Protein could adsorb on the surfaces when biomedical metals contact with body fluids and then affect the corrosion behavior of metals. In vitro results demonstrate that protein adsorption retards metal dissolution, while the detachment of metal-protein complex from the surface accelerates the corrosion or its deposition could impede the metal corrosion. Protein adsorption and its influences on the metal corrosion are related to many factors, such as the type and content of proteins as well as the pro-perty of metals. Therefore, consensus has not been made on the influences of protein on metal corrosion. However, as one of most important components in the body fluids, it should be taken into consideration for the effects of protein on the corrosion behavior of metals in vitro. So that we can find the discrepancy between in vivo and in vitro tests and find the suitable simulated environment in vitro. This will help predict reasonably the corrosion behavior of biomedical metals in the human body.
Recently, Ni-based weldments have been widely used in various industries, including aerospace, nuclear power, thermal power, and petrochemicals. In this paper, the classification and welding methods of Ni-based alloys are introduced. Fusion welding methods were mainly used for the welding of Ni-based alloys because of cost and technical limitations. The mechanism of welding cracks in Ni-based alloys and the effects of various elements on cracks are mainly reviewed. Solidification cracking, liquation cracking, ductility-dip cracking, and strain-age cracking frequently occurred in fusion welding processes. The appearance of a low-melting liquid film has been found to be the main reason for the relative clarity of the mechanisms of solidification cracking and liquation cracking. Ductility-dip cracking is still not clearly defined, and its mechanism in Ni-based alloys remains obscure. Strain-age cracking of Ni-based alloys is unique to precipitation-strengthened-Ni-based alloys and closely related to the precipitation rate. Though much research has been done, impurities and addition of elements have a major effect on welding cracks of Ni-based alloys. Therefore, the influence of most elements alone and the synergistic effects on cracks need further study.
Elastocaloric refrigeration is characterized by a high energy efficiency and drastic temperature change, and it requires no refrigerant. It is the best candidate for the non-gas-liquid compression refrigeration technology, which has the advantage of alternate absorption and release of latent heat during solid-solid phase transformation to realize refrigeration. Compared with the magnetocaloric and electrocaloric refrigeration, elastocaloric refrigeration exhibits advantages such as low cost, high cooling rate, and high efficiency. Elastocaloric refrigeration mainly employs shape memory alloys, which have been a research focus in the past decades. This study describes the mechanism and test methods of the elastocaloric effect and summarizes the research progress as well as challenges in the Ti-Ni-based, Cu-based, Fe-based, and Heusler-type shape-memory alloys as elastocaloric materials. Furthermore, a brief perspective on research directions of the elastocaloric effect based on shape memory alloys is presented herein.
Exploitation of traditional reactor structural materials tends to limits; thus, the development of novel materials is urgent. Alloying has long been used to obtain materials with desirable properties. In recent decades, a new alloying technique that combines multiple principal elements in high concentrations to fabricate new materials, termed high-entropy alloys (HEAs), has gained popularity. Refractory HEAs (RHEAs) consist of several principle refractory elements and are an important subset of HEAs. RHEAs have attracted immense attention owing to their unique mechanical, physical, and chemical properties, particularly their excellent high-temperature mechanical properties and radiation resistance. RHEAs are expected to be utilized in cladding materials for fourth-generation fission reactors and plasma-facing materials for fusion reactors. Combined with representative literature, this paper focuses on mechanical, radiation resistance, and oxidation resistance properties of RHEAs. Further, strengthening and radiation resistance mechanisms of RHEAs are explored, and the development evolution and prospects of RHEAs are proposed.
Owing to the critical role precipitation hardening plays in the improved mechanical performance of metals, understanding the formation mechanisms of precipitates is significant for the rational control of the corresponding and correlated effects. From the perspective of the synergetic variation of thermodynamics and kinetics, the current work briefly reviews the mesoscale methods for precipitation modeling based on the computational thermodynamics of CALPHAD (including the DICTRA simulation, Kampmann-Wagner numerical model, Svoboda-Fischer-Fratzl-Kozeschnik model, and diffusion field cell model) and the multiscale methods based on first-principles calculations (including the phase field model and multiscale structural modeling using the Fokker-Planck equation). On this basis, the research and development of precipitation modeling for heat-treated metals is discussed in detail.
