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

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    Effect of Annealing on Microstructure of Thermally Aged 308L Stainless Steel Weld Metal
    Xiaodong LIN,Qunjia PENG,En-Hou HAN,Wei KE
    Acta Metall Sin, 2019, 55 (5): 555-565.  DOI: 10.11900/0412.1961.2018.00365
    Abstract   HTML   PDF (22244KB) ( 694 )

    Austenitic stainless steel weld metal has been widely used as nozzle/safe-end joint and inner surface cladding of reactor pressure vessel, due to its good mechanical property and corrosion resistance. However, long-term thermal ageing at the service temperature (280~330 ℃) could induce hardening and embrittlement of the weld metal. To recover the thermal ageing embrittlement, the annealing treatment has been proposed since the annealing could affect the ageing-induced microstructural changes such as spinodal decomposition and G-phase precipitation in ferrite. However, there is still an incomplete understanding as well as a lack of nanoscale investigation about the annealing effect on the microstructural change of the weld metal. In this work, 308L stainless steel weld metal was thermally aged at 410 ℃ for 7000 h, followed by an annealing treatment at 550 ℃ for 1 h. Since the weld metal has a dual-phase structure of austenite and δ-ferrite, the phase transformation of austenite and δ-ferrite as well as the element segregation at the δ-ferrite/austenite phase boundary were investigated by TEM and atom probe tomography. The results revealed that austenite was unaffected by annealing while the ageing-induced spinodal decomposition of δ-ferrite was completely recovered. In addition, the number density of G phase in δ-ferrite was significantly reduced following annealing. This indicates that austenite has a higher stability compared with δ-ferrite. As for the δ-ferrite/austenite phase boundary, thermal ageing induced the segregation of Ni, Mn and C at the phase boundary, while the contents of Cr, Si and P remained almost unchanged. Following the annealing treatment, the segregation of all elements was eliminated. Further, only a small quantity of Ni and Mn was enriched in austenite near the phase boundary. The results suggested that the microstructure of the annealed specimen was similar to that of the unaged specimen, indicating a good recovery of the microstructure by annealing.

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    Effect of Hot Band Annealing Processes on Texture and Formability of 19Cr2Mo1W Ferritic Stainless Steel
    Houlong LIU,Mingyu MA,Lingling LIU,Liangliang WEI,Liqing CHEN
    Acta Metall Sin, 2019, 55 (5): 566-574.  DOI: 10.11900/0412.1961.2018.00540
    Abstract   HTML   PDF (13885KB) ( 611 )

    Low-cost ferritic stainless steels with excellent oxidation resistance and anti-corrosion ability are widely used in the fields of household appliances, hardware decoration, architectural structures, fuel cells and automobile exhaust systems. In order to achieve good formability of the ferritic stainless steel, the annealing process of hot-rolled sheet is crucial. As a newly developed 444-type heat-resistant ferritic stainless steel containing W and Ce, however, the influence of hot band annealing process of 19Cr2Mo1W ferritic stainless steel on its formability is not clear and need to have a deep understanding. In this work, the effect of annealing temperature of hot band on the microstructure, texture and formability of this steel was studied by means of XRD, EBSD, roughness measurement and formability test. The results indicated that although annealing processes were carried out at different temperatures after hot rolling, the characteristic of texture in the hot-rolled and annealed sheet was inherited to the cold-rolled sheet to some extent. The increased intensities of {223}<11ˉ0> and {111}<01ˉ1> texture components in the hot-rolled and annealed sheet were beneficial to improvement of the γ-fiber texture in the cold-rolled and annealed sheet. The extent of deviation from γ-fiber texture in the cold-rolled and annealed sheet was increased with increasing the intensities of {001}<11ˉ0>~{115}<11ˉ0> texture components in the cold-rolled sheet. An increased annealing temperature of the hot-rolled sheet could effectively weaken the intensities of {001}<11ˉ0>~{115}<11ˉ0> texture components in the cold-rolled sheet. In addition, the banded microstructures in the hot-rolled and annealed sheet were significantly reduced by increasing annealing temperature of the hot-rolled sheet, which improved the microstructure uniformity and formability of the cold-rolled and annealed sheet.

