The papers collected in this special issue clearly reflect the modern research trends in materials science. These fields of specific attention are high-Mn TWIP steels, high-Cr heat resistant steels, ...aluminum alloys, ultrafine grained materials including those developed by severe plastic deformation, and high-entropy alloys. The major portion of the collected papers is focused on the mechanisms of microstructure evolution and the mechanical properties of metallic materials subjected to various thermo-mechanical, deformation or heat treatments. Another large portion of the studies is aimed on the elaboration of alloying design of advanced steels and alloys. The changes in phase content, transformation and particle precipitation and their effect on the properties are also broadly presented in this collection, including the microstructure/property changes caused by irradiation.
We present a series of experimental observations on surface rumpling of an initially flat NiCoCrAlY coating deposited on a Ni-based superalloy during cyclic oxidation at 1150 °C. The extent of ...rumpling of the coating depends on the thermal history, coating thickness and exposed atmosphere. While the coating surface progressively roughens with cyclic oxidation, the bulk NiCoCrAlY alloys with the same nominal composition are much less susceptible to rumpling under the same oxidation conditions. The coatings, especially the thin ones, experience substantial degradation (e.g. β to γ phase transformation) induced by oxidation and coating/subsatrate interdiffusion. The observations together suggest that rumpling of the NiCoCrAlY coating is driven by a combination of the lateral growth of the thermally grown oxide and coating/substrate thermal mismatch. The results in this work are further discussed and compared with the rumpling behaviour of a β-(Ni,Pt)Al bond coat reported in the literature to illustrate the importance of possible factors in governing the development of rumpling in the NiCoCrAlY coating.
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Crack-free functionally graded TiC particle (TiCp) reinforced Ti6Al4V (TiCp/Ti6Al4V) composite was manufactured by laser melting deposition (LMD) technology with TiC volume fraction changing ...gradually from 0% to 50%. This research focuses on the relationship between the microstructure and mechanical properties (microhardness and tensile properties) of TiCp/Ti6Al4V composites under different TiC volume fractions. Besides the unmelted TiC particles, the granular and chain shaped eutectic TiC phases are observed in the composite with 5vol% TiC due to the melting and dissolution of TiC particles into matrix. The granular and dendritic primary TiC phases are obtained in the composite with 10vol% TiC, while the chain shaped eutectic TiC phases can scarcely be seen. The main reinforcement phases are primary TiC phases when the TiC volume fraction exceeds 15%. (i) The quantity of unmelted TiC particles, (ii) the quantity and size of primary TiC phases and (iii) the porosity of composite increase gradually when the TiC volume fraction increases. The interfaces exhibit good bonding between consecutive layers. The microhardness of the functionally graded TiCp/Ti6Al4V composite increases gradually with TiC volume fraction increasing. It is attributed to the C element in solid solution and the appearance of eutectic and primary TiC phases. The microhardness at the top layer with 50vol% TiC is improved by nearly 94% compared with that at the Ti6Al4V side. The tensile strength of TiCp/Ti6Al4V composite with 5vol% TiC is enhanced by nearly 12.3% compared with that of the Ti6Al4V matrix alloy. However, both the tensile strength and elongation of composite decrease gradually when the TiC volume fraction exceeds 5%. The reason is that the quantity of brittle unmelted TiC particles and the quantity and size of dendritic TiC phases increase with TiC volume fraction increasing. The fracture mechanism of the TiCp/Ti6Al4V composite is quasi-cleavage fracture.
Since the 1960s, it has been a common practice worldwide to pursue a homogeneous distribution of reinforcements within a matrix material, discontinuous metal matrix composites (DMMCs) in particular. ...Taking an overview of the worldwide activities in DMMC research, despite many favourable attributes such as improved specific strength, stiffness and superior wear resistance, DMMCs with a homogeneous microstructure tend to exhibit a very low room temperature damage tolerance even with a highly ductile matrix material such as aluminium. In this review, a range of uniquely multi-scale hierarchical structures have been successfully designed and fabricated by tailoring reinforcement distribution for DMMCs in order to obtain superior performance. A variety of specific microstructures that were developed in Al, Mg, Cu, Fe, Co and TiAl matrices indicate that there must be adequate plastic regions among the reinforcements to blunt or deflect cracks if one wants to toughen DMMCs. Following this path, aided by theoretical analyses, the most recent success is the design and fabrication of a network distribution of in situ reinforcing TiB whiskers (TiBw) in titanium matrix composites (TMCs), where a tailored three-dimensional (3D) quasi-continuous network microstructure displays significant improvements in mechanical properties. This resolves the brittleness surrounding TMCs fabricated by powder metallurgy. It is the large reinforcement-lean regions that remarkably improve the composite's ductility by bearing strain, blunting the crack and decreasing the crack-propagation rate. The fracture, strengthening and toughening mechanisms are comprehensively elucidated in order to further understand the advantages of such an inhomogeneous microstructure, and to justify the development of novel techniques to produce such inhomogeneous microstructures. This approach opens up a new horizon of research and applications of DMMCs and can be easily extended to general multi-phase composites with enhanced physical and mechanical properties.
Microstructure features directly reflect mechanical responses during material deformation and can influence the surface integrity of machined components, which is of great interest to both academic ...communities and industries. This paper summarizes the state of the art in modeling approaches for the effect of the microstructures and its evolution during metal-cutting processes. The aim of this paper is to analyze the advantages and drawbacks of current methods to improve modeling work and direct future orientations. First, dominant numerical modeling approaches for metal-cutting processes are reviewed. The finite element method (FEM) and mesh-free methods widely applied in professional research are discussed. Following this, approaches to modeling the effect of microstructures and their evolution are reviewed for two major categories—homogeneous field distribution and heterogeneous characteristics. Both the advantages and disadvantages of these two categories are analyzed and discussed in detail. Experimental techniques based on advanced characterization approaches to the validation of pertinent models are also discussed. Finally, a brief summary is presented and an outlook for future work is delineated.
