Compared with traditional metal elements such as iron, copper, and nickel, zirconium has a lower density and a smaller coefficient of thermal expansion. In addition, zirconium also has a low thermal neutron absorption cross-sectional area (only 0.18 × 10-28 m2) and good corrosion resistance, which makes zirconium and its alloys in the nuclear industry and aerospace and other special fields have a very wide range of application prospects. At present, zirconium and its alloys have been relatively maturely used as cladding materials in nuclear reactors. Compared with stainless steel, zirconium and its alloys can effectively reflect neutrons back into the reactor, which greatly saves uranium fuel; and the good corrosion resistance of zirconium alloys in high-temperature and high-pressure water vapor at 300 to 400 °C also makes The reactor has a long service life. Therefore, the metal element zirconium is known as the first metal of the atomic age. With the continuous development of my country's aerospace, navigation and chemical industries, traditional materials such as alloy steel have become increasingly unable to adapt to special environments such as space and ocean. In recent years, many scientists at home and abroad have turned to aluminum-based composite materials and light metal materials such as titanium alloys and zirconium alloys.
This paper briefly summarizes the current development status of zirconium alloys, and focuses on the analysis of the composition design, strengthening mechanism and application of new high-strength and toughness zirconium alloys.
1 Development status of zirconium and its alloys
The content of zirconium in the earth's crust is about 220 g/t, and its reserves surpass that of commonly used metals such as copper, nickel, lead, and cobalt, ranking 20th. my country's zirconium ore reserves ranks ninth in the world, and it is a country with a relatively wide distribution of zirconium content. The early zirconium extraction technology was immature, which greatly limited the application of zirconium materials. In 1944, Kroll successfully studied the production method of large-scale ductile zirconium, which led to the rapid development of zirconium and its alloys. Initially, zirconium alloys were mainly used as cladding materials in the nuclear industry. In recent decades, as the research on zirconium alloys has matured, zirconium and its alloys have also been widely used in the chemical industry, medical industry and some special fields.
1.1 Zirconium alloy for nuclear
Zirconium alloys have been widely used in the nuclear industry due to their extremely low thermal neutron absorption cross-sectional area and good resistance to high temperature and high pressure corrosion. , pressure tubes, component boxes, and some structural materials. France, the United States, Germany, Russia and other countries have successively developed a series of zirconium alloys for nuclear use. At present, Zr-2, Zr-4, Zr2.5Nb and zirconium alloys such as ZIRLO, E635, M5 and NDA newly developed in recent years have been successfully applied in the nuclear industry. These newly developed zirconium alloys have lower radiation creep properties and better resistance to iodine stress corrosion. In addition, they can meet the higher fuel consumption requirements of fuel assemblies, which can increase the service life of the assembly to 30 years.
In the past 30 years, Chinese researchers have developed new high-performance NZ2 and NZ8 zirconium alloys after synthesizing the advantages of ZrSn and ZrNb alloys. The mechanical properties of the alloy are better than those of the Zr-4 alloy, and the corrosion resistance of the components prepared with it in high temperature water and steam is significantly improved, and no nodules corrosion phenomenon occurs after long-term corrosion in 550 ℃ superheated steam.
1.2 Corrosion-resistant zirconium alloy
Zirconium has excellent corrosion resistance and can resist the corrosion attack of most organic acids, inorganic acids, strong alkalis and some molten salts. Therefore, some key components in corrosive environments can use zirconium to improve their service life. Another method to improve the corrosion resistance of alloy parts is surface pretreatment. In the industry, zirconium itself has high oxygen absorption characteristics, and zirconium is placed in high temperature air, so that a dense oxide film is obtained on the surface of zirconium, thereby improving the corrosion resistance and erosion resistance of zirconium and its alloys. Experiments show that the annual corrosion rate of zirconium in sulfuric acid medium after surface oxidation treatment is only 5% of that of pure zirconium, while the erosion resistance is improved by 2 times.
At present, zirconium has been widely used as a corrosion-resistant material in the chemical industry, and it has been maturely used in heat exchangers, wash towers, reactors, pumps, valves and corrosive medium pipelines. For example, concentration tubes and hydrolysis tubes made of zirconium alloys have been successfully used in hydrogen peroxide production lines, and devices such as zirconium pressure reducing valves, agitators and flow meters have also been used in fertilizer production, sewage treatment and dye industries. application. The corrosion-resistant zirconium alloys are mainly Zr702, Zr704, Zr705 and Zr706 alloys. The composition of Zr702 alloy is close to pure zirconium, mainly adding a small amount of elements such as O, H and N. Its corrosion resistance is high, but its mechanical properties are low. It is used as a chemical pipeline in a sulfuric acid medium containing FeCl3. Zr705 alloy is a zirconium-niobium alloy, and its mechanical properties are twice that of Zr702 alloy. Chemical equipment with relatively high strength and elongation requirements, such as fence heat exchangers, usually use Zr705 alloy as raw materials.
