Explore the vast range of titles available.

The evolution of pore structure during in situ underground exploitation of oil shale directly affects the diffusion and permeability of pyrolysis products. In this study, on the basis of mineral analysis and thermogravimetric results, in combination with the low-pressure nitrogen adsorption (LPNA) and mercury intrusion porosimetry (MIP) technique, the evolution of pore structure from 23 to 650 °C is quantitatively analyzed by simulating in situ pyrolysis under pressure and temperature conditions. Furthermore, based on the experimental results, we analyze the mechanism of pore structure evolution. The results show the following: (1) The organic matter of Fushun oil shale has a degradation stage in the temperature range of 350–540 °C, and there is no obvious temperature gradient between decomposition of kerogen and the secondary decomposition of bitumen. The thermal response mechanisms of organic matter and minerals are different in each temperature stage, and influence the change of pore structure. (2) Significant changes occur in pore shape at 350 °C, where thermal decomposition of kerogen begins. The ink-bottle pores are dominant when the temperature is less than 350 °C, whereas slit pores dominate when the temperature is greater than 350 °C. (3) The change in pore structure of oil shale is much less significant from 23 to 350 °C. The pore volume, porosity, and specific surface area (SSA) of samples increase rapidly with temperature varying from 350 to 600 °C. The variation of each parameter is dissimilated from 600 to 650 °C: the porosity and pore volume increases with a small gradient from 600 to 650 °C, and SSA decreases significantly. (4) The lithostatic pressure does not cause change in the evolution discipline of the pore structure, but the inhibitory effect on the pore development is significant.

Combining the advantages of a wet chemical method and spark plasma sintering, carbide-doped materials W-1wt%TiC and W-1wt%ZrC were prepared. Microstructural evolution in W-1wt%TiC and W-1wt%ZrC under irradiation of 5 keV He+ at 600 °C to fluences up to 5.0 × 1021 ions/m2 with ion flux of about 8.8 × 1017 ions/m2s was investigated by transmission electron microscopy (TEM). The dislocation loop number density of W-1wt%TiC was higher than that of W-1wt%ZrC, but the average loop size of the W-1wt%TiC was in average smaller. There were no observable helium bubbles in W-1wt%TiC and W-1wt%ZrC, exhibiting higher radiation resistance to He+ compared to pure W. He+ pre-damaged and undamaged W-1wt%TiC and W-1wt%ZrC samples were irradiated by 5 keV D2+ to estimate the D retention in doped W materials. The irradiation damage impact of He+ on deuterium retention was examined by a method of thermal desorption spectroscopy (TDS). Compared with the undamaged samples, it was illustrated that D2 retention of W-1wt%TiC and W-1wt%ZrC increased after He+ pre-irradiation.

A new, wrought Ni-Fe-based alloy with excellent creep rupture life has been developed for 700 °C-class advanced ultra-supercritical (A-USC) steam turbine rotor application. In this study, its tensile deformation behaviors and related microstructure evolution were investigated. Tensile tests were carried out at room temperature, 700 °C, and 750 °C. The results show that the Ni-Fe-based alloy has excellent yield strength at 700 °C, which is higher than that of some other Ni-based/Ni-Fe-based alloys. The fracture surface characteristics indicate trans-granular and intergranular fracture modes at room temperature, 700 °C, and 750 °C. However, the intergranular fraction mode became dominant above 700 °C. Dynamic recrystallization occurred at 700 °C and 750 °C with increasing average misorientation angles. The volume fraction of the γ′ precipitate was around 20%, and the average size of the γ′ precipitates was around 30 μm, which had no noticeable change after the tensile tests. The predominant deformation mechanisms were planar slip at room temperature, bypassing of the γ′ precipitates by the Orowan mechanism, and dislocation shearing at 700 °C and 750 °C. The tensile properties, fracture characteristics, and deformation mechanisms have been well-correlated. The results are helpful in providing experimental evidence for the development and optimization of high-temperature alloys for 700 °C-class A-USC applications.

