In recent years, non-equiatomic high-entropy alloy (HEA) has been proposed to explore the flexibility of its design rule, avoiding the strength-ductility tradeoff. For further progress, non-equiatomic HEAs doped with interstitial atoms are developed. Boron, an effective dopant in metallurgy, has been used due to the beneficial compositional effects on the interfaces of metallic materials. In this work, the effects of deformation and annealing treatments on the microstructural evolution of Fe47Mn30Co10Cr10B3 dual-phase HEAs were investigated via electron channeling contrast imaging (ECCI) and EBSD. The results show that there are three stages in the deformation mechanism with an increase in the deformation degree, which include the dominant dislocation slip in the fcc phase, joint deformation of the transformation-induced plasticity and dislocation slip, and activation of dislocation slip in the hcp phase. With an increase in the annealing holding time, the partial recrystallization transformed to complete recrystallization. Further, particles located in the grain boundary can effectively restrain grain growth, and in turn, exhibit the bimodal grain size. The amount of annealing twinning variants is influenced by the fcc grain orientation: grains with <101> orientation are prone to forming multiple twinning variants, whereas, grains with <111> and <100> orientations are prone to forming a single twinning variant. The amount of annealing twinning variants also affected the morphological characteristics of the single hcp variant; the absence of annealing twinning variant is ascribed to the formation of blocky hcp phases and the single annealing twinning variant is attributed to the formation of laminate hcp phase. Moreover, the number of hcp variants was affected by the fcc grain sizes; large-sized grains facilitated the formation of multiple hcp variants, whereas, small-sized grains facilitated the formation of the single hcp variant.
To improve the wear performance of steel without increasing its hardness, a high-Ti wear-resistant steel was reinforced with TiC particles. The effects of hot rolling deformation on the microstructure and mechanical properties of the wear-resistant steel containing 0.61%Ti after quenching and tempering were studied in hot rolling experiments with different reduction ratios. The steel products were subjected to microstructure and precipitate characterization and mechanical-property tests. Increasing the rolling deformation improved the strength, toughness, and plasticity of the tested steel. The yield strength, tensile strength, and total elongation were increased from 1202 MPa, 1437 MPa, and 7.4%, respectively, at a reduction ratio of 3∶1 to 1311 MPa, 1484 MPa, and 9.9%, respectively, at a reduction ratio of 30∶1. Meanwhile, increasing the reduction ratio from 3∶1 to 10∶1 remarkably increased the absorbed energy at room temperature (obtained in a Charpy impact test) from 11 J to 24 J. As the rolling deformation increased, the micron-sized net-like TiC particles that precipitated during solidification were gradually refined and homogenized, and the prior austenite grain size was also refined. Next, the strengthening mechanisms of the steel were quantitatively analyzed. The yield strength, calculated by adding the root mean squares of the dislocation and precipitate strengthening values, well agreed with the measured yield strength. The increasing yield strength of the tested steel at higher rolling reduction ratios is mainly attributable to increased grain-boundary strengthening and precipitation strengthening. As the strength of the steel increased, the toughness and plasticity also increased, mainly because the large TiC particles were refined and homogenized during the rolling deformation.
Fatigue failure caused by crack propagation is one of the main failure modes of H13 die steel. Because H13 die steel is mainly used under operational conditions involving high-pressure cycling or high friction, its crack initiation and propagation behavior play a critical role in fatigue failure. However, to the best of the authors' knowledge, few reports on the direct observation of deformation and crack propagation behavior of H13 die steel exist, which limits the understanding of the fracture mechanism of H13 die steel. In this work, an in situ TEM tensile study combined with post-mortem EBSD analysis was employed to investigate the microstructure evolution and crack propagation behavior of rare earth (RE) H13 steel. Results indicate that the stress concentration at the grain boundaries and coarse granular inclusions in the tensile specimen were the main sources of crack initiation. After crack initiation and during the tensile process, many cracks converged into the main crack. The main crack propagated along the direction perpendicular to the tensile direction, exhibiting zigzag-shaped features. The stress distribution in the area near a crack in a specimen was heterogeneous; the length fraction of V1/V2 inter-variant boundaries in the relatively high-stress area increased from 56.5% to 58.8% compared with the relatively low-stress area, and the length fraction of V1/V3&V5 inter-variant boundaries increased from 16.3% to 21.6%. The increase in the length fraction of V1/V2 inter-variant boundaries indicated an increase in the twin martensite fraction, which effectively relieved the stress concentration at the grain boundaries and reduced crack initiation. During the tensile process, the austenite retained at the grain boundaries underwent stress-induced phase transformation and was partially transformed into V1/V2 and V1/V3&V5 variant pairs. The dislocation propagation in martensite contributed to dislocation pile-up at high-angle grain boundaries and carbide precipitation. Moreover, the dislocation pile-up at the high-angle grain boundaries promoted the stress-induced phase transformation of the retained austenite.
