Microstructural quantification of multi-phase steels (Micro-quant) : final report

Detailed understanding of process-microstructure-property relationships for highstrength steels is a prerequisite for their optimal design. This demands the ability to perform advanced microstructural quantification of their complex (multi-phase) microstructures, addressing the full range of microstructural properties that are relevant to an appreciation of their macroscopic mechanical behaviour. In turn, this requires that conventional light-optical microscopy (LOM) is complemented with high-resolution analytical techniques and image analysis to perform advanced morphological quantification of microstructures and to determine intrinsic mechanical, chemical and crystallographic properties of phase constituents. This project has aimed to develop a range of advanced metallographic procedures to deal with morphological analysis based on scanning-electron microscopy (SEM) and electron backscatter diffraction (EBSD) together with pixel- and context-based image analysis routines. Developed procedures have been evaluated via process-microstructure- property studies on steels ranging from DP/TRIP to highly challenging complex phase (CP) and bainitic/martensitic steels. In addition, methods to determine intrinsic properties of constituents have been investigated by means of EBSD-guided nanoindentation, SEM/EDX, electron probe micro-analysis (EPMA) and high-resolution secondary ion mass spectrometry (NanoSIMS). Highlights of this project are : • a classification scheme with definitions of matrix and secondary phase constituents ; • procedures for advanced morphological quantification via LOM/SEM and pixel- or context-based image analysis ; • EBSD partitioning rules for phase constituents in DP/TRIP/CP steels ; • EBSD-guided nano-indentation study with evidence of indentation-induced phase transformation of retained-austenite in TRIP ; • visualisation of martensite tempering in DP steel by means of NanoSIMS ; • process-microstructure-property relationships for DP/TRIP/CP and bainitic/ martensitic steels.