Effects Of Elemental Additions On The Formation And Microstructural Evolution Of Zr-based Amorphous Alloys

The influence of thermodynamics,kinetics and structure on the glass forming ability(GFA),as well as formation mechanism of amorphous alloys,is one of important issues for materials science.At present,this is also one of key research directions in amorphous materials and physics.Amorphous alloys are composed of close-packed clusters with a size less than 1 nm,and this structure is closely associated with their forming ability.Minor addition(Microalloying) technique plays an effective and important role in increasing the GFA of alloys, enhancing the mechanical/thermal stability of the amorphous phase,and improving magnetic and mechanical properties of amorphous alloys.Moreover, Minor addition technique is an effective method to explore novel amorphous materials and improve their properties.In the present work,we used Zr-based binary alloys as base alloys and pertinently selected elements with different atomic size magnitude,different elements with close atomic sizes,intermetallic compound and outphase amorphous alloys as additions.Using the mechanical alloying(MA) method,we investigated the influence of different kinds of additions on the microstructural evolution behavior of MA induced Zr-Ni-based alloy powders,and further probed into the effect and mechanism of additions of elements or compounds on the GFA and mechanical/thermal stability of the amorphous alloys.In addition, we also studied the influence of elemental additions with different atomic sizes on the microstructural evolution and corrosion-resisting properties of rapidly solidified Zr-Ni-based amorphous alloys using the single-roller melt-spinning technique.The correlation between the effect of elemental additions and fabrication techniques of the amorphous alloys has also been investigated.From the viewpoint of research direction,the present study supplies general experimental and theoretical bases for the fabrication of metallic amorphous materials/composites making use of elemental additions and the improvement of their corrosion-resisting properties.This study also expands the fabrication methods for the amorphous alloys and their composites.Therefore,our work is very important for the exploration of new amorphous alloy systems and for the optimization of alloy compositions.We mainly used X-ray diffraction(XRD), differential scanning calorimetry(DSC),scanning electron microscopy(SEM), transmission electron microscopy(TEM),high resolution transmission electron microscopy(HRTEM) and energy dispersive X-ray analysis(EDX) in this work. The research contents and conclusions are described as follows.Using the MA method,the single amorphous phase of Zr_66.7-xNi_33.3C_x, Zr_66.7-xNi_33.3La_x and Zr_66.7-xNi_33.3Ag_x(x=0,1,3,5at.%) has been successfully fabricated through substitution of non-metallic C with small atomic size,Ag with intermediate atomic size and rare-earth element La with large atomic size for the base metal Zr in Zr-Ni.Under the same milling conditions,the additions of C,La and Ag has an obvious effect on the microstructural evolution behavior of Zr-Ni alloys.An amount of C additions can markedly shorten the starting time of the amorphization reaction,facilitate the amorphization process,increase the GFA of the alloys,and improve the mechanical stability of the amorphous alloys. Similarly,the La addition can greatly shorten the starting time of the amorphization reaction and improve the GFA of the Zr-Ni alloy powders. Furthermore,the addition of 1-5at.%La can obviously enhance the stability of the amorphous phase against the mechanical crystallization.With increasing La addition,however,the mechanical stability of the Zr-Ni amorphous powders decreases.The Ag addition cannot improve the GFA of Zr-Ni,but significantly extends the milling time for the stable existence of the amorphous powders and greatly improves the mechanical stability of the amorphous phase.In addition, the amorphous phase starts to mechanically crystallize with increasing milling time when subjected to shearing and collision forces of milling media.Moreover, the Zr-Ni-C and Zr-Ni-La amorphous phases gradually transform into an fcc-phase,but the Zr-Ni-Ag amorphous phase transforms into a bc-phase,with a certain amount of elemental additions.We comprehensively compare the addition effect of three elements(C,La,Ag) with quite different atomic radii under the same milling conditions.The addition of 1 at.%La shows an optimum effect on the improvement of the GFA and mechanical stability of the Zr-Ni amorphous alloys.The effect of the C addition is better than that of the Ag addition for the improvement of the GFA of Zr-Ni,but the effect of the Ag addition is better than that of the C addition for the improvement of mechanical stability of the amorphous phase.Therefore,the mechanical stability of the MA induced Zr-Ni amorphous phase increases with increasing atomic radius of the additional elements.In addition,the influence of elemental additions with different atomic sizes on the microstructural evolution behavior of mechanically alloyed Zr-Ni alloys has been discussed based upon heat of mixing between elements,mismatch of atoms,pressure produced during ball milling and atomic clusters(bond order),and so forth.We selected Pd,Ag and Au with similar atomic radii and chemical properties as additional elements,and investigated the influence of these elements on the microstructural evolution behavior of MA induced Zr-Ni alloys under the same ball milling conditions.We also probed into the mechanism of effect of elemental additions with similar atomic radii on the GFA and mechanical/thermal stability of Zr-Ni alloys.The minor additions of Pd,Ag and Au(1 at.