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

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    Orginal Article
    EVOLUTION BEHAVIOR OF LAVES PHASE IN FB2 MARTENSITIC STAINLESS STEEL DURING WELDING
    Kejian LI,Zhipeng CAI,Yifei LI,Jiluan PAN
    Acta Metall Sin, 2016, 52 (6): 641-648.  DOI: 10.11900/0412.1961.2015.00590
    Abstract   HTML   PDF (1001KB) ( 454 )

    Elevating steam parameters is the key to enhance the efficiency of fossil power plants, reducing fuel consumption and noxious emission. Therefore, a lot of new creep resistant martensitic stainless steels have been developed, among which FB2 steel (a new 9%Cr martensitic stainless steel) is the most promising candidate for manufacturing steam turbine rotors operated at temperature range from 600 ℃ to 650 ℃. In the present work, the evolution behavior of Laves phase in the as received FB2 steel was studied by thermal simulation technique. Firstly, some sparse micron-sized particles of Laves phase were observed in as received FB2 steel by SEM. It was concluded that the large Laves phase particles formed in casting due to dendritic segregation. Then constitutional liquation resulting from eutectic reaction between Laves phase and γ-Fe in the heating process of welding thermal simulation was found, suggesting a liquation crack tendency in heat affected zone of FB2 steel. In the specimens experiencing thermal simulation, some eutectic microstructures were observed by SEM. Based on the results of EDS analysis and SAED, the two eutectic constituents were identified as χ phase and γ-Fe. At last, the reason for the difference in morphology of eutectic microstructures between specimens experiencing thermal simulation with different peak temperatures was analyzed.

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    EFFECT OF Mn, Ni, Mo PROPORTION ON MICRO-STRUCTURE AND MECHANICAL PROPERTIESOF WELD METAL OF K65 PIPELINE STEEL
    Xuelin WANG,Liming DONG,Weiwei YANG,Yu ZHANG,Xuemin WANG,Chengjia SHANG
    Acta Metall Sin, 2016, 52 (6): 649-660.  DOI: 10.11900/0412.1961.2015.00453
    Abstract   HTML   PDF (2709KB) ( 583 )

    Longitudinal submerged arc welding pipeline steels with heavy caliber and large wall thickness are widely applied in the oil gas transmission to enhance the transmission efficiency and save cost. K65 pipeline steels are the main material for the Bovanenkove-Ukhta oil & gas transmission project. It is required that the -40 ℃ low temperature toughness of weld metal and heat affected zone (HAZ) are over 60 J for K65 pipelines. This standard is much stricter than that of X80 pipelines. The pipeline with superior low temperature toughness is seldom investigated. In this work, the Mn-Ni-Mo-Ti-B alloy submerged arc welding wire with high strength and high tough ness was designed, which was favorable to obtain excellent low temperature toughness. The results showed that the weld metal had a good combination of strength and low temperature toughness, the yield strength was 583~689 MPa, the tensile strength was 714~768 MPa, and the impact absorbed energy at -40 ℃ was over 90 J. The wire with a diameter of 4.0 mm was suitable for double-sided submerged arc welding with four wires, and the -40 ℃ impact energy of HAZ was over 100 J. The microstructure of weld metal was primarily comprised of fine acicular ferrite (AF), proeutectoid grain boundary ferrite (GBF), ferrite side plates (FSP) and small martensite/austenite (M/A) constituents. The weld metal with 0.2%Mo can effectively restrain the formation of GBF and FSP, significantly refining the grain size. The increased Mn and Ni contents enhanced the low temperature toughness of weld metal by increasing the amount of acicular ferrite. However, the concentration of Mn and Ni should be controlled under a critical value; much more Mn and Ni additions would promote the formation of martensite or other low temperature microstructural features, which is detrimental to weld metal toughness. The optimum combination of alloying element content was (1.5%~2.0%)Mn, (0.9%~1.2%)Ni, (0.2%~0.25%)Mo. Excellent strength and toughness can be obtained through replacing Ni by Mn in the terms of the concentration of Mn and Ni being above the Ms line.

