EQUIVALENT MODEL OF EXPANSION OF CEMENT MORTAR UNDER SULPHATE EROSION

The expansion property of cement mortar under the attack of sulfate ions is studied by experimental and theoretical methods. First, cement mortars are fabricated with the ratio of water to cement of 0.4, 0.6, and 0.8. Secondly, the expansion of specimen immerged in sulphate solution is measured at different times. Thirdly, a theoretical model of expansion of cement mortar under sulphate erosion is suggested by virtue of represent volume element method. In this model, the damage evolution due to the interaction between delayed ettringite and cement mortar is taken into account. Finally, the numerical calculation is performed. The numerical and experimental results indicate that the model perfectly describes the expansion of the cement mortar

Formation Mechanism Of Lamellar Chips During Machining Of Bulk Metallic Glass

The uniqThe unique lamellar chips formed in turning–machining of a Vit 1 bulk metallic glass (BMG) are found to be due to repeated shearband formation in the primary shear zone (PSZ). A coupled thermomechanical orthogonal cutting model, taking into account force, free volume and energy balance in the PSZ, is developed to quantitatively characterize lamellar chip formation. Its onset criterion is revealed through a linear perturbation analysis. Lamellar chip formation is understood as a self-sustained limit-cycle phenomenon: there is autonomous feedback in stress, free volume and temperature in the PSZ. The underlying mechanism is the symmetry breaking of free volume flow and source, rather than thermal instability. These results are fundamentally useful for machining BMGs and even for understanding the physical nature of inhomogeneous flow in BMGs.ue lamellar chips formed in turning–machining of a Vit 1 bulk metallic glass (BMG) are found to be due to repeated shearband

On mechanical properties of metallic glass and its liquid vitrification characteristics

A systematic survey of the available data such as elastic constants, density, molar mass, and glass transition temperature of 45 metallic glasses is conducted. It is found that a critical strain controlling the onset of plastic deformation is material-independent. However, the correlation between elastic constants of solid glass and vitrification characteristics of its liquid does not follow a simple linear relation, and a characteristic volume, viz. molar volume, maybe relating to the characteristic size of a shear transformation zone (STZ), should be involved

Dynamic Fracture Instability Of Tough Bulk Metallic Glass

We report the observations of a clear fractographic evolution from vein pattern, dimple structure, and then to periodic corrugation structure, followed by microbranching pattern, along the crack propagation direction in the dynamic fracture of a tough Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit.1) bulk metallic glass (BMGs) under high-velocity plate impact. A model based on fracture surface energy dissipation and void growth is proposed to characterize this fracture pattern transition. We find that once the dynamic crack propagation velocity reaches a critical fraction of Rayleigh wave speed, the crack instability occurs; hence, crack microbranching goes ahead. Furthermore, the correlation between the critical velocity of amorphous materials and their intrinsic strength such as Young's modulus is uncovered. The results may shed new insight into dynamic fracture instability for BMGs. (C) 2008 American Institute of Physics

Energy Dissipation In Fracture Of Bulk Metallic Glasses Via Inherent Competition Between Local Softening And Quasi-Cleavage

Compression, tension and high-velocity plate impact experiments were performed on a typical tough Zr41.2Ti13.8Cu10Ni12.5Be22.5 (Vit 1) bulk metallic glass (BMG) over a wide range of strain rates from similar to 10(-4) to 10(6) s(-1). Surprisingly, fine dimples and periodic corrugations on a nanoscale were also observed on dynamic mode I fracture surfaces of this tough Vit 1. Taking a broad overview of the fracture patterning of specimens, we proposed a criterion to assess whether the fracture of BMGs is essentially brittle or plastic. If the curvature radius of the crack tip is greater than the critical wavelength of meniscus instability [F. Spaepen, Acta Metall. 23 615 (1975); A.S. Argon and M. Salama, Mater. Sci. Eng. 23 219 (1976)], microscale vein patterns and nanoscale dimples appear on crack surfaces. However, in the opposite case, the local quasi-cleavage/separation through local atomic clusters with local softening in the background ahead of the crack tip dominates, producing nanoscale periodic corrugations. At the atomic cluster level, energy dissipation in fracture of BMGs is, therefore, determined by two competing elementary processes, viz. conventional shear transformation zones (STZs) and envisioned tension transformation zones (TTZs) ahead of the crack tip. Finally, the mechanism for the formation of nanoscale periodic corrugation is quantitatively discussed by applying the present energy dissipation mechanism

Tuning plasticity of in-situ dendrite metallic glass composites via the dendrite-volume-fraction-dependent shear banding

This work performs a systematic investigation of identifying how the volume fraction of the in-situ dendrites affects the plasticity of metallic glass composites. The quasi-static uniaxial compressions show that the global plastic strain does not follows a linear rule-of-mixture with the dendrite volume faction, instead, a slow-fast slow enhancement behaviour is observed with increasing dendrite volume fraction. It is demonstrated that the nucleation and propagation of shear bands in these composites are dependent on the dendrite volume fraction. When the dendrite volume fraction exceeds a critical value, multiple shear bands emerge in a spherical plastic zone around a dendrite. It is further proposed that the percolation of these spherical plastic zones contributes to the fast increase in the plastic strain of the glass composites. Our findings offer important implications for the microstructural optimization of the metallic glass composites with desirable mechanical properties

Joining of bulk metallic glass to brass by thick-walled cylinder explosion

This paper reports the development of a thick-walled cylinder explosion technique to weld a typical Zr-based bulk metallic glass (Vitreloy 1 (Vit 1)) to a commercial Cu-based crystalline alloy (brass). It is shown that a strong metallurgical bonding between the Vit 1 and the brass is achieved, which is due to significant atomic diffusion across the welding interface and shock wave propagation in the weldment. The dissimilar joining of the noncrystalline to crystalline alloy extends the application of bulk metallic glasses as structural and functional materials

Effect of strain rate on yielding strength of a Zr-based bulk metallic glass

Uniaxial tension and compression experiments were performed on a typical Zr52.5Cu17.9N44.6Al10Ti5 (Vit 105) bulk metallic glass over a wide range of strain rates at room temperature. It is found that the strain rate effect of the yielding strength will change from insensitive to negative with increasing strain rate above a critical value. This phenomenon can be quantitatively described by a modified cooperative-shear model of shear transformation zones that takes the adiabatic temperature rise into account. The model predicts well the present and other experimental data