Fabrication and characterization of porous NiTi Shape Memory Alloy by elevated pressure sintering

Abstract

Shape Memory Alloys (SMAs) have emerged as a class of materials with unique thermal and mechanical properties that have found numerous applications in various engineering fields. Even beyond the shape memory characteristics inherent in dense SMAs, porous SMAs with a relatively low density would expand the applicability of SMAs. In addition to the large recoverable strains observed by SMAs, porous SMAs offer the possibility of undergoing greater overall strains as well as much higher specific energy absorption under dynamic loading conditions due to the possibility of wave scattering. Porous SMAs also offer the possibility of impedance matching by grading the porosity at connecting joints with other structural materials. In biomedicine, porous SMAs have been successfully used in such applications as bone implants and dental repairs. Despite their high potential for practical use, porous SMAs have not been sufficiently studied in the U.S., and techniques for their commercial production have not been adequately developed. Currently, three methods are commonly used for producing porous NiTi SMAs from elemental powders. These methods include conventional sintering, Self-propagating High-temperature Synthesis (SHS), and sintering at elevated pressure via a Hot Isostatic Press (HIP). Conventional sintering requires long heating times and samples are limited in shape and pore size. SHS, which is initiated by a thermal explosion ignited at one end of the specimen and propagates in a self-sustaining manner, usually results in an uncontrolled porous microstructure and impurities. Sintering at elevated pressures, however, results in a decrease in the necessary sintering time as compared to conventional sintering due to consolidation of the medium and offers more control over the fabrication process and phase composition than SHS. In this work, dense and porous NiTi specimens with approximate porosity levels of 0%, 42%, and 50%, respectively, are fabricated via HIPping and characterized in terms of composition and phase transformation characteristics. Mechanical behavior of the HIPped specimens is studied through quasi-static testing in both shape memory and pseudoelastic effect. Results for the porous NiTi in dynamic testing under compressive loading are also presented to illustrate the large energy absorption and strain recovery capabilities associated with porous NiTi SMA.