With continuous demands for cost reduction and environmental protection, bainitic forging steels, which have notably higher strength and toughness combination than ferritic-pearlitic forging steels, have been developed and gained an increasingly applications in a variety of critical automotive parts. In order to optimize the microstructure and properties of bainitic forging steel, the influences of tempering temperature ranging from 200 ℃ to 500 ℃ on the microstructure and mechanical properties of a Mn-Cr type bainitic forging steel were investigated based on microstructural observations and mechanical property tests. The results show that the microstructure in the as-forged condition of the tested steel is a mixture of lower lath-bainite and granular bainite. With the increase of tempering temperature (Ttemp), the microstructure began to recover and the large blocky martensite/austenite (M/A) constituents decomposed granularly with the precipitation of fine cementites. Further increasing Ttemp to 500 ℃, the blocky M/A constituents decomposed completely and the cementites were spheroidized. Consequently, the ultimate tensile strength (UTS) decreases gradually from 1418 MPa of the as-forged specimen to 1094 MPa of the specimen tempered at 500 ℃ with increasing Ttemp, while the yield strength (YS) increases gradually with increasing Ttemp at first, reaching a peak at 400 ℃, and then decreases with further increasing Ttemp. As a result, the yield strength ratio (YS/UTS) increases continuously from 0.73 in the as-forged state to 0.93 of the specimen tempered at 500 ℃. Unlike those of the strengths, the impact energy increases at Ttemp of 200 ℃ at first, then it decreases and reaches a valley at 400 ℃, and finally it increases notably again at Ttemp of 500 ℃, an increase of about 27% than that of the as-forged one. It is concluded that suitable tempering treatment after forging can obtain better strength and toughness balance of the tested bainitic forging steel, and thus help to expand its application range.
The heat transfer component is a major component of a nuclear power plant, the safety and service life of which are determined based on the long-term creep performance of the heat-transfer pipe material. Several long-term creep tests are usually required to determine the creep life of the heat-transfer pipe materials, which considerably restrict the evaluation efficiency of the material service performance. The objective of this study is to investigate the feasibility of accelerated creep test (ACT) to reduce the time required for evaluating the creep properties of materials. A alumina-forming austenitic (AFA) stainless steel was prepared, and the ACT was performed on a Gleeble thermal simulator. Based on the ACT developed and realized on the Gleeble thermal simulator, damage accumulation was realized by applying elastic-plastic tensile and compressive strains on the ACT specimen to simulate the accelerated changes in the microstructure of the alloy that can be usually observed during a conventional creep test (CT). The average stress with respect to all the cyclic stress relaxation stages in the ACT was considered to be the initial stress of the conventional CT, and a creep fracture test was conducted on the alloy sample. Results revealed that the ACT accelerated the microstructure evolution of the precipitated phases, dislocations, twins, and so on in a short time. Nevertheless, the repeated generation and annihilation of a large number of dislocations in the AFA alloy during the ACT provided the nucleation point and reduced the driving force associated with the nucleation of the precipitated second phase, including the Laves phase. In addition, the cyclic strain applied during the ACT will reduce the strengthening effect of the nanoscale deformation twins in the AFA alloy, resulting in differences in the evaluation effect. Thus, ACT is useful for the efficient evaluation of the creep properties of materials; however, the optimal range of test parameters must be further investigated.
