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Multiple pop-in events which indicate a gradual transformation from a metastable austenite grain to martensites in steel were detected during nanoindentation. It was observed by means of an automotive mapping technique with TEM that the partial volume of prior austenite had transformed into several martensite blocks with different variants. From a finite element calculation combined with phenomenological approach for martensitic transformation, it was confirmed that each variant corresponded to those for which the transformation strain effectively accommodates external stress.

The strain-induced austenite-to-e-martensite transformation during nanoindentation of metastable austenite in a high-nitrogen stainless steel was investigated by observation of the microstructural changes underneath the indent. In the nanoindentation load–displacement curve, several small pop-ins were detected at the beginning stage of plastic deformation. The presence of e-martensite bands underneath the indent and their orientation relationship with the parent austenite strongly suggest that the pop-ins in the load–displacement curve correspond to the transformation from austenite to e-martensite. A quantitative analysis on the massive movement of Shockley partial dislocations during the transformation was performed to obtain the contraction strain under compressive stress conditions during nanoindentation. It was confirmed that the calculated contraction strain along the indenting direction matches well with the measured length of the pop-ins.

Differentiation of dynamically and statically transformed ferrites in SIDT (Strain-Induced Dynamic Transformation) steel was carried out by various image-based analyses of EBSD (Electron BackScattered Diffraction) maps. Various identification methods were tried on the basis on the grain size, and the intra- and inter-granular misorientations. The grain-size-based method was valid only when a bimodal grain size distribution in the ferrite phase, due to the large prior austenite grain size, appeared in the specimen. On the other hand, using the proposed method based on both intra- and inter-granular misorientations, the dynamically transformed ferrite could be effectively extracted from SIDTed steel, even in steel in which the grain sizes of the dynamically and statically transformed ferrites were alike.

EBSD mapping was used for phase analysis of multi-phase steels. Some products of austenite decomposition could be discriminated using a range of grain-average functions and properties derived from the map data. In DP steels, martensite could be extracted from the ferrite matrix using the average band contrast or a set of geographic information. Bainite in TRIP steel was identified with the local variations of band contrast and orientation inside a grain. As the proposed method reproduced a phase map, a visual confirmation of the result and additional analyses were possible.

Strain-induced martensitic transformation of metastable austenite was investigated by nanoindentation of individual austenite grains in multi-phase steel. A cross-section prepared through one of these indented regions using focused ion beam milling was examined by transmission electron microscopy. The presence of martensite underneath the indent indicates that the pop-ins observed on the load-displacement curve during nanoindentation correspond to the onset of strain-induced martensitic transformation. The pop-ins can be understood as resulting from the selection of a favorable martensite variant during nanoindentation.

The mechanical properties of dynamically and statically transformed ferrites were analyzed using a nanoindentater-EBSD (Electron BackScattered Diffraction) correlation technique, which can distinguish indenting positions according to the grains in the specimen. The dilatometry and the band slope and contrast maps by EBSD were used to evaluate the volume fractions of two kinds of ferrite and pearlite. Fine ferrites induced by a dynamic transformation had higher nano-hardness than the statically transformed coarse ferrites. Transmission electron microscopy (TEM) showed the dynamic ferrites to have a higher dislocation density than the statically transformed ferrites.