Grain boundary (GB) migration in polycrystalline materials necessarily implies the concurrent motion of triple junctions (TJs), the lines along which three GBs meet. Today, we understand that GB migration occurs through the motion of disconnections in the GB plane (line defects with both step and dislocation character). We present evidence from molecular dynamics grain growth simulations and idealized microstructures that demonstrates that TJ motion and GB migration are coupled through disconnection dynamics. Based on these results, we develop a theory of coupled GB/TJ migration and use it to develop a physically based, disconnection mechanism-specific continuum model of microstructure evolution. The continuum approach provides a means of reducing the complexity of the discrete disconnection picture to extract the features of disconnection dynamics that are important for microstructure evolution. We implement this model in a numerical, continuum simulation and demonstrate that it is capable of reproducing the molecular dynamics (MD) simulation results.

The results of untiring efforts by the research community over the past few decades have led to the successful development and processing of a number of advanced titanium alloys with widely varying properties that can cater to niche applications. Advanced titanium alloys on one hand challenge structural steels with their higher specific strength coupled with their low temperature capability down to 4 K and pose serious threat on the other hand to superalloys for long-term applications up to 773 K. An improved understanding of the processing-microstructure-mechanical property correlation led to the realization of large scale as well as performance critical titanium alloy products in space arena. Recent advances in additive manufacturing, wherein the desired components are directly 3D printed from pre-alloyed powders/wires have given a definite advantage for cost-prohibitive titanium alloys. This review article discusses challenges in the processing, mechanical properties and microstructure evolution of various grades of titanium alloys. It also provides useful information for researchers working on titanium alloys with a glimpse in to the recent advances in Ti alloys and transformation of scientific knowledge to technological advancements in products for space applications. Graphic Abstract

The dynamic recrystallization (DRX) features and the evolution of the microstructure of a new hot isostatic pressed (HIPed) powder metallurgy (P/M) superalloy are investigated by hot-compression tests. The sensitivity of grain dimension and DRX behavior to deformation parameters is analyzed. The results reveal that the DRX features and grain-growth behavior are significantly affected by deformation conditions. The DRX process is promoted with a raised temperature/true strain or a reduced strain rate. However, the grains grow up rapidly at relatively high temperatures. At strain rates of o.1 s−1 and 1 s−1, a uniform microstructure and small grains are obtained. Due to the obvious differences in the DRX rate at various temperatures, the piecewise DRX kinetics equations are proposed to predict the DRX behavior. At the same time, a mathematical model for predicting the grain dimension and the grain growth behavior is established. To further analyze the DRX behavior and the changes in grain dimension, the hot deformation process is simulated. The developed grain-growth equation as well as the piecewise DRX kinetics equations are integrated into DEFORM software. The simulated DRX features are consistent with the test results, indicating that the proposed DRX kinetics equations and the established grain-growth model can be well used for describing the microstructure evolution. So, they are very useful for the practical hot forming of P/M superalloy parts.

The effect of cold rolling on the microstructure and mechanical properties of an Al- and C-containing CoCrFeNiMn-type high-entropy alloy was reported. The alloy with a chemical composition (at %) of (20–23) Co, Cr, Fe, and Ni; 8.82 Mn; 3.37 Al; and 0.69 C was produced by self-propagating high-temperature synthesis with subsequent induction. In the initial as-cast condition the alloy had an face centered cubic single-phase coarse-grained structure. Microstructure evolution was mostly associated with either planar dislocation glide at relatively low deformation during rolling (up to 20%) or deformation twinning and shear banding at higher strain. After 80% reduction, a heavily deformed twinned/subgrained structure was observed. A comparison with the equiatomic CoCrFeNiMn alloy revealed higher dislocation density at all stages of cold rolling and later onset of deformation twinning that was attributed to a stacking fault energy increase in the program alloy; this assumption was confirmed by calculations. In the initial as-cast condition the alloy had low yield strength of 210 MPa with yet very high uniform elongation of 74%. After 80% rolling, yield strength approached 1310 MPa while uniform elongation decreased to 1.3%. Substructure strengthening was found to be dominated at low rolling reductions (<40%), while grain (twin) boundary strengthening prevailed at higher strains.

