The world has gradually entered the industrial 4.0 Era, which is dominated by the Internet of Things (IOT) and intelligent manufacturing. Especially, strong requirement for artificial intelligence and big data processing, the development and preparation of micro/nano electronic devices is becoming increasingly active, and much more concerns have been attracted to small-scale materials. Because of the constraint effect of geometric and microstructural dimensions of these materials, the thermal fatigue damage behavior is different from that of the bulk counterparts. At the same time, the change of the material scale from microns to nanometers also results in the transformation of the deformation mechanism, so that the materials exhibit different damage behaviors and significant size effects. In this paper, thermal fatigue testing methods, thermal fatigue damage and evolution, and the factors influencing thermal fatigue properties of metal film/line are reviewed, the corresponding mechanism of thermal fatigue and the size effect of the micro/nano-scale metals are discussed. The prospective research of this field in the future is addressed.

Steels containing bainite microstructure are widely applied in various industrial areas. Incomplete bainite transformation is frequently used to control volume fraction of retained austenite as well as its stability and is also closely related to bainite growth mechanism. It is generally accepted that incomplete bainite transformation could occur when carbide precipitation is absent. On the other hand, some new studies revealed that carbide with very fine size were observed in “carbide-free” bainite in Si added steels. Our previous study on bainite isothermal transformation kinetics together with its microstructural evolution with Fe-1.5(3.0)%Si-0.4%C alloys (mass fraction) at 400~500 ℃ found incomplete bainite transformation phenomenon for the 3.0Si alloy at 450 ℃ and for two alloys at 400 ℃. In contrast with the generally accepted view, cementite precipitation with Si depletion was observed at the beginning of incomplete transformation stage. Further analysis on three dimension atom probe results revealed that the carbide volume fraction as well as amount of C atoms in carbide hardly changes during incomplete transformation stage. Thermodynamic analysis revealed that small chemical driving force for cementite precipitation and/or the necessity of Si partition are two factors accounting for the extremely slow cementite precipitation kinetics. It is thus proposed that incomplete bainite transformation and carbide precipitation could co-exist. Conditions for incomplete bainite transformation are modified as follows. Firstly, bainitic ferrite growth is stopped before reaching equilibrium fraction. In addition, carbide precipitation should be absent or its kinetics should be slow enough.

As the core material of transaction motor for electrical/hybrid vehicles, the non-oriented silicon steel (NOSS) sheets require not only the good magnetic properties, i.e. high permeability and low iron loss, but also high yield strength to resist the centrifugal force during the high speed rotation. In this work, Nb element was added into the conventional NOSS to improve the strength without sacrificing the good magnetic properties too much. The effects of annealing process on the microstructures, magnetic and mechanical properties of Nb-containing high-strength non-oriented cold-rolled silicon steel were studied. The increases of annealing temperature and time both lead to the reduced segreation of Nb at grain boundaries and the solution and ripening of precipitates, which means the decreased suppression on the migration of grain boundaries; thus, the recrystallized grains start to grow; particularly, the density of {111}<112> texture component may increase to deteriorate the magnetic flux density, B₅₀. The best mechanical and magnetic properties cannot be achieved at the same time. The annealing process at 940 ℃ for 270 s could lead to the best combination of mechanical and magnetic properties, which include B₅₀ of 1.69 T, the iron loss P_1.5/50 of 4.86 W/kg and P_1.0/400 of 30.47 W/kg, resulting from both the segregation of solute Nb at grain boundaries and the extensive precipitation which refrains the grain growth and development of harmful γ texture. Therefore, the yield strength is increased due to both grain refinement and precipitation strengthening without greatly sacrificing the permeability and iron loss.

