Compositional Optimization And Enhanced Mechanical Performance In Bulk Amorphous Alloys

Due to the disordered structural features,bulk metallic glasses(BMGs)exhibit excellent mechanical properties and physical and chemical properties.However,the shortcomings of BMGs,i.e.the limited size/glass-forming ability(GFA)and the room-temperature brittleness,greatly restrict their possible structural application.Thus,it becomes crucial to develop BMGs with large GFA and good room-temperature ductility.Firstly,in order to predict the formation of glass formation and then fabricate large-sized BMGs,the effects of the entropy of mixing(?Smix)and the enthalpy of mixing(?Hmix)on the GFA of BMGs were studied by combining with Inoue's three empirical rules.We found that for a ternary alloy system with a fixed content of the matrix element(Matrix element:atomic radius larger than 0.158 nm;Solute elements:atomic radius difference larger than 11%;Mixing enthalpy:no positive mixing enthalpy between atomic pairs in the given system),the compositions with optimal GFA usually locate at around the inflection point of each ?Hmix vs.?Smix curve.When the compositions is far away from the inflection point,the GFA trends to become worse.By considering both calculated inflection points and experimental results,the optimizing glass-forming compositions can be roughly estimated by a formula of Di = AxB53(?)1-0.5xC47±1-0.5x,where x is the atomic fraction of the solvent element A(i.e.50 at.%? x 5 70 at.%)with an atomic radius larger than 1.58 A,B and C are solute elements.Meanwhile,the atomic radius of C is larger than B.Then the upper and lower limits of the composition swith good GFA can be represented AxB55.5(?)1-0.5xC44.5±1.0.5x to AxB50.5(?)1-0.5xC49.5±1.0.5x.Furthermore,by combing the rules above,the Inoue's three empirical rules were further modified for a ternary alloy as follows:(1)the atomic radius of the matrix element should be larger than 0.158nm;(2)no positive mixing enthalpy atomic pairs are contained and the atomic radius difference should be larger than 11%.Secondly,in order to apply the empirical rule above to multicomponent alloy systems,the "similar-element replacement" rule was also considered.It was found that the multicomponent alloy systems obtained by the similar-element replacement also follow the ?Hmix vs.ASmix rule.Then by using micro alloying method,the correlation between the micro alloying effect and the GFA was further discussed and then a series of multicomponent BMG were explored.For example,Zr56Cu4Nb4Al17.5Ni18.5 with room-temperature plasticity of about 5%was prepared by minor addition of Nb and Cu in Zr60Al17.5Ni22.5 BMGs.During deformation,multiple shear bands can be observed from the fractured samples,which results in the good ductility of the present BMGs Furthermore,Zr55Al11Nb4.sTi4.5Ni25,Zr58Al9.5Nb3.75Ti3.75Ni25,Zr60A18.5Nb3.2sTi3.25Ni25,Zr62Al7,5Nb2.75Ti2.75Ni25,and Zr65Al6Nb2Ti2Ni25 BMGs also show an average room-temperature plasticity larger than 11%,especially for Zr6sAl6Nb2Ti2Ni2s BMGs whose plasticity can reach 30%.For those compositions mentioned above,the atomic pairs such as Cu-Nb,Nb-Zr,Cu-Ni,Nb-Ti,and Zr-Ti with large positive enthalpy of mixing can cause the local chemical inhomogeneous,which is beneficial to the formation of shear transformation zones(STZs)and then leads to the formation of multiple shear bands.Based on the SEM measurements,the multiple shear bands intertwined with each other and then could delay the formation of cracks,leading to the enhancement of the ductility of BMGs.Besides,in order to further enhance the ductility of BMGs,the effect of surface corrosion on mechanical properties of BMGs has also been studied.The result show that the appropriate surface corrosion can slightly improve the plasticity of Zr-based BMGs,which is due to the formation of the corrosion pits and the corrosion products.During deformation,the corrosion pits and the corrosion products can restrict the rapid propagation of one single shear bands and then induce the formation of multiple shear bands.However,BMGs may become embrittlement after being corroded by the acid due to the formation of hydrogen compound.Therefore,it is necessary to induce the formation of the uniform point corrosion on the BMG surfaces in order to improve the room-temperature ductility.