Jishan Zhang

Oxidation and hot corrosion behaviors of HVAF-sprayed conventional and nanostructured NiCrC coatings

The oxidation and hot corrosion behaviors of HVAF-sprayed conventional and nanostructured NiCrC coatings were studied. The oxidation experiment was conducted in air, and the hot corrosion was conducted in the Na2SO4-30%K2SO4 environment, in the temperature range of 550-750℃ for periods up to 160 h. The corrosion kinetics was tested with the thermogravimetric method. The corrosion products were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD). As indicated by the results, both types of coatings possess high corrosion resistance, especially the nanostructured NiCrC coating. The enhanced grain boundary diffusion in the nanostructured coating not only promotes the formation of a denser Cr2O3 scale with a higher rate, but also helps to mitigate the Cr depletion at the metal/scale interface. The less porosity of the nanostructured coating is also thought to be beneficial to the anti-corrosion properties.

Nanostructured Al-Zn-Mg-Cu alloy synthesized by cryomilling and spark plasma sintering

Abstract Nanocrystallized Al-10.0%Zn-3.0%Mg-1.8%Cu (mass fraction) alloy powder was prepared by cryomilling, and then the nanostructured powder was consolidated into bulk material by spark plasma sintering (SPS). The microstructural evolution and phase transformation were studied. A supersaturated face-centered cubic solid solution is formed after cryomilling for 10 h, and the average grain size is 28 nm. Two typical nanostructures of the bulk nanostructured alloy are observed: primarily equiaxed grains with size of 150 nm, and occasionally occurring sub-micron grains up to 500 nm. Two types of MgZn 2 particles precipitate during consolidation. One is the sub-micron particles distributed along the boundaries of the powders, and the other is fine particles with size of several nanometers in the matrix, especially at the boundaries of sub-micron grains. These second phase particles can be completely dissolved into matrix by proper solid solution treatment.

Deposition Behavior and Characteristics of Cold-Sprayed Cu-Cr Composite Deposits

In order to explore the feasibility of cold spraying as method to prepare Cu-Cr composite, the deposition behavior was analyzed. The results show that particles parameters, such as morphology, size distribution and deformability, result in different deposition behavior. For Cu, since plastic deformation easily occurs, when a Cu particle impacts Cu substrate, both particle and substrate at the interface undergo extensive deformation, resulting in the greater bond probability. Although the velocity of Cr is higher than that of Cu, Cr particles have lower bond probability because of its hardness and fragility. Cr particles are embedded in the Cu particles. The properties of Cu-15%Cr meet industrial standard, superior to the composite prepared by explosive compaction. The results reveal cold spraying can be considered as a suitable technology for Cu-Cr composite.

Microstructural characteristics and aging response of Zn-containing Al-Mg-Si-Cu alloy

Al-Mg-Si-Cu alloys with and without Zn addition were fabricated by conventional ingot metallurgy method. The microstructures and properties were investigated using optical microscopy (OM), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), tensile test, hardness test, and electrical conductivity measurement. It is found that the as-cast Al-Mg-Si-Cu-Zn alloy is composed of coarse dendritic grains, long needle-like β/δ-AlFeSi white intermetallics, and Chinese script-like α-AlFeSi compounds. During high temperature homogenization treatment, only harmful needle-like β-AlFeSi phase undergoes fragmentation and spheroidizing at its tips, and the destructive needle-like δ-phase does not show any morphological and size changes. Phase transitions from β-AlFeSi to α-AlFeSi and from δ-AlFeSi to β-AlFeSi are also not found. Zn addition improves the aging hardening response during the former aging stage and postpones the peak-aged hardness to a long aging time. In T4 condition, Zn addition does not obviously increase the yield strength and decrease the elongation, but it markedly improves paint-bake hardening response during paint-bake cycle. The addition of 0.5wt% Zn can lead to an increment of 99 MPa in yield strength compared with the value of 69 MPa for the alloy without Zn after paint-bake cycle.

Fabrication of conventional and nanostructured NiCrC coatings via HVAF technique

The conventional and nanostructured NiCrC (with chemical composition of 80%NiCr-20%CrC) coatings with high quality were fabricated via high velocity air-fuel(HVAF) spraying technique. The microstructures of these coatings were characterized by means of metallographic microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffractometry. A Vickers microhardness tester was used to determine the mechanical properties of the as-sprayed coatings. The single-line approximation(SLA) method was employed to calculate the grain size and microstrain of as-sprayed nanostructured coating based on the XRD data. The results show that nanostructured NiCrC coating possesses a more uniform and denser microstructure, much higher microhardness and better fracture toughness than its conventional counterpart. Both TEM observation and calculation results based on XRD profile show that as-sprayed nanostructured NiCrC coating has a homogeneous nanocrystalline microstructure with an average grain size of 40 nm.

