Alloying has long been used to improve the properties of metals. Typically, the design concept starts with two metal elements as the foundation and small quantities of other elements are added to change or optimize the alloy properties. Recently, a new alloy has emerged, which combines several main elements to form new alloys, known as high-entropy alloys. Among them, refractory high-entropy alloys (RHEAs) made by mixing five or more refractory metal elements that have similar atomic ratios have wide application prospects in the field of high-temperature materials because of their stable phase structures and excellent high-temperature properties. This paper reviews the mechanical properties and microstructure of typical RHEAs, mechanism of toughening and mechanical property regulation of RHEAs, and prospects for the future development of RHEAs, starting with the current research status of RHEAs. The first section delves into the classification of RHEAs based on their constituent phases, and the microstructure and phase composition of the RHEAs are investigated. The second section summarizes the mechanical properties, strengthening, and toughening mechanisms of RHEAs at room and high temperatures. The third section illustrates and discusses three different strengthening and toughening schemes that have been used to modulate the mechanical properties of RHEAs, namely, chemical composition, process, and phase structure modulations. Finally, the future development of RHEAs have been forecasted and the following recommendations are made for key RHEA research trends in the future: simulating and calculating the materials properties and formation phases using computers and other technologies, development of a research platform and database for RHEAs, accelerating the screening of new RHEAs using combinatorial experimental methods, and acquiring top-down and bottom-up experimental methods to explore RHEAs systems with excellent properties.
Since the “emission peak-carbon neutrality” goal was proposed, coal-fired boilers, which are major power supply equipment and CO2 emission sites, have been gradually developed to zero-carbon emission biomass and low-carbon coal/biomass cofired boiler. The corrosion and erosion behavior of the sulfur and chlorine components of coal and biomass poses a serious threat to the safety and long-term operation of boilers, and protective coatings have become a convenient and efficient way to improve the corrosion and erosion resistance of boilers. This paper reviews recent research progress on high-temperature corrosion and erosion of bare materials and protective coatings used in coal-fired, biomass, and coal/biomass cofired boilers. The mechanism of sulfur corrosion and alkali chlorine corrosion in coal-burning and biomass combustion environments is summarized. The ash deposition-impaction mechanism in coal/biomass cofired environments is described. The current application status of boiler bare materials is introduced, and the design principles, preparation processes, and application status of alloy, ceramic, and metal-ceramic coatings in corrosive and erosive environments are summarized. Based on the current findings, future research on corrosion and erosion of boilers should focus on imperfect hot corrosion mechanisms, accurate corrosion-wear prediction models and types of protective coatings. Finally, material genome engineering and machine learning are proposed to accelerate material research/development and study the corrosion-erosion mechanisms as well as multifactor coupling models. There is a need to integrate powder synthesis methods, coating structure designs, and in-service performance into the development of new protective coatings.
The spontaneous growth of metal whiskers, most notably tin whiskers, has a long history, and the electronics industry has suffered greatly as a result. Pb additive had previously deactivated research on the whiskering phenomenon, but the toxicity of lead alarmed people at the beginning of the century. Nevertheless, intricate conditions involved in the whisker growth research and the multifarious phenomena have hampered the research on this topic; therefore, a comprehensive understanding of the metal whisker growth has been absent. A related whiskering phenomenon on MAX phase substrates has been investigated in recent years, and the occurrence of MAX phase, compared with the whisker growth on metallic substrates, exhibits good repeatability, a short incubation period, a fast growth rate, and rich composition varieties. Therefore, the MAX phase as a new platform of whisker growth investigation is expected to speed up the understanding of the mechanisms related to this phenomenon. Regarding the general background of spontaneous metal whisker growth and our group's current findings, this review summarizes the current status of spontaneous metal whisker growth on MAX phase substrates, discusses and describes the growth mechanism from the two main processes of crystal growth (nucleation and growth), and concludes with a perspective on future research and potential applications of spontaneous whisker growth on the MAX phase substrates.
Improving the steam temperature and the pressure of the boiler applied in the thermal power could enhance the coal-fired efficiency and reduce the emission of harmful gases. Due to the dual impact of dwindling fossil resources and an exacerbated global greenhouse effect, it is critical to develop new heat-resistant boiler materials for ultra super-critical (USC) units at temperatures of 650oC and higher. With great thermal conductivity, good fatigue resistance, and low cost, martensitic heat-resistant steel G115, based on P92 steel applied in 600oC USC units, is a promising steel to be applied to this among all candidate materials. This paper introduces the main chemical composition and the microstructure feature of G115 steel, and the research progress in the areas of microstructure stability, creep performance, fatigue resistance, steam oxidation resistance, and industrial pipe production are summarized, with a focus on the role of Cu-rich phase in G115 steel. Finally, some key points on G115 steel are proposed to provide ideas for future research.
