Effect Of Microstructural Features Research Articles

Fatigue analysis of spherulitic semi‐crystalline polymers: Unveiling the effects of microstructure and defect

AbstractA micromechanical model considering the spherulite structure of semi‐crystalline polymers was established in this study. The micro stress–strain histories were captured by combining the constitutive equations and multi‐axial fatigue criterion. The continuous damage theory was employed to describe the degradation of material properties during cycle loading. Based on the proposed model, the effects of microstructure features, such as grain anisotropy, defects, and crystallinity, on the fatigue performance was examined under multi‐axial loading condition. The local material degradation and damage accumulation were then focused on to understand the underlying fatigue mechanisms with various microstructures. Meanwhile, the crack initiation site was precisely predicted and discussed. This research provides theoretical support for understanding the failure mechanisms of spherulitic semi‐crystalline polymers, deepening the understanding of associated microstructural characteristics and strengthening the anti‐fatigue design of semi‐crystalline polymers.

Effect of microstructural features on the fatigue behavior of ultra-high strength press hardened steels

Nowadays, high-strength press-hardened steels (PHS) have gained wide applications in automotive body-in-white owing to excellent mechanical properties. Nevertheless, the fatigue properties of high-strength PHS and associated failure mechanisms have not been clearly revealed, which restricts their potential applications in other parts of automobiles involving fluctuating and repetitive stresses, such as wheels or beams. In this work, the fatigue behavior of 1900 and 2000 MPa grade PHS is systematically studied, with the main focus on the associated deformation and fatigue cracking mechanisms. The fatigue behavior of these two steels is similar, and the ratio of fatigue strength to tensile strength is around 0.5. The fatigue damage mechanism of high-strength PHS is dominated by surface cracking, as the fine martensite structures reduce the stress concentration and cracking at small-sized inclusions. This work can provide guidance for the application of high-strength PHS in alternating loads scenarios.

Chemo-mechanical ageing of paper: effect of acidity, moisture and micro-structural features

A multi-scale modeling framework is proposed for the prediction of the chemo-mechanical degradation of paper, with the particular aim of uncovering the key factors affecting the degradation process. Paper is represented as a two-dimensional, periodic repetition of a fibrous network unit cell, where the fibers are characterized by a moisture-dependent chemo-hygro-mechanical constitutive behavior. The degradation of paper occurs primarily as a result of the hydrolysis of cellulose, which causes a reduction of the degree of polymerization and a consequent decrease of the effective mechanical properties, ultimately leading to fiber embrittlement and a loss of material integrity. The interplay between the acidity of the paper, the ambient environmental conditions, and its chemo-mechanical degradation behaviour is a complex process. In the model, these interactions are accounted for by determining the coupled temporal evolution of the degree of polymerization, the acidity of the paper, and the moisture content, from which the time-dependent tensile strength of the paper is calculated. The internal stresses developing in the fibrous network under a change in moisture content lead to brittle fiber fracture once they reach the fiber tensile strength. The successive breakage of individual fibers results in damage development in the fibrous network, altering its effective constitutive properties. The temporal evolution of the effective hygro-mechanical properties of the fibrous network is calculated by employing asymptotic homogenization. For obtaining accurate model input, the strength and stiffness properties of individual fibers and the degree of polymerization of paper samples are measured at different ageing times by carrying out dedicated experiments. Subsequently, a series of numerical simulations is performed to analyze the chemo-mechanical degradation process of paper, highlighting the influence of the time-evolving acidity and moisture content. The numerical study further considers the effects of micro-structural features (i.e., the anisotropy of the fibrous network orientation and the fiber longitudinal elastic modulus) on the macroscopic degradation response of paper. The results of this work may help conservators of cultural heritage institutions determining optimal environmental conditions to limit or delay the time-dependent degradation of valuable historical paper artefacts.

Effect of microstructural features on the fatigue behavior of the valve mechanism cam rolling contact

This study introduces a methodology that incorporates gradient structural features and crystal plasticity for assessing contact fatigue in engine valve cams. It explored the influence of grain characteristics and structural gradients on fatigue failure through the lens of Plastic Strain Energy Density (PSED). Findings indicate that variability in crystal orientation significantly impacts PSED and subsequent crack initiation. Additionally, shot peening induces a gradient in grain size, promoting orderly deformation, stress alleviation, and improved fatigue resistance. The study suggests that an optimal fatigue resistance in cam contacts is achievable with a gradient layer comprising 75% volume fraction, which strikes a balance between cost and technical feasibility.

