Microstructural Origin Research Articles

Crystal Plasticity with Micromorphic Regularization in Assessing Scale Dependent Deformation of Polycrystalline Doped Copper Alloys

It is planned that doped copper overpacks will be utilized in the spent nuclear fuel repositories in Finland and in Sweden. The assessment of long-term integrity of the material is a matter of importance. Grain structure variations, segregation and any possible manufacturing defects in microstructure are relevant in terms of susceptibility to creep and damage from the loading evolution imposed by its operating environment. This work focuses on studying the microstructure level length-scale dependent deformation behavior of the material, of particular significance with respect to accumulation of plasticity over the extensive operational period of the overpacks. The reduced micromorphic crystal plasticity model, which is similar to strain gradient models, is used in this investigation. Firstly, the model’s size dependent plasticity effects are evaluated. Secondly, different microstructural aggregates presenting overpack sections are analyzed. Grain size dependent hardening responses, i.e., Hall-Petch like behavior, can be achieved with the enhanced hardening associated with the micromorphic model at polycrystalline level. It was found that the nominally large grain size in the base material of the overpack shows lower strain hardening potential than the fine grained region of the welded microstructure with stronger strain gradient related hardening effects. Size dependent regularization of strain localization networks is indicated as a desired characteristic of the model. The findings can be utilized to provide an improved basis for modeling the viscoplastic deformation behavior of the studied copper alloy and to assess the microstructural origins of any integrity concerns explicitly by way of full field modeling.

Microstructural observation of complex-shaped green ceramic compact and numerical simulation with special consideration on crack formation

The microstructure of a die-pressed green ceramic compact with a complex shape was visualized by liquid immersion and X-ray micro-computed tomography techniques. Significant microstructural inhomogeneity in terms of the pore distribution and granule shape was observed depending on the sample locations, while cracks were formed only at certain positions. The microstructural differences were explained by the von Mises stress gradient within the compact, which originated from friction between the granule and mold as well as non-uniform pressure delivery because of the complex sample shape. A numerical simulation was performed based on the modified Drucker–Prager/Cap model to explain the origin of the different microstructures, such as pore distribution and crack formation.

Vascular Complications in Individuals with Type 2 Diabetes Mellitus Additionally Increase the Risk of Femoral Neck Fractures Due to Deteriorated Trabecular Microarchitecture.

Individuals with diabetes mellitus type 2 (T2DM) have an increased risk of hip fracture, especially if vascular complications are present. However, microstructural origins of increased bone fragility in T2DM are still controversial. DXA measurement of the contralateral hip and three-dimensional microCT analyses of femoral neck trabecular microarchitecture were performed in 32 individuals (26 women and 6 men, 78 ± 7 years). The specimens were divided to two groups: T2DM individuals with hip fracture (DMFx, n = 18) and healthy controls (CTL, n = 14). DMFx group consisted of individuals with vascular complications (DMFx_VD, n = 8) and those without vascular complications (DMFx_NVD, n = 10). T-score was significantly lower in DMFx_VD and DMFx_NVD than in controls (p < 0.001). BV/TV, Tb.N, Tb.Sp, SMI, and FD varied among DMFx_NVD, DMFx_VD, and CTL groups (p = 0.023, p = 0.004, p = 0.008, p = 0.001, p = 0.007, respectively). Specifically, BV/TV of DMFx_VD was significantly lower than that of DMFx_NVD group (p = 0.020); DMFx_NVD group had higher Tb.N and lower Tb.Sp compared with DMFx_VD (p = 0.006, p = 0.012, respectively) and CTL (p = 0.026, p = 0.035, respectively). DMFx group and healthy controls showed similar BV/TV, Tb.Th, Tb.N, Tb.Sp, Conn.D, DA, and FD (p = 0.771, p = 0.503, p = 0.285, p = 0.266, p = 0.208, p = 0.235, p = 0.688, respectively), while SMI was significantly higher in controls (p = 0.005). Two distinct phenotypes of bone fragility were identified in T2DM patients: patients with vascular complications showed impaired trabecular microarchitecture, whereas bone fragility in the group without vascular complications was independent on trabecular microarchitecture pattern. Such heterogeneity among T2DM patients may explain contradicting literature data and may set a basis for further studies to evaluate fracture risk related to T2DM.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00223-021-00894-5.

