Welding overlayers of three welding wires containing various concentrations of Mo have been fabricated as the experimental materials via multiple semiautomatic gas tungsten arc welding (GTAW) with cold-wire feed. Then post-weld heat treatment is carried out at 620℃ for 29 h to reduce the welding residual stress. The corrosion resistance of the as-weld and heat-treated Ni-based weld metals is assessed in 65% nitric acid aqueous solution at 117℃ for 48 h. The results show that weld metals were suffered from several types of localized corrosion in the test medium, such as intergranular corrosion (IGC), pitting corrosion and interdendritic corrosion (IDC). Mo can promote the precipitation of Laves phase in the interdendritic region. Due to the electrochemical difference between the Laves phases and the matrix, pitting susceptibility of the Ni-based weld metals increase with the increase of Mo content. Further, the IDC takes place in the heat-treated weld metals. The electrochemical difference between the dendrite and the interdendritic region is the key factor for IDC. Mo can influence the diffusion of Ni and Cr during the post heat treatment and decrease the depletion degree of Cr and Ni in the interdendritic zone, then the degree of IDC drops for the heat-treated weld metals with addition of Mo.
Powders of LaMgAl11O19 (lanthanum magnesium hexaaluminate) were synthesized via a two-step process, i.e. chemical co-deposition for precursor powders and then high-temperature calcination for final products. The quality of the precursor powders could be improved significantly by proper adjusting the co-deposition parameters such as increasing the deposition temperature and pH value. The formation temperature of magnetoplumbite-phase, the crystallinity and grain size of the prepared powders were characterized by differential thermal analysis and X-ray diffraction. The morphology of powders calcined at 1500℃ for 5 hours for various precursors was examined by scanning electron microscope, while their grain size distribution was inspected by Malvern ZEN3600 and Manual measurement. The feasibility of preparation of various magnesium hexaaluminate was tried by replacing La2O3 with Nd2O3, Gd2O3 or Sm2O3 respectively. The results show that the precursor powders, co-deposited from solution with pH=11.5 at 60℃, could be transformed into powders of plain LaMgAl11O19-phase after calcination at 1440℃, which was 150℃ lower than those co-deposited at room temperature. The powders calcined at 1500℃ for 5 hours were nano-sized, while rising the deposition temperature and pH value may be beneficial to decrease the grain size, therewith decrease the thermal conductivity of powders. Besides the grain size of NdMgAl11O19 powders prepared with the same process parameters was slightly larger than that of LaMgAl11O19 powders.
The effect of vanadium (V) addition on the tensile- and stress rupture-property at room temperature and 750℃, and the microstructure evolution of GH4061 alloy, including grain size, carbide and γ′/γ'' phase etc. were investigated by means of tensile- and stress rupture-testing, OM, SEM, TEM and electrochemical extraction and phase analysis of precipitates. The results show that V addition can promote the precipitation of MC-type carbide and γ′/γ'' phase, and refine the grain size slightly. With the increase of V content, the lattice constant of γ matrix increases, the mismatch between γ matrix and γ′ phase decreases, and the coarsening of γ′/γ'' phase at 750℃ is restrained. The addition of V does not have significant influence on room temperature tensile property, but apparently benefits the tensile strength and rupture life at 750℃. Furthermore, the GH4061 alloy with 0.4% (mass fraction) V addition shows the best stress rupture property at 750℃.
The effect of the degree and temperature of pre-deformation on the deformation mechanism and subsequent recrystallization behavior of a high-entropy alloy CoCrFeMnNi, as well as, its microstructural evolution during deformation and post annealing treatments were investigated by using electron backscatter diffraction equipped in field emission gun scanning electron microscope. Results show that under low strain conditions, the effect of temperature on the microstructure of deformed alloy is not obvious, and the deformation mechanism is dominated mainly by dislocation slip. Moreover, at room temperature, with the increasing strain the deformation mechanisms dominated by dislocation slip and deformation twinning. In addition, in the condition of low temperature annealing, the effect of pre-deformation degree on recrystallization is not obvious, implying that the recrystallization is not easy to initiate. However, under the condition of high temperature annealing, both the refinement degree of recrystallization grains and the percentage of ∑3 boundaries increase with the increasing pre-deformation degree.
The influence of a novel heat treatment process for reducing residual stress on the microstructure evolution and mechanical properties of Al-Cu-Mg alloy was investigated by means of transmission electron microscope, scanning electron microscope, X-ray diffractometer and tensile test. The results show that the residual stress reduction rate of Al-Cu-Mg alloy (compared with the solid solution treated one) reaches 92.7% by the novel heat treatment, while an excellent combination of strength and plasticity was acquired. As a result, the yield strength, ultimate tensile strength and the elongation rate of the alloy can reach 463.6 MPa, 502.5 MPa and 12.7% respectively. TEM observations reveal that the S′ precipitates are fine and uniformly distributed in the microstructure after the novel heat treatment. The synergistic effect of the coherency stress field produced by these S' phases and the quenching residual stress field may result in a significant reduction of the residual stress, which gives rise to the high comprehensive properties of Al-Cu-Mg alloy.
