Effects of strain rate on mechanical properties and deformation behavior of an austenitic Fe-25Mn-3Al-3Si TWIP-TRIP steel

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 711; no. C; pp. 78 - 92
• Main Authors: Benzing, J.T., Poling, W.A., Pierce, D.T., Bentley, J., Findley, K.O., Raabe, D., Wittig, J.E.
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 10-01-2018; Elsevier BV; Elsevier
• Subjects: Adiabatic heating; Austenitic stainless steels; Channeling; Deformation effects; Deformation mechanisms; Electron backscatter diffraction; Elongation; Hardening rate; High-Mn [Fe-25Mn-3Al-3Si] steel; Martensite; MATERIALS SCIENCE; Mechanical properties; Mechanical twinning; Microstructure; Platelets; Room temperature; Stacking fault energy; Stacking fault energy (SFE); Strain hardening; Strain localization; Strain rate; Strain rate sensitivity; Tensile properties; Tensile strength; Thermodynamic models; Thickness; TRIP steels; Twinning and transformation-induced plasticity (TWIP-TRIP); TWIP steels; Yield strength; Yield stress; Strain rate sensitivity; Adiabatic heating; High-Mn [Fe-25Mn-3Al-3Si] steel; Stacking fault energy (SFE); Twinning and transformation-induced plasticity (TWIP-TRIP)
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2017.11.017

The effects of quasi-static and low-dynamic strain rate (ε̇ = 10−4 /s to ε̇ = 102 /s) on tensile properties and deformation mechanisms were studied in a Fe-25Mn-3Al-3Si (wt%) twinning and transformation-induced plasticity [TWIP-TRIP] steel. The fully austenitic microstructure deforms primarily by dislocation glide but due to the room temperature stacking fault energy [SFE] of 21 ± 3mJ/m2 for this alloy, secondary deformation mechanisms such as mechanical twinning (TWIP) and epsilon martensite formation (TRIP) also play an important role in the deformation behavior. The mechanical twins and epsilon-martensite platelets act as planar obstacles to subsequent dislocation motion on non-coplanar glide planes and reduce the dislocation mean free path. A high-speed thermal camera was used to measure the increase in specimen temperature as a function of strain, which enabled the use of a thermodynamic model to predict the increase in SFE. The influence of strain rate and strain on microstructural parameters such as the thickness and spacing of mechanical twins and epsilon-martensite laths was quantified using dark field transmission electron microscopy, electron channeling contrast imaging, and electron backscattered diffraction. The effect of sheet thickness on mechanical properties was also investigated. Increasing the tensile specimen thickness increased the product of ultimate tensile strength and total elongation, but had no significant effect on uniform elongation or yield strength. The yield strength exhibited a significant increase with increasing strain rate, indicating that dislocation glide becomes more difficult with increasing strain rate due to thermally-activated short-range barriers. A modest increase in ultimate tensile strength and minimal decrease in uniform elongation were noted at higher strain rates, suggesting adiabatic heating, slight changes in strain-hardening rate and observed strain localizations as root causes, rather than a significant change in the underlying TWIP-TRIP mechanisms at low values of strain.