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    EFFECTS OF N ON CREEP PROPERTIES OF AUSTENI-TIC HEAT-RESISTANT CAST STEELS DEVELOPEDFOR EXHAUST COMPONENT APPLICATIONSAT 1000 ℃
    Yinhui ZHANG,Mei LI,Larry A GODLEWSKI,Jacob W ZINDEL,Qiang FENG
    Acta Metall Sin, 2016, 52 (6): 661-671.  DOI: 10.11900/0412.1961.2015.00618
    Abstract   HTML   PDF (1573KB) ( 830 )

    To comply with more stringent environmental and fuel consumption regulations in recent years, automotive gasoline engines equipped with turbochargers are increasingly used to improve fuel efficiency. As a result, exhaust gas temperatures are now reaching 1050 ℃, about 200 ℃ higher than the conventional temperature. Hence, there is an urgent demand in automobile industries to develop novel and economic austenitic heat-resistant steels that are durable against these increased temperatures. In this study, the effects of N addition on creep behavior at 1000 ℃ and 50 MPa are investigated in a series of Nb-bearing austenitic heat-resistant cast steels, which are developed for exhaust component applications. Microstructures before and after creep rupture tests are carefully characterized to illustrate the microstructural evolution during creep deformation. The results of creep tests show approximately an order of magnitude increase in the minimum creep rate among the experimental alloys with variations of N addition. Microstructural analyses indicate that the morphology of NbC and Nb(C, N) is changed from “Chinese-script” to mixed flake-blocks, and then to faceted blocks as N additions increase. The best creep property occurs in an alloy with “Chinese-script” NbC, which could effectively strengthen the grain boundaries and interdendritic regions. The Cr-rich phases are adverse to creep properties, in particular those coarsened and coalesced phases along grain boundaries. They act as crack sources and accelerate the propagation of creep cracks. Moreover, the secondary precipitation of Cr-rich phase results in a significant decrease of C concentration in the matrix and thus reduces the solution strengthening ability during creep deformation. This study suggests that the strengthening of these austenitic cast steels can be achieved through the exploit of primary NbC and Nb(C, N) and the elimination of Cr-rich phases, and therefore, N additions should be strictly controlled.

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    CORROSION BEHAVIOR OF 6.5%Cr STEEL IN HIGH TEMPERATURE AND HIGH PRESSURE CO2 ENVIRONMENT
    Lining XU,Bei WANG,Minxu LU
    Acta Metall Sin, 2016, 52 (6): 672-678.  DOI: 10.11900/0412.1961.2015.00559
    Abstract   HTML   PDF (1029KB) ( 524 )

    In the oil and gas industry, CO2 in reservoirs generally causes severe corrosion of C steel in pipelines. Adding Cr to C steel can enhance CO2 corrosion performance by a factor of at least 3, and even up to 10 times and more, whilst maintain a cost penalty less than 1.5 times that of C steels. Corrosion behavior of 6.5%Cr steel in high temperature and high pressure CO2 environment is investigated in this work. The corrosion rate of 6.5%Cr steel at 80 ℃ and 0.8 MPa is measured by autoclave. The surface and cross-sectional morphology is studied by SEM, and the enrichment of Cr in the corrosion scale is investigated by EDS. The change of LPR and EIS results with corrosion time is studied by high temperature and high pressure electrochemical tests. According to the test results of ion concentration in the solution, the mechanism of corrosion scale growth is discussed. The results show that the corrosion rate of 6.5%Cr steel is 0.72 mm/a. The Cr enriches in the corrosion scale, and the Cr/Fe ratio is higher than 5∶1. Under different corrosion time, the concentration of Cr3+ in the solution is always low, which demonstrates that Cr seldom enters into solution.