A polarized micro-Raman study of necked epoxy fibers

Fully cured epoxy resins are typically brittle materials but according to recent research, cured epoxy fibers exhibit a singular mechanical behavior, including yielding followed by large deformation, and very high strength, toughness, and modulus. These properties appear to intensify as the fiber diameter decreases. The microstructural origin of this unusual behavior has not been fully determined. Here we use confocal polarized Raman spectroscopy to monitor the apparent molecular reorientation induced by plastic deformation of epoxy fibers, both qualitatively and quantitatively. Based on these and previous X-ray diffraction measurements, a likely molecular explanation for the extreme mechanical behavior of micro-sized epoxy fibers is proposed.

Temperature dependence and correlation of polarization processes in P(VDF-HFP) films

Polymer dielectrics are indispensable in electrical and electronic systems, being used as insulation, capacitors, gate dielectric, etc. However, a lot of polymer materials show unsatisfactory efficiency at high temperature and low frequencies, which has been found to be closely related to polarization processes. In this paper, the dielectric spectroscopy of poly (vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) films is investigated over a broad frequency range from 10 mHz to 1 MHz and temperature range from −130 °C to 130 °C based on the Dissado-Hill model. The processes in the entire response are identified and characterized quantitatively. The temperature dependence of major processes contributing to energy storage and dielectric loss are revealed. It is found that the variations of both β polarization and DC conductance coincide with Arrhenius equation, while both α polarization and quasi-DC polarization show Vogel−Fulcher−Tammann (VFT) features. Moreover, α polarization and quasi-DC polarization share the same form parameter of VFT equation, indicating that they have a common origin of microstructure. The origin and correlation of polarization processes could be used to develop a paradigm for rational design of polymer dielectrics to enhance or suppress specific processes for certain purposes.

Formation of β"-related composite precipitates in relation to enhanced thermal stability of Sc-alloyed Al-Mg-Si alloys

The Al-Mg-Si alloys added with trace Sc demonstrates an improved thermal stability, as compared with that without Sc-addition. But the microstructural origin of the enhanced property is unclear. In the present study, the precipitates formed in an Al-Mg-Si-Sc alloy were studied in detail by atomic-resolution scanning transmission electron microscopy. It is shown that the dominant precipitates formed at the peak-aging condition are the disordered β" precipitates consisting of Sc-free β" sub-units and Sc-containing disordered regions, rather than the well-ordered β". Upon further aging, these disordered β" precipitates will evolve to β"/β'/B′/U2 composite precipitates. The formation of the composite precipitates is probably associated with the strong binding between Sc and Si atoms, and with the non-preference for Sc atoms to occupy the atomic sites of the β"-structure. Our study reveals the following: It is the formation of stabilized β"-related composite precipitates that enhances the thermal stability of the Sc-added Al-Mg-Si alloy.

The mechanism for the serrated flow induced by Suzuki segregation in a Ni alloy

The occurrence of serrated flow has been well accepted as a prevailing deformation feature in Ni-based alloys during plastic deformation at intermediate temperatures, yet its microstructure origin remains poorly understood. Here, characteristics of dynamic strain aging (DSA) in a Ni alloy 230 are studied by tensile tests over a wide range of temperatures and strain rates. The activation energy, Q, of the DSA effect is calculated to be 104 kJ/mol for temperatures from 523 to 773 K, about 35%–50% of the activation energy for lattice diffusion of Cr, Co or Mo in Ni, indicating pipe diffusion to be the crucial mode for solute segregation. Enrichment of Cr and Co atoms at stacking faults investigated by the state-of-the-art energy dispersive X-ray (EDX) spectroscopy is found in the deformed samples at 773 K with the strain rate of 3 × 10−3 s−1, suggesting that dynamic interactions between dissociated dislocations and substitutional solute atoms, known as Suzuki segregation, is responsible for the DSA effect. Moreover, the calculated diffusion distances of solute atoms through bulk diffusion prove that the pipe diffusion along partial dislocation core is contributing to the tensile process.