In the recent years, lightweight and high-strength structural materials have gained much attention in engineering applications. Carbon nanotube (CNT)-reinforced Al (CNT/Al) composites are promising structural materials owing to the good mechanical properties and high reinforcing efficiency of CNTs. Previous studies on these composites mainly focused on fabricating CNT-reinforced low-strength or medium-high-strength Al alloys (such as pure Al, or 2xxx series or 6xxx series Al alloys) composites via various dispersion methods. However, only few studies investigated composites with super-high-strength Al alloys as the matrices. In the present work, CNT/7055Al composites with CNT volume fractions of 0%, 1%, and 3% were prepared by high-energy ball milling combined with powder metallurgy. The CNT distribution, grain structure, interface, and mechanical properties of the CNT/7055Al composite were investigated using OM, SEM, TEM, and tensile tests. The strengthening mechanism and anisotropy of the composite were analyzed. The results indicated that the composite had a bimodal grain structure consisting of CNT-free coarse grain zones and CNT-enriched ultrafine grain zones. CNTs were well dispersed in the ultrafine grain zones of the Al matrix, and the CNT/Al interface was clean. There were only few reaction products at the interface. The tensile strength of the 3%CNT/7055Al composite reached 816 MPa, but the elongation was only 0.5%. Grain refinement and Orowan strengthening were the main strengthening mechanisms of the CNT/7055Al composite. Because of the load transfer efficiency of CNTs and a coarse grain band structure, the composite exhibited stronger anisotropy than the matrix alloy. The tensile properties of the CNT/7055Al composite normal to the extrusion direction were weaker than those in the extrusion direction.
S31042 steel is a typical 25Cr-20Ni-type austenitic heat-resistant steel with excellent oxidation and corrosion resistance, and its creep rupture strength can be improved by the addition of Nb and N. This austenitic steel is widely used in superheater and reheater in ultrasupercritical power plants. At high temperatures, its performance is associated with the formation and evolution of Z, MX, and M23C6 phases. Till date, few studies have addressed the precipitation behavior of the Z phase in austenitic steel and the reinforcing mechanism of different M23C6 phases remains unclear. To clarify this, the ageing treatment of S31042 steel was performed at 1050oC, and the evolution behavior, thermal stability, and strengthening mechanism of the precipitates during creep tests were investigated. Furthermore, the relation between precipitate evolution and high-temperature performance was elucidated via OM, SEM, TEM, and creep tests. The supersaturation degree of the alloying components in solution-treated S31042 steel decreased after ageing at 1050oC and the driving force for M23C6 phase formation became smaller, resulting in a discontinuous distribution of the rod-like M23C6 phase along the austenite grain boundaries during the creep tests. At high stress levels, this discontinuous distribution of the rod-like M23C6 phase along the austenite grain boundaries increased the resistance to grain boundary sliding without changing the ductility, thus improving the rupture ductility of the steel. At low stress levels, the strengthening effects of the M23C6 phase discontinuously distributed along the austenite grain boundaries in aged steel were not as strong as those in solution-treated steel.
Metastable bcc β-Zr alloys generally have low elastic modulus, magnetic susceptibility, good mechanical properties, corrosion resistance, and biocompatibility, which are ascribed to co-alloying of multiple elements to enhance the structural stability of the bcc-β phase. This work systematically investigated the effects of Nb and Ti elements on the structural stability of the bcc-β phase and mechanical properties of Zr-Nb-Ti alloys. Various binary [Zr-Zr14](Zr, Nb)3 alloy compositions were designed by the cluster formula approach, based on which Ti was substituted for the base Zr to form ternary alloys. Alloy rods were prepared by the copper-mold suction-cast method with vacuum protection. The microstructure and mechanical properties of the alloys were characterized using XRD, OM, and TEM etc. The results show that the crystal structures of the Zr-Nb binary alloys could change from hcp-α to bcc-β with increasing Nb content, whereas, the ω-phase always coexists with the β-phase. An appropriate amount of Ti addition can significantly inhibit the precipitation of ω, resulting in the further improvement of the stability of the β-phase. The single β-[Ti-Zr14]Nb3 (Zr-17.37Nb-2.98Ti, mass fraction, %) ternary alloy exhibited not only a low elastic modulus of E=57 GPa but also a good tensile property with a high yield strength of σYS=557 MPa and an elongation of δ=15.5%.
