查看更多>>摘要：? 2022 Elsevier Inc.The formation and characteristics of bilamellar microstructure in Ti6242S titanium alloy under a dual heat treatment (single-phase heat treatment + two-phase heat treatment) were investigated. Heated at single β region and cooled by the air, when the grain boundary α (αGB) kept BOR (Burgers orientation relationship) with the β matrix on both sides, more grain boundary Widmansta¨tten α (αWGB) clusters were generated. The cooling modes after single-phase region heat treatment will change the source of the primary lamellar α, β → α phase transition or martensite decomposition. During two-phase heat treatment, the secondary lamellar α nucleates at the β grain boundaries or the wide interface of the primary lamellar α and the existing primary lamellar α facilitates the formation of the secondary lamellar α with same or similar orientation, showing a typical intragranular Widmansta¨tten α(αWI) microstructure. The V-shaped lamellar α with an angle of about 60° is precipitated from the martensite and shows a V-cone shape in three-dimensions. When the holding time is prolonged to 60 min, three α variants grow simultaneously and keep a mirror relationship in pairs, forming a triangular structure which has a pyramid shape in three dimensions. Variant selection occurred in the lamellar microstructure during dual heat treatment. Held at subsequent two-phase region, the primary lamellar α with the misorientation angle of 90° preferentially dissolved, increasing the variant selection. The primary lamellar α provided nucleation sites for the secondary lamellar α, leading to a decrease of variant selection.
查看更多>>摘要：? 2022The morphology of refined primary Mg2Si always exhibits the faceted crystal growing pattern in the Al-Mg2Si composites. The 2D and 3D morphologies of primary Mg2Si particles were examined by scanning electron microscopy (SEM) and field emission scanning electron microscope (FE-SEM) respectively. The morphology evolution mechanism of the faceted primary Mg2Si in Al-Mg2Si composite was investigated. The results show that there were only (100) planes and (111) planes emerging in Mg2Si particles. The morphology evolution of faceted Mg2Si has only two growing paths including Cube to Truncated Cube and Octahedron to Truncated Octahedron. The morphology evolution, critical nucleus and growth, was determined by r value which was deduced from the ratio of the surface free energy of (100) planes to the surface free energy of (111) planes. The surface free energies were identified by First-principle simulation. The validity of the above two growing path modes were verified. The configurations of the surface elements in faceted Mg2Si particles are predicted according to the simulating results.
查看更多>>摘要：? 2022 Elsevier Inc.The microstructure evolution of IN617 during creep under 30 MPa, 23 MPa, and 16 MPa at 1000 °C was characterized after creep ruptured tests and interrupted tests. As the applied stress was decreased, three different microstructure characteristics were found (i.e., dynamic recrystallization, classic creep voids, and precipitates coarsening), which were related to microstructural factors such as dislocation configuration, grain boundary (GB) weakening, and nitrogen diffusion and brittle-phase precipitation. The results indicated that the high stress of 30 MPa promoted the significant multiplication of dislocations that rearranged into high-density and low-energy substructures, leading to recrystallization and ductile rupture. When stress was decreased to 23 MPa, dislocations only formed loose tangles causing intragranular strengthening, and GB weakening was involved because most intergranular precipitates that could have exerted the pinning effect dissolved. Therefore, intergranular rupture occurred under the combined effect of intragranular strengthening and GB weakening. As stress was as low as 16 MPa, precipitates that existed in the as-received condition completely dissolved and abundant nitrogen diffused into the specimen from the furnace atmosphere through creep cracks, promoting the formation of brittle nitrides (π phase and AlN). The brittle π phase forming continuous networks along GBs was vulnerable to internal cracking, and the needle AlN precipitating inside grains caused the AlN/matrix interfacial cracking, which exerted a synergetic effect on brittle fracture.
de Medeiros Tavares M.Mashhadikarimi M.Gomes U.U.Huang S....