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    EFFECT OF HYDROSTATIC PRESSURE AND PRE-STRESS ON CORROSION BEHAVIOR OF A NEW TYPE Ni-Cr-Mo-V HIGH STRENGTH STEEL
    Lin FAN,Kangkang DING,Weimin GUO,Penghui ZHANG,Likun XU
    Acta Metall Sin, 2016, 52 (6): 679-688.  DOI: 10.11900/0412.1961.2015.00577
    Abstract   HTML   PDF (1527KB) ( 752 )

    The efforts on deep sea exploration and development have posed many challenges on the corrosion resistance and safety use of high strength steel in recent years. This has attracted a lot of attentions on the corrosion behavior of high strength steel in deep sea environment. Hydrostatic pressure has been identified as one of the most significant factors that affect pitting corrosion of materials or steel structures used in deep sea. However, pre-stress introduced by the actual service conditions is probably another critical factor of deep sea corrosion. The purpose of this work is to investigate the corrosion behavior of a new type Ni-Cr-Mo-V high strength steel under the combined stresses of hydrostatic pressure and preloaded tensile stress in simulated deep-sea environment. Corrosion rate measurement, SEM observation, statistical analysis of pitting geometry and finite element (FE) analysis were used in this work. The results indicated that corrosion rate of Ni-Cr-Mo-V high strength steel increased with the increase of hydrostatic pressure and pre-stress. The deterioration of corrosion resistance of the steel mainly reflected in pit initiation, pit growth and pit coalescence. Rather than pre-stress, hydrostatic pressure exhibited obvious effect on promoting pit initiation. Pits initiated in the form of corrosion pin-holes, which randomly distributed at the corroded surface. Both hydrostatic pressure and pre-stress facilitated pit growth, and there was an interaction between them, which was more remarkable at higher pre-stress. Hydrostatic pressure was mainly responsible for pit growth parallel to the steel surface, while pre-stress was essential to pit depth increase. Adjacent pits were inclined to coalesce in the direction perpendicular to pre-stress. With the increase of hydrostatic pressure and pre-stress, the aspect ratio of pits increased, which can lead to the formation of uniform corrosion.

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    EFFECTS OF SECOND PHASES ON MICROARC OXIDATION PROCESS OF MAGNESIUM BASE MATERIALS
    Yanqiu WANG,Kun WU,Fuhui WANG
    Acta Metall Sin, 2016, 52 (6): 689-697.  DOI: 10.11900/0412.1961.2015.00500
    Abstract   HTML   PDF (939KB) ( 569 )

    The effects of second phases on microarc oxidation (MAO, also named plasma electrolytic oxidation-PEO) behavior of Mg base materials were investigated and the related mechanism was discussed. The formation of barrier layer and its influence on sparking discharge behavior were characterized and analyzed on the base of systematic selecting and designing substrate materials. The variation of second phases at the early MAO stage was observed and analyzed by SEM and EDS, and then the effect mechanism of second phases on MAO behaviors was revealed. Voltage evolution trend during MAO were recorded to study the formation state of the barrier layer on the different Mg base materials. According to the growth mechanism of MAO film, the film growth process can be simplistically considered as a repeated breakdown and reconstruction process of a capacitor. Accordingly, the growth process of MAO film on multiphase metal materials and the effects of second phases were discussed. The results show that different second phases in substrate materials have different effects on formation process of MAO films, depending on their own characteristics. For the second phases which have the characteristics of valve metals, although selective sparking discharge occurs at the early stage of MAO, the second phases will not hinder the growth of MAO film since barrier layer can form on the second phases, and they will not induce structural defects into the film-substrate interface. If the second phases have not the characteristics of valve metals, their conductivity property will be an important influencing factor to affect the MAO behaviors. For the elecinsulating second phases which have not the characteristics of valve metals, sparking discharge just occurs on Mg matrix in the substrate, while doesn't occur on the second phases; the second phases exist in the MAO film as heterogeneous phases, do not react in MAO process, and will not hinder the growth of MAO film. For the semi-conductive second phases which have not the characteristics of valve metals, they delay the growth of MAO film because they destroy the integrity of barrier layer. For the electroconductive second phases which have not the characteristics of valve metals, they seriously hinder the growth of MAO film.