Dissimilar metal welds (DMWs) between high-Cr martensitic heat-resistant steels and nickel-based alloys with nickel-based filler metals are widely used in fossil-fired power plants. Reports of premature failures of DMW joints have attracted considerable attention as such occurrences in the field can lead to significant economic loss and safety issues. Moreover, a comprehensive understanding of the high-temperature performance of new types of DMWs is lacking. In this work, creep tests were conducted over a stress range of 140~260 MPa at of 600 and 620 ℃. A shift in the fracture location with variations in stress was observed, with three typical failure modes. At high stress levels (240~260 MPa), the DMW fractured in the base metal (BM) of martensitic steel, accompanied by a large degree of plastic deformation. At intermediate stress levels (200~240 MPa), the DMW fractured in the fine-grained heat-affected zone (FGHAZ) and the inter-critical heat-affected zone (ICHAZ), with creep cavities around coarsened carbides, indicating a typical type IV crack. At low stress levels (140~200 MPa), the DMW fractured in a mixed mode involving three stages. First, a crack initiated at the interface between the nickel-based weld metal and the martensitic steel, which was attributed to the interaction between the oxidation behavior of the martensitic steels and the thermal stress arising from the mismatch in the coefficients of thermal expansion. Second, the crack deflected into the FGHAZ/ICHAZ and developed into a type IV crack mode. Finally, the crack propagated into the adjacent BM, featuring significant plastic deformation. In addition, in the stress-LMP (Larson-Miller parameter) plot, the creep life at a relatively low stress level was shorter than that predicted by linear extrapolation of the data obtained at a high stress level, indicating a premature failure tendency at low stress. This premature failure tendency can be attributed to microstructure degradation in HAZs and preferential oxidation at the interface at low stress levels.
The abnormal shutdown of the pressurized water reactor (PWR) nuclear power plants can be primarily attributed to the rupturing of the heat transfer tube of the steam generator. Regardless, stress corrosion cracking is the most important ageing mechanism associated with the primary water of the PWR. In this work, the damage behavior of alloy 690 was systematically investigated using high-temperature and high-pressure in situ scratching and electrochemical techniques to understand its corrosion behavior and failure mode and provide a reference for controlling the manufacturing, processing, and installation of the alloy 690 tubing. Further, the polarization behavior of alloy 690 at different temperatures was investigated using the self-built high-temperature and high-pressure water circulation circuit system and the high-temperature and high-pressure in situ scratching device. Subsequently, the single-pass scratch in air and in situ reciprocating scratch of alloy 690 obtained using high-temperature and high-pressure water for 11 and 100 h, respectively, were studied. The samples after scratching were observed and analyzed via SEM and EDS. The results revealed the occurrence of microcracks at the bottom of the scratch during the single-pass scratch of alloy 690. The TiN inclusions with large particles were prone to fragmentation, whereas those with smaller particles were susceptible to cracking at the joint of the matrix. During the reciprocating scratch process in high-temperature and high-pressure water, a portion of the metal substrate debris at the bottom of the scratch groove was peeled off along with oxide particles, microcracks, and chipped debris. Further, the TiN inclusions with large particles were fragmented, whereas those with smaller particles easily cracked at the bonding interface of the substrate. The electrochemical signals of alloy 690 during the reciprocating scratch processes were measured using the high-temperature and high-pressure in situ electrochemical technology. The instantaneous peak current density at the scratch during the scratch process is 149~326 times of that associated with the substrate.
In order to investigate the effect of Er and Si on the thermal conductivity and latent heat of phase transformation of Al-based heat storage alloy, the alloys with Si contents (mass fraction) of 12%, 14% and 16% were prepared. The Er contents of the alloys were 0.2%, 0.4%, 0.6% and 0.8%, respectively. According to the specific heat capacity, thermal diffusivity and density measured by experiments, the thermal conductivity of the alloy was calculated. In addition, the latent heat of phase transformation of alloy was measured and calculated theoretically by using empirical formula. The influence of Er and Si contents on the latent heat of transformation was analyzed by variance. The results show that Er can effectively improve the morphology of primary Si and refine the microstructure in Al-Si alloy. When the content of Si is 16%, the latent heat of the alloy is 414.8 and 406.5 J/g respectively when adding 0.2% and 0.6% Er. When the contribution of the specific heat capacity difference between solid and liquid phases to entropy is considered, the calculated latent heat of phase transformation of the alloy is smaller than that not considered. The theoretical calculation models of the latent heat values of Al-12Si-xEr and Al-16Si-xEr are modified, and the latent heat values calculated by the modified model are more consistent with the measured values.The analysis of variance showed that under the condition of significant level p=0.05, the content of Si has a significant effect on the latent heat of phase transformation of the material.
