DISLOCATION STRUCTURES AND DAMAGE DEVELOPMENT IN FATIGUED X6CrNiNb 1810 AUSTENITIC STAINLESS STEEL

摘要

The understanding of crack initiation mechanisms during cyclic loading in apparently homogeneous materials like X6CrNiNb 18 10 austenitic steel requires explicit analysis of the phenomena accompanying fatigue on both atomistic and microscopic levels. The only visible microstructural details in this material on the microscopic level are austenitic grains, twins and finely distributed niobium carbides. Low cycle fatigue tests performed with different strain amplitudes show a correlation between hardening or softening behaviour of the material and external loading. Applied strain amplitudes influence also the character of micro-cracks and their density in an early stage of damage development. Assuming that micro-crack initiation is connected with inhomogeneities on smaller length scales, dislocation structures and densities have been analysed using transmission electron microscope in both undeformed and deformed specimens. Pronounced cell structures with dislocation-rich walls and dislocation-poor channels have been found in the fatigued material. Moreover, X-ray diffraction and molecular dynamics simulations confirmed martensitic transformation in cyclically loaded X6CrNiNb 18 10 austenitic steel. The impact of the martensite on the crack initiation mechanisms during cyclic loading are currently under investigation.