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THERMO-MECHANICAL CHARACTERIZATION OF HIGH-TEMPERATURE SHAPE MEMORY NI-TI-PD WIRES

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Date Issued:
2009
Abstract/Description:
Actuator applications of shape memory alloys have typically been limited by their phase transformation temperatures to around 100 degrees C. However, recently with a focus on aerospace and turbomachinery applications there have been successful efforts to increase the phase transformation temperatures. Several of these alloy development efforts have involved ternary and quaternary elemental additions (e.g., Pt, Pd, etc.) to binary NiTi alloys. Experimentally assessing the effects of varying composition and thermo-mechanical processing parameters can be cost intensive, especially when expensive, high-purity elemental additions are involved. Thus, in order to save on development costs there is value in establishing a methodology that facilitates the fabrication, processing and testing of smaller specimens, rather than larger specimens from commercial billets. With the objective of establishing such a methodology, this work compares thermo-mechanical test results from bulk dog-bone tensile Ni29.5Ti50.5Pd20 samples (7.62 mm diameter) with that of thin wires (100 μm-150 µm diameter) extracted from comparable, untested bulk samples by wire electrical-discharge machining (EDM). The wires were subsequently electropolished to different cross-sections, characterized with Scanning Electron Microscopy, Transmission Electron Microscopy and Energy Dispersive X-Ray Spectroscopy to verify the removal of the heat affected zone following EDM and subjected to Laser Scanning Confocal Microscopy to accurately determine their cross-sections before thermo-mechanical testing. Stress-strain and load-bias experiments were then performed on these wires using a dynamic mechanical analyzer and compared with results established in previous studies for comparable bulk tensile specimens. On comparing the results from a bulk tensile sample with that of the micron-scale wires, the overall thermomechanical trends were accurately captured by the micron-scale wires for both the constrained recovery and monotonic tensile tests. Specifically, there was good agreement between the stress-strain response in both the martensitic and austenitic phases, the transformation strains at lower stresses in constrained recovery, and the transformation temperatures at higher stresses in constrained recovery. This work thus validated that carefully prepared micron-diameter samples can be used to obtain representative bulk thermo-mechanical properties, and is useful for fabricating and optimizing composition and thermo-mechanical processing parameters in prototype button melts prior to commercial production. This work additionally assesses potential applications of high temperature shape memory alloy actuator seals in turbomachinery. A concept for a shape memory alloy turbine labyrinth seal is also presented. Funding support from NASA's Fundamental Aeronautics Program, Supersonics Project (NNX08AB51A) and Siemens Energy is acknowledged.
Title: THERMO-MECHANICAL CHARACTERIZATION OF HIGH-TEMPERATURE SHAPE MEMORY NI-TI-PD WIRES.
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Name(s): Fox, Matthew, Author
Vaidyanathan, Rajan, Committee Chair
University of Central Florida, Degree Grantor
Type of Resource: text
Date Issued: 2009
Publisher: University of Central Florida
Language(s): English
Abstract/Description: Actuator applications of shape memory alloys have typically been limited by their phase transformation temperatures to around 100 degrees C. However, recently with a focus on aerospace and turbomachinery applications there have been successful efforts to increase the phase transformation temperatures. Several of these alloy development efforts have involved ternary and quaternary elemental additions (e.g., Pt, Pd, etc.) to binary NiTi alloys. Experimentally assessing the effects of varying composition and thermo-mechanical processing parameters can be cost intensive, especially when expensive, high-purity elemental additions are involved. Thus, in order to save on development costs there is value in establishing a methodology that facilitates the fabrication, processing and testing of smaller specimens, rather than larger specimens from commercial billets. With the objective of establishing such a methodology, this work compares thermo-mechanical test results from bulk dog-bone tensile Ni29.5Ti50.5Pd20 samples (7.62 mm diameter) with that of thin wires (100 μm-150 µm diameter) extracted from comparable, untested bulk samples by wire electrical-discharge machining (EDM). The wires were subsequently electropolished to different cross-sections, characterized with Scanning Electron Microscopy, Transmission Electron Microscopy and Energy Dispersive X-Ray Spectroscopy to verify the removal of the heat affected zone following EDM and subjected to Laser Scanning Confocal Microscopy to accurately determine their cross-sections before thermo-mechanical testing. Stress-strain and load-bias experiments were then performed on these wires using a dynamic mechanical analyzer and compared with results established in previous studies for comparable bulk tensile specimens. On comparing the results from a bulk tensile sample with that of the micron-scale wires, the overall thermomechanical trends were accurately captured by the micron-scale wires for both the constrained recovery and monotonic tensile tests. Specifically, there was good agreement between the stress-strain response in both the martensitic and austenitic phases, the transformation strains at lower stresses in constrained recovery, and the transformation temperatures at higher stresses in constrained recovery. This work thus validated that carefully prepared micron-diameter samples can be used to obtain representative bulk thermo-mechanical properties, and is useful for fabricating and optimizing composition and thermo-mechanical processing parameters in prototype button melts prior to commercial production. This work additionally assesses potential applications of high temperature shape memory alloy actuator seals in turbomachinery. A concept for a shape memory alloy turbine labyrinth seal is also presented. Funding support from NASA's Fundamental Aeronautics Program, Supersonics Project (NNX08AB51A) and Siemens Energy is acknowledged.
Identifier: CFE0002813 (IID), ucf:48102 (fedora)
Note(s): 2009-08-01
M.S.M.S.E.
Engineering and Computer Science, Department of Mechanical Materials and Aerospace Engineering
Masters
This record was generated from author submitted information.
Subject(s): NiTiPd
DMA
Characterization
Size-Scale Effect
Turbomachinery
Persistent Link to This Record: http://purl.flvc.org/ucf/fd/CFE0002813
Restrictions on Access: private 2012-07-01
Host Institution: UCF

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