October_2021_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 2 1 4 6 FATIGUE FRACTURE OF NITINOL Nitinol fatigue fracture surfaces generally exhibit similar features to most engineering alloys, including a flat fracture plane, ratchet marks, radial lines, beachmarks, and striations. Louis G. Malito,* Matthew L. Bowers,* Paul L. Briant,* Gabriel S. Ganot,* and Brad James* Exponent Inc., Menlo Park., Calif. N itinol’s shape memory and superelastic behaviors are both enabled by a solid state, diffusionless phase transformation between the “parent” high tempera- ture (an ordered B2 or CsCl-type structure) and a “daughter” low temperature (a monoclinic B19′ structure). Phraseology from steel metallurgy is customarily adopted, referring to the higher temperature parent phase known as austenite, A , and the lower temperature daughter phase known as mar- tensite, M . *Member of ASM International The phase transformation occurs through shear stress by the formation of crystal twinning in the material. Heat is released (exothermic) during the A  M transformation and is absorbed (endothermic) during the reverse M  A trans- formation. While the names austenite and martensite are derived from the steel phases of the same names to denote the diffusionless phase transformation that relates the two, the similarities end there. Nitinol’s phase transformation is affected by deviatoric stresses while martensite in steel is affected by hydrostatic stresses. Unlike Nitinol, the diffu- sionless phase transformation in steel is irreversible due to the volume change between the phases. FEATURE 1 0 Fig. 1 — BSE micrographs of a fractured wire-form component that was fatigue tested and failed at 15.6 million cycles at 0.45% strain amplitude and 1.0% mean strain. Top: Micrograph of the overall fracture surface of the wire-form part. One can observe “feathering” lines pointing to a fatigue crack origin at the bottom of the image where the peak strain was located during cycling. Bottom: Higher- magnification micrograph of the fatigue crack origin. Fig. 2 — High-magnification micrographs of a surface nonmetallic inclusion that acted as an initiation site for the fatigue crack in the wire-form component from Fig. 1.