October AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 9 4 3 DIMENSIONALLY STABLE SHAPEMEMORY ALLOYS FORMECHANICAL ENGINEERINGAPPLICATIONS: THE OTHER SIDE OF SUPERELASTICS Newly developed NiTi alloys are well suited for bearings and gears in demanding applications. FEATURE Christopher DellaCorte NASA Glenn Research Center, Cleveland I n 2004, NASA began research to assess Nitinol 60 for use as a ball bearing material because of its documented unique combination of intrinsic corrosion resistance and harden- ability to bearing-hard levels (see sidebar), a blend of prop- erties not exhibited by any known ball bearing alloys. By applying advanced powder metallurgy processes used for en- gineered ceramics to the NiTi alloy system, researchers were able to produce fully dense, high-quality material with critical flaw sizes below required levels needed for bearings and other structural applications [3] . Figure 1 shows Nitinol 60 made us- ing advanced powder metallurgy processes (i.e., hot isostatic pressing, or HIP). Such fine-grained, homogeneous microstructures, rela- tively free from voids and other flaws are essential for highly stressed component applications like bearing balls and races. Alloy processing and materials properties are well document- ed in the literature and form the foundation for a variety of successful applications in aerospace systems [4,5] . Ball bear- ings used inside the Interna- tional Space Station (ISS) water recycling system are a notable early application [5] . Hybrid bear- ings with a 50-mm bore using Ni- tinol 60 races and silicon-nitride balls support the rotating distilla- tion assembly as shown in Fig. 2. In this space application, the bearings are subjected to high shock loads (during rocket launch to space) and exposed to highly corrosive fluids, which rapidly de- grade conventional bearing ma- terials, even corrosion-resistant stainless steels like AISI 440C. In contrast, Nitinol 60 is impervious to acidic aqueous corrosion and has comparable hardness to tra- ditional bearing materials mak- ing it an ideal candidate for this application. Fig. 1 — Microstructure of powder-metallurgy processed Nitinol 60 after HIP. Fig. 2 — ISS urine processor distillation assembly (DA) uses Nitinol 60 hybrid ball bearings in the drum assembly and compressor. 5