Three-dimensional morphology of degenerate ferrite in an Fe–C–Mo alloy
Introduction
It is well known that Fe–C–Mo alloys exhibit a peculiar transformation behavior, that is, the transformation kinetics become extremely sluggish at intermediate temperatures. This is manifested by a bay in the time–temperature–transformation (TTT) diagram for the initiation of the transformation. The mechanisms for the formation of a bay have been extensively discussed in terms of partitioning [1], [2], clustering [3] and segregation [4], [5], [6], [7], [8] of alloying elements at and near moving α:γ interphase boundaries. The bay was also regarded as a gap between the C-curves of diffusion-controlled and shear transformations [9], [10], [11], [12].
The transformation below the bay of the TTT diagram is characterized by the so-called stasis, that is, the transformation ceases temporarily before it achieves the fraction transformed predicted by the lever rule. It is well documented that the transformation product has a characteristic morphology called degenerate ferrite below the bay [5]. A similar microstructure was observed not only in the Fe–C–Mo alloys, but also in Fe–C–Cr [13] and Fe–C–V alloys [14]. The transformation product is considered to be degenerate Widmanstätten plates; in two-dimensional observation these are composed of a large number of small ferrite crystals that are aligned more or less in one direction as if they were growing as a plate [15]. TEM observations have revealed that the ferrite is actually composed of irregular, ragged-shaped crystals that are one half to a few micron in dimensions [7]. Furthermore, Nomarski interference technique in the optical microscope demonstrated that no clear surface relief or tilt was associated with the degenerate ferrite [7].
In observations under optical microscope or TEM many features of the microstructure can easily be missed because the major part of the object is buried under the surface, or removed during specimen preparation. Owing to developments in metallographical techniques using computer software for image processing the three-dimensional reconstruction of microstructures in metallic materials has attracted considerable attention. The techniques have been applied to a number of alloy systems and microstructures, as recently reviewed by Mangan et al. [16] and Kral et al. [17], [18]. In this report, using these techniques, the three-dimensional morphology of degenerate ferrite was constructed aiming at a better understanding of the microstructure and transformation behavior below the bay temperature in the Fe–C–Mo system.
Section snippets
Experimental procedures
A high purity Fe–0.28 mass% C–3.0 mass% Mo alloy was prepared by vacuum induction melting. The ingot was forged into a bar at 1200 °C and was homogenized at 1250 °C for 2 days. Specimens 10×12×0.4 mm3 in size were austenitized at 1250 °C for 600 s and isothermally reacted at temperatures ranging from 530 to 700 °C to determine the bay temperature. The specimens were polished and etched in 3% nital for optical microscopy. The prior austenite grain size was m. The fraction transformed at
Transformation behavior and microstructure
The microstructures of specimens reacted at high (⩾570 °C) and low (⩽550 °C) temperatures were distinctly different from each other. At higher temperatures aggregates of ferrite and carbides (Mo2C and Mo6C) [15], [19] were nucleated at austenite grain boundaries and grew towards the interior of the grain. At lower temperatures small ferrite particles were formed copiously at and near the prior austenite grain boundaries. An optical micrograph of a specimen reacted at 550 °C for 10 ks is shown
Summary
The three-dimensional morphology of degenerate ferrite formed below the bay temperature in an Fe–0.28 mass% C–3.0 mass% Mo alloy was observed by computer-aided reconstruction of serial sectioning images. The degenerate ferrite was initially nucleated at a prior austenite grain boundary and grew towards the grain interior. The degenerate ferrite is composed of subunits that are rod-like crystals several microns in length, rather than a lath or a plate in this alloy. The subunits are more or less
Acknowledgements
One of the authors KMW wishes to acknowledge financial support from China Scholarship Council and the Ministry of Education, Culture, Sports, Science and Technology of the Japanese government.
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- 1
On leave of absence from University of Science and Technology, Beijing, People's Republic of China.