ISSN: 1546-962X
Published Online: 14 September 2009
Page Count: 9

Crystallographic Study of Superelastic Deformation of Nitinol

Wu, Ming H.
Edwards LifeSciences LLC, Irvine,CA

Gong, Xiao-Yan
Medical Implant Mechanics LLC, Aliso Viejo,CA

Mao, Shengcheng
Institute of Microstructure and Property of Advanced Materials, Beijing Univ. of Technology, Beijing,

Han, Xiaodong
Institute of Microstructure and Property of Advanced Materials, Beijing Univ. of Technology, Beijing,

(Received 10 March 2009; accepted 22 July 2009)

Abstract

Limitations of finite element analysis (FEA) in providing accurate localized stress and strain information in superelastic nitinol are well recognized. Understanding the parent texture and the crystallography of stress-induced martensitic transformation holds the key to bridge the gap between continuum mechanics and the microscopic stress-strain condition imposed by the phase transformation in understanding the deformation mechanism of this complex material. A scanning electron microscope equipped with an electron beam back scatter diffraction detector is a powerful tool that can extract microscopic crystallographic information from bulk specimens. The technique has been employed to study the crystallography of stress-induced martensitic transformation during tensile and bend deformations of superelastic nitinol. The results suggest that for tensile deformation, the transformation variants of stress-induced martensite (SIM) inside the Lüders band follow maximum shear stress along the martensite shape change direction. The observation also confirms that the SIM transformation is incomplete, leaving a significant amount of retained B2 parent phase inside the Lüders band. As tensile deformation proceeds, the Lüders band propagates by nucleating new martensite plates instead of by thickening of the existing martensite variants. For bend deformation, SIM appears to transform much easier in the tension side than in the compression side, confirming previous studies on the asymmetrical tension-compression property in superelastic nitinol materials. Lastly, the local stress field at the tip of martensite plate has been computed by finite element (FEA) simulation based on the observed martensite morphology. The implication on local stress field and plasticity provides a rationalization in explaining why nitinol fatigue life appears to be insensitive to the mean strain effect.

Keywords:
NiTi, nitinol, superelasticity, stress-induced martensite (SIM), texture, deformation, stress-strain, finite element analysis (FEA), fatigue

Paper ID: JAI102400
DOI: 10.1520/JAI102400
ASTM International is a member of CrossRef.