Ni-Fe-Cr alloys have been widely used for petrochemical, chemical and nuclear application due to their superior corrosion resistance and good workability. Nowadays, Ni-Fe-Cr alloys with higher strength are demanded for the engineering application. Increasing the carbon content could enhance the strength of Ni-Fe-Cr alloys due to the solid-solution strengthening effect of interstitial carbon atoms. However, an increase in the carbon content would promote the precipitation of carbides, which would reduce the corrosion resistance. In order to optimize the carbon content and determine the solution treatment, microstructure evolution during solution treatment and its effects on the properties of Ni-Fe-Cr alloys with different carbon content were investigated using OM and SEM. The results show that variation in carbon content affects the carbide dissolution and grain size during solution treatment, which affects the mechanical properties and intergranular corrosion susceptibility of Ni-Fe-Cr alloys. For the Ni-Fe-Cr alloy with carbon content of 0.010%, M₂₃C₆ carbides produced during the hot-working process do not exist after solution treatment at 950 ℃. For the alloy with carbon content of 0.026%, M₂₃C₆ carbides are dissolved into the matrix when the solution temperature increases to 1000 ℃. An increase in the carbon content from 0.010% to 0.026% results in an increased tensile strength and has slightly observable effect on the elongation. The alloys with the carbon content in the range of 0.010%~0.026% have lower intergranular corrosion susceptibility. As the carbon content increases to 0.056%, M₂₃C₆ carbides could not be dissolved even at the solution temperature of 1050 ℃, and inhomogenous grain-size distribution is observed. The presence of undissolved M₂₃C₆ carbide weakens the solid-solution strengthening effect of carbon atoms, and significantly increases the susceptibility to intergranular corrosion.

In the aerospace industry, due to the increasing hardness and tensile strength of nickel-based superalloys, the traditional manufacturing methods are difficult to produce, which limits the freedom of part design and process. Selective laser melting (SLM) has great potential in this field with its additive manufacturing concept and full melting during the process. Although the dense part can be easily obtained in SLM, the residual stresses and micro-cracks in the machining process still affect the dimensional accuracy and reliability of the parts. In SLM process, rapid and complex changes of temperature and stress are observed in the vicinity of the molten pool. Understanding these changes will help to improve the quality of the process. In this work, a finite element model (FEM) is established to calculate the temperature and residual stress distribution near the weld pool during the SLM of Hastelloy X superalloy, The model uses a composite Gauss heat source to consider the influence of optical penetration depth, and implements the transformation of powder, molten pool and solid metal by changing the material properties with temperature. Comparison with the test results shows that the model can simulate the distribution of temperature field and the residual stress in SLM process well. The simulation results show that with the increase of laser power, the width, length and depth of melting pool were enlarged, the cooling rate decreases; with the increase of the scanning speed, the width and depth of melting pool decreases, the length remained unchanged, the cooling rate increase. After cooling, there is a large tensile stress on the surface of the model. As the depth increases, the tensile stress decreases rapidly and eventually becomes compressive stress.

The nickel-based Inconel Alloy 690 (Ni-30Cr-10Fe, mass fraction, %) was developed as a replacement material for Inconel Alloy 600 in the steam generator tube of pressurized water reactors nuclear power plants. Intergranular corrosion and intergranular stress corrosion cracking were the main failure reasons for steam generator tubes, which were related to the precipitation of grain boundary carbides. Hence, the precipitation of carbide at the grain boundaries and triple junctions with different characters is worthy to be studied. The morphology of carbide precipitated on grain boundaries at triple junctions with various characters in grain boundary engineering (GBE) treated Alloy 690 aged at 715 ℃ for 15 h were investigated by SEM and EBSD. The results show that, there are obvious differences in the morphology of carbides precipitated on the Σ3_c grain boundary near different types of triple junction. The size of carbide precipitated at Σ3_c grain boundary increased by the order of Σ3-Σ3-Σ9、Σ3-Σ9-Σ27、Σ3-Σ27-R、Σ3-R-R triple junctions. But the morphology of carbides precipitated at the Σ3_i and Σ9 grain boundaries was independent of the nearby triple junction characters. The precipitation morphology of carbides precipitated on the Σ27 grain boundary near the triple junction is different from that precipitated on the internal grain boundary, for example, the carbides precipitated near triple junction was more discrete and bigger than that precipitated on internal grain boundary. When the triple junction contain two random grain boundaries and one Σ3 grain boundary or Σ9 grain boundary, the size of carbide precipitated on one of random grain boundary is smaller than that of precipitated on the other one.