With the rapid development of the Chinese axle industry and the increasing demand for lightweight axle housings, the demand for a new type of hot stamping axle housing steel, which can ensure the yield strength meets the requirements after the hot stamping process, is becoming increasingly urgent. However, there are no published reports on the development of this new type of hot stamping axle housing steel. The aim of this study was to develop 500 MPa grade hot stamping axle housing steel. The effect of coiling temperature on the microstructure, precipitates, and mechanical properties of the 500 MPa hot stamping axle housing steel were studied by OM, SEM, and TEM. The results showed that the mechanical properties of the tested steel were significantly different when coiling at 600 and 570 ℃. When coiling at 570 ℃, the yield strength and tensile strength reached 538 MPa and 641 MPa, respectively, which were 165 MPa and 117 MPa higher than those at 600 ℃, whereas, the impact energy in the range of 20~-40 ℃ was lower than that at 600 ℃, especially low-temperature toughness. These are related to the differences in the microstructure and precipitates in test steel when coiling at different temperatures, especially the difference in the proportion of the high angle grain boundary (HAGB). The microstructure of the tested steel was composed of ferrite and pearlite when coiling at 600 ℃, average grain size of ferrite was 4.48 μm, proportion of HAGB was 68.1%, and average size of the precipitates was 8.4 nm, of which nanoscale precipitates with sizes below 10 nm accounted for approximately 70%. The microstructure was mainly composed of acicular ferrite, granular bainite, polygonal ferrite/quasi polygonal ferrite, and pearlite when the coiling temperature was reduced to 570 ℃, the average size of ferrite was 4.39 μm, ratio of HAGB was approximately 54.5%, average size of the precipitates was 6.4 nm, and nanoscale precipitates with a size below 10 nm accounted for 86%. The difference in the microstructure and the precipitates was mainly owing to the fact the bainite transformation temperature of the test steel was as high as 580 ℃, and nucleation rate and nucleation speed of the test steel was higher when coiling at 570 ℃ than that at 600 ℃. Thus, considering the high requirements on the shape quality, effect of the hot stamping process on the microstructure, and precipitates of the tested steel, the coiling temperatureof the test steel is more suitable at 600 ℃.
The bimetal sheet products obtained by the incremental forming are gradually gaining popularity because of their excellent properties. The forming process of the bimetal sheets is more complicated than that of the single metal sheet because it involves a complex interface between the two metal sheets. The bonding strength of the interface is an important parameter for evaluating the bonding properties of a bimetal sheet. However, if the required bonding strength is higher than the strength of the substrate, appropriate strength is difficult to achieve using only experimental methods. An improved analytical method to calculate the interface bonding strength has been proposed based on the widely used interface model of an asymmetric double cantilever beam. This improved analytical method considers the plasticity of the interface. The interface bonding strength of the explosively welded T2/A1050 copper-aluminum bimetal sheet has been assessed using the improved method, and its interface bonding strength has been found to reach 208 MPa. This bonding strength has been used in a finite element model for the forming process of the bimetal sheet. The correctness of the method has been verified by comparing the analytical results with the experimental data. Furthermore, the influence of interface bonding strength on the maximum forming depth has been explored, and it has been found that the maximum forming depth increased by 210% when the interface bonding strength increased by 38%. Moreover, a three-dimensional model including a tool, the Cu-Al bimetal sheet, and a cohesive element between the two metal sheets has been suggested for investigating the bulge defect of the forming part. The bonding strength of the interface obtained above has again been utilized in the finite element analysis. The impacts of different reductions and tool diameters on the bulge defect of the forming part have systematically been discussed. The results show that bulge defect decreased by 57% when the reduction decreased from 2.0 mm to 0.5 mm, and the bulge defect decreased by 38% when the tool diameter increased from 10 mm to 22 mm. Optimized parameters have been suggested to effectively reduce the bulge defect by 53%.
