Dual-phase steel structure visualized by extremely slow electrons

Area of Science:

• Materials Science
• Metallurgy
• Surface Science

Background:

• Complex steels, such as dual-phase (DP) steels, possess multi-phase structures that critically influence their mechanical properties.
• Characterizing these microstructures using Scanning Electron Microscopy (SEM) is essential but challenging due to difficulties in distinguishing constituent phases like ferrite and martensite.
• Existing SEM methods for phase segmentation offer various approaches and utilize a broad spectrum of electron energies.

Purpose of the Study:

• To demonstrate effective phase identification in DP steels across a wide range of primary electron landing energies, from tens of keV down to tens of eV.
• To investigate the utility of extremely low landing energies (<100 eV) for high-resolution surface visualization and phase separation in DP steels.
• To explore a novel 'cathode lens mode' for enabling SEM imaging across the full energy spectrum.

Main Methods:

• Utilized SEM to analyze DP steels at high, low, and extremely low primary electron landing energies (tens of keV to tens of eV).
• Implemented a 'cathode lens mode' by inserting an earthed detector to facilitate visualization at very low landing energies.
• Acquired images at various magnifications to assess phase separation and contrast.

Main Results:

• Extremely slow electrons (<100 eV) proved highly effective for separating martensite from the ferrite matrix due to superior surface sensitivity and visualization of fine features.
• Channelling contrast was notably suppressed at landing energies of tens of eV, enabling clear phase separation even at low magnifications.
• Phase contrast at tens of eV was attributed to differences in the native oxide layer thickness on martensite and ferrite phases.

Conclusions:

• Extremely low-energy SEM imaging, particularly in the 'cathode lens mode', offers a powerful method for detailed microstructural analysis of DP steels.
• The high surface sensitivity and reduced channelling contrast at low landing energies (<100 eV) significantly enhance the ability to distinguish ferrite and martensite phases.
• Understanding the role of native oxide layers provides a mechanistic explanation for the observed phase contrast in low-energy SEM imaging of DP steels.