Evolution Of Microstructure Characteristics Research Articles

Synergistic effect of CO2-mixing and steel slag addition on performance and microstructure of concrete

This study systematically investigated the influence of varying carbon dioxide (CO2) dosages and steel slag (SS) replacement rates on the fresh and hardened properties of concrete through the injection of CO2 gas during the mixing process. The composition of hydration products, degree of hydration, and the evolution of microstructural characteristics within the CO2-SS-cement system were analyzed. The results showed that CO2 addition during the mixing process enhanced early strength in SS concrete. For concrete with a 10 wt% SS replacement rate, the incorporation of CO2 ranging from 0.4 wt% to 1.2 wt% of the total mass of cementitious materials resulted in notable compressive strength increases of 1.26%–12.47%, 0.64%–6.14%, and 11.79%–18.24% at ages of 1 d, 3 d, and 7 d, respectively. CO2 rapidly reacted with SS to form calcite-type CaCO3 and promoted the generation of C-S-H through synergistic hydration with cement. However, a high CO2 dosage (1.2 wt%) led to issues such as microcrack propagation and interface loosening due to CaCO3 deposition, which could be mitigated to some extent by appropriately increasing the SS content.

The Microstructure Characteristics Evolution of Bulk High-Purity Silver for High Relief Application

Silver products with high relief have become popular in the silver decoration industry. However, it is difficult to obtain these products through conventional processing at ambient temperature. The aim of this work is to solve this problem by increasing the deformation temperature. Detailed studies were conducted on the evolution of microstructure characteristics in bulk high-purity silver by electron backscatter diffraction (EBSD) to achieve high-relief applications at elevated temperatures. The high temperature sample is mainly composed of recrystallized and substructured grains, exhibiting a more stable state than the ambient temperature sample. More than 70% annealing twins are observed in the hot-working sample. They are characterized by the amount of Σ3n-type triple grain boundary junctions within large grain clusters formed by multiple twinning. These particular boundaries improve the intergranular corrosion resistance and degradation, which is significantly essential for high-purity silver jewelry exposed to sweat and air. The closed multi-coining processes at different temperatures were conducted subsequently. The performance of workpieces demonstrates that increasing the deformation temperature is a viable alternative for producing durable high-relief silver products.

Evolution of microstructural characteristics during creep behavior of Inconel 718 alloy

In this work, the microstructural evolution and creep behavior of Inconel 718 alloy under different temperatures and stresses were studied comprehensively. The results show that the increase of temperature and stress significantly accelerates the creep process, steady creep rate increases by two orders of magnitude and the creep life decreased to only 11 h (690 °C/690 MPa). During creep, γ″ phase coarsened and transformed to δ phase, which was dominated by the diffusion of Nb (based on the activation energy calculation). Long holding time at high temperature/stress induced the concentration of dislocations around the δ phase, resulting in the creation of voids. With the progress of creep, the δ phase was separated from the matrix gradually and the voids coalesced to form cracks. Additionally, the crystal symmetry of twin within grains also provides energetically favorable condition for precipitation of δ phase. The increased δ phase provides more cracks, resulting in a tendency of transgranular fracture. Twinning coordinates grain deformation and obstructs dislocation motion, competing with the crack formation and propagation induced by intragranular δ phase. Based on the detailed analysis of creep fracture mechanism, a model for creep fracture was developed.

Evolution of Microstructural Characteristics During Creep Behavior of Inconel 718 Alloy

Evolution of Microstructural Characteristics During Creep Behavior of Inconel 718 Alloy

Tensile property and micro-texture evolution of the charge weld in a billet-to-billet extrusion of AA6061 aluminum profile

Charge welds occur in profiles during billet-to-billet extrusion of hollow aluminum profiles. To investigate the effects of charge welds on the tensile property of the profile, a tensile test and a fracture scanning test using the scanning electron microscope (SEM) are conducted. The results showed that charge weld plays bad roles in the tensile property of the profile. The tensile samples derived from the charge weld distribution zone are always fractured near the charge welds. In addition, the tensile strengths of the early-stage charge welds are the poorest. To validate the property variations in the charge weld zone along the extrusion directions in a microscale, the evolution of microstructure characteristics and texture in the charge weld distribution zone are discussed with the help of the electron back scattering diffraction (EBSD). The grain morphologies, misorientation, and micro-textures are not uniform along the charge weld in the extrusion direction or on different sides of itself. Incomplete dynamic recrystallization with lower distortion energy and dislocation density due to lack of frictions with dies at the beginning and middle stages of the charge weld results in strip-shaped coarse grains and lower-angle grain boundaries, further revealing the essence of the low tensile strength at the beginning and in the middle of the charge weld. The intensity of shear texture {111} on the charge weld is the largest compared with those on both sides of it, verifying the large plane strain status on the charge weld plane.

