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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. 

The Type-II Twin Boundary (TB) is a critical interface in functional materials whose irrational Miller-index identity has recently drawn significant research interest. This study establishes general structural characteristics of the Type-II twin interface, utilizing TBs in Shape Memory Alloys (SMAs) - TiPd, TiPt, and AuCd - as study targets. It is shown how the irrational identity of each TB is explained by the Terrace-Disconnection (T-D) structural topology. It is proposed that the terrace is the rational-plane nearest to the irrational TB in the reciprocal space, having in- tegral Miller-indices of least magnitude. Crystallographic-registry on this terrace requires non- trivial coherence-strains. A novel kinematic-origin of the coherence-strain is proposed, coming directly from a transformation of the classical twinning deformation-gradient. This trans- formation revealed that the classical twinning-shear partitions into the coherence-strain and a new metric termed the “terrace-shear” . It is shown that the magnitude of shear relating the twin- structure to the matrix is the terrace-shear and not the twinning-shear, contrary to classical un- derstanding. Furthermore, the Burgers vector of the twinning disconnection is shown to be related directly to the terrace-shear. The energy of each Type-II interface is determined from ab initio Density Functional Theory (DFT) calculations. It is shown that the energy-minimal atomic- structure on the terrace requires determination of a “lattice-offset” that is non-trivial and un- known apriori. In summary, this study expounds on T-D topological structure of Type-II twin interfaces, establishing methods to identify rational terraces, coherence strains, ab initio planar TB energies and revealing a unique partitioning of the twinning-shear exhibited by this class of interfaces. 

Short-range order (SRO) has a crucial impact on the mechanical strength of metallic alloys. Recent atomistic investigations defined an average SRO and attempted to correlate it with the yield strength. We propose that the local change in SRO upon slip advance must dictate the strengthening, and we elaborate the methodology to establish the “SRO change” on a slip plane considering the Wigner-Seitz cell. The model captures the variation of lattice resistance (Critical Resolved Shear Stress; CRSS) in the crystal as the SRO changes depending on the probability of neighboring atoms. The methodology was applied to Ni-V binary alloys for a wide range of compositions and stacking fault widths. The complex variation of CRSS with compositional variations shows good agreement with limited experimental results. 

This paper shows a derivation of stacking fault width for negative stacking fault energy in medium entropy alloys. 

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. 

This article is mainly concerned with summarizing the recent discoveries in the deformation physics of shape memory materials with special emphasis on (1) the need to develop fundamental understanding of twinning, slip, shear/shuffle at atomic scales, martensite twin boundary topologies, atomic stacking and stable/metastable fault structures in shape memory materials, and (2) factors that result in performance degradation and accumulation of residual strains leading to fatigue and fracture. The fundamental understanding has benefitted from ab-initio modeling of atomic-electronic structures and the fault energetics of stable/metastable crystal structures. Especially, the slip phenomenon during nucleation of martensite needs further elaboration, as it exercises a strong influence on hysteresis, functional and mechanical degradation of the shape memory alloys Finally, we discourse on studies of fatigue and fracture of shape memory alloys from the literature and outline efforts to explain the complex experimental trends with respect to fatigue thresholds and nucleation. It is in the fatigue area where advancing the understanding of cycle by cycle accumulation of the irreversibilities, the strong orientation dependence of slip resistance (i.e., non-Schmid behavior), complex evolution of elastic moduli, strain sensitive evolution of interfaces with terrace-disconnection energy minimal nanostructures, and asymmetric stress-strain response will lead to the development of a comprehensive model. In the case of fracture, continuum models employ LEFM concepts and the results need corrections for martensite-induced tractions which are rather complex with localized variants. Overall, deeper scientific activities, with potential use of lattice scale theories and ab-initio based empirical atomic potentials, are paramount to advance the field to the next level. 

Critical Resolved Shear Stress (CRSS) determination is the focus of this paper.