Understanding deformation and cyclic behavior of shape memory ceramics: a quantitative phase-field study
Lotfolahpour, Amirreza
Lotfolahpour, Amirreza
Citations
Altmetric:
Editor
Date
Date Issued
2024
Date Submitted
Collections
Research Projects
Organizational Units
Journal Issue
Embargo Expires
Abstract
Zirconia-based shape memory ceramics (SMCs) are a class of intelligent materials that can be utilized in industries such as aerospace and biomedical engineering for their remarkable superelasticity (SE) and shape memory effect (SME). These ceramics are brittle and unable to fully accommodate the large shape change due to martensitic phase transformation (MPT) and this leads to their low fracture toughness and short cyclic life.
In this Ph.D. research, we aimed to develop a reliable computational framework to study the complex interactions between phase transformation, microstructural features, fracture, and plasticity in order to design SMCs with a higher fatigue life. For this purpose, first, we proposed a modified phase-field (PF) fracture model to study cracking in brittle materials at microscopic domains. Unlike traditional models, this modified model incorporates the influence of both fracture strength and cleavage plane effects simultaneously. As a result, it accurately reproduces the mechanical response and crack propagation path and reveals that intergranular fracture is the dominant type of cracking in ceramics.
Additionally, to investigate the interaction between cracking and MPT in SE SMCs, an advanced PF-based model was developed. Unlike previous PF-MPT models, this model successfully predicts the correct elastic response in the stress-strain curve. This improvement is attributed to the proposed modified chemical free energy formulation. This is the first PF model used for simulating cracking in SE regime and accurately predicts reverse MPT behind the crack tip, a phenomenon attributed to SE regime. In addition, the model captures the effects of grain orientation and predicts a final stress drop in the stress-strain curve.
Furthermore, to explore the influence of different microstructural features on the cyclic life of SMCs prior to fatigue crack initiation, the modified chemical free energy model was integrated with a viscoplasticity model. The plastic strain accumulation (PSA) was used as a cyclic life indicator, and we aimed to lower PSA by microstructure tailoring. Simulations revealed that by controlling grain orientations, lowering the GB density, or locating pores at GBs, the PSA decreases significantly.
Finally, a new predictive numerical framework was developed incorporating the PF fracture, PF-MPT, and crystal viscoplasticity to study crystal-orientation dependent SE and SME behaviors of 3D micropillars. Through validation against experimental data, the proposed framework demonstrated the ability to accurately predict the intricate interplay between MPT, cracking, and plasticity. Our investigation revealed a broad spectrum of crystal orientations in which these ceramics undergo a complete MPT cycle without experiencing fracturing or slipping. However, we also identified certain orientations where either fracturing or slipping emerges as the dominant mechanism, with little to no observable MPT.
The findings of this Ph.D. research provided valuable insights into the crystal-orientation dependent mechanical properties of SMCs and strategies for enhancing their cyclic life, thus possibly enabling their practical applications.
Associated Publications
Rights
Copyright of the original work is retained by the author.