Hypoeutectic Al-Si alloys are extensively used in the welding industry owing to their excellent cast-ability, low coefficient of thermal expansion, and good weldability. Unfortunately, Al-Si alloys solidify under conventional cooling conditions, forming coarse dendritic α-Al grains with an eutectic structure and a flake-like morphology that has poor mechanical properties. Chemical inoculations are often used to control the size of α-Al grains and the morphology of the eutectic Si particles. The grain refiner Al-Ti-B master alloy and eutectic Si modifier Sr are commonly used in industry. In recent years, great attention has been paid to controlling the microstructure and mechanical properties of hypoeutectic Al-Si alloys through the use of the cost-effective rare earth element La. Previous studies have mainly focused on the effects of La addition on the microstructural evolution and improvements of the mechanical properties. However, to date there have been no studies on the effects of combined addition of La, Al-Ti-B master alloy and Sr on the microstructure and mechanical properties of Al-Si alloys. In this work, solidification experiments were performed to investigate the effects of the micro-alloying element La, Al-Ti-B master alloy, and Sr on the solidification microstructure and mechanical properties of hypoeutectic Al-Si alloys. These results show that synergistic effects are achieved by combinations of La, Al-Ti-B master alloy, and Sr. An addition of 0.06%La was sufficient for effective α-Al grain refinement, eutectic Si particle modification, and improved the ductility of the alloys. Excess La addition formed a coarse LaAlSi intermetallic compound, which deteriorated the ductility of the alloy. The micro-alloying element La refined the α-Al grains by acting as a surfactant that decreased the wetting angle between the TiB2 nucleation substrate and the α-Al nucleus. It modified the eutectic Si particles by promoting the formation of the multiple Si twins and changing the growth behaviors of the Si particles.
Titanium alloys and titanium-based composites are widely used in the field of aerospace owing to their advantages such as low density and high specific strength. Graphene has been found to significantly improve the mechanical properties of metal matrix composites at a lower content due to high modulus, fracture strength, and specific surface area. To achieve excellent mechanical properties, TA15 titanium alloy was fabricated via spark plasma sintering (SPS), and the effects of sintering temperature, sintering time, and sintering pressure on the densification, microstructure, and mechanical properties of the obtained alloys were investigated. The results indicate that the sintering parameters exert trivial effect on the phase composition of the sintered TA15 titanium alloy. The microstructure of the sintered alloy is mainly determined by the sintering temperature, and the prolonged sintering time will cause microstructure coarsening. Meanwhile, the sintering pressure does not have obvious effect on the sintered microstructure. Furthermore, higher sintering temperature, longer sintering time, and accurate increase in sintering pressure contribute to the densification process of TA15 titanium alloy. At room and high temperatures, the comprehensive mechanical properties exhibited by the sintered TA15 titanium alloy are determined by density and microstructure. The dense TA15 titanium alloy can be fabricated via SPS under the sintering conditions of 900oC, 50 MPa, and 5 min. Such alloy exhibits optimally comprehensive mechanical properties at room and high temperatures. Additionally, 0.5% (mass fraction) graphene reinforced TA15 composites were fabricated by SPS under the sintering conditions of 900oC, 50 MPa, and 7 min. When compared with TA15 titanium alloy, the compression yield strength and ultimate compressive strength of composites have significantly improved at room and high temperatures.
Plasma facing materials (PFMs) in future magnetic fusion devices will face various challenges, such as 14.1 MeV neutron and transmutation gas irradiation at high temperatures. W has been considered as one of the most effective candidates for a PFM in recent years. However, pure W exhibits some drawbacks that limit its applications. Conversely, W-ZrC (W-0.5%ZrC, mass fraction) alloy demonstrates excellent performance, such as a relatively low ductile-brittle transition temperature (DBTT), high ductility, and high strength, which will be particularly useful in future fusion reactors. In this work, the Doppler-broadening slow positron beam analysis (DB-SPBA) and SEM were used to characterize the W-ZrC alloy, which had been irradiated by pure H neutral beam or H+6%He (atomic fraction) neutral beam. In the DB-SPBA, parameters S and W were used to characterize the open volume defects in the samples. Under the pure H neutral beam irradiation, the defects were mainly H-V complexes with a large ratio of vacancy to H in the sample at the surface temperature of 850oC. When the surface temperature of the irradiated sample was 1000oC, there was only one kind of vacancy-type defect without any defect damage layer due to the recovery of defect damage in the sample. The surface morphology was smooth and flat at the irradiated sample surface temperature of 1000oC, and the most of pinhole damage structures disappeared compared to the surface temperature of 850oC. The S value in the sample subjected to the H+6%He neutral beam irradiation at the surface temperature of 800oC was larger than that at 700oC because of the increasing vacancy-type defect volume, and defect types were more complex in the 800oC sample. The defect damage layer in the 800oC sample was wider than that in the 700oC sample. Both the 700oC and 800oC samples presented more than one type of defects, but the sample surface damage was significantly more serious at 800oC.