Casting with superheat control is important in improving the quality and stability of steel products and reducing metallurgical defects. In recent years, the channel-type induction heating tundish is among the new technologies that have been used by the steel industry. It exhibits an effective liquid steel temperature control during continuous casting. However, in such technology, the fluid flow and heat transfer are significantly different from those in a conventional tundish. This is because of the implementation of the heating practice and action of the electromagnetic force. In this work, a mathematical model of electromagnetic-thermal-flow coupling is developed to investigate the feature of the electromagnetic force, fluid flow, and heat transfer in a six-strand H-type induction heating tundish. The flow field and temperature field characteristics in the tundish under different application modes of induction heating are compared. Moreover, the applicability of the traditional method of cold water modeling to the structure optimization of tundish with induction heating is discussed. The results indicate the eccentric distribution of the electromagnetic force in the tundish channel, pointing to the eccentric position of the channel. Additionally, the results suggest that the molten steel in the channel flows out with rotation. For case A0, an increase of 22 K in the molten steel temperature is observed after heating for 1500 s at 1000 kW power, compared with that without heating. However, due to the pinch effect of the electromagnetic force, the short-circuit flow at the outlets near the channel intensifies, and the flow consistency in tundish worsens. Compared optimized case A4 with the prototype case A0, the short-circuit flow of the outlets near the channel disappears, the temperature difference among the different flows is reduced, the flow consistency in the whole tundish is improved, and the heating rate is increased. The present study also demonstrates that the tundish structure optimization method under a cold state is still an important evidence for the induction heating state.
The grain boundary (GB) and average grain size considerably affect the properties of materials, such as the fracture strength, dielectric constant, and thermal conductivity. For instance, when subjected to irradiation at 1750 ℃, the swelling of the UO2 pellets and the release of fission gas from them decrease significantly with the increasing average grain size. However, several second-phase particles, such as pores, are inevitably introduced into a material during the solid-phase sintering or neutron radiation processes. Therefore, studying the interaction between the pores and GBs is considerably important. In this study, a phase-field model of the interaction between the pores and GBs is developed. Subsequently, the free-energy density function was modified, where the diffusion coefficient was incorporated in the tensor form. In addition, the selection of the phenomenological parameters, such as the coefficient in the free-energy density function of the phase-field model, was analyzed, and the influencing factors of interface energy and interface width were discussed. The phase-field model simulation results of the interaction between the pores and GBs show that the curvature of GB was the major driving force associated with the movement of GB and that pores resisted the movement of GB. Accordingly, the pores moved together with the GBs when the maximum pinning force exerted by the pores was larger than the driving force produced by the curvature of GB; however, the pores and GBs separated in the opposite case, during which the GB moved much faster than pores. The results of the phase-field simulation of the grain growth of the pore-containing UO2 show that the grain growth speed decreases with the increasing porosity. The average grain size of UO2 is a power function of time, the exponent of which increases with the increasing porosity.
The designed service temperature of turbine blades is rising with the increasing thrust of aircraft engines. A film cooling system is one of the promising ways to improve the service temperature of turbine blades. However, such a complex film cooling system can reduce the thickness of the blade airfoil and lead to obvious local differences in microstructure; these differences are caused by solidification and mechanical resistance between the blade airfoil and alloys such as standard solid bars. In this study, a thin-walled tube manufactured by the casting process was used to simulate the microstructure of the hollow blade airfoil. The tube was thermally exposed at a temperature range of 900~1050 ℃ for 300~1000 h, and the corresponding stress rupture properties under 975 ℃ and 225 MPa (pressure) were examined. The microstructures were investigated using OM, SEM, TEM, and XRD, and the chemical compositions of the precipitates formed were measured through physicochemical phase analysis before and after the thermal exposure. Through this analysis, the relationship between microstructural degradation and stress rupture properties was revealed. The results indicated that dissolution and coarsening of γ' precipitates, degeneration of MC carbides, and broadening of the γ' film along the grain boundaries occurred in the K465 alloy tube during thermal exposure between 900 ℃ and 1050 ℃. With increasing exposure temperatures and prolonged thermal exposure time, the degree of degradation of the γ' precipitates, carbides, and grain boundaries gradually increased. This resulted in a gradual reduction in stress rupture lives. Unlike the phenomenon observed in our previous study in which a large amount of μ phase precipitated in the solid bar following thermal exposure at 900 ℃; in the present study, the μ phase did not form in the tube. However, the degrees of microstructural degradation in the tube and bar were similar after the thermal exposure at 1000 and 1050 ℃. The stress rupture lives of the tube were significantly higher than those of the bar after the thermal exposure at 900 ℃, whereas their stress rupture lives were similar after the thermal exposure at 1000 and 1050 ℃. The thin-wall effect caused by the cooling rate on the microstructure and the corresponding stress rupture property of K465 alloy was obvious at 900 ℃, whereas it was negligible at 1000 and 1050 ℃. These results provided guidance for the manufacturing and evaluation of microstructural degradation of turbine blades made of conventionally cast polycrystalline superalloys.
