Shape Memory Ceramics Research Articles

Modeling martensitic transformation temperatures in Zirconia–Ceria solid solutions using machine learning interatomic potentials

Abstract Shape memory ceramics (SMCs), while exhibiting high strength, sizeable recoverable strain, and substantial energy damping, tend to shatter under load and have low reversibility. Recent developments in SMCs have shown significant promise in enhancing the reversibility of the shape memory phase transformation by tuning the lattice parameters and transformation temperatures through alloying. While first-principles methods, such as density functional theory (DFT), can predict the lattice parameters and enthalpy at zero Kelvin, calculating the transformation temperature from free energy at high temperatures is impractical. Empirical potentials can calculate transformation temperatures efficiently for large system sizes but lack compositional transferability. In this work, we develop a model to predict transformation temperatures and lattice parameters for the Zirconia–Ceria solid solutions. We construct a machine learning inter-atomic potential (MLIAP) using an initial dataset of DFT simulations, which is then iteratively expanded using active learning. We utilize reversible scaling to compute the free energy as a function of composition and temperature, from which the transformation temperatures are determined. These transformation temperatures match experimental trends and accurately predict the phase boundary. Finally, we compare other relevant design parameters (e.g. transformation volume change) to demonstrate the applicability of MLIAPs in designing SMCs.

Orientation-dependent deformation and failure of micropillar shape memory ceramics: A 3D phase-field study

Some microscopic samples of zirconia-based shape memory ceramics (SMCs) have shown full martensitic phase transformation (MPT) over multiple loading cycles without cracking. However, the occurrence of MPT is strongly influenced by grain orientation. Depending on the specific grain orientation relative to the loading direction, alternative mechanisms such as plastic slip and fracture may emerge. This study introduces a phase-field (PF) based framework that integrates a PF-MPT model, a PF fracture model, and a crystal viscoplasticity model to investigate the effects of grain orientation on MPT, plastic slip, and fracture mechanisms in SMC micropillars. Single crystal micropillars are created to distinguish the orientations that facilitate each mechanism. A wide range of grain orientations are found to predominantly exhibit MPT. Micropillars with grain orientations close to the (100) and (001) directions primarily experience fracture, with minimal plastic slip. Additionally, samples oriented along the (110) direction show a significant amount of plastic slip. A pole figure is constructed to elucidate the interplay between MPT, cracking, and plastic slip under compressive loading conditions. This research provides valuable insights into the intricate behavior of SMCs under different loading scenarios, crucial for optimizing their performance in practical applications.

Predictions of Lattice Parameters in NiTi High-Entropy Shape-Memory Alloys Using Different Machine Learning Models.

This work applied three machine learning (ML) models-linear regression (LR), random forest (RF), and support vector regression (SVR)-to predict the lattice parameters of the monoclinic B19' phase in two distinct training datasets: previously published ZrO2-based shape-memory ceramics (SMCs) and NiTi-based high-entropy shape-memory alloys (HESMAs). Our findings showed that LR provided the most accurate predictions for ac, am, bm, and cm in NiTi-based HESMAs, while RF excelled in computing βm for both datasets. SVR disclosed the largest deviation between the predicted and actual values of lattice parameters for both training datasets. A combination approach of RF and LR models enhanced the accuracy of predicting lattice parameters of martensitic phases in various shape-memory materials for stable high-temperature applications.

Design of Shape Memory Ceramics: Principles, Strategies and Perspectives

Design of Shape Memory Ceramics: Principles, Strategies and Perspectives

Localized plastic strain accumulation in shape memory ceramics under cyclic loading

