Fracture of precipitated NiTi shape memory alloys

Journal Article

The fracture mechanisms in single crystal and polycrystalline Ti-50.8at%Ni shape memory alloys containing Ti 3 Ni 4 precipitates are studied using the scanning electron microscope (SEM). Aged materials with three different precipitate sizes (50 nm, 150 nm, and 400 nm), which have interfaces ranging from semi-coherent to incoherent, are considered. The mechanisms of material fracture identified in the single crystal NiTi are: 1, Nucleation, growth, and coalescence of voids from the Ti 3 Ni 4 precipitates, 2. Cleavage fracture on {100} and {110} crystallographic planes, 3. Nucleation, growth, and coalescence of voids from fractured Ti-C inclusions Cleavage and ductile tearing mechanisms also operate in polycrystalline NiTi, however, since the Ti-C inclusions are an artifact of single crystal growth processes, mechanism 3 was not discovered in the polycrystalline materials. Cleavage fracture and ductile tearing are found to act in conjunction, with the relative dominance of one over the other depending on the local precipitate size and concentration. As the Ti 3 Ni 4 precipitate size increases to about 400 nm, the overall fracture is dominated by failure mechanism 1, and the cleavage markings become diffuse. Finally, we assert that the high tensile ductility of drawn NiTi polycrystals is due partially to the fact that drawn bar and wire stock usually have a strong {111} fiber texture. Such a texture promotes the initiation of the transformation at low stresses and concurrently prevents primary cleavage on the {100} or {110} planes.

Full Text

Duke Authors

Cited Authors

  • Gall, K; Yang, N; Sehitoglu, H; Chumlyakov, YI

Published Date

  • May 1, 2001

Published In

Volume / Issue

  • 109 / 2

Start / End Page

  • 189 - 207

International Standard Serial Number (ISSN)

  • 0376-9429

Digital Object Identifier (DOI)

  • 10.1023/A:1011069204123

Citation Source

  • Scopus