Fine Microstructural Features Research Articles

Revealing the interdependence of load partitioning, dislocation density evolution and mechanical behavior of medium Mn steels

The load partitioning between inter-phases and intragranular of austenite, their dislocation density evolution, and mechanical behavior were investigated in two medium Mn steels by in-situ high-energy synchrotron x-ray diffraction during tensile loading. The investigated steels were produced with different hot rolling parameters resulting in different initial microstructures. The initial microstructures were characterized using scanning electron microscope and electron backscatter diffraction analysis. Transmission electron microscope with high angle annular dark field technique was furthermore used to provide the fine microstructural features of deformed specimens in order to supplement the in-situ analysis. Despite the differences in the initial microstructure, the load partitioning from the softer ferrite to the stronger α’-martensite (α’) was observed; while different degrees of load partitioning for intragranular of γ occurred. The sample with a relatively low degree of load partitioning exhibits the intersection of two bundles of stacking faults (SF), one with high density SF, and the other with low density and limited SF, as well as the occurrence of α′ in the intersecting region. The secondary SFs and the relatively intermediate density of primary SFs bundle are characterized in γ for the sample with relatively high degree of load partitioning. The load partitioning between intragranular of γ is found to have a coordination relationship with the dislocation density evolution of γ; after the load partitioning, dislocation density of γ rapidly rises. The sample with intersection of two bundles SFs and the occurrence of α′, exhibits the higher transformation rate of γ into ε-martensite/α’, resulting in the larger work hardening rate. In addition, the early load on α’ is found to contribute to a higher yield strength. These results reveal the importance of assessing both phase-specifc strain/stress and the dislocation density evolution in order to predict the macroscopic mechanical properties of medium Mn steels.

TKD/EBSD and TEM analysis of microstructural changes ongoing in AISI 304L steel exposed to the cyclic loading

The study deals with microstructural evolution of the 304L steel exposed to the low cycle straining. The two states of material were studied, the as-received and solution annealed state. Solution annealing caused grain growth from 29 ± 10 μm to 48 ± 30 μm, reduction of chemical heterogeneity and decrease of overall dislocation density. Cyclic hardening/softening curves were measured and three stages of fatigue life were distinguished. Initial cyclic hardening was followed by cyclic softening. The pronounced secondary hardening was measured in the last stage of fatigue life. Advanced techniques of electron microscopy were used to enlighten origin of the material behavior in particular stages. The material response in individual stages is thoroughly discussed in terms of microstructure evolution. Electron channeling contrast imaging visualized the fine microstructural features and revealed structure evolution, however was not able to identify the features. Transmission electron microscopy is useful for individual structural features characterization, however, it is quite complicated to be performed. Transmission Kikuchi diffraction provided similar results to transmission electron microscopy while characterization of evolved microstructural features revealed the deformation mechanism acting under loading at lower time and relatively smaller effort. Particular damage mechanisms operating in material are linked to the mechanical data.

Resolution-enhanced OCT and expanded framework of information capacity and resolution in coherent imaging

Spatial resolution in conventional optical microscopy has traditionally been treated as a fixed parameter of the optical system. Here, we present an approach to enhance transverse resolution in beam-scanned optical coherence tomography (OCT) beyond its aberration-free resolution limit, without any modification to the optical system. Based on the theorem of invariance of information capacity, resolution-enhanced (RE)-OCT navigates the exchange of information between resolution and signal-to-noise ratio (SNR) by exploiting efficient noise suppression via coherent averaging and a simple computational bandwidth expansion procedure. We demonstrate a resolution enhancement of 1.5 × relative to the aberration-free limit while maintaining comparable SNR in silicone phantom. We show that RE-OCT can significantly enhance the visualization of fine microstructural features in collagen gel and ex vivo mouse brain. Beyond RE-OCT, our analysis in the spatial-frequency domain leads to an expanded framework of information capacity and resolution in coherent imaging that contributes new implications to the theory of coherent imaging. RE-OCT can be readily implemented on most OCT systems worldwide, immediately unlocking information that is beyond their current imaging capabilities, and so has the potential for widespread impact in the numerous areas in which OCT is utilized, including the basic sciences and translational medicine.

