Formation Mechanism Of Phase Separated Metallic Glass And Its Plastic Deformation

The metallic glasses?MGs?show excellent mechanical,physical and chemical properties.Therefore,they are used as functional and structural materials.However,the poor plastic deformation at room temperature caused by the special microstructure has become a bottleneck which limits its application.It is found that the nano-scale phase separated MG is an effective way to improve the room temperature plasticity,with the mutually exclusive property between the components in phase separated?immiscible?alloys.At present,the phase separated MG formation mechanism and the plastic deformation mechanism are not understood.Fe-Cu alloy is a typical phase separated alloy,so the phase separated MG system formed by Fe-Cu is representative.A systematic investigation on Fe-Cu phase separated MG is made by both computer simulation and experiments.Molecular dynamic?MD?is used to characterize the structure and thermodynamics properties in miscible and immiscible alloy systems.We discuss the liquid-solid transition,phase separation mechanism and its driving force in Fe-Cu immiscible alloy.At the same time,nano-scale phase separated Zr-?Cu-Fe?-Al bulk metallic glasses?BMGs?are prepared by rapid solidification technology,which used to reveal the room-temperature plasticity,phase separated structure formation mechanism and deformation mechanism.Firstly,Cu-Zr and Fe-Cu alloy systems are used as the typical illustration of the miscible and immiscible melt,respectively.The structure and thermodynamics properties are discussed.It is found that demixing alloys are very different from the mixing alloys at the microstructure.In miscible Cu-Zr liquid alloy,the partial pair correlation functions?PPCF?of liquid Cu₅₀Zr₅₀ show the highest peak for heterogeneous atom pairs and the coordination number?CN?of Cu-Zr is much higher than that of Cu-Cu and Zr-Zr,which implys the heterogeneous particles locating around the center atoms.There is no obvious fluctuate in the Bhatia-Thornton?B-T?structure factor when q?0.In immiscible alloy,Fe-Cu melt,the first peak in PPCF for homogeneous atom pairs is higher than that for heterogeneous atom pairs,and CNs of Cu-Cu and Fe-Fe are much higher than that of Fe-Cu,which means the nearest neighbor structure consisted by homogeneous atoms.The B-T structure factor Scc?q?sharply increases when q?0,due to the exquisite concentration fluctuation in the liquid alloy.Further investigation indicates that the excess volume is related to the microscopic interaction between particles,and it could be negative and positive for both miscible and immiscible liquid alloys.For the miscible systems,enthalpy of mixing is positive while it is negative for the immiscible systems.The above research provides a basis for the design of phase separated MG composition.The relaxation behaviors of liquid Fe₅₀Cu₅₀ at different temperature are calculated by MD.The phase separation mechanism and its driving force are analyzed with the change of domain size,concentration fluctuation and potential energy in liquid alloy.The results show that the liquid-liquid phase separation?LLPS?process in Fe₅₀Cu₅₀ melts controlled by spinodal decomposition mechanism,which undergoes the following stages:forming interconnected structure,coarsening and the migration and aggregation of droplets.The liquid phase separates into Fe-rich and Cu-rich regions.The contents of Cu atoms in Cu-rich region and Fe in Fe-rich region in 1₅₀0K system are 90%and 95%respectively,which means obvious up-hill diffusion happens in melt.The abrupt decrease of potential energy and domain size can be discovered at the initial stage of phase separation and the aggregation and coarsening of droplets,and then it keeps unchanged.The above phenomenon implies that the reduction of potential energy is the driving force of LLPS.Cu atoms in Fe-rich region are forced to move toward Cu-rich region due to decreasing of Fe potential energy.Due to the larger potential energy difference between Cu-rich region and Fe-rich region in the lower temperature system,the atomic diffusion rate is higher and the phase separation in lower temperature system is more obvious.The rapid solidification processes of Fe₅₀Cu₅₀ melt from 3₅₀0K to 300K are performed by MD simulation to explore the phase separation mechanism and the formation process of glass and crystal.Fe₅₀Cu₅₀ liquid alloys separate with spinodal decomposition mechanism,and then solidify into amorphous or crystal state.Correspond to the different cooling rates,there are three kind of microstructure configurations in the finai alloy:in the relative higher cooling rate system,the interconnected-type separated particles distributed in the glass;in the middle rate system,the crystal formed with interconnected-type separated particles;in the relative lower cooling rate system,Fe-rich droplets embedded in the Cu-rich matrix.The solid nuclei only emerge in Fe-rich region?clusters?.There are many twinning planes formed in final crystal with homogeneous atoms stack in both side of them.The rapid solidification processes of Fe₇₅Cu₂₅ melt with different cooling rates are simulated with MD.At the same time,Fe₇₅Cu₂₅ and Fe₅₀Cu₅₀ thin belts are prepared by single roller rapid quenching.When the cooling rate reaches 6.4×1012K/s,the liquid alloy solidifies into glass.Diverged from spinodal decomposition mechanism,LLPS in Fe₇₅Cu₂₅ melt is controlled by nucleation and growth mechanism.So the phase separation structure in whole system is Cu-rich droplets can be found in the Fe matrix,which comports with the microstructure in thin belts.The crystal structure is similar with Fe₅₀Cu₅₀ melt.The simulation results indicate the content of alloy has influence on the phase separation mechanism and final micro-morphology,but the cooling rates only determine the final micro-morphology.The LLPS controlled by nucleation and growth mechanism do not form the interconnected-type particles at all times,which conductive to obtain the uniform phase separation structure in phase separated MGs.With the demixing properties of Fe-Cu binary alloy and the excellent glass forming ability?GFA?of Zr-Cu-Al alloy,nano scale phase separated Zr_65-xCu₃₀Fe_xAl₁₀?x =5 and 7.5 at.%?BMGs are prepared by rapid solidification.And then the plastic properties of the BMGs are studied.The results reflect that Zr₆₀Cu₂₅Fe₅Al₁₀ has better GFA than Zr_57.5Cu₃₀Fe_7.5Al₁₀.The multi-step LLPS in BMG controlled by nucleation and growth lead to formation of nano scale phase separated particles and cause inhomogeneous microstructure.Because of the coupling effects of the formation of shear bands around and away from major shear bands during the deforming process,the prepared BMGs show high yield strength and obvious macroscopic plasticity.The complementary cumulative probability distribution of plastic serration in compresses curves follows power-law distribution,during the deformation.The power-law scaling exponent of stable deformation regime is larger than later deformation regime,reflecting the former regime located in a more stable self-organized critical?SOC?state,because of the formation of multiple shear bands far away from the major shear bands.In the later deformation regime multiple shear bands arising around the major shear bands delay the slip-to-failure transition in the alloy.So both kind of multiple shear bands in phase separated BMGs results in the enhancement of room-temperature plastic property.The experimental results verify the phase separation structure and its formation mechanism.It proves that,during plastic deformation,the nanoscale phase separated particles can improve the stability of shear bandings propagation,which helps to increase the plasticity of BMGs.