ISSN 0412-1961
CN 21-1139/TG
Started in 1956

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 , Volume 54 Issue 3 Previous Issue    Next Issue
Orginal Article
 Select Progress in Thermal Fatigue of Micro/Nano-ScaleMetal Conductors Guangping ZHANG, Honglei CHEN, Xuemei LUO, Bin ZHANG Acta Metall Sin, 2018, 54 (3): 357-366.  DOI: 10.11900/0412.1961.2017.00371 Abstract   HTML   PDF (3600KB) ( 762 ) 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.
 Select Incomplete Bainite Transformation Accompanied with Cementite Precipitation in Fe-1.5(3.0)%Si-0.4%C Alloys Huidong WU, Goro MIYAMOTO, Zhigang YANG, Chi ZHANG, Hao CHEN, Tadashi FURUHARA Acta Metall Sin, 2018, 54 (3): 367-376.  DOI: 10.11900/0412.1961.2017.00262 Abstract   HTML   PDF (4225KB) ( 666 ) 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.
 Select Influence of Annealing Process on Microstructures, Mechanical and Magnetic Properties of Nb-Containing High-Strength Non-Oriented Silicon Steel Jun HUANG, Haiwen LUO Acta Metall Sin, 2018, 54 (3): 377-384.  DOI: 10.11900/0412.1961.2017.00326 Abstract   HTML   PDF (4904KB) ( 625 ) 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 P1.5/50 of 4.86 W/kg and P1.0/400 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.
 Select Microstructure Evolution During Solution Treatment and Its Effects on the Properties of Ni-Fe-Cr Alloy Shenghu CHEN, Lijian RONG Acta Metall Sin, 2018, 54 (3): 385-392.  DOI: 10.11900/0412.1961.2017.00210 Abstract   HTML   PDF (9858KB) ( 709 ) 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.
 Select Finite Element Simulation of the Temperature Field and Residual Stress in GH536 Superalloy Treated by Selective Laser Melting Shu WEN, Anping DONG, Yanling LU, Guoliang ZHU, Da SHU, Baode SUN Acta Metall Sin, 2018, 54 (3): 393-403.  DOI: 10.11900/0412.1961.2017.00284 Abstract   HTML   PDF (4416KB) ( 1132 ) 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.
 Select Effects of Triple Junction and Grain Boundary Characters on the Morphology of Carbide Precipitation in Alloy 690 Xirong LIU, Kai ZHANG, Shuang XIA, Wenqing LIU, Hui LI Acta Metall Sin, 2018, 54 (3): 404-410.  DOI: 10.11900/0412.1961.2017.00141 Abstract   HTML   PDF (6176KB) ( 528 ) 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 Σ3c grain boundary near different types of triple junction. The size of carbide precipitated at Σ3c 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 Σ3i 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.
 Select Magnetic Viscosity of Anisotropic Rare Earth Permanent Films Yachao SUN, Minggang ZHU, Rui HAN, Xiaoning SHI, Nengjun YU, Liwei SONG, Wei LI Acta Metall Sin, 2018, 54 (3): 457-462.  DOI: 10.11900/0412.1961.2017.00211 Abstract   HTML   PDF (1056KB) ( 474 ) 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 $Hc⊥$ 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.