Strain induced martensitic transformation at high strain rate in two austenitic stainless steels

Two grades of unstable austenitic stainless steels, AISI 301LN and 201 were tested under uniaxial tension over a wide temperature range [-150 degrees C; 150 degrees C] and a large strain rate range [3.10(-4) s(-1); 200 s(-1)]. A special device was designed to run interrupted tests at high strain rates [50 s(-1) & 200 s(-1)]. The gamma ->alpha' phase transformation was characterized by magnetic measurements. The strain-induced phase transformations are different in these materials. Grade 301LN transformation sequence appears essentially to be gamma ->alpha' whereas grade 201 sequence is gamma ->epsilon ->alpha'. The nucleation of epsilon martensite is promoted by high strain rate, as shown by TEM observations. It is demonstrated that at high strain rates ( approximate to 50 s(-1)) the kinetics of the martensite transformation is largely reduced due to adiabatic heating. It is also shown that it is possible to account for the large reduction in the amount of strain-induced alpha' martensite by taking into account the adiabatic effect. However, an accelerating effect of high strain rates is found when the materials are tested at an initial temperature close to Md. This observation is discussed in the light of the effect of strain rate on gamma ->epsilon transformation and, more generally, on planar deformation modes.