Because the compressor thin-blades of aero-engine often fractured in service, laser shock processing was suggested to be applied as a surface strengthening technology. Aim at the problem of compressive residual stress relaxation in laser-peened compressor thin-blades, TC11 titanium alloy thin-components were treated by laser shock processing and then conducted in axial tensile-tensile fatigue test and thermal insulation in vacuum. X-ray diffraction tests were carried out to obtain the relaxation rules of residual stress under fatigue loading and thermal stress loading. In addition, the relaxation mechanisms of residual stress were indicated. Experiment results demonstrate that surface compressive residual stress relaxes by 53%, and 95% of stress relaxation occurs in the previous 5 fatigue cycles under the fatigue loading (maximum stress σ_max=500 MPa, stress ratio R=0.1). The surface relaxation degree and severely-relaxed depth increase with fatigue loading, and the relaxation mechanism is that plastic deformation of local area material results in residual stress redistribution. Surface compressive residual stress relaxes by 3%, 29% and 48% respectively after thermal insulation for 120 min under the constant temperature of 200 ℃, 300 ℃ and 400 ℃. Surface compressive residual stress relaxes by 18% and 58% respectively after thermal insulation for 120 min under the altering temperature of 200 ℃+400 ℃ and 300 ℃+400 ℃. The relaxation all occurs in the previous 60 min. There is a similar trend with temperature in the aspect of severely-relaxed depth. The relaxation mechanism under thermal stress loading is that dislocations and grain-boundaries are activated to move and annihilated, and then plastic deformation recovery occurs. Due to the distinction of relaxation mechanisms, there is an obvious superimposed effect under the combined action of fatigue loading and thermal stress loading.

Seeding technique is a promising method for growing single crystal superalloy blade. However, sometimes stray grains nucleate in the transformation process of single crystal structure from a seed, which always cause failure of single crystal growth. In order to obtain single crystal with high perfection structure, Ni-based single crystal superalloy was prepared with low-segregated seeds by high rate solidification (HRS) method in the dual heating zone furnace. The melt-back zones of seeds were investigated systematically, and the results showed that a fusion zone without microsegregation exists in front of the melt-back equilibrium interface of seeds, in which solidification interface transited from planar to cellular. Further experiments showed that increasing the W content of seeds or the solidification rate can both accelerate the whole non-steady transition process and make fusion zone shrink. Compared with the traditional seeding method, the low segregated heterogeneous seeding technique can increase the casting yield by avoiding the nucleation of stray grains in the fusion zone, which caused by the pinched-off secondary dendrites and constitutional undercooling.

Nanotwinned (NT) metals are promising structural materials due to their excellent combination of strength and ductility. These superior properties are strongly dependent on the microstructures i.e. the twin length (grain size), the twin thickness and the twin orientation. Understanding the synthesis process and growth mechanism of NT metals is essential for their structure design. In this work, the effect of electrolyte temperature on the microstructures of highly oriented NT Cu samples, including twin thickness and twin length (grain size), are systematically studied. The NT Cu samples were prepared by means of the direct-current electrodeposition at 293, 298, 303, 308 and 313 K, respectively, while other deposition parameters such as current density, concentration of additive and pH value were kept constant. With decreasing the temperature from 313 K to 293 K, the average grain size decreases from 27.6 μm to 2.8 μm and the average twin thickness decreases from 111 nm to 28 nm, which results in an increment of hardness from 0.7 GPa to 1.5 GPa. This is because with decreasing the temperature, the overpotential of cathode for depositing metal elevates, leading to the nucleation rate of both the grain and twin enhanced.

