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Mechanical Behaviors of Zr-based Bulk Metallic Glasses

Author: KongChao
Tutor: LiFuShan
School: Zhengzhou University
Course: Materials Processing Engineering
Keywords: bulk metallic glasses glass-formation ability sand blasting elastic deformation plastic
CLC: TG139.8
Type: Master's thesis
Year: 2009
Downloads: 24
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Abstract

In recent years,metallic glasses can be made in bulk by casting,so people may make use of the property index of conventional structural materials to characterize the mechanical properties of bulk metallic glasses.Although metallic glasses generally exhibit relatively excellent properties,such as higher strengh,larger elastic limit,and better corrosion resistance and so on,but the essential brittleness of metallic glasses seriously has limited the applications of metallic glasses.This subject is advoted to make metallic glasses plastic.Zr₆₅Cu₁₅Ni₁₀Al₁₀ and Zr₅₅Cu₂₅Ni₅Al₁₀Nb₅ bulk metallic glasses were chosen in this paper,which have larger glass-formation ability and relatively better plastic.This paper first analysised conventional mechanical behaviors of Zr-based bulk metallic glasses,then deformation behaviors and mechanism of metallic glasses were further studied by surface sand blasting,shot peening,preloading and predeformation treatments.The results as follows:1) According to compression curves and stress-deflection curves of Zr₆₅Cu₁₅Ni₁₀Al₁₀ bulk metallic glass,work harding didn’t exist in plastic deformation, and the metallic glasses can exhibit better plastic.Increased with the loading rate,the deflection values get smaller.Shear bands didn’t propagate along the direction of theoretical maximum stress because of complex stresses.Typical vein-like patterns are observed on the fracture surface in compressive stress regions,which are the same as fracture surface in compressive tests.Complex reticular patterns were showed under different stress states in tension regions,and micro-scale voids and cracks exited along the shear bands,showing that the formation of shear bands was associated with free volume.Shear bands which were found not only in surface of samples but also in fracture surface.2) Shot-peening and sand-blasting treatments may improve the plastic of Zr-based bulk metallic glasses,and the compression properties of specimens after sand blasting in surface can exhibit better plastic than that after shot peening.The compression strain of specimens after sand blasting can up to be 17.9%.3) By microhardness tests,it is indicated that sand blasting and shot peening both make bulk metallic glasses softening,but degrees of softening are different,and the softening layers of specimens after sand blasting are rather thicker than which after shot peening in the same condition.Stress fields can generate in surface of specimens by shot-peening and sand-blasting to reduce stress concentration in surface,on the other hand,pre-existing shear bands,which can make the deformation of specimens homogeneous and plastic improved.However the surface imperfections can not be ignored,texturing effect caused from sand blasting can make the surface of specimens generate a large number of homogeneous folds,and microcracks can be formed by interaction of folds,then the microcracks will generate fast shear bands in compression deformation.We can conclude that the interaction between fast shear bands by surface imperfections and pre-existing shear bands is taken as the reason which can improve specimens by sand blasting.4) Zr₆₅Cu₁₅Ni₁₀Al₁₀ bulk metallic glass exhibits obvious viscoelastic behavior in the processes of preloading and pre-deformation:creep deformation generate under constant stess with holding time increasing;stress relaxtion generate under constant strain.Creep deformation and sress relaxtion can exist under smaller stress and strain respectively.5) Proper preloading or pre-deformation in elastic deformation region can obviously improve bulk metallic glass plastic.When dwell time is 30mins in elastic deformation region,with the increasing of dwell strain and dwell stain,comprssion strength and compression plastic get higher,and plastic can be significantly improved.According to results of the experiment,it is indicated that after keeping strain with smaller strain and longer time,plastic of the specimens can significant been improved meanwhile strength value increases a few.6) Microhardness tests show that softening phenomenon were found on the surface of specimens as the larger dwell pressure or longer dwell time in elastic region under the same dwell pretreatment,which indicated that the aggregation of free volume happened in macroscopic view.The decreasing of microhardness value can show the increasing of the aggregation degree of free volume.Micro shear bands can generate by proper aggregation of free volume in compression deformation of specimens, which can hinder the instantaneous fracture of materials and improve materials plastic.

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