Guo-lu Li

Microvoid evaluation of ferrite ductile iron under strain

According to the principle of microdamage mechanics, the evolutional law of graphite void in ferrite ductile cast iron was studied under action of strain. The results showed that strain has a remarkable influence on growth of graphite void. The mathematical model of growth law of void volume was obtained. The interaction between nodular graphite has a great effect on void growth. The nodularity of graphite has a little influence on critical fraction of void volume. Therefore, the critical fraction could act as a fracture parameter of ductile iron. The theory and test proved that large nodular graphite results in rapid growth of void. This provides the theoretical basis for controlling of microstructure and mechanical properties of ductile iron.

Ultrasonic measurement of fatigue damage of nodular cast iron

According to the principles of damage mechanics, the evolutional law of fatigue damage of pearlite nodular cast iron under uni-axial stress control was studied by measuring the ultrasonic speed and attenuation. The results show that the fatigue damage variable D of pearlite nodular cast iron as a function of fatigue cycle ratio varies according to the index law. The critical fatigue damage variable was obtained. By comparing with ultrasonic speed, the attenuation is more sensitive to fatigue damage of ductile cast iron. At the early stage of fatigue, the variation of ultrasonic speed and attenuation comes from the movement and proliferation of dislocation in matrix. This is not a real reflection to fatigue damage in ductile cast iron.

Effect of flame remelting on microstructure and wear behaviour of plasma sprayed NiCrBSi-30%Mo coating

ABSTRACT The effects of flame remelting on the microstructural features, mechanical properties and dry sliding wear properties of supersonic plasma sprayed NiCrBSi-30 wt-% Mo coating were evaluated. The modified microstructure of the flame remelted coating, such as reduction in micro-defects, decrease in line roughness, laminar architecture elimination and material homogenisation was obtained. The Vickers micro-hardness and elastic modulus measurement showed an enhancement for flame remelted coating. The main wear mechanism of the as-sprayed coating was adhesive/delaminative wear. While the main wear mechanism of the flame remelted coating was adhesive wear. The wear resistance of flame remelted coating surpassed that of the as-sprayed coating and was by far superior to the substrate.

Microstructure and Wear Resistance of TIG Remelted NiCrBSi Thick Coatings

The self-fluxing NiCrBSi coatings with 800 μm thickness were prepared on the surface of AISI1045 steel substrate by plasma spraying. And the remelted coating was obtained using by the tungsten inert gas (TIG) arc process. The microstructure, surface roughness, hardness, phase composition, and wear resistance of the sprayed coating and remelted coating were systematically investigated. The results demonstrate that TIG remelted treatment can significantly eliminate the microscopic defects in thick coating and improve its density. The surface roughness (Ra) of the remelted coating is only 18.9% of the sprayed coating. The hardness of the remelted coating is 26.8% higher than that of the sprayed coating. The main phases in the sprayed coating are changed from γ-Ni, Cr7C3, and Cr2B to γ-Ni, Cr23C6, CrB, Ni3B, and Fe3C. The wear mass loss of the remelted coating is only 17.1% of the sprayed coating. Therefore, a Ni-based thick coating with good wear resistance can be obtained by plasma spraying and remelted technique.

The erosion effect of Kapton film in a ground-based atomic oxygen irradiation simulator

In order to investigate the effect of space environmental factors on spacecraft materials, a ground-based simulation facility for space atomic oxygen (AO) irradiation was developed in our laboratory. Some Kapton film samples were subjected to AO beam generated by this facility. The Kapton films before and after AO exposure were analyzed comparatively using optical microscopy, scanning electronic microscopy, atomic force microscopy, high-precision microbalance, and X-ray photoelectron spectroscopy. The experimental results indicate that the transmittance of Kapton film will be reduced by AO irradiation notably, and its color deepens from pale yellow to brown. Surface roughness of the AO-treated sample is already increased obviously after AO irradiation for 5 hours, and exhibits a flannel-like appearance after 15 hours’ exposure in AO beam. The imide rings and benzene rings in kapton molecule are partially decomposed, and some new bonds form during AO irradiation. The mass loss of kapton film increases linearly with the increase of AO fluence, which is resulted from the formation of volatile products, such as CO, CO2 and NOx. The breakage in structure and degradation in properties of AO-treated Kapton film can be attributed to the integrated effect of impaction and oxidization of AO beam. The test results agree well with the space flight experimental data.