A Simulation Study On The Heredity And Evolution Of Clusters In Liquid Pd-Si Alloys During The Rapid Solidification

Amorphous alloy or metallic glass?MG?have attracted much attention because of its unique physical,chemical and mechanical properties.However,its limited glass forming ability?GFA?restricts the wide application of this new alloy materials.GFA of metallic glass has always been a hot and difficult issue in the field of amorphous materials.Since metallic glasses are essentially"frozen"liquids,their local atomic structures are inherited from metallic liquids to some extent.Therefore,to study the hereditary characteristics of clusters during the rapid solidification of liquid alloys will be helpful to understand the micro-structure mechanism of GFA.Molecular dynamics?MD?simulation can give the position and velocity of any particle in the system at any time,it is a powerful tool to study the heredity and evolution of local atomic structures of the rapidly solidified alloys.In this thesis,therefore,the rapid solidification of liquid Pd-Si alloys was simulated by MD,and the microstructures of rapidly solidified alloys were characterized and analyzed by pair correlation function,bond-pair index and cluster type index method?CTIM?.The heredity of characteristic cluster of rapily solidified alloys were tracked and analyzed,and the correlation of GFA with heredity of cluster was also established.The following provides the main contents of this thesis.Firstly,the local atomic structures and hereditary characteristics of liquid Pd₈₀Si₂₀ alloy during rapid solidification were studied.It was found that the main local atomic structures in Pd₈₀Si₂₀ amorphous alloys are Kasper clusters and their distorted configurations.The bi-capped Archimedean antiprism?BSAP?structures plays a critical role in the formation of Pd₈₀Si₂₀ glassy alloys,and the Si-centered BSAP clusters are manily isolated each other in the glassy alloys,only some Pd-centered Kasper clusters can be formed into medium-range orders?MROs?by intercross-sharing?IS?.The heredity of Kasper clusters indeed emerges in the supercooled liquid region,and the dominant BSAP clusters have the highest onset temperature T_onset of herdtity and fraction F of direct heredity of suppercooled liquids.The T_onset of Kasper clusters has a linear relationship with the F at T_g.Secondly,based on the heredity and evolution of basic clusters,the binding energy and electronic structure of the characteristic clusters of Pd₈₂Si₁₈ alloy were systematically analyzed by first-principles calculation.The results show that the Si-centered Kasper clusters remain play a key role in the formation of Pd₈₂Si₁₈8 glassy alloys.The BSAP clusters are the characteristic clusters of Pd₈₂Si₁₈ alloy,which possess the largest number in glassy solids and the highest fration of heredity.Relative to other canonical Kasper clusters,BSAP clusters have the lowest binding energy and the overlapping charges between shell Pd atoms and central Si atoms.There exits a good correlation of heredity with the average binding energy for Kasper clusters:the lower the binding energy,the better the heredity.Then,the effect of Si concentration on the micro structures of Pd_100-x00-x Si_x MGs was investigated.It was found that the lower the Si content,the fewer the basic clusters,and the more the number of characteristic clusters and the higher the order degree of glassy alloys.In a wide concentration range of Si,the Si-centered Kasper clusters play an important role in the formation of MGs,especially the BSAP clusters.The numbers of Z9,Z10 and Z11 canonical Kasper clusters and their extended configurations correspond well to the reduced glass transition temperature T_rg of the rapidly solidified Pd_100-xSi_x alloy.In the glassy solids with different Si concentration,only a small number of non-characteristic clusters with coordination number Z>12 are connected by IS,and their number decreases with the increase of Si concentration.Finally,the effect of high pressure on the structure and its hereditary characteristics of Pd₈₂Si₁₈ alloy were investigated.The results show that with increasing pressure,the number of low-coordination clusters such as Z9 and Z10 in glassy solids decreases significantly,while the number of high-coordination clusters such as Z11,Z12,Z13 and?142/1441 8/1551 2/1661?increases obviously.Under the pressure P>10GPa,the dominant role of BSAP cluster are replaced by Z11 Kasper cluster in glassy alloys.Higher pressure does not significantly affect the linkage model of low-Z Kasper clusters such as Z9 and Z10 in MGs,but can promote the formation of high-Z clusters such as Z13 and?142/14418/15514/1661?.Increasing pressure can significantly increase the fraction and onset temperature of the staged heredity of Kasper clusters.Still,the Z10 and Z11 Kasper clusters play the critical role in the GFA of Pd₈₂Si₁₈ alloys.