Additive manufacturing of NiTi shape memory alloys has attracted attention in recent years, due to design flexibility and feasibility to achieve four-dimensional (4D) function response. To obtain customized 4D functional responses in NiTi structures, tailorable phase transformation temperatures and stress windows as well as one-way or two-way shape memory properties are required. To achieve this goal, various heat treatments, including direct aging, annealing and annealing followed by aging, were optimized for the Ti-rich NiTi (Ni49.6Ti (at. %)) fabricated by laser powder bed fusion (L-PBF). Microstructural evolution, phase transformation, precipitation and shape memory behaviour were systematically investigated by multiscale correlative microstructural, differential scanning calorimetry analysis and thermomechanical analysis. Based on optimized heat treatments, ∼25 K phase transformation temperature windows and ∼90 MPa stress windows were achieved for the one-way shape memory effect. Solutionized annealing was found to be the most effective way to improve one-way shape memory degradation resistance, due to the reduction of defects and solid solution strengthening. One of the main findings of this study is that the heterogonous microstructures between hard intergranular Ti2NiOx and soft NiTi matrix, induced by solutionized annealing with subsequent aging, result in strain partitioning and enclosing the internal stress state, which was found to promote a pronounced two-way shape memory effect response. The results of this work provide in-depth knowledge on tailoring and designing functional shape memory characteristics via heat treatments, which contributes to expanding L-PBF NiTi application fields, such as biomedical implants, aerospace components, and other advanced engineering applications.@en

Superelastic metallic materials possessing large recoverable strains are widely used in automotive, aerospace and energy conversion industries. Superelastic materials working at high temperatures and with a wide temperature range are increasingly required for demanding applications. Until recently, high-temperature superelasticity has only been achievable with multicomponent alloys fabricated by complex processes. In this study, a novel framework of multi-scale models enabling texture and microstructure design is proposed for high-performance NiTi fabrication via laser powder bed fusion. Based on the developed framework, a Ni-lean Ni(49.4 at.%)-Ti alloy is, for the first time, endowed with a 4% high-temperature compressive superelasticity. A 001 texture, unfavorable for plastic slip, is created to realize enhanced functionality. The unprecedented superelasticity can be maintained up to 453 K, which is comparable with but has a wider superelastic temperature range (∼110 K) than rare earth alloyed NiTi alloys, previously only realizable with grain refinement, and other complicated post-processing operations. At the same time, its shape memory stability is also improved due to existing textured 100 martensite and intergranular precipitation of Ti2NiOx. This discovery reframes the way that we design superior performance NiTi based alloys through directly tailoring crystallographic orientations during additive manufacturing.@en

The pursuit of enhancing NiTi superelasticity through laser powder bed fusion (L-PBF) and [001] texture creation poses a challenge due to increased susceptibility to hot cracking in the resulting microstructure with columnar grains. This limitation restricts NiTi's application and contributes to material waste. To overcome this, we introduce a pioneering approach: utilising spark plasma sintering (SPS) to heal directional cracks in [001] textured L-PBF NiTi shape memory alloy. Diffusion bonding and oxygen utilisation for Ti2NiOx formation was found to successfully heal the cracks. SPS enhances mechanical properties, superelasticity at higher temperatures, and two-way shape memory strain during thermomechanical cycling. This work provides an alternative solution for healing cracks in L-PBF parts, enabling the sustainable reuse of cracked materials. By implementing SPS, this approach effectively addresses hot cracking limitations, expanding the application potential of L-PBF NiTi parts while improving their functional and mechanical properties.@en

Boron doped MoSi2 particles have been envisioned as sacrificial particles for self-healing thermal barrier coatings (TBCs) but their oxidation behaviour is yet not well understood. In this work, oxidation of MoSi2 based particle is studied in the temperature range of 1050–1200 °C. The oxidation proceeds from a transient to a steady-state oxidation stage. The kinetics during steady-state oxidation is captured with a thermal diffusion-based model. As compared to the oxidation of pure MoSi2 particles, the addition of boron strongly enhances the silica formation. Also, a finer dispersion of MoBx in the MoSi2 matrix accelerates the formation of silica. The oxide growth rate constant increases proportional with the boron content of the MoSi2 particles. This enhanced oxidation is related to the microstructure of the oxide scale. Upon oxidation, boron yields B2O3, which promptly merge with SiO2 to form amorphous borosilicate, hindering the formation of crystalline SiO2. Consequently, the migration of oxygen in the borosilicate oxide scale is faster than in the silica oxide scale on pure MoSi2 particles.@en

Boron doped MoSi2 particles have been envisioned as sacrificial particles for self-healing thermal barrier coatings (TBCs) but their oxidation behaviour is yet not well understood. In this work, oxidation of MoSi2 based particle is studied in the temperature range of 1050–1200 °C. The oxidation proceeds from a transient to a steady-state oxidation stage. The kinetics during steady-state oxidation is captured with a thermal diffusion-based model. As compared to the oxidation of pure MoSi2 particles, the addition of boron strongly enhances the silica formation. Also, a finer dispersion of MoBx in the MoSi2 matrix accelerates the formation of silica. The oxide growth rate constant increases proportional with the boron content of the MoSi2 particles. This enhanced oxidation is related to the microstructure of the oxide scale. Upon oxidation, boron yields B2O3, which promptly merge with SiO2 to form amorphous borosilicate, hindering the formation of crystalline SiO2. Consequently, the migration of oxygen in the borosilicate oxide scale is faster than in the silica oxide scale on pure MoSi2 particles.@en