||The amorphous alloys have attracted great attention due to their characteristics and future potential. This research is intended to synthesis new amorphous alloy with high glass forming ability as well as low density. The addition of lighter-weight elements such as Al, Ti, Zr, Ni and Cu are tried. The selected vitrification methods in this study are solid-state accumulated roll bonding (ARB) and arc-melting of multi-element alloys. Although the procedures of solid-state reaction are more complicated than that of casting, the influence of cooling rate on amorphization process is not important. |
Various Zr based binary, ternary, and pentanary alloys are synthesized by the ARB method. Besides, two pentanary alloys are also developed by arc melting method for the properties comparison with those made by ARB.
The evolutions of hardness, strain accumulation, the enhanced diffusion, nanocrystalline phase size, amorphous volume fraction, elastic modulus, and relative energy states in various Zr based alloy systems during ARB are characterized and analyzed by transmission electron microscopy (TEM), in correlation with X-ray diffraction results. It appears that compatible initial foil hardness would be most beneficial to the nanocrystallization and amorphization processes during the room temperature ARB; the influence would overwhelm the atomic size effect (i.e., the anti-Hume-Rothery rule) applicable for solidification processing such as drop casting or melt spinning. Meanwhile, the estimated diffusion rates during ARB are higher by several orders of magnitude than the lattice diffusion in bulk materials and the hardness is seen to increase with increasing ARB cycles. The last stage for the nanocrystalline phase to suddenly transform into the amorphous state is examined, coupled with thermodynamic analysis. From the experimental observations and interfacial energy calculations for multilayered films, it is demonstrated that the rapid increase of interfacial free energy of the nanocrystalline phases with increasing ARB cycles appears to be a determining role in enhancing amorphization process. The local spatial distributions of the nanocrystalline and amorphous phases are seen under TEM to be non-uniform, varying significantly in size and quantity in different regions. The diffraction spots and rings in the TEM diffraction patterns are still originated from the pure elements, meaning that the nanocrystalline phases are those unmixed hard particles left from the previous severe deformation and diffusion processes. A critical size of the nanocrystalline phases around 3 nm is consistently observed in all binary, ternary, and pentanary Zr-X based alloys, below the critical size a sudden transformation from the nanocrystalline to amorphous state would occur. Finally, the hardness and Young’s modulus of the nanocrystalline and amorphous materials are estimated based on the microhardness results.
On the other hand, a pentanary alloy (according to the composition of the synthesized ARB specimens) is also made by the arc melting method for comparison. The sharp peaks are still observed in XRD pattern of the as-melted alloys. Hence, the melt spinning method is followed. A nearly completely amorphous state is obtained in the melt spun alloy. The hardness readings of the prepared alloys are all significantly higher than those typically for metallic alloys. Moreover, the resulting Zr based amorphous alloys made by ARB possess glass transition and crystallization temperatures similar to those processed by melt spinning or drop casting.