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    RESEARCH ON THE MORPHOLOGY AND FRACTALDIMENSION OF PRIMARY PHASE IN SEMISOLIDA356-La ALUMINUM ALLOY BY ELECTRO-MAGNETIC STIRRING
    Zheng LIU,Lina XU,Zhaofu YU,Yangzheng CHEN
    Acta Metall Sin, 2016, 52 (6): 698-706.  DOI: 10.11900/0412.1961.2015.00496
    Abstract   HTML   PDF (1311KB) ( 567 )

    In order to obtain the fine, round and uniform distribution primary α phase in semisolid A356 alloy, the different amount of La was added into the alloy melt, and the melt was poured at 650 ℃ and slightly electromagnetically stirred under the condition of 30 Hz and 15 s, then, it was isothermally held at 590 ℃ for 10 min. The microstructure of the samples was observed by OM and SEM. The influences of La and electromagnetic stirring on morphology of primary α phase in semisolid A356 alloy were studied, and the symbolization of the characteristics of morphology of primary α phase by the fractal dimension was discussed in this work. The results showed that the morphology of primary α phase in semisolid A356 alloy was effectively improved by the suitable addition of La, no matter whether the semisolid slurry of A356-La alloy was prepared by electromagnetic stirring or not, the morphology of primary α phase showed better at first and then worse as the amounts of La increases, and the morphology and grain size of primary α phase reach the optimal state when the content of La was 0.4% (mass fraction). At the same time, the average equal-area circle diameter of the morphology of primary phase in semisolid A356-La alloy by electromagnetic stirring was finer than that without stirring, on the other hand, the shape factor was bigger than that without stirring. It implies that the primary α phase in semisolid A356-La alloy by electromagnetic stirring was smaller and more rounded than that without stirring, that is, the morphology of primary α phase in semisolid A356-La alloy by electromagnetic stirring was better than that without stirring. In addition, the real microstructure has fractal characteristics, and it was feasible to describe and analyze the change regularity and even the formation mechanism of the morphology of primary α phase in semisolid aluminum alloy by the principle of fractal geometry. The morphology of primary α phase in semisolid A356 alloy by the different process parameters had different fractal dimension. The fractal dimension of the semisolid primary α phase gradually became smaller with its morphology changed from dendritic-like to particle-like or globular-like.

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    MECHANISMS OF NON-UNIFORM MICROSTRUC-TURE EVOLUTION IN GH4169 ALLOYDURING HEATING PROCESS
    Jianguo WANG,Dong LIU,Yanhui YANG
    Acta Metall Sin, 2016, 52 (6): 707-716.  DOI: 10.11900/0412.1961.2015.00551
    Abstract   HTML   PDF (1783KB) ( 628 )

    The Ni-Fe-based superalloy GH4169 (Inconel718) is widely used for several critical gas-turbine components which are hot forged. Its microstructure and property are sensitive to the parameter adjustment during hot working process. To obtain required low-cycle fatigue and fracture properties, it is essential that the microstructure is controlled during preheating and heat treatment. The evolution of non-uniform microstructure during hot working is more complicated than that of uniform microstructure. On the other hand, various secondary phases can be observed in GH4169 alloy, thus it is important to investigate the effect of secondary phases on the microstructure evolution during forging process. In this work, the mechanisms of non-uniform microstructure evolution in GH4169 alloy were studied by analyzing the evolution of secondary phases, grain boundary misorientation, grain size and interactions of dislocation. It is found that the volume fraction of δ phase increases with the increasing of temperature and heating time at the lower temperature. While at the higher temperature, it decreases monotonously with the temperature increasing, but increases first and then decreases to stable value with time increasing. The pinning effect of secondary phases in GH4169 alloy can be concluded that the γ" phase and δ phase precipitated within the grains retain movement of dislocation, the δ phase precipitated at the grain boundary hinders the nucleation and growth of recrystallized grains, and the carbides limits the grain growth. The frequency of low angle grain boundary decreases with temperature and time increasing, and the mobility of low angle grain boundary increases with temperature increasing. The uniformity of microstructure and the size of equaxied subgrain increases with heating temperature and time increasing. Continuous recrystallization of elongated grain occurs at specific conditions. The mechanisms of non-uniform microstructure evolution during heating process can be concluded as subgrain growth, recrystallized grain growth, and anneal twinning nucleation and growth. The recrystallized grains are formed by the growth of subgrains conducted by the rotation of low angle grain boundary and the movement of dislocation. When the grain growth is pinned, the mechanisms for the energy dissipation is the nucleation and growth of anneal twinning. And the growth of anneal twinning promotes the generation of low angle grain boundaries at the tip of partial anneal twinning.

