The role of texture and restoration mechanisms in defining the tension-compression asymmetry behavior of aged NiTi alloys fabricated by laser powder bed fusion

Tension-compression asymmetry is an inherent property of superelastic NiTi alloys linked to the microstructural evolutions and the stress induced martensite transformation during different deformation modes. Deep understanding of NiTi's asymmetry, a topic that is often overlooked in additive manufacturing, is imperative in designing functional parts that experience multiple deformation modes. Accordingly, in this paper, the asymmetry parameters, namely the critical stress for martensite transformation, transformation strain, and strain recoverability were measured for an aged material with optimum superelasticity, which was fabricated by two different scanning strategies. The acquired results were correlated to the changes in the developed texture components during laser powder bed fusion and subsequent annealing processes, highlighting the role of dynamic and static restoration mechanisms, respectively. The deviation of texture from <001>||building direction, caused by the occurrence of the restoration mechanisms during fabrication and subsequent aging, was found to be dissimilar for samples fabricated by 67° or 90° rotation angles, effectively impacting the behavior of the material in different deformation modes. For a sample with enhanced superelasticity, aged at 550°C for 6 h, 90° scanning strategy resulted in lower asymmetry and superior superelasticity in compression, while 67° rotations led to highest strain recoverability in tension, but an intensified asymmetrical behavior.