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Rationale and Objectives The purpose of this study is to improve accuracy of near-term breast cancer risk prediction by applying a new mammographic image conversion method combined with a two-stage artificial neural network (ANN)-based classification scheme. Materials and Methods The dataset included 168 negative mammography screening cases. In developing and testing our new risk model, we first converted the original grayscale value (GV)-based mammographic images into optical density (OD)-based images. For each case, our computer-aided scheme then computed two types of image features representing bilateral asymmetry and the maximum of the image features computed from GV and OD images, respectively. A two-stage classification scheme consisting of three ANNs was developed. The first stage included two ANNs trained using features computed separately from GV and OD images of 138 cases. The second stage included another ANN to fuse the prediction scores produced by two ANNs in the first stage. The risk prediction performance was tested using the rest 30 cases. Results With the two-stage classification scheme, the computed area under the receiver operating characteristic curve (AUC) was 0.816 ± 0.071, which was significantly higher than the AUC values of 0.669 ± 0.099 and 0.646 ± 0.099 achieved using two ANNs trained using GV features and OD features, respectively ( P Conclusion This study demonstrated that applying an OD image conversion method can acquire new complimentary information to those acquired from the original images. As a result, fusion image features computed from these two types of images yielded significantly higher performance in near-term breast cancer risk prediction.

Variant selection under specific applied stresses during precipitation of α plates from prior-β matrix in Ti–6Al–4 V was investigated by 3D phase field simulations. The model incorporates the Burgers transformation path from β to α phase, with consideration of interfacial energy anisotropy, externally applied stresses and elastic interactions among α variants and β matrix. The Gibbs free energy and atomic mobility data are taken from available thermodynamic and kinetic databases. It was found that external stresses have a profound influence on variant selection, and the selection has a sensitive dependence, as evidenced by both interaction energy calculations and phase field simulations. Compared with normal stresses, shear stresses applied in certain directions were found more effective in accelerating the transformation, with a stronger preference to fewer variants. The volume fractions of various α variants and the final microstructure were determined by both the external stress and the elastic interaction among different variants. The α clusters formed by variants with Type2 misorientation ([1 1 -2 0]/60°) relation were found more favored than those with Type4 ([-10 5 5 -3]/63.26°) under certain applied tensile stress such as along β. The mechanical properties of different microstructures from our phase field simulation under different conditions were calculated for different loading conditions, utilizing crystal plastic finite element simulation. The mechanical behavior of the various microstructures from phase field simulation can be evaluated well before the alloys are fabricated, and therefore it is possible to select microstructure for optimizing the mechanical properties of the alloy through thermomechanical processing based on the two types of simulations.

Two-step nucleation, in which a metastable intermediate phase acts as a precursor for nucleating a thermodynamically stable phase, has been widely observed in many materials systems and solid-state reactions. Among the advantages of two-step nucleation is that the stable phase may nucleate heterogeneously at the hetero-phase interface between the original and the precursory phases. Although heterogeneous nucleation (HN) theories for homo-phase grain boundaries and inert surfaces are well established, our understanding of HN at reactive hetero-phase interfaces remains incomplete. This deficiency stems from the discontinuity of the chemical potential driving force across the hetero-phase interface, which profoundly affects the fundamental properties of the nucleus in a way that is not properly accounted for in existing models. Herein, we incorporate these effects to extend the classical nucleation theory to HN at hetero-phase interfaces. Our extended model demonstrates that the nucleus shape along the minimum energy path is strongly size-dependent, and this additional degree of freedom can result in the reduction of the critical nucleus volume and associated activation energy barrier by orders of magnitude relative to conventional predictions. The simulation results are used to construct a sensitivity map in the parameter space of interfacial energy and bulk driving force ratios, which quantifies the difference in nucleation barriers predicted by different models.