Microwave sintering response of different grade stainless steels and its influence on metallurgical properties

ABSTRACT Ultra-rapid microwave sintering of powder metals will provide fine microstructural features that improve mechanical properties. In present study, three grades of stainless steel powder compacts (316L, 430L, 410) were produced using a uniaxial compaction unit. These compacts were sintered by microwave hybrid heating method at 1300°C (super-solidus region). The densification response, microstructural attributes, and mechanical properties were compared at 30, 45 and 60 min holding times. The compositional analysis was performed with the help of optical emission spectroscopy (OES) and scanning electron microscopy equipped with energy dispersed spectroscopy(SEM-EDS). The results obtained from both spectroscopies are compared for the sintered samples. The correlation of mechanical properties is analysed with evolved microstructural attributes (pore volume, pore shape and pore distribution). The excellent strength of 457 ± 16 MPa, 466 ± 6 MPa and 476 ± 26 MPa with 23 ± 1.3%, 14 ± 1.5% and 11 ± 1% of ductility is observed for sintered AISI 316L, 430L and 410, respectively.

Microstructure evolution during AlSi10Mg molten alloy/BN microflake interactions in metal matrix composites obtained through 3D printing

Utilization of metal/ceramic powders opens new possibilities for 3D printing of metal matrix composites of complex shape with high strength, but it is still a great challenge. In this work, an AlSi10Mg matrix composite embedded with 1 wt% of hexagonal BN phase microflakes (h-BN) was obtained by means of 3D printing. Then the present study elucidated microstructure evolutions occurring at the h-BN/melt interface during selective laser melting (SLM) of an h-BN-AlSi10Mg powder mixture. During short-term (0.15 ms) high-temperature (∼2900 K) processing the BN inclusions partly dissolved in the Al–Si melt. This process was accompanied by the formation of an AlN phase at the BN surfaces. The AlN crystallites, 100–200 nm in size, had spherical/semispherical shape and formed a continuous layer along the BN/metal grain boundaries. The peculiar growth of AlN grains along the metal/BN interfaces was governed by the specific features of localized N diffusion in the vicinity of interfaces. By contrast, B atoms, released from the dissolved BN phase, were randomly distributed over the melt. AlB2 nanocrystallites (∼10 nm in size) precipitated from the supersaturated Al–Si melt during cooling stage. With the addition of h-BN microflakes, the composite hardness and tensile strength increased by 32% and 28%, respectivelly. The observed experimental results were supported by ab initio molecular dynamics simulations. Our study demonstrates the possibility and wide prospects of obtaining a dense BN/AlSi10Mg material reinforced with h-BN, AlN, and AlB2 phases via SLM 3D printing and sheds a new light on fine morphological and microstructural features of thus obtained new composites.

Disclosing small scale length properties in core-shell structured B4C-TiB2 composites

This work explores the fine microstructural features of B4C-TiB2 composites and suggests an overall microstructure evolution from powder processing to sintering. In addition, small-scale grain structures are correlated to local properties measured by nanoindentation, thus providing trends in mechanical behavior. Dense ceramics were typified by development of a core/shell structure of the boride grains, with the shell comprising a (Ti,W)B2 solid solution with different assemblage and variable amount of W guest cation depending on the processing route. TEM analyses revealed chemical and morphological differences that were associated to the presumed densification mechanisms. Nanoindentation was used to extract the overall and single phase properties of TiB2 core, shell and B4C phase highlighting for the first time hardness and modulus variation as a function of the lattice perturbation, i.e. of nominally pure core boride and shells region.This work provides new experimental findings fundamental for the development and synthesis of high-performance structural materials starting from a small-length scale perspective.

Optimization of post-weld heat treatment condition of arc-welded T91 steel tubes

Modified 9Cr–1Mo ferritic/martensitic (T91) steel tubes have been welded with gas tungsten arc welding (GTAW) and shielded metal arc welding (SMAW) processes. The welded tubes have been post-weld heat-treated (PWHT) at selected temperatures (i.e., at 720 °C, 760 °C and 775 °C for different times) below their Ac1 critical temperature (~791 ± 5 °C). The PWHT microstructures, mechanical properties and fracture behavior of welded samples have been investigated. The results suggest that the PWHT temperatures closer to Ac1 critical temperature (i.e., 760 °C and 775 °C) has a strong influence on the properties of the T91 welds. They have shown uniform hardness and enhanced tensile strength and percent elongation as against as-welded and 720 °C – 120 min PWHT condition. Further investigation suggests that the strength and percent elongation for 775 °C – 30 min condition were highest among all the PWHT conditions examined. The observed variation in mechanical properties has been related with the finer microstructural features (mainly grain size and precipitate size). This work further confirms that for PWHT temperatures closer to Ac1, as the holding time increases the failure shifts from base metal (BM) to weld metal (WM) region, predominantly due to de-cohesion at the Mn, Cr, Mo rich coarse precipitate/matrix interface. From this study, a novel PWHT condition at 775 °C – 30 min is being proposed for optimum microstructure and mechanical properties for the T91 steel tube welds.

