Study On Thermodynamic Parameters And Bending Plasticity Of Zr₆₅Cu_17.5Ni₁₀Al_7.5 Amorphous Alloy

Amorphous alloys have excellent mechanical properties, physical properties,chemical properties due to their unique disordered structure and do not have crystal defects, so they attracted the wide attention of researchers. Amorphous alloys are metastable on thermodynamics, and they can transform spontaneously to steady state under a certain condition, which lead to changes of the properties of amorphous alloys. During deformation of amorphous alloys under the temperature far below glass transition temperature, the plastic deformation of amorphous alloys are highly localized in shear bands, and then extend quickly along the shear zone, and result in a catastrophic failure. All of these have limited its wide application as engineering materials.The formation of amorphous alloys are closely related with cooling rate,different microstructures can be obtained under different cooling rates, which can influence the properties of amorphous alloys. In addition, the free volume content and crystallization fraction of amporphous alloys can change during heating, and result in variation of properties. In order to discover the influence of variation of microstructure on the properties of amorphous alloys, we take the Zr₆₅Cu_17.5Ni₁₀Al_7.5 amorphous alloy as the research object in this work, and the samples with different thickness?1 mm, 2 mm and 3 mm? were prepared, and the relationships among the cooling rates during solidification and thermodynamic parameters and bending plasticity were investigated. The isothmeral annealing treatment of 1mm sample were performed at 571-771 K,and the relationships among the free volume content and the crystallization fraction and thermodynamic parameters and bending plasticity.In this work, the structure of samples were detected by XRD, and the thermodynamic parameters of samples was characterized by DSC, and three point bending tests were carried out for the samples by Instron universal testing machine, the fracture morphology was observed by SEM. The results were analysed and discussed, and the main conclusions drawn are as follows.The crystallization mechanism of Zr₆₅Cu_17.5Ni₁₀Al_7.5 amorphous alloy with different cooling rates is a diffusion-controlled three-dimensional growth process, whose nucleation rate increases with time. Among these samples, 3 mm sample has the highest crystallization activation energy, and 1 mm and 2 mm samples have similar apparent activation energies of crystallization. With decreasing of cooling rate, the crystallization onset temperature and the peak temperature first decrease and then increase, but glass transition temperature first increases and then decreases.With the decrease of cooling rate, the free volume content decrease, which leads to a gradually decreasing of the bending plasticity of the amorphous alloy.1 mm samples did not fracture before the largest displacement that can be realized with the present testing machine, while the fracture of 2 mm and 3 mm samples are typical cup-cone fracture. It is found that the plasticity of amorphous alloy is closely related to the shear band density, critical shear stage and the vein pattern through the observation of fracture morphology. In addition,the crystallization fraction of Zr₆₅Cu_17.5Ni₁₀Al_7.5 amorphous alloy increases gradually with the decrease of cooling rate, which leads to a transition of“large-small-large” in the bending strength.After isothermal annealing treatment at different temperatures, the characteristic temperatures of Zr₆₅Cu_17.5Ni₁₀Al_7.5 amorphous alloys have no obvious change compared with that of as-cast, and the enthalpy of structure relaxation decreases with the increase of annealing temperature. The plasticity of as-annealed samples are lower than that of as-cast 1 mm sample due to free volume reduction. Isothermal annealing treatment at Tg and above Tg result in appearance of crystallization phase. When the crystallization fraction is small,crystallization phase can improve the plasticity of amorphous alloy to a certain extent, but can not compensate the decrease of plasticity due to the reduction of free volume. When the crystallization fraction is large, the plasticity of amorphous alloy deteriorates directly. The results of three point bending tests of the amorphous alloys with different cooling rate and isothermal annealing treatment show that higher free volume content leads to better plasticity.