Recent experiments suggest that Suzuki segregation may play an important role during deformation in Ni-base superalloys at intermediate temperatures. In this study, a segregation isotherm model incorporating segregation enthalpy from ab initio calculations is proposed to predict quantitatively solute enrichment at superlattice intrinsic stacking faults (SISF) within the γ’ precipitates in Ni-base superalloys. A sublattice model is employed to describe the γ’ phase. A strong correlation between segregation enthalpy and solute enrichment is found. Even though the segregation enthalpy is relatively small, the predicted solute enrichment is consistent with experimental observations. The simulation predictions also suggest a strong cross-correlation among different alloying elements. For example, it is found that segregation of Co on the Ni sublattice at the fault draws segregation of Cr that has a positive segregation enthalpy at the fault without the presence of Co. Such quantitative predictions of equilibrium segregation of solutes at stacking faults in γ’ precipitates and its effect on the stacking fault energy could aid the investigation of deformation mechanisms and help the design of superalloys.

In addition to conventional dislocation plasticity, mechanical twinning and structural phase transformations are another two important plasticity carriers. Although both are symmetry-breaking processes and theories to treat each of them individually have been well-established, the intrinsic coupling between the two has not been investigated. Here we employ a phase transition graph approach to analyze systematically deformation modes arising from the interplay between mechanical twinning and phase transformations. Using metastable β Ti-alloys as an example, we show that mechanical twinning and phase transformations are intrinsically coupled in the symmetry-breaking processes, which results in multiple interconnected transformation and non-transformation deformation pathways and characteristic twinning modes. This work not only reveals the physical origin of unique twinning modes (e.g., {332}, {5 8 11} and {3 9 10} twins) and extended core structures of twin boundaries (e.g., nested twins) observed in experiments, but also provides a new insight into the enhanced plasticity of metastable β Ti-alloys through coupled twinning and transformation pathway engineering.

Materials with certain heterogeneous microstructures have been shown to hold a synergistic combination of strength and ductility. In this study, we demonstrate novel transformation pathways for creating such heterogeneous microstructure in Ti-alloys by integrating thermodynamic databases with phase field simulations. The results show that the concentration modulations at different length scales produced by (a) precursory spinodal decomposition in the parent phase and (b) interdiffusion in multi-layers having different alloy compositions can generate effectively hierarchical and gradient α + β two-phase microstructures, with a mixture of fine α precipitate regions and α precipitate-free-zones or coarse α precipitate regions. The novel microstructures produced include “inverted globular α” bi-modal microstructures and gradient microstructures with controlled spatial gradients in particle size and number density of α precipitates. This study may shed light on how to design novel hierarchical and gradient two-phase microstructures with tunable size and density of precipitates as well as the length scale of their spatial heterogeneity for desired properties.

Deformation twinning in crystals is one of the major strain carrier during plastic deformation, which plays a critical role in determining the mechanical properties of metals and alloys. One of the key issues to understand deformation twinning mechanisms is the determination of deformation path, which is critical for the calculations of twinning strain and critical shear stress. This work provides a new perspective on deformation twinning by systematically investigating the relationship between symmetry breaking and deformation path, with the purpose of establishing a theoretical foundation to identify the broken symmetries associated with twinning and predicting the twinning modes using group theory and graph theory. From a physical point of view, deformation twins can be regarded as one type of the so-called topological defects that are induced by symmetry-breaking. Taking BCC crystals as an example, we demonstrate how the presence of intermediate high symmetry states in the deformation strain space (strain calculated through lattice distortion) along the twinning path, such as FCC, HCP or orthorhombic state, can lead to different characteristic twinning modes. Our predictions not only agree well with classical theoretical analyses and experimental observations in BCC metals and alloys, but also reveal the origin of recently observed high-index twinning modes (such as {5 8 11} and {3 9 10} twins). This work may open a new avenue for analyzing deformation twinning through the symmetry breaking along twinning pathways.