The premature failure of shape memory ceramics (SMC)s under cyclic loading is a critical issue limiting their applications as actuators and thermal protection layers. Martensitic phase transformation (MPT), essential for superelasticity and shape memory functionalities in SMCs, induces localized plastic deformations due to phase expansion. In polycrystalline materials, the accumulation of localized plastic strain serves as the primary mechanism for fatigue crack initiation under cyclic loading. In this research, a phase-field (PF) phase transformation model coupled with a viscoplasticity model is presented to study the effects of microstructural features, engineered pores, and sample size on plastic strain accumulation (PSA) during cyclic loading of SMCs. Our findings highlight that the grain boundaries (GB)s are critical regions with high PSA, with noticeable reductions observed by decreasing the grain boundary density (GBD). Additionally, we found that engineered pores effectively reduced cyclic PSA, however, we identified a threshold on the volume fraction of pores. Also, textured microstructures with certain characteristics demonstrate significant influence on PSA. A cross-correlation data analysis approach is employed to study the relationship between the studied microstructural features and PSA to facilitate the identification of the most important factors controlling the irreversible PSA during compressive cyclic loading. By mitigating the rate of PSA, significant enhancements in cyclic life before fatigue crack initiation can be achieved, enabling superior performance in practical applications.

Realizing reversible phase transformation of shape memory ceramics constrained in aluminum

Small-scale shape memory ceramics exhibit superior shape memory or superelasticity properties, while their integration into a matrix material and the subsequent attainment of their reversible tetragonal-monoclinic phase transformations remains a challenge. Here, cerium-doped zirconia (CZ) reinforced aluminum (Al) matrix composite is fabricated, and both macroscopic and microscopic mechanical tests reveal more than doubled compressive strength and energy absorbance of the composites as compared with pure Al. Full austenitization in the CZ single-crystal clusters is achieved when they are constrained by the Al matrix, and reversible martensitic transformation triggered by thermal or stress stimuli is observed in the composite micro-pillars without causing fracture in the composite. These results are interpreted by the strong geometric confinement offered by the Al matrix, the robust CZ/Al interface and the local three-dimensional particle network/force-chain configuration that effectively transfer mechanical loads, and the decent flowability of the matrix that accommodates the volume change during phase transformation.

4D Additive–Subtractive Manufacturing of Shape Memory Ceramics (Adv. Mater. 39/2023)

4D Additive–Subtractive Manufacturing of Shape Memory Ceramics (Adv. Mater. 39/2023)

4D Additive-Subtractive Manufacturing of Shape Memory Ceramics.

The development of high-temperature structural materials, such as ceramics, is limited by their extremely high melting points and the difficulty in building complicated architectures. Four-dimensional (4D) printing helps enhance the geometrical flexibility of ceramics. However, ceramic 4D printing systems are limited by the separate processes for shape and material transformations, low accuracy of morphing systems, low resolution of ceramic structures, and their time-intensive nature. Here, a paradigm for a one-step shape/material transformation, high-2D/3D/4D-precision, high-efficiency, and scalable 4D additive-subtractive manufacturing of shape memory ceramics is developed. Original/reverse and global/local multimode shape memory capabilities are achieved using macroscale SiOC-based ceramic materials. The uniformly deposited Al2 O3 -rich layer on the printed SiOC-based ceramic lattice structures results in an unusually high flame ablation performance of the complex-shaped ceramics. The proposed framework is expected to broaden the applications of high-temperature structural materials in the aerospace, electronics, biomedical, and art fields.

Viewpoint: Tuning the Martensitic Transformation Mode in Shape Memory Ceramics via Mesostructure and Microstructure Design

Viewpoint: Tuning the Martensitic Transformation Mode in Shape Memory Ceramics via Mesostructure and Microstructure Design

A phase-field model for interactive evolution of phase transformation and cracking in superelastic shape memory ceramics

This work presents a modified phase-field model for accurate coupling of phase transformation and cracking in shape memory ceramics. The existing phase-field models underestimate the elastic response at the beginning of the mechanical response. We modified the chemical free energy to control the rate of phase transformation and consequently obtain a physical elastic response before initiation of phase transformation. First, the forward and reverse martensitic phase transformation in a superelastic single crystal 3 mol% yttria-stabilized tetragonal zirconia is studied. Then, the interaction between phase transformation and fracture under displacement-controlled loading condition is investigated. The model predicts a realistic mechanical response and the experimentally observed microstructure and crack deflection due to the phase transformation. In addition, the model captures the reverse phase transformation and the stress drop due to the crack propagation.

