Analysis Of The Viscoelastic Behaviors Of Bulk Amorphous Alloys

In recent years, bulk amorphous alloys (BAAs) have intrigued wide interest at home and abroad owing to their superior mechanical properties and perspective applications. However, structure and deformation mechanism of these alloys are not yet understood, impeding their applications. In this paper, a Zr-based and Fe-based bulk amorphous alloy was chosen as the research object. By combining numerical analysis with experimental testing, this project thus aims at systematic and quantitative investigation on the above problems in several alloy systems. The main results are summarized as follows:1. In the loading stage, the formula ε=1/mt has been proposed to characterize the variation of strain rate in the loading stage. It is shown that the formula can characterize the variation of strain rate in the loading stage well. In addition, a fast loading rate leads to a great strain rate at the same depth.2. In the holding stage, the influence of the loading rate and the maximum load on the creep behavior of Zr-based bulk amorphous alloys has been taken into account. The results show that the creep displacement increases with the increase of both loading rate and maximum load. An empirical equation was used to model the depth and time curves in the holding stage, with which we can calculate to obtain stress index (n).It is founded that the increasing maximum load will result in larger stress index and the increasing loading rate leads to a decresed stress index.The phonomenon that the stress index changes with the lodading rate and the maximum load is further explained by free volume theory.3. The properties of fractional order pot model were discussed based on the Riemann-Liouville definition of fractional differential. Furtherly based on creep and loading curves, and followed by nonlinear fitting for the parameter in The fractional order derivative rheological constitutive model, we determine the parameter α (fractional-order). The results show that a presents loading rate sensitivity and its trend is opposite to the stress index’s, suggesting that α associates with the evolution of amorphous alloy microstructure under external loads.4. The internal structure of amorphous alloys presents a dynamic non-uniformity, that is, amorphous alloys consist of liquid-like and solid-like region on the nanoscale. Then, a "core-shell" model to describe the structure of amorphous alloys was proposed. Furtherly, we establish an expression of depth and load before yielding for "core-shell" model from Hertz theoretical solution and three-parameter model. Finally, the expressionof depth and load derived was used to fit the loading curves for Zr-based and Fe-basedbulk amorphous alloys under different loading rate. The results show that the "core-shell" model can describe the viscoelastic and viscoplastic deformation behavior ofamorphous alloys in the loading stage well, and the model parameters have a clearphysical meaning. And the impact of the non-uniformity of the amorphous alloy structureon the shear modulus is discussed as below.