Undergo Phase Transformation Research Articles

Quantification of the Dislocation Density, Size, and Volume Fraction of Precipitates in Deep Cryogenically Treated Martensitic Steels

Two groups of martensitic alloys were examined for changes induced by deep cryogenic treatment (DCT). The first group was a range of binary and ternary compositions with 0.6 wt % carbon, and the second group was a commercial AISI D2 tool steel. X-ray diffraction showed that DCT made two changes to the microstructure: retained austenite was transformed to martensite, and the dislocation density of the martensite was increased. This increase in dislocation density was consistent for all alloys, including those that did not undergo phase transformation during DCT. It is suggested that the increase in dislocation density may be caused by local differences in thermal expansion within the heterogeneous martensitic structure. Then, samples were tempered, and the cementite size distribution was examined using small angle neutron scattering (SANS) and atom probe tomography. First principles calculations confirmed that all magnetic scattering originated in cementite and not carbon clusters. Quantitative SANS analysis showed a measurable change in cementite size distribution for all alloys as a result of prior DCT. It is proposed that the increase in dislocation density that results from DCT modifies the cementite precipitation through enhanced diffusion rates and increased cementite nucleation sites.

Effect of shading and hydrothermal aging on optical properties of partially and fully stabilized zirconia

Statement of problem: Zirconia undergoes phase transformation due to low temperature degradation LTD which could deteriorate its optical properties. Objectives: Current study was conducted to evaluate the effect of shading and hydrothermal aging on optical properties of partially and fully stabilized zirconia. Materials and methods: Two commercially available zirconia products, high-translucent PSZ (Prettau®, Zirkonzahn) and ultra-translucent FSZ (Prettau®, Anterior, Zirkonzahn) were assessed. 80 disc shaped specimens (12×1 mm) were prepared; half of the specimens were shaded using acid-free coloring solution. Half of the specimens were subjected to accelerated aging using autoclave (134°c at 2 MPa) for 5 hours. The specimens were divided into 4 subgroups. Non-shaded, non-aged (control), (non-shaded, aged group), (shaded, non-aged group) and (shaded, aged group). Contrast ratio CR and translucency parameter TP were evaluated in each subgroup using reflective spectrophotometer. X-ray diffraction analysis was used to measure the amount of tetragonal to monoclinic phase transformation. Microstructural analysis was evaluated using atomic force microscope. SPSS program was used to evaluate the data (P≤0.05). Results: FSZ recorded higher translucency parameter and lower contrast ratio than PSZ; which were significantly affected by shading and aging in all subgroups except for non-shaded aged subgroups of PSZ. Conclusions: Shading and aging induced tetragonal to monoclinic transformation in PSZ and FSZ which lead to changes in their optical properties.

Fe(II)-induced transformation of iron minerals in soil ferromanganese nodules

Recrystallization of iron (hydr)oxides by ferrous iron [Fe(II)] is an important part of the biogeochemical cycling of iron in earth surface environments. Ferromanganese nodules are complex assemblages of natural Fe and Mn minerals, which are widely found in soils and marine environments, and critically impact the geochemical behavior of trace metals and nutrients. However, little is known regarding the Fe(II)-induced recrystallization of ferromanganese nodules, in comparison to pure iron (hydr)oxides. Accordingly, the objective of this study was to elucidate the reaction processes of aqueous Fe(II) with ferromanganese nodules, and to use complementary spectroscopic techniques to characterize the products. Goethite in the nodule that underwent phase transformation to magnetite or Fe(II) oxidation to form magnetite has not been reported in previous studies with respect to pure goethite. 57Fe-enriched isotope tracer experiments confirmed Fe atom exchange between aqueous Fe(II) and structural Fe(III) of the nodule and was enhanced at higher pH conditions. The coexistence of Mn oxides and other minerals possibly make the goethite in the nodule more reactive for phase transformation to magnetite than pure synthetic goethite. In addition, Fe(III) precipitates were formed due to Fe(II) oxidation by Mn(III) of the nodule, and magnetite was likely consequently produced through further Fe(II)-induced reaction. Considering the enrichment of trace metals and nutrients in ferromanganese nodules, the observed Fe(II)-induced recrystallization of nodules in the study is expected to exert important effects on the geochemical behavior of the elements, especially in nodule-enriched soils.