Low-hysteresis shape-memory ceramics designed by multimode modelling.

Zirconia ceramics exhibit a martensitic phase transformation that enables large strains of order 10%, making them prospects for shape-memory and superelastic applications at high temperature1-5. Similarly to other martensitic materials, this transformation strain can be engineered by carefully alloying to produce a more commensurate transformation with reduced hysteresis (difference in transformation temperature on heating and cooling)6-11. However, such 'lattice engineering' in zirconia is complicated by additional physical constraints: there is a secondary need to manage a large transformation volume change12, and to achieve transformation temperatures high enough to avoid kinetic barriers6. Here we present a method of augmenting the lattice engineering approach to martensite design to address these additional constraints, incorporating modern computational thermodynamics and data science tools to span complex multicomponent spaces for which no data yet exist. The result is a new zirconia composition with record low hysteresis of 15 K, which is about ten times less transformation hysteresis compared to typical values (and approximately five times less than the best values reported so far). This finding demonstrates that zirconia ceramics can exhibit hysteresis values of the order of those of widely deployed shape-memory alloys, paving the way for their use as viable high-temperature shape-memory materials.

Phase transformation and incompatibility at grain boundaries in zirconia-based shape memory ceramics: a micromechanics-based simulation study

Zirconia-based shape memory ceramics (SMCs) exhibit anisotropic mechanical response when undergoing elastic deformations as well as during austenite–martensite phase transformation. This behavior results in different types of strain incompatibility at grain boundaries, which we study here using a micromechanical model. A single-crystal model is implemented to provide a full mechanistic three-dimensional description of the anisotropic elastic as well as martensitic transformation stress–strain response, including non-Schmid behavior caused by the significant volume change during martensitic transformation. This model was calibrated to and validated against compression tests of single-crystal zirconia micro-pillars conducted previously, and then used to model bi-crystals. Upon the introduction of a grain boundary, the simulation provides detailed information on the nucleation and evolution of martensite variants and stress distribution at grain boundaries. We identify bi-crystal configurations which result in very large stress concentrations at very low deformations due to elastic incompatibility, as well as others where the elastic incompatibility is relatively low and stress concentrations only occur at large transformation strains. We also show how this approach can be used to explore the misorientation space for quantifying the level of elastic and transformation incompatibility at SMCs grain boundaries.Graphical abstractMicromechanics models provide insights on grain boundary elastic and phase transformation strain incompatibility in shape memory zirconia

Shape memory effect in Na0.5Bi0.5TiO3-based ferroelectric ceramics

Shape memory materials (SMMs) are a class of smart materials that generate recoverable residual deformation. Among the family of SMMs, shape memory ceramics (SMC) are more applicable to extreme situations with high temperature, high stress, or corrosive environments. Most of the well-known SMCs are zirconia-based ceramics. Herein, we demonstrate the shape memory behavior of ferroelectric ceramics through a case study on Na0.5Bi0.5TiO3-BaTiO3 (NBT-BT) ceramics. The results show that after heat treatment under stress, there was a large residual strain left in the NBT-BT ceramics. After they were annealed at a temperature above the phase-transition temperature, the residual strain recovered. The maximum recoverable residual strain observed in the NBT ceramic plate was approximately 0.369%. Because most ferroelectric ceramics have a smaller critical stress regarding the stress-induced structural orientation or transformation compared with that of the zirconia-based SMCs, they should be more suitable for SMC applications at a medium stress level.

Phase Transformation and Incompatibility at Grain Boundaries in Zirconia-Based Shape Memory Ceramics: A Micromechanics-Based Simulation Study

Zirconia-based shape memory ceramics (SMCs) exhibit anisotropic mechanical response when undergoing elastic deformations as well as during austenite-martensite phase transformation. This behavior results in different types of strain incompatibility at grain boundaries, which we study here using a micromechanical model. A single-crystal model is developed to provide a full mechanistic three-dimensional description of the anisotropic elastic as well as martensitic transformation stress-strain response, including non-Schmid behavior caused by the significant volume change during martensitic transformation. This model was calibrated to and validated against compression tests of single-crystal zirconia micro-pillars, prior to being used to model bi-crystals. Upon the introduction of a grain boundary, the simulation provides detailed information on the nucleation and evolution of martensite variants and stress distribution at grain boundaries. We identify bicrystal configurations which result in very large stress concentrations at very low deformations due to elastic incompatibility, as well as others where the elastic incompatibility is relatively low and stress concentrations only occur at large transformation strains. We also show how this approach can be used to explore the misorientation space for quantifying the level of elastic and transformation incompatibility at SMCs grain boundaries.

