：

2025年5月1日

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T. A. Velikanova,
A. V. Kotko,
A. M. Zaslavskii,
V. V. Kuprin,
S. D. Kharchenko

摘要

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 solidified 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 alloy consisted of the π_Fe,Mo,Cr,C and χ_Fe,Mo,Cr,C phases, corresponding to the β- and α-Mn-type polymorphs, 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 _{F e,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 → χFe,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 solidified Fe–Mo–C alloys and 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 forms. 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 microstructure is preserved from the crystallization temperature of the π_Fe,Mo,Cr,C phase to the observation temperature in regions adjacent to the melt-spun ribbon surface. The other microstructural form 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 nearequilibrium 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 original 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. Like the metastable π_Fe,Mo,Cr,C phase, the metastable χ_Fe,Mo,Cr,C solid solutions exhibit high kinetic stability at room temperature.