Fig.2 TiAl automotive turbine wheel
(SUBARU "LEGACY" 2003)

Fig.3 Stress dependence of minimum creep rates of the three PST orientation variants together with a polycrystal of Ti-42.3mol%Al

Microstructural Design of TiAl based Intermetallics

TiAl based alloys have a low density and high strength at elevated temperatures. Because of these attractive properties they have been used for turbocharger rotors and exhaust valves in automobile and will be applied to blades and vanes of aircraft engine. The TiAl alloys consist of a₂ Ti₃Al and g TiAl phases and take a fine lamellar microstructure. A stable lamellar microstructure is essential to have good mechanical properties at elevated temperatures. We have been working on lamellar boundary design of the TiAl alloys. The lamellar structure contains a g/a₂ boundary and three types of g/g boundaries, and their thermal stability is different among the four types. We have proved that a high density of g/a₂ boundaries increases stability of the lamellar microstructure and improves creep deformation resistance of TiAl alloys. We have proposed several heat treatment procedures to raise the density of g/a₂ boundaries.

Fig.4 Lattice image of g/a₂ boundary

Failure Mechanism and Residual Life Evaluation of Heat Resistant Steels for USC Electric Power Plant

Increase in operation temperature of electric power plants raises their thermal efficiency, and thereby reducing CO₂ emission from the power plants. Ultra super-critical (USC) power plants have been developed to fulfill the requirement, and high Cr ferritic heat resistant steels are widely used in the plants. Long term properties of the steels are predicted from their short term creep data, and actual failure sometimes occurs earlier than the prediction. The premature failure may cause unexpected accident such as the one that occurred at Mihama nuclear power plant, and should be avoided. The premature failure is expected to occur in base metal as well as welded joint of high Cr ferritic steels. We have been studying the mechanism of the failure to predict and prevent the premature failure.

High Temperature Deformation and Strengthening Mechanism of Mg based Alloys

CO₂ emission from automobile is a serious environmental problem, and we are urged to reduce the CO₂ emission. Mg based alloys draw increasing attention from automobile industries due to their lowest density among the conventional structural materials. Application of Mg based alloys to engine parts is a hot issue in automobile industries. We have working on high temperature deformation of Mg-Y alloys. They exhibit incredibly good creep resistance as compared to conventional Mg alloys such as AZ (Mg-Al-Zn) alloys. We have discovered that an addition of very small amount of Zn further improves the good property of the Mg-Y alloys. The good creep resistance is attributed to low stacking fault energy of the Mg-Y-Zn alloys. Alloy design for heat resistant Mg alloys is a subject under investigation.

Fig.9 Mg-Al-Ca based Thixomolded® alloys which show thousandfold creep strength by Ca addition and stabilizing second phase on grain boundary.