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04.12.2019 | SMST2019 | Ausgabe 4/2019

Shape Memory and Superelasticity 4/2019

Microstructural and Thermomechanical Comparison of Ni-Rich and Ni-Lean NiTi-20 at.% Hf High Temperature Shape Memory Alloy Wires

Zeitschrift:
Shape Memory and Superelasticity > Ausgabe 4/2019
Autoren:
A. W. Young, R. W. Wheeler, N. A. Ley, O. Benafan, M. L. Young
Wichtige Hinweise
This article is an invited submission to Shape Memory and Superelasticity selected from presentations at the Shape Memory and Superelastic Technology Conference and Exposition (SMST2019) held May 13–17, 2019 at The Bodensee Forum in Konstanz, Germany, and has been expanded from the original presentation.

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Abstract

The processability of Ni-rich Ni50.8Ti29.2Hf20 and Ti–rich Ni49.8Ti30.2Hf20 (at.%) high temperature shape memory alloy wires was investigated. Hot-extruded NiTiHf rods with an initial diameter of 6.35 mm, were hot-rolled (HR) at 750, 775, 800, or 825 °C to a square cross section with ~ 1.9 mm side lengths, over a series of 33 non-consecutive hot passes. The thermomechanical behavior of HR, solutionized, and aged NiTiHf wires was examined with thermal cycling using differential scanning calorimetry and tensile testing to assess the viability of the hot-rolling process. The ideal rolling temperature for Ni-rich NiTiHf wires lies just over the H-phase dissolution temperature (near 800 °C), to avoid wire embrittlement caused by H-phase precipitates and to limit compositional instability at the wire surface caused by HfO2 internal oxidation. The characteristic oxide layers formed during the rolling process and subsequent heat treatments were compared across rolling temperatures and compositions to address several challenges associated with NiTiHf wire processing.

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