Author
席瑞、姜浩、李桂川、张志辉、韦辉亮、赵国群、Jan Van Humbeeck、王协彬
Institutions
Shandong University
Citation
Xi R, Jiang H, Li G C, Zhang Z Z, Wei H L, Zhao G Q, Humbeeck J V, Wang X B. 2024. Effect of solution treatment on the microstructure, phase transformation behavior and functional properties of NiTiNb ternary shape memory alloys fabricated via laser powder bed fusion in-situ alloying. Int. J. Extrem. Manuf. 6 045001.
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https://doi.org/10.1088/2631-7990/ad35fc
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Article guide
NiTi-based shape memory alloys have unique "shape memory effect" and "superelasticity", and have broad application prospects in aerospace, automotive industry and biomedical fields. Among them, the NiTiNb ternary alloy has attracted much attention in the field of pipe joints due to its wide hysteresis characteristics. However, the popularization and application of traditional NiTiNb alloys face bottlenecks such as melting difficulty (large difference in melting point of elements) and machining difficulty (strong work hardening and poor welding performance). Laser Powder Bed Fusion (L-PBF), as an additive manufacturing technology, is an ideal method to solve the processing problem of NiTiNb alloy, and on the other hand, the unique micrometallurgical process of L-PBF is also a promising "in-situ metallurgy" method to solve the problem of NiTiNb alloy smelting. However, the in-situ synthesis of NiTiNb alloys by L-PBF usually exhibits problems such as inhomogeneous microstructure and poor mechanical properties. Therefore, it is necessary to improve the mechanical properties and functional properties of NiTiNb alloy by optimizing the microstructure and improving the inhomogeneity of the microstructure through post-processing.
近期，山东大学材料科学与工程学院王协彬教授与吉林大学、南京理工大学、比利时鲁汶大学（KU Leuven）等研究团队联合在SCI期刊《极端制造》（International Journal of Extreme Manufacturing, IJEM）上共同发表《Effect of solution treatment on the microstructure, phase transformation behaviour and functional properties of NiTiNb ternary shape memory alloys fabricated via laser powder bed fusion in-situ alloying》的研究论文。 该团队提出了一种“L-PBF原位合金化”与“后处理工艺”相结合的NiTiNb合金制备路线。 以预合金NiTi和纯Nb粉末为原材料，通过L-PBF技术制备近无缺陷的(NiTi)91Nb9 (at.%)合金。 通过高温固溶处理调节共晶相的形貌分布，提高合金的力学/功能特性。 最终成功制备了兼具宽滞后特性与良好力学性能的NiTiNb合金（图1）。 此工艺路线也可适用于其它NiTi基三元形状记忆合金的设计与制备。
keyword
Shape memory alloys, NiTiNb, laser powder bed melting, in-situ alloying, heat treatment
Bright spots
- The feasibility of in-situ preparation of NiTi-Nb alloy by laser powder bed melting technology was verified.
- The high-temperature solution treatment could adjust the morphological distribution characteristics of the eutectic phase and improve the mechanical/functional properties of NiTiNb alloy.
- NiTiNb alloys with wide hysteresis and good tensile mechanical properties were prepared.
Fig.1. Microstructure, phase transformation characteristics and mechanical/functional properties of NiTi 91Nb9 alloy prepared by laser powder bed fusion (NiTi) before and after solution treatment.
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Background:
In recent years, scholars at home and abroad have carried out preliminary research on the in-situ preparation of NiTi-based shape memory alloys by L-PBF. The idea of mixing pre-alloyed NiTi powder with the third component elemental powder is usually adopted, including NiTi+HfH2, NiTi+Fe, NiTi+Nb, etc. This preparation method ensures the formation of the host NiTi phase, so the in-situ synthesis of NiTi-X alloy can exhibit typical phase transformation behavior and mechanical properties. However, the inherent defects of the in-situ alloying process, such as inhomogeneous microstructure, unmelted inclusions, etc., lead to poor mechanical/functional properties of the alloy. The results show that adjusting the distribution of the second phase (e.g., Ni4Ti3, Ti2Ni) by post-processing can significantly improve the mechanical/functional properties of binary NiTi alloys prepared by L-PBF. However, as a pseudo-binary eutectic alloy, the mechanical/functional properties of NiTiNb alloy are mainly affected by the morphology, size and distribution of eutectic phases. Due to the influence of rapid solidification characteristics and complex thermal history, the eutectic microstructure and morphology of eutectic alloys (such as Al-12Si and Al-5.7Ni) prepared by L-PBF are quite different from those of conventional alloys. Based on this, the effects of solution treatment on the microstructure, phase transformation behavior and mechanical/functional properties of L-PBF prepared (NiTi)91Nb9 (at.%) alloy were systematically studied.
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Latest developments
Effect of solution treatment on microstructure: The authors first studied and revealed the mechanism of microstructure evolution by scanning electron microscopy (SEM), electron probe microscopy (EPMA), and transmission electron microscopy (TEM) (Fig. 2). The printed (NiTi)91Nb9 alloy is a multi-level heterogeneous microstructure, including micron-sized unmelted Nb particles, submicron-sized cellular-dendrite structure, nano-scale Nb-rich precipitate phase and NiTi(Nb) matrix. After high-temperature solution treatment, the microstructure gradually changed from non-equilibrium state to equilibrium state, which included the following three aspects: 1) the Nb-rich precipitate phase precipitated from the supersaturated NiTi(Nb) matrix, 2) the NiTi/Nb eutectic was broken and spheroidized, and the network morphology changed from rod-like and spherical particles, and 3) the β-Nb phase and the Nb-rich precipitate phase were coarsened with the extension of holding time.
