Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-31 Cooperative journals: 《中国腐蚀与防护学报》
Abstract： In fact, the metallic material corrosion process may intrinsically involve subprocesses such as the exchange of valence electron between metallic atoms and ions in corrosive medium, the formation of oxide scale, the migration of corrosive species through the formed oxide scale, and the interaction between oxidation scale and corrosive medium. Therefore, the energy state of the valence electron of components of metallic material may play an important role in the corrosion process, thus for reveal which, ultraviolet photoelectron spectroscope (UPS) and X-ray photoelectron spectroscope (XPS) may become useful tool. Herewith new progress in this respect is subsequently introduced. The valence electron energy state of components for bulk nanocrystalline materials 304 stainless steel (BN-SS304), industrial pure aluminum (BN- Al) and ingot iron (BNII), as well as their counterparts of conventional microcrystalline ones (CP-SS304), (CP-Al) and (CPII) was characterized by UPS at room temperature. The valence electron energy state of components of the oxide scale formed on these metallic materials due to corrosion in hydrochloric acid solutions, and the oxide scales formed on BN-SS304 and CP-SS304 due to air oxidation at 900 ℃ were comparatively studied by XPS. The above aquired results may enable one to establish the relationship between the corrosion performance with the valence electron energy state of components of these metallic materials, and to figure the electron structure of components of the corresponding formed oxide scales, as well. Furthermore, a new concept of intrinsic parameter related with metallic material corrosion was proposed.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-31 Cooperative journals: 《中国腐蚀与防护学报》
Abstract： Three high- entropy alloys Co1.5CrFeNi1.5Ti0.5Mo0.1, AlCoCrFeNiSi0.1 and TaNbHfZrTi were prepared by arc melting. Their phase structure, microstructure and chemical composition were studied by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). A commercial alloy 690TT was used as the contrast material. The repassivation kinetics of the three high entropy alloys in high temperature pressurized water was investigated by means of electrochemical test. The results show that Co1.5CrFeNi1.5Ti0.5Mo0.1, AlCoCrFeNiSi0.1 and TaNbHfZrTi high entropy alloys are all composed of single phase. The crystallographic structure of Co1.5CrFeNi1.5Ti0.5Mo0.1 alloy is fcc, while that of the alloys AlCoCrFeNiSi0.1 and TaNbHfZrTi is bcc. The SEM results show that the Co1.5CrFeNi1.5Ti0.5Mo0.1 alloy showed a typical dendritic microstructure, of which the dendrite riches in Cr and Fe, but the interdendrite zone riches in Ti and Ni. There is no obvious element segregation observed in the AlCoCrFeNiSi0.1 alloy. The TaNbHfZrTi alloy also exhibited a dendritic microstucture, of which the dendrite riches in Ta and Nb, and the interdendrite zone riches in Hf, Zr and Ti. The repassivation rates of these four alloys in high-temperature pressurized water may be ranked as the following sequence: TaNbHfZrTi＞Co1.5CrFeNi1.5Ti0.5Mo0.1＞690TT＞AlCoCrFeNiSi0.1.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-31 Cooperative journals: 《腐蚀科学与防护技术》
Abstract： Flakes fell from the tooth surface of the small gear in the gearbox of a 1.5 MW wind turbine in service. The design lifetime of the gear is 20 a. However, it failed 5 a later since it was installed into the gearbox. The material is 20CrNiMo and the gear was carburized and quenched during the manufacturing process. Macro- and micro- observation of the fracture surface as well as some necessary physical and chemical analysis were applied to analyze the cause for the failure of the small gear. Besides, results from the macro-observation showed that only two teeth failed, on the one of which flakes fell from the tooth surface only left some pits, and on the other tooth surface some cracks can be observed obviously. The results also showed that pits and cracks could be attributed to the same reason. Tool marks on the tooth surface increases the risk of cracking. In addition, the abnormal load introduced by emergency braking led to cracks occurred along the machining marks on the tooth surface. Then, fatigue cracks nucleated at the tip of the crack and extend slowly driven by the working load. Finally, the hardened layer fell from the tooth surface and left some pits. It was suggested that the machining quality and the braking technology should be improved in order to avoid the cracking of the teeth surface under the abnormal impact load.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-31 Cooperative journals: 《材料研究学报》
