On the Lüders band formation and propagation in NiTi shape memory alloys

Abstract

• Both the B19′ martensite and B2 austenite can form Lüders band. • Lüders band initiation strain rate is two orders of magnitude of the applied strain rate. • Lüders band forms with a finite minimum width. • Lüders band propagates with a transition zone half of the band minimum width. • Transition zone local strain rate is more than one order of magnitude of the applied strain rate. Near-equiatomic NiTi shape memory alloys are known to exhibit Lüders-type deformation associated with a stress-induced transformation. Many studies have been conducted in the past mainly focusing on the macroscopic characteristics of the phenomenon and some theories have been proposed in the literature to explain its mechanisms, but some aspects of this phenomenon are still unclear, particularly at the microscopic scale. This study investigated the local strain evolution during the initiation and propagation of Lüders band in a pseudoelastic NiTi alloy during tensile deformation by means of digital image correlation (DIC) analysis. Based on the evidence collected, distinct stages of Lüders band formation and propagation are defined and the corresponding local strain rates are obtained. These local strain rates are much higher than the global strain rate of the testing, giving insight to the mechanism of this phenomenon.