Martensite Shear Band Formation Under Pseudoelastic Compression-Shear Testing

It is well known that pseudoelastic NiTi shape memory alloys exhibit a mode of localized deformation (formation of martensite bands) under uniaxial tension. This is associated with the stress-induced martensitic transformation from austenite to martensite during loading, and its reversal during unloading. While previous studies have characterized microstructural aspects, like texture effects, or thermo-mechanical aspects, such as a detailed analysis of strains near the propagating bands, it is not yet entirely clear why the formation of martensite bands is mainly limited to tensile loading. In this study, pseudoelastic deformation is investigated under different load cases including uniaxial tension, compression and particularly a combined compression–shear loading. Ti-50.9at. %Ni alloy bar specimens were aged at 623 K for 3.6 ks. The formation of Ti₃Ni₄ precipitates results in an increased yield strength and therefore excellent pseudoelastic recovery. Under uniaxial, tensile, and quasi-static conditions, the material exhibits a flat stress-plateau, associated with nucleation of propagation of well-defined martensite bands. Using the in situ optical technique known as digital image correlation (DIC), the formation and growth of these bands are characterized in detail. Moreover, the strain fields are documented under quasi-static uniaxial compression (where, from a simple macroscopic point of view, the deformation appears to be homogeneous) and under compression-shear loading, where the superimposed shear stresses trigger localized deformation even under compressive loading. We also study the effect of different strain rates up to 1 s⁻¹ on the formation and propagation of martensite bands under the different types of loading conditions. Based on our experimental results we argue that the material may well be prone to localized deformation even under predominantly compressive loading, but martensite bands simply cannot form because of the subtle differences of specimen designs for the different load cases.