The effect of heating history, such as those related with standard heat treatment, hot isostatic pressing (HIP), coating and brazing thermal processes, on the microstructure and creep properties (at 760 oC/790 MPa and 980 oC/248 MPa) of a first generation nickel-based single crystal superalloy DD413 was investigated. The results show that the microstructure and creep behavior of nickel-based single crystal superalloy DD413 are significantly affected by different thermal processes. Compared with the standard heat treatment, HIP reduces the size of γʹ and γ channel, but increases the volume fraction of γʹ phase. Meanwhile, micro-pores in the alloy are effectively eliminated by HIP, which improves the creep life. The coating thermal process preparation can coarsen the γʹ and γ channel, reduce the volume fraction of γʹ phase, and delay the rafting kinetics of γʹ phase at high temperature, therewith, reduce the creep property of the alloy. The brazing thermal process not only causes the coarsening of γ′ phase but also destroys the cubic degree of γ′ phase, leading to significant reduction in creep property of the alloy.
A Ni-based alloy Ni-(12~18) Cr, (10~14) Co, (2~6) W, (2~6) Mo, (2~6) Ti, (2~6) Al was prepared via hot isostatic pressing by 150 MPa at 1150 oC with its argon gas atomization powders as raw material. Then its oxidation behavior in air at 950 oC was studied by intermittent weighing method in terms of the oxidation kinetics, phase composition, microstructure and elemental distribution of oxide scale and characteristics of oxygen-affected zone etc. The results show that the oxidation resistance of the as-HIPed superalloy at 950 oC has reached the full oxidation resistance grade in accord with the national standards. Both the oxidation kinetics curve and the depth variation curve of oxygen-affected zone follow a parabolic law. In the early stage of oxidation, Ti and Al atoms in the matrix diffuse rapidly along the prior particle boundaries (PPB) towards the alloy surface and form a continuous mixed oxide layer of Cr2O3, TiO2 and Al2O3. However, oxygen atoms diffuse inward along the PPB and undergoes in-situ oxidation reaction with secondary phases to form granular internal oxide Al2O3. After 24 hours oxidation, the surface oxide scale consists of the outmost layer of mixed oxides TiO2 and Al2O3, and the inner layer of Cr2O3 oxide. The continuous internal oxides TiO2 and Al2O3 appear in Cr-depleted zone. In the later stage of oxidation (> 100 h), the outmost TiO2 oxide layer peels off from the alloy surface, and the internal oxides TiO2 and Al2O3 distributed in Cr-depleted zone in a continuous root-like manner. Finally, the high temperature oxidation mechanism and formation process of internal oxides in as-HIPed nickel-based superalloy was also discussed.
Domestically produced Aramid fiber III are extensively utilized in aerospace and other military industries on account of their advantages like high specific strength and high specific modulus. Nevertheless, the drawbacks of its smooth surface, scarcity of active groups, and poor bonding performance with the resin matrix restrict the outstanding performance of its composite materials. In view of the above shortcomings, the surface of AF III was modified via argon plasma, and then monofilament composites of epoxy resin with the untreated and argon plasma treated aramid fiber III was fabricated respectively. The influence of argon plasma treatment time on the surface composition, surface morphology, surface wetting properties, monofilament tensile strength of the fiber and the interfacial bonding strength of composite material were investigated respectively by X-ray photoelectron spectroscopy (XPS), Scanning Electron Microscopy (SEM), Optical Microscopy (OM), Atomic Force Microscopy (AFM), Dynamic Contact Angle Analyzer (DCAA), Monofilament Tensile Strength and Micro-droplet Debonding Test, as well as interface strength test. The structural dissociation energy of the plasma-treated fiber was calculated using Materials studio (MS) software. The results indicated that after plasma treatment for 5 min~30 min, new groups (―C―O―, O＝C―O, ―NH2) were introduced on the fiber surface; while the fiber surface roughness increased from 134 nm untreated to 214 nm; and the fiber surface wettability property was enhanced from 46.14 mJ/m2 for the bare fiber to 68.52 mJ/m2 for the argon plasma treated one, representing an increase of 48.44%. The surface of plasma treated fibers showed uneven morphology and changed periodically with the extension of treatment time; the strength of fiber monofilament decreased gradually with the increasing plasma treatment time. Besides the results of the microdroplet debonding test demonstrated that, after plasma treatment for 10 min, the interfacial shear strength (IFSS) of AF III/epoxy was increased from 28.51 MPa for the untreated fiber to 38.02 MPa for the treated one, which was improved by 33.36%.