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查看更多>>摘要：? 2022 Elsevier Inc.This study investigates the influence of milling parameters and sintering cycle on the microstructure and properties of cemented carbides with an overal WC–11.5wt%Fe–4wt%NbH–0.5wt%C starting powder composition. The raw material powders were milled and mixed in a ring mill or planetary ball mill and sintered at 1150, 1200 or 1250 °C using spark plasma sintering (SPS). The cemented carbide density, Vickers hardness and fracture toughness were evaluated. The microstructure was investigated by optical and scanning electron microscopy using wavelength dispersive X-ray spectroscopy for elemental mapping. The transition from NbH to NbC was identified for all sintered materials. Moreover, microstructural gradients were formed creating bi-layered structured materials with higher hardness edges and tougher cores. The highest microstructural homogeneity and hardness were found for planetary ball milled starting powder sintered at 1250 °C. For these samples, the highest density recorded was 13.69 g/cm3, which also obtained highest hardness values of 1620 Hv and 1589 Hv, on the surface and center regions, respectively.
查看更多>>摘要：? 2022The Pt-modified aluminide coating is applied on the superalloy in advanced turbine aero-engines due to its excellent oxidation resistance. However, the interdiffusion between the coatings and substrates would occur under high service temperatures. It could deteriorate the microstructure and mechanical performance of the substrates. The present study investigates the creep properties of coated samples with different holding times at 1100°C (0–500 h). The bare alloys are exposed and tested at the same temperature with various stresses to build the Larson-Miller curves. Owing to the topological inversion and the precipitation of TCP phases, the creep life of the bare alloys reduces with the increased holding time. The coated samples show an elevated creep rate compared with bare alloys. Only a slight degradation in the creep life of the coated samples is observed within 100 h thermal exposure, but a further deterioration happens with the extended exposure time. The load-bearing capacity of the coats shows a dependence on the exposure time, and it can be attributed to the microstructure and elemental distribution changes in the coating during thermal exposure.
查看更多>>摘要：? 2022 Elsevier Inc.A novel self-induced ultrasonic tungsten inert gas (U-TIG) welding has been successfully applied to weld the Inconel 718 sheet with thickness of 2 mm to reduce hot cracking. The welding arc, weld formation, microstructure and micro-hardness distribution between TIG and U-TIG welding were analyzed systematically. The relationship between micro-hardness and characteristics of grain size, grain orientation, grain boundary, segregation and recrystallization proportion in transition zone (from weld metal zone to heat affected zone) was investigated. The characteristics of sizes, disorientations and boundaries of adjacent grains, as well as the micro-strain distributions in weld metal zones of TIG and U-TIG welds were analyzed contrastively. The results indicate that the welding arc with symmetrical shrinkage induced by ultrasonic pulse current greatly intensifies the central depression of the weld bead surface, which benefits the release of welding thermal stress. With the promoted transformation of coarse columnar dendrites to fine equiaxed dendrites with similar orientation in U-TIG welds, the segregation in grain boundaries is greatly reduced, which reduces potential of the hot cracking initiation and propagation. In addition, the improvement of micro-hardness distribution is contributed by grain refinement and segregation dispersion in U-TIG welds.
查看更多>>摘要：? 2022 Elsevier Inc.Fe-21Cr-4Al-xGd (x = 0, 1, 2, 4 wt%) alloy was prepared by spark plasma sintering (SPS) in this study. The effect of Gd on the microstructure, mechanical properties and damping properties of Fe-21Cr-4Al alloy was investigated. The results show that the compressive strength and hardness of the alloy containing Gd are improved by fine grain strengthening and second phase strengthening, and the hardness and compressive strength of the Gd alloy increase with increasing Gd content. When Gd content is 4 wt%, the average grain size is 7.20 μm, which is much smaller than Fe-21Cr-4Al (9.15 μm). The hardness and compressive strength of the alloy are increased by 16.38% and 9.87%, respectively, compared with those of the alloy without Gd. The microstructure and phase composition show that the alloy is mainly composed of the body-centered cubic (BCC) structure α-Fe phase, Gd and Gd2O3. A large number of Gd and Gd2O3 are scattered in the grain interior and grain boundary. The peak intensity of the α-Fe phase decreases with increasing Gd content. The Gd containing alloys maintain high damping properties in a wide range of strain amplitudes, and the maximum damping capacity of the Fe-21Cr-4Al-2Gd alloy is Qmax?1=0.072. In addition, after Gd is added to the Fe-21Cr-4Al alloy, ferromagnetic damping and dislocation damping exists in the appropriate strain amplitude region to jointly increase the internal friction, which may enhance the damping capacity of the alloy.
