Block and sub-block boundary strengthening in lath martensite

Well-defined uniaxial micro-tensile tests were performed on lath martensite single block specimens and multi-block specimens with different number of block boundaries parallel to the loading direction. Detailed slip trace analyses consistently revealed that in the {110}<111> slip system with the highest Schmid factor is activated. Both block and sub-block boundaries act as barriers to dislocation motion, whereby a Hall-Petch like behavior is observed. Sub-block boundary strengthening appears to be only slightly less effective than block boundary strengthening, even though fracture analyses indicates that dislocation motion can cross sub-block boundaries but not block boundaries.