As a new ferroelastic state, strain glass, has attracted a lot of recent attentions and, most importantly, strain glass transitions (SGTs) could underpin many phenomena that have puzzled the physics community for decades, including the quasi-linear superelasticity and Invar and Elinvar anomalies. However, there has been a lack of fundamental understanding at the atomistic level beyond the phenomenological Landau theory. In this paper, we propose a way to obtain quantitatively the continuous strain/stress fields distribution caused by doped point defects through molecular statics calculations by incorporating a Gaussian probability distribution function. By using the quantitative strain/stress fields distribution to inform phase field simulations, we reproduce quantitatively the experimentally observed critical defect concentrations separating the normal martensitic phase transition from strain glass transition at different temperatures and critical temperatures for spontaneous strain glass to martensitic transition at different defect concentrations. Based on percolation theory, we demonstrate how the strain network created by point defects with a critical concentration regulates nucleation and growth of martensitic domains, suppresses autocatalysis by strain frustration, and changes the sharp first-order martensitic transformation into a continuous SGT. A general temperature- and defect-concentration-dependent percolation criterion is formulated for accurate prediction of SGT, which could enable high throughput computations for systematic search of new strain glass systems using simply molecular static calculations.

Implementing the phase-field model at finite strains is usually considered unattainable through the spectral method, largely because of the nonlinearity in the transformation-induced microelasticity. Here we present a phase-field microelasticity (PFM) theory at finite strains with a representation in the reference configuration, allowing the spectral method to be readily incorporated. Following the spirit of Khachaturyan’s PFM theory at small strains, the elastic energy is formulated as a functional of microstructure (order parameters) solely, which should automatically satisfy the mechanical equilibrium. Thermodynamic consistency of the current theory under multiplicative decomposition of the total deformation gradient (into elastic and inelastic parts) and in conjunction with hyperelasticity and the time-dependent Ginzburg-Landau equation is shown rigorously. The new theory is first applied to the classical Eshelby’s inclusion problem, where shear-dilation coupling due to geometric nonlinearity is shown and a convergence study between small strain and finite strain theories is also carried out. The effects of geometric nonlinearity on the co-evolution of micromechanics and microstructure is further studied through modeling the growth of { 1 0 1 ¯ 2 } 〈 1 ¯ 0 1 1 〉 deformation twins in magnesium. The simulation results suggest significant differences in terms of the shape of and the stress field around the deformation twin. In particular, the current finite strain PFM theory predicts a deviation of the twin boundary plane from the theoretical K1 plane, which is not captured in the small strain theory nor in the crystallographic theory. A parametric study further reveals that the observed deviation is caused by the tip effect of the finite-sized twin plate when the aspect ratio is relatively small. The symmetry of the stress field distribution around the twin tip is also found to be drastically different between the small strain and finite strain based phase-field modeling. The sharp twin tip observed in experiments is also shown to be likely related to the anisotropy in twin/matrix interface mobility.

The mechanical properties of two-phase γ-TiAl alloys depend strongly on the scale of their lamellar microstructures. In this study, we investigate the effect of alloy composition on lamellar thickness in Ti100-xAlx by simulating microstructural evolution during the α2′ → α2 + γ phase transformation using the phase field method. From both existing experimental data and our simulation results we find that the average thickness of γ lamellae first decreases and then increases with increasing Al content. Further analyses reveal that this unique composition-dependence of the lamellar thickness could be associated with non-conventional pseudospinodal decomposition and congruent structural transformation mechanisms that generate ultra-high densities of nuclei at the initial stage, which impacts the later growth processes. The interplay between the number density of nuclei and the number of coalescence events during growth results in the non-monotonic dependence of the lamellar thickness on the alloy composition. These findings may offer new insights on the design of ultrafine lamellar microstructures for γ-TiAl alloys.

The microstructure in a multifunctional β-titanium alloy, Ti–24Nb–4Zr–8Sn (wt%), before and after cold rolling was investigated using conventional and aberration-corrected scanning/transmission electron microscopy. With the presence of nano-scaled orthorhombic phase (O′) and hexagonal phase (ω), primary {332} β deformation twins were activated during cold rolling with a thin layer of orthorhombic martensite (α″) occupying every twin boundary. In the interior of primary twins, nano-scaled α″ particles were observed, together with secondary {112} β twins. Our work may indicate that the pre-formed nano-scaled O′ phase assists in activating the {332} β deformation twining and O′ phase transforms to nano-scaled stress-induced α″ during cold rolling.

