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This paper describes the main findings from an experimental investigation into local and overall strength and fracture behavior of a microstructurally flexible, quadruplex, high entropy alloy (HEA), Fe42Mn28Co10Cr15Si5 (in at%). The alloy consists of metastable face-centered cubic austenite (g), stable hexagonal epsilon martensite (ε), stable body-centered cubic ferrite (a), and stable tetragonal sigma (σ) phases. The overall behavior of the alloy in compression features a great deal of plasticity and strain hardening before fracture. While the contents of diffusion created a and σ phases remain constant during deformation, the fraction of ε increases at the expanse of g due to the diffusionless strain induced γ→ε phase transformation. High-throughput nanoindentation mapping is used to assess the mechanical hardness of individual phases contributing to the plasticity and hardening of the alloy. Increasing the fraction of the dislocated ε phase during deformation due to the transformation is found to act as a secondary source of hardening because g and ε exhibit similar hardness at a given strain level. While these two phases exhibit moderate hardening during plasticity, significant softening is observed in σ owing to the phase fragmentation. While the phase transformation mechanism facilitates accommodation of the plasticity, the primary source of strain hardening in the alloy is the refinement of the structure during the transformation inducing a dynamic Hall-Petch-type barrier effect. Results pertaining to the evolution of microstructure and local behavior of the alloy under compression are presented and discussed clarifying the origins of strain hardening. While good under compression, the alloy poorly behaves under tension. Fracture surfaces after tension feature brittle micromechanisms of fracture. Such behavior is attributed to the presence of the brittle σ phase.
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A gradient regularized model for shape memory alloys
In this work a transformation strain gradient enhancement is introduced into a phenomenological constitutive model for the pseudoelastic behavior of shape memory alloys. The constitutive model is able to capture several unique features of the constitutive response of these materials during the transformation between austenite and martensite during the pseudoelastic response. These features include the asymmetry in the initial transformation stresses in tension versus compression, the asymmetry in the transformation strains in tension and compression, and finally the asymmetry in the hardening behavior in tension and compression. In fact, experiments have shown that untrained NiTi exhibits hardening during its transformation in compression, but softening for tensile loading. It is this softening behavior that motivates the need for the introduction of the transformation strain gradient into the constitutive modeling. Transformation strain gradient effects are introduced via a phase variable that describes the extent of transformation. The free energy of the material then depends on gradients of the phase variable, which introduces a material length scale into the theory. The governing equation for the phase variable is developed from a microforce balance and continuum thermodynamics analysis. The model is implemented in the commercial finite element software Abaqus through user defined subroutines and several numerical simulations are performed to illustrate the model response and lack of numerical mesh-dependency of the results.
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- Award ID(s):
- 1762389
- PAR ID:
- 10479637
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Mechanics of Materials
- Volume:
- 183
- Issue:
- C
- ISSN:
- 0167-6636
- Page Range / eLocation ID:
- 104689
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.mechmat.2023.104689
