*2.3. Sample Characterizations*

The specimen surface aspect was observed using an LEXT OLS4000 confocal microscope (Olympus, Tokyo, Japan), and the Rq surface roughness parameter was measured using a Mitutoyo SJ-400 probe (Mitutoyo, Kawasaki, Japan). The surface and subsurface hardening states were analyzed on a cross-section using a microhardness test with a measurement step of 50 μm. Phase identification (γ austenite and α' martensite) was done by EBSD (Electron Back-Scattered Diffraction) technique on specimen cross-sections in order to link the hardening state with the observed microstructural features.

On the RB samples, the surface residual stresses were evaluated using a Proto iXRD X-ray diffraction apparatus (PROTO Manufacturing Inc., Taylor, MI, USA), operating under a 20 kV tension and a 3 mA intensity, before cyclic loading, as well as after fatigue tests on the run-out samples. Two X-ray wavelengths were used to measure the residual stresses in each phase: (i) a chromium tube (CrK<sup>α</sup> = 2.291 Å) for the {211} α' martensite, and (ii) a manganese tube (MnK<sup>α</sup> = 2.103 Å) for the {311} austenite. Residual stresses were calculated by the classical sin<sup>2</sup> ψ method [30].

On the TC samples, the surface residual stresses were evaluated with a Pulstec μ-X360 X-ray diffraction equipment (Pulstec, Hamamatsu, Japan) operating under a 30 kV tension and a 1.5 mA current. A chromium tube (CrK<sup>α</sup> = 2.291 Å and CrK<sup>β</sup> = 2.085 Å) was selected for the measurements, and the cos α method [30] was used for the calculation of residual stresses in both γ and α' phases before and after fatigue tests.

In order to quantify the self-heating of the specimens during TC tests, a few samples were painted with black heat-resistant paint to allow a good emissivity, and their surface temperature was measured using an Infratech infrared camera (Infratech, Kennesaw, GA, USA).

Testing conditions and surface treatments used are summarized in Table 1.


**Table 1.** Summary of all testing and surface treatment conditions with their associated designations. SMAT—surface mechanical attrition treatment.
