| Apparent structure relaxation of bulk metallic glasses (BMGs) will occur inpractical service due to its metastable structure, which results in the change ofcorrosion properties and mechanical properties. Hence, the study on the effect ofstructural relaxations on BMG properties is important both in theories and practicalapplications. In this paper, different relaxation states of Zr52.5Cu17.9Ni14.6Al10Ti5(Vit105) BMGs were obtained by controlling annealing temperatures. Thecorresponding corrosion resistance and room temperature compression propertieshave been investigated.The Zr52.5Cu17.9Ni14.6Al10Ti5(Vit105) BMG rods were prepared by arc meltingand vacuum suction technique with Ar protection. The XRD pattern confirms theamorphous structure of as-cast rods. The DSC analysis show that the glass transitiontemperature Tgof the as-cast sample is685K. Different relaxation states of sampleswere obtained by annealing for15minutes at403,503,553,593,623and653Krespectively, which are all below Tg. The influence of annealing temperatures, typesand concentration of the solutions on the corrosion properties of BMG rods has beenobtained by immersion and electrochemical corrosion tests. The effect of structuralrelaxation on the mechanical properties has been observed by compression tests atroom temperature. Combining with the SEM observations and EDS analysis, theinfluence of structure relaxation on the corrosion properties and compressionperformance of Vit105BMG rods has been studied.XRD patterns of all annealed Vit105BMG rods show that only a broad diffusiondiffraction peak appears, and no crystal diffraction peaks can be observed. It indicatesthat all annealed samples still keep amorphous structure. However, with increasingannealing temperatures, the position of the diffuse peak shifts slightly to higher anglewhile the full width at half maximum (FWHM) of the peak apparently decreases. Itsuggests that the structural relaxation indeed occurs although no crystallization can beobserved for annealing below653K for15minutes. DSC analysis show that theannealing temperature has an obvious effect on Tg, Tgapparently decreases from685 K for as-cast sample to666K for sample annealed at403K. With further increasingannealing temperature, Tgcontinuously increases to683K for553K annealing, andthen slightly decreases. Unfortunately, Tgis difficult to determine once the annealingtemperature is higher than623K. In addition, two crystallization peaks can beobserved in Vit105BMG rods. The effect of annealing temperatures on thecrystallization temperature can be hardly observed. However, the crystallizationexothermic effects are weakened with increasing annealing temperatures; The secondcrystallization peak disappears for annealing temperature above623K.The study on the structural relaxation of materials by impedance spectroscopyhas been tried here although it’s few reported. It is found that the impedance spectrumat the same testing condition of Vit105BMG rods is obviously affected by annealingtemperatures. For the same annealed sample, the impedance spectrum also varies withtest temperatures. The change of impedance spectrum is mainly observed in the highfrequency range. The height and position of phase angle peaks change significantly.The reason for the change still needs further investigation.The immersion corrosion of Vit105BMG rods and304L stainless steel in0.6MNaCl, l M HCl, and5M NaCl, HCl, H2S04and NaOH solutions were measured.Comparing with304L stainless steel, Vit105BMG rods exhibit better corrosionresistance. The corrosion rate decreases slowly with increasing annealing temperatureup to593K, and rapidly increases with further increasing annealing temperature. Inaddition, the corrosion rate increases with increasing concentration of solutions whileit decreases in the sequence of HCl, NaCl, H2SO4and NaOH solutions for the sameconcentration. In all, the best corrosion resistance of Vit105BMG rods can beobtained by annealing at593K for15minutes in all corrosion solutions.The anodic polarization curves in NaCl and HCl solutions show that the pittingfollowing passivation can be observed in NaCl (0.6and5M) and HCl (1M) solutionsfor all annealed samples while no obvious passivation can be found in5M HClsolution, which show that Vit105BMG rods own the worst corrosion resistance in5M HCl solution. With increasing annealing temperatures, the corrosion currentdensity first decreases then increases while the pitting potential shows opposite trend. The critic annealing temperature is in the vicinity of593K. The results furtherconfirm that the593K annealed sample owns the best corrosion resistance. Theelectrochemical impedance spectroscopy (EIS) of Vit105BMG rods in Cl-containingsolutions contains only one capacitive loop. With increasing annealing temperatures,both the diameter of the semicircle and electrochemical transfer resistance areincreasingly bigger till593K then continuously decreases. In addition, theelectrochemical transfer resistance in NaCl solution is much higher that in HClsolution. It suggests that the Vit105BMG rods have much higher corrosion resistancein NaCl solution than that in HCl solution.SEM observations show that corrosion pits with different sizes and irregularshape can be observed in the corrosion surface. With the increasing annealingtemperature till593K, the size and number of corrosion pits gradually becomesmaller and fewer, respectively. Once the annealing temperature is higher than593K,corrosion pits obviously become larger and more. EDS analysis show that O contentmarkedly increases in the corrosion pits. In addition, the relative content of Cuapparently increases while that of other elements decreases in the corrosion pits.The room temperature compression test of Vit105BMG shows that the fracturesurfaces of samples are typical amorphous “veins”. The plasticity at roomtemperature is very low. The best plasticity (1.6%) and strength (~2000MPa) can beobtained by annealing at403K for15minutes. No plasticity can be observed forannealing temperature higher than593K. |
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