A non-local damage model-based FFT framework for elastic-plastic failure analysis of UD fiber-reinforced polymer composites

A novel computational framework combining fast Fourier transform (FFT) with the non-local damage model is proposed to simulate the nonlinear mechanical behaviors of unidirectional composites. Integral-type non-local damage model and gradient-regularized non-local damage model are integrated to analyze brittle materials and ductile materials, respectively. Under transverse tensile load, the failure of matrix in the composites is characterized by non-local maximum principal stress damage model or the elastic-plastic damage model obeying the parabolic yield criterion, respectively. It is verified that predicted stiffness, strength, and ultimate crack morphologies obtained by the proposed model are in good agreement with traditional finite element method (FEM) and phase field method (PFM) in the literature. The effects of microstructure features on the mechanical behaviors and the suitability of multiple crack propagation are further studied effectively and accurately.

On the Role of Microstructure and Defects in the Room and High-Temperature Tensile Behavior of the PBF-LB A357 (AlSi7Mg) Alloy in As-Built and Peak-Aged Conditions.

Additive processes like Laser Beam Powder Bed Fusion (PBF-LB) result in a distinctive microstructure characterized by metastability, supersaturation, and finesse. Post-process heat treatments modify microstructural features and tune mechanical behavior. However, the exposition at high temperatures can induce changes in the microstructure. Therefore, the present work focuses on the analyses of the tensile response at room and high (200 °C) temperature of the A357 (AlSi7Mg0.6) alloy processed by PBF-LB and subjected to tailored T5 (direct aging) and T6R (rapid solution treatment, quenching, and aging) treatments. Along with the effect of microstructural features in the as-built T5 and T6R alloy, the role of typical process-related defects is also considered. In this view, the structural integrity of the alloy is evaluated by a deep analysis of the work-hardening behavior, and quality indexes have been compared. Results show that T5 increases tensile strength at room temperature without compromising ductility. T6R homogenizes the microstructure and enhances the structural integrity by reducing the detrimental effect of defects, resulting in the best trade-off between strength and ductility. At 200 °C, tensile properties are comparable, but if resilience and toughness moduli are considered, as-built and T5 alloys show the best overall mechanical performance.

Mechanical Behavior of Spheroidized Steel Sheets: Effects of Microstructural Features, Loading, and Boundary Condition Modeling

Mechanical Behavior of Spheroidized Steel Sheets: Effects of Microstructural Features, Loading, and Boundary Condition Modeling

Uncertainty Quantification for Microstructure-Sensitive Fatigue Nucleation and Application to Titanium Alloy, Ti6242

Microstructure of polycrystalline materials has profound effects on fatigue crack initiation, and the inherent randomness in the material microstructure results in significant variability in fatigue life. This study investigates the effect of microstructural features on fatigue nucleation life of a polycrystalline material using an uncertainty quantification framework. Statistical volume elements (SVE) are constructed, where features are described as probability distributions and sampled using the Monte Carlo method. The concept of SVE serves as the tool for capturing the variability of microstructural features and consequent uncertainty in fatigue behavior. The response of each SVE under fatigue loading is predicted by the sparse dislocation density informed eigenstrain based reduced order homogenization model with high computational efficiency, and is further linked to the fatigue nucleation life through a fatigue indicator parameter (FIP). The aggregated FIP and its evolution are captured using a probabilistic description, and evolve as a function of time. The probability of fatigue nucleation is measured as the probability that the predicted FIP exceeds the local critical value which represents the ability of material to resist the fatigue load. The proposed framework is implemented and validated using the fatigue response of titanium alloy, Ti-6Al-2Sn-4Zr-2Mo (Ti-6242).