Abstract： The evolution of the secondary phase in G18CrMo2-6 heat-resistant steel induced by tempering at 680℃ for a series of durations was investigated by optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive X-ray detector (EDX). It demonstrates that martensite/austenite (M/A) particles and M3C precipitate on bainite matrix after normalizing. During the tempering, decomposition of M/A particles into the M3C carbides in bainite matrix can be found. The increasing of tempering time results in the precipitation of MC, the spheroidization and refinement of M3C carbide as well as the precipitation and coarsening of M23C6 carbide at the grain boundaries.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-31 Cooperative journals: 《材料研究学报》
Abstract： A gradient and nanostructured (GNS) surface layer was formed on a 316L stainless steel sample by using surface mechanical rolling treatment (SMRT). The effect of SMRT on the evolution of phase composition and microstructure was studied of the GNS surface layer. The results show that deformation- induced martensite transformation occurs in the surface layer after SMRT, and the martensite amount increases with the increasing penetration depth of SMRT. The microstructural refinement mechanism includes subsequently the formation and interaction of various dislocations, deformation twinning, deformation- induced martensite transformation, and martensite refinement. Finally, nanostructure with mostly martensite and a mean grain size of ca. 55 nm was achieved in the topmost surface layer of the 316L sample.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： The directionally solidificated (DS) Co-based superalloys are widely used in aircraft turbine vanes due to the good stress-rupture parameters and excellent hot corrosion resistance. The cyclic change of temperatures and complex stress state thermal fatigue (TF) cracks happen frequently in vanes during service. However, most of the work are conducted in Ni- based superalloys and there is rare report concerning the TF behavior of DS Cobased superalloys. Furthermore, due to the residual strain accumulated during processing, shot peening, grinding and recrystallization (RX) frequently occur when the DS components are exposed to high temperatures. It is believed that RX may change the microstructure, especially adding more grain boundaries to DS alloys, and result in the reduction of the mechanical properties of DS superalloys. Therefore, in this work, V-notch plate specimens with notch direction perpendicular and parallel to the DS orientation are machined from the DS plate. Local RX grains are prepared (local indented and then heat treated) in the notch areas of some samples. TF test is conducted between 1000 ℃ to room temperature. The effect of DS orientation and RX on TF properties of a DS Co-based superalloy is investigated. The results indicate that the cracks propagate along the interdendritic regions in the samples with notches parallel to the DS direction, which exhibites lower TF properties than samples with notches vertical to the DS direction. TF cracks initiate and propagate along RX boundaries in samples containing RX grains. Precipitation of M23C6 carbides is found along the RX boundaries during TF tests. Due to the oxidation at the tip of crack, M23C6 desquamates and leads to the formation of micro voids, which accelerates the crack propagation and decreases TF properties of tested alloy. In samples with notches parallel to the DS direction, cracks preferentially propagate along the RX grain boundaries.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： A gradient nanostructured (GNS) surface layer was fabricated on a Z5CND16-4 martensitic stainless steel by means of surface mechanical rolling treatment (SMRT). The microstructure in the GNS surface layer was characterized by using SEM and TEM. The results showed that the mean grain size increases with depth, from about 25 nm at the topmost surface layer to the initial value in the matrix. The total thickness of the grain-refined layer is about 150 mm. The electrochemical corrosion property of the SMRT sample was compared with that of the as-received sample in a 3.5%NaCl aqueous solution. It is shown that the pitting corrosion potential increases from about 0.179 V in the as-received sample to about 0.313 V in the SMRT sample, and the self-corrosion potential also increases evidently. The formation of nanostructures, the increased structural homogeneity, and the introduction of compressive residual stresses in the GNS surface layer, as well as the decreased surface roughness, were discussed to promote the pitting corrosion resistance of the SMRT sample.