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    Influence of Shielding Gas Composition on Microstructure Characteristics of 1000 MPa Grade Deposited Metals
    Tongbang AN,Jinshan WEI,Jiguo SHAN,Zhiling TIAN
    Acta Metall Sin, 2019, 55 (5): 575-584.  DOI: 10.11900/0412.1961.2018.00375
    Abstract   HTML   PDF (27651KB) ( 481 )

    In recent years, high and ultra-high strength steels have been developed and used in light-weight constructions such as the structural members of mobile equipment in order to reduce weight and fabrication costs and to enhance the performance. Welding of steels with yield strength of more than 900 MPa is particularly challenging because of the toughness requirements for the weld metal, which calls for welding consumables of high strength and good toughness. Weld metals have been produced for a variety of welding methods with yield strength up to or above 1000 MPa, but their impact toughness remained only at medium yield strengths. Proper microstructure is the key to meeting this requirement, and its final microstructure depends on the chemical composition and cooling rate. For the deposited metal produced by gas metal arc welding (GMAW), the composition is dependent on the welding wire and shielding gas. The cooling rate of the weld metal is controlled by a combination of heat input and heat extraction. It is known that the addition of CO2 to argon based shielding gas is effective for improvement of productivity in GMAW welding of steel. Through the chemical reaction in the welding arc, CO2 in the shielding gas can affect the chemical composition of the weld metal, and its microstructure. The 1000 MPa grade deposited metals was welded with GMAW, and the effects of shielding gas composition (Ar+(5%~30%)CO2, volume fraction) on the general compositional and microstructural characteristics of deposited metals, including nonmetallic inclusions, were experimentally characterized with SEM, EBSD and TEM. The microstructure of the deposited metals is mainly composed of martensite and bainite. With the increase of CO2 content (5%~30%), the strength of the deposited metals decrease slightly and the impact toughness increases first and then decreases. Meanwhile, the transformation range (B50-Ms) of the deposited metal increases, the bainite content increases (8%~29.6%) with the quantity of inclusions that are suitable for bainite nucleation increases, and the nucleation position changes from the original austenite grain boundary to the common nucleation on the original austenite grain boundary and inclusions within the grain. At the same time, the microstructure morphology of the deposited metal changes from parallel to intertexture, which presented an intersected configuration and microstructure refinement.

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    Effect of Low Temperature Annealing on Microstructure and Mechanical Properties of Ultra-Heavy Cold-DrawnPearlitic Steel Wires
    Hanchen FENG,Xuegang MIN,Dasheng WEI,Lichu ZHOU,Shiyun CUI,Feng FANG
    Acta Metall Sin, 2019, 55 (5): 585-592.  DOI: 10.11900/0412.1961.2018.00319
    Abstract   HTML   PDF (48884KB) ( 713 )

    Ultra-heavy cold-drawn pearlite wires provide an excellent combination of ductility and strength. Therefore, they have been widely used in engineering applications, such as suspension bridge cables, automotive tyre cords and cutting wires. In this work, the effects of low temperature annealing on the microstructure and mechanical properties of ultra-heavy cold-drawn pearlitic steel wires were investigated. The mechanical properties have been determined by tensile testing and the structures analyzed by TEM and HRTEM. The overall carbon contents in the detected volumes as well as the carbon concentrations in ferrite and cementite were measured by 3DAP. Experimental results show that, for the steel wires with strain (ε) less than 4, annealing in the range of 120~170 ℃ could effectively increase the strength of steel wires and remain most of the plastic performance. The tensile strength of wire with a strain of 3.0 can be increased about 150 MPa after annealing at 150 ℃ for 8 min. However, both of strength and toughness of steel wires with a strain 4.5 decreased after annealed at 170 ℃. After the steel wire is deformed by excessive strain (ε=4.5), the cementite decomposed obviously. DSC analysis showed that there is an obviously exothermic peak between 150 ℃ and 170 ℃ in the DSC curve. The TEM diffraction pattern analysis reveal the phenomenon of tailing at diffraction pattern, which is mainly caused by segregation of carbon atom at the dislocation after annealed at 150 ℃. However, HRTEM images show that annealing temperature as low as 170 ℃ could result in the transformation of partial cementite from amorphous state to nano-crystalline state. It could effectively pin and hinder the movement of dislocations. The underlying mechanism responsible for changes in microstructure and mechanical properties after annealing at low temperature are closely related to C-segregation and "crystal-amorphous" cementite transformation in heavy cold-drawn pearlitic steel wires.

