H. X. Li

Corrosion behaviors of coatings fabricated using bulk metallic glass powders with the composition of Fe68.5C7.1Si3.3B5.5P8.7Cr2.3Mo2.5Al2.0

Two different types of coatings were prepared, by a high velocity oxy-fuel spraying method and a laser spraying method, respectively, using bulk metallic glass powders with the nominal composition of Fe68.5C7.1Si3.3B5.5P8.7Cr2.3Mo2.5Al2.0. The corrosion behaviors of the two coatings in 1M HCl, H2SO4, NaCl and NaOH solutions were investigated based upon the microstructural differences originating from the different coating methods. The amorphous coating layer formed by the high velocity oxy-fuel spraying method exhibited higher, excellent corrosion resistance in the 1M HCl solution. The coating layer formed by the laser spraying method exhibited a high pitting tendency attributed to the dendritic microstructure with various borides and carbides. Due to a great number of pores, the HVOF coating exhibits slightly lower corrosion resistance than the LS coating in alkaline solution.

Effects of electrolyte ph on the electrochemical behavior of Fe-based bulk metallic glass

The effects of electrolyte pH on the electrochemical behavior of Fe-based bulk metallic glass with a composition of Fe68.8C7.0Si3.5B5P9.6Cr2.1Mo2.0Al2.0 were investigated at an ambient temperature. The results indicate that corrosion behavior is strongly dependent on the pH values. The corrosion current densities and capacitance values decrease with an increase in pH values in acidic electrolytes, while the opposite tendencies are obtained in alkaline electrolytes. While the corrosion product of the outer layer in low pH conditions is an amorphous structure, crystalline ferric oxide is obtained in the electrolyte with pH=14. The electrochemical behavior is discussed on the basis of the results of electrochemical and microstructural analysis.

The Influences of Grain Size and Morphology on the Hot Tearing Susceptibility, Contraction, and Load Behaviors of AA7050 Alloy Inoculated with Al-5Ti-1B Master Alloy

The influences of grain size and morphology on the hot tearing susceptibility of AA7050 alloy inoculated with Al-5Ti-1B master alloy were investigated by the authors. It was found that with the optimal addition of Al-5Ti-1B, coarse columnar grains were transformed into fine globular equiaxed grains. Moreover, due to the changes of grain size and morphology, the hot tearing susceptibility was decreased remarkably, which was attributed to the lower mechanical coherency temperature, better feeding ability, lower strain, and strain rate imposed to the mushy zone and more meandering propagation paths of hot tears. But the excess Al-5Ti-1B additions did not affect the grain structure, and greatly promoted hot tearing susceptibility due to the agglomerations of secondary phase particles from Al-5Ti-1B master alloy. The formation of massive secondary phases at grain boundaries hindered the interdendritic liquid flow and substantially deteriorated the feeding ability in the last stage of solidification. Meanwhile, TiB2 agglomerates would also act as stress raisers and cause the formation of voids. The contraction and load behaviors of AA7050 alloy influenced by grain size and morphology would be explored and connected with the hot tearing occurrence in this study.

Roles of Alloy Composition and Grain Refinement on Hot Tearing Susceptibility of 7××× Aluminum Alloys

During the production of high-strength 7××× aluminum alloys, hot tearing has set up serious obstacles for attaining a sound billet/slab. In this research, some typical 7××× alloys were studied using constrained rod casting together with the measurement of thermal contraction and load development in the freezing range, aiming at investigating their hot tearing susceptibility. The results showed that the hot tearing susceptibility of an alloy depends not only on the thermal contraction in freezing range, which can decide the accumulated thermal strain during solidification, but also on the amount of nonequilibrium eutectics, which can effectively accommodate the thermally induced deformation. Our investigations reveal that Zn content has very profound effect on hot tearing susceptibility. The Zn/Mg ratio of the alloys also plays a remarkable role though it is not as pronounced as Zn content. The effect of Zn/Mg ratio is mainly associated with the amount of nonequilibrium eutectics. Grain refinement will considerably reduce the hot tearing susceptibility. However, excessive addition of grain refiner may promote hot tearing susceptibility of semi-solid alloy due to deteriorated permeability which is very likely to be caused by the heavy grain refinement and the formation of more intermetallic phases.

A modified hot tearing criterion for direct chill casting of aluminium alloys

Using the good criteria to predict hot tearing is very important during DC casting of aluminium alloys. Among all the hot tearing criteria, a fracture-mechanics based SKK criterion proposed by Suyitno et al. has made considerable improvements in the hot tearing prediction. However, its obtained hot tearing susceptibility (HTS) evolution during solidification is also not completely consistent with real industrial production circumstances, especially when approaching the solidus temperature. In this paper, some further modifications are made based on the SKK criterion to emphasise the important effect of solid bridging/grain coalescence on hot tear propagation. It is proved that the HTS evolution in freezing range predicted by the modified hot tearing criterion is in good agreement with casting practice.

