Local Atomic Structure And Polyamorphic Transition Of Pd-Ni-P Metallic Glasses

As a typical phase transformation of crystalline materials,the polymorphic transition?PT?is often recognized as one of the effective approaches to regulating the properties of materials.Although amorphous materials lack the long-range periodic order that characterizes a crystal,similar PTs have also been reported in these glassy substances,including metallic glasses?MGs?.Nevertheless,all the reported PTs in MGs are reversible,which makes it difficult to retain the products of the PT and thus limits the in-depth understanding of the underlying mechanism of the PTs.In this work,the pressure-induced PT,thermal-induced reentrant glass transition and the related mechisms,as well as the compositional effects on the PTs and crystallization behavior of the typical Pd-Ni-P MG system were systematically investigated via the combination of advanced experimental techniques and simulation analyses.The main findings of this thesis are as follows:?1?A unique pressure-induced PT was discovered in the Pd_41.5Ni_41.5P₁₇ MG.In-situ high-pressure XRD results show that an unexpected glass-glass transition occurs in the Pd_41.5Ni_41.5P₁₇ MG at around 6 GPa.This transition is characterized by a decreased modulus after the PT,indicating that the compressibility of the amorphous alloy increases with the increasing pressure.First-principle calculation results reveal that some covalent P-Ni?Pd?bonds in the as-cast MG transform into the metallic bonds under high pressure,thereby resulting in this unique PT and improving the compressibility of the MG.?2?The reentrant glass transition and its atomic mechanism were also discovered in the Pd_42.5Ni_42.5P₁₅ MG.Dynamic evolution of local atomic structures upon heating of the Pd_42.5Ni_42.5P₁₅ MG uncovers that Glass ? first goes into its supercooled liquid and subsequently transforms into a new Glass ?,accompanied by a precipitous drop in its volume,electrical resistance and specific heat,as well as a clear signal of local structural ordering at the short-to-medium-range scale.Atomistic simulations indicate that thermal agitation facilitates the dissociation of P-P bonds and then induces the formation of additional?Ni,Pd?-P bonds due to the chemical interaction,increasing the number of energetically favorable Ni?Pd?2P-and Ni?Pd?3P-like clusters.These clusters connect to form superclusters at the medium-range scale,leading to substantially ordered structure,lowering the free energy of Glass ?,and eventually achieving a thermodynamically and kinetically ultrastable MG.?3?The compositional effect of phosphorus on the reentrant glass transition behavior of the Pd-Ni-P MGs was discussed in detail.It is found that the reentrant glass transition would take place in the Pd_50-xNi_50-xP_2x MGs when the P content is between 13 at.%and 19 at.%.EXAFS results show that the occurrence of reentrant glass transition in Pd-Ni-P MGs is closely related to the rearrangement between the P and Ni/Pd atoms to form new?Ni,Pd?-P bonds in the system.When the P content is low?less than 13 at.%?,the fraction of the P-P bonds is too low to be decomposed,thus suppressing the bonding between the extra P and Pd/Ni atoms.When the P content is high?more than 19 at.%?,the?Ni,Pd?-P bonds have already been formed due to the excessive P atoms available.Therefore,when the P content is either too low or too high,the reentrant glass transiton is difficult to take place because of the suppression of forming?Ni,Pd?-P bonds.?4?Alloying effects of Pd on the glass formation and crystallization behaviors of the Ni₈₀P₂₀ alloy were investigated.The primary crystallization phases for the Ni₈₀P₂₀ and Pd₄₀Ni₄₀P₂₀ MGs are metastable superlattices and multiple phases,respectively.The introduction of Pd not only successfully inhibits the formation of metastable superlattice phase in the binary MG,but also changes the crystallization mode from the primary to eutectic mode,thus incresing the crystallization temperature and the crystallization activation energy,and promoting the stability of the ternary MG.Eventually,the glass formation ability is enhcnaced by the introduction of Pd.In summary,the pressure-induced PTs and thermal-induced reentrant glass transition in Pd-Ni-P MGs were systematically investiaged in this work,which would establish the basis for understanding PTs of amorphous materials in general.The ultrastable MG obtained through the reentrant glass transition can be easily retained to ambient conditions,which would provide a key model material for understanding the nature of the PTs and paves a new route for regulating the properties of MGs.