Quantitative Analysis of Martensitic Microstructures

EDX experiments were performed using a LaB₆ JEOL JSM 840A type SEM (Tokyo, Japan) operating at 30 kV. EDX maps were recorded from areas of 0.25 mm². To obtain high-quality images of martensitic microstructures and to study the distributions of the ξ-angles to differentiate between different types of martensite, an FEI Quanta FEG 650 SEM (Hillsboro, OR, USA) was used. During the EBSD measurements, the specimens were tilted by 70° and an acceleration voltage of 30 kV was used at a working distance of ~17 mm. Investigations were performed on vibro-polished metallographic cross-sections. Areas of 100 × 100 µm² in the center of large prior austenite grains (~mm size) were scanned using a step size of 100 nm resulting in 500,000 crystallographic data points per material state. EBSD measurements were conducted using an EDAX Inc. type Hikari XP camera (Mahwah, NJ, USA). The EBSD data were evaluated following the approach which was recently documented in Ref. [8 (link)] and implemented into the MATLAB [60 ] toolbox MTEX [61 (link),62 (link)]. In the present work, two procedures are used which were introduced in previous studies, see Ref. [8 (link)]. First, the distribution of the three ξ-angles, which represents the deviation between the actual orientation relationship between the austenite parent lattice and the ideal Bain orientation relation is considered. The three angles are denoted as ξ₁, ξ_2, and ξ₃, where indices ₁, _2, and ₃ correspond to the x, y, and z-axes of the Bain cell respectively., see Figure 3a. The symmetry of the austenite-martensite transformation yields 24 possible martensite variants which can form in one austenite grain. Secondly, a martensite variant color coding which was introduced in [8 (link)], Figure 3b, is applied to visualize information about the emerging variants forming during the martensitic transformation.