More Like this

1. Orientation Relationships in FeMnNiAl Governed by Martensitic Substructure

   Mohammed, Ahmed; Sidharth, Ravi; Abuzaid, Wael; Akamine, Hiroshi; Nishida, Minoru; Sehitoglu, Huseyin (August 2023, Shape Memory and Superelasticity)

   This study establishes the Orientation Relationship (OR) between the austenitic and martensitic phases of the new Shape Memory Alloy (SMA) FeMnNiAl from both experiments and analytical modeling. Through Transmission Electron Microscopy (TEM) and Electron Back-Scatter Difraction, three distinct ORs, namely the Nishiyama-Wassermann (N-W), Pitsch, and Kurdjumov–Sachs (K-S) ORs are established. The observations of non-unique ORs are explained using the energy-minimization theory of martensite revealing dependence of OR on the internal morphology of the martensitic phase, whether twinned or stackingfaulted. It is shown that the twin-variants of an internally twinned martensitic structure individually explain the Pitsch and K-S ORs. The N-W OR was observed in a stackingfaulted substructure of martensite. Through a novel extension to the energy-minimization theory for stacking-faulted substructures, the N-W OR is explained. Thus, the current study challenges the notion of OR as a material-characteristic and reveals a dependence of the OR on the internal substructure of the martensitic phase in SMAs, further establishing the OR for the new SMA FeMnNiAl. 

   more » « less
2. Nontrivial nanostructure, stress relaxation mechanisms, and crystallography for pressure-induced Si-I → Si-II phase transformation

   https://doi.org/10.1038/s41467-022-28604-1  

   Chen, Hao; Levitas, Valery I.; Popov, Dmitry; Velisavljevic, Nenad (February 2022, Nature Communications)

   Abstract Crystallographic theory based on energy minimization suggests austenite-twinned martensite interfaces with specific orientation, which are confirmed experimentally for various materials. Pressure-induced phase transformation (PT) from semiconducting Si-I to metallic Si-II, due to very large and anisotropic transformation strain, may challenge this theory. Here, unexpected nanostructure evolution during Si-I → Si-II PT is revealed by combining molecular dynamics (MD), crystallographic theory, generalized for strained crystals, and in situ real-time Laue X-ray diffraction (XRD). Twinned Si-II, consisting of two martensitic variants, and unexpected nanobands, consisting of alternating strongly deformed and rotated residual Si-I and third variant of Si-II, form$$\{111\}$$ $\left\{111\right\}$interface with Si-I and produce almost self-accommodated nanostructure despite the large transformation volumetric strain of$$-0.237$$ $-0.237$. The interfacial bands arrest the$$\{111\}$$ $\left\{111\right\}$interfaces, leading to repeating nucleation-growth-arrest process and to growth by propagating$$\{110\}$$ $\left\{110\right\}$interface, which (as well as$$\{111\}$$ $\left\{111\right\}$interface) do not appear in traditional crystallographic theory. 

   more » « less
3. An exact formulation for exponential-logarithmic transformation stretches in a multiphase phase field approach to martensitic transformations

   https://doi.org/10.1177/1081286520905352  

   Basak, Anup; Levitas, Valery_I (February 2020, Mathematics and Mechanics of Solids)

   A general theoretical and computational procedure for dealing with an exponential-logarithmic kinematic model for transformation stretch tensor in a multiphase phase field approach to stress- and temperature-induced martensitic transformations with N martensitic variants is developed for transformations between all possible crystal lattices. This kinematic model, where the natural logarithm of transformation stretch tensor is a linear combination of natural logarithm of the Bain tensors, yields isochoric variant–variant transformations for the entire transformation path. Such a condition is plausible and cannot be satisfied by the widely used kinematic model where the transformation stretch tensor is linear in Bain tensors. Earlier general multiphase phase field studies can handle commutative Bain tensors only. In the present treatment, the exact expressions for the first and second derivatives of the transformation stretch tensor with respect to the order parameters are obtained. Using these relations, the transformation work for austenite ↔ martensite and variant ↔ variant transformations is analyzed and the thermodynamic instability criteria for all homogeneous phases are expressed explicitly. The finite element procedure with an emphasis on the derivation of the tangent matrix for the phase field equations, which involves second derivatives of the transformation deformation gradients with respect to the order parameters, is developed. Change in anisotropic elastic properties during austenite–martensitic variants and variant–variant transformations is taken into account. The numerical results exhibiting twinned microstructures for cubic to orthorhombic and cubic to monoclinic-I transformations are presented. 

   more » « less
4. Origins of functional fatigue and reversible transformation of precipitates in NiTi shape memory alloy

   https://doi.org/10.1016/j.actamat.2024.119990  

   Sidharth, R; Celebi, TB; Sehitoglu, H (August 2024, Acta Materialia)

   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
5. Phase-field description of fracture in NiTi single crystals

   https://doi.org/10.1016/j.cma.2023.116677  

   Kavvadias, D.; Baxevanis, Th. (February 2024, Computer Methods in Applied Mechanics and Engineering)

   A phase-field model for thermomechanically-induced fracture in NiTi at the single crystal level, i.e., fracture under loading paths that may take advantage of either of the functional properties of NiTi–superelasticity or shape memory effect–, is presented, formulated within the kinematically linear regime. The model accounts for reversible phase transformation from austenite to martensite habit plane variants and plastic deformation in the austenite phase. Transformation-induced plastic deformation is viewed as a mechanism for accommodation of the local deformation incompatibility at the austenite–martensite interfaces and is accounted for by introducing an interaction term in the free energy derived based on the Mori–Tanaka and Kröner micromechanical assumptions and the hypothesis of martensite instantaneous growth within austenite. Based on experimental observations suggesting that NiTi fractures in a stress-controlled manner, damage is assumed to be driven by the elastic energy, i.e., phase transformation and plastic deformation are assumed to contribute in crack formation and growth indirectly through stress redistribution. The model is restricted to quasistatic mechanical loading (no latent heat effects), thermal loading sufficiently slow with respect to the time rate of heat transfer by conduction (no thermal gradients), and a temperature range below 𝑀𝑑, which is the temperature above which the austenite phase is stable, i.e., stress-induced martensitic transformation is suppressed. The numerical implementation of the model is based on an efficient scheme of viscous regularization in both phase transformation and plastic deformation, an explicit numerical integration via a tangent modulus method, and a staggered scheme for the coupling of the unknown fields. The model is shown able to capture transformation-induced toughening, i.e., stable crack advance attributed to the shielding effect of inelastic deformation left in the wake of the growing crack under nominal isothermal loading, actuation-induced fracture under a constant bias load, and crystallographic dependence on crack pattern. 

   more » « less