研究紹介
First-principles prediction experimental verification of materials properties
Research Group of Materials Design
First-principles calculations and Designing of Materials
With the developments of condensed matter theory, efficient algorithm and powerful computational resources, we are now at the stage of designing materials based solely on the knowledge of quantum mechanics and statistical mechanics. In the first-principles calculation, all the relevant thermodynamic and physical quantities are derived only from the atomic numbers of constituent elements. Hence, one can even create a hypothetical material which can not be found in the nature, examine its properties and optimize the manufacturing process inside a computer.
Shown above are the L10-disorder phase diagrams for Fe-Ni(left) and Fe-Pt (right) systems in the vicinity of 50at%. L10 ordered phase has been attracting broad attentions as a high density magnetic recording medium. For Fe-Pt system, the first principles calculation predicts 1610K as the transition temperature which is in excellent agreement with the experimental value of 1600K. For Fe-Ni system, the L10 ordered phase is missing in a conventional phase diagram, while the first principles calculation suggests the existence of this phase below 500K, and this is recently confirmed by experiment.
It is realized that microstructural controlling is essential to design and develop both a structural material with an excellent combination of strength and toughness and a functional material with desired properties. Recently, the first principles calculation is extended to calculate temporal evolution process of microstructures, which subsidize the experimental information of transmission electron micrograph.
Shown above are the time evolution process of Anti Phase Boundary of L10 ordered phase in Fe-Pd system which is subject to ageing operation after the rapid quenching from a high temperature. It is emphasized that these results are obtained based only on two numbers, 26 and 46, as atomic numbers of Fe and Pd.
Combinaition of Experiment-base study and Theoretical approach for Materials Design
Materials respond to various kinds of input, such as heat and pressure. Materials having proper properties such as high stiffness, quick phase change and low thermal expansion will be picked up for production of goods.
In order to understand the properties of materials properly, microscopic view points are very useful and important. Recent Ni-base super alloys are strengthened by introducing L12-type Ni3Al phase as a fine dispersoids. Upper figure shows the effect of the deviation of composition from the most stable 75Ni-25Al on its strength at low temperature. With increasing Al concentration (when the composition shifts to right side), the strength is much higher than that of 75Ni-25Al composition. However in the left side, the strengthening rate is not so much. The reason of this asymmetric feature has not been well understood.
As shown in the figures at the middle, the excess Ni is expected to occupy a different position from that of the excess Al. Excess Ni is surrounded by Ni, but Al is surrounded by both Ni and Al. However, this has not been proved by experimental methods. This difference results in the difference of the distortion of lattice, and this distortion is expected to produce an extra strengthening of Al-rich alloy. The figure of the bottom shows the result of a first-principles (FLAPW) calculation which suggests the distortion produces no volume change. This result is in good agreement with an experimental study by X-ray diffractometry, and such a distortion is expected to explain the extra strengthening. As can be seen from this example, the combination of experimental and theoretical studies provides us with a deeper understanding of material responses.
Figures shown below present the microstructure evolution of Nb-base alloys expected for ultra-high temperature use above 1500oC. According to the Nb-Si binary phase diagram, a high temperature Nb3Si decomposes into Nb and Nb5Si3 at 1770 oC in the manner of eutectoid reaction. The decomposition kinetics is strongly affected by the decomposition temperature and third elements; at a high temperature above 1500 oC no obvious crystallographic orientation relationship between Nb and Nb5Si3 is observed, but at relatively low temperature it exists. Moreover, small amount of Zr addition accelerates the decomposition (decomposition time becomes about two order faster by adding 1.5%Zr). It can be expected that details of these phenomena can be understood by combining microstructure observations using latest facilities such as EBSD (Electron Back-Scattering Diffraction pattern) and EPMA (Electron Prove Micro-Analysis), and theoretical approaches.
Such observation methods have potentials to reveal local, not averaged, phenomena. In order to understand some macroscopic properties, important are observations of phenomena affected by the microstructure in the size of the order of grains under various conditions. An advanced optical microscope such as a confocal laser scanning microscope is also a powerful tool to observe various phenomena which occur at high temperature and/or hazardarous atmosphere. Combining these facilities mentioned above, local phenomena such as the phase transition temperature of each grain, path of crack propagation and the sequence of twin formation during deformation at a certain grain boundary will be revealed, and such information will help to establish theoretical models.