A comprehensive investigation of phase evolution of Al-Si coating during the prolonged aging at 650 °C

Microstructural evolution of silicon-modified aluminide coating during 10,000 h aging at 650 °C was systematically investigated. Upon aging, the coating was gradually transformed from a multilayered structure to a double-layer structure, with the matrixes of Al-Fe-Si compounds and β-(Fe, Ni)Al in the two layers, respectively. An internal oxide layer formed below the coating, while the thickness of interdiffusion zone (IDZ) increased as the inward diffusion of Al. Precipitate evolutions in both the coating and the IDZ were discussed, which clarifies the microstructural origin of long-term stability of the coating. The phase evolution of Al-Si coating in prolonged aging was further discussed.

Thermally‐Driven Changes to Porosity in TATB‐Based High Explosives

AbstractUltra‐small‐ and small‐angle neutron scattering techniques were employed to quantify thermally driven changes to the microstructure of the TATB‐based composite high explosive, PBX 9502. Samples of PBX 9502 were studied in‐situ at temperatures ranging from 25 to 220 °C and after re‐equilibration at ambient temperature. Significant changes to the void size and morphology within the PBX 9502 microstructure were observed upon heating and are consistent with the known increase in the shock sensitivity of thermally‐treated PBX 9502. More extensive microstructural changes were found after the thermally treated samples were re‐equilibrated at ambient temperature. Utilizing a model of ramified porosity, the size and the volume and surface dimensions of voids in the PBX 9502 microstructure were quantified as a function of temperature. The temperature dependence of the void radius of gyration is well correlated with the known increase in the shock sensitivity of PBX 9502 with increasing temperature, providing a microstructural origin for the increased shock sensitivity of PBX 9502 at elevated temperatures.

The interfacial energy penalty to crystal growth close to equilibrium

AbstractUnderstanding the origin of rock microstructure is critical for refining models of the geodynamics of the Earth. We use the geometry of compositional growth zoning of a population of garnet porphyroblasts in a mica schist to gain quantitative insight into (1) the relative growth rates of individual crystals, (2) the departure from equilibrium during their growth, and (3) the mobility of the porphyroblast-matrix interface. The driving force for garnet growth in the studied sample was exceedingly small and is comparable in magnitude to the interfacial energy associated with the garnet-matrix interface. This resulted in size-dependent garnet growth at macroscopic length scales, with a decrease in radial growth rates for smaller crystals caused by the penalty effect of the interfacial energy. The difference in growth rate between the largest and the smallest crystal is ~45%, and the interface mobility for garnet growth from ~535°C, 480 MPa to 565°C, 560 MPa in the phyllosilicate-dominated rock matrix ranged between ~10−19 and 10−20 m4 J−1 s−1. This is the first estimation of interface mobility in natural rock samples. In addition to the complex structural and chemical reorganization associated with the formation of dodecahedral coordination polyhedra in garnet, the presence of abundant graphite may have exerted drag on the garnet-matrix interface, further decreasing its mobility.

On two‐phase modeling of dewatering pulp suspensions

AbstractAn experimental study of the dewatering of wood‐pulp fiber suspensions by uniaxial compression is presented, to rationalize their dewatering dynamics within a two‐phase framework. Twenty‐seven pulp suspensions are examined, encompassing materials with different origins, preparation methodologies, and secondary treatments. For each suspension in this library, the network permeability and compressive yield stress are calibrated at low rates of dewatering. Faster compressions are then used to verify that a solid bulk viscosity is essential to match two‐phase model predictions with experimental observations, and to parameterize its magnitude. By comparing the results with a suspension of nylon fibers, we demonstrate that none of the wood‐pulp suspensions behave like an idealized fibrous porous medium. Nevertheless, the properties of pulp fiber networks can be reconciled within a two‐phase framework, and comparisons made between different wood‐pulp suspensions and between wood‐pulp and nylon fibers, by appealing to potential microstructural origins of their macroscopic behavior.

