Hydrogen embrittlement of additively manufactured NiTi shape memory alloys: Review on interactions between microstructure and mechanical properties

Abstract

Hydrogen embrittlement poses a formidable challenge to the structural integrity and functional performance of high-performance NiTi applications in biomedical, aerospace, and hydrogen energy sectors, where hydrogen exposure is inevitable, and structural reliability is paramount. There is a growing interest in fabricating these smart alloys in additively manufactured (AM) parts to harness additional features and functionality. It is therefore important to systematically study the effect of hydrogen-rich environments on the superelasticity and shape memory, the main functional properties of these alloys. Despite extensive research on hydrogen embrittlement in conventionally manufactured (CM) metallic alloys, the impact of AM-induced microstructural heterogeneities, such as residual stresses, fine grains, and porosity, on hydrogen-microstructure interactions remains underexplored. There is a need to study hydrogen embrittlement to enhance our understanding of the intricate hydrogen-material interactions, phase transformation behaviors, and HE-assisted failure mechanisms, especially of AM NiTi shape memory alloys. This review addresses this need by systematically analyzing how AM-specific microstructures influence (1) hydrogen trapping and diffusion kinetics, (2) multiple active hydrogen embrittlement mechanisms, (3) phase stabilization, and (4) hydrogen embrittlement-provoked mechanical degradation. These issues are not well characterized in the current literature. To this end, the complex interplay between hydrogen diffusion and trapping processes, phase stability, and mechanical degradation is examined, with a particular focus on hydrogen-induced martensitic stabilization, active hydrogen embrittlement mechanisms/models in NiTi shape memory alloys: hydrogen enhanced localized plasticity (HELP), and hydrogen enhanced decohesion (HEDE), including their synergy (HELP + HEDE model), and hydride embrittlement. The classical single-mechanism hydrogen embrittlement models are inadequate in capturing the intricate diffusional-mechanical coupling in AM NiTi shape memory alloys. We have discussed this issue and explained the necessity of novel multi-scale modeling and experimental frameworks. These proposed frameworks provide the basis for understanding the interplay of hydrogen embrittlement and hydrogen damage mechanisms. This understanding leads to mitigating the hydrogen embrittlement in AM NiTi shape memory alloys and similar alloys. Furthermore, the review highlights the challenges in designing AM NiTi shape memory alloys with improved hydrogen embrittlement resistance, identifying gaps in predictive modeling and real-time characterization techniques. Future research directions emphasize the need for real-time in situ characterization techniques, integrated computational-experimental approaches, and innovative hydrogen embrittlement mitigation strategies such as microalloying, surface engineering, and post-processing treatments to enhance hydrogen embrittlement resistance.