Effects of microstructural mechanisms on the localized oxidation behavior of NiTi shape memory alloys in simulated body fluid

Localized oxidation and corrosion behavior of a nickel–titanium (NiTi) shape memory alloy (SMA) was investigated via static immersion experiments in a simulated body fluid solution. Detailed electron microscopy examinations on the sample surfaces revealed preferential formation of local oxide particles around dislocation networks, which constitute high-energy zones. Moreover, various intermediate phases were detected in addition to the parent NiTi phase around dislocation networks. These are also areas with enhanced diffusion, which promotes Ni release. These findings emphasize the significant role of fine microstructural features, such as dislocation networks, on the oxidation and Ni release, and thus, the biocompatibility of the NiTi SMAs.

Different composite voxel methods for the numerical homogenization of heterogeneous inelastic materials with FFT-based techniques

FFT-based homogenization methods aim at calculating the effective behavior of heterogeneous materials with periodic microstructures. These methods operate on a regular grid of voxels, and hence require an appropriate spatial discretization of periodic microstructures. However, when different microstructural length scales are involved, it is not always possible to have sufficient spatial resolutions to explicitly consider the influence of fine microstructural features (e.g. voids, second-phase particles). To circumvent this difficulty, one solution consists of using composite voxel methods to define the effective properties and the effective internal variables of heterogeneous voxels.In this work, different composite voxel methods are proposed to deal with inelastic materials with multiple length scales. These methods use simple homogenization rules to calculate the effective behavior of heterogeneous voxels. The first part of this paper is dedicated to the description of the composite voxel methods, which are based either on the Voigt, laminate structure or Mori–Tanaka approximations. In the second part, these methods are used to model the elasto-plastic behavior of a pearlitic steel polycrystalline aggregate. According to the results, the Voigt approximation, which ignores morphological features, is not appropriate for treating heterogeneous voxels. When morphological information is accounted for, with either the laminate structure or Mori–Tanaka approximations, a better agreement with experimental observations is obtained. Though none of these methods is universal, they offer some possibilities to investigate the mechanical behavior of heterogeneous materials involving multiple length scales.

High quality transmission Kikuchi diffraction analysis of deformed alloys - Case study

Modern scanning electron microscopes (SEM) equipped with thermally assisted field emission guns (Schottky FEG) are capable of imaging with a resolution in the range of several nanometers or better. Simultaneously, the high electron beam current can be used, which enables fast chemical and crystallographic analysis with a higher resolution than is normally offered by SEM with a tungsten cathode. The current resolution that limits the EDS and EBSD analysis is related to materials' physics, particularly to the electron-specimen interaction volume. The application of thin, electron-transparent specimens, instead of bulk samples, improves the resolution and allows for the detailed analysis of very fine microstructural features. Beside the typical imaging mode, it is possible to use a standard EBSD camera in such a configuration that only transmitted and scattered electrons are detected. This modern approach was successfully applied to various materials giving rise to significant resolution improvement, especially for the light element magnesium based alloys. This paper presents an insight into the application of the transmission Kikuchi diffraction (TKD) technique applied to the most troublesome, heavily-deformed materials. In particular, the values of the highest possible acquisition rates for high resolution and high quality mapping were estimated within typical imaging conditions of stainless steel and magnesium-yttrium alloy.

Grain refinement of Al–Si alloys by Nb–B inoculation. Part II: Application to commercial alloys

The potency of Nb–B inoculation for the refinement of Al–Si cast alloy has been demonstrated in Part I of this work by the systematic analysis of binary Al–xSi alloys (where x = 1–10 wt.%). In Part II of this work the effect of Nb–B inoculation on commercial Al–Si alloys is assessed. Specifically, hypo-eutectic alloys such as LM24 (A380) and LM25 (A356) as well as near-eutectic LM6 (A413) Al–Si alloys are considered. The aim is to quantify the grain refinement and detect possible interaction with alloying elements commonly present in Al cast alloys, such as Mg, Fe, Cu, Mn and Zn. The in-depth analysis of the alloys solidified under wide range of cooling rates indicates that Nb–B inoculation does not only lead to a much finer microstructural features but also makes the final grain size far less sensitive to the cooling rate employed to solidify the material. Finally, the mechanism essential for the grain refinement of commercial Al cast alloys by Nb–B inoculation is determined on the base of SEM and thermal analysis results. It is found that in-situ formed Al3Nb and NbB2 intermetallic particles (forming from the interaction of Al alloy/Nb powder/KBF4 flux) are the heterogeneous nuclei responsible for the grain refining of Al cast alloys.

