Stability and optimization of P-phase precipitates in nickel-titanium-palladium shape memory alloys.

摘要

The motivation for this research is the understanding of phase transformations that lead to an increase in the shape memory effect (SME) transformation temperature in a Ni-Ti-Pd shape memory alloy (SMA). The research addressed three major parts of this transformation---(1) The phase stability of the P-phase precipitate previously discovered with an emphasis on its stoichiometric limits by changing the Ni and Pd content with the Ti11(Ni,Pd)13 ratio; (2) The effects of P-phase precipitation on the martensitic transformation temperature in near-equiatomic Ti(Ni,Pd) alloys; and (3) The effects of dilute additions of Hf (0.1-1 at.%) to the precipitation and shape memory transformation temperature in Ni-Ti-Pd.;P-phase stabilization: The compositional limits of the P-phase have been systematically studied by varying the Pd and Ni content in the P-phase's Ti11(Ni+Pd)13 stoichiometry. Each alloy was solutionized at 1050°C followed by water quenching, and aging at 400°C for 100 hours. Four distinct phases were identified---Ti 2Pd3, B2 Ni-Ti, P- and P1-phases dependent on alloy composition---by electron and x-ray diffraction. The latter precipitate phases become more stable with increasing Ni at the expense of Pd content. Atom probe tomography revealed the P-phase composition to be 45.8Ti-29.2Ni-25Pd (at.%) or Ti 11(Ni7Pd6) as compared to the P1-phase 44.7Ti- 45.8Ni-9.4Pd (at.%) or Ti5Ni5Pd.;Optimization of P-phase precipitation: The effect of aging time and temperature on precipitation and subsequent martensitic transformation temperatures for a series of Ni-(50.5-49.2)Ti-32Pd (at.%) shape memory alloys has been studied. Structure-property relationships were developed through detailed microstructural characterization involving transmission electron microscopy, diffraction analysis, and atom probe tomography with links to microhardness measurements and transformation temperatures established by differential scanning calorimetry. The Ti-rich alloy contained relatively coarse Ti2Ni in a B19 matrix and had the highest martensitic transformation temperatures (Ms ~ 280°C) independent of aging condition. After aging, the two Ti-lean alloys contained P-phase precipitates in a B19 matrix. The increase in transformation temperature was associated with a near 50 at.% Ti composition in the matrix phase.;Quaternary alloys addition to P-phase: The effect of Hf (0 -- 1 at. %) additions in a Ni-Ti-Pd alloy on P-phase precipitation and martensitic transformations were studied. The addition of hafnium promoted an increase in strain within the matrix which resulted in the refinement of precipitates upon precipitation with a corresponding increase in number density. The overlapping strain fields of the precipitates created from the decrease in inter-precipitate spacing reduced the matrix volume to be less than the critical free volume size required for the martensitic transformation over the temperature range studied (183 K to 573 K), hence a deleterious suppression of the transformation temperature. Hafnium was also noted to delay the aging time to achieve peak hardness, suggesting a reduction in growth and coarsening kinetics.