An official website of the United States government
The work clarifies several key questions in shape memory research that have eluded previous studies. The f indings show that dislocation slip emanates at austenite-martensite interfaces during unloading and aligns with the internal twin boundary interface of martensite. It was observed that the type II internal twins of the martensite become parallel dislocations in the austenite. During reloading, these dislocations act as nucleation sites for the martensitic twins, reducing the nucleation barrier and the transformation stress. The precipitates facilitate martensite nucleation but also act as an obstacle to martensite front motion, restrict detwinning, and pin the interfacial dislocations during unloading, thereby contributing to residual strains and martensite stabilization. Martensite nucleation is not suppressed by the size of the thin film, which is of the order of 85 to 105 nanometers thick, and repeated transformation occurred cycle after cycle. Single crystals deformed in the <101> LD exhibited the best recoverability of up to 5.5 % and tensile stresses of up to 1.4 GPa. It was demonstrated for the first time that, when favorably oriented, Ni 4 Ti 3 precipitates undergo a reversible phase transformation to R-phase and can accommodate up to 4 % reversible strains.
more »
« less
Thermodynamic rationale for transformation-induced dislocations in shape memory alloys
Shape Memory Alloys (SMAs) undergo stress-induced martensitic phase-transformation affording a “superelastic” behavior with functional applications. Such behavior is undermined by the formation and accumulation of irreversible residual strains in each cycle. These residual strains arise from transformation-induced dislocation- emission and current understanding has postulated the microstructural role of emitted dislocations to accommodate lattice-mismatch while also observing a preference to occur during reverse-transformation. This study develops a thermodynamic framework to offer a causal explanation for dislocation-emission from Gibbs’ free energy considerations. Superelastic stress-strain curves for a reversible pathway without emitted-dislocations and for an irreversible pathway with emitted-dislocations are derived. The role of emitted-strain in relaxing the transformation-strain of the martensitic-inclusion and in accruing residual strain is proposed. It is shown that both pathways obey the first law of thermodynamics but it is the second law of thermodynamics that dictates the path preference. It is shown that the irreversible path achieves the critical condition for spontaneous reverse- transformation at a higher stress-level than the reversible path. Thus, the irreversible pathway initiates earlier during unloading and is thermodynamically selected during reverse-transformation. The driving-forces associated with the irreversible pathway are analyzed to establish the cause of emitted-dislocations and why it is thermodynamically preferred despite offering a higher lattice-friction barrier. Consequently, a new approach to target fatigue-resistant SMA properties is offered, focusing on the interplay of the individual driving-forces coming from the elastic strain-energy, work-interaction, and lattice-friction, as revealed by the thermodynamic framework.
more »
« less
- Award ID(s):
- 2104971
- PAR ID:
- 10587443
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Acta Materialia
- Volume:
- 274
- Issue:
- C
- ISSN:
- 1359-6454
- Page Range / eLocation ID:
- 119998
- Subject(s) / Keyword(s):
- Shape memory alloys Phase transformations Micromechanics Martensitic transformations
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The newly developed FeMnAlNiTi shape memory alloy (SMA) holds significant promise due to its desirable properties including ease of processing, room temperature superelasticity, a wide superelastic window of operation, and high transformation stress levels. In this study, we report single crystals with tensile axis near h123i exhibiting transformation strains of 9% with a high trans- formation stress of 700 MPa. The functional performance revealed excellent recovery of 98% of the applied strain in an incremental strain test for each of the 40 applied cycles. Concomitantly, the total residual strain increased after each cycle. Accumulation of residual martensite is observed possibly due to pinning of austenite/martensite (A/M) interface. Subsequently, under structural fatigue loading with a constant strain amplitude of 1%, the recoverable strains saturate around 1.15% in local residual martensite domains. Intermittent enhancement of recoverable strains is observed due to transformation triggered in previously untransformed domains. Eventually, fatigue failure occur- red after 2046 cycles and the dominant mechanism for failure was microcrack initiation and coalescence along the A/M interface. Thus, it is concluded that interfacial dislo- cations, which play a crucial role in the superelastic (SE) functionality, invariably affect the structural fatigue per- formance by acting as the weakest link in the microstructure.more » « less
-
Abstract BackgroundSuperelastic shape memory alloys (SMAs) such as nickel-titanium, also known as Nitinol, recover large deformations via a reversible, stress-induced martensitic transformation. ObjectivePartitioning the deformation into the contributions from superelasticity and plasticity and quantifying the interaction between these mechanisms is key to modeling their fatigue behavior. MethodsWe capture these microscopic interactions across many grains using a combination of scanning electron microscopy digital image correlation (SEM-DIC) and electron backscatter diffraction (EBSD). Modeling our data as a statistical distribution, we employ a Gaussian Mixture Model (GMM) soft clustering framework to understand how these mechanisms interact and evolve as a function of global strain. ResultsOur findings show that, under globally-applied uniaxial tensile loading, plasticity bridges deformation in regions where competing positive and negative martensitic shear bands intersect. Early stage transformation-induced plasticity is concentrated at these intersections and forms concurrently with the Lüders-like martensitic transformation front, often appearing with a zig-zag pattern that is linked to compound twinning at the martensite-martensite interface. At higher strains, austenite slip is activated as a second mechanism of plastic deformation. ConclusionsWe propose that this plastic bridging mechanism underpins the prestrain effects previously reported in the literature, where higher prestrains can enhance the fatigue strength of superelastic materials within a given loading mode.more » « less
-
Abstract Magma ascent and eruption are driven by a set of internally and externally generated stresses that act upon the magma. We present microstructural maps around melt inclusions in quartz crystals from six large rhyolitic eruptions using synchrotron Laue X-ray microdiffraction to quantify elastic residual strain and stress. We measure plastic strain using average diffraction peak width and lattice misorientation, highlighting dislocations and subgrain boundaries. Quartz crystals across studied magma systems preserve similar and relatively small magnitudes of elastic residual stress (mean 53–135 MPa, median 46–116 MPa) in comparison to the strength of quartz (~ 10 GPa). However, the distribution of strain in the lattice around inclusions varies between samples. We hypothesize that dislocation and twin systems may be established during compaction of crystal-rich magma, which affects the magnitude and distribution of preserved elastic strains. Given the lack of stress-free haloes around faceted inclusions, we conclude that most residual strain and stress was imparted after inclusion faceting. Fragmentation may be one of the final strain events that superimposes stresses of ~ 100 MPa across all studied crystals. Overall, volcanic quartz crystals preserve complex, overprinted deformation textures indicating that quartz crystals have prolonged deformation histories throughout storage, fragmentation, and eruption.more » « less
-
A thermodynamically consistent multiphase phase-field approach for stress and temperature-induced martensitic phase transformation at the nanoscale and under large strains is developed. A total of N independent order parameters are considered for materials with N variants, where one of the order parameters describes A ↔ M transformations and the remaining N − 1 independent order parameters describe the transformations between the variants. A non-contradictory gradient energy is used within the free energy of the system to account for the energies of the interfaces. In addition, a non-contradictory kinetic relationships for the rate of the order parameters versus thermodynamic driving forces is suggested. As a result, a system of consistent coupled Ginzburg-Landau equations for the order parameters are derived. The crystallographic solution for twins within twins is presented for the cubic to tetragonal transformations. A 3D complex twins within twins microstructure is simulated using the developed phase-field approach and a large-strain-based nonlinear finite element method. A comparative study between the crystallographic solution and the simulation result is presented.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.actamat.2024.119998