Laser-deposited CoCrMo alloy: Microstructure, wear, and electrochemical properties

Abstract

Gelation and In Vitro Digestibility of Soybean Protein Isolate Treated by a Ternary System Containing Horseradish Peroxidase, Glucose Oxidase, and Glucose

The aim of the present study was to investigate the potential application of a ternary system containing horseradish peroxidase, glucose oxidase, and glucose for the oxidative cross-linking of soybean protein isolate. Protein content, reaction temperature, and original pH were fixed at 30 g/L, 37°C, and 7.0, while suitable glucose, glucose oxidase, horseradish peroxidase addition, and reaction time selected from single factor trials for the one-step treatment were 0.1 mmol, 4.0 U, 200 U/g protein, and 3 h, respectively. In the two-step treatment, glucose, glucose oxidase, and horseradish peroxidase addition were the same, but a reaction time of 1 or 2 h was used for the glucose oxidase or horseradish peroxidase treatment. Two modified soybean protein isolates obtained by the one- and two-step treatment contained some protein polymers, had relative dityrosine content of 414 and 381, respectively, and showed lower in vitro digestibility and less free sulfhydryl than the control soybean protein isolate. During acidification induced by glucono-δ-lactone, the modified soybean protein isolates showed shorter gelation time (57 and 61 versus 72 min), lower loss tangent and gelation point temperature (52.9 and 53.2 versus 62.7°C) but higher final storage modulus than the control soybean protein isolate. The acidified gels prepared from the modified soybean protein isolates also exhibited enhanced hardness, springiness, cohesiveness, and chewiness, and finer microstructural features. The ternary system was capable of treating protein ingredients to improve their gelation properties.

A transmission electron microscopy and three-dimensional atom probe study of the oxygen-induced fine microstructural features in as-sintered Ti–6Al–4V and their impacts on ductility

Recent systematic experimental studies involving the benchmark Ti–6Al–4V alloy fabricated from powder have established that there exists a critical level of oxygen around 0.33mass%, beyond which the tensile ductility of the alloy drops dramatically until reaching total brittleness. To understand the fundamental mechanisms behind this critical oxygen content, three-dimensional atom probe tomography, transmission electron microscopy and other analytical means have been used to identify and characterize the fine microstructural changes induced by the increased oxygen content beyond the critical level. Three fine microstructural features were identified in as-sintered Ti–6Al–4V when the interstitial oxygen content was increased from 0.25mass% to 0.49mass%. These are: (i) the formation of fine acicular α precipitates in the β phase; (ii) the formation of α2-type (Ti3Al) nanometric clusters in the α matrix; and (iii) grain boundary α–β–α-layered structures between the α grains. The impacts of these microstructural changes on the tensile ductility are discussed.

Suspension and solution plasma spraying

Suspension and solution plasma spraying makes it possible to achieve coatings with fine microstructural features and is becoming a common route in laboratories to elaborate coatings a few tenths to a few hundreds of micrometres thick. This paper presents the recent developments in direct current plasma spraying of suspensions or solutions. It begins with a short description of the main plasma torches used for liquid feedstock spraying as well as the techniques used to experimentally observe droplets and particles in the plasma jet and characterize the void network of nanostructured plasma-sprayed coatings. The paper then turns to the momentum and heat transfers between fine particles and the plasma jet and the interactions between the plasma jet and a liquid in the form of a jet or drops. It concludes by linking some characteristic features of coating microstructures with the liquid processing in the plasma jet.