A356 aluminum alloy has been widely used in semisolid processing because of its wide range of liquidus and solidus temperatures. The flow of the melt during solidification has a certain influence on the composition, solute distribution, phase morphology and crystal defects of the alloy in the solidification structure. The flow of melt is an important factor that influences the overall performance of the solidification process. The researchers use various external fields to act on the melt to induce the melt flow. Electromagnetic stirring has the characteristics of no contact, no pollution, light oxidation, less gas content and easy to control stirring parameters. It is the most popular method to fabricate semisolid alloy slurry. The electromagnetic force of the electromagnetic field can be used to study the flow phenomena of the melt. Numerical simulation combined with experimental research can get better results. The effects of crucible size and electromagnetic frequency on flow during fabrication of semisolid A356 aluminum alloy slurry under electromagnetic stirring through numerical simulation as well as the influence of crucible size on the primary phase of semisolid A356 aluminum alloy slurry induced by electromagnetic field were investigated. The results show that with the increasing of the major and minor axial ratio of crucible (R), the maximum electromagnetic force and maximum flow rate of the semisolid A356 aluminum alloy at the minor axis firstly increase and then decrease, and the maximum electromagnetic force and maximum flow rate of the semisolid A356 aluminum alloy at the major axis increase first, then decrease and then increase. The higher the electromagnetic frequency, the electromagnetic force difference and the flow rate difference of the semisolid A356 aluminum alloy at the minor axis and the major axis are apparent, so that occurs the phenomenon of "acceleration-deceleration-acceleration" in the melt flow. When the electromagnetic frequency and R are 30 Hz and 1.1 respectively, the maximum flow rate at the major axis and the minor axis of the crucible are 153.6 and 143.2 mm/s respectively, and the flow rate difference is the smallest, better semisolid A356 aluminum alloy slurry can be fabricated at this condition.

Due to its combination of outstanding characteristics, such as superior biocompatibility, excellent mechanical properties as well as good corrosion resistance, Ti-6Al-4V alloy has gained much attention as one of the most popular load-bearing biomedical metals in the area of orthopedic and dental. Unfortunately, Ti-6Al-4V alloy suffers from the localized corrosion damage in human body ?uids containing high chloride ion concentrations, which leads to the release of metal ions into the human body. The released ions (e.g., Al and V) are found to not only cause allergic and toxic reactions but also exhibit potential negative effects on osteoblast behavior. To improve the corrosion resistance of Ti-6Al-4V alloy in simulated body ?uids, a 40 μm thick Ta₂N nanocrystalline coating with an average grain size of 12.8 nm was engineered onto a Ti-6Al-4V substrate using a double cathode glow discharge technique. The hardness and elastic modulus of the Ta₂N coating were determined to be (32.1±1.6) GPa and (294.8±4.2) GPa, respectively, and the adhesion strength of the coating deposited on Ti-6Al-4V substrate was found to be 56 N. There is no evidence of crack formation within the coating under loads ranging from 0.49 N to 9.8 N, implying that the Ta₂N nanocrystalline coating has a high contact damage resistance. Moreover, the corrosion resistance of the Ta₂N nanocrystalline coating is significantly greater than that of Ti-6Al-4V alloy when tested in naturally aerated Ringer's solution at 37 ℃. This is due to that the passive film developed on the coating has superior compactness compared with that formed on the uncoated Ti-6Al-4V alloy. XPS analysis indicated that at a low polarized potential, the passive film consisted of TaO_xN_y, which would be converted to Ta₂O₅ at a higher polarized potential. The analysis of Mott-Schottky curves suggested that the passive film formed on the coating exhibits n-type semiconductor properties and, as such, the density and diffusivity of carrier for the coating was considerably lower than that for the uncoated Ti-6Al-4V alloy.