Fig.2 Microstructure and composition distribution of (NiTi)91Nb9 alloy prepared by L-PBF: (a) printed alloy, (b) 1273K solution treatment for 15 minutes, (c) 1273K solution treatment for 60 minutes, and (d) 1273K solution treatment for 120 minutes.
Effect of solution treatment on phase transformation behavior: The authors used differential scanning calorimetry (DSC) to characterize the phase transformation behavior of the prepared NiTiNb alloys (Fig. 3). The martensitic phase transition temperature of the printed (NiTi) 91Nb9 alloy is low, such as the Mp temperature is about 171K. After solution treatment, the phase transition temperature increased (Mp=245K), which was similar to that of as-cast Ni46Ti45Nb9 alloy. Comparing the microstructure characteristics of NiTiNb alloys before and after solid solution, the authors analyzed the mechanism of phase transition temperature. As shown in Figure 4, the increase in phase transition temperature is attributed to the transition of the microstructure from non-equilibrium to equilibrium, mainly including: (i) the expulsion of Nb atoms from the supersaturated B2-NiTi phase, which weakens the lattice distortion in the matrix, and (ii) the expulsion of Ti from the β-Nb phase, which leads to a decrease in the Ni/Ti ratio of the matrix. After 20% compression pre-deformation, the phase transformation hysteresis of the solid solution (NiTi)91Nb9 alloy is about 145 K, which is comparable to that of the traditional Ni47Ti44Nb9 alloy (130 K~170 K).
Fig.3 Phase transformation behavior of (NiTi)91Nb9 alloy prepared by L-PBF.
Fig.4 Schematic diagram of the phase transformation behavior of (NiTi)91Nb9 alloy prepared by L-PBF: (a) the variation of martensitic phase transition temperature with Nb content, and (b) the variation of martensitic phase transition temperature with holding time.
Effect of solution treatment on mechanical/functional properties: The authors tested the mechanical and functional properties of L-PBF prepared (NiTi)91Nb9 alloy (Fig. 5). After 1273K-30min solid solution, (NiTi)91Nb9 alloy exhibited good tensile mechanical properties (breaking strength of about 781MPa and breaking strain of about 9.5%), which was better than the published results of L-PBF preparation of NiTiNb alloy. The improvement of mechanical properties after solution treatment is mainly attributed to the transformation of NiTi/Nb network eutectic into rod and spherical particles. Compared with the mesh eutectic, rod and spherical β-Nb have better dislocation storage capacity, which is more conducive to maintaining the continuity of the matrix. In addition, all specimens showed a good tensile shape memory effect, with a shape recovery rate of more than 90% (pre-strain of 5%).
Fig.5 Mechanical and functional properties of (NiTi)91Nb9 alloy prepared by L-PBF: (a-b) low-temperature tensile properties and (c-d) shape memory effect.
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Future outlook
In order to solve the melting and processing problems of NiTiNb ternary shape memory alloys, NiTiNb alloys with typical phase transformation characteristics and good mechanical properties were prepared by using the process route of L-PBF in-situ alloying combined with post-treatment. Combining the advantages of additive manufacturing for the preparation of complex shapes and the potential of in-situ metallurgy, it provides a new idea for the integrated preparation of controlled and shaped control of NiTi-based shape memory alloys. In addition, the process route has the advantages of flexible composition design and short preparation cycle, which is expected to accelerate the development of high-performance NiTi-based memory alloys.
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About the Author
Wang Xiebin
Shandong University
Xiebin Wang is a professor at the School of Materials and Science Engineering, Shandong University. He received his bachelor's degree from Shandong University and his Ph.D. from KU Leuven University in Belgium. He has published more than 40 academic papers in related fields, including 23 first-author/independent corresponding author papers, including Acta Materialia, Additive Manufacturing (2 papers), Journal of Materials Science & Technology, and Scripta Materialia (7 articles), etc., applied for 13 national invention patents, authorized 7 patents, participated in the formulation of 1 national standard, presided over/participated in the formulation of 3 group standards, served as a young editorial board member of International Journal of Extreme Manufacturing, Materials Research Letters, Materials Engineering and other magazines, as well as the National Natural Science Foundation of China, He was selected as a young expert of Taishan Scholars in Shandong Province and funded by the Outstanding Youth Fund of Shandong Province, and won the third prize of the 2021 China Machinery Industry Science and Technology Progress Award (ranked 2).
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Hatps://doi.org/10.1088/2631-7990/akfd5
About the journal
The International Journal of Extreme Manufacturing (Chinese "Extreme Manufacturing"), abbreviated as IJEM, is committed to publishing high-quality and up-to-date research results related to the field of extreme manufacturing. Since its inception in 2019, the journal has been included in more than 20 international databases such as SCIE, EI, and Scopus. In 2023, the latest impact factor of JCR is 14.7, ranking first in the engineering/manufacturing subject field. Chinese Academy of Sciences Division Engineering Technology Zone 1.
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