Influences of extrinsic factors on the Lüders-type deformation behaviour of NiTi

NiTi shape memory alloys exhibit Lüders-type deformation in tension. We propose a new explanation of the mechanism of this behaviour based on a mechanical criterion of sample length change conservation among three contributions, including machine displacement, Lüders band elongation and elastic strain contraction, and established a mathematical model to describe the behaviour. This model predicts the influences of several parameters some intrinsic to the testing alloy and some extrinsic by the testing conditions. In this work, we investigated the effects of machine speed and sample gauge length (L₀) on the elastic strain contraction (Δε_el) and the stress drop (Δσ) of the Lüders-type deformation, and the characteristic length of the Lüders band at its nucleation (l_L^*). It was found that Δσ is independent of L₀ and at the same time l_L^* increased with increasing L₀. This coordinated variation of l_L^* with L₀ demonstrates a “self-regulation” mechanism of the Lüders-type deformation behaviour to assure a constant Δσ, which further indicates that the upper (peak) and the lower (plateau) stresses of the Lüders-type deformation are intrinsic to the metallurgical conditions of the material used. This reveals a new insight to the behaviour of the Lüders-type deformation of NiTi. This study also clarified a common misperception (or confusion) in mechanical testing for Lüders-type deformation, between local strain rate and global strain rate conditions.