Phase Transition Of Metallic Droplets And Amorphous Particles Under Extremely Non-equilibrium Conditions By Nanocalorimetry

With the development of science and engineering,phase transition in metallic materials occurring under extremely non-equilibrium conditions attracts much attention.Nanocalorimetry,whose scanning rates can be up to 107 K/s with the sensitivity of nJ/K,has been developed to quantify those phase transitions in a controllable way.Combined with other characterization methods,it is not only promising to demonstrate the classical theories but also reveal the mechanisms behind some anomalous phase transitions under extreme conditions,which provides direct experiment evidence for the development and modification of new materials.In this dissertation,the solidification behavior of Sn-based alloys including Sn3.5Ag?wt.%?and Sn₃.0Ag₀.5Cu?wt.%?are revealed by nanocalorimetry.In addition,the glass transition,crystallization and melting behavior of Ce₆8Al₁0Cu₂0Co₂?at.%?bulk metallic glass are quantified based on this novel technique.The solidification behavior of SnAg and SnAgCu droplets is investigated by nanocalorimetry.Quenched at 15,000 K/s,undercooling of 0.22 Tm is obtained in a Sn3.5Ag droplet.Ag₃Sn are less than 100 nm in spite of some nanosized needle-like ones.There is a composition gradient in the order of 10⁹ /m between growing b-Sn and Ag₃Sn,which is able to decrease the driving force of nucleation and stabilize the undercooled liquid.By quenching,the undercooled liquid can turn to be amorphous layer.A nano-sized diffusion couple is put forward to illustrate the formation of this amorphous structure.In terms of Sn₃.0Ag₀.5Cu quenched at 20,000 K/s,the morphology of the Ag3 Sn is closely related to its position.There are more defects in the grain boundaries in which atoms can diffuse at a higher rate,leading to larger precipitates.While in grains,precipitates form at lower temperature and the atom diffusion is limited,causing the much smaller precipitates.A large number of precipitates with different sizes are distributed in the solidification structures,based on which the growth mechanism can be revealed.As initially precipitated,Ag₃Sn is spherical.Later,????02?plane grows along [???03] direction,forming the needle-like ones.Combined with differential scanning calorimetry?DSC?and nanocalorimetry,the crystallization of Ce₆8Al₁0Cu₂0Co₂ bulk metallic glass at heating rates from 0.083 K/s to 14,000 K/s is realized.At higher heating rates,a reduction of activation energy is observed,which is contrary to the assumption of Kissinger equation.By calculating the crystal growth rate between Tg and Tm,a decoupling of growth rate from viscous flow is observed because of the breakdown of Stokes-Einstein equation.In contrast,Ediger equation with D??^-0.865 can describe this decoupling effect at low temperature.Based on the classical nucleation theory,the nucleation rate is plotted accompanied with crystal growth rate to conclude that Al-based intermetallics rather than Ce crystals are the priority phases on crystallization.Due to the discrepancy in heating rates,crystallization products are different in DSC and nanocalorimetric measurements.At 10 K/min,AlCe3 and Ce are dominant after crystallization.While at 5,000 K/s,Al₁3Co₄ and Ce are the main crystalline phases.Al₁3Co₄ is quasicrystal and possesses structure similarity with Al-centered icosahedron in metallic glass,which can precipitate firstly attributing to the reduction of nucleation barrier.This metastable phase can bridge the local configurations in metallic glass and crystallization counterparts.By nanocalorimetry,amorphous Ce₆8Al₁0Cu₂0Co₂ can be in situ prepared.At cooling rates from100 K/s to 50,000 K/s,the solidification structure is tuned,which can be revealed by following reheating.It is concluded that glass forming ability is not only about the suppression of crystallization but also relating to the nucleation behavior.Specifically,crystallization can be suppressed at 10,000 K/s while homogeneous nucleation can be avoided at 50,000 K/s.Heterogeneous nucleation,on the other hand,is unavoidable under this condition.Following the formation of amorphous phase,annealing near glass transition temperature from 10-3 s to 25,000 s is performed.Relaxation,nucleation and crystallization occur continuously.By calculating the crystallization enthalpy and overall latent heat on following reheating,the kinetics of isothermal nucleation and crystallization is quantified.During annealing,a lot of nuclei form from the amorphous matrix and display size-dependent melting behavior on the followed reheating,which is demonstrated by the evolution of low-temperature melting peak.In terms of nucleation,both homogeneous and heterogeneous nucleation work in glass while the crystallization of undercooled liquid is dominated by heterogeneous nucleation respectively.As nanocrystals are widely distributed in the amorphous matrix,a transition layer exists between ordered and disordered phases,which can increase the glass transition temperature and improve the kinetic stability of residual amorphous phase.