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    Effect of Mn Composition on the Nanometer Cu-Rich Phase of Fe-Cu-Mn Alloy by Phase Field Method
    Baojun ZHAO,Yuhong ZHAO,Yuanyang SUN,Wenkui YANG,Hua HOU
    Acta Metall Sin, 2019, 55 (5): 593-600.  DOI: 10.11900/0412.1961.2018.00506
    Abstract   HTML   PDF (8374KB) ( 624 )

    The precipitation of nanometer Cu-rich phase can be observed in Fe-Cu alloy systems during isothermal ageing. The existence of Cu-rich phase is one of the reasons for the embrittlement of reactor pressure vessel (RPV) steel. The phase-field method applies a set of field variables defined by functions of space and time to describe the temporal evolution of composition and structural parameter, characterizing microstructure evolution during phase transformation. This work uses phase-field model to simulate the three-dimensional morphology, the volume fraction, number density and average particle radius of Cu-rich phase in Fe-Cu-Mn alloy at 823 K. The chemical free energy is derived from the thermodynamic database of the calculated phase diagram (CALPHAD), so the microstructure evolution of precipitation changes are directly corresponded to phase diagram of the real alloy system. The simulation results show that nanometer Cu-rich phase are formed by the spinodal decomposition mechanism in the early stage of phase separation. Meanwhile, Mn atoms segregate to the center of the Cu-rich phase. During the process of Ostwald coarsening, Mn atoms migrate from core to the interface of Cu-rich phase, finally forming Mn-rich ring distributed in the exterior of Cu-rich phase. Its existence can decrease the rates of diffusion growth and coarsening of Cu-rich phase. The Cu-rich phase is bcc structure and disperses in the matrix with spherical shape in the early stage of ageing. As the Cu-rich phase continues to grow, it will transform into fcc structure with ellipsoid or rod shapes. Meanwhile, increasing Mn content of Fe-Cu-Mn alloy accelerates the precipitation of Cu-rich phase and facilitates the growth and coarsening of Cu-rich phase.

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    Effect of Re-Heat Rejuvenation Treatment on γ′ Microstructure of Directionally SolidifiedSuperalloy Damaged by Creep
    Wenshu TANG,Junfeng XIAO,Yongjun LI,Jiong ZHANG,Sifeng GAO,Qing NAN
    Acta Metall Sin, 2019, 55 (5): 601-610.  DOI: 10.11900/0412.1961.2018.00364
    Abstract   HTML   PDF (29463KB) ( 832 )

    As the core hot section components in gas turbine systems, the turbine blades are inevitably subjected to various microstructure creep damages after long time service, which seriously affect their service life. The hot isostatic pressing (HIP)-combined rejuvenation heat treatment process has been developed as a critical step in refurbishment of degraded blades with equiaxed structure, and there is a common view that HIP process has positive impact on healing creep cavities, however, the turbine blades in current gas turbine systems are widely made of directionally solidified superalloys with excellent resistance of creep voids due to the minimal number of oriented grain boundaries, which indicates that it is likely to use simple re-heat rejuvenation treatment consisting of solution and ageing treatments as a cheaper refurbishment method in recovering microctructures and properties of directionlly solidified superalloys. In this work, the interrupted creep test was conducted on directionlly solidified GTD111 superalloy to simulate the service damage of turbine blades. The effect of re-heat rejuvenation treatment on γ′ precipitates microstructure of creep degraded GTD111 superalloy and the evolution process of γ′ precipitates under different stages of re-heat rejuvenation treatment were investigated. The results show that solid solution treatment at the temperature range of 1180~1220 ℃ can effectively dissolve the coarsened and rafted primary γ′ precipitates and promote uniform precipitation of fine size secondary γ′ precipitates in the damaged alloy, meanwhile the size of secondary γ′ precipitates decreases with the increase of solution temperature and cooling rate. However, when the solid solution temperature increases to 1240 ℃, incipient melting in the interdendritic region ocurrs. High temperature ageing results in continued growth of the secondary γ′ precipitates and precipitation of tertiary γ′ precipitates. The size and cubic degree of the secondary γ′ precipitates increase with the increase of ageing temperature and soaking time. The tertiary γ′ precipitates continue to precipitate and grow during low temperature ageing process. The suitable re-heat rejuvenation parameters are 1220 ℃, 2 h, AC+1121 ℃, 2 h, AC+843 ℃, 24 h, AC. The rupture life of rejuvenated alloy under the condition of 750 ℃ and 843 MPa is up to 65 h, which is about 1.3 times of that of virgin alloy, due to its more volume fraction of duplex size γ′ precipitates after re-heat rejuvenation treatment.

