Mechanical and corrosion behavior of additively manufactured NiTi shape memory alloys

A team from Lanzhou University of Technology have developed a novel NiTi shape memory allow (SMA) with harmonic microstructures fabricated via selective laser melting (SLM). This work explores the relationship between microstructural evolution at various deformation stages and corrosion behaviour in seawater environments. The study reveals that in its initial states, the alloy exhibits superior corrosion resistance, primarily owing to dense and stable passivation films composed mainly of TiO₂ and NiO. Post-fracture, the formation of fragmented amorphous phases and nanocrystalline grains accelerates corrosion processes. Leveraging first-principles calculations and electrochemical analysis, the team provides insights into microgalvanic reactions and phase interactions that influence corrosion resistance, paving the way for advanced smart materials in marine applications.

Nickel-titanium shape memory alloys (NiTi SMAs) are widely renowned for their superelasticity, shape memory effects, high strength and biocompatibility, making them promising candidates for seismic, vibration damping and biomedical applications. Particularly in marine engineering, their ability to withstand impact and seismic events is highly valued. However, their high toughness and work hardening characteristics make it difficult to fabricate complex and precision components via traditional processes. Moreover, existing studies mainly focus on non-loaded conditions, examining corrosion behaviour in inert environments or static states, with limited understanding of performance under mechanical strains and real-world seawater conditions.

As an advanced additive manufacturing technology, selective laser melting (SLM) enables near-net shaping of complex structures, and its rapid solidification feature is conducive to optimizing microstructures. Nevertheless, the influence mechanism of strain-induced phase evolution (such as transformations between B2, B19’ and amorphous phases) on corrosion resistance during SLM fabrication remains unclear. Therefore, exploring the correlation between microstructure and corrosion resistance of NiTi SMAs under strain regulation is of great significance for promoting their application in marine engineering.

The Solution: To address the research gap, a research team from Lanzhou University of Technology introduced a harmonic-structured NiTi SMA fabricated via SLM, designed to optimize both mechanical properties and corrosion resistance. in strain-related corrosion and traditional manufacturing challenges of NiTi SMAs in marine engineering applications, this study proposes a systematic solution. Selective Laser Melting (SLM) technology is adopted to fabricate alloys with harmonic structures. By controlling the microstructure through tailored deformation stages, initial SLM state, initial plastic deformation (IPT), and after fracture (FS), the researchers demonstrate how microstructural features influence corrosion behavior. The alloy’s microstructure transitions from predominantly B2 phase with minor B19’ and amorphous lamellae to nanocrystalline B2 grains with fragmented amorphous phases after fracture.

Comprehensive multiscale characterization including: TEM, EBSD, nanoindentation, XPS and electrochemical analyses (Tafel curves, EIS, Mott-Schottky) revealed that the initial SLM and IPT states possess dense, uniform passive films with high stability, mainly consisting of TiO₂ and NiO, providing improved seawater corrosion resistance. Conversely, fracture-induced structures feature uneven, porous passive films rich in Ti₂O₃ and Ni(OH)₂, vulnerable to localized corrosion. First-principles calculations elucidate the microgalvanic interactions between phases, highlighting the B2 phase’s superior corrosion resistance and the potential for galvanic acceleration at phase boundaries.This integrated approach unlocks new pathways for designing NiTi SMAs tailored for marine environments, balancing high damping capacity with durability against seawater corrosion under mechanical strains

The Future: Future research will optimize SLM parameters, explore multi-factor corrosion, promote marine applications, refine mechanism models and develop composites.

The harmonic-structured NiTi SMA boasts notable advantages: SLM technology enables precise fabrication of complex components, with excellent mechanical properties, micropillar compressive strength up to 5.38 GPa and superelastic recovery strain of 4.96%. It exhibits optimal corrosion resistance in the IPT state, featuring a dense and uniform passive film. Multi-scale characterization and first-principles calculations clarify the corrosion mechanism, providing theoretical support for optimization, and it combines engineering practicality with academic value.

The Impact: This study fills the research gap in strain-related corrosion of NiTi SMAs, provides a new solution for seismic and vibration damping materials in marine engineering, and promotes the development of additively manufactured high-performance alloys.

The research has been recently published in the online edition of Materials Futures, a prominent international journal in the field of interdisciplinary materials science research.

Reference:  Huwen Ma, Yanchun Zhao, Yu Su, Junhui Luo, Jiacheng Xiang, Tengfei Zheng, Honghui Wu, Peter. K Liaw, Yuan Wu. Mechanical and corrosion behavior of additively manufactured NiTi shape memory alloys[J]. Materials Futures. DOI: 10.1088/2752-5724/ae293f