Inclusion Effects on the Lifetime Performance of Superelastic Nitinol Wires

Miniaturization of Nitinol-based medical devices requires increased performance from less material, and therefore necessitates investigation of the connection between material processing and performance. Moreover, it is imperative to understand how impurities that result from processing affect its lifetime performance. Research on the microcleanliness (i.e. presence of nonmetallic inclusions, intermetallic particles, etc.) of fine Nitinol wires and its effect on fatigue performance are not frequently found in literature.

This study compared a manufacturer-defined standard (SP) and high (HP) purity, Nitinol superelastic (SE) fine (< 140 μm) wire. Nonmetallic inclusion (NMI) chemistry was determined to be Ti_xNi_yO_z with varied morphology, occurring with/without pores. Combined NMI/pore area percentages ranged from 0.08% ± 0.04% (HP) to 1.44% ± 0.26 (SP) when measured with scanning electron microscopy (SEM). Plasma focused ion beam (PFIB) serial sectioning was also used characterize the combined NMI/pore volume percentages yielding 0.09% (HP) and 0.47% (SP).

Differential scanning calorimetry was used to identify SE transformation temperatures. Microindentation hardness measurements showed gradients with the center of the wire measuring 359 ± 6.4 HK (HP) and 329 ± 7.2 HK (SP) while mid-radius values were similar for both at 366 ± 5.1 HK and 361 ± 6.0 HK, respectively. Higher tensile and upper plateau strengths were observed in SP (1573 MPa and 515 MPa, respectively) than HP (1420 MPa and 392 MPa, respectively) with greater reduction in area for HP (92.2%) vs. SP (80.2%). Fatigue enhancement was observed in HP wires in flex bending fatigue at strain amplitudes (ε_a) from 0.67-11% with a run-out at 10⁶ cycles and rotating bending fatigue (ε_a= 1.2-1.5%) though run-out at 10⁸ cycles. Fractography showed crack initiation attributed to NMIs/pores at the surface or sub-surface with feature areas ranging from submicron to 20.4 μm².

Characterization techniques showed that HP contained less large-dimension NMIs/pores than SP leading to less potential stress concentrations and points for crack initiation. These differences in relative purity were reflected in the fatigue results where the HP exhibited enhanced fatigue resistance in both low cycle fatigue and high cycle fatigue regimes.