Computational Design of Precipitation-Strengthened TiNi-Based Shape Memory Alloys

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Motivated by performance requirements of future medical stent applications, experimental research addresses the design of novel TiNi-based, superelastic shape-memory alloys employing nanoscale precipitation strengthening to minimize accommodation slip for cyclic stability and to increase output stress capability for smaller devices. Using a thermodynamic database describing the B2 and L21 phases in the Al-Ni-Ti-Zr system, Thermo-Calc software was used to assist modeling the evolution of phase composition during 600C isothermal evolution of coherent L21 Heusler phase precipitation from supersaturated TiNi-based B2 phase matrix in an alloy experimentally characterized by atomic-scale Local Electrode Atom Probe (LEAP) microanalysis. Based on measured evolution of the alloy hardness (under conditions stable against martensitic transformation) a model for the combined effects of solid solution strengthening and precipitation strengthening was calibrated, and the optimum particle size for efficient strengthening was identified. Thermodynamic modeling of the evolution of measured phase fractions and compositions identified the interfacial capillary energy enabling thermodynamic design of alloy microstructure with the optimal strengthening particle size. Extension of alloy designs to incorporate Pt and Pd for reducing Ni content, enhancing radiopacity, and improving manufacturability were considered using measured Pt and Pd B2/L21 partitioning coefficients. After determining that Pt partitioning greatly increases interphase misfit, full attention was devoted to Pd alloy designs. A quantitative approach to radiopacity was employed using mass attenuation as a metric. Radiopacity improvements were also qualitatively observed using x-ray fluoroscopy. Transformation temperatures were experimentally measured as a function of Al and Pd content. Redlich-Kister polynomial modeling was utilized for the dependence of transformation reversion Af temperature on B2 matrix phase composition. Biocompatibility is likely improved as Pd substitution of Ni alleviates concerns of Ni toxicity in TiNi alloys. The B2/L21 interphase misfit was calculated to facilitate the design of optimized alloys with low-misfit coherent precipitation strengthening, desired transformation temperatures and enhanced radiopacity absorption, within constraints of practical processing temperatures. Enhanced cyclic and thermal stability in this alloy was demonstrated through differential scanning calorimetry and compression testing, respectively, as indication of improved cyclic fatigue life. A proposed Ni20Pd30Ti46Al4 (at.pct.) alloy with a 950C solution treatment for 20 h and a 600C aging for 4 h is predicted to provide high strength, a low 0.71% misfit, an Af transformation temperature of 10C for desired superelastic behavior at body temperature, and a radiopacity absorption 142% greater than current TiNi alloys.

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  • 09/13/2018
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