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" Oxidation Analysis of Additive Manufacturing Shape Memory Alloys "
Dabbaghi, Hediyeh
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Latin Dissertation
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| Language of Document
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English
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| Record Number
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1112076
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TLpq2515703138
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| Main Entry
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Dabbaghi, Hediyeh
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| Title & Author
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Oxidation Analysis of Additive Manufacturing Shape Memory Alloys\ Dabbaghi, Hediyeh
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| College
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The University of Toledo
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| Date
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2020
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| student score
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2020
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| Degree
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M.S.
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| Page No
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118
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| Abstract
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Shape memory alloys (SMAs) are considered as interesting engineering smart materials due to their ability to memorize shapes through a thermally induced solid-state phase transition. NiTi-base shape memory alloys are the most prevalent SMAs due to their unique shape memory effect (SME), superelasticity (SE), and damping capacity (DC). The unique properties of SMAs have widely used in the applications with operating temperatures below 100 °C. Since this commercially available NiTi alloy cannot be utilized at a temperature higher than 100 °C, many scientists have paid more attention to the development of a new shape memory material with higher strength and transformation temperatures (TTs). These materials can operate at temperatures above 100°C due to the demand for a broad range of applications from automotive, aerospace, oil, and many other industries. This demand led to the emergence of a new shape-memory alloying system as a High-Temperature Shape Memory Alloys (HTSMAs). HTSMAs have been widely investigated not only for their high transformation temperature but also for good cycling stability, creep, and plastic deformation resistance. In recent years, HTSMAs have been introduced via ternary elemental additions to the base alloy. Alloying is one of the most powerful methods to enhance the properties of NiTi alloys. Addition of a third element can affect the phase transformation behavior, the mechanical properties and oxidation resistance of binary NiTi alloys. Among the potential HTSMAs, NiTiHf alloys have received considerable attention due to its low cost and emerged as potential materials for applications requiring high transformation temperatures (> 100 °C) with high strength and work output. Even though there is a high demand in NiTi and NiTiHf alloys, two main challenges are remaining; the inability to produce complex components and the lack of repeatable processes to provide the desired thermomechanical properties. In recent year, additive manufacturing techniques (AM) have been widely used to fabricate complex components without any tooling and pave the way to tailor the microstructure and critical properties of the alloys by choosing the appropriate processing parameters. Selective laser melting (SLM) is one of the AM processes for fabrication parts which has been utilized in this study. It is well known that SMAs usually have experienced hot rolling during high-temperature, which leads to the formation of high-temperature oxides on the surface. Oxidation causes loss of some elements that have effects on the TTs and shape memory effect. Therefore, the evaluation of oxidation characteristics and the interpretation that this oxidation has a beneficial or destructive effect on the alloys is an important issue. So far, many studies investigated the oxidation behavior of NiTi alloys at high temperatures, but there are a few numbers of research that evaluated the effect of additive elements on the oxidation kinetics which have fabricated conventionally, and just one study has reported the investigation of the oxidation kinetics of NiTiHf. No study has been reported the evaluation of the oxidation behavior of AM shape memory alloys. An understanding of the AM NiTiHf alloy oxidation and how it is affected by alloy characteristics and exposure conditions is demanded and should be evaluated. In this study, to analyze the oxidation kinetics of NiTiHf and NiTi, which are fabricated through SLM, a series of samples was built. The effect of Hf addition on the high-temperature oxidation kinetics and the structure of oxides that formed on the surface of AM NiTiHf alloy was evaluated to estimate the design life of this alloy in terms of its oxidation resistance as components used at elevated temperature. Thermogravimetric analyses (TGA) were performed at high temperature for different times in dry air. Furthermore, the oxidation behavior of conventional Ni50.4Ti29.6Hf20 and Ni50.8Ti49.2 was measured for comparison to see the differences between the oxidation resistance of samples that fabricated additively with those produced conventionally. After oxidation, the microstructure and chemical composition of oxide formed were analyzed by SEM/EDS. It is found that the addition of 20 at. % Hf decreases the oxidation resistance of NiTi alloys. The oxidation behavior of NiTi alloys is well fitted with parabolic rate law, while for NiTiHf alloys, the trend is different. In the early stage of oxidation, these alloys obey logarithmic rate law, and after a while, they follow parabolic rate law. The activation energy of NiTiHf alloys was lower than NiTi alloys, which proved that ternary alloy oxidized faster than binary alloys. It was observed that six main oxide layers formed during the oxidation. The Hf-rich oxide layer, which had the highest thickness, played a significant role in mass-gain and oxidation kinetic. The results of the study pave the way for estimating the service life of high-temperature actuators and components made of these functional alloys.
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Mechanical engineering
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