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    Microstructural Evolution and Mechanism of Solidified TiAl Alloy Applied Electric Current Pulse
    Zhanxing CHEN,Hongsheng DING,Ruirun CHEN,Jingjie GUO,Hengzhi FU
    Acta Metall Sin, 2019, 55 (5): 611-618.  DOI: 10.11900/0412.1961.2018.00504
    Abstract   HTML   PDF (15521KB) ( 1246 )

    As a new type of lightweight and high temperature structural material, TiAl alloy has become the most ideal candidate in the fields of aerospace, military and civil products, and it has a good perspective in the industrialization. Refining and improving the microstructure of TiAl alloys has higher theoretical significance and engineering value. In this work, the solidified Ti-48Al-2Cr-2Nb alloy applied electric current pulse is studied, and its microstructural evolution and mechanism are analyzed. The results show that the electric current pulse refines the primary dendrite arm spacing, columnar crystal size and interlamellar spacing of the Ti-48Al-2Cr-2Nb alloy. The primary phase is α without electric current pulse, the angle of the Ti-48Al-2Cr-2Nb alloy that between the lamellar orientation and the growth direction is usually bigger, even perpendicular to the growth direction approximately. The applied electric current pulse causes the dendrite to melt and break, and promotes the occurrence and increase of the primary β phase, the lamellae orientation having a small angle or 45° between the growth direction is further increasing. The electric current pulse reduces the solid-liquid phase free energy and atomic diffusion activation energy, reduces the nucleation barrier and the critical nucleation energy, thereby atomic diffusion and the crystallization nucleation is promoted to a certain extent, the primary dendritic spacing and columnar crystals are remarkably refined. The electric current pulse causes the transformation of the primary phase and its corresponding crystal orientation relationship is the main reason for the change of lamellar orientation.

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    Dendrite Growth and Orientation Evolution in the Platform of Simplified Turbine Blade for Nickel-Based Single Crystal Superalloys
    Dejian SUN,Lin LIU,Taiwen HUANG,Jiachen ZHANG,Kaili CAO,Jun ZHANG,Haijun SU,Hengzhi FU
    Acta Metall Sin, 2019, 55 (5): 619-626.  DOI: 10.11900/0412.1961.2018.00426
    Abstract   HTML   PDF (15431KB) ( 717 )

    Ni-based single crystal (SX) superalloys are widely used in key hot section parts of advanced aero engine and industrial gas turbines (IGTs) because of their superior mechanical performance at high temperature. During directional solidification process of SX blades, high angle grain boundaries that degrade the creep and fatigue properties significantly might be highly likely to occur in the platform region, thus the analysis of multi-influencing factors, such as platform dimension, withdrawal rate and seed orientation, need to be studied. However, most of these works are conducted from the perspective of heterogeneous nucleation induced by the extreme concave shape of liquid-solid interface, and there is rare report concerning that whether dendrite deformation could also induce the high angle grain boundaries. Therefore, in the present work, the SX castings with three platforms were directionally solidified in a Bridgman-furnace, to investigate dendrite growth and the associated orientation evolution. It was observed that the whole platform consisted of three types of regions: blade body, secondary dendrite spread zone, and the "circuit-like" dendrite growth zone. The convergent boundaries of dendrite arms (CBDAs) were formed between secondary dendrite spread zone and the "circuit-like" dendrite growth zone. With increasing withdrawal rate, the area of "circuit-like" dendrite growth zone was increased, and the area of secondary dendrite spread zone was reduced. Moreover, the monotonically increased misorientation angle along CBDAs as a result of the dendrite deformation around platform edge was identified. As withdrawal rate increased, the misorientation angle along CBDAs was increased, and thus the tendency of high angle grain boundary formation on the CBDAs was enhanced. Unlike the nearly constant misorientation angle of the high angle grain boundary between grains and the low angle grain boundary between subgrains, the misorientation angle of the high angle grain boundary and low angle grain boundary formed on the CBDAs varied regularly.

