Fig. 4: Microstructures at different tensile strains.

a Engineering stress-strain curve of the Stage 3 RAFM steel. Markers indicate the strain to which interrupted tensile tests were conducted (curve taken as representative of 3 different repeat tests). b–d Sample strained to 9%: (b) grain boundary map, black lines indicate high angle grain boundaries (HAGBs) (>5˚) and red lines indicate low angle grain boundaries (LAGBs). c STEM-BF micrograph of martensite and (d) STEM-BF micrograph of precipitates and dislocations in the ferrite. e–i Sample strained to 16%: (e) grain boundary map; k Transmission Kikuchi Diffraction (TKD) boundary map and its corresponding Kernel Average Misorientation (KAM) in l; STEM-BF micrographs showing h martensite and i dislocation cell formation in ferrite. j–n Sample strained to 38%: (j) grain boundary map; (k) TKD boundary map with red arrows pointing to the nucleation of low orientation gradient ferrite grains along a ferrite/martensite boundary and corresponding l KAM map; STEM-BF micrographs showing m martensite and n elongated dislocation cell formation in ferrite. o–s Sample strained to 49%, i.e. failure. o grain boundary map; p TKD boundary map showing periodic formation of strain-free ferrite grains with corresponding (q) KAM map; STEM-BF micrographs of (r) newly formed strain-free grains at a ferrite/martensite boundary and s elongated subcell formation in ferrite.