Kangying Zhu

Characterization and quantification methods of complex BCC matrix microstructures in advanced high strength steels

The development of comprehensive and reliable microstructure characterization tools is very necessary for the understanding and modelling of both the formation of different phases during processing and the relationship between microstructure and mechanical properties. A series of samples containing different phases with a BCC structure have been characterised using different techniques including optical microscopy (OM), field emission gun scanning electron microscopy (FEG-SEM) with secondary electron (SE) and back scattered electron (BSE) modes, electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). It is shown that it is difficult to distinguish ferrite from bainite especially granular bainitic ferrite using conventional OM and FEG-SEM (SE) techniques, whereas FEG-SEM (BSE) and EBSD are the most suitable techniques to differentiate them. A new EBSD method has been developed to dissociate and quantify ferrite, bainite and martensite in multi-phase AHSS steels. This method has been proven to be pertinent and has been validated using reference specimens.

On the nanoindentation behaviour of complex ferritic phases

Systematic nanohardness measurements on martensite, lower bainite, upper bainite, granular bainite and ferrite were carried out using nanoindentation technique. The variation of nanohardness among these ferritic phases is ascribed to the different capacity of strengthening factors in confining the geometrically necessary dislocation within indentation plastic zone. The large scatter of nanohardness distribution in lath martensite may be due to the uneven block boundary strengthening, inhomogeneous dislocation forest strengthening and the different extent of auto-tempering.

Effect of Al on martensite tempering: comparison with Si

Both Si and Al are known to have negligible solubility in cementite and therefore retard cementite precipitation. The effect of Si on carbides formation during martensite tempering has been extensively studied, whereas that of Al has attracted little attention. The aim of the present study is to shed light on the effect of Al on martensite tempering. Various advanced characterization techniques like FEG-SEM, TEM, in situ synchrotron XRD and APT have been employed to investigate the microstructural evolution during martensite tempering of 0.25C–2.1Mn steels with or without Al and Si additions. It is revealed that Al addition promotes ε-carbide formation, whereas Si addition has no significant effect. Al addition has weaker influence than Si addition to retard θ-carbides formation. At lower tempering temperature or short tempering time, Si addition retards more efficiently θ-carbide growth, whereas at higher temperature tempering or longer tempering time, Al addition becomes more efficient to retard θ-carbide growth. As a consequence, Si addition resists better martensite softening during tempering at lower temperature or for shorter time, whereas Al addition resists more strongly martensite softening during tempering at higher temperature or for longer time. Based on the present experimental results, mechanisms are proposed to explain the effect of Al on θ-carbide formation and growth as compared with Si.