Abnormal Austenite-Ferrite Transformation Kinetics of Ultra-Low-Nitrogen Fe-N Alloy
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THE properties of steels are highly sensitive to and predominantly controlled by their chemical composition and microstructure. For the production practice of steels, two kinds of alloying components, substitutional (i.e., Mn, Co, Ni, and Cr) and interstitial (i.e., C and N) elements, can be distinguished for manipulation of the final microstructure in order to optimize the mechanical, physical, and chemical properties. The microstructure of hot-rolled steel grades, for example, is essentially formed during the austenite (c) to ferrite (a) phase transformation. So, the c fi a transformation in steels has been investigated extensively from both a technological point of view and a fundamental scientific point of view.[1–9] In general, the c fi a phase transformation of steels comprises three overlapping mechanisms: nucleation, growth, and impingement. Upon nucleation, a new interface is generated that separates the product ferrite phase from the parent austenite phase. This interface migrates into the surrounding parent phase during the subsequent growth, and the migration rate of the interface principally is determined by the diffusion of YONGCHANG LIU, formally Guest Scientist, Max Planck Institute for Metals Research, is Professor, College of Materials Science and Engineering, Tianjin University, 300072 Tianjin, People’s Republic of China. FERDINAND SOMMER, Professor and Research Associate, is with the Max Planck Institute for Metals Research, D-70569 Stuttgart, Germany. Contact e-mail: f.sommer@ mf.mpg.de ERIC J. MITTEMEIJER, Director, Max Planck Institute for Metals Research, is Professor, Institute for Physical Metallurgy, University of Stuttgart, D-70569 Stuttgart, Germany. Manuscript submitted October 2, 2006. Article published online July 15, 2008 2306—VOLUME 39A, OCTOBER 2008
alloying elements away from the interface and/or the mobility of the interface.[1,4–7] The thus formed microstructure is influenced by the impingement of the growing ferritic particles. Hence, occurrence of various nucleation and growth mechanisms would bring diversity in the prevailing c fi a transformation mechanism and thereby the resulting microstructure (i.e., a transition from interfaced-controlled to diffusion-controlled growth in isothermal annealing,[10,11] as well as a transition from diffusion-controlled to interfacecontrolled growth in isochronal annealing,[12] were observed during the c fi a transformation in ultra-low carbon Fe-C alloys). Very recently, abnormal c fi a transformation behavior in pure iron and substitutionally alloyed Fe-Co alloys characterized by the occurrence of more than one maximum in the transformation rate as a function of transformed fraction and transition from diffusioncontrolled to interface-controlled growth in ultra-lowcarbon Fe-C alloy was recognized for the first time, with the aid of high-resolution dilatometry and differential thermal analysis.[13–15] Microscopic evidence and kinetic analysis on the basis of phase-transformation models demonstrated that the occurrence of the abnormal transformati
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