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ALLOY DESIGN LABORATORY
for Extreme Environments
Hydrogen Embrittlement
Hydrogen embrittlement in high-entropy alloys and steels
We investigate the hydrogen behaviors in high-entropy alloys or various automotive steels to reveal the embrittlement mechanism.
We introduce the hydrogen into the materials by high-pressure gas charging or electro-chemical charging. The embrittlement is evaluated by tensile tests or immersion test after deep drawing / bending. Then, we investigate the effect of stress/strain levels, residual stress/strain field, hydrogen solubility, hydrogen diffusivity, and microstructures on the hydrogen embrittlement.
Cryogenic Applications
1. Transformation-assisted high-entropy alloys
2. Transformation/Twinning-assisted high-Mn steels
Development of new energy sources such as liquefied natural gas and hydrogen gas demands advanced structural materials to transport, preserve, and utilize them safely. Since these gases are generally carried at cryogenic temperatures such as 111 or 20 K, designing of materials available under extreme environments is essentially needed.
We design the FCC-based high-entropy alloys and high-Mn steels for the cryogenic applications. We aim to develop those alloys triggering the transformation-induced & twinning-induced plasticity (TRIP & TWIP), leading to the high strain-hardening capability.
- The alloy design is based on the thermodynamic and ab initio calculation.
- Tensile, impact toughness, and fracture toughness are evaluated.
- Then, we investigate the mechanism by in-depth characterizations.
High-temperature Applications
1. Alloy and Microstructure Design for Turbo-charger Applications
To endure high-temperature atmosphere as well as extreme damage in the turbo-charger (thermo-mechanical fatigue) the materials are required to present high corrosion resistance and moderate strength levels.
We thus design the austenitic stainless cast steel containing various carbides based on the thermodynamic calculation.
We quantify the composition, size, fraction, and distribution of constituent phases, then evaluate high-temperature properties.
2. In-situ Observation (heating-straining)
It is very challenging to define the deformation/fracture mechanism at high-temperature because we couldn't "see" the behaviors directly during the tests.
However, we've adopted recently the very fascinating system, confocal laser scanning microscopy, enabling them to be defined by the in-situ observation.
We are setting the system up now!
Dynamic Deformation
To use designed materials for automotive steel sheet applications effectively, e.f., collision-absorption components, the detailed information of dynamic deformation should be obtained. This is because automotive steel sheets require high resistance to impact energy upon vehicle collision as well as high strength and fracture toughness for sustaining sufficient structural stability. However, phenomena under dynamic loading conditions are hardly investigated.
Accurate evaluation should also be performed on how safe the steel sheets are under worst conditions like vehicle collision. In vehicle-collision tests, actual vehicles are generally used, but restrictions remain in applying the evaluation data to the vehicle body safety itself and in styling new car designs. Accordingly, the vehicle body safety data are essentially needed before the evaluation of actual vehicle products.
We conduct dynamic and quasi-static tensile tests under strain rates of about 3000 /s and 0.001/sec using a split Hopkinson tensile bar and a universal testing machine, respectively. Detailed deformation mechanisms are examined by analyzing how the quasi-static deformation mechanisms vary under the dynamic loading. Finally, we correlate them with the microstructural evolution processes.
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