Electrochemical performance and stability of LSFMn+NiSDC anode in dry methane

Anode deactivation over prolonged SOFC operation represents a major issue towards stable and reliable energy production from hydrocarbon-containing fuels. In this respect, Ni-YSZ replacement with oxide-based anodes, less prone to coking, is commonly regarded as a viable solution. Nevertheless, most studied oxides cannot compete with Ni-based state-of-the-art anodes in terms of conductivity and catalytic activity. In this work a novel MIEC anodic composite based on La0.6Sr0.4Fe0.8Mn0.2O3-δ (LSFMn) perovskite and Ce0.85Sm0.15O2-δ containing 5 wt.% of Ni (NiSDC) was tested in dry methane. In reducing conditions, LSFMn undergoes phase transformation to Ruddlesden-Popper structure, exsolving Fe0 nanoparticles and retaining proper conductivity. The small amount of Ni, well-dispersed on the SDC matrix, forms a Ni-Fe alloy with the exsolved iron from the perovskite. While Ni-Fe alloy activates CH4, SDC provides enough O2− ions to electrochemically oxidize any cracking products. Endurance cell test at 800°C in dry CH4 shows a stable current density output for over 33h and both SEM and Raman Spectroscopy do not reveal any anode fouling.

Correlated noise effect on the structure formation in the phase‐field crystal model

A phase‐field crystal model (PFC model) which takes into account exponential relaxation of the atomic flux and its fluctuations is developed. The model corresponds to a system undergoing phase transformation described with a partial differential equation of hyperbolic type. Such a model covers slow and rapid regimes of interface propagation at small and large driving forces during melting and solidification. The analysis is done for the evolution of atomic crystal lattices appearing from a metastable homogeneous liquid for the chemically pure system supercooled below its critical temperature. Numerical simulation of the system “Liquid ‐ Body Centered Cubic (BCC) crystal lattice” allows us to formulate the hypothesis about the formation of metastable periodic solutions (atomic configurations which are not in the thermodynamic equilibrium) during the relaxation of atomic configurations to the stable equilibrium. These metastable states may be destroyed (or even avoided) due to the action of colored noise. Namely, considering spatiotemporal correlations of the atomic flux fluctuations, we have found that the temporal correlations promote selecting long‐range atomic lattices, whereas the spatial correlations corresponding to the periodic structure scales decelerate such processes.

A Novel Concept of Hybrid Treatment for High‐Hardenability Steels: Concomitant Hardening and Paraequilibrium Thermochemical Treatment to Produce Interstitially Hardened/Stabilized Austenite Surfaces

A novel concept of hybrid treatment for steels is presented. The goal is to produce a hard interstitially hardened/stabilized austenite layer, with an expected improved wear and corrosion resistance, in the surface of high‐hardenability steels. The novelty is in the fact that the thermochemical treatment is performed in the steel metastable austenite field, above the martensite start temperature, during the isothermal step of the martempering or austempering hardening treatment. As a proof of concept, CA6NM martensitic stainless steel samples are subjected to the novel hybrid treatment. The whole treatment is plasma assisted and the chosen thermochemical treatment is nitriding. In the presented case, the produced hard layer is constituted of N‐stabilized austenite. During the paraequilibrium thermochemical treatment step, the metastable austenite surface is converted into a hard interstitially hardened/N‐stabilized austenite. Once concluded the whole treatment, as the component is cooled down to the room temperature, surface does not undergo phase transformation, whereas the nonchemically altered substrate bulk undergoes metastable austenite to martensite phase transformation. So, by means of this case study, the feasibility and potentiality of this novel concept of hybrid treatment is proved.