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    STUDY ON REJUVENATION HEAT TREATMENT OF A DIRECTIONALLY-SOLIDIFIED SUPERALLOYDZ125 DAMAGED BY CREEP
    Jing ZHANG,Yunrong ZHENG,Qiang FENG
    Acta Metall Sin, 2016, 52 (6): 717-726.  DOI: 10.11900/0412.1961.2015.00505
    Abstract   HTML   PDF (1571KB) ( 604 )

    The degradation of microstructure and property in turbine blades of aircraft engines is inevitable during their service. Usually, rejuvenation heat treatment is applied to regenerate the original microstructure for extending the service life of blades and improve economic returns. To date, systematic investigations about rejuvenation heat treatment of the directionally-solidified superalloys are limited. In this work, the effect of rejuvenation heat treatment on the degraded microstructure and property of DZ125 superalloy damaged by creep was investigated. The interrupted creep test was first conducted on DZ125 superalloy to simulate the damage of turbine blades during their service. Three rejuvenation heat treatments with the solution temperature at 1230, 1240 and 1250 ℃ were applied to the interrupted creep specimen. Then, the rejuvenated specimens were retested, and their microstructures as well as creep properties were compared with those of the initial interrupted creep tests. The results showed that no recrystallization occurred after the interrupted creep tests at 1.0% and 3.5% strain followed by rejuvenation heat treatment, and the critical strain for the formation of the recrystallization was between 3.5%~10.0%. The solution treatment at 1230 ℃ partially dissolved the coarse γ' phase caused by creep deformation, caused a nonuniform microstructure of γ /γ' matrix after aging treatments, and resulting in partially recovering the baseline creep property. However, the solution treatment at 1240 and 1250 ℃ could not only dissolve the coarse γ' phase but also reduce the fraction of residual γ' eutectic significantly, and then regain a uniform microstructure of γ /γ' matrix after aging treatments. Hence, the effect of rejuvenation was further improved, maintaining or exceeding the baseline creep property.

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    EFFECTS OF Mo CONTENT ON THE MICRO-STRUCTURE AND TRIBOLOGICAL PROPERTIES OF CrMoAlN FILMS
    Baiyang LOU,Yuxing WANG
    Acta Metall Sin, 2016, 52 (6): 727-733.  DOI: 10.11900/0412.1961.2015.00493
    Abstract   HTML   PDF (857KB) ( 550 )

    In recent decades, CrAlN coatings have been widely used for cutting tools due to their high hardness, good wear resistance, especially excellent thermal stability and oxidation resistance. However, the rapid development in high speeds and dry cutting applications demands further improvement in hardness and wear properties of CrAlN coatings. Mo nitrides coatings are commonly used as protective surface layers against wear and corrosion. The combination of CrAlN and Mo may lead to the development of new composite coatings with superior wear properties. In this study, the CrMoAlN multilayer coatings with different Mo contents were deposited on M2 tool steel and silicon wafers substrates by closed-field unbalanced magnetron sputtering ion plating (CFUMSIP) technique in a gas mixture of Ar+N2. The chemical composition, surface and cross sectional morphologies, microstructure, mechanical and tribological properties of coatings were studied by EDS, SEM, XRD, XPS, nano-indentation and pin-on-disk tribometer, respectively. The results indicate that the CrMoAlN coatings exhibit fcc structure. Mo atoms substitute Cr and/or Al atoms in CrAlN lattice forming the solid solution CrMoAlN coatings. The surface and cross-sectional morphologies of the CrMoAlN coatings show that the grain size and the column width decrease with the increasing of Mo content. Nano-indentation result reveals a promoted hardness and elastic modulus of the CrMoAlN coatings with enhanced Mo content from 0 to 19.47% (atomic fraction) due to the solid solution strengthening and grain size diminishment. A maximum hardness and elastic modulus of the coatings are found to be 29.70 GPa and 427.53 GPa when the Mo content reached to 19.47%. The average friction coefficient and wear rate were observed to decrease with the increase of Mo content and the lowest values were 0.271 and 1.2×10-16 m3/(Nm), respectively, at 19.47%Mo.

