Associate Professor, Faculty of Engineering, Yokohama National University, Japan

Biography: Dr. Makoto Hasegawa obtained his Doctor of Engineering from Yokohama National University, Japan in 2002. He worked as a postdoctoral research fellow and research associate in The University of Tokyo from 2002 to 2006 and a research associate in Yokohama National University from 2006 to 2010. From 2011, he became an associate professor in Yokohama National University. His is currently focused on the researches of the high temperature processing on intermetallic compounds and alloys for preferential orientation control and an orientation control of metals and ceramics coatings by aerosol deposition method which will form the coatings under room temperature processing.

Speech Title: Toughening of TiAl intermetallic compound by β phase precipitation

Aims: TiAl intermetallic compounds having fully lamellar structure composed by ( α₂+γ ) two phases has drawn considerable attention because of their superior high temperature strength and creep properties. However, low fracture toughness at room temperature restricts the TiAl intermetallic use for practical applications. In this study, the effects of heat treatments at ( γ+β ) two phase state on the microstructure of fully lamellar Ti-46Al-7Nb-0.7Cr-0.2Ni-0.1Si (mol%) intermetallic compounds are researched.

Methods: Ti-46Al-7Nb-0.7Cr-0.2Ni-0.1Si (mol%) alloy are used for the tests. All the specimens are held at α single phase region (1643 K) for 3.6 ks to homogenize the microstructure. Then, the specimens were heat treated at ( β+γ ) two phase region (1273 K and 1373 K) from 1.8ks to 72.0ks. Microstructure was observed by a SEM. Fracture toughness has evaluated by three point bending test where the chevron notch is introduced at the center of the specimen.

Results: The homogenized specimen shows fully lamellar structure where the average colony size is about 1.1 mm. The colony size is almost constant during the heat treatment at ( γ+β) two phase state. Precipitation of the β phase particles and partial collapse of the fully lamellar structure could be seen with the increase in heat treatment time. The fracture toughness initially increased up to 3.6 ks and then decreased with the increase in heat treatment time at 1273 K. Heat treatment at 1373 K decreased the toughness of the specimens. This change in toughness may be due to the difference in size and distribution of the β phase precipitates.

Conclusions: Heat treatment under ( γ+β ) two phase region can improve the fracture toughness of the TiAl intermetallic compounds by the precipitation of the β phase. In this case, size and distribution of the β phase precipitates seem to be important.