Titanium Aluminides are being used in aeroengine applications. Different structures of titanium aluminides can be achieved by heat treating these alloys in binary phase region (Ti3Al+TiAl). Two basic microstructures produced from heat treatments are duplex and lamellar structure. The lamellar structure provides strength at high temperature so is the case with duplex structure but it provides good properties at room temperature as well i.e. ductility of duplex structure is superior than the lamellar structure. My question is that what's the mechanism behind it? And what would be the interaction and resulting effect of interfaces of 'lamellar and equiaxed grains' within the duplex structure?
Dr. Hashimi:
I looked at duplex vs. lamellar gamma-based TiAl alloys some time ago (Campbell et al., Metall. Mater. Trans A, 30A, 1999, p. 563) and my impression was that the duplex alloys showed generally better ductility properties due to their much finer-scale microstructure. However, this did not work in their favor with respect to damage-tolerant properties, specifically their fracture toughness and fatigue crack propagation resistance, which were considerably better in the coarser lamellar structures due to extensive extrinsic toughening. In view of the use of these alloys in aerospace, I think that I would have prefer to have the superior fracture resistance of the lamellar alloys to the somewhat higher ductility of the duplex ones.
Dr. Hashimi:
I looked at duplex vs. lamellar gamma-based TiAl alloys some time ago (Campbell et al., Metall. Mater. Trans A, 30A, 1999, p. 563) and my impression was that the duplex alloys showed generally better ductility properties due to their much finer-scale microstructure. However, this did not work in their favor with respect to damage-tolerant properties, specifically their fracture toughness and fatigue crack propagation resistance, which were considerably better in the coarser lamellar structures due to extensive extrinsic toughening. In view of the use of these alloys in aerospace, I think that I would have prefer to have the superior fracture resistance of the lamellar alloys to the somewhat higher ductility of the duplex ones.
Hello Luqman,
according to my understanding of micro structural processes during deformation (my PhD thesis) this is due to the following aspects: grain structure should accommodate the outer deformation somehow, right? So the duplex structure is of course more "flexible" in this case because of presence of the small components. Some small restructuring and micro rotations of a duplex structure I suppose are always possible providing that the matrix phase in the duplex structure is ductile and the second is brittle. Hence higher ductility. In case of big lamellae it is less possible to make some small rotations of microcristallites (subgrains). They should be at first destroyed. The most important thing is that a new grain boundary should be created (the role of interfaces) to accommodate the strain. This mechanism needs higher energy investment. A release of the excessive accumulated energy during formation of grain boundary occurs locally in the place of a new grain boundary origination and of course often leads to the opposite effect to ductility. Due to energy accumulation phase it provides higher strength). In fact the low angle boundary originates a micro crack and the process develops in that way to low ductility and earlier destruction of the material.
I hope you appreciate my effort of explanation. English is not my native.
Regards,
Anton
Duplex - two structure together (alpha + beta)
1. fine grain strengthening mechanism
2. Composite structure - beta phase as matrix and ductile alpha phase reinforcement
reason for strength with ductility
http://slideplayer.com/slide/3815776/
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