Generalization For Fragility Of Alloy Melts

It is of great significance to investigate the properties of alloy melts,especially the dynamic and thermodynamic properties.The properties and microstructures of alloys depend on that of their melts to a large extent.The microstructure of the solid alloy could be intrinsically improved by melt treatment.Hence,the best performance of application could be acquired eventually.From the physical point of view,the dynamic viscosity is strongly sensitive to structural changes with respect to temperature as well as the sample composition.Thus, besides the mentioned technological aspect,determining the dynamic viscosity of liquid alloys will contribute to understanding the relations between the atomic structure and physical properties.The theory of fragility of melts is one of the important and valuable methods to analyze the temperature-dependent viscosity of alloys.The concept of fragility in supercooled liquids proposed by Angell gives new insights into glass transition,structural relaxation phenomenon,glass or supercooled liquid structure,etc. The fragility index m indicates the glass forming ability(GFA) of the liquids.In general, the smaller the index m,the greater the GFA.However,the parameter T_g(the glass transition temperature) is dependent on the scanning rate of DSC test.The differences of T_g are evident when the scanning rate varies violently.Fortunately,Professor Bian introduced the index M for superheated melts,which is calculated by the T_L-scaled temperature(T_L is the liquidus temperature).The accuracy and certainty of the melt fragility is assured according to this definition.It is very convenient for the researchers to determine the GFA of alloys according to the corresponding parameters of superheated melts without acquisition of metallic glasses.The fragility theory has been widely applied to the relaxation of glasses and the estimation for GFA of alloys,in order to prepare bulk amorphous alloys in recent twenty years.The researchers have found that the correlation between the fragility of alloy melts and their solid physical properties,bulk elastic modulus and shear elastic modulus,recently.But the studies on such relation are insufficient.So,it is very significant and crucial to discover the inherence of the transition from liquid to solid of alloys through the connection between fragility and solid physical properties.The research focused on the high purity eutectic Al-12Si alloys.The temperature-dependent viscosity is obtained by high-temperature viscometer.The influence of P-modification on the viscosity is obvious according to viscosity-temperature plots.The fragility concept is introduced to analyze the changing law of melts viscosity and to establish the mechanism of P-modification.The thermal stability of alloys is well reflected by the coefficient of thermal expansion(CTE)α, which is the result of anharmonic vibration of atoms.Viscosityηreflects the jump and move of the flow units in liquid alloy.The parametersηandαare both concerning thermodynamic property.Therefore,the solid microstructure of alloys inherited from their liquid microstructure can be determined qualitatively by the correlation betweenηandα.The experimental results show that P-modification increases the viscosity of Al-Si alloys.The viscosity varies obviously with P-additional level.The viscosity rises violently when the P-concentration couldn't modify the alloy or slightly when the primary silicon precipitates.The forms of modifier P also alter the viscosity to different extents.When added in the form of red phosphorus,the viscosity of the alloy is almost 2 times higher than that of unmodified alloy.The viscosity of Al-12Si alloy increases about when added in the form of Al-2.5P master alloy.By comparing the values of M of alloy melts,we can find the correlation between P-concentration and modifying effect. The value of M of the melt that is not effectively modified is the largest,while the value of M of the melt effectively modified is much less.The equation M=2 can be taken as the critical value to judge the modification effect of P.It is found that the activation energy for viscous flow E_αand the interatomic distance r change with the modification process.The correlation between r and M can be shown as r=1.9+1.6M.Only when M＜2 can the stable Si-Si clusters originate in the Al-Si melts according to the lattice constants of Si and Al,then the primary silicon precipitates and the modification accomplishes. Some peaks in the CTE-T curves result from the dilatometric experiments in the temperature zone 500K to 620K.Alloying elements Ge,Ni,Ce and P all increase the maximum valueα_max,of CTE,but Ge,Ce and P decrease the CTE in the high temperature region.The changes of CTE are synthetically caused by the elastic modulus, crystal interface effect and vacancies of the alloys.The ordinary heat-treatment process, solid-solution and aging,is probably attributed to the changes of microstructures in 500K to 620K.So,we suggest that the efficiency and effect of aging treatment should be improved greatly to strengthen the properties of alloys when the heat-treatment temperature rises to 500 to 620 K.And we conclude that the thermal stability of the alloy is the best after it is modified with 200ppm phosphorus.The parameter M(m) not only characterizes the stability of alloy melts,but also is the key connecting the liquid properties of alloys with their solid ones.The little change of M reflects the greater alteration of the microstructure of the melt.The fragility M is negative linear dependent on bulk elastic modulus K and positive linear dependent on E_α/V.And the correlations among M,m,E_α,αcan be formulated as follows:αE_α/R∝m,αE_α/R∝M,respectively.The fragility index is a generalized physical parameter representing the microstructures of alloy melts and the properties of their transformation from liquid to solid.