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Ni- based single crystal superalloys have been widely used for blades and vanes in gas turbine. However, recrystallization (RX) induced by residual strain has been a serious problem for the application of single crystal superalloys. In previous work, effect of microstructure, such as g', g/g' eutectics and carbides, as well as heat treatment parameters, on the RX behavior have been studied. However, the effect of alloy elements on the RX behavior has rarely been reported. Therefore, in this work, the effect of the important solution strengthening elements, W and Re, on the deformation and RX of solution heat treated Ni-based single crystal superalloys was investigated. At first, two single crystal superalloys were prepared, and W and Re were added into one alloy among them. After solution heat treatment, these two single crystal superalloys were deformed by shot-peening or Brinell indentation. Then these deformed samples were heat treated to observe the microstructure of RX. It indicated that RX depth decreased with the addition of W and Re irrespective of deformation mode and heat treatment temperature. Short time heat treatment experiment of indented and shot-peened samples both indicated that incubation period of RX was prolonged and nucleation of RX was slowed with the addition of W and Re, which verified that RX was suppressed by W and Re. After shot-peening, micro-hardness of the alloy with W and Re increased, but the depth of deformation zone was obviously reduced. Higher density of dislocation was found in the single crystal superalloy with W and Re, and also lots of dislocation tangles were observed. So, in this alloy, dislocation annihilated slowly, that is, recovery was slowed down, which prolonged the incubation period of RX. During the process of RX grain growth, the maximum RX grain boundary migration velocity was reduced with the addition of W and Re. Moreover, the change of mean RX grain boundary migration velocity showed the same trend with the micro-hardness along the direction of RX depth.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Amorphous alloy is a new type of material that exhibits exceptional properties or combinations of properties that are often not achievable in conventional crystalline materials. Fe-based amorphous alloys has attracted significant attention over the last few decades because of their low cost and enhanced mechanical performance. However, they are more suitable for the industrial application of coatings due to the fatal disadvantages of poor toughness. High velocity oxygen-fuel (HVOF) spraying is a good way to make amorphous alloy coatings (AMCs), for the individual droplets are cooled at a rate of around 107 K/s which is much higher than the critical cooling rate of the amorphous alloys during the thermal spraying. Fe-based AMCs obtained by using the HVOF spray method are important materials for industrial applications because of high glass- forming ability and exceptional performances, such as excellent corrosion resistance, high hardness, and superior wear resistance. In this work, Fe-CrMoMnWBCSi AMCs were prepared by HVOF thermal spray. The microstructure and amorphous characteristics of AMCs were characterized by SEM and XRD. Electrochemical corrosion behavior of AMCs was investigated in different concentration of NaCl and H2SO4 solutions compared with that of 304 stainless steel and ND steel. The surface film of materials after immersed in two solutions was analysed by XPS. The results indicated that HVOF thermal spraying Fe-based AMCs presented dense layered structure, high amorphous phase content and low porosity. The composite structure of AMCs was formed with some nanocrystallite phases embedded in the amorphous matrix. AMCs exhibited better resistance to pitting corrosion and relatively low uniform corrosion resistance due to the porosity, while the pitting potential of 304 stainless steel was sensitive to NaCl concentration. XPS results revealed that the presence of Cr, Mo and W oxides in the passive film of AMCs may result in the better corrosion resistance. The enrichment of Mo4+ oxides on the surface favored the formation of a more stable and protective layer which could be assumed to be responsible for the observed high stability of passive film. The diminishing or avording pores may be beneficial to further improve the pitting corrosion resistance of AMCs in NaCl solution. In all cases, AMCs showed better resistance to H2SO4 solutions corrosion due to the high stability of passive film. 304 stainless steel and ND steel presented stable passivation behavior only in high concentration of H2SO4 solution. In the lower concentration solution of H2SO4, the amorphous structure of the thinner coatings could facilitate the formation of thicker passivation film and lead to the higher corrosion resistance. The corrosion resistance of AMCs in H2SO4 solution could be enhanced significantly by formation of high amorphous phase.