Research On Crystallization And Corrosion Resistance Property Of FeCo Based Bulk Amorphous Alloy

In this thesis, FeCo based bulk amorphous alloy samples were prepared bywater-cooling coppe mold suction casting under a vacuum non-consumable arc melting.Crystallization behaviors of Fe28Co20Cr15Mo14C15B6Y2amorphous alloy samples withdifferent annealing temperature were studied by using thermal analyzer, X-raydiffraction and high-resolution transmission electron microscopy, respectively. Thecorrosion resistance properties of Fe24Co24Cr15Mo14C15B6Y2bulk amorphous alloy thatwere analyzed in3.5%NaCl solution by electrochemical polarization curve andalternating current impedance test through the electrochemical workstation, as well asthe relative surface work function of the alloy samples was tested using the electronicscanning Kelvin probe.The results show that there are two crystallization peaks in the DSC curve of theamorphous alloy samples under the continuous heating process, which mean that thecrystallization process has two steps. The crystallization activation energy is calculatedby using the Kissinger method and Ozawa method, respectively. The results show thesame change trend: the higher the annealing temperature, the lower the crystallizationactivation energy. Under the annealing treatment of Fe28Co20Cr15Mo14C15B6Y2bulkamorphous alloy, with the increase of annealing temperature, the types of precipitatedphase are increased. The atom of the amorphous alloy is rearranged during the processof structural relaxation under a low temperature. Grain size increases as the temperatureraising. When the alloy is completely crystallized, there are other grains which restrictmutually each other during the process of the grain growth, and the grain size becomessmaller. As the annealing temperature is570°C, there are a few Cr23C6phase precipitatedafter one hour heat preservation. As the one is620°C, phase Cr23C6numbers obviouslyincreased accompanying with the precipitation of new phase Fe3B. As the one is720°C,there are not noly Cr23C6, Fe3B phases but also a new phase-Fe. As the one is920°C,the main precipitation phases are-Fe, Cr23C6, Fe3B as well as the new precipitatedphase Fe23B6.Based on the electrochemical polarization curve and alternating current impedanceof Fe24Co24Cr15Mo14C15B6Y2amorphous alloy samples in the3.5%NaCl solutiontesting results, it can be found that the corrosion resistance property of amorphous alloy under the as-cast conditions is higher than that of the crystallized alloy, because thecharge transfer resistance of amorphous alloy is higher. The self corrosion potential ofamorphous alloy is-0.5153V, and the charge transfer resistance is9.459×104Ω·cm2. Withthe increase of annealing temperature, the self corrosion potential gradually reduce, thecharge transfer resistance gradually decreases. Based on the testing results of scanningKelvin probe, it can be obtained that with increasing the annealing temperature, thesurface the electronic work function gradually declines, and there is a certainrelationship between the alloy corrosion resistance and surface electron work functio n.The relative surface electron work function of amorphous alloy reaches-98meV and theone after being crystallized completely reaches-180meV. It can be known that alloyhas the higher surface the electronic work function, the better electrochemical corrosionresistance property.