| Titanium aluminides have excellent oxidation resistance, good corrosion resistance and wear resistance, high temperature strength, low density and high melting point, which are widely used in structural materials. Recently, workers dedicated to employ titanium aluminide as strengthening coatings on the surface of aluminum alloy. In addition, titanium based nitrides with high hardness could form due to high affinity between the two elements. Based on this, a duplex surface treatment involving two procedures(magnetron sputtering and post plasma nitriding) was carried out to fabricate thermo-reactive diffusion coatings on surface of 2024 and 5083 Al alloy for improving their wear and corrosion resistance.According to the method, first principles calculations combined with the thermo calculations were used to study the possibility of expected reactions and structure stability of the possible formation phase, the mechanical properties of which were also predicted. Based on this, the parameters of the diffusion treatment were determined by the previous calculations. The effects of deposition parameters and diffusion parameters on the microstructures of the surface layers of Al alloys were investigated by using X-Ray diffraction(XRD), scanning electron microscopy(SEM) equipped with energy dispersive X-ray analyzer(EDS) and transmission electron microscopy(TEM). The mechanical properties and corrosion resistace of the surface layer were tested by microhardness tester, pin-on-disc tribometer and potentiodynamic polarization method in 3.5 wt. % Na Cl solution.Thermodynamic analysis shows that the reaction in Ti-N system and Ti-Al-Mg system under low temperature could lead to the formation of α-Ti N0.3, Al3 Ti and Al18Ti2Mg3 respectively. According to first principles calculation, Al3 Ti shows most negative formation energy while α-Ti N0.3 has most negative cohesive energy among the three phases. All of the three kinds of phases show a mixture of covalent, ionic and metallic bond. In addition, the Ti-N bond shows the strongest covalent bond and Al3 Ti shows stronger covalent bond than Al18Ti2Mg3. The Young’s modulus of α-Ti N0.3 and Al18Ti2Mg3 are firstly predicted, i.e., 199 GPa and 122 GPa, respectively. The values of B/G ratio of the three kinds of compounds are 1.84, 1.56 and 1.25. Especially, α-Ti N0.3 and Al18Ti2Mg3 exhibit less brittle than Al3 Ti. Finally, the structure comprised of multiphase and multilayer could decrease the residual thermo stress at the interface between the coating and substrate.The as-deposited Ti films on surface of Al alloy crystallize in a hexagonal structure, with(002) preffered plane. Under the lower pulse, the Ti particles grow in hexangular shape and some voids form in the Ti film. With higher pulse and target power, longer deposition time, the(002) preffered orientation decrease. After plasma nitriding, multilayer structure formed on surface of aluminum alloy, i.e., α-Ti N0.3/Al3 Ti /Al18Ti2Mg3 for 2024 Al and α-Ti N0.3/Al18Ti2Mg3 for 5083 Al, which is in agreement with the theoretical calculation results. The thickness vs time of Al3 Ti layer and Al18Ti2Mg3 layer obeyed the power function relationship. The growth index for Al3 Ti layer and Al18Ti2Mg3 layer is 0.797 and 1.133, respectively, indicating that the intermetallic layer exhibits approximately linear growth characteristic.The nitriding temperature and time play an important role in the phase structure and thickness of the diffusion layer. The α-Ti N0.3 layer on surface of 2024 Al alloy showed the preferred(002) orientation under low nitriding temperature. With the increase of nitrding temperature, α-Ti N0.3 is inclined to grow in(100) plane. While the α-Ti N0.3 layer on surface of 5083 Al alloy always grow in(002) plane and the grow tendency become intense with the increase of the nitriding temperature. The thickness of titanium aluminide layers increases with raising temperature, especially, the thickness of the Al18Ti2Mg3 layer grow faster than that of the Al3 Ti layer. The α-Ti N0.3 layer on surface of 2024 Al alloy exhibit preferred(100) orientation under low nitrogen content condition. With higer N content in the atmosphere, α-Ti N0.3 tends to grow in preferred(002) orientation. However, nitriding atmosphere has little effect on the formation of titanium aluminide layers as well as the phase composition on surface of 5083 Al alloy. As for 2024 Al alloy, the nitriding time has little effect on the phase composition. With the increasing of nitriding time, the thickness of α-Ti N0.3 keeps nearly constant while that of the titanium aluminide layer increase. As for the 5083 Al alloy, the α-Ti N0.3 layer exhibits in preferred(002) orientation at the intial stage of nitriding. With the increase of nitriding time, the(100) preferred orientation become intense. Under the condition of low temperature and low nitrogen content, the α-Ti N0.3 particles grow in nano-scale. The coating surfaces become denser with larger α-Ti N0.3 particles with the increase of nitriding time and the N content of nitrding atomsphere.The microhardness of the untreated 2024 and 5083 Al alloy are HV98 and HV61, respectively. The surface hardnesses of the Al alloy are improved significantly by the multiphase layer fabricated by duplex treatment, which can reach HV631 for 2024 Al alloy and HV880 for 5083 Al alloy. The prefferd(002) orientation ofα-Ti N0.3 leads to obvious hardness increase of the modified layer. The results of nanoindentation show that the Young’s modulus of α-Ti N0.3 and Al18Ti2Mg3 is 172 GPa and 131 GPa, respectively, which is close to the calculation results of first principles. The nano-hardness of α-Ti N0.3 and Al18Ti2Mg3 is 10.35 GP and 8.97 GPa, respectively, which is much larger than the soft Al alloy.The friction coefficient and wear rate of Al alloy are decreased by the modified layer, the friction coefficients of the untreated 2024 and 5083 Al alloy are 0.52 and 0.71, respectively. After the duplex treatment, the surface friction coefficients of the two kinds of Al alloy can be reduced to 0.31 and 0.28, respectively. Compared with the untreated Al alloy, the wear rate can be can be reduced by 59.4% and 68.8%, respectively. The uncoated Al alloys exhibit predominant adehssive wear, along with oxidation wear. The coated Al alloys exhibit predominant abrasive wear, accompanied with oxidation wear and slight adhesive wear. |
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