Additively Manufactured NiTi and NiTiHf Alloys: Estimating Service Life in High-Temperature Oxidation.

Hediyeh Dabbaghi,Parisa Bayati,Keyvan Safaei,Mohammadreza Nematollahi,Mohammad Elahinia

doi:10.3390/ma13092104

Abstract

In this study, the effect of the addition of Hf on the oxidation behavior of NiTi alloy, which was processed using additive manufacturing and casting, is studied. Thermogravimetric analyses (TGA) were performed at the temperature of 500, 800, and 900 °C to assess the isothermal and dynamic oxidation behavior of the Ni50.4Ti29.6Hf20 at.% alloys for 75 h in dry air. After oxidation, X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were used to analyze the oxide scale formed on the surface of the samples during the high-temperature oxidation. Two stages of oxidation were observed for the NiTiHf samples, an increasing oxidation rate during the early stage of oxidation followed by a lower oxidation rate after approximately 10 h. The isothermal oxidation curves were well matched with a logarithmic rate law in the initial stage and then by parabolic rate law for the next stage. The formation of multi-layered oxide was observed for NiTiHf, which consists of Ti oxide, Hf oxide, and NiTiO3. For the binary alloys, results show that by increasing the temperature, the oxidation rate increased significantly and fitted with parabolic rate law. Activation energy of 175.25 kJ/mol for additively manufactured (AM) NiTi and 60.634 kJ/mol for AM NiTiHf was obtained.

Highlights

NiTi is a shape memory alloy (SMA) with unique properties, such as biocompatibility, wear, and corrosion resistance, low modulus of elasticity, and high actuation work output [1,2,3,4]
For the first time, we have evaluated the oxidation kinetics of additively manufactured (AM) NiTi and NiTiHf alloys and compared them with conventionally (CON) made Ni-rich NiTiHf
These ingots were from the same pool that were atomized to make the powder for the selective laser melting (SLM) fabricated samples

Summary

Introduction

NiTi is a shape memory alloy (SMA) with unique properties, such as biocompatibility, wear, and corrosion resistance, low modulus of elasticity, and high actuation work output [1,2,3,4]. These characteristics make NiTi useful for functional and smart structures in primary areas of biomedical and aerospace applications. Adding the third element to NiTi is an approach for changing the transformation temperatures (TTs). Hf, Pd, and Au are some of the candidates added to NiTi for altering the TTs and other thermomechanical behaviors

Methods

Results

Conclusion