Design of Fatigue-resistant NiTi-based Shape Memory Alloys for Additive Manufacturing

Liu, Chuan

doi:https://doi.org/10.21985/n2-5g2n-8v62

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Design of Fatigue-resistant NiTi-based Shape Memory Alloys for Additive Manufacturing

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NiTi shape memory alloy (SMA) has drawn a great deal of attention for its various applications in the medical field (orthodontics, cardiovascular stents technology, etc.) and in other engineering fields (aerospace, aircraft, automotive, etc.) as it shows shape memory effect, superelasticity and biocompatibility. The fatigue-related issues, however, are pronounced in NiTi SMA due to the dynamic mechanic cycling in its most applications. Unlike other structural materials, these fatigue-related issues are even complicated with its unique thermo-mechanic characteristics of shape memory effect and superelasticity with reversible martensitic transformations. In the present study, improved high cycle fatigue resistance of SMAs is approached through a combination of inclusion nucleation potency control and efficient nanoscale precipitation strengthening to reduce accommodation slip. As the advanced processing methods, additive manufacturing, is promising to suppress fast coarsening of oxides, a computational design strategy was applied to optimize the precipitates-strengthened SMAs with desired printability. To better understand the effect of inclusions, the microstructures of commercial cast-wrought NiTi tubes were characterized in terms of inclusion chemistry, morphology, and size distribution. An empirical model was developed addressing the stressed volume effect in fatigue tests of NiTi wires and diamond specimens. It predicts a (-1/2) power-law size dependence of fatigue strength to 10$^{7}$ cycles: $\varepsilon (10^{7})\sim d^{-1/2}$. This power-law dependence was also validated by a crystal plasticity finite element (CPFE) modeling of NiTi high cycle fatigue behavior. With the concept of materials design, minor amounts of Sc and Y were added to binary NiTi as both of them could be strong oxide formers. Although 0.1 wt\% Y or Sc additions effectively decreased the amount of oxide by 2/3, Sc addition was more promising as the maximum or average oxide size was also reduced. To improve fatigue resistance via strengthening, a Ni$_{50}$(Ti,Hf,Al)$_{50}$ shape memory alloy was under design with nano-size L2$_{1}$ Ni$_{2}$TiAl aluminide dispersion strengthening. Supported by electron microscopy and atom-probe microanalysis, phase equilibrium and precipitation kinetics of Ni$_{50}$(Ti,Hf,Al)$_{50}$ alloys was investigated at 600 C and 650 C, and the precipitation competence between L2$_{1}$ and H-phase was confirmed in the composition space. Hardness tests were conducted on the aged non-transforming Ni$_{50}$Ti$_{31}$Hf$_{15}$Al$_{4}$ to obtain changes in B2 strength in relation to aging treatment. By correlating the dispersion evolution with the strength, a precipitation strengthening model was established, and the optimal precipitate radius for maximum strengthening is identified. The thermal transformation behavior of aged Ni$_{50}$Ti$_{24}$Hf$_{22}$Al$_{4}$ was measured by differential scanning calorimetry to quantify the relationship between transformation temperature, matrix composition, and precipitate phase fraction. The compression tests for solution heat-treated and aged Ni$_{50}$Ti$_{24}$Hf$_{22}$Al$_{4}$ alloys at various temperature proved their fully superelastic responses in a wide temperature range. Improved mechanical and thermal cyclical stability was observed for peak-aged specimens. Prior to the additive manufacturing process, a series of laser scanning experiments were conducted to evaluate the printability of Heusler-strengthened NiTiHfAl and PdNiTiAl prototypes. A feasible window of processing parameters was found without obvious defects for both of NiTiHfAl and PdNiTiAl, while NiTiHfAl seemed more susceptible to hot cracking. Nevertheless, a systematic design was conducted for high-strength NiTiHfAl alloys with acceptable printability. In the preliminary test of the final design prototype, Ni$_{50}$Ti$_{23.9}$Hf$_{21.8}$Al$_{4.3}$, it was confirmed to be perfectly superelastic at room temperature. Further validation on the additive manufacturing of this prototype will be performed in the future.

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