Calorimetric energetics and localized strain recovery in electron beam powder-bed fusion built NiTi shape memory alloy

Nickel-Titanium (NiTi or NiTiNOL) is widely recognized for its shape memory effect recovery, which is governed by the thermoelastic martensitic phase transformation. Its limited machinability has restricted its widespread adoption in applications requiring complex geometries. Additive manufacturing is attractive, but ubiquitous laser-based methods result in porous and micro-cracked microstructures that require extensive post-built thermal treatments to barely show functionality. Electron beam powder bed fusion (EB-PBF) presents a viable alternative, enabling near-net-shape fabrication while preserving the material's functional integrity due to high build temperatures and vacuum. This work quantifies the thermoelastic martensitic transformation energetics and links them to local deformation and recovery behavior in as-printed and annealed EB-PBF NiTi. Martensitic transformation behavior is analyzed using differential scanning calorimetry (DSC), which characterizes the transformation temperatures and enthalpies, which are then used to determine the elastic strain energy and irreversible/frictional energy for multiple cycles. We then use digital image correlation (DIC) to map localized reversible deformations that are responsible for the shape memory recovery. The results indicate that high-temperature annealing worsens thermoelastic potential, with as-printed samples exhibiting higher elastic strain energy. We also report large reversible localized deformation islands of detwinned martensite in a slurry of elastically deformed twinned martensite that are ultimately responsible for the global deformation that is then recovered upon heating, returning to its original shape for the as-printed material. The annealed sample on the other hand evolves a localized region of large plastic deformation that results in the fracture of the sample prior to useful detwinning of martensite taking place. These findings reinforce the role of DSC in quantifying phase transformation energetics and underscore EB-PBF's potential in creating NiTi components, without the need for post-annealing, for shape memory-based active applications.