The unprecedented properties of multi-functional metastable β-Ti alloys, including superelasticity over a wide temperature range, ultra-low modulus, and Invar and Elinvar anomalies, have attracted a great deal of attention. Persistent research efforts have been made towards the understanding of the origins of these unique properties. In this article we report a novel shuffle-nanodomain regulated strain glass transition in a metastable β-Ti alloy, Ti-24Nb-4Zr-8Sn (wt.%, Ti2448), which could be the dominant transformation pathway that offers these unique properties. Using the ex-situ aberration-corrected scanning transmission electron microscopy and in-situ cooling transmission electron microscopy, we find that randomly distributed {011} 〈 0 1 ¯ 1 〉 β O′ phase (orthorhombic, shuffle only) nanodomains embedded in the β phase (BCC) matrix at room temperature transform to α″ phase (orthorhombic) with a continuous increase in the amount of {2 1 ¯ 1 } 〈 1 ¯ 1 ¯ 1 〉 β shear upon cooling or loading. Crystallographic analysis shows that the shuffle of the O′ phase will restrain the twelve possible shears that transform a BCC lattice to α″ martensite to only two. Thus, the randomly distributed O′ nanodomains prevent the formation of long-range-ordered, self-accommodating transformation-strain domain patterns seen in normal martensitic transformations and suppress completely the sharp first-order, auto-catalytic and avalanche-like martensitic transformation into a high-order-like (continuous) strain glass transition. Such a continuous β → O′ → α″ strain glass transition has been confirmed by dynamic mechanical analysis, resistivity and differential scanning calorimetric measurement. This unique transition pathway allows us to offer new insights into the unique properties found in this alloy.

The elastic strain limit of most metals are less than 0.2% except for whiskers or freestanding nanowires whose elastic strain limit could reach 4–7%. Ferroelastic metals such as shape memory alloys (SMAs) do exhibit giant recoverable strains (up to ∼13%). However, the strong non-linear pseudo-elasticity of SMAs leads to mechanical instability. By taking advantage of the strong composition-dependent critical stress for stress-induced martensitic transformation (MT) in NiTi SMA, this work demonstrates a novel design approach to achieve linear-superelasticity (∼4.6%) and ultralow modulus (8.7 GPa) of a NiTi single crystal. These unprecedented properties are realized through precisely controlling strain release during the MT via nanoscale concentration-gradient engineering. The computer simulation results and theoretical analyses reveal that the stress–strain behavior of NiTi and other SMAs can be regulated effectively by fine-tuning the concentration gradient. This may open a new avenue for the design of next generation ferroelastic materials.

Thanks to its small hysteresis, large electrostrictive strain in relaxor ferroelectrics is superior than piezoelectric strain for applications in precision microactuators. Although relaxor ferroelectrics exhibit the largest electrostrictive strain in ceramics, the magnitude of the strain is limited to ∼0.20% at room temperature due to the large amount of non-ferroelectric defects existing in relaxors. In this work, we develop a relaxor with a morphotropic phase boundary (MPB) by doping a rhombohedral (R3m) ferroelectric BaZr0.2Ti0.8O3 into a tetragonal (P4 mm) ferroelectric 0.89Bi0.5Na0.5TiO3-0.11BaTiO3. A high electrostrictive strain of 0.27% is achieved at room temperature in the relaxor sample. Experimental results illustrate that the composition is near the MPB and exhibits the existence of nanodomains, favoring the achievement of high electrostrictive strain. Moreover, phase field simulations show that the high electrostrictive strain obtained at this composition originates from the low defect fields needed to induce relaxor as a result of small polarization anisotropy at the phase boundary as compared to conventional relaxors away from phase boundaries. Our work provides a new design strategy for the next generation of high-performance ferroelectric relaxors.