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•Numerical modeling approaches with the consideration of microstructure features towards metal cutting are reviewed.•Modeling through treating microstructures as homogeneous characteristics and pertinent validation techniques are reviewed.•Modeling of heterogeneous microstructure evolution in metal-cutting and pertinent characterization techniques are reviewed.•Future orientations of modeling the effect of the microstructure and its evolution in metal-cutting are recommended.
Liquid metal embrittlement (LME) has emerged as a major concern when developing high-strength automotive steels. However, information regarding the impact of initial microstructure on LME severity is ...limited in the Fe/Zn couple. Specifically, there is no consensus as to which ferritic and austenitic microstructures are more susceptible to LME cracking. The present study aims to examine the LME cracking behavior of fully ferritic and austenitic microstructures under the same thermomechanical conditions. It was shown that the ferritic microstructure has a higher sensitivity to LME crack initiation, whereas the austenitic specimen displayed a much longer average crack length, which indicates higher crack propagation rate than the ferritic specimen. It has been determined that in-situ austenite to ferrite transformations during Zn diffusion, as well as grain boundary segregation of alloying elements such as Cr and Ti, contribute to the LME propagation rate.
In this article, we report a novel but simple method for the phase transformation of ZnO sub(2) to flower-like ZnO microstructures hydrothermally at 90 degree C with and without the assistance of ...hexadecylamine as surfactant. The generation of zincate ion ZnO \documentclass{article}\usepackage{amssymb}\begin{document}\pagest y le{empty}$2-}_{2}$\end{document}Imageomitted as a growth unit from the reaction between ZnO sub(2) and peroxide ion O \documentclass{article}\usepackage{amssymb}\begin{document}\pagest y le{empty}$2-}_{2}$\end{document}Imageomitted in situ plays a key role in the phase transformation of ZnO sub(2) to ZnO. The morphology, structure, and composition of the products have been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Powder X-ray diffraction (PXRD) and energy dispersive X-ray analysis (EDX). It has been demonstrated that the as-fabricated ZnO flowers are composed of self-assembled brooms and rods in the presence and absence of hexadecylamine respectively. On the basis of experimental results, a possible reaction mechanism and the growth processes involved in the formation of flower-like ZnO microstructures are discussed. In this article, we report a novel but simple method for the phase transformation of ZnO sub(2) to flower-like ZnO microstructures hydrothermally at 90 degree C with and without the assistance of hexadecylamine as surfactant. The generation of zincate ion ZnO \documentclass{article}\usepackage{amssymb}\begin{document}\pagest y le{empty}$2-}_{2}$\end{document}Imageomitted as a growth unit from the reaction between ZnO sub(2) and peroxide ion O \documentclass{article}\usepackage{amssymb}\begin{document}\pagest y le{empty}$2-}_{2}$\end{document}Imageomitted in situ plays a key role in the phase transformation ...
Two MOX powders containing 11 mol% Pu/(U+Pu) have been synthesized using the freeze granulation or dry-cogrinding routes. The sintering behaviour of green compacts has been investigated up to 1700 °C ...in an Ar/4.3 vol% H2 humidified with 1200 vpm H2O. The evolution of the sintering trajectory (grain size as a function of relative density) is identical up to approximately 94% relative density. Above this value, grain growth becomes significant in compacts made with the powder obtained by freeze granulation and densification becomes more modest. As for it, the sintering trajectory relating to the compacts shaped from the co-grinded powder continues up to approximately 97.5% relative density, without remarkable grain growth. Beyond this value, grain growth occurs and densification slows down. It has also been shown that the plutonium distribution is more homogeneous in the case of a sample sintered from the powder obtained by freeze-granulation (relative density around 97–99%).
This paper introduces neurite orientation dispersion and density imaging (NODDI), a practical diffusion MRI technique for estimating the microstructural complexity of dendrites and axons in vivo on ...clinical MRI scanners. Such indices of neurites relate more directly to and provide more specific markers of brain tissue microstructure than standard indices from diffusion tensor imaging, such as fractional anisotropy (FA). Mapping these indices over the whole brain on clinical scanners presents new opportunities for understanding brain development and disorders. The proposed technique enables such mapping by combining a three-compartment tissue model with a two-shell high-angular-resolution diffusion imaging (HARDI) protocol optimized for clinical feasibility. An index of orientation dispersion is defined to characterize angular variation of neurites. We evaluate the method both in simulation and on a live human brain using a clinical 3T scanner. Results demonstrate that NODDI provides sensible neurite density and orientation dispersion estimates, thereby disentangling two key contributing factors to FA and enabling the analysis of each factor individually. We additionally show that while orientation dispersion can be estimated with just a single HARDI shell, neurite density requires at least two shells and can be estimated more accurately with the optimized two-shell protocol than with alternative two-shell protocols. The optimized protocol takes about 30min to acquire, making it feasible for inclusion in a typical clinical setting. We further show that sampling fewer orientations in each shell can reduce the acquisition time to just 10min with minimal impact on the accuracy of the estimates. This demonstrates the feasibility of NODDI even for the most time-sensitive clinical applications, such as neonatal and dementia imaging.
► Proposed an experimental design and analysis framework for imaging neurite morphology. ► First in vivo demonstration of neurite orientation dispersion and density mapping. ► NODDI estimates disentangle the key factors contributing to fractional anisotropy. ► NODDI protocol is clinically feasible: imaging the whole brain in 30 minutes or less. ► NODDI protocol is simple to implement, consisting of just two HARDI shells.
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