Rare earth permanent thin films are useful for magnetic microdevices such as micromotors, since its excellent magnetic properties are able to raise the performance of the devices. In order to judge the reliability of permanent magnet materials, it is quite theoretical and practical to study the time dependence behavior of magnetization, that is, magnetic viscosity or magnetic after-effect. In this work, NdFeB, CeFeB and NdFeB/CeFeB films were fabricated on the Si substrates by direct current (DC) magnetron sputtering. A Ta underlayer of 50 nm and a coverlayer of 40 nm were sputtered at room temperature to align the easy axis of the RE₂Fe₁₄B grains perpendicular to the film plane and to prevent oxidation of the magnetic films, respectively. NdFeB and CeFeB magnetic films were deposited at 903 and 883 K, respectively, and submitted to an in-situ rapid thermal annealing at 948 K for 30 min. The microstructure and magnetic properties of the films were characterized by XRD and physical property measurement system (PPMS). The results indicate that the films show excellent perpendicular anisotropy. A coercivity \begin{array}{c}{H}_{c\perp }\end{array} of 1377.4 kA/m is obtained for NdFeB monolayer film at room temperature. The magnetic viscosity coefficient (S) of the films was studied over a range of temperatures (5~300 K). It is found that the values of S for all films are less than 1, and are quite similar at low temperature (5 K). Both weakened thermal agitation and strengthened anisotropy energy barriers are supposed to decrease transition frequency (f) and prolong relaxation time (τ) at low temperature, which lead to S decreasing. The magnetic viscosity of NdFeB/CeFeB thin film is as similar as that of the CeFeB monolayer thin film, and both are much smaller than that of the NdFeB film. It is shown that the dual-hard magnetic layer structure can effectively reduce the viscosity coefficient and improve the time stability of the NdFeB/CeFeB thin film. Furthermore, the temperature dependence of the initial decay rates (dM/dt) from 0 s to 10 s was discussed. The initial magnetic decay of the film demonstrates a similar temperature behavior as the magnetic viscosity coefficient S.

In the process of depositing coatings on the surface of metal substrates via ion plating, substrate temperature increases due to the bombardment of deposited particles and the heat radiation of discharge target, forming "heat affected zone" where substrate temperature gradually reduces from the surface. In this work, quenched 40CrNiMoA was prepared as substrate to discuss about the influence of target power density on the temperature rising range, region scale of "heat affected zone" and microstructure of Ti coating. The results show that the traditional metal heat treatment method can accurately characterize temperature rising range and region scale of "heat affected zone". And, with target power density increases from 20.61 W/cm² to 143.01 W/cm², substrate temperature ranges from 310 ℃ to 525 ℃, the region scale of "heat affected zone" reaches to 2.51 mm. Also, the preferential orientation of Ti coating changes from (002) to (110), the average grain size significantly increases from 9.9 nm to 19.5 nm, the surface roughness declines first and then increases slightly. In addition, the internal stress releases gradually for elimination of lattice defects when substrate temperature is above 300 ℃.

High performance Cu-Al composites have widely applied in aviation, aerospace and other fields, at the same time the continuous casting as one of composite forming technologies has been also developed in recent years. Obviously, it is an effective and cheap way to numerically simulate the solidification process of short process continuous casting for manufacturing Cu-Al composites before fabricating them. To meet the need of simulation, in this work, a numerical method for theoretically describing the Cu-Al composite forming in continuous casting processes was proposed. The vertical continuous casting of copper clad aluminum bar billet and the horizontal continuous casting of copper and aluminum composite plate were performed. Based on this method, the steady state temperature fields in solidification processes in the above two kinds of casting technologies were numerically simulated by using proCAST software package. In this work the effects of the theoretical parameters on the steady state temperature fields and then on the performance of Cu-Al composites fabricated by using the above two casting technologies were carefully discussed. It is found that the experimental and simulated results are in good agreement. For the cases of the copper clad aluminum bar billet with a cross section of 100 mm×100 mm, and the copper or aluminum plate with a thickness of 20 mm and a width of 75 mm (coat thicknesses of 4~7 mm), the feasible parameters for producing high performance Cu-Al composites, for examples, are as follows: for the former the temperature of copper liquid is 1250 ℃, the temperature of aluminum liquid is 750 ℃, the length of crystallizer is 200 mm, the length of graphite mandrel tube is 290 mm, the flux of the first cooling water is 1600~2000 L/h, the flux of the second cooling water is 900~1300 L/h, the distance from the second cooling water to the exit of crystallizer is 30 mm, and the withdrawing speed is 60~80 mm/min. For the latter the temperature of copper melt was 1250 ℃, the temperatures of aluminum melt are 760~800 ℃, the withdrawing speed is 40~80 mm/min, and the length of aluminum duct is 20 mm.