Nanocrystal is one of the classic formulation strategies for poorly water-soluble drugs. In this study, nitrendipine (NTD) nanocrystals were prepared using a media milling method for the first time. Comprehensive physical characterizations of NTD nanocrystals were carried out including hydrodynamic diameter, polydispersity index, zeta potential, transmission electron microscope, scanning electron microscope, differential scanning calorimetry, powder X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, and optical microscope. The optimized nanocrystals stabilized by 1.25% (w/v) HPMC-E5 and 0.4% (w/v) SDS were oblong-shaped with a particle size of 256.5 ± 6.6 nm. The crystal structure of NTD nanocrystals was maintained during the milling process, but a portion of crystal was transformed to amorphous state. No obvious interaction was detected between the drug and stabilizers. In addition, the physical stability and in vitro dissolution were assessed. NTD nanocrystals were physically stable during the storage at 25 ± 2 °C for 30 days. In the dissolution test, for NTD nanocrystals, the accumulative release after 20 min was over 80% in the media of 0.3% SDS, which was 4-fold higher than those of physical mixture and commercial tablets. The results indicated that NTD nanocrystals exhibited superior dissolution due to the reduced particle size. In conclusion, drug nanocrystal produced by wet media milling is a promising strategy for improving the solubility and dissolution rate for NTD.

Nanoscale concentration modulation (CM) is a novel and effective approach of manipulating martensitic transformations (MTs) for developing next-generation high-performance shape memory alloys (SMAs). Spinodal decomposition is one of the most economic methods to obtain bulk compositionally modulated materials for practical applications. The wavelength, amplitude, and statistical distribution of CM generated by spinodal decomposition are tunable via adjusting the aging temperature, or the aging time. However, how these features influence the effect of CM on MTs still remains largely unexplored. In this study, theoretical analyses and computer simulations are combined to dissect the influence of these features on the kinetic process of MTs and mechanical properties of SMAs. The findings of this study provide insights and guidance on the design of SMAs for desired mechanical properties via CM engineering. Moreover, the findings are applicable to not only SMAs but also other materials that have MTs, e.g. steels and high-entropy alloys.

We investigate mechanisms of ω-assisted α nucleation for microstructure refinement in a metastable β titanium alloy, Ti-20V (wt%). In particular, we analyze quantitatively the individual and combined effects of concentration and stress fields associated with a growing isothermal ω precipitate and the potency of the ω/β interface on the activation energy barrier for α nucleation as a function of the ω precipitate size and coherency state. We find that the complex interplay among these three factors determines the most potent ω particle size for inducing α nucleation and the most favorable nucleation site.

Traditional viewpoints state that defects, such as pores or cracks, that generally exist in plasma sprayed thermal barrier coatings (TBCs) can provide fast infiltration channels for molten calcia-magnesia-alumino-silicate (CMAS). In contrast with this viewpoint, results from this study suggest that the effect of defects on the CMAS infiltration of the yttria stabilized zirconia (YSZ) coating is limited, even though the dense YSZ bulk material still cannot inhibit the infiltration of CMAS. However, a few additions of Sc2O3 can significantly improve the CMAS resistance of the YSZ coating owing to the lower solubility of Sc3+ in the CMAS. Our findings provide key insights for the development of CMAS-resistant TBCs.

Slip transmission across α / β interfaces is of great significance in understanding the strength of Ti-alloys, but currently a detailed mechanistic understanding of the process is still lacking. Here we develop a microscopic phase-field framework that incorporates the generalized stacking fault energy and the interface crystallography, which are, respectively, calculated by atomistic methods and revealed by crystallographic theories of phase transformations and experimental characterization. The model is then applied to studying the transmission of a constant flux of discrete dislocations across multiple α / β interfaces at micron-scale. The simulations predict interesting slip transmission mechanisms that have not been reported before, wherein Shockley partials play a critical role in assisting the dislocation transfer across the interfaces. The dislocation configurations generated by these mechanisms seem to agree well with experimental characterizations. Spatial cross-over between full dislocations in α is also seen from the simulations, which is again attributed to a reaction mechanism involving Shockley partials. A parametric study further reveals that stacking fault energy can influence the slip transmission in terms of transmitted dislocation types, transmission rate, and the residual dislocation content, suggesting a new strengthening strategy at the α / β interface level. This work offers new understanding of the complex slip transmission process in Ti-alloys and demonstrates a new computational tool complementary to advanced electron microscopy analysis of plastic deformation.