Friction stir welding of Ti-6Al-4V alloy: Friction tool, microstructure, and mechanical properties

Abstract In this work, 2 mm-thick Ti-6Al-4V plates were successfully friction stir welded using a newly designed friction tool, and the microstructure evolution and mechanical behaviors of weld were investigated. The process parameters of friction stir welding (FSW) were explored and defect-free welds were produced. The cross-sections of the weld showed typical bowl profiles, which can be divided into four regions: base metal (BM), shoulder-affected zone (SAZ), stir zone (SZ), and heat-affected zone (HAZ). The peak temperature in the SZ exceeded the β-phase transition temperature and the materials in the SZ underwent phase transformation during FSW. The final microstructure of the SZ was a lamellar structure with (α + β) phase and the HAZ has a bimodal microstructure consisting of equiaxed prior α and lamellar (α + β) grains. The maximum tensile strength and yield strength of the FSWed samples reached up to 92 % of the base metal and the tensile failures occurred at the SZ. The micro-hardness varied significantly along the thickness of the plate. While the micro-hardness value of SZ was lower than that of BM due to the grain coarsening and dynamic recrystallization during the welding. The tensile fracture morphologies exhibited a typical quasi-cleavage fracture.

A Study of Recrystallization Behavior of Two Ni-Cu–Containing High-Strength Steels with Different Mn Contents

Abstract The Defence Metallurgical Research Laboratory (DMRL) has been investigating several high-strength and high-toughness steels in the past few decades for various defense applications. As a part of this work, two novel low-carbon steels—each containing 4–6 wt% Ni + Cu of similar strength level, one with high Mn (∼1 wt%) and one with low Mn (∼0.3 wt%)—were investigated. In order to optimize the rolling schedules during production, it is necessary to understand the recrystallization behavior of these two steels during hot deformation. Usually, the recrystallization behavior of materials that do not undergo phase transformations (viz. aluminum, copper, etc.) is commonly studied using microstructural techniques for identification of fraction of recrystallized grains. But, as the steels in the present study undergo phase transformation wherein the austenitic structure at deformation temperature is transformed to martensitic/bainitic structure during cooling, it is challenging to track the recrystallization process directly from the microstructure. Therefore, the recrystallization behavior was investigated by conducting double-hit compression tests (DHCTs) using Gleeble. Fractional softening values were determined from DHCT data using various methods, such as the offset method, back-extrapolation method, and mean flow stress method. The mean flow stress method was found to be more reliable and thus was adopted in this study. The effects of different deformation temperature (850°C–1,050°C) and intermediate static recrystallization time (ISRT) (10, 100, and 500 s) on fractional softening were studied. Temperature for 50 % recrystallization (T0.5), determined as the temperature for 50 % fractional softening, was seen to reduce with an increase in ISRT. Steel with lower manganese content showed relatively lower fractional softening values and thus relatively higher T0.5, as compared with that for the steel with higher manganese content, for all three ISRTs.

Microstructure and properties of Al2O3-ZrO2-Y2O3 coatings during high temperature and thermal shock resistance

Al2O3-ZrO2 composite coatings were fabricated by plasma spraying Al2O3-ZrO2 composite powders with and without Y2O3 addition to learn the influence of Y2O3 on high temperature stability of the coating. The two coatings both underwent phase transformation in the process of heat treatment. Specifically, XRD results showed that in the Al2O3-ZrO2-Y2O3 coating, Y3Al5O12 (YAG) phase existed after heating at 800 °C, and γ-Al2O3 all transformed to α-Al2O3 after heating at 1200 °C, so the composition of the coating was t-ZrO2, α-Al2O3 and YAG after heating at 1200 °C, while in the Al2O3-ZrO2 coating, the final coating consisted of t-ZrO2 and α-Al2O3 beacuse γ-Al2O3 all transformed to α-Al2O3 after heating at 1200 °C. The amorphous phase in the two as-sprayed coatings had been crystallized completely after heat treatment at 1200 °C. Heat treatment can reduce the porosity and improve the hardness of the two coatings, and YAG phase can also improve the hardness. The coating with TC4 substrate had better thermal shock resistance than it with Q235 steel substrate. The thermal shock resistance of the Al2O3-ZrO2-Y2O3 coating was better than that of the Al2O3-ZrO2 coating at same condition. NiCrAlY bonding layer can improve thermal shock resistance of the coating.

Phase transitions, percolation, fracture of materials, and deep learning.

