Functional fatigue in functionally graded shape memory alloys.

F. M. Braz Fernandesa, E. Camachoa

aCENIMAT / I3N - Departamento Ciência dos Materiais, Universidade Nova de Lisboa / FCT, 2829-516 Caparica, Portugal

These functional characteristics of shape memory alloys are a consequence of phase transformations that take place within well-defined temperature ranges or stress ranges, depending on being thermal or stress-induced. These temperature/stress ranges are a function of chemical composition and heat treatment of the material. For applications requiring a wider controllable range, a wider temperature/stress range than that associated to a specific composition/heat treatment may be required. In such a situation, the possible solution will be to use a functionally graded material. Functional gradient may be introduced by: (i) a geometrical gradient (variable cross-section along a specific direction [1]), (ii) a chemical composition gradient (either along the longitudinal direction [2], or along the thickness [3]), (iii) a graded heat treatment [4], or (iv) may indirectly arise from a processing technique, such as laser welding [5,6].

Fatigue of shape memory alloys can be generally understood in two broad categories: (a) functional fatigue and (b) structural fatigue [5]. Functional fatigue refers to the drifting of superelastic responses over cycles, which results from persistent accumulation of slip and martensite. Structural fatigue, on the other hand, means nucleation of a crack (due to excessive localization of plasticity and residual martensite) and its gradual progression into a catastrophic fracture. In order for an SMA to retain its strain recoverability i.e. the phase reversibility, it is not sufficient for the parent lattice to be able to only transform. It is crucial to ensure the absence of factors that might hamper the two-way conversion e.g. considerable slip activities. An enhanced slipping propensity may adversely affect the reversibility of transformation.

The aim of the present study was to correlate the heat treatment conditions with the functional characteristics presented during cyclic tensile loading: (i) critical stress for direct (austenite-martensite) and reverse (martensite-austenite) transformation and (ii) irrecoverable strain.