A large electrostrain usually occurs at ferroelectric-ferroelectric phase boundaries. However, in the Ba(Ti1 – xZrx)O3 (BT-xZr) system prepared by the conventional solid-state reaction method, we discovered a local electrostrain maximum in the single rhombohedral phase region at the quasi-quadruple point with a composition of BT-0.12Zr that had a relatively weak temperature sensitivity below 50 °C. Mechanisms of the local electrostrain maximum in the single-phase region of the BT-xZr system were studied. The large coercive field and large domain size of BT-0.12Zr indicate that domain switching was not facilitated. Nevertheless, the large lattice distortion of BT-0.12Zr suggests a large intrinsic lattice strain, which predominantly contributed to the observed macroscopic electrostrain enhancement at the quasi-quadruple point composition in the single-phase region. Compared to nearby sample compositions, the largest lattice distortion of BT-0.12Zr was found to be correlated with the highest electrostrictive coefficient at the tricritical quasi-quadruple point, as deduced from phenomenological Landau analysis. Our work may shed light on the development of weakly temperature-dependent large electrostrain materials by identifying the tricritical quasi-quadruple point in ferroelectric phase diagrams.

The microstructure and bonding strength of plasma sprayed coating are highly dependent on flattening and solidification behavior of in-flight droplets. In the present study, the flattening and solidification behavior of subsonic/supersonic droplets after impacting on a flat substrate was comparatively studied. Results show that a dimensionless Somerfeld number (K) of flattening droplets can be used as the splashing threshold. Due to the extremely rapid spreading and solidification of supersonic-droplet, some large-sized bubbles and grains that usually formed at the bottom of flattened subsonic-droplet were effectively restrained, leading to the improvement of splat/substrate micro-shear strength. The effect of grain size and growth feature on the macro-bonding property was further elaborated. It was found that the simultaneous increase of both velocity and temperature of in-flight droplets was an effective approach for the improvement of bonding strength of as-sprayed coating.

Strain glass, a glassy state of lattice strain, has been found in Ti50-xNi50+x alloys and later in many metallic ferroelastic/martensitic systems, where shear or shuffle serves as a primary order parameter (POP). Another class of non-metallic ferroelastic ceramics are also known to widely exist and commonly possess a polyhedral tilt as the POP. So far, it is unclear whether a “tilt” strain glass exists. Here, we report a finding of a tilt strain glass in La1-xSrxAl0.95Nb0.05O3-δ ceramics. With increasing Sr2+ dopants, the ferroelastic transition from cubic to rhombohedral phases is gradually suppressed. At a critical concentration (xc ∼10%), a strain glass transition emerges, characterized by five sets of evidence: (I) an invariance of average structure; (II) frequency dependence of elastic moduli at a strain glass transition temperature Tg; (III) non-ergodicity; (IV) formation of rhombohedral nanodomains; (V) a gradual increase of tilt angle upon cooling. Surprisingly, the established phase diagram shows an increase of Tg with increasing dopants (a positive correlation), which is different from previous strain glass phase diagrams. The positive and negative correlations can be explained as a balance between two factors of strain glass transition: a global transition factor producing a negative contribution competes with a local field one producing a positive contribution. Our discovery of strain glass in ceramics may also bring novel properties as in metals.

A correct description of interfacial dislocations is critical to the understanding of physical and mechanical properties of the interfaces and their impact on phase transformation and deformation mechanisms. The O-lattice theory provides a general theoretical framework for determining interfacial dislocation structures. However, for systems having an invariant line (IL) or an invariant plane, the O-elements do not always exist in the three-dimensional (3D) space, which impedes the capability of the O-lattice theory to the interfaces in these systems. To determine the dislocation structures in interfaces for which the O-elements do not extend in the 3D space, we introduce a generalized O-element approach by employing the Moore-Penrose pseudoinverse. The generalized O-elements, as the least square solutions to the O-lattice, play a parallel role as that of the ideal O-elements and extend the candidate locations for the coherent regions between dislocations. Worked examples for dislocation structures in both homo-phase and hetero-phase systems are presented. The predicted interface structures are in good agreement with experimental observations and molecular statics (MS)/molecular dynamics (MD) simulations.

