Nano-indentationstudies On Micro-mechanical Behaviors Of Fe-based,Pd-based And ZrCu-based Bulk Amorphous Alloys

Because of their unique internal microstructure,bulk amorphous alloys?BAAs?process excellent physical,chemical and mechanical properties,as well asbroad application prospects in machinery,electronics,national defense,aerospace and other fields.In recent years,with the continuous development of BAAs,these alloysare getting more and more applications in practical engineering,and thus they have caused widespread concern in academic circles.Currently,a variety of bulk amorphous alloy systemshave been successfully developed,and some of them have been well applied in practical engineering,but most BAAs show low plastic deformation ability at room temperature.Thisbottleneck has seriously affected the large-scale engineering applications of BAAs?Thus,how to improve the mechanical properties of BAAs becomes one of the hottest issues in recentresearches.In this paper,using the nanoindentation method,the mechanical properties of four kinds of BAAs,i.e.Pd₄₀Cu₃₀Ni₁₀P₂₀?Zr₄₈Cu₃₂Ni₄Al₈Ag₈?Zr₄₈Cu₃₄Pd₂Al₈Ag₈ and [(Fe_0.6Co_0.4)_0.75B_0.2Si_0.05]₉₆Nb4werestudied.The hardness,reduced modulus,serrated flow behavior,creep,creep compliance and retardation spectrum of the above-mentionedBAAs under different loading conditions are characterized in order to get some usefulconclusion forimproving the mechanical properties of BAAs.The research results are summarized as follows:1.Nanoindentation experiments were done to study the effects of loading rate and the maximum load on the hardness and the reduced modulus of four kinds of BAAs.It was found thatas the maximum load increases,the indentation depth of the four kind of BAAs increase while their hardness and reduced modulus decrease,especially the [(Fe_0.6Co_0.4)_0.75B_0.2Si_0.05]₉₆Nb4BAA.For the studied BAAs,their hardness change present a similar trend in the loading rate range of 0.2-10 mNs^-1,i.e.,when the loading rate is no more than 1mNs^-1,the hardness has a little decrease,however,when the loading rate continues to increase,it has almost no effect onthehardness.2.With the increasing of the loading rate,the variation of the reducedmoduli of the fourkinds of the studied BAAs with loading rate exhibits various change trends.For BAA Zr₄₈Cu₃₂Ni₄Al₈Ag₈,when the loading rate is no more than 1mNs^-1,the reduced modulus drops rapidly,and when the loading rate continues to increase,it has almost no effect onits reduced modulus.the reducedmoduli of Zr₄₈Cu₃₄Pd₂Al₈Ag₈,Pd₄₀Cu₃₀Ni₁₀P₂₀ and Zr₄₈Cu₃₄Pd₂Al₈Ag₈ have a significant dependence on loading rate.Interestingly,for [(Fe_0.6Co_0.4)_0.75B_0.2Si_0.05]₉₆Nb4,its reduced modulus has a little increasewhen the loading rate exceeds 10mNs^-1.3.The dependence of the loading rate andchemical composition on the serrated behavior of the studied four BAAs has been discussed.The results show that the serrated plastic flowsof four BAAs are differentat the same loading rate.The plastic flow of BAA Pd₄₀Cu₃₀Ni₁₀P₂₀ is more obvious than BAA Zr₄₈Cu₃₂Ni₄Al₈Ag₈,while BAA Zr₄₈Cu₃₄Pd₂Al₈Ag₈ and BAA [(Fe_0.6Co_0.4)_0.75B_0.2Si_0.05]₉₆Nb4 hardly have serrated plastic flow;The plastic flow of BAA Pd₄₀Cu₃₀Ni₁₀P₂₀ is more severe at lower loading rate,the pop-in platform increaseswith decreased loading rate,and pop-in sizeof BAA Pd₄₀Cu₃₀Ni₁₀P₂₀ also increases with the increasing depth of the indentation;Under the same indentation condition,the strain ratepeaks of BAA Pd₄₀Cu₃₀Ni₁₀P₂₀ is more significant than those of BAA Zr₄₈Cu₃₄Pd₂Al₈Ag₈,and their magnitude is affected by indentation strain rate.4.Effects ofthe maximum load and the loading rate on nanoindentation creep behaviorsof Zr₄₈Cu₃₂Ni₄Al₈Ag₈ and Zr₄₈Cu₃₄Pd₂Al₈Ag₈ BAAs at room temperature were analyzed in detail.5.Based onthe empirical formula and the generalized Kelvin model,the creep experimental data on two kinds of ZrCu-based BAAswere fitted;According tothe curvesfitted with the empirical formula,the stress exponentsfor Zr₄₈Cu₃₂Ni₄Al₈Ag₈ and Zr₄₈Cu₃₄Pd₂Al₈Ag₈ BAAs werecalculated;According to the curves fitted with a generalized Kelvin model,creep compliance and retardation spectrum for Zr₄₈Cu₃₂Ni₄Al₈Ag₈ and Zr₄₈Cu₃₄Pd₂Al₈Ag₈ BAAs were also calculated;The variationof the stress exponent,creep compliance and retardation spectrumwith the loading rate and the maximum load for Zr₄₈Cu₃₂Ni₄Al₈Ag₈ and Zr₄₈Cu₃₄Pd₂Al₈Ag₈ BAAs and the differences between them were discussed.Finally,these phenomenaare interpreted in terms of free volume and “shear transformation zone”?STZ?theories.