Elucidation of deformation mechanisms of Ti-6Al-4V lamination moulding materials by trans-scale mechanical evaluation technique

The ability to simultaneously control the shape and material property parameters of a Ti-6Al-4V alloy produced by lamination moulding can answer a variety of needs in the aerospace and medical equipment industries and contribute greatly to achieving a sustainable society. The understanding of deformation mechanisms in Ti-6Al-4V lamination moulding materials has thus far been inhibited by the anisotropic properties of the HCP structure as well as by the hierarchical microstructure, which develops during moulding, appearing as a result of overlaying. This research carries out a trans-scale elucidation of the mechanical properties and deformation mechanisms of Ti-6Al-4V lamination moulding materials by integrating micro-tensile tests and crystal plasticity finite element analysis. The results form the basis for investigating guiding principles for high-strength alloy design.

Elucidation of fatigue crack propagation mechanism and fatigue strengthening design of martensitic steels by micromechanical testing

This research will elucidates the fatigue crack mechanism of martensitic structures in steel that has a complex hierarchical microstructure and propose guiding principles pertaining to the design of steel materials that have excellent fatigue crack resistance. Crack propagation inside a single block of martensitic steel will be observed directly, furthermore deformation behaviour at the crack tips and slip systems will be numerically analysed, and the fatigue crack mechanism will be elucidated by comparison with experimental results. There is great academic value in specially developing mechanical testing equipment that can perform tensile, compression, and bending tests for micron-sized ultramicro test pieces, directly observing crack propagation inside a single block of martensitic steel, and quantitatively elucidating fatigue crack mechanisms. Moreover, this research can be expected to contribute to the invention of steel materials that have excellent resistance to fatigue crack propagation.

Pursuit of guiding principles for designing hydrogen-resistant toughening by multi-scale mechanical testing

By overcoming hydrogen embrittlement in metastable austenitic stainless steel, the weight of structural members of hydrogen systems can be reduced and resources of rare metals can be saved, thus contributing greatly to achieving a sustainable society. Dynamic phase transition during deformation of this kind of stainless steel has thus far inhibited the understanding of the hydrogen embrittlement mechanism. This research utilises micromechanical testing technology to elucidate the hydrogen embrittlement mechanism of austenitic steel in which microstructural changes during deformation processes pose a problem. In particular, this research focuses on the role of twin boundaries, which determine hydrogen-enhanced fatigue crack growth, and the effect of a banded structure caused by component segregation. Furthermore, the aim is to propose guiding principles pertaining to hydrogen-resistant toughening design of materials based on multi-scale modelling in conjunction with crystal plasticity analysis.

In-situ observation during fracture and fatigue testing of micro single crystal

It is difficult, by conventional material testing methods, to analyse crack growth behaviour in an alloy that has a complex hierarchical microstructure. We therefore developed a crack growth testing technique which uses ultra micro specimens that have been reduced in size to 1/50th of a standard specimen. Using this technique we examined the process of fatigue crack growth in martensite, which is an important structure in advanced high-strength steels. Furthermore, we elucidated the relationship between crack growth and martensite that develops in metastable austenite steel, leading to the problem of hydrogen embrittlement.