Three-dimensional phase field modeling of fracture in shape memory ceramics

Despite the vast applications of transformable ceramics, such as zirconia-based ceramics, in different areas from biomedical to aerospace, the fundamental knowledge about their mechanical degradation procedure is limited. The interaction of the phase transformation and crack growth is crucial as the essential underlying mechanism in fracture of these transformable ceramics, also known as shape memory ceramics. This study develops a three-dimensional (3D) multiphysics model that couples the variational formulation of brittle crack growth to the Ginzburg-Landau equations of martensitic transformation. We parameterized the model for the 3D single crystal zirconia, which experienced stress- and thermal-induced tetragonal to monoclinic transformation. The developed 3D model considers all 12 monoclinic variants, making it possible to acquire realistic microstructures. Surface uplifting, self-accommodated martensite pairs formation, and transformed zone fragmentation were observed by the model, which agrees with the experimental observations. The influence of the crystal lattice orientation is investigated in this study, which reveals its profound effects on the transformation toughening and crack propagation path.

The mechanism of thermal transformation hysteresis in ZrO2-CeO2 shape-memory ceramics

Hysteresis is an important phase transformation property to understand and control in shape-memory materials. While a detailed understanding of hysteresis has been developed in shape-memory alloys (SMAs), it is far less understood in emerging shape-memory ceramics (SMCs). Here, we systematically study thermal hysteresis of the tetragonal-monoclinic martensitic transformation in ZrO2-CeO2 SMCs. We present calorimetry measurements of thermal hysteresis, which reveal a non-monotonic trend with CeO2 content as well as significant rate dependence, and correlate these findings with the transformation crystallography measured by in situ X-ray diffraction. We quantitatively analyze these results in the framework of martensite nucleation theory and conclude that the initial decrease in hysteresis with CeO2 content can be attributed to improving interface compatibility (λ2) for coherent nucleation whereas the sharp increase in hysteresis at higher CeO2 contents (below temperatures of ~773 K) is a result of interface friction consistent with thermally-activated interactions with the Peierls barrier of the crystal. Our analysis gives an activation energy of ~136 kJ/mol, which is consistent with previously reported activation energies of isothermal transformation in these materials. These findings clarify contradicting reports in the ZrO2 literature and confirm that the same controlling factors of hysteresis in SMAs, namely λ2 and thermal friction, are also operative in SMCs.

The Mechanism of Thermal Transformation Hysteresis in Zro 2-Ceo 2 Shape-Memory Ceramics

Hysteresis is an important phase transformation property to understand and control in shape-memory materials. While a detailed understanding of hysteresis has been developed in shape-memory alloys (SMAs), it is far less understood in emerging shape-memory ceramics (SMCs). Here, we systematically study thermal hysteresis of the tetragonal-monoclinic martensitic transformation in ZrO2-CeO2 SMCs. We present calorimetry measurements of thermal hysteresis, which reveal a non-monotonic trend with CeO2 content as well as significant rate dependence, and correlate these findings with the transformation crystallography measured by in situ X-ray diffraction. We quantitatively analyze these results in the framework of martensite nucleation theory and conclude that the initial decrease in hysteresis with CeO2 content can be attributed to improving interface compatibility (λ2) for coherent nucleation whereas the sharp increase in hysteresis at higher CeO2 contents (below temperatures of ~773 K) is a result of interface friction consistent with thermally-activated interactions with the Peierls barrier of the crystal. Our analysis gives an activation energy of ~136 kJ/mol, which is consistent with previously reported activation energies of isothermal transformation in these materials. These findings clarify contradicting reports in the ZrO2 literature and confirm that the same controlling factors of hysteresis in SMAs, namely λ2 and thermal friction, are also operative in SMCs.