A typical γʹ-strengthened Co-based superalloy was focused on in this research, and various characterization methods were employed to investigate the microstructure of the as-cast alloy. Subsequently, a heat treatment regime was developed based on the characteristics of the as-cast microstructure, and the effect of heat treatment on the microstructure and room temperature/high-temperature tensile properties of the alloy was assessed. The results showed that: the microstructure of the as-cast alloy consisted of γ matrix, γʹ phases, MC carbides, γ/γʹ eutectic and M3B2 borides. After solution and aging heat treatments, the γ/γʹ eutectic disappeared, and the M3B2 borides were mostly dissolved, with uniformly sized γ′ phases precipitating in the matrix. In comparison to the as-cast alloy, the room temperature tensile strength of the heat-treated alloy decreased, but plasticity slightly improved. Tensile tensile at 950 °C showed that the strength and plasticity of the heat-treated alloy were improved to some extent. Both the as-cast and heat-treated alloys exhibited a characteristic of cleavage fracture during room temperature tensile testing, while the fracture mechanism under high-temperature tensile conditions was a ductile fracture of the micropore aggregation type.
316 stainless steel is widely utilized due to its exceptional corrosion resistance and processibility. However, the broad composition range specified by the industrial standards can lead to obvious property variations. In this study, we first classify the solute elements of substitutional type into Cr-like ferrite stabilizer (Cr, Si, Mo) and Ni-like austenite stabilizer (Ni, Mn). Then we employ the “cluster-plus-glue-atom” model to obtain its composition unit, which contains 16 atoms. Accordingly, the GB standard is interpreted as being enclosed by five 16-atom formulas, corresponding respectively to the lower and upper limits of Cr- and Ni-like elements (Cr, Si, Mo)3.0625, 3.5-(Ni, Mn)1.75, 2.25-Febal and the mid-value (Cr, Si, Mo)3.25-(Ni, Mn)2-Febal. According to the above classification, five kinds of test steels were designed and melted by Ar-atmosphere arc furnace. Then in vacuum heat furnaces, they were homogenized (1150 oC/2 h/furnace cooling), cold-rolled (50% deformation) to 5mm sheets, and finally solutioned (1050 oC/0.5 h/water quenching). The steels containing the lowest Ni-like content of 1.75 in the 16-atom formulas (10.9%, atom fraction), form ferrite in austenite matrix, corresponding to the composition range of (21.2~18.5) (Cr, Si, Mo)-11.4(Ni, Mn)-Fe (%, mass fraction). The other three test steels consist of only single austenite phase. The average hardness value after rolling and solutioning is 160HV approximately, satisfying the GB requirement (< 200HV). The electrochemical tests in 3.5% NaCl solution demonstrates that the steel Cr3.5-Ni2.25-Febal (Fe-17.8Cr-0.6Si-2.7Mo-14.0Ni-0.8Mn-0.021C), with the highest Cr-like element content, possesses the best corrosion resistance, next to it are alloys Cr3.0625-Ni2.25-Febal and Cr3.25-Ni2-Febal, containing Ni-like element above 2 in the formulas, covering a composition range of (18.4~21.1) (Cr, Si, Mo)-(14.7~13.0) (Ni, Mn)-Fe (%). The steels containing the highest Cr-like contents of 3.5 show the best pitting corrosion potential 0.211 V. It follows that the steel with proper amounts of alloying elements Cr3.25-Ni2-Febal (Fe-16.7Cr-0.4Si-2.7Mo-11.9Ni-1.2Mn-0.021C), falling in the middle of the formula zone, can not only form single-phase austenite, but also meet the standard requirements of Vickers hardness (~160HV), while its corrosion resistance is also high (free-corrosion potential -0.082 V, corrosion current density 1.83 × 10-6 A·cm-2, PREN 25.6, and pitting corrosion potential 0.19 V).
