Carbon Enrichment in Austenite During Bainite Transformation in Fe-3Mn-C Alloy
- PDF / 964,111 Bytes
- 6 Pages / 593.972 x 792 pts Page_size
- 108 Downloads / 213 Views
I.
INTRODUCTION
TRANSFORMATION stasis is a phenomenon that is frequently observed in bainite transformations.[1,2] It refers to the temporary cessation of bainitic ferrite formation, and the transformation cannot resume until carbide starts to form. Carbon enrichment in austenite during the stasis is crucial to understand the mechanism of bainite transformations. Even now, the understanding on bainite transformations is still in a fierce controversy. One holds the opinion that the formation of bainitic ferrite is a diffusional process and then carbon enrichment at the cessation of bainite growth should be given by paraequilibrium model (PE) or partitioning local equilibrium/negligible partitioning local equilibrium model (PLE/NPLE) with certain deviation due to free energy dissipation at interface.[3– 5] Nevertheless, the other insists that the growth of bainitic ferrite is a martensite-like diffusionless transformation. Then, the carbon enrichment should be given by the T0 model or the T0¢ model,[6] which takes into account the strain energy associated with the diffusionless growth. To provide evidence on the formation mechanism of bainitic ferrite, carbon enrichment in austenite after the cessation of bainitic ferrite growth has been evaluated using lever rule,[7] X-ray diffraction,[8] transmission electron microscopy (TEM),[9] and atom probe.[10–12] Recently, our research group has conducted a direct measurement of local carbon content using field-emisZHEN-QING LIU, Ph.D. Student, is with the School of Materials Science and Engineering, Key Laboratory of Advanced Materials of Ministry of Education, Tsinghua University, Beijing 100084, P.R. China, also with the Department of Metallurgy, Tohoku University, Aoba-ku, Sendai 980-8577, Japan. Contact e-mail: [email protected] GORO MIYAMOTO, Associate Professor, and TADASHI FURUHARA, Professor, are with the Institute for Materials Research, Tohoku University. ZHI-GANG YANG, Professor, and CHI ZHANG, Associate Professor, are with the School of Materials Science and Engineering, Key Laboratory of Advanced Materials of Ministry of Education, Tsinghua University. Manuscript submitted September 12, 2014. Article published online 21 January 2015 1544—VOLUME 46A, APRIL 2015
sion electron probe microanalyzer (FE-EPMA) which is able to quantify carbon content accurately in much wider area than TEM and atom probe techniques. Then, the measured carbon content was compared with the predictions of the above-mentioned models.[13,14] In the previous studies on ferrite transformations in Fe-2Mn-C[13] and Fe-1.5Mn-0.05Si-C alloys,[14] it was found that carbon enrichment in austenite agrees relatively well with the PLE/NPLE transition line at high transformation temperatures. But as the transformation temperature decreases to bainite formation range,[14] the carbon enrichment deviates from PLE/ NPLE transition line to lower carbon content. This deviation can be explained by solute drag effects and/or finite interface mobility in diffusional mechanism, which cause carbon enrichment in austenite
Data Loading...