We predict structure and energy of low-angle ( 1 1 ¯ 0 ) pure twist grain boundaries (GBs) in five BCC transition metals ( β -titanium, molybdenum, niobium, tungsten, and tantalum) using a combination of atomistic and microscopic phase-field (MPF) modeling. The MPF model takes as inputs solely the generalized stacking fault energy surfaces (i.e., the γ -surface) and elastic constants obtained from the atomistic simulations. Being an energy-based method, the MPF model lifts the degeneracy of the geometric models in predicting GB structures. For example, the multiple indefinite solutions offered by the Frank-Bilby equation are shown to converge to exactly the same equilibrium structure. It predicts a transition of the equilibrium GB structure from a pure screw hexagonal network (Mo and W) to mixed hexagonal networks (Nb and Ta) to a rhombus network (β-Ti) of dislocations. Parametric simulation studies and detailed analyses of the underlying dislocation reactions that are responsible for the formation of the rhombus and hexagonal structures reveal a close correlation between material properties (including the elastic anisotropic ratio and the local curvature on the γ -surface) and the GB structure and energy in BCC metals. This integrated approach allows one to explore, through high throughput calculations, the potential to tailor the structure and energy of special GBs in BCC metals by alloying.

We show in this paper how strain engineering alters the fundamental characteristic of a martensitic transformation (MT) and gives it a new set of properties including large quasi-linear elastic strain response with nearly vanishing hysteresis and low elastic modulus. The work is motivated and inspired by a recent experimental study on elastic and inelastic (transformation) strain matching in a pre-strained nano-composite with Nb nanowires embedded in a NiTi shape memory alloy matrix. In particular, we demonstrate by computer simulation that dislocations at Nb/NiTi interfaces produced by the pre-straining are responsible for the unprecedented properties. Microstructural evolution captured in the simulations reveals that local stress fields associated with the dislocations regulate the nucleation and growth of martensite, turning the otherwise sharp, strong first-order transition into a continuous, high-order like transition. The simulations predict that the stress-strain hysteresis and modulus of the composite decrease with increasing amount of pre-strain, which agrees well with the experimental measurement. This study suggests a design strategy by introducing non-uniform stress fields for enhanced properties of shape memory alloys.

Precipitation of the γ ' ' phase on { 100 } facets of preceding γ ' precipitates is found to prevent the latter from overaging upon slow cooling from solution treatment in Ni-base superalloys based on the composition of alloy 718. By computer simulation using a multi-phase-field model, we find that the growth of a coprecipitate involves several concurrent and closely coupled processes, including thickening and lengthening of γ ' ' shells, growth of the γ ' core along the " open=" 001 γ ' , " open=" 011 γ ' and " open=" 111 γ ' directions, hard impingement between γ ' and γ ' ' precipitates, and soft impingement among γ ' ' precipitates of different variants. These processes at different stages of growth are analyzed systematically as a function of coprecipitate size and configuration, and the results show that the growth kinetics of the γ ' core in a coprecipitate is controlled by the interplay among: (1) partial removal of supersaturated γ matrix surrounding the γ ' core by coprecipitation of γ ' ' shells, (2) cooperative growth of γ ' and γ ' ' in the coprecipitates and (3) atomic mobility of γ ' -formers in the γ ' ' phase. To maximize the effect of coprecipitation on preventing γ ' from overaging upon slow cooling, the alloy composition and heat treatment schedule should be optimized to minimize the size of γ ' cores at which coprecipitation of γ ' ' shells occurs and to reduce diffusion of γ ' -formers through γ ' ' .

