Evaluation of Elastic-Plastic Deformation Behavior of Lath Martensite by Nanoindentation Technique

Abstract

It is well known that ferritic steels yield discontinuously with a clear yield point, while martensitic steels yield continuously with a low elastic limit followed by significantly large strain hardening. To evaluate the unique elastic-plastic deformation behavior of martensitic steels easily and accurately, nanoindentation tests were conducted using martensitic steels with lath martensitic structure, and the obtained results were then compared with that of ferritic steel while taking into account the pop-in phenomenon. In the normal load-displacement curves, ferritic steel had pop-in clearly, but no pop-in was observed for martensitic steels, regardless of carbon content. However, the analysis based on Hertz’s contact theory made it possible to quantitatively evaluate the elastic limit of martensitic steel as well as ferritic steel. As a result, it was found that the elastic limit of martensitic steel is much lower than that of ferritic steel, and the plastic strain at yielding is also quite small. The plastic deformation behavior based on dislocation theory suggests that the yielding of ferritic steels is governed by dislocation nucleation and subsequent dislocation avalanche. In contrast, the yielding phenomenon of martensitic steels might be greatly influenced by the motion of pre-existing mobile dislocations introduced through martensitic transformation.