Coherent Precipitates as a Condition for Ultra-Low Fatigue in Cu-Rich Ti53.7Ni24.7Cu21.6 Shape Memory Alloys

Abstract

Sputtered Ti–rich TiNiCu alloys are known to show excellent cyclic stability. Reversibility is mostly influenced by grain size, crystallographic compatibility and precipitates. Isolating their impact on cyclic stability is difficult. Ti2Cu precipitates for instance are believed to enhance reversibility by showing a dual epitaxy with the B2 and B19 lattice. Their influence on the functional fatigue, if they partly lose the coherency is still unknown. In this study, sputtered Ti53.7Ni24.7Cu21.6 films have been annealed at different temperatures leading to a similar compatibility (λ2 ~ 0.99), grain size and thermal cyclic stability. Films annealed at 550 °C exhibit a superior superelastic fatigue resistance but with reduced transformation temperatures and enthalpies. TEM investigations suggest the formation of Guinier–Preston (GP) zone-like plate precipitates and point towards a coherency relation of the B2 phase and finely distributed Ti2Cu precipitates (~ 60 nm). Films annealed at 700 °C result in the growth of Ti2Cu precipitates (~ 280 nm) with an irregular distribution and a partial loss of their coherency. Thus, GP zones are assumed to cause the reduction of transformation temperatures and enthalpies due to increased internal stresses, whereas the coherency relation of both, Ti2Cu and GP zones, help to increase the superelastic stability, well beyond 107 cycles.

Structural-Functional Changes in a Ti50Ni45Cu5 Alloy Caused by Training Procedures Based on Free-Recovery and Work-Generating Shape Memory Effect

“…The coexistence of B19 and B19 martensites at RT was reported in several studies, although it has been demonstrated that B19 orthorhombic martensite can form in Ti 50 Ni 45 Cu 5 SMAs, but it is instantly replaced by B19 monoclinic martensite [5]. According to 00-035-1281 and 00-044-0113 databases, the main diffraction maxima are (111) B19 , located at 2θ = 41,365 • , and (020) B19 , located at 2θ = 42,675 • , respectively, which makes it rather difficult to distinguish between the peaks of the two martensitic phases [42]. Nevertheless, with the increase in Cu content, stable B19 orthorhombic martensite was obtained together with B19 monoclinic martensite.…”