Thick plates of 7055 aluminum alloy are widely used as structural components, especially in the aerospace industry, due to their high strength, low density, excellent hot workability, and high stress-corrosion resistance, which are dependent on the type of thermal treatment the alloy is subjected to. Because of the heating and cooling stages in such components, non-isothermal ageing has attracted a lot of research interests. Replacing isothermal ageing with non-isothermal ageing is needed for higher efficiency and practicability. Herein, a novel isothermal-ageing technique based on double ageing is developed. Hardness test, electrical conductivity test, room-temperature tensile test, exfoliation corrosion test, DSC, and TEM analyses were employed to study the influence of non-isothermal double ageing on microstructure and properties of the 7055 aluminum alloy. The results showed that in the heating stage of the second ageing treatment, inner grains of the microstructure evolved from a three-phase coexistence state containing the GP zone, η′ phase, and α-Al to that containing η′ phase, η phase, and α-Al. On the other hand, in the continuous cooling stage of the second ageing, GP zone and η′ phase re-precipitated, resulting in improved hardness. The η phase on the grain boundary became coarse and discontinuously distributed, which resulted in a progressive improvement of the electrical conductivity. The heating rate and highest ageing temperature (Tp) of the second ageing stage determined the final properties. With a standard electrical conductivity of 22 MS/m, 1 ℃/min heating rate corresponds to the Tp of 215 ℃, while Tp of 225 ℃ is needed when heating by 3 ℃/min. After pre-aged by 105 ℃, 24 h and non-isothermal ageing including heating and cooling stages, the strength and exfoliation corrosion resistance of approximately 610 MPa and EB level were achieved, respectively. The alloy showed a better comprehensive performance than the T6 and T73 state ones. Additionally, the non-isothermal ageing removing the heat preservation stage realized the short process preparation.
Degenerative and inflammatory joint disease including osteoarthritis, rheumatoid arthritis and chondromalacia affect more and more people. The standard treatment nowadays is arthroplasty, such as total hip replacement (THR). The material selection for the combination of bearing surfaces is a critical issue to determine the life quality of patients with THR. High entropy alloy (HEA) is expected to be a candidate material owing to its appreciated mechanical properties. A new HEA, (TiZrNbTa)90Mo10 alloy was developed recently, but its tribological behavior is still unclear. Using ball-on-plate reciprocating sliding approach, dry sliding wear behavior of arc-melted (TiZrNbTa)90Mo10 HEA against Al2O3 was investigated, together with two conventional implant alloys, Ti6Al4V and Co28Cr6Mo, for comparison. The wear mechanism of these alloys was revealed by characterizing the morphology of worn track, wear debris and scar on counterpart ball. It was shown that, under dry sliding condition, the coefficient of friction (f) of the HEA was determined to be 0.8~0.9, which is nearly double of that of either Ti6Al4V or Co28Cr6Mo, and is insensitive to the applied loading. Meanwhile, specific wear rate of this HEA is approximately 2.3 and 90 times of that of Ti6Al4V and Co28Cr6Mo, respectively, which means that wear resistance of the former is inferior to the two latter under the current conditions. As indicated, wear mechanism of the HEA is dependent on the applied loading. The abrasive wear is predominant under lower-level loading, whereas the third-body abrasive wear took place and played a remarkable role as the loading increased. It is noteworthy that such a complex mechanism is considerably different from that of either Ti6Al4V or Co28Cr6Mo. Furthermore, it is of interest to note that the advantage in mechanical properties of the current HEA, such as high strength and high hardness, is not necessarily to offer its excellent wear resistance, at least under the current tribological condition. It is proposed as future work to screen the appreciate materials as counterpart for this HEA and to characterize its wear behavior under a condition containing lubricant medium such as physiological fluid.