Friction-assisted plastic deformation at the joint interface is essential for achieving the desired joint properties in dissimilar friction stir lap welding (FSLW) of aluminum alloy (Al) with steel (Fe). This plastic deformation can be precisely controlled by using an adjustable tool, where shoulder and probe rotation speeds are independently controlled. This study explores the effect of microstructure evolution and probe rotation speed on the weld’s interface morphology and tensile properties of FSLW joints between an aluminum alloy and steel using an adjustable tool. Microstructure evolution is influenced by inherent material properties and process-induced stress inhomogeneity. Grain refinement in Fe is more gradual compared to Al, which exhibits a homogenous microstructure. Lower probe rotation speeds lead to more significant grain refinement in Al, while increasing probe rotation speeds promote the formation of intermetallic compounds. An intercalated structure with varying fractions and morphology is observed across the joint interface in all welds. The controlled evolution of microstructure and the differences in intercalated structure formation at the weld interface are attributed to variations in shear strength. This study demonstrates the ability of an adjustable tool to tailor the microstructure and tensile properties of FSLW joints, providing a promising approach for enhancing joint performance in dissimilar metal joining applications.

Friction stir processing (FSP) was used to modify the surface layer of the AZ91 magnesium alloy. The treatment was carried out using a jet cooling nozzle, generating a stream of cold air and enabling intensive cooling of the friction stir processed (FSPed) zone. Single-pass FSP was carried out using a tool rotational speed of 500 rpm and travel speed of 30 mm/min. The treatment was conducted using a truncated cone-shaped tool with a threaded side surface. Strong grain refinement and microstructural changes typical for FSP were found in all the samples. Very fine, equiaxed recrystallized grains dominated in the stirring zone. In the samples modified with the jet cooling nozzle, greater grain refinement was obtained than in the case of naturally cooled material. The average grain size in the surface part of the stirring zone was 1.4 μm and 9 μm in the samples with air-cooling and with natural cooling, respectively. Both the naturally cooled specimen and air-cooled specimen were characterized by a distinctly higher hardness than the base material. The average Vickers hardness in the stirring zone was 91 HV0.1 in the FSPed sample with the air-cooling system and 85.5 HV0.1 with natural cooling, respectively. The average Vickers hardness of the as-cast alloy was 64 HV0.1. Slightly higher wear resistance of the FSPed samples using a jet cooling nozzle was found in relation to the naturally cooled sample. Based on the conducted research, high efficiency of the jet cooling nozzle in cooling the modified zone during friction stir processing was found.

Machined surface with ultrafine grained microstructure can be obtained through severe plastic deformation in high-speed machining (HSM) process. The aim of this paper is to investigate the evolution of grain size and micro-hardness during HSM of Ti-6Al-4V alloy using the finite element method (FEM) and user subroutine VUSDFLD of Abaqus/Explicit. Firstly, a FE model to simulate the cutting process of Ti-6Al-4V is proposed. The proposed cutting simulation model is verified by high-speed machining experiments in terms of cutting force and chip morphology. Secondly, a novel user subroutine VUSDFLD based on equations of Zener-Hollomon and Hall-Petch is developed to simulate the modifications of grain size and micro-hardness in chip formation and machined surface generation under different cutting speeds. Parameters in the equations of Zener-Hollomon and Hall-Petch are modified for Ti-6Al-4V for simulation of the change of grain size and micro-hardness during HSM. Lastly, the simulation results of the microstructure evolution in chips and machined surfaces are compared with experimental results obtained by optical microscopy, scanning electron microscopy (SEM), and measurement of micro-hardness. The comparison results show that the evolution of grain size and micro-hardness of Ti-6Al-4V in HSM can be accurately predicted by the modified Zener-Hollomon and Hall-Petch equations. This research indicates that smaller grain sizes are produced into both chips and machined surfaces due to more severe deformations with increasing of the cutting speed. The findings validate that HSM is a reliable approach to generate refined grains if proper machining parameters are selected. HSM can also be applied as a novel material test method to study the relationship between microstructure evolution and deformation parameters.