Evaluation of the boundary strength of ferrous martensite through a micromechanical test

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We investigated the block boundary and sub-block boundary strengths of martensite evaluated through a micromechanical test. The sub-block boundaries contribute very little to the macroscopic strength of martensite. In contrast, the existence of a block boundary in the specimen greatly increased the strength. In addition, the block boundary induced a serrated flow and load drop in the load-displacement curve. The load drop and serrated flow were attributed to dislocation pile-up and subsequent propagation of dislocations across the block boundary. In a microstructural observation of specimens after deformation, we found that a block boundary significantly restricts the motion of dislocations, while sub-block boundary does not. We concluded that the block boundary is the most effective grain boundary for strength in martensite.