Researchers from Shandong University propose an alloy design approach to develop NiTiNb shape memory alloys

May 29, 2024

Schematic illustration of the microstructure evolution of (a) the fine eutectic region and (b) the coarse eutectic region for the LPBF-fabricated (NiTi)91Nb9 alloy before and after solution treatment (Courtesy International Journal of Extreme Manufacturing)
Schematic illustration of the microstructure evolution of (a) the fine eutectic region and (b) the coarse eutectic region for the LPBF-fabricated (NiTi)91Nb9 alloy before and after solution treatment (Courtesy International Journal of Extreme Manufacturing)

A study published in the International Journal of Extreme Manufacturing proposes an alloy design approach to develop NiTiNb shape memory alloys (SMAs). The researchers from Shandong University, Jinan, China, suggested using Laser Beam Powder Bed Fusion (PBF-LB) in-situ alloying and post-heat treatment.

The research looks to tackle issues faced when fabricating NiTiNb ternary alloy structures. The in-situ alloyed NiTiNb alloy exhibits typical transformation features, for example, wide hysteresis, and excellent mechanical/functional qualities.

Post-heat treatment is commonly employed to improve the microstructural homogeneity and enhance the mechanical performances of additively manufactured metallic materials. This research produced (NiTi)91Nb9 (at.%) by Laser Beam Powder Bed Fusion using pre-alloyed NiTi and elemental Nb powders. The effect of solution treatment on the microstructure, phase transformation behaviour and mechanical/functional performances was investigated.

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NiTi-based shape memory alloys (SMAs) are attracting growing attention in biomedical, aerospace, and automotive industries due to their unique shape memory effect and superealsticity. Adding ternary elements could potentially alter the transformation characteristics of binary NiTi alloys and expand their applications, especially in some cases that are incompatible with the binary NiTi alloys. Among the NiTi-based ternary SMAs, NiTiNb alloy exhibits a wide thermal hysteresis (>130 K) and excellent mechanical properties, which has been widely applied in sealing and coupling fields. Moreover, the excellent biocompatibility and non-toxicity of Nb also provide the potential for the biomedical applications of NiTiNb alloys.

Laser Beam Powder Bed Fusion is capable of producing complex NiTi parts, such as scaffolds. PBF-LB could also act as an in-situ alloying (ISA) method when using blended powders, which has been applied in the development of Ti-, Al-based and high entropy alloys. Compared with PBF-LB pre-alloyed powders, ISA enables flexible compositional and microstructural design, as well as shortened lead times.

Post-heat treatment has been commonly adopted to enhance compositional uniformity and modulate the microstructure, and thus stabilises the service performance of the additively manufactured parts. Previous studies have shown that the mechanical/functional performances of the LPBF-fabricated binary NiTi alloys could be improved by modulating the distribution of the secondary phases (e.g. Ni4Ti3, Ti2Ni) through post-heat treatment. The differences in initial microstructure of additively manufactured alloys means that existing heat treatment regimes for conventionally manufactured alloys are not always compatible. There is a need to understand the influence of post-heat treatment on the NiTiNb alloys manufactured via PBF-LB and to establish the inter-relationship between the microstructure, PT behaviour and mechanical/functional performances.

This research investigated the effect of solution treatment on the microstructure, PT behaviour and mechanical/functional performances of the (NiTi)91Nb9 (at.%) alloy produced by PBF-LB using pre-alloyed NiTi and elemental Nb powders. Targeting the classical Ni47Ti44Nb9 alloy, a high Nb content of 9 at.% was selected for ISA to obtain a comparable thermal hysteresis. The mechanical/functional performances of (NiTi)91Nb9 alloys were improved by heat treatment. As a result, NiTiNb alloys with good tensile performances and wide hysteresis were obtained. This study provides insights into the development of high-performance NiTi-based SMAs through PBF-LB ISA and post-heat treatment.

Read the full article, here.

www.en.sdu.edu.cn

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