Microstructure and mechanical behavior of an as-drawn MP35N alloy wire

Microstructure and mechanical behavior of annealed MP35N alloy wire

“…As previously shown [3], the as-drawn MP35N low Ti alloy wire contained a hierarchical twinned microstructure with twin spacings ranging in size from 1 nm to 1 µm within fcc grains of 1-4 µm size; furthermore, it was also shown that the twin spacing increased from the surface to the center of the wire, and the as-drawn wire exhibited a strong <111> fiber texture. The effect of annealing temperature (1h at temperature) on the microstructure evolution in the asdrawn wire is captured in Figure 1a-f with the as-drawn wire microstructure illustrated in Figure 1a and serving as the reference structure.…”

“…%) and a diameter of 100 µm; ~40 % cold work in the wire) is the same as that used in our recently published work and more details can be found there [3]. For in-situ transmission electron microscopy (TEM) straining experiments, an MP35N low Ti alloy in ribbon form (30% cold-rolling of a previously cold-drawn wire), approximately 2.5 mm wide x 0.28 mm thick, was provided by Medtronic and was originally processed at Fort Wayne Metals in Fort Wayne, IN, USA.…”

“…The microtexture evolution in the wire upon annealing was characterized by collecting Kikuchi patterns at 20 kV using a NordlysF EBSD detector and HKL fast acquisition software. Specimens for TEM examination were obtained from the annealed wire segments by the FIB lift-out method using an OMNIPROBE TM tip and the procedure is described in more detail in [3]. These specimens were examined at 200 kV in a Philips CM20 TEM in order to characterize at the fine scale, microstructural changes that had occurred during annealing of the cold-drawn wire.…”

“…When the MP35N low Ti alloy wire in the cold-drawn state was deformed under cyclic loading conditions (R = -1; RBBF; see data in Table I in [3]), the material with its high strength, low hardening rate and limited ductility exhibited fatigue life of <5 × 10 5 cycles at stress amplitudes, σ a > 550 MPa (and corresponding to σ a /σ y > 0.33; note that when R=-1, σ a = σ max ).…”

“…The alloy derives its high yield and fracture strengths from cold working and when used as a lead wire in pacemaker devices, the wire is in the cold-drawn state. The monotonic and cyclic properties of a low-Ti version of the MP35N wire in the cold drawn state were examined in detail and recently reported [3]. The study confirmed that the alloy in this state exhibits i) a hierarchical twinned structure with twins ranging in width from micrometers to nanometers, and ii) a yield strength of around 2 GPa that is accompanied by ~3% tensile elongation to failure and negligible strain hardening; deformation twinning and de-twinning appeared to be the primary carriers of plasticity, while in the pronounced necked region of the tensile specimens where extensive deformation had occurred, twin intersections coupled with shearing resulted in twin fragmentation and the development of nanocrystalline grains analogous to those observed in severe plastic deformation [4].…”