The influence of Fe- and Cu-content on the microstructure and mechanical properties of TC10 alloy bars was investigated by means of scanning electron microscopy, EDS analysis of transmission electron microscopy, and mechanical testing machine. The results indicate that for the as rolled and annealed TC10 Ti-alloys, with the increasing Fe- and Cu-content, their yield strength and tensile strength increase, while the changes in elongation and cross-sectional shrinkage at break are not significant. For the solid solution aged TC10 alloy, Ti is evenly distributed in both βt and αp phases, Al is less distributed at grain boundaries, while V, Fe, and Cu elements are more abundant at the grain boundaries in βt phase. For TC10 alloys with the same Fe- and Cu-content, with the increasing solid solution temperature, their yield strength and tensile strength increase, while the elongation and cross-sectional shrinkage decrease. For TC10 alloys subjected to solid solution treatment at the same temperature, with the increase of Fe and Cu content, their yield strength and tensile strength increase, while the elongation and cross-sectional shrinkage at break decrease. The higher the solid solution temperature, the more significant the influence of alloying elements Fe and Cu on the strength and plasticity of the alloy. Being subjected to solid solution treatment at 900 oC, the alloy with 0.65% Fe and 0.65% Cu can achieve superior comprehensive mechanical properties, with yield strength, tensile strength, elongation, and cross-sectional shrinkage at break of 1392 ± 3 MPa, 1435.5 ± 0.5 MPa, (8 ± 1)% and (21.5 ± 1.5)%, respectively.
Surface modification treatment of 7Cr7Mo2V2Si steel was conducted by using plasma nitriding, and the effect of post diffusion treatment on its microstructure, hardness, and friction and wear properties was investigated. The results showed that the nitriding layer on 7Cr7Mo2V2Si steel generated by plasma nitriding at 520 oC of about 186 μm in thickness, presented a microstructure with abundant vein-like network along grain boundaries. After diffusion treatment at 550 oC, the microstructure of vein-like network in the nitriding layer was gradually shrinked or disappeared, while the thickness of nitriding layer increased. The thickness of the nitriding layer was increased by 15.59%, 24.73% and 36.02% respectively, corresponding to the post diffusion time for 10 h, 20 h and 30 h. After post-diffusion treatment, tempered martensite was formed in the steel, which prevents significantly grain slip and plastic deformation, and improves the strength and toughness of the steel. The average hardness of the bare steel was 674.8HV, while the highest hardness of 1276.4HV was for the nitriding layer. Plasma nitriding can significantly improve the wear resistance of 7Cr7Mo2V2Si steel. A large number of plowing grooves and spalling pits appeared on the wear surface of the steel without nitriding treatment, while after plasma nitriding treatment, the wear degree is reduced, the plowing grooves became shallower, and the spalling pits also reduced. Compared with the bare 7Cr7Mo2V2Si steel, the wear loss amount was reduced by 59.48% for the steel being plasma nitride at 520 oC, and the friction coefficient was reduced by 26.63%. After plasma nitriding and post diffusion treatment for different time, the hardness of 7Cr7Mo2V2Si steel was decreased. Compared to the as nitriding steel, the maximum surface hardness decreased from 1276.4HV to 881.5HV for the steel subjected to plasma nitriding plus post diffusion treatment for 30 h, and the average friction coefficient and the wear loss amount decreased from 0.4731 and 4.7 mg to 0.5939 and 9.3 mg respectively.
In order to solve the problems of low efficiency and high cost of catalyst in the treatment of phenol-containing wastewater by traditional photocatalytic technology, a new composite material of α-Fe2O3/TiO2 was prepared by high temperature calcination. The prepared materials were characterized by X-ray diffractometer and other instruments, and their photocatalytic degradation of phenol was studied under the irradiation of 500W high voltage mercury lamp. The results of XRD and SEM showed that the composite material was successfully prepared, and the nano-α-Fe2O3 was attached to the nano-TiO2 to form a heterostructure. FT-IR and UV-Vis detection showed that the composite material had good photocatalytic performance, its visible light absorption range was obviously widened, and the band gap was narrowed (2.02 eV to 1.81 eV). The specific surface area and pore volume were calculated to be 66.4725 m2/g and 0.3213 m3/g by BET test, which were much higher than that of single catalyst, further indicating that the photocatalytic performance of the composite was better than that of α-Fe2O3. In addition, the factors such as the composite ratio of α-Fe2O3 to TiO2, the dosage of composite material, the concentration of phenol, pH and illumination time were optimized. The results show that the degradation efficiency of phenol can reach 94.79% in case that the test solution with phenol concentration of 20 mg/L and pH = 8, a dosage 0.6 g/L of the composite material with a mass ratio 1∶10 for α-Fe2O3 to TiO2 is adopted under the illumination of high voltage mercury lamp for 120 min. Which is much higher than that of single material.