Cast Austenitic Stainless Steel Reinforced with WC Fabricated by Ex Situ Technique

In this study, the process of reinforcing austenitic stainless steel with tungsten carbide (WC) particles prepared by an ex situ technique was investigated. More specifically, the effect of microstructural features on the properties of the resulting WC-metal matrix composite (WC-MMC) was studied. For that purpose, porous Fe-WC preforms, prepared by the ex situ technique, were fixed in the mold cavity where they reacted with the molten steel. As confirmed by scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), the resulting composite showed a compositional and microstructural gradient in depth. The microstructure next to the surface is essentially martensite with large WC particles. From this region to the base metal, the dissolution of the original WC particles increased, being closely related to the formation of new carbides: (Fe,W,Cr)6C, (Fe,Cr,W)7C3, and (Fe,Cr,W)23C6. At the interface bonding, a sound microstructure free of discontinuities was achieved. Furthermore, the mechanical tests indicated that the WC-MMC is four times harder and more wear-resistant than the base metal.

Effect of Microstructural Features on Magnetic Properties of High-Carbon Steel

Effect of Microstructural Features on Magnetic Properties of High-Carbon Steel

The influence of grain size on the onset of the superelastic transformation in Ti–24Nb–4Sn–8Zr (wt%)

Superelastic Ti alloys have potential across a wide range of industrial sectors, but each application requires specifically tailored transformation behaviour. Composition is well known to influence transformation properties but for a given alloy, the effect of microstructural features, such as grain size, is far less clear. Within the literature, no consistent relationship between grain size and key parameters, such as the transformation stress, σSIM, exist. This lack of clarity is a major barrier to engineering uptake of these materials. Therefore, in the present work, the transformation behaviour of Ti–24Nb–4Sn–8Zr (wt%) from two different starting stocks has been studied, with grain sizes varied by changing solution heat treatment time. Irrespective of the material stock, σSIM was shown not to vary as a function of grain size. As such, this observation is believed to be a general effect, and as a result, microstructures can be tailored to improve other parameters such as yield stress, without altering the onset of transformation.

Small punch testing of heat resistant ultrafine-grained Al composites stabilized by nano-metric Al2O3 (HITEMAL©) in a broad temperature range

The mechanical properties of thermally stable ultrafine-grained Al metal matrix composites dispersion strengthened and stabilized by nano-metric in-situ Al2O3 (named HITEMAL©) fabricated by powder metallurgy (PM) were tested by small punch testing (SPT) in a broad temperature range from a room temperature (RT) to 500 °C. By changes to PM approach and applied processing parameters four specific HITEMAL© materials with different Al grain structure, and different morphology and distribution of Al2O3 dispersoids were fabricated and characterized by transmission electron microscopy and energy-dispersive X-ray spectroscopy. The effect of microstructural features on the mechanical properties of HITEMAL© materials was pursued by SPT. The SPT results were compared and correlated with the results obtained by conventional tensile testing. It was confirmed that a mutual correlation of tensile test and SPT results strongly depended on the particular testing temperature. As a result of microstructural differences between HITEMAL© materials the correlation between tensile test and SPT results changed quite significantly for each studied material. The analysis of fractured SPT discs by scanning electron microscopy (SEM) revealed the transformation in fracture mechanism during SPT from ductile to brittle behavior as the testing temperature increased from RT to elevated temperatures. If the testing temperature was considered as a parameter, the precision of yield strength calculation from SPT results improved significantly in whole temperature range. Data AvailabilityThe raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Multi-scale pseudoelasticity of NiTi alloys fabricated by laser additive manufacturing

Pseudoelasticity behaviors have been found in the NiTi alloys fabricated by laser additive manufacturing (LAM) processes. Reported investigations of pseudoelasticity in LAM fabricated NiTi alloys mainly focus on the effects of laser input energy and post-heat treatments on phase transformation behaviors. The pseudoelasticity behaviors can be affected by the significantly different load-contact conditions in various industrial applications. However, there are no reported investigations of pseudoelasticity behaviors of NiTi alloys fabricated by LAM in different load-contact conditions. For the first time, multi-scale indentation tests at three scales (nanoscale, microscale, and macroscale) are conducted to evaluate and compare the pseudoelasticity behaviors of the NiTi alloys fabricated by LAM in this investigation. In addition, the effects of microstructural features and phase constituents on pseudoelasticity are discussed. Overall, pseudoelasticity at the nanoscale is the best in terms of the smallest remnant depth ratios, followed by that at the macroscale and that at the microscale. Moreover, pseudoelasticity at the nanoscale improves with the increase of load or the number of cycles. As a comparison, pseudoelasticity at the microscale stays steady with the increase of load and decreases with cycling, and pseudoelasticity at the macroscale stays steady with the increase of load or the number of cycles. It is also found that the improvement in pseudoelasticity leads to decreases in apparent hardness and Young's modulus.