Percolation and fracture propagation in disordered solids represent two important problems in science and engineering that are characterized by phase transitions: loss of macroscopic connectivity at the percolation threshold p_{c} and formation of a macroscopic fracture network at the incipient fracture point (IFP). Percolation also represents the fracture problem in the limit of very strong disorder. An important unsolved problem is accurate prediction of physical properties of systems undergoing such transitions, given limited data far from the transition point. There is currently no theoretical method that can use limited data for a region far from a transition point p_{c} or the IFP and predict the physical properties all the way to that point, including their location. We present a deep neural network (DNN) for predicting such properties of two- and three-dimensional systems and in particular their percolation probability, the threshold p_{c}, the elastic moduli, and the universal Poisson ratio at p_{c}. All the predictions are in excellent agreement with the data. In particular, the DNN predicts correctly p_{c}, even though the training data were for the state of the systems far from p_{c}. This opens up the possibility of using the DNN for predicting physical properties of many types of disordered materials that undergo phase transformation, for which limited data are available for only far from the transition point.

Derived crystal structure of martensitic materials by solid-solid phase transformation.

A mathematical description of crystal structure is proposed consisting of two parts: the underlying translational periodicity and the distinct atomic positions up to the symmetry operations in the unit cell, consistent with the International Tables for Crystallography. By the Cauchy-Born hypothesis, such a description can be integrated with the theory of continuum mechanics to calculate a derived crystal structure produced by solid-solid phase transformation. In addition, the expressions for the orientation relationship between the parent lattice and the derived lattice are generalized. The derived structure rationalizes the lattice parameters and the general equivalent atomic positions that assist the indexing process of X-ray diffraction analysis for low-symmetry martensitic materials undergoing phase transformation. The analysis is demonstrated in a CuAlMn shape memory alloy. From its austenite phase (L21 face-centered cubic structure), it is identified that the derived martensitic structure has orthorhombic symmetry Pmmn with the derived lattice parameters ad = 4.36491, bd = 5.40865 and cd = 4.2402 Å, by which the complicated X-ray Laue diffraction pattern can be well indexed, and the orientation relationship can be verified.

Structural Evolution and Redox Mechanism of O3-NaNi1/3Fe1/3Mn1/3O2 Layered Cathode for Na Rechargeable Batteries

Sodium-ion batteries (SIBs) have great potential to alternate Li-ion batteries (LIBs) for large-scale energy storage systems in view of easy accessibility to Na resources and low cost [1–4]. Since Na ions are similar chemical characteristics of Li ions, the knowledge from research in LIBs can be easily applied to Na-based systems. Derived from the equivalent structures of Li analogue, various electrode materials such as oxides, polyanionic compounds, and sulfates, have been researched in SIBs to date [2,4–8]. One of the cathode candidates for SIBs, layered transition metal oxides (Na x TMO2, x ≤ 1, TM = transition metals) are of great interest due to their potential of relatively high capacity, simple structure, and easy synthesis [9,10].In this study, layered sodium-ion battery cathode, O3-type NaNi1/3Fe1/3Mn1/3O2, has been systematically investigated by synchrotron-based analyses to characterize the structural behavior during electrochemical reaction. X-ray absorption spectroscopy shows reversible redox process upon cycling and clearly proves that both Ni and Fe are active in Na1–x Ni1/3Fe1/3Mn1/3O2 and that redox couples of Ni2+/Ni4+ via Ni3+ and Fe3+/Fe4+ are responsible for charge compensation. Specifically, the capacity is mainly realized with Ni2+/Ni4+ and slightly from Fe3+/Fe4+ under charging voltage of 4.0 V. At high voltage (> 4.0 V), however, Feredox reaction is dominant and Ni contributes slightly to capacity. In terms of structural evolution, Na1-x Ni1/3Fe1/3Mn1/3O2 undergoes phase transformation from O3 to P3 structure below 4.0 V and further reaches OP2 structure above 4.0 V along with a significant contraction of d-spacing. Moreover, quantitative analysis of extended X-ray absorption fine structure suggests that disorder of local structure for Fe is greatly increased in high voltage region. Accordingly, collapse of d-spacing can be considered as being caused by Fe migration in the TM layer into the neighboring Na layer. This study will give a better understanding of phase transformation and clear charge compensation of NaNi1/3Fe1/3Mn1/3O2 layered cathode during Na+ deintercalation/intercalation. Furthermore, we propose the factor to bring the structural distortions under high voltage region by examining the local environment changes of each transition metal.From these experimental results, we will discuss structural evolution behavior and particular redox reaction of layered NaNi1/3Fe1/3Mn1/3O2 cathode material. More detailed results and discussion will be presented in the 237th ECS meeting.

