Melting Behaviours Of Grain Boundaries Within Bulk Colloidal Crystals

Grain boundary（GB）melting is of basic importance to condensed matter science and material science.However,even the foremost question of whether GBs premelt is far from conclusive,due to the great difficulty of direct experimental tests.In this paper,the melting mechanism of tilt GBs with different misorientations in three dimensional crystals is studied.We focused an optical beam to locally heat single<111>tilt GBs within bulk face-centered cubic polycrystals consisted of thermosensitive hard-sphere-like colloids and observed the melting dynamics at single particle resolution using real-time video microscopy.The melting point is determined through analyzing both the Lindemann parameter and the critical nucleus size for homogeneous nucleation.We found that GBs can be superheated and melt via a heterogeneous nucleation mechanism,including high-energy GBs.All the measuredγ_b?(2γ_sl)≤0.95near melting point.Based on the classical nucleation theory of GBs,we measured the incubation time and contact angle of critical nucleus to compute all relevant kinetic factors,energy barrier,nucleation rate,as well as diffusion coefficient at solid-liquid interface,for each particular degree of superheating and misorientation pair under weak superheating.The superheat limits of GBs in various misorietations have also been measured for further investigation of the instability mechanism of GBs.For low-energy GBs consisted of dislocation array,each of dislocations attracts nucleation around its core and can be simply treated as isolated ones as misorientation decreases to～1°.Under traditional uniform heating,premelting occurs only at triple junctions,whereas GBs retain original structure up to melting point.The premelted region intrude into high-energy GBs by uniform liquid layers to interrupt them from superheating,but the premelted region cannot penetrate the low-energy GBs.The GB between two close triple junctions could exhibit pseudo“premelting”,corresponding to a grain size less than～10⁴.Our experiments provide insights into the melting scenario and help predict the evolution of polycrystalline materials during sintering,coarsening,crystal growth and glacial movement.