Role of V on the coercivity of SmFe12-based melt-spun ribbons revealed by machine learning and microstructure characterizations

We employed machine-learning of a dataset extracted from literature on Sm(Fe,X)12-based alloys to understand the most influential parameters for coercivity. V-addition is found to be most influential to coercivity. The microstructure origin for coercivity in SmxFe11Ti and SmxFe10TiV (1.0≤x ≤ 2.1) melt-spun ribbons was investigated experimentally. Unlike V-free alloys, ThMn12-type structure can be stabilized in a wide range of x =1.03-1.62 in SmxFe10TiV ribbons. Coercivity increases from 0.51 T for Sm0.99Fe11Ti to 1.1 T for Sm1.06Fe10TiV optimally-annealed ribbons. The enhanced coercivity originates from formation of a Sm-enriched intergranular phase enveloping the SmFe12-based grains, increasing pinning force against the magnetic domain-wall propagation.

Effects of high-temperature treatment and iron reduction index on tensile strength of basalt continuous fiber

Iron plays an important role in enhancing the tensile strength of basalt fibers (BFs); however, the origins of tensile-strength variation at high temperatures are not well understood. In this work, we prepared BFs with different iron reduction indices (IRI = Fe2+/Fetotal) and explored the effects of high temperature and IRI on tensile strength. The results show that BFs with higher IRIs retained better tensile strength after high-temperature treatment at 600°C for 1 h. The surface morphology was analyzed by scanning electron microscopy to observe the nanocrystalline layers on the surface of BFs with higher IRIs, and the main composition of the crystals was CaO. The formation of nanocrystalline layers increased the tensile strength of the BFs. Moreover, results from transmission 57Fe Mössbauer spectroscopy and Fourier transform infrared spectroscopy indicate that BFs with lower IRIs formed more ferric tetrahedral structures (Fe3+(tet)) after high-temperature treatment, which depolymerized Si(Al)-O-Si(Al) tetrahedral structures, resulting in lower tensile strength. A better understanding of the origin of microstructures in different iron coordination states and the tensile strength of BFs at high temperatures will assist in the development of advanced iron-bearing glass fiber products suitable for high-temperature environments.

Influence of microstructure on replacement and porosity generation during experimental dolomitization of limestones

Replacement reactions commonly alter the multiscale pore structures of rocks during fluid-rock interactions. Analysis of these processes in various model fluid-rock systems during controlled laboratory experiments provides insights into the origins of microstructures found in natural materials. This study focused on understanding the effects of initial starting material permeability and resultant differences in transport pathways on porosity and mineralogical changes during limestone dolomitization.A series of replacement experiments (32–317 days in duration) have been conducted in which 1.59 cm (5/8 in.) diameter cores of two different limestones were reacted with saturated MgCl2 solutions at 200 °C. The Texas Cream (Austin Chalk) is a high-porosity, high-permeability limestone, whereas both the porosity and permeability of the Carthage Marble (Burlington Limestone) are relatively low. Altered limestones were imaged using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), Time-of-Flight Secondary Ion Mass Spectrometry (ToF–SIMS) and electron microprobe analysis (EMPA). A representative grain boundary of the low-porosity limestone was targeted for a focused ion beam (FIB) lift-out and characterized using transmission electron microscopy (TEM). These results were coupled with analyses of radial changes in the porosity distribution of the core derived from X-ray and neutron small- and ultra-small angle scattering ((U)SANS/(U)SAXS).The high-porosity/permeability limestone showed a four times faster bulk replacement rate than the lower-porosity/permeability material, and a different mechanism of porosity development. For the low-porosity limestone, a two-stage replacement occurred, with the reacted region of the core consisting of an inner rim in which the limestone was replaced by two calcite-dolomite solid solutions, and an outer rim in which the dolomite was replaced by magnesite. Elongated pores formed along grain boundaries at the initial limestone/dolomite reaction interface, and additional nanometer-scale porosity was formed at the secondary magnesite replacement rim. Grain boundaries were identified as preferential pathways for transport leading to dolomitization and a grain boundary diffusion rate was calculated based on microstructural characterization. In contrast, replacement in the high-porosity limestone was accompanied by porosity generation through replacement of individual grains by dolomite throughout the sample and, in longer runs, magnesite in outer parts of the core. These observations emphasize that both the mechanisms of the replacement reaction and the microstructure and chemistry of the replaced product are contingent on the initial structure of the starting material.