TiB2 coating comprises a large number of ionic and covalent bonds, conferring it with excellent properties such as high melting point, high hardness, and good oxidation and corrosion resistances. However, its application to cutting tool surfaces is limited due to high brittleness. When doped with N atoms, TiB2 coating forms a nanocomposite structure with improved toughness. However, the hardness of the resulting coating is significantly impaired by the abundant amorphous BN (a-BN) phase. The addition of metal ions and reactive N2 increases the proportion of hard nitrides and improves the coating hardness. However, the addition of N2 increases the amount of soft a-BN phase, which largely negates the strengthening effect. To further improve the mechanical properties of Ti-B-N coating, a series of Ti-B-N coatings were prepared by pulsed direct current magnetron sputtering in this work. The content of soft-phase a-BN in the coating was reduced by decreasing the flow of reactive gas N2. Meanwhile, the amount of hard TiB2 phase was increased by increasing the sputtering power of the TiB2 target. Consequently, a noncrystalline (nc)-(Ti2N, TiB2)/a-BN nanocomposite coating with significantly improved toughness and strength was formed. The influence of TiB2 target sputtering power on the composition, microstructure, and mechanical and tribological properties of the Ti-B-N coatings were systematically investigated by EDS, TEM, SEM, XRD, and nano-indentation, scratch, and ball-on-disk tribological testings. As the sputtering power of the TiB2 target increased, the microstructure of Ti-B-N coatings gradually evolved from nc-Ti2N/a-BN to hexagonal-close-packed TiB2/a-BN, and the nanohardness also increased gradually. The particle size on the coating surface was significantly increased, and all Ti-B-N coatings were uniform and compact without pinholes and other defects. The coating with highest hardness of about 33.8 GPa was achieved under a sputtering power of 2.4 kW at the TiB2 target. This coating also exhibited the lowest friction coefficient (0.55), lowest wear rate (2.1×10-4 μm3/(N·μm)), and best wear resistance.
Steel materials are highly sourced construction materials owing to their robust mechanical properties, and they are widely used in the construction industry for building bridges, tunnels, skyscrapers, towers, ship-metal parts, and other industrial metal applications. However, as steel has poor surface wear resistance, parts are susceptible to failure due to friction damage. To improve the surface wear resistance of steel materials, Ni-based WC coating was prepared by ultra-high-speed laser cladding. Using low-speed laser cladding as a reference, the surface morphology, microstructure, and wear resistance of ultra-high-speed laser cladding of Ni-based WC coatings were studied using SEM, EDS, and XRD, respectively. Experimental results revealed that the Ni-based WC coating prepared by ultra-high-speed laser cladding exhibited better surface quality compared with that prepared by low-speed laser cladding. Comparatively, ultra-high-speed laser cladding requires a smaller heat input and a faster cooling rate. However, the dilution rate of the coating is significantly reduced. In addition, ultra-high-speed laser cladding significantly reduces thermal damage in the WC coating; it inhibits the precipitation of carbides and formation of porosities and promotes the uniform distribution of the WC in the coating, thereby significantly reducing stress localization in the coating and also inhibits crack nucleation in the coating. Because of the reduction of porosities, cracks, and other surface defects in the coating and uniform distribution of WC particles, the Ni-based WC coating prepared by ultra-high-speed laser cladding possesses better wear resistance than that prepared by low-speed laser cladding, and the wear mechanism is abrasion.
Poly (3,4-ethylenedioxythiophene) (PEDOT) is one of the most promising anticorrosive materials due to its outstanding conductivity, stability, and environmental compatibility. From the standpoint of corrosion protection of aluminum alloys, PEDOT coatings are a good substitute for the traditional toxic chromium-based coatings. Electrochemical deposition, as a convenient and clean synthesis approach, has been widely employed for direct preparation of PEDOT coatings. Herein, cyclic voltammetry and constant-current method were used to electrodeposit PEDOT coatings on 2024 aluminum alloy substrates. The effects of polymerizing 3,4-ethylenedioxythiophene (EDOT) in three electrolyte solutions (lithium perchlorate (LiClO4) and sodium dodecyl sulfate (SDS), sodium hydrogen phthalate (C8H5NaO4) and SDS, and tetrabutylammonium hexafluorophosphate (TBAPF6) acetonitrile) on the growth and morphology of the PEDOT coatings were investigated. The interactions between the coating and the aluminum substrate were studied through galvanic corrosion, electrochemical impedance spectra (EIS), and scanning vibration electrode technology (SVET). The results show that TBAPF6 exhibited passivation and corrosion-inhibition effects on the substrate and significantly reduced the oxidation potential of EDOT. The surface morphology of the coating prepared via constant-current method showed complete and dense agglomerated spherical particles. The PEDOT coating formed a passivation layer on the substrate, and thus protected it from the corrosive medium. The maximum resistance was achieved in DHS solution (3.5 g/L (NH4)2SO2+0.5 g/L NaCl) after 3 d. The scratched PEDOT coating could promote surface charge delocalization and avoid charge concentration, resulting in electrochemical protection of the aluminum alloy.