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    INTERFACIAL MICROSTRUCTURE AND MECHANI-CAL PROPERTIES OF INDIRECT BRAZED GRAPHITE/COPPER JOINT
    Wei FU,Xiaoguo SONG,Long LONG,Jianhang CHAI,Jicai FENG,Guodong WANG
    Acta Metall Sin, 2016, 52 (6): 734-740.  DOI: 10.11900/0412.1961.2015.00502
    Abstract   HTML   PDF (807KB) ( 482 )

    Graphite and metal composite structures were widely used in aerospace, electrical engineering and electronics. Because of its conveniences and less cost, brazing was widely used to bond graphite and metals. Due to the differences in microstructure, graphite was difficult to be wetted by traditional braze alloys. To improve the wettability of traditional brazing alloys on graphite, active brazing process and indirect brazing process were developed to braze graphite to metals. As to active brazing process, active elements (such as Ti, Cr, Zr) were added into traditional brazing alloys, a high brazing temperature, was essential to guarantee the reaction of active elements with graphite. However, the mechanical properties of metals will degrade under high temperature. Electroplating and chemical plating were the general techniques for indirect brazing process. The covered coating had a mechanical combination which decreased the joint strength, rather than metallurgical bonding with graphite. Therefore, in this work, a new metallization method was proposed. On the one hand, a metallurgical bonding was formed between metallization layer and graphite substrate. On the other hand, graphite could be brazed to metal at a relatively low temperature. Firstly, graphite was metalized by Ti-containing Sn0.3Ag0.7Cu metallization powder at 950 ℃ for 30 min. Then metalized graphite was brazed with copper by Sn0.3Ag0.7Cu successfully. The typical interfacial structure of brazed joint was copper/Cu3Sn/Cu6Sn5/β-Sn/TiC/graphite. Element Ti of metallization powder played an important role in metallization process for a reaction layer TiC was formed on the interface of graphite and metallization layer. Nevertheless, Ti contents had no effect on interfacial structure and shear strength of brazed joint. With the increase of brazing temperature, more and more element Cu dissolved into molten solder and formed Cu-Sn compounds by reacting with Sn. Furthermore, shear strength was improved slightly. Fracture analysis reveals that cracks extended along β-Sn layer and presented ductile fracture. When Cu-Sn compounds occupied the entire brazing seam (joint brazed at 600 ℃), shear strength improved remarkably and reached 30 MPa. Additionally, the joint was fractured in graphite entirely.

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    PREPARATION OF Ti1-xAlxN COATING IN CUTTING TITANIUM ALLOY AND ITS CUTTING PERFORMANCE
    Xudong SUI,Guojian LI,Qiang WANG,Xuesi QIN,Xiangkui ZHOU,Kai WANG,Lijian ZUO
    Acta Metall Sin, 2016, 52 (6): 741-746.  DOI: 10.11900/0412.1961.2015.00454
    Abstract   HTML   PDF (851KB) ( 470 )

