Investigation On Two Nanocrystallization Methods On Metallic Surfaces And Characterization And Properties Of Obtained Layers

Obtaining nanocrystalline layers in the surface of metallic materials have attracted great attention of researchers over the past decades. Two kinds of deformation processes which can be applied in the industry were successively developed, which can obtain the nanocrystalline layers in the surface of various metallic materials to overcome the limitations of existing severe plastic deformation methods. Microstructures and properties of the obtained surface layers were investigated in this dissertation.The main research and conclusions are as follows:(1) Ultrasonic impact peening (UIP) process, which was conventionally used to eliminate welding residual stress, was firstly introduced to induce surface nanocrystallization of metallic materials. Nanocrystal layers with the grain size ranging from 10 to 35 nm,30-100μm thick, on the surface of 35 steel were obtained by ultrasonic impact peening process.(2) A novel normal pulse spinning (NPS) method and cryogenic treatment technique were developed for synthesizing a nanostructured surface layer on metallic materials. Controllable microstructures of lamellar nano-twins or equiaxed nanocrystalline copper were produced by NPS process at the temperature of-196℃.(3) Microstructure features of various sections in the nanocrystalline surface layer synthesized by UIP and NPS processes were systematically characterized to investigate the grain refinement mechanism. The analyses of microstructural evolution revealed that plastic deformation of 35 steel surface layers by UIP process were induced by dislocation sliding, while the deformation twinning was the main carrier in copper during NPS.(4) Distribution of microhardness and properties of fatigue, friction and wear were experimentally investigated for nanostructured surface layer on 35 steel induced by UIP method. The results showed that surface nanocrystallization can increase the hardness of the plastic deformation layer and fatigue strength, decrease the surface friction coefficient, and improve the anti-wear property.(5) Experimental investigation on wettability and condensation heat transfer characteristics from nanocrystalline metallic surfaces, which were produced via severe plastic deformation, was conducted. To our knowledge, this has been the first one ever reported. The dynamics behavior and parameters of condensed droplets in the dropwise condensation were quantitatively characterized by the method combining the motional visualization and microscopic analysis. The results showed that the condensation heat transfer coefficient from the nanocrystalline copper surface was 2-3 times that from the untreated copper surface, which remarkably enhances condensation heat transfer. This is because that the contact angle of the nanocrystalline copper surface is two times higher than that of the untreated copper surface. The nanocrystallization of the copper surface significantly decreased the surface wettability, and consequently, the vapor condensation mode transferred from the filmwise mode to the dropwise or dropwise-dominant mode.(6) With the test facility designed and established by our lab, the experiment of pool boiling heat transfer characteristics from the nanocrystalline copper surface was carried out for the first time. The entire process of bubble growth and departure from the copper surfaces before and after nanocrystallization was observed and recorded with the high-speed visualization method. The bubble morphology, growth period and departure frequency from the two types of surfaces were analyzed and compared to reveal the boiling heat transfer enhancing mechanism of the nanocrystalline surface. It was shown by results that the incipient wall superheat for nucleate boiling from the copper surface was decreased from around 10℃to 5～6℃, after the nanocrystallization of the surface. At a given wall superheat, the nanocrystallne surface could transfer much higher heat flux, and therefore yielded a boiling heat transfer coefficient 1.6 times higher than the untreated surface. The main reason for the enhancement of boiling heat transfer is that bubles on the heated nanocrystalline surface have smaller smaller departure diameter and shorter growth period compared with that on the untreated surface.