The effect of solution treatment temperature (STT) (ranging from 935°C to 995°C) on the microstructure evolution of α"-phase, α'-phase and microhardness of TC11 titanium alloy were investigated systematically by means of optical microscope, electron microscope with energy dispersive spectroscope, X-ray diffractometer and microhardness tester. Results show that the crystal structure of α"-phase gradually correspond to the crystal structure of the α' phase, and the phase composition of TC11 alloy changes with increasing STT (α+α", α+α"+α', α+α', α'). Microhardness of the alloy solution treated in the temperature range from 935~985°C increased with the solution temperature, whereas the microhardness reduced for further increase of solution temperature up to the range of 985~995°C. Microstructural features resulting from different STTs were correlated with corresponding microhardness values. With the increment of STT the microhardness increased, because the thickness and spacing of α'-lamellae increased slowly and the β-transformed structure grew up slowly. Besides, due to phase transition strengthening (PTS) the α"-phase and α'-phase are precipitated in the β-transformed structure, and the α'-lamellae contents in the β-transformed microstructure increased, eventually reaching a maximum at 985°C. Above 985°C the microhardness decreased, because the thickness and spacing of α'-lamellae increased significantly and the β-transformed structure became coarser at the expense of α'-phase and α"-phase contents.
Superplastic performance of the submerged friction stir processed Mg-Y-Nd alloy was assessed by initial strain rates in range of 2×10-2 to 4×10-4 s-1 at temperatures in range of 683 to 758 K, aiming to reveal the correlation of the microstructure evolution and the superplastic performance of the alloy. Results show that due to the fine-grained and stable microstructure, the alloy exhibits the maximum elongation of 967% by strain rate of 3×10-3 s-1 at 733 K, and the excellent high strain rate superplasticity of 900% by 2×10-2 s-1 at 758 K respectively. The average size of α-Mg grains and secondary phase particles remarkably increased when the alloy subjected to high temperature tensile tests for long time, as a result, the elongation of the alloy significantly decreased. Cavities easily formed at grain boundaries instead of the interface of secondary particles and matrix, which may be responsible to the good deformation compatibility between particles and matrix.
Technora fiber was surface modified by plasma treatment, and then characterized by means of scanning electron microscope and single fiber tensile strength tester in terms of the fiber surface morphologies and the properties of fiber itself, respectively. Then the influence of plasma treatment on the interfacial property of Technora fiber/epoxy resin in their composites in both room temperature dry (RTD) and elevated temperature wet (ETW) conditions was assessed based on the relevant values of interlamilar shear strength and water absorption. The results show that the plasma treatment had a great influence on the surface morphology of Technora fiber. Therefore, the interlamilar shear strength of Technora/Epoxy composites with the surface modified fiber is 24.93 MPa, which increases by 58.4% in comparison to 15.74 MPa for the composite with the as received fiber, correspondingly, the ability of water absorption of the composite decreased. However, the surface modification exhibits little effect on the property of the fiber itself. It is concluded that the surface plasma treatment of the Technora fiber is a significantly favored means for the enhancement of the interfacial performance of Technora fiber/epoxy resin within their composites.
Silicon oxide films were prepared on silicon- and quartz-substrate by plasma enhanced chemical vapor deposition (PECVD) technique. The dependence of composition, structure and properties of the films were investigated on the location of substrates in the reaction chamber, namely, which were fixed onto either cathode- or anode-electrode plate. Meanwhile, the preparation of functional decorative silicon oxide films with high transparency and scratch resistance was assessed in terms of processing parameters. The results show that the film synthesized on the substrate attached to anode is organosilicon oxide of Si (CH3)nO with transmittance of as high as 90%~98% in the wavelength range of 380-780 nm, unfortunately, the film is loose with hardness of only 2 GPa. However, the hardness of the film can be increased to 6 GPa by increasing the substrate temperature, as a result, the transmittance of the film decreases slightly; The film synthesized on the substrate attached to the cathode composes of inorganic silicon oxide and amorphous carbon. That film is compact with hardness of up to 15 GPa, but poor transmittance in the wavelength range of 380~780 nm. Increasing the O2-flux can promote the reaction of carbon and oxygen to produce carbon dioxide, thereby to eliminate the amorphous carbon. Therefore, the transmittance of the film increases to 99%, but the hardness decreases down to 9 GPa.
FeSiAl alloy flakes were coated with glass powder D250 of low melting point via a two-step process, i.e. FeSiAl alloy flakes and glass powders D250 were firstly blended by ball milling to prepare D250 powders covered FeSiAl alloy flakes and then which was heat treated at proper elevated temperature. The phase composition, surface topography and composition of the coated flakes were characterized by using XRD, SEM, XRF and EDS. The electromagnetic parameter and reflection loss were also assessed in the frequency range of 1~18 GHz by using vector network analyzer. The results show that a dense coating formed on the flakes with uniform appearance, which present a lower real part of their complex permittivity of about 8. The glass D250 coated FeSiAl alloy flakes with optimal performance could be acquired when the heat treatment process was conducted at 700℃, namely the maximum reflection loss depresses to -40.10 dB, and the effective bandwidth reaches to 3.76 GHz, respectively.