Nanocrystalline copper coatings produced by cold spraying

Copper powder was cryomilled for 12 hours to achieve particle size in the range of 2 μm to 25 μm, which powder was subsequently used as feedstock for the deposition of nanocrystalline (nc) Cu coating via cold spraying. The as-milled copper powder was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The particle size of the cryomilled Cu powder was detected by laser scattering. The microstructure of the coating was analyzed using SEM and scanning transmission electron microscopy (STEM). The XRD and HRTEM analysis showed that the grain size of the cryomilled Cu powder was about 5 to 40 nm. This nanoscale structure was retained after the cold spraying. The nanoindentation analysis showed that the nc Cu coating hardness value reached 3.3 GPa, which was higher than that of its coarse grained counterpart.

Synthesis of nanocrystalline NiCrC alloy feedstock powders for thermal spraying by cryogenic ball milling

Abstract Nanocrystalline NiCrC alloy powders with a qualified particle size distribution for thermal spraying were synthesized using the cryogenic ball milling (cryomilling) method. The morphology, microstructure, size distribution, and phase transformation of the powders were characterized by scanning electron microscopy (SEM), laser scattering for particle size analysis, X-ray diffraction (XRD), and transmission electron microscopy (TEM). After cryomilling for 20 h, the average grain size of the as-milled powders approached a constant value of 30 nm by XRD measurement. The average particle size slightly increased from 17.5 to 20.3 μm during the 20-h milling. About 90vol% of the powders satisfied the requirement for thermal spraying with the particle dimension of 10-50 μm, and most of the powders exhibited spherical morphology, which were expected to have good fluidity during thermal spraying. The Cr2O3 phase formed during the cryomilling process as revealed in the XRD spectra, which was expected to enhance the thermal stability of the as-milled powders during the followed thermal spraying or other heat treatment.

Synthesis and characterization of nanocrystalline Ni produced by cryomilling in liquid nitrogen

Abstract Nanocrystalline Ni powders were successfully fabricated by mechanically milling at cryogenic temperature (cryomilling) with 1 wt.%Y2O3 particles. The experimental results have shown that the Ni grain size is reduced to 25 nm after 2 h of cryomilling in the presence of the Y2O3 particles. The cryomilled Ni/Y2O3 powders can maintain their nanocrystalline structure up to 900°C, or 62% of the melting point of Ni. A bulk nanocrystalline Ni/Y2O3 material with a thermally stable grain size of approximately 100 nm was produced by cryomilling, cold isostatic pressing, followed by hot isostatic pressing. The microhardness of bulk nanocrystalline Ni/1wt.%Y2O3 is 315 DPH, which is two times as high as that of conventional Ni.

Thermal Stability of Nanostructured NiCrC Coating Prepared by HVAF Spraying of Cryomilled Powders

Thermal stability of nanostructured NiCrC coating prepared by high velocity air-fuel (HVAF) spraying of cryomilled feedstock powders was investigated. Transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) were utilized for characteristic analysis. Recrystallization and normal grain growth occur when isothermal treatment is performed at 923 K (0.55 T (subscript M)) for up to 100 h, and the average grain size increases from initial 41 nm for as-deposited state to around 100 nm for nearly equilibrium state. Isochronal treatment at 823 K and 1023 K was also conducted for comparison. Accordingly, for 0.49 to 0.61 T/T(subscript M), the time exponent n deduced from D(superscript 1/n)-D(superscript 1/n subscript 0)=kt increases from 0.15 to 0.30. The observed high thermal stability is attributed primarily to a Zener pinning mechanism arising from the fine Cr2O3, dispersions and the solute drag effect as well.

Deformation twins and twinning at ambient temperature in cryomilled Al-Zn-Mg-Cu alloy powders

The nanocrystalline Al-Zn-Mg-Cu alloy powders were synthesized using cryomilling. Deformation twins and twinning at ambient temperature were observed in Al-Zn-Mg-Cu powders. Experimental results indicate that high strain rate at low temperature during cryomilling induces the formation of deformation twins by the climbing of the restricted dislocations in nano-scale grains, and the cryomilled powders at high energies and nonequilibrium state reduce the grain boundary energies by twinning.