Observations of grain-boundary phase transformations in an elemental metal.

While the theory of grain boundary (GB) structure has a long history1 and the proposition that grain boundaries can undergo phase transformations was made already 50 years ago2,3, the search for these GB transitions has been in vain. The underlying assumption was that multiple stable and metastable states exist for different grain boundary orientations4,5,6. The terminology complexion was recently proposed to distinguish between those interfacial states that differ in any equilibrium thermodynamic property7. Different types of complexions and transitions between them have been characterized mostly in binary or multicomponent systems 8,9,10,11,12,13,14,15,16,17,18,19. Novel simulation schemes have provided a new perspective on the phase behavior of interfaces and showed that grain boundary transitions can occur in a multitude of material systems20,21,22,23,24. However, their direct experimental observation and transformation kinetics in an elemental metal remained elusive.Here, we show atomic scale grain boundary phase coexistence and transformations at symmetric and asymmetric [11-1] tilt grain boundaries in elemental copper (Cu). We found the coexistence of two different grain boundary structures at Σ19b grain boundaries by atomic resolution imaging. Evolutionary grain boundary structure search and clustering analysis21,25,26 finds the same structures and predicts the properties of these GB phases. Furthermore, finite temperature molecular dynamics simulations explore their coexistence and transformation kinetics. Our results demonstrate how grain boundary phases can be kinetically trapped enabling atomic scale room temperature observations.Our work paves the way for atomic scale in situ studies of grain boundary phase transformations in metallic grain boundaries. In the past, only indirect measurements have indicated the existence of interfacial transitions9,15,27,28,29. Now, their atomic scale role on the influence of abnormal grain growth, non-Arrhenius type diffusion or liquid metal embrittlement can be explored.

Synthesis of Ge-based nanosheet bundles using calcium germanides as templates in IP6 aqueous solution

Ge-based nanosheet bundles were synthesized by de-intercalating Ca atoms from the lattice of CaGe2 and Ca-Germanide (synthesized and commercially available) using inositol hexakisphosphate (IP6) acid, with and without ultrasonication assistance. Ca-Germanides were synthesized via the thermal annealing and the mechanical alloying (MA) processes. Multiple binary phases of Ca–Ge system with low X-ray diffraction (XRD) intensities were obtained after the MA operation along with high XRD intensity elemental Ge phases, whereas with the thermal annealing process yielded phase transformation from elemental Ge to mixed binary mixture of Ca–Ge system. The obtained Ge-based nanosheet bundles were subjected phase and morphological characterization. It revealed that CaGe2 underwent phase transformation after IP6 treatment and a layered morphology was obtained. This paper describes a simplistic approach to synthesize Ge-based nanosheets using Ca-Germanide crystals as templates. A detailed morphological and phase evolution of the template Ca-Germanide and exfoliated Ge-based nanosheet bundles have been presented in this work.

In Situ Neutron Diffraction Study of Phase Transformation of High Mn Steel with Different Carbon Content

In situ neutron diffraction was employed to examine the phase transformation behavior of high-Mn steels with different carbon contents (0.1, 0.3, and 0.5 wt.%C). With increasing carbon contents from 0.1 C to 0.5 C, the austenite phase fraction among the constituent phases increased from ~66% to ~98%, and stacking fault energy (SFE) increased from ~0.65 to ~16.5 mJ/m2. The 0.1 C and 0.3 C steels underwent phase transformation from γ-austenite to ε-martensite or α’-martensite during tensile deformation. On the other hand, the 0.5 C steel underwent phase transformation only from γ-austenite to ε-martensite. The 0.3 C steel exhibited a low yield strength, a high strain hardening rate, and the smallest elongation. The high strain hardening of the 0.3 C alloy was due to a rapid phase transformation rate from γ-austenite to ε-martensite. The austenite of 0.5 C steel was strengthened by mechanical twinning during loading process, and the twinning-induced plasticity (TWIP) effect resulted in a large ductility. The 0.5 wt.% carbon addition stabilized the austenite phase by delaying the onset of the ε-martensite phase transformation.

