Kinetics of Martensite Formation in Substitutional Fe-Al Alloys: Dilatometric Analysis
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INTRODUCTION
THE martensitic transformation (MT) in solid-state materials has received considerable attention over the years because of its many potential applications (e.g., References 1–10). The MT belongs to a special class of diffusionless solid-state phase transitions which occur in crystalline materials by transforming a parent-phase to a product-phase crystal structure, conventionally named austenite (c) and martensite (a¢), respectively. Austenite is characterized by a highly symmetric unit cell and is stable at relatively high temperature. Upon MT, the austenite lattice typically undergoes a displacive type of transformation to a martensite lattice, characterized by a unit cell of lower symmetry and (meta)stability at relatively low temperature. The MT takes place upon cooling (quenching) of the high-temperature austenite at a rate larger than a critical value. Although the influence of cooling/quenching rate has been generally discussed in numerous previous publications and classic textbooks on phase transformations, a kinetic description of the YONGCHANG LIU, Professor, and LIFANG ZHANG, Ph.D. Student, are with the School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, P.R. China. FERDINAND SOMMER, Professor, is with the Max Planck Institute for Intelligent Systems, Heisenbergstr. 3, 70569 Stuttgart, Germany. Contact e-mail: [email protected] ERIC JAN MITTEMEIJER, Professor and Director, is with the Max Planck Institute for Intelligent Systems, and also with the Institute for Materials Science, University of Stuttgart, Stuttgart, Germany. Manuscript submitted February 27, 2012. Article published online November 10, 2012 1430—VOLUME 44A, MARCH 2013
very fast martensite transformation (c/a¢ interface velocities are of the order ranging from 104 to 102 m s1[11–13]), as function of cooling/quenching rate, is unknown to us. Particularly, in most studies the martensite transformation was investigated employing one cooling/quenching rate, for example, 10 K (10 C) s1 as e.g., in Reference 4. Two experimental difficulties appear responsible for this lack of knowledge until now: (1) the necessity of imposition of a high cooling/quenching rate, to induce the martensite transformation, makes the temperature calibration/homogenization very difficult, and (2) the high speed of the martensite transformation requires a very high sensitivity of the devices used to measure the accompanying (thermal and/or volume) effects (i.e., a high-resolution displacement sensor is imperative for dilatometric measurements). It is generally considered that the martensite reaction in steels normally occurs athermally (i.e., the transformation is not thermally activated and the transformed fraction depends on the extent of the undercooling below a martensite-start temperature MS ). The degree of transformation, f ð0 f 1Þ, then is virtually independent of time at constant temperature. Thus, the classic Koistenen and Marburger equation has often been used to describe the progress of transformation upon cooling
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