TC4-DT alloy is developed based on TC4 alloy, with medium strength and high damage-tolerance, showing a great promise to be widely used in aerospace field. However, the traditional method to fabricate large and complicated parts has the problems of hot working difficulty, long processing cycle, low "buy-to-fly" ratio and high-cost. Additive manufacturing (AM) technology is a good alternative and has been used to manufacture titanium parts since year 2006. Compared to other AM technologies, cold metal transfer mode wire and arc additive manufacturing (CMT WAAM), as a kind of gas metal arc welding (GMAW) technology, has several advantages like simple structure, good spatial accessibility and high efficiency. In this study, a TC4-DT deposit was fabricated by CMT WAAM method. The macrostructure and microstructure, texture, and tensile properties in the overlapping zone and ordinary deposition zone were investigated and compared. The bottom of the ordinary deposition zone consisted of columnar and equiaxed prior β grains, and in higher zone the coarse equiaxed prior β grains were in the majority. The microstructure of the ordinary deposition zone was mainly characterized by basketweave α phase platelets. Microstructure of both sides of the overlapping line was characterized by a mixture of fine basketweave, lamellar and coarse basketweave α phase due to temperature gradient. Transformed α texture from {001}β//Z silk texture existed in different zones, and the texture of overlapping zone was complicated due to the complexity of heat dissipation conditions, including transformed α texture and other complicated texture. EBSD results showed that there was a strong <0001>α //X texture at the overlapping line, resulting in low Schmid factors for both prismatic slip and basal slip at the overlapping line, hindering the slip of dislocation. Combined with the Hall-Petch relationship, it was concluded that the prior β grain boundary and the overlapping line were main factors affecting the uniformity of mechanical properties. The average effective dislocation slip distances in different zones had the following relationship: overlapping zone<bottom of the ordinary deposition zone<top of the ordinary deposition zone, leading to different yield strengths in different zones of the ordinary deposition zone.
Currently, with the gradual depletion of onshore resources, more efforts are being devoted to both scientific and resource exploitation of the ocean and the deep sea. Compared with the onshore environment, marine habitats are complex and characterized by high hydrostatic pressure, high salinity, and high marine population. The ocean is a unique aquatic environment, and it has a large population of microorganisms. There is a need to exploit the ocean for new energy sources. The significant challenges of exploiting oil, gas, and minerals have forced the people to innovate and develop advanced exploration tools. Al alloys are attractive for use in marine environments due to their low densities, high strengths, good plasticity, excellent electrical and thermal conductivities, and excellent corrosion resistance. The high chloride concentrations and microorganisms in the ocean have a significant effect on the corrosion resistance of many metallic materials. In this work, the corrosion behavior of 5754 Al alloy in seawater containing B.subtilis was investigated. The corrosion rate was analyzed by the weight loss method. The morphologies of the corrosion products and the corrosion profiles were observed by SEM and white light interferometer, respectively. The corrosion products were analyzed by energy dispersive spectroscopy and XRD. Finally, the corrosion mechanism of the Al alloy was studied using electrochemical impedance spectroscopy. The results show that the corrosion rate of the Al alloy in the seawater with B.subtilis was 12.5 mg/(dm2·d), which was only 1/6 times that in the seawater without the bacteria. A protective film comprising of CaMg(CO3)2 was gradually formed on the surface of the alloy in the presence of the bacteria. The bacteria promoted the formation of the CaMg(CO3)2 film, which protected the alloy from the seawater, and consequently, inhibited the pitting corrosion of the Al alloy in the marine environment.
In recent years, there has been a rapid development in microelectromechanical systems owing to their lightweight and integration with small volume. The microactuater bears low-frequency vibrations during operations, which affects the safety and stability of the device. Ti-Ta-Zr-Fe thin film is a high-temperature shape memory alloy film, which has good shape memory effect and thermal stability. The addition of Fe can improve the plasticity of the film and make it promising to use in high-temperature miniature damping devices. Ti70-xTa15Zr15Fex (x=0.3, 0.6, and 1.0, atomic fraction, %) shape memory thin films were prepared by direct-current magnetron sputtering. The effect of the Fe content on the microstructures, martensitic transformation, mechanical properties, and damping behavior was studied by XRD, TEM, bending test, and dynamic mechanical analysis. It was found that the room-temperature phase composition of the alloy films with different Fe contents were in the parent and martensite phases. The reverse martensitic transformation temperature of the thin films was above 100 ℃, and the films exhibited the two-way shape memory effect. The addition of Fe enhanced the ductility and strength of the film. When the Fe content was 1.0%, the elongation could reach up to 12.8%. During the heating and cooling processes, the relaxation type friction peak was observed. The damping capacity during the relaxation processof the film containing 1.0%Fe could reach up to 0.116.
