On the influence of interactions between phases on the mechanical stability of retained austenite in transformation-indu
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INTRODUCTION
THE transformation-induced plasticity (TRIP) effect, i.e., the mechanically induced martensitic transformation of metastable austenite, has been proven for many years to contribute very effectively to the deformation process in a large variety of fully austenitic iron-based alloys. Numerous studies have shown that the TRIP effect improves strength and ductility by helping to maintain a high work-hardening rate during straining.[1–11] This phenomenon is commonly ascribed to two different mechanisms:[12,13] (1) the stressassisted nucleation of martensitic variants favorably oriented with respect to the applied stress (Magee effect[14] or orientation effect); and (2) the plastic straining of the surrounding phases due to the volume and shape changes associated with the displacive transformation (Greenwood–Johnson effect[15] or accommodation effect). In Fe-Ni-Cr alloys,[2,16,17] Fe-Ni alloys,[17,18] or in other highly alloyed fully austenitic steels,[3,19] tensile strength and elongation are enhanced when the chemical composition is such that the Ms temperature is just below the testing temperature. The TRIP effect is also thought to be the main phenomenon responsible for the improved balance of strength and ductility exhibited by the new and so-called “TRIP-assisted multiphase steels.” These low-alloy C-Mn-Si steels are quite different from the previous fully austenitic TRIP steels. They present a multiphase microstructure consisting of an intercritical ferrite matrix with a dispersion of bainite, metastable retained austenite, and martensite. In the present case, retained austenite is a minor phase (⬃5 to 20 pct[20]) dispersed in a ferrite-based microstructure. Previous studies[21–24] have shown that the enhancement of both strength and ductility P.J. JACQUES, Associate Researcher, and F. DELANNAY, Professor, Division of Physical Chemistry and Engineering of Materials, and J. LADRIE`RE, Professor, Division of Inorganic and Nuclear Chemistry, are with the Catholic University of Louvain, B-1348 Louvain-la-Neuve, Belgium. Manuscript submitted May 16, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
in these TRIP-aided steels is related to the presence of this retained austenite, which transforms into martensite during mechanical straining. In the case of cold-rolled steel sheets, the particular microstructure of the TRIP-assisted multiphase steels is obtained by carrying out a two-stage heat treatment. As for dualphase steels, the first stage consists of an intercritical annealing, during which part of the initial microstructure transforms into austenite. While dual-phase steels are directly quenched to room temperature, TRIP-aided steels are rapidly cooled to a temperature at which an isothermal bainitic holding is conducted (typically in the range from 350 ⬚C to 450 ⬚C). During this second isothermal dwell, part of the intercritical austenite transforms to bainite, whereas the remaining austenite is stabilized in such a way that it does not transform to martensite during the final water quench to room te
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