Control of NiAl-W Eutectic Self-organization Structure and Research on the Extraction and Property of Micro/Nano Filaments and Pore Arrays Structure

Author:Gao Jian Jun

Supervisor:zhao zhi long


Degree Year:2018





The research of micro/nano particles,wire,sieve pore and other structural materials has been paid much attention.The micro/nano pore can be used as a template to prepare novel micro/nano structure or as a micro/nano filter to filter impurities.Micro/nanowires have excellent electrical transport performance and field emission performance.The directionally solidified(DS)eutectic alloys has a self-organizing structure.By controlling on the heat flux and composition collocation,the morphology and size of self-orgnizing eutectic can be effectly controlled.The corronsion potentials in eutectic phases are different.Micro/nano pores and wires can be obtained by selective etching in one of the phases by proper corrosion liquid.In this paper,DS NiAl-W eutectic alloy was carried out by Bridgman directional solidification method.The microstructure of self-orgnaized NiAl-W eutectic alloys with the growth rates of2-25μm/s were studied.Based on the modified dynamic supercooling model,the transformation mechanism of W section shape from facted to non-facetd phase was discussed in detail.NiAl porous and W wire were prepared by selective etching of DS NiAl-W eutectic alloy.The Co particle arrays with different structure morphology was electrodeposited by using NiAl porous template.The correlation between electrodeposition process parameters and magnetic properties of Co particles arrays was studied.The relationship between elastic modulus of single W wire and its characteristic structure was studied by AFM indentation method.The W wire nano/micro-device was prepared by mask method.The relationship beween the electrical transport performance and its characteristic structure was studied.The main points of the study are as follows:Firstly,the characteristic self-organised structure of DS NiAl-W with the growth rates of2-25μm/s was studied.With the increase of growth rate,the solid/liquid(S/L)interface of DS NiAl-W eutectic alloys tanslated from planar to cellular,and fiber spacing and size of W fibers decreased.During the initial growth stage of NiAl-W eutectic alloys,growth of dendrites W competed with eutectic phase.However,eutectic growth rate was faster than that of dendrite due to diffusion coupling effection.This resulted in the primary dendrites W were gradually phased out,and the directional solidification structure in the steady-state area was completely changed into eutectic structure.The microhardnessHVof DS NiAl-W eutectic alloys decreased with the increase of fiber spacing d and fiber sizea.Their relationships meet:HV=371.58d-0.09,HV=297.70a-0.09.Secondly,the microstructure of NiAl phase and W phase was studied by high resolution transmission electron microscopy(HTEM),and the transformation mechanism of W phase from faceted to non-faceted was related with the dynamic undercooling.NiAl phase and W phase had different crystallographic orientation at different growth rates.When the growth rate was 6μm/s,the crystallographic orientation between NiAl phase and W phase was[1-11]NiAl//[200]W.While in the growth rate of 8μm/s,the crystallographic orientation between NiAl phase and W phase was changed to[-111]NiAl//[-111]W.Based on the modified dynamic undercooling model,the calculation results by dynamic analysis were as:both for the NiAl and W phase,the dynamic undercooling(Δ(TK,Wc、ΔTK,NiAlcwas lower than the critical undercooling(Δ(TK,NiAlc、ΔTK,Wcat the growth rate of 6μm/s.The growth modle of W phase was facted,and its cross-sectional morphology was hexagonal.When the growth rate increase to 8μm/s,Both in the NiAl and W phase,the dynamic undercooling(Δ(TK,Wc、ΔTK,NiAlcwas higher than the critical undercooling(Δ(TK,NiAlc、ΔTK,Wc.The growth modle of W phase was non-facted,and its cross-sectional morphology was elliptical.Then,a selectice etching W fibers for preparing porous NiAl or a remove of NiAl matrix for fabricating W fibers was carried out by a selective etching method.The growth rate greatly affected the morphology of porous NiAl.With the increase of gowth rate,the cross-sectional morphology of porous NiAl transformed from hexagon(2-6μm/s)to ellipse(8-25μm/s).The microhardnessHV of porous NiAl decresed with the increase of pore spacing(9,pore diameter(6 and the etching depth?.Their relationships meet:HV=342.55(9-0.06、HV=299.01(6-0.06、HV=349.66?-0.03.With the increase of growth rate,the distribution of W wires exposed in the NiAl matrix changed from unifrom to cystiform.With the increase of etching time,the length of W wires increased.When the etching time was 14 h,the length of W wires could be 300μm.Then,the porous NiAl was used as template to electrodeposition Co particles arrays.The coral flake Co present ferromagnetic property.With the increase of depsotion time,the coercivity decreased,the remaining magnetization and saturation magnetization increased for the coral flake Co.At the same deposition time,with the increase of gowth rate in the template,the coercivity,remaining magnetization and saturation magnetization increased.The Coral ball Co particles present superparamagnetism.With the increase of depsotion time,the size of coral ball Co paricles increased and they present multiple domain state.Affected by the grain boundary nailing effect between grains,the coercivity and remaining magnetization increased with the extention of deposition time.Under a deposition of-0.8 V,during the depositon process of coral falke or coral ball Co,the nucleation and growth of Co paritcles satisfied with the instantaneous nucleation model.The coercivity,remaining magnetization and saturation magnetization of both coral flake and coral ball Co paricles decreased with the increase of tempareture.Last,the single W wire was fastened to the surface of SiO2 by metal mask method.Based on the AFM indentation method,the Young modulus of W wire was calculated by Sneddon contact model,and its value was 260-460 Gpa.Based on the metal mask method,the W micro/nano-device was fabricaetd.Both the resistivity and electrical resistivity of W wires increased with increasing temperature.The relationship between the resistivity and fiber diameter conformed to FS+MS model.