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The Role of Microstructure in Fracture Resistence Control of TiAl Based Intermetallics
Dlouhý, Ivo ; Haga, H. ; Chlup, Zdeněk ; Hadraba, Hynek ; Hasegawa, M. ; Fukutomi, H.
The analysis of the role of microstructure in fracture performance of TiAl intermetalics at room and elevated temperatures has been carried out. For Ti-46Al-0.7Cr-0.1Si-7Nb-0.2Ni alloy different regimes of thermal and thermal-mechanical treatment were applied enabling to control substantially microstructural parameters. Attention was paid to grain (colony) size effect, effect of lamellas thickness and morphology and, in addition, to β-phase effect. For separate treatment regimes the flexural strength and fracture toughness were evaluated in addition to microstructural parameters. Fractographic analysis enabled to evaluate fracture micromechanisms; development of shear ligament toughening was observed in almost all cases. This is the key mechanism controlling the fracture behaviour. Based on findings obtained key aspects of the effect of separate microstructural components have been discussed.
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Effect of Microstructure Change on the Crack Formation of Heat Treated Ti-Al Intermetallic Compound
Hasegawa, H. ; Nomura, M. ; Haga, H. ; Dlouhý, Ivo ; Fukutomi, H.
Heat treatments were conducted under several conditions in order to clarify the effect of holding temperature, time and cooling rate on microstructure of Ti-46Al-7Nb-0.7Cr-0.2Ni-0.1Si and Ti-45Al-10V (mol%) alloys. Indentation tests were performed in order to understand the effect of beta phase precipitation on crack formation of microstructure controlled Ti-45Al-10V alloy. As for Ti-46Al-7Nb-0.7Cr-0.2Ni-0.1Si alloy, phase condition on several temperatures was estimated experimentally. It is found that the cooling rate where the massive transformation occurs ranges from -300K/s to -5K/s. Furthermore, in order to obtain the fully lamellar structure, the alloy should be cooled slower than -1K/s. As regards the heat treatment on Ti-45Al-10V alloy, fully lamellar structure was formed by cooling from (alfa+gama) two phase state. However, beta precipitates were observed near the lamellar colony boundaries when the material was cooled from (beta+gama) two phase state. Though the cracks are not observed at the specimen cooled at -8x10-2K/s from (alfa+gama) two phase state, the cracks induced by Vickers indentation are observed in most of the heat treated specimens.
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