Bulk NiTiCuCo shape memory alloys with ultra-high thermal and superelastic cyclic stability

Shape memory alloys sustaining stable functional properties for millions of cycles have been reported in NiTiCuCo thin films. The mechanism behind such ultralow functional fatigue was proposed to originate from the epitaxy strains surrounding coherent Ti2Cu precipitates. Despite having a well-understood microstructural origin, such fatigue-resistant property has never been achieved in bulk NiTiCuCo counterparts. In this study, we first show that the semi-coherent Ti2Cu precipitates help satisfy closely the compatibility criteria under thermal phase transformation. Assisted by aging at 600 °C for 1 h, an unprecedented thermal stability is reported in bulk coarse-grained Ti54Ni31.7Cu12.3Co2, where the transformation temperatures migrate by ∼ 0.1 °C for 200 cycles. To achieve stable superelastic response, a thermomechanical processing route is proposed that results in a uniform nanocrystalline microstructure with embedded Ti2Cu precipitates. Such a microstructure exhibits much improved superelastic cyclic stability as evidenced by ∼ 22 MPa shakedown of plateau stress for 200 superelastic cycles.

On the microstructural origins of improvements in conductivity by heavy deformation and ageing of Al-Mg-Si alloy 6101

This work has examined an Al-0.54Mg-0.38Si (at.%) conductor alloy (6101) subjected to two different thermomechanical processing routes. Conventional solution treatment, quenching, and artificial ageing (170 °C) was compared to a process applying solution treatment, quenching, pre-ageing (7 h at 170 °C), 50% thickness reduction by cold-rolling, and re-ageing at 170 °C. Re-ageing after rolling caused a rapid increase in electrical conductivity. After 2 h re-ageing the rate of change in conductivity had slowed to a level comparable to that of the undeformed material after the same total ageing time. From this point on the deformed material maintained a 2–3%IACS gain in conductivity with further ageing. It is shown that the improvement in conductivity could largely be explained by two precipitate formation mechanisms, leading to increased solute depletion of the Al matrix in the deformed material, which was quantified by atom probe tomography. Clear differences between the deformed and undeformed material were also seen in precipitate distributions as shown by transmission electron microscopy results. The findings and the discussion presented are of importance to future alloy and process development for Al alloy conductor materials.

The correlation between the mixed-mode oligo-cyclic loading induced mechanical and microstructure changes in HDPE

Two grades of HDPE (one is for PE-100 pipeline application and one is for blow molding) were processed to vary their crystallinity and molecular topology in order to investigate and understand the microstructural origin of the seismic performance of HDPE materials. For a given grade, the quenched sample exhibits a lower crystallinity, thinner crystallites, and a higher density of entanglement/tie molecules compared to the isothermal one. The HDPE samples are submitted to mixed-mode, oligo (plastic)-cyclic tensile deformation (that simulates the seism) with the applied maximum strain below or at the onset of necking propagation. The changes of numerous macroscopic mechanical metrics with increasing cycles were monitored (e.g. stress at maximum strain, residual strain at zero stress, secant modulus, and hysteresis loop area). These mechanical changes not only depend on the maximum imposed cyclic strain, but also the microstructure of HDPE. A physical scenario is proposed to describe the microstructural changes during the cyclic deformation and explicate the macro-micro correlations. Moreover, the post-cyclic tensile (reloading) curve can return to the original one rapidly for each sample, this phenomenon indicates the neglecting effect of the following cycles on the further loading path. It is also found that the material designed for the PE-100 pipeline shows limited variation between the initial and post-cyclic tensile toughness, indicating that the pre-loaded materials require nearly the same energy to rupture as the pristine ones. This is evidence that the material for PE-100 pipeline has a good resistance against the damage induced by seismic events.

