Many load-bearing industrial settings require light-weight structural materials with adequate strength. Although commercial aluminum (Al) alloys are suitable for room-temperature applications, their strength at elevated temperatures (300-500oC) is largely reduced by coarsening of the strengthening precipitates. However, high-temperature alternatives such as titanium alloys are much heavier and more expensive than Al alloys. Creating microstructures that remain stable over 300oC is an important goal of the aluminum-manufacturing community. This article focuses on the recent development of high-temperature resistant Al-based alloys. Especially, it discusses the unique microstructural features, selection criteria of the strengthening phase, alloying effects, and microstructural stabilization of aluminum. The strategies summarized in this review are expected to realize the new microstructural architectures of light-weight alloys, which are currently limited to low-temperature service.
High-energy particle irradiation can often cause microstructure damage, resulting in different types of defects in metal-based structured materials. These irradiation-induced defects can accumulate and evolve, leading to the deformation and reduction of the structural integrity of the materials. Finally, this causes the degradation of the mechanical and physical properties of the aforementioned materials. These defects can be shielded, absorbed, and annihilated by introducing interfaces in materials, alleviating the radiation damage. In the previous two decades, metal-based nanostructured materials have attracted considerable attention in designing irradiation-resistant materials because of its high density of internal interfaces. This review aims to investigate the effect of the interface microstructure and energy on strengthening the irradiation resistance of metal-based nanostructured materials, with special emphasis on the interface responses of low- and high-energy interfaces. Furthermore, this review provides the theoretical and scientific foundation for optimizing the interface structure design and exhibits delicate balance between the interface microstructure, interface energy, interface stability, and irradiation resistance. In addition, the recent research progress on irradiation-resistant carbon-/metal-based nanostructured materials that consider such interface characteristics is reviewed in detail. Finally, the prospect of future irradiation-resistant metal-based nanostructured material development is discussed.
The susceptibility of sintered NdFeB to corrosion in harsh environments limits their wide variety of applications. Improved resistance to corrosion of NdFeB magnets and advancement of surface coatings are critical directions in permanent magnet material fields. Significant work has been undertaken on the development of long-life NdFeB magnets. However, comprehensive research of NdFeB corrosion, from process technology to fundamental theory, is lacking, mainly owing to the hysteresis in the study of the elementary works of corrosion and protection related to magnetism, and improving the quality and diversified market demands. This analysis covers recent research that has led to advances in anti-corrosion material technology for NdFeB magnets, including corrosion factors, contemporary methodologies from technology to fundamentals, the basic framework for surface protection and key techniques in the industry application. Finally, prospects and existing challenges in the field of corrosion and protection are reviewed, attempting to identify future developments and directions.
High temperature protective coatings are involved in a wide variety of applications including aero engines and gas turbines. Reactive elements, including all rare-earth elements as well as Ti, Zr, and Hf, are increasingly used to modify high temperature protective coatings, and their main effects are reducing scale growth and improving scale adhesion. The mechanism through which reactive elements work is yet to be clearly understood. The current mechanism comprises “enhanced scale plasticity”, “graded seal mechanism”, “modification to growth process”, “chemical bonding”, “the vacancy sink model”, “oxide pegging”, “dynamic segregation theory”, and “the sulfur effect theory”. Among these, oxide pegging is perhaps the most important one, although some people may disagree. Oxide pegging is the result of the mechanical joining of an oxide to its corresponding alloy; it is a result of either the internal oxidation of added reactive elements or dispersed oxide particles growing in size and extending into the alloy. This paper offers an overview of the research progress on oxide pegging, including its proposed, the relationship between the peg and scale adherence, improving the “key-on” effect, and the peg formation and growth under different conditions (the doped reactive element with a low or high solid solute in the alloy, dispersed oxide added in the alloy, and two reactive elements doped into the alloy). Moreover, it sheds light on the model’s inability to explain the surface application of reactive elements on the alloy. Finally, the paper suggests future studies on this model, like focusing on how to obtain the ideal oxide pegging, developing a new model for oxide peg formation and growth with two or more reactive elements added to the alloy, and the cooperation effect between the oxide pegging and other mechanisms. The paper’s objective is to offer a better understanding of oxide pegging and to provide theoretical support for the studies on the reactive element effect and the design of materials in thermal barrier coating systems.
