Ivas software

Samples for APT analyses were prepared by electropolishing, followed by treatment with a FIB (FEI, Helios Nano-Lab 600). APT measurements were taken using a local electrode atom probe (LEAP 4000X HR, CAMECATM) in the voltage-pulsing mode. The experimental parameters were set to maintain a 0.2% detection rate, 20% pulse fraction, and 200-kHz pulse repetition. All measurements were performed at 40 K at < 10⁻⁷ Pa pressure. A minimum of two successful measurements were performed. The APT data sets were mapped in 3D using IVAS software (version 3.6.10) supplied by Cameca Instruments. Reconstruction was calibrated by determining the APT sample geometric parameters, such as tip radius and shank angle. Statistical errors for measured atom counts were calculated as σ = (C_i × (1 − C_i)/N)^−1/2, where C_i corresponds to the measured atomic concentration fraction of the individual element i, and N is the total number of atoms collected in the bin.

The evolved crystal structure was identified via X-ray diffraction (XRD, X’Pert PRO-MRD, PHILIPS, Netherlands). The grain structure and phase distribution were investigated via EBSD using field-emission scanning electron microscopy (FE-SEM, S-4300SE, HITACHI, Japan). Specimens were mechanically polished using SiC papers of up to 4000 grit size, and electropolished in a mixed solution of 92% acetic acid and 8% perchloric acid. Transmission electron microscopy (TEM) was performed using a JEOL JEM-2100F instrument operated at 200 kV. The TEM specimens were prepared via focused ion beam lift-out using a FEI Helios NanoLab 450 F1 instrument. The chemical composition of each phase was measured via atom probe tomography (APT, Cameca LEAP 4000X HR) using the pulsed laser mode at a specimen base temperature of ~50 K. The pulse frequency and energy were 200 kHz and 50 pJ, respectively. The acquired APT data were reconstructed and analysed using the commercial IVAS® software by Cameca. To reveal the deformation structures, TEM was conducted for the 1%-deformed tensile specimen.

Site-specific needles for APT analysis were extracted from the 1 minute (cementite midrib formation), 3 minutes (inverse bainitic ferrite formation), 5 minutes (inverse bainitic ferrite and secondary carbide formation), and 10 minutes (degeneration of inverse bainite) isothermal hold samples using a FEI Nova 200 dual beam FIB/SEM^{36 (link)}. Regions of interest were annularly milled and cleaned at low voltage (2 kV) with a Ga ion beam to make the needle shaped specimens suitable for field evaporation. Atom probe analyses were performed using a CAMECA Instruments LEAP 4000X HR local electrode atom probe (LEAP). The atom probe needles were field evaporated in laser mode with a 200 kHz pulse repetition rate, 35 K specimen temperature, 50–70 pJ laser power, and a detection rate of 0.5%. The resulting data was reconstructed and analyzed using CAMECA’s IVAS software.