Preparation And Investigation On Mechanical Properties Of Nanocrystalline Aluminum

When compared with common coarse-grained Al, bulk nanocrystallinealuminum (Al) has several advantages, including high strength, largemicro-hardness and outstanding wear resistance, due to their ultrafine and uniformcrystal, high concentrations of grain-boundary, clean grain surface. However, thepreparation conditions of nano-aluminum bulk compact are rigorous. Nano-sizedAl powder can not be used directly in air due to its large surface area and the hightendency to react with oxygen. It is difficult to obtain nanocrystalline Al throughordinary sintering process because it will result in grain growth within severalsintering hours.The nano-Al powders can not only keep the purity but also keep the highactivity when coated with an organic layer, so it can be used in air directly. sparkplasma sintering (SPS) fulfills rapid sintering by utilizing a pulsed direct currentand Joule heat, so it enables the production of nanocrystalline Al possible byproviding a lower sintering temperature, a much more rapid heating rate andshorter holding time. It is apt to obtain materials with high density, ultrafine anduniform grain and excellent mechanical properities.Organic-coated aluminum (Al) nano-powders were consolidated by SPStechnique in present study. The effects of sintering temperature, heating rate,holding time on microstructure, mechanical and physical properties weresystematically investigated. The results indicate that the sintering temperature hasthe greatest influence on properties of nanocrystalline Al. The density, grain size,and strain increase with an increase in temperature, however, the micro-hardness,compressive strength, and tensile stress of the bulk compact increase initially andthen decrease with increasing sintering temperature. At the same time, the porosityincrease as the heating rate increases, the grain size increases with the extensionof holding time. The nanocrystalline Al compact bulk with an average grain sizeof80nm, high density of99.5%and best mechanical properties can be prepared through SPS technique with low initial pressure of1MPa, heating rate of40℃/min, high holding pressure of300MPa, holding time of6min, and sinteringtemperature of500℃. Then the annealing treatment was conducted on the samplessintered at this condition. The compact bulks before and after annealing werecharacterized in respect of their residual stress, compressive and tensile propertiesby short-wavelength X-ray diffraction, compressive and tensile tests, respectively.The results indicate that the SPS sintered nanocrystalline Al has big residual stressinside and the residual stress can be eliminated through annealing treatment. Themechanical properties (compressive strength, tensile stress and strain) wereimproved after annealing. The effect of sintering temperature on thermal diffusioncoefficient and resistivity was investigated as well. The results indicate that theresistivity decreases with an increase in sintering temperature. However, thethermal diffusion coefficient increases as the temperature rises. The above resultswill lay a good foundation in application of SPS technology on preparation of bulknanomaterials.