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Advanced aero and power generation industry needs high-performance gas turbine. As key parts of gas turbine directionally solidified (DS) columnar grain and single crystal (SX) blades operate in heavy stress and high temperature conditions. The continuous demand for increasing turbine inlet temperature and aggressive environment has pushed alloy designers to develop DS and SX Ni-based blade alloys that contain high amount of alloying elements. DS process of blades using such alloys has become a challenging task. The small DS and SX blades are usually produced by high rate solidification (HRS) process. However, the growth of large DS and SX blades requires directional solidification with a sustained thermal gradient along the DS direction. By increasing the thermal gradient, the dendrites are refined, which results in a mechanically-superior DS and SX with reduced defects. One method to achieve consistent and higher thermal gradients is the utilization of the liquid metal cooling (LMC) process. In this method, heat extraction from the outer surface of the mold during DS relies on heat conduction rather than radiation in the conventional HRS process. The optimization of the LMC process is difficult and costly by experimental methods, especially for the complexly shaped industry gas turbine (IGT) blades because of the complicated process parameters associated with the technique. Numerical simulation is an efficient method to solve this problem. In this work, directionally solidified industry gas turbine hollow blades were prepared by high gradient LMC process. Liquid Sn was used as cooling medium. The temperature fields, macrostructures, primary dendrite arm spacing (PDAS) at various withdrawal rates during LMC process have been calculated with ProCAST software. The impact of withdrawal rate on formation of stray grains and freckles was predicted. The calculated results and the experimental observations agreed well. The solidification rates and cooling rates were found to increase with the increase of withdrawal rate. The axial thermal gradient was high and stable during the LMC process. It was found that stray grains would not block the growth of original grains at optimized withdrawal rate. No freckles were observed in the industry gas turbine hollow blades prepared by LMC technique due to the high cooling rate. Though the mean diameters of columnar grains in LMC blades were almost identical to that observed in HRS blades, the PDAS were more than 50% refined in LMC blades than those in HRS blades.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Annealed 7B04 Al sheets in thickness of 2 mm were subjected to friction stir welding (FSW) under three rotation rate and welding speed parameters of 1600 r/min, 200 mm/min; 800 r/min, 200 mm/min and 400 r/min, 400 mm/min, respectively. The effect of welding parameters on the tensile property and microstructure of the FSW joints were investigated, with more efforts focusing on the low-temperature superplasticity of the nugget zones (NZs). The results showed that FSW joints with high quality could be produced by controlling welding parameters, with a joint strength coefficient of 100% being obtained. Dynamic recrystallization took place in the NZs with fine and equiaxed grains generated. The grain size of the base material was about 300 μm, while it was significantly decreased in the NZs with decreasing the rotation rate: about 2, 1 and 0.6 μm for the above three samples, respectively. The fine grain structure of the NZs could facilitate their superplastic deformation. The NZs exhibited superplastic elongations ranged from 160% to 590% at 300 ℃ at strain rates of 1×10-3 and 3×10-4 s-1. The maximum superplasticity of 790% was obtained at 350 ℃ at the strain rate of 1×10-3 s-1. The ability to superplastic deformation disappeared in the NZs at 400 ℃.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： 2.25Cr-1Mo-0.25V steel is the most popular material used for pressure-vessel applied at elevatedtemperature in hydrogen environment. For higher process efficiencies in future coal- conversion plants, chemical processing plants, and petrochemical-refining plants, much thicker cross-section component are necessary for constructing much larger pressure- vessel for these plants. Because of the thick cross- section, the cooling rate in the central region of the component is insufficient to obtain low bainite during quenching treatment, and a large amount of granular bainite appears in the central region. Previous studies have shown that good impact toughness can be achieved by appropriate tempering for 2.25Cr-1Mo-0.25V steel with low bainite microstructure. However, the impact toughness of 2.25Cr-1Mo-0.25V steel with granular bainite after tempering always cannot satisfy the demanding requirement due to the unclear understanding of the evolution of microstructure and mechanical