Inconel 718 alloy is a popular material used for additive manufacturing. Corrosion involvement is crucial for its application. The corrosion behaviors of the additive manufacturing Inconel 718 alloy in neutral NaCl solution and acidic solution have been thoroughly investigated and documented. However, information available in the literature regarding the corrosion behaviors of additive manufacturing Inconel 718 alloy in alkaline solution is insufficient. In this paper, the corrosion behavior of Inconel 718 alloy fabricated through selective laser melting (SLM Inconel 718) in NaOH solution was studied using open circuit potential (OCP), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), potentiostats polarization, Mott-Schottky analysis, and X-ray photoelectron spectroscopy (XPS). The results were compared with a commercial rolled Inconel 718 alloy (R Inconel 718). Pitting corrosion was observed in both SLM Inconel 718 alloy and R Inconel 718 alloy. SLM Inconel 718 alloy has lower activity and corrosion rate compared with R Inconel 718 alloy. Because the passive film formed on the surface of SLM Inconel 718 alloy has a lower content of porous NiO and a higher content of compact Cr2O3, the passive film is more compact; however, the donor/acceptor density is lower in the passive film.
Recently, the fatigue failure of aluminum alloy components of high-speed railway catenary is becoming increasingly serious, which causes a threat to the normal operation of high-speed trains. In this study, the effect of micro-arc oxidation (MAO) coating on the high-cycle fatigue properties of 6082-T6 aluminum alloy for the catenary cantilever of the high-speed railway was studied. First, the rotating bending fatigue tests of untreated (UP) and MAO specimens of 6082-T6 aluminum alloy were performed. Then, the surface morphology and roughness, cross-section morphology, nanoindentation hardness, and elastic modulus gradient distribution, phase composition of MAO coating, and fatigue fracture morphology of the fatigue samples were studied by a confocal laser microscope, nanoindentation, XRD, and SEM. The experimental results showed that after MAO treatment, a large number of micro-cracks and pores were formed on the surface of the samples, and the morphology of the samples severely deteriorated. The XRD results indicated that the closer the coating surface, the stronger was the diffraction peaks of α-Al2O3 and γ-Al2O3. Besides, there was the higher tensile residual stress at the coating-substrate interface, which led to a significant reduction in fatigue properties. The fatigue strength decreased by 26.7% at 2 × 107 cyc. Finally, the formation and relaxation mechanisms of tensile residual stress under cyclic stress were analyzed using mismatch strain theory, and the effects of the coating on the fatigue properties of the substrate under high and low cyclic stresses were discussed further.
To reduce the weight of a car body, Al-Mg-Si-Cu alloys have been extensively studied for outer body panels of automobiles owing to their high strength-to-weight ratio, recyclability, and good formability. Moreover, the strength of Al-Mg-Si-Cu series alloys can be enhanced using the bake-hardening treatment. However, compared with steel, the formability and final strengths of the alloys need further improvement, which is a major challenge to the large-scale application of Al alloys in the automotive fields. In this study, the non-isothermal precipitation behavior of Al-0.8Mg-1.2Si-0.5Cu-0.3Mn-0.5Fe(-3.0Zn) (mass fraction, %) alloy was systematically investigated using DSC, TEM, tensile test, and hardness measurements. The results show that adding Zn can simultaneously increase the formation and redissolution of solute clusters in the alloys during low- and high-temperatures non-isothermal heat treatments and promote precipitation. The kinetic equations of precipitation in the two alloys were established based on the activation energy of precipitation obtained through DSC analyses and other material parameters, which can effectively predict the corresponding precipitation rates. Further, adding 3.0%Zn to the alloy can effectively increase the nucleation rate of the precipitates in the alloy during non-isothermal heat treatment, resulting in higher hardness. Additionally, with the increase of ageing temperature, the hardness increased gradually, but a hardness plateau appeared at approximately 100oC, and the peak hardness values appeared at approximately 250oC, followed by a decrease in hardness. TEM microstructural characterization showed that non-isothermal heat treatment could result in the formation of multiscale β'' precipitates in the two alloys in the peak ageing state. In comparison, adding Zn to the alloy increased the number density of the precipitates and significantly changed the lattice parameters of β'' phases formed in the peak aged alloy. Finally, based on the obtained microstructure and mechanical properties, the relationship between the precipitate distribution and microhardness of the two alloys was established.