Columnar grain and hot crack restrict the performance of laser melting deposited René 88DT superalloy when applied to turbine disks, requiring equiaxed grain to improve the fatigue property. Selective laser melting can fabricate material with low composition segregation, reduced defects, and equiaxed or near-equiaxed grains due to the fast cooling rate and vastly varied local heat flow direction during processing. In this study, a selective laser melted René 88DT was fabricated under variable processing parameters. The formation mechanism and control methods of grain structure and metallurgical defects were investigated. Results showed that changing the processing parameters can affect the preferred growth direction of crystal; thus, affecting the grain morphology and size. Processed with low heat input and 67° scan vectors rotation between deposited layers, cellular dendrites with different orientations grow competitively with each other, leading to the formation of near-equiaxed grains. The cellular dendrites can grow epitaxially across multiple deposited layers due to high heat input and 0° scan vector rotation, forming columnar grains. The columnar grains with a relatively low aspect ratio can be fabricated with 90° scan vectors rotation. However, anisotropy exists between the two orthogonal scanning directions due to the shielding effect of metal vapor dust. The primary defects in specimens are solidification cracks along grain boundaries caused by remelting of low-melting-point eutectic. Specimens with equiaxed grains showed much better ability in preventing solidification cracks from propagation than that with columnar grains. The defect density in the columnar grains is about 21 times higher than the equiaxed grains. Suitable processing conditions for selective laser melted René 88DT superalloys are low heat input and 67° scan vectors rotation based on the forming quality and service conditions.
The mechanism of microstructure evolution and its effect on the mechanical properties of nickel-based single-crystal superalloys during creep at high temperatures and low stresses are critical to the development of advanced single-crystal superalloys for aeroengines with high thrust: weight ratios. In this work, the microstructural evolution of a fourth-generation nickel-based single-crystal superalloy during creep at 1100oC for 200 h at various stress levels was investigated using a specially designed sample with variable cross-sections, with the aim of obtaining different applied stresses synchronously on a single sample. The effects of applied stress on γ/γ' microstructure, interfacial dislocation configuration, alloy element partitioning behavior, and lattice misfit of γ/γ' phases of the used single-crystal superalloy were also studied, as were the effects on room temperature Vickers hardness. The results indicated that the typical rafting microstructure was formed during creep over the 200 h period at 1100oC under various stress levels. With increasing applied stress, the volume fraction and rafted thickness of the γ' phase gradually decreased, while the rafting degree of the γ' phase and the channel width of the γ phase gradually increased. A dense interfacial dislocation network was formed at the γ/γ' interface, and interfacial dislocation spacing decreased with increasing applied stress. Simultaneously, increased partitioning of solution-strengthening elements Re, Mo, and Cr to the γ phase and increased partitioning of γ'-strengthening element Ta to the γ' phase resulted in a larger absolute value of γ/γ' lattice misfit at higher stress. In addition to the decreases in volume fraction and rafted thickness of the γ' phase and the increase in channel width of the γ phase, another important factor in the strength decline of the single-crystal superalloy was the pile-up of dislocations at bent γ/γ' interface boundaries, mainly caused by the dissolution of the γ' phase and promotion of dislocation shear into the γ' phase. This work provides a basis for quickly establishing the relationship between creep conditions and microstructure evolution of nickel-based single-crystal superalloys.
The high temperature strength of Ni-based cast superalloys can be significantly improved by adding tungsten (W), a solid solution strengthening element. Hence, superalloys with high W content have been developed as key materials for the preparation of aircraft engine blades. However, the high segregation coefficient of W results in inconsistent composition and microstructure during the solidification process, which can be difficult to eliminate via heat treatment leading to deteriorated mechanical properties. The Ni-based superalloy K416B contains approximately 16.5%W (mass fraction) and exhibits a high tendency to precipitate W-rich phases, such as the α-W and M6C phases, which not only consume a large amount of W in the matrix but also reduce the solid solution strengthening ability of the alloy. W-rich precipitates also become the origin and propagation paths of cracks during stress-rupture testing. Much research on high W-containing, Ni-based superalloys has focused on the effects of W content on W-rich phase formation and mechanical properties. However, the roles of casting temperature and cooling rate on the formation of the W-rich phase are still unclear. In this work, five groups of K416B alloy test bars with the same composition were prepared with different processes. Three casting temperatures were chosen, and the cooling rate was controlled by burying sand in the thick shell and single shell, respectively. The relationship between the precipitation behavior of the W-rich phase in the K416B alloy and casting temperature, and solidification rate under different casting processes were analyzed using SEM and EDS. When the casting temperature is lowered from 1500°C to 1450°C, the grain size is significantly reduced. Results show massive α-W phases in the residual eutectic of the alloy at different casting temperatures, and the morphology of the α-W phase show few differences. The larger M6C phase in the alloy exists with residual eutectic, and the small M6C phase is embedded at the edge of the residual eutectic. At a high solidification rate, the precipitation of the W-rich phase is inhibited, which is primarily manifested by the decreased number and size of the W-rich phase in the alloy. When casting high W-containing Ni-based superalloys, choosing an appropriate casting temperature and adopting an appropriate heat preservation system to accelerate the cooling rate during solidification will affect the precipitation and transformation of W-rich phases, and optimize the properties of the alloy.
