Shu Guo

Liquid-solid joining of bulk metallic glasses

Here, we successfully welded two bulk metallic glass (BMG) materials, Zr51Ti5Ni10Cu25Al9 and Zr50.7Cu28Ni9Al12.3 (at. %), using a liquid-solid joining process. An atomic-scale metallurgical bonding between two BMGs can be achieved. The interface has a transition layer of ~50 μm thick. The liquid-solid joining of BMGs can shed more insights on overcoming their size limitation resulting from their limited glass-forming ability and then promoting their applications in structural components.

In-situ tensile testing of ZrCu-based metallic glass composites

ZrCu-based bulk metallic glass composites (BMGCs) are well known for their plastic deformability, superior to traditional metallic glasses (MGs), which is attributed to a unique dual-phases structure, namely, the glassy matrix and unstable B2 phase. In the present study, in-situ tensile testing is used to trace the deformation process of a ZrCu-based BMGC. Three deformation stages of the BMGC, i.e., the elastic-elastic stage, the elastic-plastic stage, and the plastic-plastic stage are identified. In the elastic-elastic and elastic-plastic stages, the yield strength and elastic limit are major influenced by the volume fraction of the B2 crystals. In the plastic-plastic stage, the B2 phase stimulates the formation of multiple shear bands and deflects the direction of shear bands by disturbing the stress field in front of the crack tip. The deformation-induced martensitic transformation of the metastable B2 phase contributes to the plasticity and work hardening of the composite. This study highlights the formation and propagation of multiple shear bands and reveals the interactions of shear bands with structural heterogeneities in situ. Especially, the blocking of shear bands by crystals and the martensitic transformation of the B2 phase are critical for the mechanistic deformation process and illustrate the function of the B2 phase in the present BMGCs.

Formation Mechanism of Surface Crack in Low Pressure Casting of A360 Alloy

A surface crack defect is normally found in low pressure castings of Al alloy with a sudden contraction structure. To further understand the formation mechanism of the defect, the mold filling process is simulated by a two-phase flow model. The experimental results indicate that the main reason for the defect deformation is the mismatching between the height of liquid surface in the mold and pressure in the crucible. In the case of filling, a sudden contraction structure with an area ratio smaller than 0.5 is obtained, and the velocity of the liquid front increases dramatically with the influence of inertia. Meanwhile, the pressurizing speed in the crucible remains unchanged, resulting in the pressure not being able to support the height of the liquid level. Then the liquid metal flows back to the crucible and forms a relatively thin layer solidification shell on the mold wall. With the increasing pressure in the crucible, the liquid level rises again, engulfing the shell and leading to a surface crack. As the filling velocity is characterized by the damping oscillations, surface cracks will form at different heights. The results shed light on designing a suitable pressurizing speed for the low pressure casting process.

Improving the microstructure of ITZ and reducing the permeability of concrete with various water/cement ratios using nano-silica

The interfacial transition zone (ITZ) between an aggregate and a cement matrix is known to be the weakest component of concrete; hence, its microstructure is the key factor determining its performance in terms of permeability. The objectives of this study are to strengthen the ITZ and reduce the permeability of concrete using nano-silica, and explore its improvement features under different water-to-cement ratios (W/Cs) between 0.3 and 0.5. An innovative testing and characterization methodology is adopted to quantitatively evaluate the properties of the ITZ. The results indicate that nano-silica can reduce both the width and formation of abrasion cracks in the ITZ. Further, the role of nano-silica is especially effective when the W/C of concrete is reduced. Small hydration products growing on the surface of the aggregate and cement matrix of nano-concrete with a reduced W/C can overlap each other and effectively fill the gap, resulting in enhanced densification of the ITZ by the nano-silica at reduced W/C ratios. Further, the enhancement rate of the anti-permeability of concrete also becomes significant. It can be concluded that nano-silica can effectively improve the performance of concretes, especially those with reduced W/C.

Understanding the structure-Poisson’s ratio relation in bulk metallic glass

Abstract Here, we employ isothermal annealing on a ZrCuNiAl bulk metallic glass (BMG) to obtain different structural states. The nanohardness of the studied Zr-based BMG shows a clear transition from heterogeneous to homogeneous distribution and the Poisson’s ratio gradually decreases with prolonging the isothermal annealing time. Then, the structural change is analyzed using high-resolution transmission electron microscope. The structure–Poisson’s ratio relation can be quantitatively illustrated by a rule of mixture. These findings have implications for better understanding the structure–property relation from atomic level and thus exploring high-performance BMGs of excellent strength and ductility.

Secondary Phases Present in Spray Formed Al-10.5Zn-2.0Mg-1.5Cu Alloy

Al-10.5Zn-2.0Mg-1.5Cu alloy was produced by spray forming (SF) technique. The initial microstructure, particularly secondary phases, present in the as-SFed alloy was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with attached energy dispersive X-ray spectroscopy (EDS). The results indicated that the equiaxed α-Al grains have an average size of about 18-20 μm in diameter. The intergranular phase was identified as MgZn2 by selected area electron diffraction (SAED) pattern, which was also distributed in the grain interior as well as Al2Mg3Zn3. A needle-like Al23Cu(FeCrMn)4 precipitate was detected. The combined analysis of morphology and crystallographic structure suggested that the Al23Cu(FeCrMn)4 grew along its long axis of <001> orientation. The refinement of the intergranular phases occurred, which is probably due to a decreased amount of eutectic liquid phase finally solidifying on a large area of grain boundaries.