Mechanism And Kinetics Of Liquid-liquid Phase Transition In Metallic Liquids

The theory of simple liquids is a crowning achievement of condensed matter physics in the second half of the 20th century.It captures the key role of repulsive interactions between atoms or molecules in determining liquid structures as elucidated by the hard sphere model and the van der Waals picture of liquids.By contrast,the effect of attractive interactions were simply treated in terms of a mean field—a spatially uniform background potential,which solely provides the cohesive energy to stabilize the liquid system at a certain density and pressure,and has negligible effect on the liquid structures and dynamics.However,evidences of complexity in simple liquids and the essential role of attractive interactions are out there and growing.Repulsive interactions alone and trivial treatment of attractive interactions as in the van der Waals picture are inadequate,not only quantitatively but qualitatively,to describe some of the complex properties of“simple”liquids.Liquid-liquid transition(LLT)and the complex dynamics of glass-forming liquids are the representatives of such complexity.Further studies of these issues are of crucial importance in the development of theories of complex liquids,glass transition and crystallization.Therefore,in this dissertation,the liquid-liquid transitions in the equilibrium melts of Pd-Ni-P,pure aluminum and aluminum alloys are systematically studied using the methods of nuclear magnetic resonance(NMR),flash differential scanning calorimetry(FDSC),high-energy synchrotron X-ray diffraction(XRD),molecular dynamics(MD)simulation and high-resolution transmission electron microscopy(HRTEM),and the effects of liquid-liquid transition on the glass-forming ability and crystallization are also explored.Since the LLT is usually very subtle,a sensitive technique is essential to reveal this phenomenon.NMR is able to capture the subtle changes in local structures of liquids and hence is a powerful technique to detect LLT.In order to perform the measurements on liquid metals which are usually at high temperature,a high-temperature NMR probe is required.In this dissertation,each part of a homemade high-temperature NMR probe,together with the temperature calibration of the probe,is introduced in detail at first.The LLT in the equilibrium liquid of Pd_42.5Ni_42.5P₁₅ and its effects on the glass-forming ability are studied using high-temperature NMR,FDSC and high-energy XRD.In-situ NMR experiments reveal a first-order LLT above the liquidus temperature of Pd_42.5Ni_42.5P₁₅,with no detectable change in density at the phase transition temperature T_LL=1063 K.Further analyses suggest that the discontinuous jump of the number of locally favored structures may be the mechanism of this LLT,consistent with the two-order-parameter model.Two corresponding glasses are formed from the two liquids,and the high-energy XRD results indicate that the glass obtained from the low-temperature liquid(stable liquid phase below T_LL)is more ordered in structure than the glass obtained from the high-temperature liquid(stable liquid phase above T_LL).The glass-forming ability of high-temperature liquid is much better than that of low-temperature liquid as revealed by the FDSC.Further analyses suggest that the crystallization of high-temperature liquid is a two-step process,i.e.,first transforms to the low-temperature liquid and then crystallizes.The glass-forming ability of high-temperature liquid is hence determined by the kinetics of LLT.The poor glass-forming ability of low-temperature liquid could be attributed to the significant medium-range ordering in its undercooled state.These results are of great significance to the understanding of the crystallization and the glass formation,and also to the optimizing of the structures of metallic glasses.To reveal the prevalence of the LLT,the high-temperature NMR and MD simulation are used to study the LLT in the monatomic liquid aluminum.The NMR results indicate that an LLT takes place at T_LL=1163 K,also with no detectable change in density at T_LL.Below T_LL,the structural relaxation time of high-temperature liquid towards the low-temperature liquid decreases exponentially with the increasing of the degree of undercooling.MD simulations suggest that this phase transition is associated with the percolation of cooperatively aggregated icosahedral-like clusters.The existence of LLT in monatomic liquid system and the well-established icosahedral-like clusters in metallic liquids suggest that LLT could be prevalent in nature.Based on the above results of pure liquid Al,the liquid properties of aluminum alloys Al-Cu and Al-Si are further studied using high-temperature NMR.Liquid-liquid transitions are observed in the hypoeutectic,eutectic and hypereutectic Al-Cu alloys,and they become more pronounced as the content of Cu increases.While for the Al-Si system,weak LLT is only observed in the hypoeutectic alloy melt.Such difference may be because that the icosahedral-like clusters are promoted in Al-Cu liquids while they are inhibited in the Al-Si liquids.These results provide important theoretical guidance for the superheating treatment used in the casting of aluminum alloys.