Microstructure formation and mechanical behaviour of titanium aluminides during torsion

Two-phase Ti-47Al-4(Cr, Nb, Mn, Si and B) alloys (at.%) with fine-grained globular and coarse-grained lamellar structures were deformed in torsion at 1000 °C under 400 MPa hydrostatic pressure at a maximum shear strain rate of 2 × 10−4 s−1 in a Paterson-type rock deformation machine. The evolution of microstructural characteristics during thermomechanical processing was investigated by transmission electron microscopy (TEM). It is observed that the γ-TiAl + α2-Ti3Al lamellar structure breaks down, and a fine-grained duplex globular structure with a grain size on the order of 5 μm develops. TEM observation indicates that the phase transformation of α2 to γ occurring during torsion shows diffusional character. Ordinary dislocations 1/2[1 1 0] and superdislocations 1/2[1 1 ] are observed. The microstructural and mechanical characteristics indicate that superplastic flow occurs during torsion of titanium aluminides.

Evolution of Microstructural Characteristics of Different Cement Materials Caused by Carbonation

The objective of this work was to examine the influence of carbonation on the microstructure of cement materials. Different materials, which were CEM I mortar and paste, CEM II mortar and paste, were carbonated at 20°C, 65% relative humidity and 20% of CO2 concentration. The specific surface area and pore size distribution were determined from the method of nitrogen adsorption. The results showed that the materials based on CEM II seemed to be more sensible to a creation of mesoporosity after carbonation than the CEM I based materials. The results of this study also helped to explain why observations in the literature diverge greatly on the influence of carbonation on specific surface area.

Effects of HIP Temperature on the Microstructural Evolution and Property Restoration of a Ni-Based Superalloy

The effects of hot isostatic pressing (HIP) temperature on the evolution of microstructural characteristics around the creep cavities were investigated using five different soaking temperatures during HIP. The restoration of microstructural and material properties under these HIP schedules followed by the same rejuvenation heat treatment (RHT) procedure was then evaluated. The results showed that HIP temperature played a dominant role in the cavity healing process, determining the evolution features of cavities and hence the extending ratio of rupture life. With gradually increasing the adopted HIP temperature, the cavity healing behaviors were revealed. Concentrically oriented N-type γ′ rafting structure in the vicinity of the healing cavity was observed at an appropriate soaking temperature. The driving force for the cavity healing was considered to be the chemical potential gradient induced by the stress gradient field around the cavity. The appropriate HIP schedule followed by RHT process significantly improved the rupture life of damaged alloy, and the extending ratio varied with the adopted HIP temperature.

Nanocrystallization process and mechanism in a nickel alloy subjected to surface severe plastic deformation

Bulk components made of a Ni-base C-2000 alloy with a face-centered cubic crystal structure and a very low stacking fault energy have been severely plastically deformed at the surface region to attain a grain size gradient ranging from nanocrystalline at the surface to coarse grained in the bulk. The evolution of microstructural characteristics has been studied as a function of the processing time employing a variety of analytical techniques, including extensive conventional and high-resolution transmission electron microscopy analyses. The thickness of the nanocrystalline surface layer is found to increase with the processing time. Deformation twinning is ubiquitous and occurs at the earliest stage of deformation and the deepest region of the material where plastic deformation has taken place in the surface severe plastic deformation process. A grain-refinement mechanism led by deformation twins and complemented by dislocation activity has been put forth to explain the nanocrystallization of the coarse-grained material employed in this investigation.

Evolution of microstructure characteristics during the electron-beam-induced phase transition of aluminum trihydroxide

We report interesting microscopic features that evolve as a function of electron dose from the starting aluminum trihydroxide to the resulting transition aluminas with layered mesoporous structures. Remarkably, the sizes of pores and crystals in the porous structure were reduced as the electron dose was increased. The properties of the layered structure from the aluminum trihydroxide were maintained through the transition process, and hence the final mesoporous structures (mainly γ-alumina) were of lamella-type microstructure.