Abstract
Direct microscopic observation of the isothermal bainite evolution in terms of nucleation events, the location of the nuclei, as well as their growth is very valuable for the refinement of models predicting the kinetics of bainite transformation. To this aim, the microstructural evolution in a Fe-0.2C-1.5Mn-2.0Cr alloy during isothermal bainite formation at temperatures between 723 K and 923 K is monitored in situ using high temperature laser scanning confocal microscopy (LSCM). Both the nucleation and the growth kinetics of the bainitic plates are analyzed quantitatively. Bainitic plates are observed to nucleate on three different types of locations in the grain: at austenitic grain boundaries, on newly-formed bainite plates and at unspecific sites within the austenite grains. Grain boundary nucleation is observed to be the dominant nucleation mode at all transformation temperatures. The rate of nucleation is found to vary markedly between different austenite grains. The temperature dependence of the average bainite nucleation rate is in qualitative agreement with the classical nucleation theory. Analysis of plate growth reveals that also the lengthening rates of bainite plates differ strongly between different grains. However, the lengthening rates do not seem to be related to the type of nucleation site. Analysis of the temperature dependence of the growth rate shows that the lengthening rates at high temperatures are in line with a diffusional model when a growth barrier of 400 J mol−1 is considered.
Highlights
The wish for better performing automobiles with improved fuel efficiency, low CO2 emission, crashworthiness and rigidity has directed the automotive industry’s efforts towards the adoption of advanced high strength steels (AHSS) as the primary structural material
This study reports on the bainitic nucleation and growth observed at different isothermal temperatures of 723 K, 773 K, 823 K and 923 K (450 ◦ C, 500 ◦ C, 550 ◦ C and 650 ◦ C)
Nucleation was wasfound foundtototake takeplace place(a)(a) grain boundary; in the grain interior the of a recently-formed bainite plate and in the at a location not showing any specific side of a recently-formed bainite plate(c)and (c) grain in theinterior grain interior at a location not showing any feature
Summary
The wish for better performing automobiles with improved fuel efficiency, low CO2 emission, crashworthiness and rigidity has directed the automotive industry’s efforts towards the adoption of advanced high strength steels (AHSS) as the primary structural material. The first generation of AHSS (1st Gen AHSS) includes dual phase (DP), transformation-induced plasticity (TRIP), complex-phase (CP) and martensitic (MART) steels with a high allotriomorphic ferrite fraction. The second generation of AHSS (2nd Gen AHSS) includes twinning-induced plasticity (TWIP), Al-rich lightweight steels (L-IP® ) and shear band-strengthened steels (SIP) containing a mass fraction of Mn of about 20%. These 2nd Gen AHSS display significantly higher strength values compared to the 1st Gen
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