Untitled research work

The effect of pre-deformation on the martensitic transformation induced plasticity (TRIP) in austenitic Fe-Ni-C alloys (their Ms temperatures are below room temperature) was studied by tensile tests between -196°C∼80°C. Pre-deformation up to 65% in reduction was given by rolling at 200°C. Since the rolling temperature was above Md temperature, all the specimens consisted of a single austenitic phase even after rolling. The TRIP was observed in the pre-deformed specimens as well as in the non-deformed ones, showing the maximum elongation at a certain temperature between Ms and Md. The value of maximum elongation (εmax) decreased with the degree of pre-deformation. The amount of the increase in elongation by TRIP (ΔεA), which was defined as the difference between εmax and the elongation of austenite itself (εA), was found to be independent on the degree of pre-deformation in alloys having less than 0.2% carbon, but in an alloy with 0.4% carbon, the ΔεA was decreased by pre-deformation. When εA, εmax, and the elongation of martensite (εM) which was obtained by the subzero treatment, were compared with each other, the εM was scarcely decreased by pre-deformation. So, both the strength and elongation of martensite formed from pre-deformed austenite by cooling the specimen to a temperature below Ms temperature were higher than those of pre-deformed austenite itself (without TRIP). The difference (ΔεM) between εmax and εM decreased with pre-deformation even in 0.2% C alloys, being different from the trend of ΔεA. Finally, the Lüders type deformation, which appeared in heavily pre-deformed specimens, was observed in detail and discussed in comparison with other phenomena caused by the stress induced martensitic transformation.