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    Arc Erosion and Degradation Mechanism ofAg/Ti2AlC Composite
    Jianxiang DING,Wubian TIAN,Dandan WANG,Peigen ZHANG,Jian CHEN,Zhengming SUN
    Acta Metall Sin, 2019, 55 (5): 627-637.  DOI: 10.11900/0412.1961.2018.00534
    Abstract   HTML   PDF (44932KB) ( 471 )

    Ag-based contact is widely used in low-voltage switch (contactor, relay and breaker), which determines the safety and stability of the circuit. Toxic Ag/CdO goes against the development of environmentally friendly materials and will be excluded from future market. Ag/10%Ti2AlC (mass fraction, Ag/10TAC) composite shows excellent arc erosion resistance, and has the potential to replace Ag/CdO. Dynamic electric arc discharging experiment was performed on the Ag/10TAC contact surface to investigate its arc erosion mechanism. Inhomogeneous arc erosion generates three featured regions (unaffected, transitional, affected) on the contact surface. The various microstructure and chemical composition of Ag are attributed to the melting and vaporization of Ag, absorption of O2, deposition of Ag-O vapor, and interdiffusion of Ag-Al. The rapid "decomposition-oxidation" process of Ti2AlC accounts for the microstructure evolution and oxidation behavior of Ti2AlC during arc erosion. The changes of structure and function on the contact surface lead to the degradation of Ag/10TAC composite.

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    Graphene Nanoplatelets Reinforced Magnesium Matrix Composites Fabricated by Thixomolding
    Ting ZHANG,Yuhong ZHAO,Liwen CHEN,Jianquan LIANG,Muxi LI,Hua HOU
    Acta Metall Sin, 2019, 55 (5): 638-646.  DOI: 10.11900/0412.1961.2018.00301
    Abstract   HTML   PDF (19229KB) ( 572 )

    As a new type of carbon material reinforcement, graphene nanoplatelets (GNPs) have excellent mechanical, electrical, thermal and optical properties. Adding GNPs with a large specific surface area to the magnesium matrix can significantly improve the mechanical properties, the thermal and electrical properties of the magnesium matrix. However, so far, few reports focused on GNPs reinforced magnesium matrix composites, especially for lack of feasible fabrication technologies. Thus, in the present work, a new method for fabricating GNPs reinforced magnesium matrix composites is presented. First, the GNPs were dispersed by ultrasonic dispersion. Subsequently, the AZ91D magnesium alloy particles and the uniformly dispersed GNPs were mixed in a V-type powder mixer. Finally, GNPs reinforced magnesium matrix composites were prepared by semi-solid thixomolding. The effects of GNPs contents (0.3%, 0.6%, 0.9%, mass fraction) on the microstructure and properties of magnesium matrix composites were studied. The results show that the GNPs were uniformly distributed in the matrix, which were well combined with the matrix, and the addition of GNPs could refine the grain size and reduce porosity. Compared with AZ91D magnesium alloy, the addition of GNPs improved the tensile strength and hardness of the material. When the content of GNPs was 0.6%, the mechanical properties of the composites were the best, and the hardness and tensile strength reach up to 92.3 HV and 245 MPa.

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    Preparation of Ti2AlC Coating by the Combination of a Hybrid Cathode Arc/Magnetron Sputtering with Post-Annealing
    Wentao LI,Zhenyu WANG,Dong ZHANG,Jianguo PAN,Peiling KE,Aiying WANG
    Acta Metall Sin, 2019, 55 (5): 647-656.  DOI: 10.11900/0412.1961.2018.00285
    Abstract   HTML   PDF (16185KB) ( 527 )

