Investigations Of The Non-density-driven Liquid-liquid Transition Of Metallic Liquids

Liquid-liquid transition(LLT)refers to a first-order transition from one liquid to another liquid without ungoing any compositional changes.LLTs have been reported in many types of the liquids including the metallic liquids.Studying LLT is essential to understand the fundamentals of the glass and liquid.In this thesis,the exisiting LLTs(or amorphous-to-amorphous transitions,AATs)convering a variety of glass systems are firstly reviewed from the perspectives of order-parameter,thermodynamics and kinetics.Classical liquid theory describes the simple liquid by pair correlation function that is hard to explain the complex liquid and the non-density driven LLT.Two-order-parameter model(TOM),however,adds a second order-parameter of the volume fraction of the locally favored structures besides the density to describe complex liquid and is successful in understanding LLT and the glass transition.The TOM enlightens this work and has been conveyed to explain the discoveries.This thesis investigates a series of the La(or Ce)-Ni(or Co)-Al metallic glasses(MGs)and finds evidence of a first-order LLT.On the differential scanning calorimetry heating curves,an exothermic peak has been discovered when MG firstly enters the supercooled liquid.As confirmed by high-energy X-ray scattering and high-resolution transmission electron microscopy(HRTEM),this exothermic peak is not a crystallization process but a first-order LLT.Using fast differential scanning calorimetry(FDSC),the reversible“melting-like”endothermic process is discovered at high heating-rate where crystallization is bypassed.The product of the LLT can be preserved through vitrification,which is named as metallic glacial glass(MGG).In comparision with the original MG,MGG has a lower configurational entropy,a higher glass transition temperature,a larger hardness,a higher Young's modulus and unique relaxation behaviors.The MGG existence is a fingerprint of LLT in metallic liquid.This thesis explores the kinetics of the non-density-driven LLT of the MG-forming liquids.In-situ vitrification,isothermal annealing,and fast heating is performed on the Pd₄₀Ni₄₀P₂₀and Pd_42.5Ni_42.5P₁₅ MG systems.The Pd₄₀Ni₄₀P₂₀ exhibits relaxation behaviors of one-endothermic peak after thermal annealing,whereas the Pd_42.5Ni_42.5P₁₅shows an abnormal two endothermic peaks during the glass transition.The two endothermic peaks are contributed by both glass transition and the reverse process of LLT indicating a forward LLT are induced during annealing.With the decreasing annealing temperature,LLT kinetics is slowed down and stoped when reaching a critical temperature.Such critical temperature seprates two modes of the LLT:above the critical temperature is the nucleation-growth LLT,and below the critical temperature is the spinodal-decomposition LLT.The spinodal-decomposition type LLT can occur not only above but also below the glass transition temperature suggesting a true glass-to-glass transition.This thesis further studies the order-parameter of the non-density-driven LLT of Pd-Ni-P MG system.It is found that the P alloying element plays an important role on LLT of metallic liquid.For(Pd_0.5Ni_0.5)₁₀₀–_xP_x(x=15-20),the decrease of P concentration decides the occurence of LLT but not on the vitrification tempearature.Ab-initio simulation reveals that the most prominent clusters existing in the(Pd₅₀Ni₅₀)₁₀₀–_xP_xliquid is the P-centered<0360>and<0440>clusters.S-electron density on Fermi surface of<0360>cluster is higher than that of<0440>.The volume fraction of<0360>cluster increases upon cooling and saturates after vitrification.Comparing Pd_42.5Ni_42.5P₁₅ with Pd₄₀Ni₄₀P₂₀,the<0360>increasing rate against the decreasing temperature of the former is larger than the latter.These results indicate that the volume fraction of<0360>cluster may be the order-parameter governing LLT in Pd-Ni-P.According to the TOM,the cooperative stimulation of the locally favored structure is the key to LLT.From the temperature-order parmeter phase diagram,whether LLT can be observed depends on whether the liquid coexistence line is above the glass transition temperature.