Fracture of precipitated NiTi shape memory alloys

Ken Gall, Nancy Yang, Huseyin Sehitoglu, Yuriy I. Chumlyakov

Research output: Contribution to journalArticlepeer-review

Abstract

The fracture mechanisms in single crystal and polycrystalline Ti-50.8at%Ni shape memory alloys containing Ti3Ni4 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 Ti3Ni4 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 Ti3Ni4 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.

Original languageEnglish (US)
Pages (from-to)189-207
Number of pages19
JournalInternational Journal of Fracture
Volume109
Issue number2
DOIs
StatePublished - May 1 2001

Keywords

  • Cleavage fracture
  • Growth and coalescence
  • Scanning electron microscopy
  • Single crystal and polycrystalline NiTi
  • Solutionized and aged
  • Void nucleation

ASJC Scopus subject areas

  • Computational Mechanics
  • Modeling and Simulation
  • Mechanics of Materials

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