    High-strength lightweight titanium alloy structural materials have been widely used in aerospace and other industry. However, the titanium is hard to machine due to its characteristics of low thermal conductivity, high chemical affinity and low elastic modulus. Coating tools provide a solution to overcome the problem of cutting titanium alloy. Ti1-xAlxN coating is one of the most popular candidates in cutting titanium alloy. However, the cutting performance and wear mechanism of the sputtering Ti1-xAlxN coating should be studied further in order to meet the demands of cutting titanium alloy. In this work, Ti1-xAlxN coatings with different Al contents have been prepared by magnetron sputtering. Microstructure and mechanical properties of the coatings were examined by XRD, SEM, EDX and nanoindenter. Results show that the coatings is a single fcc structure with a (111) preferred orientation when x is in the range of 0.50~0.58 (atomic fraction). When the Al content is 0.63, the hexagonal AlN is formed in the coating and the hardness declines. In addition, the surface particle size of Ti1-xAlxN coatings increases and the coating density decreases with increasing the Al content. The results of titanium cutting experiment indicate that the tool wear is mainly adhesive wear and chipping. The cutting performances of Ti0.50Al0.50N coated tool is slightly better than uncoated tool and are much better than those of Ti0.42Al0.58 and Ti0.37Al0.63N coated tools at a lower cutting speed (65 m/min). The cutting performance of Ti0.50Al0.50N coated tool is the best at a higher cutting speed of 100 m/min and is four times larger than that of uncoated tool. The excellent cutting performance of Ti0.50Al0.50N coating is mainly due to its high surface density and high hardness, which lead to the formation of regular and dense built-up edge during titanium cutting. Therefore, Ti0.50Al0.50N coating with a (111) preferred orientation, dense surface and relatively low Al content is recommended in high speed turning titanium.

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    MICROSTRUCTURE AND WEAR RESISTANCE OF TiAlZrCr/(Ti, Al, Zr, Cr)N GRADIENT FILMS DEPOSITED BY MULTI-ARC ION PLATING
    Shilu ZHAO,Zhen ZHANG,Jun ZHANG,Jianming WANG,Zhenggui ZHANG
    Acta Metall Sin, 2016, 52 (6): 747-754.  DOI: 10.11900/0412.1961.2015.00522
    Abstract   HTML   PDF (871KB) ( 561 )

    Nowadays, the cutting tools are exposed to much more severe operating conditions, i.e. high cutting speed, high feed rate, aggressive mechanical and thermal loading. As a result, the existing hard films have frequently encountered wear-related failures. Such situation requires the new generation hard films concurrently displaying superior hardness, excellent adhesive strength and outstanding wear resistance. Previous studies have demonstrated some promising mechanical properties (hardness and adhesion strength) of TiAlZrCr/(Ti, Al, Zr, Cr)N quaternary gradient films as compared to those of the (Ti, Al)N binary and (Ti, Al, Zr)N or (Ti, Al, Cr)N trinary nitride films. However, the research on wear resistance of hard films under the conditions of high speed and dry friction has been seldom reported. In this work, using combined Ti-Al-Zr alloy and pure Cr targets, TiAlZrCr/(Ti, Al, Zr, Cr)N quaternary nitride films were deposited on high speed steel W18Cr4V substrates by multi-arc ion plating (MAIP) process at various bias voltages of -50, -100, -150 and -200 V. Surface morphology and crystalline struc ture of the gradient films were analyzed by SEM and XRD. Wear resistance of the films was evaluated by abrasion tester at both ambient (15 ℃) and elevated (500 ℃) temperatures. The worn surface morphology was then investigated by SEM. The results show that the deposited TiAlZrCr/(Ti, Al, Zr, Cr)N quaternary nitride films exhibited TiN-type (fcc-NaCl type) structure. The films have uniform and dense columnar morphologies. Furthermore, it was confirmed that the primary wear mechanism was adhesive wear (caused by plastic deformation) accompanied by a slight abrasion. The average values of friction coefficient varied at 0.25~0.30 at ambient temperature and 0.30~0.35 at elevated temperature, respectively. Finally, the best wear resistance was achieved when the bias voltage increased to -200 V.