查看更多>>摘要：? 2022The nanoscale precipitation and the mechanical properties of the two CuNiAl-containing steels with different Cu contents aged at 500 °C were investigated in this study. Results of mechanical property indicated that the co-precipitation of Cu-rich and NiAl phases during aging contributed to the precipitation strengthening effects. The influence of Cu content on the evolution of precipitates was studied by atom probe tomography (APT). The results demonstrated that the increasing of Cu content enhanced the number density of precipitates and accelerated the precipitation process. The two steels showed two kinds of precipitation sequences on account of different Cu contents. The steel 1 with low Cu content exhibited a precipitation sequence of “supersaturated solid solution → NiAl → NiAl + Cu”, while the steel 2 with high Cu content presented the process of “supersaturated solid solution → Cu → Cu + NiAl”. After further aging, the precipitates of both steels were transformed into Cu/NiAl co-precipitates without isolated Cu-rich phases and isolated NiAl phases.
查看更多>>摘要：? 2022Functionally graded and composite materials are known as new materials, which exhibit higher performances and provide a comprehensive application. This paper introduces a bi-layer Ni-rich/NiTiCu alloy with a composite structure comprising nano-crystal, glassy and amorphous microstructure. The successive crystallization of amorphous Ni-rich/NiTiCu bi-layer thin films deposited at room temperature was studied by annealing at 773 K for 15, 30, and 60 min. Different experiments, including scanning electron microscopy (SEM), grazing incidence X-ray diffraction (GIXRD), high-resolution transmission electron microscopy (HRTEM), and thermodynamics calculations (CALPHAD) were conducted to analyze the microstructure. The crystal nucleation, glassy/amorphous thermal stability against alloying elements, and related interfaces were studied. The as-deposited thin films were fully amorphous and had a rounded nano-glassy structure. HRTEM images indicated no long-range periodic lattice and the interface between layers was flat and sharp. Short-time annealing depicts non-spherical crystal nuclei in the amorphous NiTiCu layer with a diffuse interface. In contrast, after 60 min annealing, the NiTiCu and NiTi layers were fully and mostly crystallized with different structures, respectively. The results confirm the higher thermal stability of the amorphous NiTi binary alloy. Thermodynamic and diffusion studies indicate that the higher driving force of amorphous to B2 ordered structure against Cu addition along with the higher Cu diffusivity compared with Ni and Ti are responsible for lower thermal stability of NiTiCu.
查看更多>>摘要：? 2022Unlike the recrystallization-induced shear band mechanism which has been well accepted for the RE-free Mg alloys during rolling, an understanding of deformed microstructure evolution (twin mode/variant selection and dislocations) at different strains is crucial for interpreting the shear band mechanism in the RE-containing Mg alloys upon hot rolling. In this study, a Mg-1.1Zn-0.76Y-0.56Zr alloy ingot was subjected to hot rolling at 440 °C with single-pass reductions of 10–50%, and the microstructure evolution and texture development of this hot-rolled plates at different reductions were investigated, with aim to disclose the roles of different types of twins/variants and dislocation mechanisms in the shear band (SB) formation and their correlation with the macro-texture characteristics. As the dominate twinning mode for most parent grains at lower reductions, and the primary {10?12} extension twins/variants together with basal <a > slip activated in twins contribute to formation of basal texture in the twinned grains, thus subsequently facilitating the operation of secondary {10?11} and {10?13} compression twins and following {10–12} twins. Afterwards cross-grain boundary (GB) paired twins for compression twin and their double twins were frequently observed at the nearby basal-oriented parent grains with low misorientation (≤34.5°), which should correspond to the predecessor of SBs. Further EBSD observation at higher reductions (R0.4 and R0.5) reveals that most of the banding structures are distributed homogenously and are likely transformed from the {10–11}-{10–12} twins, which are the predominant twin types under such state. The ‘RD-split’ textures start to be formed at lower reductions (R0.3) and keeps stable at higher reductions (R0.4-R0.5), which were explained by the correlations with some variant pairs of {10–12} twins and/or {10–11}-{10–12} twins predicted by Schimid factor analysis as well as shear bands. Nevertheless, the contributions of <c + a > slip to the ‘RD-split’ texture could be ignored on basis of corresponding EBSD analysis and reappearance of such texture after rolling at a low temperature (170 °C).
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