Effects of mechanical constraint on thermally induced reverse martensitic transformation in granular shape memory ceramic packings

Zirconia-based ceramics exhibit shape memory and superelastic effects based on the reversible martensitic transformation between tetragonal and monoclinic crystal structures. In the form of granular packings, these shape memory ceramics can be scaled up for bulk applications despite their intrinsic brittleness, while displaying drastically different transformation characteristics than the monolithic counterparts. Here, we present a comparative study to understand the thermally induced reverse martensitic transformation in granular packings and the influence of mechanical constraints. This study employs ZrO2–CeO2 shape memory ceramics of the same composition but with different degrees of mechanical constraints. The explored material forms include loose and jammed granular packings, themselves consisting of polycrystalline or single crystal particles, as well as sintered bulk polycrystals. Except for the latter, no endothermic peak is observed in the heat flow measurement of the reverse transformation process. This unusual behavior is shown to stem from the weak inter-particle mechanical constraint and the transformation heterogeneity among individual particles, rather than stress relaxation or particle rearrangement. To compare, conspicuous endothermic peaks only appear in bursting-type transformations under a strong mechanical constraint. For granular packings, the intra-particle mechanical constraint does not affect the presence of any endothermic peaks in thermal reversion but can influence the austenite start temperature.

Concurrent modeling of martensitic transformation and crack growth in polycrystalline shape memory ceramics

Mechanical degradation of shape memory materials (SMM) has been a long-lasting challenge that has prevented the wider range of high-cycle applications of SMM. The core of the challenge is the limited current knowledge of how crack growth and martensitic transformation (MT) interact concurrently. In this paper, we study the dynamic interaction of MT and crack propagation in polycrystalline shape memory ceramics. We construct a multiphysics phase-field model that couples the Ginzburg-Landau theory of MT to the variational formulation of brittle fracture. The model is parameterized for tetragonal polycrystalline zirconia, and the experimental data from literature were used to validate the model. The model predicts the three dominant crack propagation patterns which was observed experimentally including the secondary crack initiation, crack branching, and grain bridging. The model shows the critical role of texture engineering in toughening enhancement. Polycrystalline zirconia samples with grains that make low angles between a-axis in tetragonal phase and the crack plane, show higher transformation toughening, due to maximum hydrostatic strain release perpendicular to the crack tip. The model also shows the grain boundary engineering as a way to enhance the transformation toughening. The maximum fracture toughness occurs at a specific grain size, and further coarsening or refinement reduces the fracture toughness. This optimum grain size is the consequence of the competition between the toughening enhancement and MT suppression with grain refinement.

Cyclic martensitic transformations and damage evolution in shape memory zirconia: Single crystals vs polycrystals

AbstractIn zirconia‐based shape memory ceramics (SMCs), cracking during the martensitic transformation can be avoided in structures that reduce the relative presence of grain boundaries where high levels of transformation mismatch stress develop. This approach has been well established in single crystals, but only for sample sizes below about 5 microns. In this paper, we extend the strategy of eliminating grain boundaries to bulk specimen scales by fabricating mm‐scale single crystal SMCs via cold crucible induction melting. For 1.5 and 2.0 mol% Y2O3‐ZrO2, we study the cyclic martensitic transformation of both single crystal and polycrystal structures. Whereas single crystals have very repeatable transformation behavior in terms of hysteresis and strain amplitude, polycrystals degrade dramatically as they accumulate cracking damage with repeated cycling. As the polycrystal evolves from a pellet to granular packing of loose single crystals/grains, the energy dissipation converges with that of the single‐crystal structure, and the energy spent on cracking throughout that process is captured by calorimetry analysis. These results verify that grain boundaries play a key role in damage evolution during martensitic transformation and that microstructural control can extend the size‐scale of viable single crystal or oligocrystal SMCs from the micro‐ to the millimeter scale.