Abstract
The NbTiAl system offers many intermetallics that can be potentially used for high temperature structural applications. The objective of this study has been to improve the room temperature toughness of the σ phase (Nb 2Al + Ti), which has a high melting point and good high temperature strength, by inclusion of semiductile phases. The incorporation of second-phase particles has been found to enhance the toughness of the σ phase considerably. Fractographic analysis has revealed that when the interface between the particles and the matrix is strong, the second-phase particles impede the crack front by trapping of the crack tip due to plasticity. The crack propagation continues by a reinitiation process ahead of the crack tip. Consequently, the crack is bridged by the second-phase particles. The cleavage of the second-phase particles results in the final separation of the crack faces. In the case of weakly bonded particles the level of internal stresses has been found to influence the crack path and therefore the toughness. In this paper the effects of the volume fraction, size and distribution of the particles as well as their nature on the fracture path and toughness are discussed.
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