Phase-field simulation of γʹ coarsening behavior in cobalt-based superalloy

Because the microstructure characteristics of γʹ strengthened cobalt-based superalloys are critical for anticipated next generation superalloys, the morphological evolution and coarsening behavior of γʹ precipitates during isothermal aging in Co-Al-W ternary alloys are investigated using the phase-field method. Accurate γʹ evolution over the entire aging process is simulated through coupling with the antiphase domain and the elastic energy. With a change in γʹ volume fraction from 38% to 66%, the coarsening rate of γʹ precipitates shows an approximately linear increase, primarily due to the increased diffusion potential difference between the γ matrix and γʹ precipitate. The coalescence between adjacent precipitates is promoted in the high γʹ volume fraction, resulting in polyhedral precipitates and particle size distribution (PSD), which agrees with MLSW theory. The morphology transition mechanism of γʹ is showed through the evolutions of interfacial energy and elastic energy, and the effects of microstructure features on the coarsening rate and coarsening mode are discussed. Results of this study provide guidance for the development of Co-based superalloys with optimized microstructures.

Effect of microstructural features on the corrosion behavior of severely deformed Al–Mg–Si alloy

AbstractThe aim of this study was to determine the influence of severe plastic deformation processing and the changes in microstructure resulting therefrom on the corrosion resistance of an Al–Mg–Si alloy. The alloy was processed using incremental equal channel angular pressing, which caused a reduction in grain size from 15 to 0.9 µm. The grain refinement was accompanied by an increase in the number of grain boundaries and dislocations, and by changes in grain orientation. However, there was no change in the size and number of intermetallic particles, which presumably resulted in a constant number of galvanic couplings. Electrochemical experiments revealed only slight differences between the samples before and after processing. Higher potential transients/oscillations upon immersion and increased corrosion currents in the vicinity of corrosion potential point to slightly higher reactivity of the most refined material. This indicates that intermetallic particles are the most crucial microstructural elements in terms of corrosion resistance. Their impact exceeds that of grain boundaries, in particular, at the stage of corrosion initiation. The development of corrosion attack is controlled more by the microstructure of the matrix as the grain refinement resulted in a less pronounced corrosion attack in comparison with the coarse‐grained sample.

Multi-scale indentation creep behavior in Fe-based amorphous/nanocrystalline coating at room temperature

This work reports first ever investigation on multi-scale room temperature creep behavior of Fe-based amorphous/ nanocrystalline coatings, synthesized via plasma spraying at varying plasma power. Micro- and nano-indentation testing at different loading scales was carried out to reveal the combined effect of microstructural heterogeneities viz. porosity and constituent phases, as well as the individual contribution of amorphous and crystalline phases on the creep behavior of the coatings. Microindentation creep tests revealed higher creep resistance for the coating deposited at elevated power, ascribed to lower porosity and higher degree of crystallinity. Nanoindentation creep tests showed that mixed (amorphous and intermetallic) phase possessed lower creep displacement and retardation spectra compared to fully amorphous one, indicating better creep resistance. The present work provides valuable insight into the effect of microstructural features on the creep deformation behavior of amorphous-nanocrystalline coatings.

Microstructural influences on the high cycle fatigue life dispersion and damage mechanism in a metastable β titanium alloy