A strategy for the microstructural design to achieve a critical upper limit of uniform nano α precipitates by controlling the amount of dislocations in the pre-deformed matrix after short time aging treatment was proposed in Ti–10Mo–8V–1Fe–3.5Al (all in wt.%, TB3 alloy) β alloy. The optimal processes focused on the interaction of defects, mainly point defects and dislocations that are generated during cold rolling (CR). Texture evolution of α and β phases was also specified. Amount of nano-scaled α precipitates was characterized by the number density of precipitates observed by SEM, which is saturated at ∼204.1 μm−2. For the CR 20% sample, a good combination of tensile strength (∼1400 MPa) and elongation (∼12%) was achieved after secondary aging at 550 for 1 h, which provides an enlightenment for the re-engineering of traditional precipitate-hardening alloys. Keywords: β-Ti alloy, Short-time aging saturation, Uniform α precipitates, Mechanical properties

Phase instabilities in a recently developed gum like alloy, Ti-24Nb-4Zr-8Sn (Ti2448), were investigated using conventional and aberration-corrected scanning/transmission electron microscopy. Athermal ω (hexagonal) and O′ (orthorhombic) phases were found to be present in the alloy following solution treatment. A partial collapse of {111}β atom planes was observed in the ω phase, and a shuffle of every other {011}β atom planes along 0 1 ¯ 1 β was observed in the O′ phase particle. The presence of both the ω and O′ phases in Ti2448 may influence the transition behavior under loading and may have significant impact on its mechanical properties.

Variant selection during solid–solid phase transformation in titanium alloys affects greatly the microstructure and mechanical properties of the alloys. Theoretical investigations of the variant selection were generally performed by using phase field simulation that considers solely the elastic coherency strain energy. In the present work, we develop a model to determine the variant selection directly from first-principles calculations. The source of the variant selection is considered to be the varying free energy gains (including both bulk and interface contributions) induced by the transformation from the parent phase to different variants of the product phase. This model is applied to investigate the effects of shear stress and alloy composition on ω variant selection during athermal β → ω transformation in binary Ti-xMo ( x ≤ 25 at.%) alloys. The random distribution of the atoms in the alloy is described by using virtual crystal approximation (VCA). We show that the tendency of variant selection becomes stronger with increasing shear stress. With increasing Mo concentration, the favorable ω variant transfers from one to another, except for the ω phase with very small particle size (e.g., radius R ≤ 1 nm) where one variant is always selected. The critical Mo concentration for the transfer of the favorable ω variant approaches to 5 at.% with increasing size of the ω phase particles and remains almost unchanged against the shear stress. At the critical Mo concentration, the considered variants have equal free energy gains and there is no variant selection. This finding opens the possibility of controlling the variant selection and the associated microstructure by changing the composition of the alloys.

Voids and dislocation loops are two major types of damages in irradiated structural materials, which are mainly responsible for the degradation of material properties. Here we use the phase-field model and rate theory to simulate the microstructural evolution of voids and dislocation loops, respectively, in irradiated bcc iron and vanadium. The temperature-dependent material parameters of iron and vanadium are derived from ab initio calculations. The simulated results at different temperatures (513 K, 623 K and 722 K) and irradiation doses (1∼20 dpa) are analyzed to reveal the impact of irradiation conditions on the formation of irradiation-induced defect clusters. A comparison of the results shows larger void porosity and void/loop size in iron and higher void/loop density in vanadium. Then, a dispersed-barrier hardening model is used to correlate the mesoscale simulation results on microstructure with the yield stress change of the materials.

We report the first direct quantification of the structural heterogeneity in metallic glasses using intensity variance and angular correlation analyses of the 4-dimensional (4-D) scanning transmission electron microscopy (STEM) data. We demonstrate that the real-space reconstruction and analyses of the 4-D nanodiffraction data acquired using a pixelated fast STEM detector enables quantitative determination of the details of local structural heterogeneity, including the type, size, volume fraction and spatial distribution of local ordering at the nano- to meso-scale, beyond the limits of the previous measurements using conventional detectors. We show that different types of local ordering are present in Zr55Co25Al20 glass, leading to a high degree of structural heterogeneity, with the total volume of locally ordered regions making up to ∼14% of the entire volume. These findings are significant, as the structure-property relationship in metallic glasses and other amorphous materials has been difficult to establish because of the lack of detailed structural information from experiments.