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Study On Glass Forming Ability, Crystallization And Properties Of Al-TM-RE Based Amorphous Alloys

Posted on:2011-06-22Degree:DoctorType:Dissertation
Country:ChinaCandidate:G H LiFull Text:PDF
GTID:1101360305951324Subject:Materials science
Abstract/Summary:PDF Full Text Request
In the present thesis, Al-TM-RE-based (TM:transition metal; RE:rare earth) amorphous alloys were mainly chosen for the study of the relationships between glass-forming ability (GFA) and elements addition, thermal expansion behavior and corrosion resistance, respectively. In addition, the effect of different annealing atmospheres (H2, air, Ar and N2) on crystallization behavior, microhardness and corrosion resistance of Al-Ni-La amorphous alloy was also investigated.The effect of Si addition on the GFA and thermal stability of melt-quenched Al-Fe-La alloys has been investigated systematically by using X-ray diffraction (XRD), differential scanning calorimetry (DSC) and viscometer. Results show that all the Alg88-xFe6La6Six (x= 0,0.5,1-4) alloys are full amorphous structure, moreover Al86Fe6La6Si2 alloy has the best GFA, and Al87Fe6La6Si1 alloy has the best thermal stability. That is because appropriate Si addition can induce:(1) larger packing density of the Al-Fe-La amorphous alloys due to large atomic size difference; (2) a larger negative heat of mixing; (3) the passivation of impurities (such as oxygen and carbon) which trigger heterogeneous nucleation in the melt, which increases the GFA. However, when Si content (csi) is beyond 2 at.%, the addition of Si reduces the GFA and thermal stability of the alloys. The reason is that the redundant Si atoms incline to form silicides with Al and Fe, i.e. increase the number of final phases which may serve as heterogeneous nucleation sites, then deteriorate the GFA. In addition, it is also found that the addition of Si influences the crystallization behaviors of the alloys, transforming from two-step crystallization process to three-step crystallization process.The effects of similar element pairs:La-Ce and Ni-Co, on the GFA, magnetic properties and hardness of Al-Ni-La alloy were investigated using XRD, transmission electron microscopy (TEM), DSC, magnetometer and hardness-tester. The results show that the GFA of the samples in the order of Al86Ni0.5Co0.5)9(La0.5Ce0.5)< Al86Ni9La5< Al86Ni9(La0.5Ce0.5)5, implying that similar element substitution has a limited enhancing effect on the GFA of the present Al-Ni-La alloy. In addition, the measured samples display a diamagnetic behavior at room temperature. The variations of diamagnetic behavior as well as the microhardness of the samples are strongly dependent on the microstructure, i.e. the amounts of the icosahedral structure and precipitates, after the similar element substitution in Al-Ni-La alloy.The dynamic viscosities and thermodynamic dilatometric behaviors of Al86Ni9La5 and Al86Ni9(La0.5Ce0.5)5 amorphous alloys were investigated using viscometer and conventional dilatometer. Results show that based on free volume theory, less free volume is found in Al86Ni9(La0.5Ce0.5)5 alloy than Al86Ni9La5 alloy in both melt and glassy states, suggesting that the partial substitution La by Ce decreases the quantity of free volume in Al-Ni-La system by improving the continuous degree of atomic size, which leads to the improvement of GFA. Meanwhile, it can be confirmed than the properties of a glass are closely related to the properties of a liquid. In addition, the dilatometric of Al-Ni-RE (RE=Y, Ce, Gd and Er) amorphous alloys were also investigated. It is found that the ratio of the linear thermal expansion coefficient (aas) and the liquidus temperature (T1) (aas/T1), defined byθ', has a good positive correlation with GFA in Al86Ni9La5, Al86Ni9(La0.5Ce0.5)5 and Al-Ni-RE (RE=Y, Ce, Gd and Er) amorphous alloys. A similar phenomenon is also found in Gd-and Cu-based amorphous alloys. Consequently, the parameterθ' can be applied to predict the GFA in the system with same main element. Based on the free volume model, it is found that the amorphous alloy which exhibits better GFA contains less free volume.The GFA and corrosion behaviors of Al94-xNixGd6 (x=6,8,10,12) amorphous alloys were investigated using XRD, DSC, electrochemistry experiment, SEM, EDS and XPS. The results show that:(1) with the increasing of Ni content, the thermal stability and GFA of Al-Ni-Gd alloys increase continually, and the GFA is in the order of Al82Ni12Gd6> Al84Ni1oGd6> Al86Ni8Gd6> Al88Ni6Gd6; (2) the corrosion resistance of Al-Ni-Gd alloys is better in 3.5 wt.% NaCl and 0.1 mol/L NaOH solutions than in 0.1 mol/L HC1 solution due to the formation of wide passive region; (3) the higher the GFA of Al-Ni-Gd alloy, the better the corrosion resistance; (4) the current density peak at-0.5 -0.25 V always appears in the Cl- containing solutions, which results from the co-function of the oxidation process of Ni2+ and Cl-.The Al86Ni9La5 amorphous ribbons were annealed in different annealing atmospheres (H2, air, Ar and N2), and the precipitated phases were investigated using XRD and TEM. Results show that:(1) during annealing at 523 K (about the peak temperature of first crystallization Tp1), the primary crystallized Al is independent on the annealing atmospheres; (2) during annealing at 584 K (about the onset temperature of second crystallization Tx2), the final crystalline phases, i.e. Al+Al11La3+Al3Ni, are also independent on the different annealing atmospheres; (3) during annealing at 523 K, H2 and air can promote the eutectic crystallization process, and induce the formation of metastable Al3Ni2 phase, and the promoting effect of different annealing atmospheres is in the order of H2> air> Ar> N2. In addition, the microhardness and corrosion resistance in the 3.5 wt.% NaCl solution of the annealed samples were also investigated by micro-hardness tester and electrochemistry experiment. Comparing with the ribbons annealed at 523 K in Ar and N2, the ribbons annealed in H2 and air show a higher microhardness and a better corrosion resistance. The property promotion caused by annealing process can be ascribed to the formation of nano-crystalline phases, which is possibly helpful to develop the alloy's application in the seawater environment.
Keywords/Search Tags:Al-based amorphous alloy, glass forming ability, thermal expansion, corrosion resistance, crystallization
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