In this work, the effect of microstructure features on the high-cycle fatigue behavior of Ti-7Mo-3Nb-3Cr-3Al (Ti-7333) alloy is investigated. Fatigue tests were carried out at room temperature in lab air atmosphere using a sinusoidal wave at a frequency of 120 Hz and a stress ratio of 0.1. Results show that the fatigue strength is closely related to the microstructure features, especially the αp percentage. The Ti-7333 alloy with a lower αp percentage exhibits a higher scatter in fatigue data. The bimodal fatigue behavior and the duality of the S-N curve are reported in the Ti-7333 alloy with relatively lower αp percentage. Crack initiation region shows the compound αp/β facets. Faceted αp particles show crystallographic orientation and morphology dependence characteristics. Crack-initiation was accompanied by faceting process across elongated αp particles or multiple adjacent αp particles. These particles generally oriented for basal slip result in near basal facets. Fatigue crack can also initiate at elongated αp particle well oriented for prismatic <a> slip. The β facet is in close correspondence to {110} or {112} plane with high Schmid factor. Based on the fracture observation and FIB-CS analysis, three classes of fatigue-critical microstructural configurations are deduced. A phenomenological model for the formation of αp facet in the bimodal microstructure is proposed. This work provides an insight into the fatigue damage process of the α precipitate strengthened metastable β titanium alloys.

A three-dimensional microstructure-based crystal plasticity model for coarse-grained and harmonic-structured Ti–6Al–4V under monotonic and cyclic shear loading

This paper develops a microstructure-based numerical model to simulate the mechanical behavior of homogeneous coarse-grained (CG) and harmonic-structured (HS) Ti–6Al–4V under monotonic and cyclic simple shear loading conditions. This model incorporates a crystal plasticity model describing the deformation behavior of lamellar $$\alpha +\beta $$ colonies and a scale transition rule called ‘ $$\beta $$ -rule’ which enables to deal with the huge contrast between grain sizes in fine-grained and coarse-grained regions in HS Ti–6Al–4V. Besides, the numerical model can be used to consider the effects of microstructural features, such as the slip geometry for lamellar $$\alpha +\beta $$ colonies, length scale dependence of hardening, or anisotropic strength of the slip systems. The model is implemented into a finite element code (Cast3M) via a UMAT subroutine. The strengthening effect of harmonic structure design is studied based on the numerical results for HS Ti–6Al–4V and homogeneous CG Ti–6Al–4V. The simulation results are in good agreement with the experimental observations.

Effects of Wire Drawing and Annealing Conditions on Torsional Ductility of Cold Drawn and Annealed Hyper-Eutectoid Steel Wires

The effects of microstructural features on torsional ductility of cold drawn and annealed hyper-eutectoid steel wires were investigated. The patented wire rods were successively dry drawn to ε = 0.79 (54.7%) ~ 2.38 (90.7%). To examine the effects of hot-dip galvanizing conditions on torsional ductility, steel wires with ε = 1.95 were annealed at 500 °C for 30 s for ~1 h in a salt bath. In cold drawn wires, the number of turns to failure increased steadily, showing the maximum peak, and then decreased with drawing strain. During the post-deformation annealing at 500 °C, torsional ductility of steel wires decreased with annealing time, except for the rapid drop due to the occurrence of delamination for 10 s annealing. The decrease of the number of turns to failure would be attributed to the microstructural evolutions, accompanying the spheroidization and growth of cementite particles and the recovery of ferrite in cold drawn steel wires. From the relationship between microstructural evolution and torsional ductility, it was found that among microstructural features, the shape and orientation of lamellar cementite showed the significant effect on torsional ductility of cold drawn and annealed hyper-eutectoid steel wires.

The effect of microstructural features on the ferromagnetism of nickel oxide nanoparticles synthesized in a low-pressure arc plasma

Abstract Nickel oxide nanoparticles were first synthesized by sputtering high-purity nickel in an oxygen plasma of a low-pressure arc discharge. The structure, morphology, and optical and magnetic properties of NiO nanoparticles were studied by XRD, TEM, FTIR, UV-VIS, and VSM. TEM images showed that the obtained NiO nanoparticles have a narrow particle size distribution and an average particle size of 12 nm. The XRD results and the processing of diffractograms by the Rietveld method showed that the obtained nanoparticles have a face-centered cubic lattice with an average particle size of 13 nm. With decreasing temperature, residual stresses increase and peaks corresponding to the superstructure appear. The band gap of NiO was determined from the optical absorption spectrum and amounted to 3.21 eV. Magnetic measurements showed that, at temperatures of 200 and 300 K, NiO nanoparticles, unlike bulk particles, exhibit ferromagnetic behavior, and at 5 K a magnetic hysteresis loop appears. Based on the studies, a dendritic model of the nanoparticle microstructure is proposed.