Microstructural Study of the Phase Transformations Upon Cooling to Room Temperature of High Mn Steels
- PDF / 742,559 Bytes
- 8 Pages / 593.972 x 792 pts Page_size
- 49 Downloads / 224 Views
.
INTRODUCTION
THE properties of a class of austenite steels with high concentrations of Mn and Al have been characterized for decades.[1–4] Mn and Al are austenite and ferrite formers in steels, respectively. Higher Mn content in steels causes a higher proportion of the austenite phase. In Mn-Al steels with high concentrations of Mn, the single phase of austenite at high temperature can be preserved, even at room temperature. However, as the concentration of Al increases, the proportion of the austenite in the ferrous alloys decreases. Thus, the constituent phases of Mn-Al steels could be austenite, ferrite, or dual phases. It depends on the relative concentrations of the Mn and Al in the steels.[3,4] For austenite-to-ferrite phase transitions in steels during cooling from high temperatures, Widmansta¨tten plate, massive, and martensitic phases form in the austenite matrix.[5] In contrast to the phase transformations of the steels during cooling, various ferrite-toaustenite transformations have been found in Mn-Al steels. Product phases such as Widmansta¨tten sideplate,[6] massive,[7] and 18R martensitic phases[8,9] exist in the ferritic matrix of Mn-Al steels. For phase transformations in Mn-Al steels isothermally held at low temperatures, j-carbide precipitates in the austenite matrix. The j-carbide, with an L12 crystal structure, appears in the austenite matrix homogeneously via Spinodal decomposition.[10–12] The hexagonal close-packed (HCP) e-martensitic side-plates appear in austenite steels during cooling or deformation.[13–19] Face-centered cubic (FCC) micro-twins have also been observed along with the e-martensite in deformed austenite steels.[20,21] WEI-CHUN CHENG, Professor, and TUNG-YI LIN, Graduate Student, are with the Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan R.O.C. Contact e-mail: [email protected] Manuscript submitted December 17, 2010. Article published online February 9, 2012 1826—VOLUME 43A, JUNE 2012
Furthermore, two types of martensitic transformations, i.e., c fi e and c fi a, occur in austenite steels. As the nucleation of body-centered cubic (BCC) a-martensite is within the e-martensitic plates, the e-martensite is believed to be an intermediate phase for the formation of a-martensite, i.e., c fi e fi a[13,22–26] Stacking faults in the FCC matrix have been proposed as the major mechanism for the transition of austenite to e-martensite in austenite steels, such as Fe-based shape memory alloys.[19,27–29] Stacking faults were created as a result of the gliding of a/6h112i Shockley partial dislocations along the {111} planes of the austenite matrix. Therefore, all {111} compact planes of the FCC phase are possible shear planes for the gliding of the partial dislocations. Various stacking sequences of the stacking faults generate regions that allow the growth of the nuclei for the HCP phase or for the FCC microtwins in the FCC matrix.[5,13,18] The strain energy field can force the gliding of Shockley partial dislocations t
Data Loading...