Comparison of creep rupture behaviour of type 316L(N) austenitic stainless steel joints welded by TIG and activated TIG welding processes

► In the present study, Creep rupture behaviour of 316L(N) base metal, and its weld joints were investigated at 923 K. ► Both the weld joints possessed lower creep rupture strength than the base metal. ► A-TIG joint had higher creep rupture strength than the MP-TIG joint. ► Minimum steady state creep rates of the base metal, A-TIG and MP-TIG weld joints were comparable. ► Reduced cavitation in A-TIG joint has been attributed to lower δ-ferrite content and lesser extent of strength heterogeneity. ► The columnar grains along the applied stress direction in weld metal leading to improvement in creep strength of A-TIG joint. Creep rupture behaviour of type 316L(N) austenitic stainless steel base metal and its weld joints fabricated both by single-pass activated TIG (A-TIG) and multi-pass conventional TIG (MP-TIG) welding processes were studied at 923 K over a stress range of 160–280 MPa. Both the weld joints possessed lower creep rupture lives than the base metal. The A-TIG weld joint displayed higher rupture lives than the MP-TIG weld joint. Failure in the weld joints occurred in the weld metal. Progressive localization of creep deformation in the weld metal of both the joints led to the premature failure. Accumulation of creep deformation at higher rate was observed in the weld metal of the MP-TIG joint than in the A-TIG joint. Finer microstructural features and higher amount of δ-ferrite was observed in the weld metal of MP-TIG joint than in the A-TIG weld joint. Orientation of the columnar grains and δ-ferrite was nearly transverse to the welding direction in the A-TIG joint, whereas it was towards short transverse to the welding direction in the MP-TIG weld joint. TEM investigation of creep exposed weld metal showed the extensive formation of M 23 C 6 carbides, σ-phase and Laves phase along the boundaries in MP-TIG joint, which were less prevalent in the A-TIG joint. With creep exposure, the δ-ferrite transformed to σ and Laves phases, and creep cavitation was found to be associated with the intermetallic phases. Creep cavitation was more pronounced in the MP-TIG weld joint than in the A-TIG weld joint.

Sinterability and microwave dielectric properties of nano structured 0.95MgTiO 3–0.05CaTiO 3 synthesised by top down and bottom up approaches

0.95MgTiO 3–0.05CaTiO 3 (MCT) nano powders were synthesised using sol–gel method and high energy ball milling (HEBM). Synthesised powders were characterised using X-ray diffraction analysis to ensure phase purity and HRTEM to determine the fine microstructural features like particle size, interplanar spacing, etc. The powder pellets were heat treated to study the sinterability and microwave dielectric properties and these properties were then compared with the microwave dielectric properties of micron sized sample. Nano powder synthesised using HEBM shows better dielectric properties, sinterability and gets densified to 90% of theoretical density (TD) at 1200 °C/2 h. Dielectric resonators prepared using chemically synthesised nano powder showed poor sinterability and microwave dielectric properties, but, dielectric properties of HEBM samples were very near to that of solid state synthesised samples. Sintered HEBM powders retain the microwave dielectric properties almost to the same level as the solid state synthesised powder with considerable lowering of sintering temperature.

Potentiodynamic Polarization Aspects of the As-cast and Sprayed Al-Si, Al-Sn and Al-Sn-Si Alloys in a Sodium Chloride Solution

The present study compares the corrosion behavior of Al-Sn, Al-Si and Al-Sn-Si alloys processed by spray forming with that of the conventional chill cast ones in aqueous 0.1 N NaCl solution. Spray forming resulted in finer microstructural features with uniform distribution of second-phase particles. The spray formed Al-Si samples showed improved corrosion resistance as compared to the chill cast ones. The Sn containing alloys showed inferior corrosion resistance in the neutral electrolyte. The addition of 12.5 wt.% Si to Al-Sn alloys improves the corrosion resistance.

Heterogeneity of Deformation in Pure Ni Single Crystal of Cube Orientation Deformed by Equal Channel Angular Extrusion

Texture and microstructure were investigated after 1 pass equal channel angular extrusion (ECAE) of a single crystal cube oriented Ni billet. Neutron and X-ray diffraction were used to determine global and local textures, respectively. EBSD and TEM investigations were also carried out in order to study fine scale microstructural features. Substantial variations in texture and microstructure, resulting from different efficiencies in the shearing process, were locally recorded through the billet height. The deformation was heterogeneous on both micro and macro scales.

Pure Ni Single Crystal of Cube Orientation Deformed by Equal Channel Angular Extrusion

Texture and microstructure were investigated after 1 pass equal channel angular extrusion (ECAE) of a single crystal cube oriented Ni billet. Neutron and X-ray diffraction were used to determine global and local textures, respectively. To investigate the fine scale microstructural features EBSD was used. Substantial variations in texture and microstructure resulting from different efficiencies in the shearing process were locally recorded through the billet height.