Local strain fluctuations enable sluggish martensitic transformation in additively manufactured NiTi alloys with 〈001〉 growth texture under tensile loading

Abstract

Implementing additive manufacturing to NiTi (Nitinol) alloys typically enables a preferred 〈001〉_B2 texture along the building direction. Unfortunately, this growth orientation always possesses a high critical stress level to induce the martensitic transformation and experiences premature failure before the formation of martensite during tensile testing. By utilizing in situ characterization technologies, in this study, we demonstrate that by fabricating a NiTi sample with complete 〈001〉_B2 texture using wire-fed electron beam directed energy deposition, a sluggish martensitic transformation can be achieved to retard the initiation of fracture under tensile loading. To discern the origins of this tensile response, we combine experiments with molecular dynamics simulations to systematically analyze the micro-scale details on how internal lattice defects can select the variety of martensite variants. Using both quasi in situ transmission electron microscopy analysis and calculations of the different atomic configurations, our results indicate that the pre-existing precipitates and accumulated dislocation defects, rather than columnar boundaries, can have a positive influence on the sluggish formation of variants that can couple with plastic deformation within a much wider stress interval. Specifically, only the variant favored by both internal strain/stress fluctuations around local defects and external tensile load will overcome the high-energy transition barrier of 〈001〉_B2-oriented tension to nucleate and grow sluggishly. The current findings not only show how the mechanical responses can be controlled in additively manufactured NiTi alloys with 〈001〉_B2 texture, but also regard this understanding to be a step forward in decoding the salient underlying mechanisms for the correlating texture, defects, and phase transformation of these functional materials.