High temperature microstructure evolution and thermal shock resistance of plasma sprayed Al2O3-ZrO2-CeO2 coatings

Al2O3-ZrO2-CeO2 composite coatings were prepared by plasma spraying Al2O3-ZrO2 composite powders with and without CeO2 addition. The microstructure and properties of the Al2O3-ZrO2-CeO2 coatings at high temperature and thermal shock resistance were investigated. The results show that an amorphous phase was formed in the plasma sprayed Al2O3-ZrO2 and Al2O3-ZrO2-CeO2 coatings. The Al2O3-ZrO2 coating consisted mainly of t-ZrO2, α-Al2O3, and γ-Al2O3. The Al2O3-ZrO2-CeO2 coating consisted mainly of t-ZrO2, α-Al2O3, γ-Al2O3, and CeO2. The Al2O3-ZrO2-CeO2 coating was more uniform and denser than the Al2O3-ZrO2 coating. The results after heat treatment showed that the two coatings were stable at 800 °C. With the increase of temperature, the coatings underwent phase transformation at high temperature (1000 and 1200 °C). At 1000 °C, the amorphous phase was obviously reduced, and the amorphous phase was crystallized to form nanocrystals. When the heat treatment temperature was raised to 1200 °C, the intensity of the diffraction peaks of t-ZrO2 and α-Al2O3 increased, the amorphous phase was completely crystallized, and γ-Al2O3 was transformed into α-Al2O3. The porosity of the two coatings decreased and the hardness increased after heat treatment. The thermal shock resistance of the coatings on the TC4 titanium alloy substrate was better than that on the Q235 steel substrate. The thermal shock resistance of the Al2O3-ZrO2-CeO2 coating was better than that of the Al2O3-ZrO2 coating.

Prediction of strain path and forming limit curve of AHSS by incorporating microstructure evolution

Austenitic stainless steel is known as second-generation advanced high strength steel (AHSS) due to its very high strength and elongation. The stability of austenite phase under deformation at room temperature depends on alloying elements such as Ni and Mn. Low-Ni austenitic stainless steel undergoes phase transformation, austenite to martensite, during plastic deformation. The phase transformation and crystallographic texture development are strain and strain path dependent. Consequently, material properties especially work hardening exponent (n) and normal anisotropy ($$ \overline{r} $$) are altered during plastic deformation. To understand the effect of dynamic mechanical properties on the formability of low-Ni austenitic stainless steels, limiting dome height (LDH) tests were performed. Strain path diagrams (SPD) and forming limit diagrams (FLD) were plotted for this material. Finite element (FE) analyses were carried out with constant and varying (with strains and strain paths) n and $$ \overline{r} $$ values. The work hardening exponent was calculated for uniaxial strain path and correlated with in-grain misorientation and martensite phase fraction at different strains. This relationship is further extended to estimate the work hardening parameter for other strain paths. Strain path–dependent instantaneous $$ \overline{r} $$ values were determined by using bulk texture analysis. The strain path–dependent n and $$ \overline{r} $$ values were incorporated during FE simulation which remarkably improved the SPD and FLD predictability of low-Ni austenitic stainless steel.

Water dispersible ligand-free rare earth fluoride nanoparticles: water transfer versus NaREF4-to-REF3 phase transformation.