The coprecipitation of carbides and copper (Cu) particles is an effective technique for strengthening microalloyed steel. In this study, OM and TEM techniques were used to investigate the coprecipitation behavior of carbides and ε-Cu in Ti-Mo-Cu microalloyed steel at different isothermal temperatures. A solid solution precipitation model and the classical nucleation theory of precipitates were used to calculate the precipitation kinetics in the Ti-Mo-Cu microalloyed steel. The results show that (Ti, Mo)C and ε-Cu precipitated independently, and they showed the N-W and K-S orientations with the ferrite matrix, respectively. The dominant precipitates at 600oC are ε-Cu. (Ti, Mo)C and ε-Cu were coprecipitated at 620oC. At 640-660oC, (Ti, Mo)C was mainly precipitated in the form of interphase precipitation. Thermodynamic calculations showed that in the range of 600-660oC with an increase in temperature, the Ti/Mo atomic ratio in (Ti, Mo)C increases from 2.5 to 4.5, and the carbide changes from Ti0.71Mo0.29C to Ti0.79Mo0.21C. The precipitation-temperature-time (PTT) curves of (Ti, Mo)C and ε-Cu intersect at 616oC, indicating simultaneous precipitation of (Ti, Mo)C and ε-Cu. (Ti, Mo)C and ε-Cu preferentially precipitate below and above 616oC, respectively. The calculation and experimental results are consistent.
A rare-earth Ce microalloyed non-quenching and tempering steel was designed. The morphology, quantity, and size of inclusions in non-quenched and tempered steel with different Ce contents were characterized by SEM, EDS, and ASPEX, as well as the metallography of the test steels. The formation process of Ce sulfide inclusion was analyzed by Thermo-Calc thermodynamic software, and the element distribution at grain boundary and phase interface was characterized by 3DAP. The results showed that Ce combined with S to form Ce3S4 inclusion which then transformed into Ce2S3 inclusion, and finally formed the composite inclusion with Ce2S3 as the core and Ti4C2S2 and MnS as the cladding growth. The aspect ratio of more than 90% inclusions in the steel test with Ce element is less than 2.5; the microstructure of the steel was the smallest with an average grain size of 4.17 μm when the Ce content was 0.019% (mass fraction). The results of 3DAP prove that Ce segregates at the grain boundary and phase boundary, which hinder the diffusion of C and inhibit the growth of grain. In addition, the finely dispersed Ce inclusions as nucleation particles also refine the microstructure of the non-quenched and tempered steel.
Dislocation slip and twinning are the main deformation mechanisms dominating plastic behavior of crystalline materials, such as twinning-induced plasticity steel, Cu, Mg, and their alloys. The influence of twinning and interaction between dislocations and twins on the plastic deformation of crystal materials is complex. On the one hand, a sudden stress drop in the stress-strain curve during twin nucleation, propagation, and growth (TNPG) of crystal materials, i.e., the twinning softening effect, is evident. On the other hand, the interaction between twins and dislocations demonstrates the strengthening effect of plastic deformation. Polycrystalline materials are used in engineering applications, and twin nucleation corresponds to different strains in each grain. Therefore, determining the influence of twin softening and strengthening effects on plastic deformation of polycrystalline materials is difficult. In this work, a crystal plastic finite element model of Cu, considering the twinning softening effect, was developed to describe the TNPG process based on the crystal plasticity theory. The method was used to reveal the influence of twins' activation and their interaction with dislocations on strain hardening during the tension of Cu single crystal and polycrystal. The results show that twinning has an evident orientation effect. Under twinning favorable orientation, a sudden stress drop in the stress-strain curve caused by twinning propagation during plastic deformation of Cu single crystal is evident, and the total plastic deformation can be divided into three stages: slip, twinning, and interaction between dislocations and twins. Compared with Cu single crystal, the stress-strain curve changes smoothly and the strain hardening rate is higher during the tension of Cu polycrystal. Meanwhile, the dislocation density is concentrated at the grain boundary, and twins are easy to form at the grain boundary during the plastic deformation of Cu polycrystal.