    Nuclear power generation provides a reliable and economic supply of electricity, due to low carbon emissions and relatively few waste. However, the reaction between zirconium and steam at high temperature is accompanied by the release of large amounts of hydrogen gas, which will bring serious consequences. After the Fukushima nuclear accident, the concept of accident-tolerant fuels (ATF) has been proposed and widely investigated. In terms of nuclear claddings, one key requirement is reduced oxidation kinetics with high-temperature steam and hence significantly reduced heat and hydrogen generation. An economical and simple method could be the preparation of protective coatings on the surface of zirconium alloys to improve the oxidation resistance. The MAX phase has been considered to be one of the most promising coating materials for nuclear cladding coatings. In this work, Ti-Al-C coatings with different Ti/Al ratios have been deposited on Zirlo alloy using a hybrid arc/magnetron sputtering method, and the Ti2AlC coatings were obtained by post-annealing. The effects of Ti/Al ratios and annealing temperatures on the phase and microstructure of Ti-Al-C coatings after annealing were studied by SEM, EDS, XRD, Raman spectrometer and TEM. It is found that Ti-Al-C coatings with different Ti/Al ratios deposited by the hybrid cathode arc/magnetron sputtering are a multi-layer structure of an alternative Al-rich layer and TiCx layer. The as-deposited coatings are compact with a small amount of large particles. The Ti/Al ratio has an important influence on the phase structure of the annealed coating. When the Ti/Al ratio is 2.04, the highest purity and crystallinity of Ti2AlC are obtained. TiC and Ti3AlC impurities will form within the coating at a higher Ti/Al ratio (3.06), while the purity and crystallinity of Ti2AlC will decrease at a lower Ti/Al ratio (0.54). In addition, the annealing temperature affects the formation of Ti2AlC to a great extent. When the Ti/Al ratio is 2.38, the optimum temperature for Ti-Al-C coatings to Ti2AlC coatings is at 750 ℃. The atom cannot diffuse fully at a lower annealing temperature (600 ℃), which is difficult to form the Ti2AlC phase, while a higher annealing temperature (900 ℃) will enable the formation of Ti2AlC coatings with more TiC, TiAlx and other impurities.

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    Performance Research of Magnesium Base Lanthanum Hexaaluminate Prepared by Co-Precipitation
    Ying LI,Chao SUN,Jun GONG
    Acta Metall Sin, 2019, 55 (5): 657-663.  DOI: 10.11900/0412.1961.2018.00448
    Abstract   HTML   PDF (21357KB) ( 401 )

    Thermal barrier coatings are widely used on turbine blades to provide high temperature insulation, oxidation and corrosion protection. Thermal barrier coatings are composed of matrix, oxide layer, bonding layer and ceramic layer. The lanthanum magnesium hexaaluminate with magnetoplumbite structure have a high aspect ratio, large specific surface area and strong resistance to high temperature sintering, and it can be used as ceramic layer of thermal barrier coatings. In this work, the former powders of lanthanum magnesium hexaaluminate was prepared at synthesized temperature of 60 ℃, pH=11.5. Comparing with the conventional chemical co-preparation synthesis, the synthesized temperature was raised and the pH value for synthesizing was reduced, which resulted in improving production efficiency of former powders. And the lanthanum magnesium hexaaluminate powders for ceramic layer of thermal barrier coating was prepared after the precursor powders were calcinated at 1500 ℃ for 5 h. The phase structure of reaction products, morphology of the powders, full width at half maximum and spectral intensity were analyzed by XRD, SEM, TEM and XPS. The results showed that the magnetoplumbite structured formation generation was elevated much more efficiency if the former powders were precipitated at higher temperature. Since the bonding energy of La, Mg, Al and O atoms increased, the kinetic energy of electrons decreased, therefore, chemical composition of the reaction products were steady near 1500 ℃. During the crystallization process of lanthanum magnesium hexaaluminate, the spinel layer was generated firstly and mirror layer was consequently produced. In the process of producing pure magnetoplumbite phase powders, the full width at half maximum of LaAlO3 formula and MgAl2O4 formula were increased and the activation energy of the crystal structure was higher than that before reaction, which was beneficial to improve anti-sintering performance and thermal stability of the ceramic coating.

