As most of the failures in engineering components initiate from the surface layer, applying surface treatments can play a crucial role in controlling material performance and lifetime. In this study, ...different surface severe plastic deformation techniques including severe shot peening, laser shock peening and ultrasonic nanocrystal surface modification have been considered. The effects of process parameters and the kinetic energy of each treatment on the microstructure, mechanical properties and fatigue behavior of nickel-based super-alloy Inconel 718 have been investigated. The results revealed that using the proper parameters to increase the kinetic energy of the applied surface treatments, it is possible to effectively promote surface grain refinement and induce a deep compressive residual stress field in Inconel 718 samples. Among the applied treatments, ultrasonic nanocrystal surface modification was found to be the most efficient one in improving the mechanical properties as it led to the most significant fatigue performance, followed by severe shot peening and laser shock peening.
High pressure torsion (HPT) was employed to fabricate nanostructured α-uranium successfully. The evolution of microstructures (grain size and dislocation density) and related mechanical properties ...(hardness and elastic modulus) varied with strain were quantified and analyzed. The steady-state microstructures and microtextures of the nanostructured α-uranium were characterized by transmission electron microscopy (TEM) and transmission Kikuchi diffraction (TKD) analysis in scanning electron microscopy (SEM), respectively. The grain/subgrain size of the nanostructured α-uranium is below 100 nm, and a weak microtexture with four components is presented. The microstructure evolution of α-uranium during severe plastic deformation (SPD) could be divided into three stages, and it resonates with the variation of creep resistance as a function of strain. On account of the unique crystal structure of α-uranium, the grain refinement limit was discussed and compared with 25 pure metals from a statistical view.
The addition of 1.08wt% Sc to an Al-Mg-Zr alloy produced by selective laser melting modifies the highly coarse columnar grain structure to significantly refined columnar grains separated by ...sub-micron equiaxed grains at the melt pool boundaries. The latter nucleate mainly from the Al3Sc particles in the remelting zone. An almost fully equiaxed grain structure was achieved by increasing the applied volumetric energy density from 77.1J/mm3 to 154.2J/mm3 and the platform temperature from 35°C to 200°C, due to the combined effects of potent nuclei, increased remelting zone volumes and reduced thermal gradients.
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Effect of solutes on grain refinement Fan, Z.; Gao, F.; Wang, Y. ...
Progress in materials science,
January 2022, 2022-01-00, 20220101, Letnik:
123
Journal Article
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Grain refinement not only enhances the mechanical performance of as-cast metallic materials but also provides an effective mechanism for controlling cast defects, such as macro-segregation, porosity ...and coarse second phase particles. Therefore, understanding grain refinement of alloys with different solute additions is of both theoretical and practical importance. Although extensive research has been carried out over many decades and significant progress has been made on the subject, such historical research has not delivered the desirable scientific understanding, and many critical questions remain open. Adopting a hybrid approach between review and overview, in this paper, we firstly provide a brief review on the historical research on the solute effect on grain refinement in the literature, then present the recent advances in the understanding of the subject in a holistic manner, and finally offer a summary of the factors that hindered progress in the past, key advances made in recent years and some suggestions for future research directions.
Grain refinement through severe plastic deformation enables synthesis of ultrahigh-strength nanostructured materials. Two challenges exist in that context: First, deformation-driven grain refinement ...is limited by dynamic dislocation recovery and crystal coarsening due to capillary driving forces; second, grain boundary sliding and hence softening occur when the grain size approaches several nanometers. Here, both challenges have been overcome by severe drawing of a pearlitic steel wire (pearlite: lamellar structure of alternating iron and iron carbide layers). First, at large strains the carbide phase dissolves via mechanical alloying, rendering the initially two-phase pearlite structure into a carbon-supersaturated iron phase. This carbon-rich iron phase evolves into a columnar nanoscaled subgrain structure which topologically prevents grain boundary sliding. Second, Gibbs segregation of the supersaturated carbon to the iron subgrain boundaries reduces their interface energy, hence reducing the driving force for dynamic recovery and crystal coarsening. Thus, a stable cross-sectional subgrain size <10 nm is achieved. These two effects lead to a stable columnar nanosized grain structure that impedes dislocation motion and enables an extreme tensile strength of 7 GPa, making this alloy the strongest ductile bulk material known.
The improvement in the hardness of Sn-3.0Ag-0.5Cu solder alloy reinforced with 1.0 wt % TiO2 nanoparticles was evaluated by nanoindentation. A specific indentation array was performed on four ...different horizontal cross sections of the composite solder with different heights and diameters, in order to verify the mixing homogeneity of TiO2 nanoparticles within the Sn-3.0Ag-0.5Cu solder paste during the ball milling process. The phase analysis indicated successful blending of the Sn-3.0Ag-0.5Cu with the TiO2 nanoparticles. According the scanning electron microscopy micrographs, presence of the TiO2 nanoparticles reduced the size of the Cu6Sn5 and Ag3Sn intermetallic compound phases. Incorporation of the 1.0 wt % TiO2 nanoparticles improved the hardness values up to 26.2% than that of pure SAC305. The hardness values increased gradually from the top cross sections towards adjacent to the solder/substrate interface. The mechanism of the hardness improvement attained by the TiO2 nanoparticles addition were also investigated on the horizontal cross sections of the samples.