Biomedical materials are a new high-tech material emerging in recent years, and biomedical alloys must have good compatibility with the biological fluid environment and good corrosion resistance. Ti6Al4V alloy is an implanted titanium alloy used in human hard tissue earlier, but its elastic modulus close to 110 GPa far exceeds the elastic modulus of human natural bones 15-30 GPa. Zirconium is valued by researchers because of its good biocompatibility, elastic modulus similar to bone, and good corrosion resistance. In the early 1990s, Smith & Nephew Richards developed a ZrTiNb alloy, which not only has an elastic modulus similar to that of human bone, but also has complete biocompatibility. Williams et al. also confirmed that the degradation degree of ZrTiNb alloy under the combined action of corrosion and friction and wear is significantly smaller than that of Ti6Al4V alloy. Subsequently, a series of medical zirconium alloys were developed, such as ZrNb, ZrMo, ZrCu, ZrMoTi and ZrSi alloys.
In recent years, researchers have found that α+β dual-phase and β single-phase zirconium alloys have the best compatibility with human muscle, bone and brain tissue. In addition, β single-phase alloy has better corrosion resistance and wear resistance than α single-phase alloy, and is a promising alloy for surgical implantation, which can be used in various medical devices and other biomedical materials. used in.
1.3 High-strength and tough zirconium alloys
In the fields of space exploration, deep-sea exploration, and high-speed railways, there are often some special use environments, such as -200-200 ℃ alternating temperature environment, continuous space irradiation and relative motion between structural parts and so on. In these special environments, structural components in long-term service often face problems such as fatigue damage, dimensional instability, atomic oxygen erosion, and friction and wear. At present, the structural parts used in these special fields are mainly made of alloy steel materials such as 20Cr and GCr15, which often have the problems of poor radiation resistance, easy damage to movable parts, high density and high cost. Compared with traditional alloy steel and other materials, zirconium and its alloys have several important potentials: 1) small thermal expansion coefficient, stable dimensional structure, and the potential to prepare precision structural components; 2) the potential to resist space radiation damage; 3) It has the potential of proton oxygen erosion. Therefore, zirconium and its alloys are expected to adapt to unconventional environmental conditions in special fields, and have the potential to be used as structural parts in special environments. The tensile strength of pure zirconium is low, only about 300 MPa, and it is impossible to use it directly as a structural part. Strengthening and toughening treatment will become an important link in the use of zirconium as a structural part. At present, researchers have developed several typical zirconium alloys, such as ZrTi, ZrCr, ZrB, ZrBe, ZrAl, ZrTiAl[38] and ZrTiAlV alloys. The tensile strength of these zirconium alloys is significantly higher than that of pure zirconium, and the tensile strength of ZrTiAlV alloys even exceeds 1 600 MPa, which has a very broad application prospect.
2 Research status and application of new high-strength and tough zirconium alloys
2.1 Design and preparation of new high-strength and tough zirconium alloys
Pure zirconium has two main phases, hexagonal close-packed (HCP) α phase (normal temperature and pressure) and body-centered cubic (BBC) β phase (high temperature), in addition to a large number of metastable phases. These allotropes with different structures are the basis for the design of new zirconium alloys, so it is critical to fully understand the structure of the different phases and their differences in properties. It is found that α phase has more obvious anisotropy (mechanical and physical properties), lower self-diffusion coefficient, better creep resistance and higher strength than β phase. In addition, the relationship between the macroscopic properties of the zirconium basic phase and the microscopic electronic structure can also be established based on the electron density topology, thereby providing important theoretical guidance for the design of new high-strength and tough zirconium alloys.
For single-phase disordered solid solution zirconium alloys, the addition of alloying elements can better control the phase content and mechanical properties. The solid solution strengthening effect of Ti, Al, V, Cr, C, Sn, Mo is systematically studied through a large number of experiments and theoretical calculations. The results show that the solid solution strengthening effect of Ti element with similar physical and chemical properties to Zr is the most obvious. The solid solution strengthening effects of elements are Al, V, C, Cr in sequence.