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    Dendrite Coarsening and Secondary Arm Migration in the Mushy Zone During Directional Solidification:
    Hui FANG,Hua XUE,Qianyu TANG,Qingyu ZHANG,Shiyan PAN,Mingfang ZHU
    Acta Metall Sin, 2019, 55 (5): 664-672.  DOI: 10.11900/0412.1961.2018.00427
    Abstract   HTML   PDF (7613KB) ( 622 )

    Directional solidification is a common and important process in both scientific research and industrial practice. Dendrites are the most frequently observed microstructures in the directional solidification. It is known that dendrite coarsening in mushy zones is an unavoidable phenomenon that influences microstructures and thereby properties significantly. Moreover, the presence of temperature gradients during directional solidification leads to temperature gradient zone melting (TGZM), which yields dendrite arm migration towards higher temperatures. In the present work, the evolution of dendrite microstructures in the mushy zone during directional solidification is investigated through in situ experiments and cellular automaton (CA) simulations for a transparent succinonitrile-acetone (SCN-ACE) alloy. The phenomena of dendrite coarsening and the secondary dendrite arm migration toward high temperature direction due to TGZM have been observed by both experiment and simulation. Dendrite coarsening is found to be caused by three modes, including the remelting of small dendrite arms, and dendrite arm coalescence through the advancement of interdendritic grooves and joining of dendrite arm tips. The experimental measurements indicate that the average migration velocity of the secondary dendrite arm increases with increasing the temperature gradient. For a fixed temperature gradient, dendrite arm migration becomes slower with time. The experimental data agree reasonably well with the analytical predictions. The present CA model involving the mechanisms of both solidification and melting can effectively reproduce the typical features of secondary dendrite arm migration and dendrite coarsening as observed in experiments. The simulation results show that the local liquid concentrations near the lateral side of big arms and in the "valleys" between side arms are relatively higher than that at the tips of small arms. This drives solute diffusion and leads to the dissolution of small arms, the growth of thick arms, and advancement of interdendritic groove bases. However, at the groove between two relatively narrow and long adjacent side arms, the solute diffusion is obstructed. In this case, dendrite arm coalescence through joining arm tips together with an entrapped liquid droplet in the solid can be observed. The role of melting for microstructure evolution in mushy zones is investigated by comparing the simulation results using CA models with and without melting effect. It is demonstrated that remelting is one of the dominant mechanisms for dendrite arm migration and dendrite coarsening by the mode of small dendrite arm remelting. Moreover, remelting also promotes dendrite coarsening by the mode of dendrite arm coalescence.

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    First-Principles Study on the Impact of Antisite Defects on the Mechanical Properties of TiAl-Based Alloys
    Zongwei JI,Song LU,Hui YU,Qingmiao HU,Levente Vitos,Rui YANG
    Acta Metall Sin, 2019, 55 (5): 673-682.  DOI: 10.11900/0412.1961.2018.00349
    Abstract   HTML   PDF (4967KB) ( 947 )

    Microalloying is an effective approach to improve the mechanical properties of TiAl-based alloys which have been applied as high-temperature structure materials. The antisite defects may be regarded as special alloying elements. However, the detailed information about the effect of antisite defects on mechanical behavior (full slip and twinning), which may be described theoretically by generalized stacking fault energy (GSFE), of TiAl-based alloys are scarce. In this work, the composition dependent GSFEs of off-stoichiometric γ-TiAl were calculated by using the first-principles exact muffin-tin orbitals method in combination with coherent potential approximation. With the calculated GSFE, the energy barriers for various deformation modes including twin (TW), ordinary dislocation (OD), and superlattice dislocation (SDI and SDII) were determined. The selection of the deformation mode under external shear stress with various directions was analyzed. The effects of the TiAl and AlTi antisite defects on the mechanical properties of γ-TiAl were then discussed. The results showed that the TiAl antisite defect decreases the energy barrier for the TW deformation leading by the superlattice intrinsic stacking fault (SISF) partial dislocation and increases the angle window of the applied shear stress within which TW deformation may be activated. Therefore, TiAl antisite defect is expected to improve the plasticity of γ-TiAl. The effect of AlTi antisite defect is opposite. The AlTi antisite defect decreases the energy barriers for the OD and SDII deformations leading by complex stacking fault (CSF) partial dislocation and increases their operating angle window, indicating that AlTi facilitates the slip of OD and SDII. Considering that the energy barrier for CSF is much higher than that for SISF, the plasticity induced by OD and SDII should be lower than that induced by TW. Calculations in this work explain the experimental finding that TiAl antisite defect improves the plasticity of γ-TiAl more significantly than AlTi antisite defect.

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