Design And Solidification Behavior Of Zr-Ce-Co-Cu Based Phase-Separated Metallic Glasses

The liquid-liquid separated two-phase metallic glasses obtianing two kind of amorphous phases are synthesised by liquid-liquid phase transformation.When a homogenous single-phase liquid is cooled into the miscibility gap,the liquid-liquid phase separation occurs.Subsequently,if the two coexisting liquid phases obtain a good glass-forming ability,the coexisting liquids undergo liquid-to-glass transition and thereby result in glass/glass composites.Previously,the two-phase metallic glasses were designed based on an amorphous system with a good glass-forming ability by addition of an element.The addition element should satisfy two conditions:(ⅰ)the addition element and the main component in the original amorphous alloy form an atom pair exhibiting a positive heat of mixing,meaning that the liquid-liquid phase separation will occur;(ⅱ)the addition element and the other components in the amorphous alloy also have a good glass-forming ability,ensuring the formation of the secondary glassy phase upon solidification.Up to now,the evolution of microstructure and chemical composition of coexsting liquids for liquid-liquid phase separated metallic glasses alloy during liquid-liquid separation process is unclear.It is very difficult to design a two-phase bulk metallic glasses with a critical size up to 1 mm due to the coexsting liquids with bad glass-forming ability.After understanding the problem of the two-phase metallic glasses and the process of liquid-liquid phase separation,we optimal design two-phase metallic glasses based on the interactions of component atoms,eutectic point criteria and the liquid system free energy minimization principle.Two-phase metallic glass may be obtained better ductility than single phase metallic glass at room temperature.The main research work and results are summarized as follows:The two-phase metallic glasses with the Zr-Ce-Co-Cu quaternary alloy are designed.The effects of A1 on the two-glass-forming ability(the ability of formation two glass phase)are explored.The liquid-liquid phase separation behaviors of the Zr-Ce binary system during solidification are investigated.It reveals that the Zr75Ce25 alloy undergoes a liquid-liquid phase decomposition and thereby results in Zr-rich crystalline layer/Ce-rich crystalline layer prepared by arc melting.The effects of alloying elements,i.e.Cu and Co,on the liquid-liquid phase separation are explored.The results show that the elements Cu and Co distributed in the Ce-rich and Zr-rich liquids,respectively,when both the Cu and Co are added together into the binary Zr-Ce system.The rapid solidification characteristics of the quaternary(Zr0.89Ce0.11)100-x(CocCud)x alloys are investigated.The effects of preparation methods and chemical composition on the microstructure formation have been studied.The results indicate that the Ce-Cu poor layer formed at the surface of a cylindrical sample(φ2 mm)prepared by copper mold casting.This is attributed to the Marangoni migration of the Ce-Cu-rich liquid droplets.For the alloy samples prepared by the melt spinning method,the average size of the Ce-Cu-rich particles near the wheel side is smaller than the center of the sample due to a higher cooling rate near the wheel side.The EDX analysis shows that when the atomic ratio and total mole fraction of Co and Cu are 4:1 and 38.5 at%,the compositions of the Zr-Co-rich and Ce-Cu-rich liquids approaches to their individual eutectic compositions,i.e.Zr65;Co35 and Ce72Cu28,respectively.The two coexisting liquids may undergo the liquid-to-glass transition and form two-phase metallic glasses with Ce-Cu-rich glassy particles(volume fraction = 9.4%)embedded in Zr-Co-rich glassy matrix.The effects of addition element Al on the liquid-liquid phase separation and solidification microstructure of the quaternary Zr-Ce-Co-Cu alloys have been investigated.The results show that the Al distributes uniformly in coexisting liquids when the content of A1 is less than 15 at.%.When the A1 content is 15 at.%,the addition element Al can enhance the two-glass-forming ability,and the two-phase metallic glass containing Ce-Cu-Al-rich glassy particles embedded in the Zr-Co-Al-rich glassy matrix can be prepared by copper mold casting with a diameter of 1.5 mm.The two-phase metallic glasses with the RE-Cu-Al-rich glassy particles embedded in the Zr-Co-Al-rich glassy matrix are designed.For the coexisting liquids produced by liquid-liquid phase separation,the Zr-Co-Al-rich liquid has a better glass-forming ability in contrast to the Ce-Cu-Al-rich liquid.In order to improve the glass-forming ability of the Ce-Cu-Al-rich liquid,the effects of partial substitution of Ce with similar rare earth elements La.Pr and Nd on the liquid-liquid phase separation and formation of two glasses of Zr-Ce-Co-Cu-Al alloy have been explored.The results indicate that the liquid-liquid phase separation still occurs when the component Ce is partially substituted with the La,Pr and Nd.The rare earth elements(Ce,La,Pr and Nd)mainly are enriched in the Ce-Cu-Al-rich liquid,resulting in increasing the entropy and glass-forming ability of the Ce-Cu-Al-rich liquid.It means that partial substitution of Ce with La,Pr and Nd can improve the two-glass-forming ability of the Zr-Ce-Co-Cu-Al alloy.The two-phase metallic glass[(Zr68Co32)0.86(RE54CU48)0.14]0.85Al0.15 corntaining RE-Cu-Al-rich glassy particles(volume fraction =7.95%)in Zr-Co-Al-rich glassy matrix have been successfully prepared by copper mold casting with a dimeter of 2 mm.In addition,the two-phase metallic glasses with the Zr-Co-Al-rich glassy particles embedded in the RE-Cu-Al-rich glassy matrix are designed.The effects of Al content(7.7-16.7 at.%)on the thermal stability of Zr-Co-Al-rich and RE-Cu-Al-rich glassy phases have been studied.The hardness and deformation mechanism of the two-phase metallic glasses have been investigated.The results indicate that the Zr-Co-Al-rich and RE-Cu-Al-rich glassy phases show a good thermal stability when the Al content is 10.7 at.%.The Zr-Co-Al-rich glassy phase has a higher hardness in contrast to the RE-Cu-Al-rich glassy phase.The Zr-Co-Al-rich glassy particles can effectively impede the direct propagation of shear bands in the glassy matrix during the plastic deformation,resulting in the initiation of multiple shear bands and the increases of ductility and strength.