Haishun Liu

Mechanical properties and structural features of novel Fe-based bulk metallic glasses with unprecedented plasticity

Fe-based bulk metallic glasses (BMGs) have attracted great attention due to their unique magnetic and mechanical properties, but few applications have been materialized because of their brittleness at room temperature. Here we report a new Fe50Ni30P13C7 BMG which exhibits unprecedented compressive plasticity (>20%) at room temperature without final fracture. The mechanism of unprecedented plasticity for this new Fe-based BMG was also investigated. It was discovered that the ductile Fe50Ni30P13C7 BMG is composed of unique clusters mainly linked by less directional metal-metal bonds which are inclined to accommodate shear strain and absorbed energy in the front of crack tip. This conclusion was further verified by the X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy experiments of Fe80-xNixP13C7 (x = 0, 10, 20, 30) and Fe72-xNixB20Si4Nb4 (x = 0, 7.2, 14.4, 21.6, 28.8) glassy systems. The results also indicate a strong correlation between the p-d hybridization and plasticity, verifying that the transition from brittle to ductile induced by Ni addition is due to the change of bonding characteristics in atomic configurations. Thus, we can design the plasticity of Fe-based BMGs and open up a new possible pathway for manufacturing BMGs with high strength and plasticity.

Soft magnetic properties and microstructure of novel Nb poor Finemet type alloys

Abstract Series of Nb poor Finemet type alloys were fabricated by substitution of Nb by Al for the first time, and soft magnetic properties of them have been researched by using permeability spectroscopy and X-ray diffraction. It is found that with 0·5 at.-% substitution of Nb by Al, the initial permeability is improved drastically, and the relaxation frequency is enhanced as well, and the loss factor reduces to a very low value of the order of 10−6 in the low frequency range compared to conventional Finemet. The improved soft magnetic properties are related to the lower crystalline anisotropy of FeSiAl compared to FeSi and the proper crystalline volume fraction. This research shows that the substitution of Nb by Al in Nb poor Finemet type alloys can bear ultrasoft magnetic properties at less cost.

Correlation of atomic packing with the boson peak in amorphous alloys

Boson peaks (BP) have been observed from phonon specific heats in 10 studied amorphous alloys. Two Einstein-type vibration modes were proposed in this work and all data can be fitted well. By measuring and analyzing local atomic structures of studied amorphous alloys and 56 reported amorphous alloys, it is found that (a) the BP originates from local harmonic vibration modes associated with the lengths of short-range order (SRO) and medium-range order (MRO) in amorphous alloys, and (b) the atomic packing in amorphous alloys follows a universal scaling law, i.e., the ratios of SRO and MRO lengths to solvent atomic diameter are 3 and 7, respectively, which exact match with length ratios of BP vibration frequencies to Debye frequency for the studied amorphous alloys. This finding provides a new perspective for atomic packing in amorphous materials, and has significant implications for quantitative description of the local atomic orders and understanding the structure-property relationship.

Correlation between fractal dimension and strength of brittle bulk metallic glasses

Abstract In this paper, the strength of brittle bulk metallic glasses (BMGs) was investigated, and a corresponding theoretical model was proposed by modification of Griffith theory using fractal geometry. It was shown that the fracture energy and strength of brittle BMGs not only are related to the plastic zone size, the length scales of atomic order range and the surface energy but also has significant relationship with the fractal dimension in fracture surface. The strength of typical BMGs was also calculated by our improved model, which is in excellent agreement with the experimental data. This study provides a compelling approach for investigating and understanding the high strength of BMGs.

Determine optimal annealing temperature of Fe based nanocrystalline alloys from their melting point

Abstract The dependence of permeability and grain size on annealing temperature was investigated, and a corresponding model was proposed. A surprising phenomenon wherein the grain size first decreases and then increases with increasing annealing temperature was analysed and explained by using this model. Moreover, it was also found that excellent soft magnetic properties were obtained when annealed at temperature near 0·6 times that of the melting point. Theoretical analysis results are in excellent agreement with experiments.

Plastic Deformation Mechanism of Ductile Fe 50 Ni 30 P 13 C 7 Metallic Glass

Shear band (SB) multiplication is considered as an essential characteristic of the plastic deformation in metallic glasses (MGs). In this work, the evolutionary characteristics of SBs and serrated behavior in ductile Fe50Ni30P13C7 MGs were studied by the finite element method simulation. The study demonstrated that the stress field would redistribute and become inhomogeneous during SB sliding, where the stress perpendicular to the original SB gradually accumulates until reaching the magnitude of yielding strength and triggering new SBs. The results of simulation are in good agreement with the SBs intersection morphologies observed in SEM images and the serration flows on the stress–strain curves. Furthermore, several factors affecting stress field distribution in MGs, such as contact friction, aspect ratio, and boundary confinement, were also analyzed and discussed. The overall results indicate that the ductility of Fe-based MGs could be achieved without any inhomogeneous structure, and scientifically explicate the dependence of SB multiplication on both material properties and loading condition, which can provide us with a deeper understanding on plastic deformation mechanism of MGs.

Ductile Fe-based bulk metallic glasses at room temperature

ABSTRACT In this work, Fe70Ni10P13C7 and Fe60Ni20P13C7 bulk metallic glasses (BMGs) with high ductility are synthesised through the appropriate adjustment of flux conditions. Microstructure analyses indicate that the ductility of these two Fe-based BMGs is primarily determined by Fe–Ni and Ni–Ni metal–metal bonds, which enable the glasses to undergo a strain of more than 50% in the absence of fracture, as well as extensive bending ductility, similar to metals such as Fe and Ni. This study aims to find a solution to the brittleness problem in Fe-based BMGs, and the results have implications for understanding the deformation mechanism in Fe-based BMGs.