Photocatalytic technology is gaining increasing attention as one of the key technologies in solving environmental pollution problems owing to its ability to completely decompose organic contaminants. In addition, the development of visible-light-driven photocatalysts has always been an important topic in photocatalytic fields. In this work, Ni(HNCN)2 and BiVO4 were synthesized using a simple chemical precipitation process, which was followed by the preparation of Ni(HNCN)2-BiVO4 composite particles using a simple ultrasonic blending method. The structure of the resulting composite photocatalysts was characterized via XRD, SEM, FT-IR, and UV-Vis spectra, and its photocatalytic activities in the degradation of Rhodamine B were tested. The experimental results revealed that the smaller Ni(HNCN)2 particles produced via ultrasonic treatment were deposited on the surface of BiVO4 to form a heterostructure. The bandgap of single Ni(HNCN)2, BiVO4, and Ni(HNCN)2-BiVO4 are 2.64, 2.41, and 2.37 eV, respectively. Compared to the single Ni(HNCN)2 and BiVO4 particles, there was improved sensitivity to visible light in Ni(HNCN)2-BiVO4 composites due to the narrower bandgap of the composite particles. During the photocatalytic degradation of Rhodamine B, the composite particles synthesized with 1∶2 mole ratio of Ni(HNCN)2 and BiVO4 demonstrated the best visible-light photocatalytic activity. The mechanism of the photocatalysis suggested that the matched band structure promotes the flow of the photogenerated electrons and holes at the interface, thereby improve the photocatalytic efficiency.
Fe-based amorphous alloys are well known for their excellent soft magnetic and mechanical properties such as high saturation magnetization (Bs), very low coercive force (Hc), high magnetic permeability (μ), low core loss, and high strength, and they are suitable for application as transformer-core materials and have potential applications as structural materials. The minor addition of early transition metal (ETM) such as Zr, Nb, Mo, Hf, Ta, or W can effectively improve the glass-forming abilities, thermal stability, soft magnetic and mechanical properties of Fe-based metallic glasses. The beneficial effects of the minor addition on the glass-forming ability can generally be classified into three aspects: (1) it favors the formation of the unique atomic dense configurations with small free volumes, strong liquid behavior, and high viscosity, which are significantly different from those for conventional metallic glasses; (2) it makes the melts energetically closer to the crystalline state than other metallic melts due to their high packing density in conjunction with a tendency to develop short-range order; (3) it makes the melts more viscous, which leads to slow crystallization kinetics. Despite these advantages, the fundamental theory about the mechanism of the minor addition of ETM in glass formation and properties tailoring is yet to be fully established. In this study, a "cluster plus glue atom" local structure model has been proposed to explore the local structure-property correlation of metallic glasses. The accessibility of calorimetric glass transition (Tg), glass-forming ability, thermal glass stability, and the mechanical properties of metallic glasses are explained in terms of the intra- and inter-atomic cluster correlations in the amorphous structures. Based on the local structure model, the Tg and its composition dependence micro-hardness and strength have been attributed to the inter-cluster correlation, and the enhancement of intra-cluster correlation due to minor alloying would contribute to the enhanced thermal glass stability. The experimental results were verified by alloying the Fe-B-based glassy alloy with Si and alloying the Fe-B-Si-based glassy alloy with ETMs (Zr, Hf, Nb, or Ta) and rare-earth metals (Y, Ce, Pr, Nd, Sm, Gd, or Dy). The experimental results correspond well with theoretical analysis. This study provides a novel understanding of the local structure-property correlation and minor alloying beneficial effects on amorphous alloys.