%) do not improve the GFA of Zr-Ni alloys,but the minor addition of Au can markedly increase the mechanical stability of the amorphous alloys. Furthermore,the Zr_65.7Ni_33.3Au₁ amorphous alloy has higher thermal stability. According to the variation(with increasing milling time) of nearest-neighbor atomic distance corresponding to the position of the first maximum peak(2θ) on the XRD patterns of the amorphous alloy,it has been found that the structural relaxation or atomic rearrangement of the Zr_65.7Ni_33.3Au₁ amorphous alloy always takes place during the entire MA process.For the Zr_66.7Ni_33.3, Zr_65.7Ni_33.3Pd₁ and Zr_65.7Ni_33.3Ag₁ amorphous alloys,however,the microstructural transition occurs only at the initial milling stage.The local microstructure of these amorphous alloys almost keeps invariable with further prolonged milling time.Therefore,it is reasonable to assume that the initial crystallization of these three amorphous alloys proceeds through the interchange of the sites of neighboring atoms,keeping the relative atomic distance of the involved atoms invariable.During the later crystallization process,the long-range diffusion of atoms occurs resulting in the ordered atomic configuration.The influence of Nb with intermediate atomic size on the microstructural evolution of mechanically alloyed Zr_66.7-xCu_33.3Nb_x(x=0,2,4at.%) alloys has been investigated.At the lower speed of 200 rpm,the Nb addition does not lead to the formation of a single amorphous phase,but an amount of Nb addition obviously accelerates the amorphization process and shortens the starting time of the amorphization reaction,indicating the improvement of GFA.Moreover,the GFA of mechanically alloyed Zr-Cu-Nb alloys increases with increasing Nb addition.At the higher speed of 350 rpm,a single Zr-Cu-Nb amorphous phase can be obtained.Furthermore,the Nb addition can also accelerate the amorphization process and shorten the starting time of the amorphization reaction.The mechanical stability of the Zr-Cu-Nb amorphous phase increases with increasing Nb content.In addition,the Zr_66.7Cu_33.3 amorphous phase mechanically crystallizes with increasing milling time,gradually transforming into the fcc-Zr₂CU phase.The intermetallic compound TiC was selected as enhanced particles,and the solid-solid reaction occurred during MA of Zr-Ni-based alloy powders.We investigated the influence of the TiC addition(1-5wt.%) on the microstructural evolution during the crystalline-amorphous transformation and on the mechanical crystallization behavior.The TiC enhanced amorphous composites have been successfully synthesized,and the TiC addition markedly affects the microstructural evolution behavior of Zr-Ni alloy powders.Based upon the EDX analysis,we have found that the diffusion of TiC among the atoms of Zr and Ni is inhomogeneous,leading to the increase of the disorder degree of atoms in local regions.Therefore,the TiC addition improves the GFA and stability of the Zr-Ni alloys.The addition of 5 wt.%TiC not only shortens the starting time of the amorphization reaction,but also improves the GFA of Zr-Ni alloy powders and greatly enhances the mechanical stability of the amorphous composites.The DSC results demonstrate that the effect of the addition of 3 wt.%TiC is better than that of the addition of 5 wt.%TiC on the improvement of thermal stability of the amorphous phase,suggesting that there is no correlation between thermal stability and mechanical stability of MA induced Zr-Ni-based amorphous alloys.We used MA-synthesized Zr_66.7Ni_33.3 amorphous phase as the base alloy and outphase Cu₅₀Ti₅₀ amorphous phase with different composition as the addition. The MA induced solid-solid transition mechanism of mixing amorphous powders has been studied using different testing instruments.We probed into the influence of the addition of the outphase amorphous phase on the forming ability, crystallization behavior and thermal stability of the mixing amorphous powders, and further discussed the interaction relationship of different atomic clusters and the effect mechanism.The addition of the outphase Cu₅₀Ti₅₀ amorphous phase can markedly influence the microstructural evolution behavior of the Zr_66.7Ni_33.3 amorphous alloy.Moreover,the interaction between two kinds of atomic clusters with different types can increase the disorder degree of atoms or improve the mechanical stability of the disorder.The addition of 3 wt.%Cu₅₀Ti₅₀ amorphous alloy can give rise to the cyclic amorphization transformation with increasing milling time.In addition,the diffusion of Cu and Ti atoms is inhomogeneous with increasing Cu₅₀Ti₅₀ addition,resulting in the occupation of Ni sites by Ti in the icosahedral Zr₉Ni₄ cluster and the increase of interatomic affinity.Therefore, the disorder of atoms increases and the mechanical stability of the mixing amorphous powders can be improved.Under the same milling conditions, however,the thermal stability of the mixing amorphous powders decreases with increasing addition of the outphase amorphous alloy.In comparion to the MA method,we investigated the amorphization mechanism of Zr-Ni-M(M=Si,Pd or La) with the additions of Si with small atomic size,Pd with intermediate atomic size and La with large atomic size under rapid solidification conditions,using the single-roller melt-spinning technique.In combination with the MA results,we systematically discussed the influence of elemental additions with different atomic radii on the GFA,thermal stability and microstructure of Zr-Ni-based alloys.In addition,the corrosion-resisting experiments were carried out to study the correlation between the addition effect of different elements and the corrosion-resisting properties of the amorphous alloys,using the emerging method.It has been found that the addition of La with large atomic size can markedly improve the GFA of Zr-Ni amorphous alloys.In terms of atomic model for metallic glasses proposed by Miracle,the R value of Ni-La is possibly closest to the R_N^* value,leading to the most effective arrangement of atoms in clusters.This explains the good effect of the La addition,similar to the MA results.The additions of Si,Pd and La can improve the thermal stability of Zr-Ni-M amorphous alloys,and this has been further confirmed by the evaluation of activation energy of the amorphous alloys. Moreover,the thermal stability of the amorphous alloys increases with increasing addition,and the addition of 3 at.%Si exhibits the optimum effect on the improvement of the thermal stability of the amorphous phase.However,the variation tendency of thermal stability of the amorphous phase does not change with increasing cooling rate.Additionally,the corrosion-resisting experiments show that the addition of Pd with intermediate atomic size greatly decreases the corrosion-resisting properties of the amorphous ribbons.The substitution of La for Zr or Ni may lead to the passivation of Zr-based amorphous alloys.Therefore, the addition of La(3at.%) can markedly improve the corrosion-resisting properties of Zr-Ni amorphous ribbons.