Therefore, the new high-strength and tough zirconium alloy should be based on Zr-Ti, and other alloying elements should be added appropriately, and then the alloy should be strengthened by solution treatment and the formation of metastable β phase should be controlled. For the dual-phase zirconium alloy, in addition to the change of phase structure, it also has abundant microstructure, so the properties can be optimized by microstructure design. On the one hand, the dual tough phase composed of α phase and β phase in the dual phase zirconium alloy can ensure the plastic deformation ability of the alloy; play a significant strengthening role. In order to further clarify the strengthening effect of the α/β phase interface and the strength design method, on the basis of the strengthening and toughening design of the single-phase disordered solid solution, the microstructure, grain size, defect and phase content of the dual-phase zirconium alloy are combined with influencing factors. , and established the Hall-Petch method for strength design of dual-tough phase materials. Combining the strength and plastic change law of single-phase solid solution zirconium alloy, the strength design of dual-ductile phase materials and the Hall-Petch method, a variety of new high-strength and tough zirconium alloy materials have been designed and developed (some new zirconium alloys are shown in Table 1). Compared with traditional zirconium alloys such as ZrSn and ZrNb, the strength of the new high-strength and tough zirconium alloy is increased by 1 to 5 times, and it can maintain good plasticity.
Due to the high melting point of zirconium alloy (1 400 ~ 1 800 ℃), the microstructure after high temperature solidification will be very coarse and uneven, which will lead to a sharp deterioration of the mechanical properties of the alloy. Therefore, it is necessary to adjust the microstructure and obtain excellent comprehensive mechanical properties through subsequent deformation recrystallization and various heat treatment processes. Microstructure ultra-refinement can effectively improve the strength of the alloy, and equiaxed structure can make the alloy have good plasticity. Therefore, ultra-refinement and tissue isometrics are at the heart of tissue optimization. The new zirconium alloy also inherits the traditional titanium alloy thermal deformation and heat treatment methods (such as forging, hot rolling, annealing, solution aging, etc.) to optimize the microstructure of the alloy.
Researchers have recently developed a new heat treatment process for composite deformation of zirconium alloys, that is, low-temperature large plastic deformation in metastable β/α″ martensitic phase combined with long-term low-temperature aging composite process. Figure 1 shows the composite process optimized by composite The equiaxed and dual-state zirconium alloy structure obtained by the technology. The new high-strength and tough zirconium alloy through the optimization of the structure can reach the strength of 1 500 ~ 1 700 MPa, and has a plasticity of 5% to 12%.
As shown in Fig. 2, after the ZrTiAlV alloy is treated by the composite optimization technology, the tensile strength can reach 1 600 MPa under the premise of ensuring a certain plasticity. The tensile strength of the ZrTiAlV alloy after aging at 650 ℃ reaches 1 MPa. Above 400 MPa, and the elongation is greater than 12%. The development of new high-strength and tough zirconium alloys breaks the limitations of traditional zirconium alloys in terms of mechanical properties and greatly expands the application range of zirconium alloys.
2.2 Strengthening mechanism of new high-strength and tough zirconium alloys
2.2.1 Solid solution strengthening
Alloying elements such as Ti, Al, V, and Nb have higher solid solubility in Zr matrix. The size difference between the solute atoms and the Zr atoms causes the Zr matrix lattice to distort, resulting in solid solution strengthening.
In addition, the more alloying elements added, the greater the lattice distortion of the alloy as a whole, the stronger the interaction force between atoms, and the more obvious the solid solution strengthening effect of the zirconium alloy. Figure 3 shows the XRD patterns of ZrAl alloys with different Al contents. It can be observed that the diffraction peaks of α phase gradually shift to the high-angle direction. The atomic radius of Al (0.143 nm) is smaller than the atomic radius of Zr (0.162 nm). After the Al atoms are dissolved into the Zr matrix, the value of the lattice parameter a of the α phase decreases gradually, while the value of c/a increases gradually. , which causes the lattice distortion of the Zr matrix to increase with the increase of Al content, and the effect of solid solution strengthening is gradually enhanced.
2.2.2 Second phase strengthening
Alloying elements such as B, Be, Cr, and C have low solid solubility in the Zr matrix, and mainly exist in the form of the second phase, which further strengthens the alloy. Liang Shunxing et al. improved the surface hardness of the alloy by adding C element to the Zr alloy to form a compound and achieved an ideal effect. In addition, when the addition amount of solute atoms (such as Al, V, etc.) is lower than the solid solubility of the β phase and higher than that of the α phase, the Zr alloy can obtain a supersaturated high-temperature phase solid solution through solid solution treatment. During the subsequent low-temperature aging treatment, the precipitated compounds will be strengthened by the second phase. In general, the compounds obtained by the solution + aging method can be uniformly distributed in the alloy matrix, which has a great contribution to the improvement of the strength of Zr alloys.
2.2.3 Grain Refinement Strengthening
The addition of alloying elements (such as B, Be, Cr and Ti, etc.) or through suitable thermal deformation and heat treatment methods can refine the microstructure of Zr alloys, resulting in grain refinement strengthening. Figure 4 shows the microstructure pictures of pure Zr and Zr alloys after adding 1.0% Be element.
