Investigations in phase stability and mechanical attributes in nickel-rich Nitinol with and without hafnium additions

Abstract

In this work, the phase stability and mechanical attributes in Ni-rich Nitinol with and without Hf additions are investigated through an array of microanalysis techniques. The work has two distinct areas of interest: (1) establishing the structure-properties relationship for a slightly Ni-rich ternary shape memory alloy with macro-additions of Hf for use as solid-state actuators and (2) determining the primary strengthening mechanisms and its compositional limit in very Ni-rich alloys with and without solute additions of Hf for bearing applications. For the first area of interest, a 50.3Ni-29.7Ti-20Hf (at.%) alloy was aged at 550°C out to 300 hours. The shape memory transformation temperature and mechanical response were measured for samples aged to specified times with the microstructure yielding these transformation temperatures and mechanical response characterized via transmission electron microscopy and atom probe tomography. These instruments allowed for the role precipitation plays to be thoroughly established while resolving the optimal microstructure by determining precipitate number density, size, and composition. The alloy exhibited near optimum response for thermal and dimensional stability after aging for three hours. The second area of interest focused on the lesser studied systems of very Ni-rich alloys with their extremely low transformation temperature. However they possess high hardness values on par with tool steels, corrosion resistance, and stably non-magnetic behavior making them promising candidates for bearing applications. A 55Ni-45Ti (at.%) alloy, initial found to possess such hardness, had its strengthening mechanism elucidated. The alloy was solutionized and aged at 400°C to 300 hours with no considerable drop in hardness registered. This consistent hardness was revealed to be the result of a microstructure composed of a high volume fraction of Ni4Ti3 precipitates within narrow channels of B2 matrix. Generalized stacking fault energies for the two phases composing the microstructure were determined confirming the difficulty to slip in such a microstructure. The second study focusing on these bearing alloys was investigation of the compositional limit of this strengthening mechanism for a series of Ni-rich binary compositions ranging from 53NiTi to 58NiTi (at.%) solutionized and aged at various times and temperatures. The alloy set was found to increase in hardness with increasing Ni content until reaching a maximum at 56NiTi with its hardness equivalent to the 55NiTi alloy. This hardness was contributed to a high volume fraction of Ni4Ti3 precipitates encased in narrow B2 matrix channels. Additionally, the precipitation sequence of these lesser studied Ni-rich alloys was observed at 625° and 750°C to better establish the decomposition pathways between the secondary precipitates phases. Finally, a set of Ni-rich ternary alloys with solute additions of Hf were investigated to ascertain their practicality as second generation bearing alloys. The alloys containing 1-2% Hf were found to have a traditional parabolic aging behavior with hardness below those of binary 55NiTi and 56NiTi, interestingly the H-phase precipitates formed after extended aging at 400°C. The precipitation of H-phase appears to alter the typical precipitation sequence seen in Ni-rich alloys and expands the compositional limits of H-phase. Finally, the 4% Hf containing alloy's hardness was greater than 55NiTi and further increased with aging. This behavior was the result of H-phase forming in the narrow channels of B2 matrix phase seen in the binary alloy versions.