• Jinschek, Joerg R.
• Bastos Da Silva Fanta, Alice
• Sneppen, Thor Bjerregård
• Popovich, Vera
• Yang, Yi-Chieh
• Zhu, Jia-Ning

Abstract

<p class="chapter-para">Shape memory alloys (SMAs) [1, 2] are gaining attention in many applications, such as in actuators [3], sensors [4], and dampers [5], due to their attractive property of shape memory effects (SME). SME is a capability of SMAs to regain the original shape of a deformed material upon heating through the reversible martensitic transformation. According to the stress-strain curve for the SMAs, the applied strain and the working temperature are used to determine the stress of the SMA and its phase. For optimizing the working properties of SMA, the corresponding phase transformation temperature is an essential parameter, and it strongly depends on local microstructure characteristics, such as chemical composition [6], precipitates [6, 7], dislocations [8] and grain size [9]. As a result, an in-depth understanding of the correlation between the structural variation and the applied temperature is essential for optimizing fabrication parameters to control the application conditions.</p><p class="chapter-para">Here, NiTi (Nitinol) SMA is used to fabricate a sample using laser powder bed fusion (L-PBF), a metal additive manufacturing technique [10, 11]. The ability to build parts with complex geometries [12] and <em>in situ</em> tailorable microstructures [13] makes L-PBF a great choice for fabrication. However, since laser rastering in L-PBF introduces an inhomogeneous heating profile, in each scanning point a melt pool with non-uniform composition distribution perpendicular to the build direction is introduced which results in metastable phases within in the melt pool, and thereby influencing the structural and shape memory effect stability.</p><p class="chapter-para">In order to capture the correlation between phase transformation and the local inhomogeneity, <em>in situ</em> heating experiments in transmission electron microscopy (TEM) are used to study the SME in L-PBF Nitinol SMAs. To study the variation in phases with increasing temperature, TEM samples from different areas of the melt pool were prepared by focused ion beam (FIB) and placed on the MEMS-based microheaters for in-situ TEM heating experiments.</p><p class="chapter-para">Observing the phase transition upon <em>in situ</em> heating in L-PBF Nitinol SMAs shows a higher phase transformation resistance in the melt pool boundaries, due to the fine cellular structure and high-density dislocations. Further segregation at the grain boundaries also causes the change in the phase transition temperature. Our results indicate the capability to apply in-situ TEM heating experiments to study microstructural transformations and providing essential insights to further optimize process parameters in (additive) manufacturing, such as controlling the functional anisotropy [14].</p>

Topics

• density
• impedance spectroscopy
• grain
• grain size
• experiment
• melt
• stress-strain curve
• selective laser melting
• phase transition
• chemical composition
• focused ion beam
• transmission electron microscopy
• dislocation
• precipitate
• metastable phase