Phase transformations in FeMnAlC austenitic steels with Si addition
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.A. SIMAO, and L.C. PEREIRA, Associate Professors, and C.A. ACHETE, Professor, are with the Metallurgy and Materials Engineering Department, Federal University of Rio de Janeiro, CEP 21941-972, Brazil. Contact e-mail: [email protected] I.S. KALASHNIKOV, formerly Visiting Professor, Metallurgy and Materials Engineering Department, Federal University of Rio de Janeiro, is with the All-Russia Institute for Aviation Materials (VIAM), Moscow, Russia. E.M. SILVA, formerly Graduate Student, Metallurgy and Materials Engineering Department, Federal University of Rio de Janeiro, is with the Federal Center for Technological Education, CEFET-CE, Fortaleza, Brazil. Manuscript submitted December 28, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
solution treated at 1050 ⬚C for 2 hours and water quenched. Subsequent aging was performed at temperatures ranging from 300 ⬚C to 700 ⬚C in salt baths followed by water quenching. Metallographic specimens were subjected to etching in a 5 pct solution of nitric acid in ethyl alcohol. A Zeiss DSM960 microscope was used for scanning electron microscopy (SEM). Atomic force microscopy (AFM) and magnetic force microscopy (MFM) were performed with a TMX 2010 Discoverer (Topometrix, USA) scanning probe microscope in the noncontact mode. A 100 kV SUMY EMV-100L (JEOL Ltd, Japan) microscope was used for transmission electron microscopy (TEM), and a Siemens-F (Siemens, Germany) X-ray diffractometer with Cu K␣ radiation was used for structure and phase analysis. Electrical resistance was measured with a setup made of standard modules, using a fourpoint method with current excitation up to 0.1 A along the main axis of a 2 ⫻ 2 ⫻ 10 mm specimen. Contacts were established by pressing stainless steel terminals to clean and polished surfaces. The observed results of phase transformations in Fe28Mn-8.5Al-1C steels with 1.25 pct Si allowed the construction of the diagram shown in Figure 1. Since the early works of Schmatz,[1,10] dealing first with ternary FeMnAl alloys, it is clear that at high temperatures, above 1000 ⬚C, there is only a single austenitic phase field. However, conventional quenching of FeMnAlC does not ensure a fully austenitic structure at room temperature.[11] In the present study, specimens have been water quenched from 1050 ⬚C to room temperature. The electron diffraction pattern corresponding to the bright-field image of the austenitic phase in Figure 2 shows weak superstructure reflections corresponding to the existence of ordered regions, in addition to the basic austenite 001 reflections. Therefore, the initial (as-quenched) austenite shows clear signs that some decomposition process has already started. During early decomposition stages, when neither TEM nor X-ray structure analysis provides reliable results,[12,13] the nature of phase transformations is better revealed by the variation of the specific electrical resistivity of the steel.[14,15,16] The observed dependence of the electrical resistivity of the steel on aging temperature and time within the temperature range
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