On the microstructural origin of premature failure of creep strength enhanced martensitic steels

The creep strength enhanced martensitic steels are key material for the main power generating units in ultra-supercritical plants. Studies on the evaluation of their creep rupture life show there is an overestimation of rupture life after long-term creep, which is known as premature failure. However, the microstructural origin of the premature failure remains unclear. Here in this study, we have carefully investigated the microstructural transformations and their influences on creep rupture behavior, showing that the evolution of martensite and M23C6 carbides as well as Laves phase are responsible for the premature failure. By using multi-step TTP-LMP method, we confirmed a three-stage creep rupture behavior under different stress regions. Further quantitative analysis showed that the coarsening of M23C6 carbides and recovery of martensite exert equal and dominant effects on the premature failure in the medium stress region, while precipitation and coarsening of Laves phase are responsible for the premature failure in the low stress region.

Microstructural origin of locally enhanced CO2 electroreduction activity on gold.

Understanding how the bulk structure of a material affects catalysis on its surface is critical to the development of actionable catalyst design principles. Bulk defects have been shown to affect electrocatalytic materials that are important for energy conversion systems, but the structural origins of these effects have not been fully elucidated. Here we use a combination of high-resolution scanning electrochemical cell microscopy and electron backscatter diffraction to visualize the potential-dependent electrocatalytic carbon dioxide [Formula: see text] electroreduction and hydrogen [Formula: see text] evolution activity on Au electrodes and probe the effects of bulk defects. Comparing colocated activity maps and videos to the underlying microstructure and lattice deformation supports a model in which CO2 electroreduction is selectively enhanced by surface-terminating dislocations, which can accumulate at grain boundaries and slip bands. Our results suggest that the deliberate introduction of dislocations into materials is a promising strategy for improving catalytic properties.

Tailoring precursor and medium of ball milling to optimize magnetic properties in Mn-Al bonded permanent magnets

Ball milled Mn-Al powders are ideal raw materials for fabricating bonded Mn-Al permanent magnets, and selecting proper precursor phase and milling medium for ball milling are of great importance to realize optimal magnetic properties in the final magnets. In this work, we studied the effect of different precursors and mediums of ball milling on the magnetic properties in the bonded magnets and the corresponding microstructural origins. ε-phase as the precursor leads to a serious decomposition behavior during the subsequent annealing after ball milling, resulting in a strong reduction in magnetization in the powders but just slightly better coercivity, so τ-phase is a superior choice as the precursor. In the case that τ-phase is adopted as the precursor, the milling medium of ethanol (wet-milling) or argon (dry-milling) has a remarkable influence on the morphologies of the milled powders. The former condition can fabricate thin polycrystalline flakes, and form stronger texture inside simultaneously, while the latter condition produces isotropic polycrystalline particles. The bonded magnets prepared using wet-milled powders thoroughly show superior magnetic properties including magnetization, remanence and coercivity than the magnets based on dry-milled particles. This work may promote the understanding of the nanostructures of Mn-Al ball milled powders, and provide the reference for the fabrication of Mn-Al rare-earth-free bonded magnets.Prime Novelty Statement.Ball milled Mn-Al powders are ideal raw materials for fabricating bonded Mn-Al permanent magnets, and selecting appropriate precursors and milling mediums for ball milling are important to obtain optimal magnetic properties in bonded magnets. In this work, ball milling with two precursors, ε phase (high temperature phase) and τ phase (metastable ferromagnetic phase), are performed. Powders from τ-precursor after annealing treatment showing good properties (M1200kA/m of 73 Am2/kg and Hc of 250 kA/m). While for ε-precursor, during the subsequent annealing treatment for the transformation from ε to τ, obvious decomposition occurs, inducing the distinct magnetization deterioration (36 Am2/kg) as well as slightly higher coercivity (331 kA/m). In comparison, the τ-precursor is superior to ε-precursor. In case that τ-phase is adopted as the precursor, the milling medium of ethanol (wet-milling) and argon (dry-milling) has a remarkable influence on the morphologies of the milled powders. Wet-milling condition can fabricate polycrystalline thin flakes with stronger texture, while the dry-milling condition produces isotropic particles. The bonded magnets prepared using wet-milled powders thoroughly show superior magnetic properties including magnetization, remanence and coercivity than the magnets based on dry-milled particles. This work may promote the understanding of the nanostructures of Mn-Al ball milled powders, and provide the reference for the fabrication of Mn-Al rare-earth-free bonded magnets.