properties during tempering. In this work, the influence of tempering on the microstructure and mechanical properties of 2.25Cr-1Mo-0.25V steel with granular bainite microstructure was investigated by OM, XRD, SEM, TEM and EPMA. The results show that the normalized 2.25Cr-1Mo-0.25V steel with granular bainite microstructure is composed of bainite ferrite and island of martensite and austenite (M-A island). Nanoindentation test indicates that MA island is much harder than that of metrix bainite ferrite, because of the high concentration of carbon in M-A is-lands. The synergistic effect of the decomposition of M-A islands and softening of bainite ferrite determined that Charpy absorbed energy at -18 ℃ increases first and then decreases with the increasement of tempering temperature. The degree of decomposition of M-A islands and the morphology, size and distribution of carbides in granular bainite, coupled with the softening effect of bainite ferrite recrystallization are the key factors determining low-temperature impact toughness of 2.25Cr-1Mo-0.25V steel.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： In this paper, research progresses on gradient nanostructured materials in recent years is briefly reviewed. It includes classification of gradient nanostructures, properties and processing techniques of the gradient nanostructured materials. Perspectives and challenges on scientific understanding and industrial applications of gradient nanostructured materials are addressed.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： In this work, the hot rolling process of SiCp/2009Al composites is simulated using the fully coupled thermal-stress analysis in Abaqus/Explicit. By the investigation of formation process for rolling along with different fields of temperature, strain rate, strain and stress and their evolutionary history, the hot rolling mechanisms under complicated stress states is achieved. The results show that the maximum principal stress changes from compressive stress to tensile stress at the stage of rolling entrance and a reverse trend replaces it at the exit, and that the compressive stress is dominant in the deformation zone at the steady rolling stage. The temperature drop effect due to heat transfer is far greater than the temperature rise effect due to friction on the plate surface while the temperature rise is embodied in the center due to plastic deformation. Besides, the effect of strain rate on flow stress plays a leading role at the entrance and exit stage, and the flow stress on the plate surface in the deformation region is mainly determined by strain and temperature except the stick zone which is controlled by strain rate, however, the center flow stress in deformation is mainly affected by temperature.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Bulk metallic glass (BMG) composites containing B2-CuZr phase are of interest due to they behave large plastic strain and apparent work hardening in tension. Nevertheless till now most BMG composites containing B2-CuZr phase are based on Cu47.5Zr47.5Al5 or Zr48Cu47.5Al4Co0.5 BMG, which has limited glass forming ability (GFA). The prepared sample size is small, which restricts their potential engineering structural applications. In this work, Zr-Cu-Al-Y quaternary system is selected due to its high GFA. By tuning composition close to CuZr alloy in Zr-Cu-Al-Y quaternary system, Zr46.9Cu45.5Al5.6Y2.0 BMG is selected because it has proper GFA (critical diameter Dc=5 mm) and relatively large fracture toughness (KQ=(49±3) MPa·m1/2). By decreasing the cooling rates of the melt via increasing diameter of casting rods, large-sized in situ Zr46.9Cu45.5Al5.6Y2.0 BMG composites containing 13% and 25% volume fractions spherical B2-CuZr phase were prepared in the casting rods with 6 and 7 mm in diameters, respectively. In compression testing, the in situ BMG composites containing 25%B2-CuZr phase promote multiple shear bands within glass matrix and remarkable global plastic deformation, accompanied by a large compressive plastic strain as 6.5%. Nevertheless in tension testing no obvious global ductility was achieved, which attributes to the low mode I fracture toughness and small plastic zone size (RP=88 mm, RP=(1/3π)(KQ/sy)2 ) of glass matrix. Three point bending test results show that Y has an adverse effect on the fracture toughness and plastic zone size of Zr-Cu-Al BMGs. In contrast to Zr46.9Cu45.5Al5.6Y2.0 BMG, fatigue pre-cracked Zr48Cu45Al7 BMG plate samples can be prepared and exhibit a high fracture toughness (KQ=(62±3) MPa·m1/2) and a large plastic zone size (RP=150 mm) in plane strain state. Our results show that GFA and fracture toughness of glass matrix should be balanced when designing new BMG composites containing B2-CuZr phase.
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