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Finite Element and Experimental Studies on Nanoindentation Deformation Behaviors of a Zr-based Amorp

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Tutor: LongZhiLin
School: Xiangtan University
Course: General and Fundamental Mechanics
Keywords: Bulk amorphous alloy,Nanoindentation,Yield criterion,Room-temperature creep,Stru
CLC: TG139.8
Type: Master's thesis
Year:  2013
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With the rapid development of preparation technology and research of bulkamorphous alloys (BAAs), their excellent physical, chemical and mechanicalproperties have been found, which laid a solid foundation for broad applications ofthese alloys. In recent years, some high-precision nanoindentation techniques and avariety of theoretical models were used to explore the correlation betweendeformation behavior and microstructure of BAAs and their room-temperaturerheological mechanism. On other hand, some recent studies on the randomdistribution of the elastic modulus in BAAs indicated that their modulus exhibitnano-scale heterogeneity. In this thesis, the nanoindentation deformation behaviorsand the effect of elastic heterogeneity on meso-mechanical behaviors of aZr48Cu32Ni4Al8Ag8(Zr-BAA) BAA were investigated by depth-sensingnanoindentation experiments and atomic force microscopy as well as finite elementsimulations. The main results are summarized as follows:1. Zr-BAA exhibited ¡°pop-in¡± event during room-temperature nanoindentation,which was significantly dependent on loading rates. These discrete ¡°pop-in¡± eventscorrespond to the activation of individual or multiple shear bands; Slower indentationrates promote more conspicuous serrations, wherea rapid indentation suppressesserrated flows. The rate dependency of the indentation creep (the larger creep, thefaster loading become) might be explained according to the excess free volumecreated during loading and the ratio of loading deformation to total deformation.2. The displacement-time curve during holding segment can be fitted well by anempirical power-law equation. Based on the fitted curve, the room temperature creepstress exponent of Zr-BAA can be extracted. The calculated results show that thevalue of stress exponent decreases with increasing loading rate. At high loading rate,The creation process of free volume dominates the diffusion and annihilation of freevolume. This results in the growing amount of excess free volume during loading,which gives rise to the formation of many shear transformation zones (STZs) and thusa permanent homogeneous flow in the following creep segment.3. Finite element (FE) methods was used to simulate the nanoindentation loading-displacement curve of the Zr-BAA. Calculated results show that the observedexperimental results are consistent with FE simulations using a Mohr-Coulomb yield criterion and/or a Drucker-Prager yield criterion, suggesting the influence of a normalstress component. The contour plot of Mises equivalent stress under loadingconditions illustrates that the Mises equivalent stress reaches the maximum value atthe position located beneath the indenter, indicating that the effective stress is relatedto the initiation of shear bands.4. The nano-indents were examined using atomic force micrograpy (AFM).Some shear bands observed around the indents corresponds to the maximum effectivestress beneath the indenter.5. Effects of elastic heterogeneity (i.e. Gaussian distribution) on thenanoindentation behavior of a Zr-BAA were investigated. Simulation results showthat von Mises effective stress exhibits uniform distributions when there is no theelastic heterogeneity in this alloy. With the increase of elastic heterogeneity, theeffective stress distribution become more dispersed, and the unequal stress displaysthe alternative distribution. On other hand, the influence of the elastic heterogeneityon the nanoindentation behavior of the BAA can be eliminated when the level ofheterogeneity falls below a certain value.
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