HT9, a ferritic/martensitic steel, is a candidate structural material for next-generation advanced reactors. Its microstructure is a typical tempered martensite showing a hierarchical lath-block-and-packet structure. We investigate the specimen size effect and strengthening contribution of various microstructural boundaries manifested in the compression tests of micropillars with diameters ranging from 0.5 to 17 μm. It is observed that micropillars with diameters larger than 3 μm show uniform deformation and plastic flow curves comparable to the bulk flow curve. Localized deformation by a few pronounced slip bands occurs in micropillars with diameters smaller than 1 μm, and the yield strength is reduced. Careful examination of the sizes of the microstructural features and cross-sections of the micropillars shows that the block boundaries are the most effective strengthening boundaries in tempered martensitic microstructure. The bulk mechanical properties of HT9 can be evaluated from a micropillar with diameter as low as 3 μm.

HT9是一种铁素体/马氏体钢，是下一代先进反应堆的候选结构材料。它的微观结构是典型的回火马氏体，显示出分层的板条块和包结构。我们调查了直径在0.5至17μm范围内的微柱的压缩试验中所显示的试样尺寸效应和各种微结构边界的强化作用。可以观察到，直径大于3μm的微柱显示出均匀的变形，塑性流动曲线可与整体流动曲线相媲美。在直径小于1μm的微柱中会出现一些明显的滑移带引起的局部变形，从而降低了屈服强度。仔细检查微结构特征的尺寸和微柱的横截面可知，块体边界是回火马氏体微结构中最有效的强化边界。HT9的整体机械性能可以通过直径低至3μm的微柱进行评估。