The chemical stability of oleate-capped sub-10 nm α- and β-NaREF4 NPs (RE = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu for α- and RE = Pr, Nd, Sm, Eu, Gd, Tb, Dy for β-phase NPs) was evaluated under the acidic conditions used for ligand removal towards water dispersibility. It was found that for such small NPs, a pH lower than 3 was necessary for the water transfer to be efficient and to yield well-dispersed ligand-free NPs. In stark contrast to the generally considered good chemical stability of NaREF4, these conditions were observed to pose a risk to phase transformation of the NaREF4 NPs into much larger, hexagonal- or orthorhombic-phase REF3, depending on the NP composition. A correlation between the thermodynamic stability of the α/β-NaREF4 and the hexagonal/orthorhombic REF3 phases - dictated by the RE ion choice - and the chemical stability of the NPs was found. For instance, β-NaGdF4 NPs remained stable, while α-NaGdF4 NPs underwent phase transformation into hexagonal GdF3. More general, NaREF4 NPs based on lighter RE ions were more prone towards phase transformation, while those based on heavier RE ions exhibited stability. Herein, within the RE series, the borderline for phase transformation was identified as Tb/Dy for α-NaREF4 NPs and Sm/Eu for β-NaREF4 NPs, respectively. Also, given the large interest in luminescent NPs for, e.g. biomedical applications, optically active Ln3+ ions (Ln = Nd, Eu, Tb, Er/Yb) were doped into α/β-NaGdF4 host NPs, and the dopant influence on the chemical stability was evaluated. Steady state and time-resolved spectroscopy unveiled spectral features characteristic for Ln3+ f-f transitions, i.e. downshifting and upconversion, before and after ligand removal. Overall, the results herein described emphasise the importance of minding the chemical procedure used for ligand removal of NaREF4 NPs of different crystalline phases and RE compositions.

Modeling deformation induced anisotropy of light-activated shape memory polymers

Light-activated shape memory polymers (LASMPs) are a new generation of active materials that undergo phase transformations upon light irradiation at different wavelength and intensities. This type of material shows much promise in its use in adaptive and shape reconfiguration structures for applications in biomedical and aerospace engineering. LASMPs exhibit nonlinear viscoelastic behavior due to the rearrangements in the macromolecular networks of the polymers when they are mechanically deformed. The deformed configuration can have a different symmetry group than the original undeformed configuration. In this study we assume that the body has multiple natural configurations in order to incorporate the microstructural changes in the viscoelastic polymers due to mechanical loading and light irradiation. We also account for the directional preference of the new network that is formed due to stretching the polymers. In this paper, the constitutive models developed for elastic and viscoelastic LASMPs in our previous work (Yuan and Zhi, 2017; Zhi and Yuan, 2017) are modified to include the changes in the symmetry group of the new network due to mechanically stretching the polymer. The models are implemented within ABAQUS finite element (FE) through the use of user-material subroutines (UMAT).

In situ heating TEM observations of evolving nanoscale Al-Mg-Si-Cu precipitates.

This research concerns the precipitation and subsequent dissolution of precipitate phases in an Al-0.86Mg-0.62Si-0.1Cu (at.%) alloy subjected to varying thermal exposure during an in situ heating transmission electron microscope (TEM) experiment. The distribution and crystal structure of precipitates were determined by a scanning diffraction approach at multiple stages, pinpointing the precipitates that underwent phase transformations during heating. Obtained results were compared with TEM studies of the material heated ex situ. This revealed differences in the transformation kinetics of precipitates in an electron transparent lamella (thickness ) to that of macroscopic bulkspecimens. LAY DESCRIPTION: The precipitate phases that are formed in age-hardenable Al alloys are conventionally studied using the TEM, with electron transparent specimens prepared from bulk materials heat treated for various times. We demonstrate here that the growth, phase transformation and subsequent dissolution of precipitate phases in the Al-Mg-Si-Cu system can be studied successfully by in situ heating TEM. Imaging and scanning diffraction combined with data postprocessing were used to study the distribution and structure of transforming precipitates. This enabled us to track the precipitates that underwent phase transformations during heating, hence offering a direct quantitative assessment of the precipitation occurring in these materials. The narrow thickness of the electron transparent specimen (≈90 nm) may potentially have affected the resulting precipitation. Therefore, thickness effects were assessed by comparing obtained results with TEM studies of ex situ heated bulkspecimens.