Investigation of fatigue degradation mechanisms in shape memory alloys: A multiscale experimental approach

Ravi, Sidharth

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https://hdl.handle.net/2142/120485 Copy

Description

Title

Investigation of fatigue degradation mechanisms in shape memory alloys: A multiscale experimental approach

Author(s)

Ravi, Sidharth

Issue Date

2023-01-30

Director of Research (if dissertation) or Advisor (if thesis)

Sehitoglu, Huseyin

Doctoral Committee Chair(s)

Sehitoglu, Huseyin

Committee Member(s)

• Bellon, Pascal
• Ertekin, Elif
• Krogstad, Jessica

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Shape Memory Alloys
• Fatigue
• Austenite
• Martensite
• Superelasticity
• Transformation Induced Plasticity
• Crack Growth
• Functional Fatigue
• Structural Fatigue

Abstract

Martensitic phase transformations have been of interest to researchers for more than a century. These phase transformations assert a ubiquitous presence in nature and are found in metallic, ceramic and even biological systems aiding life functions. Reversible stress-induced austenite to martensite phase transformations found in shape memory alloys (SMAs) bear unique functional properties like recoverability of large deformations and elastocaloric cooling enabling them to be utilized in earthquake resistant structures, solid state refrigerants, medical stents, orthopedic implants, micro-valves, micro-pumps, MEMS devices, to name a few. However, like any other material, SMAs degrade under repeated (fatigue) usage. This thesis is aimed at uncovering fatigue degradation mechanisms using NiTi and FeMnAlNi SMA systems by employing millimeter scale crack growth studies to nanometer scale insitu TEM studies. Firstly, we establish the relationship between functional fatigue degradation of functional properties) and structural fatigue (material failure via cracking) utilizing superelastic FeMnAlNi and FeMnAlNiTi SMAs. With insitu deformation mapping via digital image correlation and exsitu electron microscopy, we show that, in notched samples of FeMnAlNi cycled under stress control, crack growth occurs along the preferred martensitc variants upon exhaustion of functionality i.e., recoverable strains near the notch. During crack growth, the martensitic variants near the crack tip led to local crack deflection imparting resistance however increased the crack tip stress-intensity which is detrimental. Strain control fatigue tests in FeMnAlNiTi revealed crack nucleation (structural fatigue) along the austenite-martensite interface that doesn’t recover (functional fatigue) upon fatigue loading thereby making it the weakest link in the microstructure. In the second part, by comparing NiTi, and FeMnAlNi SMA, we show that NiTi is highly rate sensitive due to large latent heat of transformation. At increasing loading frequencies in the range of 0.01Hz to 10 Hz, excessive self-heating owing to latent heat release during the stress induced martensitic transformation dynamically modifies the stress levels during loading/unloading and results in a reduction in superelastic hysteresis for NiTi. Due to the consequent reduction in transformation induced defects, functional fatigue degradation is also minimized at increasing loading frequencies. By utilizing exsitu transmission electron microscopy (TEM), we characterize the transformation induced defects as parallel dislocations, possibly originating from the internal twins of the stress induced martensite. Moreover, these dislocations seem to interact and penetrate the Ni4Ti3 nano-precipitates present in the microstructure. We also demonstrate that these defects act to modify the functional characteristics like the transformation stress, hysteresis and elastocaloric cooling capacity upon continued cycling. The third part of this thesis is concerned with discovering the origins of functional fatigue. For this we employ insitu TEM deformation experiments on nanoscale tensile samples of NiTi to observe defect generation in real time. We observe dynamic aspects of martensite nucleation, growth and shrinkage. Ni4Ti3 precipitate tips and interfaces act as nucleation points but also hinder the mobility of the martensite interface. We show that internal twins of the martensite convert to parallel dislocations in the austenite as the martensite shrinks during unloading and could potentially pin the interface thereby making it sessile. Martensite then nucleates from these parallel dislocations in the subsequent cycles explaining the reduction in transformation stress due to the reduction in nucleation barrier. Depending on the loading orientation, martensite was observed to nucleate and move along the interface of the precipitate and even shear into it leading to transformation of the precipitate that reverses upon unloading. Finally, fracture was observed to occur along the austenite-martensite interface pinned by interfacial dislocations that originate from the internal twins of the martensite hence clarifying the mechanisms behind functional fatigue and structural fatigue in SMAs. Finally, preliminary results on tailoring superelastic properties and improving the functional fatigue resistance of FeMnAlNiTi SMAs with a BCC disordered austenite matrix with DO3 ordered nanoprecipitates are presented. The compressive mechanical response of FeMnAlNiTi single crystals heated at 200°C for 0.5 hours to 200 hours were probed. It is shown that ultrahigh transformation stress, with a high elastic limit, can be attained with simultaneous reduction in the superelastic hysteresis and a consequent improvement in functional fatigue resistance. Preliminary microstructural investigation by conventional and atomic resolution TEM of samples heated up to 24 hours show no change in the lattice constant, volume fraction, size or composition of the DO3 nano-precipitates. However, additional diffraction spots and moire contrast that disappear upon heating were observed, suggesting local modulation due to short range clustering of atoms which evolves with heating. Modulated domain sizes are <1nm. Therefore, it is hypothesized that low temperature annealing modifies the atomic stacking sequence and improves the degree of ordering. Future studies are proposed to evaluate this hypothesis.

Graduation Semester

2023-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Sidharth Ravi

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