Research On Glass Forming Ability And Crystallization Behavior Of Al-TM-RE Amorphous Alloys

Al-based amorphous alloys are promising materials due to its high specific strength,good toughness and excellent corrosion resistance.However,at present,the maximum size of Al-based amorphous bar sample that can be fabricated by melt casting is merely 2.5 mm in diameter,which greatly limits its engineering application.Thus,deeply understanding the factors that restrict the glass forming ability of Al-based amorphous alloys,and searching for better glass former in Al-based system have become essential in the field of amorphous alloys.Therefore,this thesis firstly summarized and analyzed the existing theories designing Al-based amorphous alloys,and then proposed a new method to predict the optimal glass forming composition.Then,the influence of similar element substitution on crystallization behavior of Al-based amorphous alloys was studied,aiming at knowing more about the regulation of primary phase selection,thermal stability and the possible relation between them and glass forming ability.The main results are as follows:Combining the stability theory of electronic system(KP=2KF criteria,where KP is the diameter of pseudo Brillouin zone interface,KF is the radius of Fermi sphere),short range cluster packing in the efficient cluster packing model(ECP)and simultaneous efficient packing model(SEP),and considering the composition characteristics of Al-based amorphous alloys,a new approach of predicting the best glass former in Al-TM-RE system was proposed.That is:the alloy compositions that satisfy the stability theory of electronic system locate on a curve in the composition triangle,and the alloy compositions that satisfy the efficient atomic packing lie on a straight line.Thus,the alloy at the intersection of these two curves will be stable both in electronic and topological aspects,and should be the best glass former.The optimal glass forming composition of Al-Ni-La alloys predicted by this method is quite close to the experimental results in the literature,and the prediction method is also effective to a certain content in Al-Co-Y system.By gradually substituting La with Gd in the Al-Ni-La base alloys,the experimental best glass former(Al85.5Ni9.5La5)and its six surrounding alloys(Al85Ni10La5,Al85.5Ni10La4.5,Al86N19.5La4.5,Al86Ni9La5,Al85.5Ni9La5.5 and Al85Ni9.5La5.5),the effect of rare earth atomic substitution on the primary phase and thermal stability of the amorphous alloys during crystallization was studied.The results show that the gradual replacement of La by Gd generally reduces the thermal stability of Al-Ni-(La,Gd)amorphous alloys,and promotes the precipitation of single ?-Al phase in the first crystallization stage.Comprehensively considering the size of solute atoms,melting point of Al-rich compounds and coordination number of solute atoms according to the ECP model,a new parameter(?)has been proposed to characterize the thermal stability of Al-TM-RE(TM—transition elements,RE—rare earth elements)amorphous alloys.The ? parameter shows a high linear fitting coefficient of 0.89 with the onset crystallization temperature(Tx)of Al-Ni-(La,Gd)alloys in the present work,and also shows a linear fitting coefficient of 0.75 with Tx of Al-(Fe,Co,Ni,Cu)-(La,Y)alloys in the literature.By analyzing the type of primary phase of various Al-TM-RE amorphous alloys,it is concluded that the type of primary phase of Al-TM-RE amorphous alloys is related to the atomic size,content and corresponding Al-rich compounds of the solute elements.The larger the solute atom,the higher the solute content in the alloy,and the lower the solute content in the Al-rich compound,the easier for metastable compounds to precipitate.On the contrary,it is favorable for the precipitation of primary ?-Al phase.In analyzing the influence of Ti or Zr substitution of Ni on the thermal stability and primary crystallization phase of Al86Ni9La5 amorphous alloy,it is found that the thermal stability of amorphous alloy first increases and then decreases with the increase of Ti or Zr contents.Thereinto,a little substitution of Ni by Ti significantly improves the thermal stability and makes the metastable phase appear as primary phase,while Zr substitution of Ni does not improve the thermal stability as significantly as Ti,and the primary phase remains unchanged.The reason may be that Zr is more inclined to bond with solute atoms of Ni and La than Ti,so that part of the Al atoms that should be bounded with Ni and La are replaced and aggregated,and then the resulting heterogeneous structure will affect the crystallization behavior.