S. Yi

Amorphous phase formation in a Ni−Zr−Al−Y alloy system

Quaternary Ni-based amorphous alloys containing only metallic elements were developed through systematic alloy design. The importance of the phase equilibria information for the development of amorphous alloys was demonstrated through experimental results. Ni−Zr−Al ternary alloys having low liquidus temperature tend to have high GFA. Partial replacements of Zr with Y in the ternary alloys significantly enhanced the GFA of the quaternary alloys. The alloy Ni60Zr25Al8Y7 could be cast into a fully amorphous rod through an injection casting method. Since most Ni-based amorphous alloys reported to date contain non-metallic elements, the Ni-based amorphous alloys developed in the alloy system Ni−Zr−Al−Y are of interest.

Formation of a bimodal eutectic structure in Ti–Fe–Sn alloys with enhanced plasticity

Microstructural investigations on a series of (Ti70.5Fe29.5)100−xSnx alloys with x=5, 7, and 9 reveal that Sn addition is effective in introducing both structural and spatial heterogeneities in ultrafine eutectic composites stemming from a large temperature difference between two eutectic temperatures upon solidification. The microstructural heterogeneities in these ultrafine eutectic composites strongly enhance the room temperature compressive plasticity up to ∼15.7%.

Work hardening ability of ductile Ti45Cu40Ni7.5Zr5Sn2.5 and Cu47.5Zr47.5Al5 bulk metallic glasses

Ductile Ti45Cu40Ni7.5Zr5Sn2.5 and Cu47.5Zr47.5Al5 bulk metallic glasses (BMGs) present different work hardening abilities under compression. Microstructural investigations reveal that nanoscale chemical heterogeneities occur throughout the samples. The morphology of the chemically heterogeneous domains in the as-cast Ti45Cu40Ni7.5Zr5Sn2.5 BMG is irregular and significantly interconnected. In contrast, the as-cast Cu47.5Zr47.5Al5 BMG exhibits a spherical morphology of the chemically heterogeneous regions. Furthermore, the distribution of the nanoscale chemical heterogeneity is macroscopically inhomogeneous throughout the material. These findings suggest that the different work hardening abilities of the Ti45Cu40Ni7.5Zr5Sn2.5 and Cu47.5Zr47.5Al5 BMGs possibly originate from the different morphologies and distributions of the chemically heterogeneous regions.

Influence of a bimodal eutectic structure on the plasticity of a (Ti70.5Fe29.5)91Sn9 ultrafine composite

Systematic investigations on the microstructural evolution of a bimodal eutectic (Ti70.5Fe29.5)91Sn9 ultrafine composite containing Ti3Sn dendrites upon compression reveal that local deformation of the dendrites dominates the early stage of deformation with a plastic strain of ep=5.8%. After further deformation (ep=10.2%), a wavy propagation of shear bands indicative of dissipation of the shear stress is caused by a rotation of the coarse eutectic structure along the interfaces of the bimodal eutectic structure.

Effect of Sn on microstructure and mechanical properties of (Ti–Cu)-based bulk metallic glasses

A Ti43Zr7Cu43Ni7 bulk metallic glass (BMG) exhibits a plastic strain of less than 0.2% under room-temperature compression. However, a Ti45Zr5Cu40Ni7.5Sn2.5 BMG developed by addition of Sn to Ti43Zr7Cu43Ni7 shows a significant improvement of ∼14.8% to the plastic strain with work hardening. Microstructural comparison of these BMGs reveals that the addition of Sn leads to formation of nano-scale chemical heterogeneities throughout the material. These nano-scale chemical fluctuations play an important role in enhancing the plastic strain of (Ti–Cu)-based BMGs.

Solidification microstructure of as-cast Mg–Zn–Y alloys

Composites consisting of icosahedral (i) phase and ductile α-Mg can be fabricated by controlling the alloy composition. With increasing Mg content, the primary solidification phase changed from the i phase to the α-Mg phase and single eutectic structure can be obtained at a composition of Mg72Zn23.5Y3.5. The i phase showed a variation in structural order from the well-ordered icosahedral phase to the 1/1 rhombohedra1 approximant with lattice constants a=27.2 Å and α = 63.43°. The structural change in the i phase can be explained by microdomain formation due to compositional change during solidification. Annealing treatment improves the structural order of the i phase by homogenization. By controlling the alloy composition, a single eutectic structure consisting of a hard i phase and a ductile α-Mg phase could be obtained.

Dry Sliding Tribological Properties of Fe-Based Bulk Metallic Glass

The dry or unlubricated sliding friction and wear properties of as-cast and annealed BMG (bulk metallic glass) with nominal composition of Fe66.7C7.0Si3.3B5.5P8.7Cr2.3Al2.0Mo4.5 against Si3N4 ceramic ball was studied, along with a conventional material, using a ball-on-disk tribotester at room temperature. The overall average coefficient of friction value of the as-cast BMG was in the range of 0.26–0.42, which was better than the conventional material SUJ2 (0.36–0.46) and comparable with SUS304 (0.31–0.40). The wear mechanism of the Fe-based BMG changed with wear condition. The wear rate increased with increasing load. The hardness of the BMG increased during annealing, however, the wear resistance did not increase proportionally.

Effect of local chemistry, structure and length scale of heterogeneities on the mechanical properties of a Ti45Cu40Ni7.5Zr5Sn2.5 bulk metallic glass

We report on distinct variations in local chemistry, structure and length scale of heterogeneous regions in Ti45Cu40Ni7.5Zr5Sn2.5 fully glass rods of different diameters, i.e. rods subjected to different cooling rates. The present investigations indicate that the mechanical properties of the Ti45Cu40Ni7.5Zr5Sn2.5 bulk metallic glass can be modified within a wide range of strength and plastic deformability by controlling the scale of the heterogeneous regions in the glass through appropriate variation of the cooling rate applied for solidification.

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.

Sliding friction and wear behavior of Fe-based bulk metallic glass in 3.5% NaCl solution

The corrosion wear behaviors of a Fe-based bulk metallic glass Fe66.7C7.0Si3.3B5.5P8.7Cr2.3Al2.0Mo4.5 against Si3N4 ceramic ball was studied in a 3.5% NaCl solution and using a unidirectional tribometer with the ball-on-disk configuration. During friction process, the Fe-based BMG showed tribochemical reaction with the 3.5% NaCl solution. Wear occurred mainly through an abrasive wear mechanism and corrosive wear effects. The overall average friction coefficient was in the range of 0.18 to 0.25 according to the test conditions and slightly decreased with increasing load. The wear resistance of the BMG was better than that of the conventional bearing steel, demonstrating the potential of the BMG as a viable engineering alloy for anti-corrosive wear applications.