It is difficult to form the precipitated phase of duplex stainless steel (DSS) with low nickel content, and a low Cr content of 18.7% (mass fraction) during hot working. However, the stability of the austenite phase changes by substituting Mn for Ni, which can increase the difference between the softening mechanisms of the two phases under high temperature and deformation conditions. Under a temperature and strain rate of 1123-1423 K and 0.1-10 s-1, respectively, the large thermal compression deformation behavior (70%) of 18.7Cr-1.0Ni-5.8Mn-0.2N DSS was investigated. The thermal deformation microstructures were analyzed by OM, SEM, and EBSD. The results show that the dynamic recrystallization (DRX) of the ferrite phase mainly occurred at a lower deformation temperature of 1123 K, and that the degree of grain refinement increased, and the degree of grain inhomogeneity decreased with an increase in strain rate. The strain rate was observed to have a large impact on the ferrite phase DRX, while the austenite phase DRX was more sensitive to deformation temperature. The ferrite phase underwent continuous dynamic recrystallization (CDRX) with the transition from low-angle to high-angle grain boundaries under deformation at 1223 K and 10 s-1, while the austenite phase was dominated by discontinuous dynamic recrystallization (DDRX) deformed at 1323 K and 0.1 s-1. DDRX can be easily induced by increasing the temperature at a low strain rate, while CDRX can be induced at a higher strain rate. The crystal orientation of the austenite phase is mainly characterized by the recrystallization texture of the (001) and (111) planes at higher temperatures and lower strain rates. In the ferrite phase, there is a competitive relationship between the recrystallization texture of the (001) and (111) planes. The critical stress (strain) was obtained by data fitting and its relationship with the peak stress (strain) was determined. As the strain increased, the flow instability domain of hot compression decreased, and the stability zone gradually moved toward higher temperature and strain rate. Furthermore, the optimum hot working conditions, 1323-1423 K and 0.01-6.05 s-1, were obtained.
In the fields of aviation, aerospace, and military, Ta-based coatings are of wide prospects in applications owing to their remarkable characteristics. Compared with other preparation methods, plasma spraying has obvious technical advantages in preparing Ta-based coatings. In this work, Ta2O5 in situ composite nanocrystalline Ta-based coatings via plasma spraying were fabricated. Effects of the spraying power, main gas (Ar) flow rate, and spraying mode on the fine surface microstructure and friction, as well as on the wear properties of Ta-based coatings, were investigated using several techniques. Such techniques included SEM, micro-beam XRD, fretting friction and wear test, and computational analysis. Moreover, related rules and causes were discussed. Results indicated that the Ta2O5 content at the surface of Ta-based coatings initially decreased and then increased with an increase in the spraying power. Conversely, the crystal size and lattice distortion of α-Ta at the coating surface initially increased and then decreased. With an increase in the Ar flow rate, the Ta2O5 content decreased in general and reached the lowest value when the flow rate was 2.17×10-3-2.33×10-3 m3/s. A negative correlation between the variation of the crystal size and lattice distortion with Ar flow rate has been observed. The Ta2O5 content, crystal size, and lattice distortion slightly decreased due to intermittent spraying. Moreover, a significant correlation between the Ta2O5 content and coating microhardness was observed. Under dry friction, the wear rate is closely related to both the oxide content and crystal size. Low oxide content and large crystal size results in high wear rate. Under boundary lubrication, coatings with higher levels of hardness exhibit better anti-wear and friction-reducing performance in the same series of Ta-based coatings. The coating exhibiting the best tribological properties can be achieved via intermittent spraying. Mechanical properties of Ta-based coating are significantly correlated with their fine microstructure characteristics. Aside from microhardness, the grain size, lattice distortion, and oxide content of the coatings can be utilized as important indexes to control the coating quality.
Metal additive manufacturing three-dimensional printing technology is widely used in the manufacturing industry. This technology is an important scientific and technological achievement in the field of world-class manufacturing, and it has influenced and changed the ways of production, which has brought great changes in the manufacturing field. The plasma spheroidization process can provide a better rate of spheroidization for refractory metals. Smoothed particle hydrodynamics (SPH) is a mesh-less Lagrangian method to simulate a flow field and enable heat transfer. This method has shown great potential for replacing the traditional numerical methods and has been successfully used to simulate the collision and fusion process of metal droplets during plasma spheroidization. In this study, the SPH technique has been employed to simulate the collision and fusion processes of multiple metal droplets of a W-Ni-Fe ternary alloy in plasma spheroidization with the consideration of the effect of surface tension. The spheroidization process has been visualized with fluid evolution in terms of the flow field and temperature distribution. A high spheroidization quality is found in a system with the a high amount of tungsten, small tungsten particle size, high processing environment temperature, and large Marangoni force. To check the simulation results, spheroidization experiments have been conducted using the following raw materials: tungsten powder with an average particle size of 1.5 μm, nickel powder with an average particle size of 4.1 μm, and iron powder with an average particle size of 2.4 μm. The raw materials were mixed with a mass ratio of W∶Ni∶Fe=90∶7∶3 and then spheroidized at 8000oC. The achieved ternary alloy powder has high sphericity and dense internal structure. The flowability of the powders is 11.62 s/50 g. The apparent density is 10.66 g/cm3. The resulting products show high precision and good efficiency. The experiments confirm the consistency of SPH simulation. The simulation results can be used to guide development in the fabrication process of the spheroidized powder of refractory metals, such as tungsten.
