PSI - Issue 13

Xiaofei Guo et al. / Procedia Structural Integrity 13 (2018) 1453–1459 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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3. Results 3.1. Microstructure and mechanical properties

The investigated materials revealed fully recrystallized austenitic microstructures with an average grain size of 10 µm for 22Mn and 14 µm for 17Mn_Al, as shown in Figure 1a). Figure 1b) shows their engineering stress-strain curves at a strain rate of 10 -3 s -1 . The 22Mn material has higher yield strength due to the refined grain size. It also exhibits high strain hardening capacity with an early onset of serrated flow starting at ~6% elongation. The serration is assumed due to the impedance of dislocation gliding by deformation twins and the Mn-C short range ordering [1]. The 17Mn_Al material exhibits a lower strain hardening capacity and a delayed onset of serrated flow at the elongation of ~35%. It obtains a uniform elongation of 69%. Both materials have very high ECO index (defined as the multiplication of uniform strength and elongation) close to 60000 MPa·%, which indicates excellent strength ductility combination.

Figure 1. a) Microstructure characterization by EBSD and b) Tensile properties at the strain rate of 10 -3 s -1 .

3.2. Hydrogen concentration Figure 2 describes the hydrogen desorption curves measured by TDS at the constant heating rate of 20 °C/min starting from RT, and up to 800 °C. The first peak at low temperature is associated with the diffusion of hydrogen in lattice defects with low hydrogen trapping energy. These defects are assumed to be interstitial sites, vacancies, dislocations and grain boundaries.

Figure 2. Thermal desorption curves of the 166-hour hydrogen pre-charged and 3-hour RT homogenized 22Mn and 17Mn_Al materials.