Graphene and its derivatives have attained a considerable amount of popularity as effective reinforcements in the electrodeposited nickel (Ni) matrix composites for lubrication. However, the ...composites suffer from certain challenges, such as the agglomeration of graphene nanosheets in the electrodeposition process and the uncertain role of graphene on the friction reduction and wear resistance during sliding. By using a polyvinylpyrrolidone assisted reduction method to improve the dispersity of reduced graphene oxide (RGO) in the electrolyte, we prepare the bulk RGO/Ni composites with different RGO adding amount and perform a complete research on the tribological behaviours. Our work reveals that although an extremely low amount of RGO nanosheets (carbon content below 0.018%) have been incorporated into the Ni matrix, it induces significant grain refinement and friction reduction effects. Compared with the RGO-free Ni deposit, the friction coefficient and wear rate of the composite is reduced by 25.6% and 27.5%, respectively. The improved tribological properties are ascribed to the fine-grain strengthening effect and the formation of a continuous easy-shear nickel oxide film on the contact surface. This finding offers a new view on the wear mechanism of graphene/Ni composites with low graphene content, for which the fine-grain strengthening rather than the formation of carbon-rich transfer layer will dominate the lubricating and anti-wear performances.
•Stable RGO dispersion in the electrolyte is obtained by surfactant-assisted reduction process.•RGO nanosheets induce a significant grain refinement effect in Ni matrix.•The improved tribological properties of RGO/Ni composites are ascribed to the ultrafine grain structure.
•Sm2Co17 sintered magnets with high flexural strength and high magnetic properties were prepared by doping Al2O3 powders.•The magnets doped with 0.1 wt% Al2O3 increase the flexural strength by 20 %, ...while the decrease of magnetic properties is not more than 5 %.•The decrease in flexural strength (for c//b and c//h cases) of the magnets introduced beyond 0.2 wt% Al2O3 is related to the larger cell sizes and incomplete cell boundaries.
In this work, a small amount of Al2O3 powders (≤0.3 wt%) were incorporated into the Sm2Co17-type sintered magnets, obtaining both high mechanical and magnetic properties. It is found that 0.1 % weight percentage of Al2O3 doping is enough to enhance the flexural strength by about 20 % (∼ 180 MPa for the case of the c-axis parallel to height). Meanwhile, the (BH)max remains around 219 kJ/m3, and Hcj is 2052 kA/m, which is over 95 % of that of the original magnets without doping. The promising improvement in flexural strength is mainly attributed to the grain size effective refinement caused by Sm2O3 particles including newly-formed ones from the reaction of the Al2O3 powder and Sm in the matrix. Furthermore, the grain size of the magnets decreases significantly with increasing of Al2O3 doping up to 0.3 wt%. Especially, the grain size of 0.3 wt% Al2O3 doped magnets is refined by 37 %. However, the flexural strengths (for the c-axis parallel to height and the c-axis parallel to width cases) of the magnets decrease sequentially and are even lower than that of the original magnet. The microstructure investigations indicate that the decrease in flexural strength may closely be correlated to the larger cell size and the incomplete cell boundaries phase. The obtained results infer that the flexural strength is susceptible to not only grain size but also the cellular structure of the magnets.
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Bulk nanostructured (ns)/ultrafine-grained (UFG) metallic materials possess very high strength, making them attractive for high strength, lightweight and energy efficient applications. The most ...effective approach to produce bulk ns/UFG metallic materials is severe plastic deformation (SPD). In the last 30 years, significant research efforts have been made to explore SPD processing of materials, SPD-induced microstructural evolutions, and the resulting mechanical properties. There have been a few comprehensive reviews focusing mainly on SPD processing and the mechanical properties of the resulting materials. Yet no such a review on SPD-induced microstructural evolutions is available. This paper aims to provide a comprehensive review on important microstructural evolutions and major microstructural features induced by SPD processing in single-phase metallic materials with face-centered cubic structures, body-centered cubic structures, and hexagonal close-packed structures, as well as in multi-phase alloys. The corresponding deformation mechanisms and structural evolutions during SPD processing are discussed, including dislocation slip, deformation twinning, phase transformation, grain refinement, grain growth, and the evolution of dislocation density. A brief review on the mechanical properties of SPD-processed materials is also provided to correlate the structure with mechanical properties of SPD-processed materials, which is important for guiding structural design for optimum mechanical properties of materials.
Metastable β-Titanium alloys containing a high content of segregated β-stabilising elements such as Cr make segregation defects (known as β-flecks or β-freckles) difficult to avoid during ...solidification processing. This work investigates the formation of β-flecks in the high strength Ti-3Al-8V-6Cr-4Mo-4Zr (Beta-C/ASTM Grade 19) produced by Wire Arc Additive Manufacturing and explores whether the introduction of grain refiner (La2O3) to the alloy can prevent the segregation defects from forming. Promoting the columnar to equiaxed transition and refining the grain size with La2O3 nucleant particles was found to significantly reduce the presence of β-flecks by 99.9% compared to the un-refined (standard) alloy. Analytical and numerical modelling is used to provide insights into how grain refinement can achieve this significant defect reduction.
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