Mechanical Properties and Constitutive Behavior of as-Cast High Strength AA7xxx Alloys below Solidus Temperature

High strength AA7xxx alloys have been extensively used in aerospace industry. However, experience in cast house demonstrates that such alloys are particularly prone to cold cracking and ingot distortion during direct chill (DC) casting, which leads to big amount of scraping or even total rejection of ingot. Those stress induced defects are greatly affected by casting process, and fine tuning of casting parameters is critical to improve the quality and productivity. Recently, numerical modeling has been widely utilized in direct-chill casting for the purpose of thermal mechanical analysis and cracking evaluation. Parameter optimization has become convenient. The model needs the input of constitutive properties of the AA7xxx alloys, of which the microstructure should resemble those formed during DC casting. Unfortunately, these constitutive data are not yet available in literatures. In this study, the mechanical properties of two high strength AA7xxx alloys were measured at temperature range from solidus down to room temperature through on-cooling compressive tests and the effect of strain rate on stress-strain behavior was also studied. The results were fitted to extended Ludwik equation which has been proved to be able to properly describe the stress-strain response of aluminum alloys. The results of the current work shed some light on the evolution of mechanical behavior of high strength AA7xxx alloys during cooling from high temperature.

Hot Tearing Evaluation and Contraction Behaviors of Four AA7xxx Alloys

The hot tearing susceptibilities (HTS) of some AA7×××alloys, AA7050, AA7055, AA7085 and AA7022 were evaluated with constrained rod casting (CRC). Thermal contraction behaviors during solidification were measured as well in a T-shaped setup. The results showed that alloys with HTS from high to low were AA7055, AA7085, AA7050 and AA7022. Zn content in 7××× aluminum alloys seemed to play a major role with respect to the HTS index. Remarkable differences could be seen on thermal contraction behaviors within solidification range for each alloy. The rate and amount of thermal contraction for AA7055 was most prominent, followed by AA7085 and AA7050, while contraction curve of AA7022 was very flat together with least amount of thermal contraction. There was a well consistency between the amount of thermal contraction and HTS. Despite complex interactions of many variables in the formation of hot tear, thermal contraction behaviors within solidification range could give a quick evaluation of hot tearing susceptibility.

Effect of Minor Si and Ag Additions on Glass-Forming Ability of Ti-Cu-Co-Zr-Sn-Be Bulk Metallic Glass

Effect of Minor Si and Ag additions on glass-forming ability (GFA) of the base Ti44Cu38.9Co4Zr6Sn2Be5.1 (at. %) alloy are studied. (Ti44Cu38.9Co4Zr6Sn2Be5.1)100-xRx (R = Si, Ag, x=0, 1, 2, 3 at. %) metallic glasses are formed by splat-quenching and copper mold suction casting. It is found that the minor Si and Ag additions enhance the glass-forming ability of Ti44Cu38.9Co4Zr6Sn2Be5.1 greatly. The maximum size of fully amorphous structure is increased from 4 mm for base alloy to be larger than 6 mm for alloys containing 1-2 at. % Si/Ag, while Si/Ag more than 2 at. % additions decrease the GFA.

Bulk Metallic Glass Ti42Cu37.1Co4Zr8Sn2Ag2Be4.9 with High Glass Forming Ability and Ductility

Bulk metallic glass Ti42Cu37.1Co4Zr8Sn2Ag2Be4.9 that can be cast into a fully amorphous rod of more than 6 mm in diameter by copper molder casting has been developed through systematic alloy design. The bulk metallic glass exhibits high fracture strength (f＝2071 MPa) and good ductility (f＝5.83％) under compression.

TEM INVESTIGATION ON CRYSTALLIZATION BEHAVIOR OF A Fe68C10.5Si4.4B6.5P8.6Al2 BULK METALLIC GLASS

TEM investigations on crystallization behavior of a Fe68C10.5Si4.4B6.5P8.6Al2 bulk metallic glass reveals that the nano-scale fcc Fe phase formed by primary crystallization is homogeneously embedded in the amorphous matrix after heat treatment at 783 K for 30 min. With increasing heat treatment time (783 K for 45 min), a tetragonal Fe3(C,B,P) phase encapsulates the primary fcc Fe phase suppressing the growth of the fcc Fe phase. The Fe3(C,B,P) phase decomposes into a mixture of orthorhombic Fe3(C,P) and tetragonal Fe3(B,P) upon further annealing treatment at 783K.