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    CHARACTERISTIC TEMPERATURE AND PERFOR-MANCE OF THE Ge30Se70 CHALCOGENIDE GLASS
    Tingting JIA,Zengyun JIAN,Junfeng XU,Man ZHU,Fang'e CHANG
    Acta Metall Sin, 2016, 52 (6): 755-760.  DOI: 10.11900/0412.1961.2015.00501
    Abstract   HTML   PDF (635KB) ( 757 )

    Chalcogenide glass is an ideal infrared wave-transparent material, and it has the advantages of low cost, high production efficiency, high glass transition temperature and good mechanical properties, etc.. It is a candidate material for thermal imaging system. The block sample of Ge30Se70 chalcogenide glass was prepared by the method of the melt-quenched. In this work, XRD was used to determine whether the sample was amorphous material. With the DSC thermal analysis method, the glass transition temperature Tg and the initial crystallization temperature Tx of the sample were measured. The dynamics ideal glass transition temperature T0 of the specimen was fitted by VFT equation. The method of segmented step heating is used to analyze the calorific value for the glass and congruent crystal of Ge30Se70 sample in setting temperature range. Then from the calculated calorific values of the glass and crystalline samples, the specific heat capacity relationships were obtained, i.e., cp,l=0.0002T+0.3337 and cp,c=0.00006T+0.4594. The results show that Tg and T0 of Ge30Se70 sample is 590 and 581 K, respectively. And Tg will increase with the increasing of the heating rate R. The average value of the specific heat capacity of the Ge30Se70 glass sample is about 11.8 J/(molK) below the glass transition temperature. The infrared transmittance is about 60% indicating that the infrared performance is good. The glass reduced temperature Trg of Ge30Se70 sample is between 0.5~0.667, and the nucleation rate is very low, which indicates that the glass forming ability of Ge30Se70 glass is good.

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    INVESTIGATIONS ON THERMAL STABILITY OF FATIGUE DISLOCATION STRUCTURES IN CONJUGATE AND CRITICAL DOUBLE-SLIP-ORIENTED Cu SINGLE CRYSTALS
    Weiwei GUO,Chengjun QI,Xiaowu LI
    Acta Metall Sin, 2016, 52 (6): 761-768.  DOI: 10.11900/0412.1961.2015.00572
    Abstract   HTML   PDF (1137KB) ( 317 )

    It is well known that the cyclic deformation behavior and dislocation structures of Cu single crystals with different orientations have been systematically investigated and understood. However, there is as yet no general and unequivocal knowledge of the thermal stability of fatigue-induced dislocation structures in Cu single crystals, which is particularly significant for the further improvement of low energy dislocation structure (LEDS) theory. In previous work, the thermal stability of fatigue dislocation structures in 18 41] single-slip and coplanar double-slip Cu single crystals have been reported. For deeply understanding the orientation-dependent thermal stability of fatigue dislocation structures, in the present work, conjugate and [017] critical double-slip-oriented Cu single crystals were cyclically deformed at different plastic strain amplitudes γpl up to saturation, and then annealed at different temperatures (300, 500 and 800 ℃) for 30 min, to examine the thermal stability of various fatigue-induced dislocation structures. It was found that an obvious recovery has occurred in various dislocation structures at 300 ℃. At the higher temperatures, e.g., 500 and 800 ℃, a remarkable recrystallization phenomenon takes place together with the formation of many annealing twins. The thermal stability of various dislocation structures produced in fatigued Cu single crystals with different orientations from high to low are on the order of vein structure, persistent slip band (PSB) structure, labyrinth structure and dislocation cells. The annealing twins formed in Cu single crystals with different orientations all develop strictly along the dislocation slip planes, which have been operated under fatigue deformation. The more serious the fatigue-induced slip deformation, the greater the amount of annealing twins would be. Furthermore, an over high annealing temperature, e.g. 800 ℃, would greatly speed up the migration of boundaries of recrystallized grains to restrain the formation of annealing twins, leading to, more or less, the decrease in the amount of twins.

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