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

Abstract
Two grades of unstable austenitic stainless steels, AISI 301LN and 201 were tested under uniaxial tension over a wide temperature range [−150°C; 150°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 γ → α’ 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 γ → α’ whereas grade 201 sequence is γ → ε → α’. The nucleation of ε martensite is promoted by high strain rate, as shown by TEM observations. It is demonstrated that at high strain rates (≈ 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 α’ 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 γ → ε transformation and, more generally, on planar deformation modes.