Tatsuya Morikawa

To clarify the contribution of martensite increase to work-hardening during transformation-induced plasticity (TRIP), the changes in local strain distribution with tensile deformation were investigated for dual-phase steels with different volume fractions of martensite in ferrite using the precise marker method. Particular attention was paid to the changes in strain and stress bearing by martensite and ferrite with tensile deformation. The precise marker method is especially useful for local strain analysis of multiphase materials. Three types of steels with volume fractions of 25%, 50%, and 75% martensite were subjected to local strain analysis at several stages of deformation. The important results are as follows: (1) in steels with a large volume fraction of martensite, the contribution of the plastic deformation of martensite to the overall tensile strain is large from the beginning of the tensile deformation, and (2) the difference in strain bearing by martensite and ferrite increases through tensile deformation in both 25% and 50% martensitic steels. Using the constitutive equations for the stress-strain response, the strain distributions in each phase were translated into stress distributions. Then, the stress-strain response was numerically estimated by applying the general rule of mixtures using the average values of strain and stress, and compared with the experimental results. The relationship between the controlled martensitic transformation and tensile deformation behavior in TRIP steel is also discussed.

The nature of crack tip dislocations and their multiplication processes in silicon crystals have been examined by using high-voltage electron microscopy. Cracks were introduced by the Vickers indentation method at room temperature, and the specimen indented was annealed at high temperatures to induce dislocation generation around the crack tip under the presence of residual stress due to the indentation. In the specimens annealed, fine slip bands with the step heights of around 1 nm were formed along the {111} slip planes near the crack tip. The crack tip dislocations observed were characterized by matching their images to those simulated, and it was found that two different slip systems were activated even in the early stage of dislocation emission. With the increase in the number of crack tip dislocations, more complicated dislocation configurations such as dislocation tangles were formed around the crack tip, showing the beginning of multiplication of crack tip dislocations which causes effective crack tip shielding.