Study On The Sintering Process And Densification Kinetics Of W-Mo-Cu Alloy By Large Current Electric Field Sintering

W-Mo-Cu alloy has broad application prospects in electric contact materials,electronic packaging materials,electrode materials for EDM and weapons shields,aerospace materials and other fields due to its high strength and hardness,excellent electrical and thermal conductivity and resistance to ablation and high temperature oxidation.In addition to the field-assisted sintering methods used by our group,including microwave sintering,sparkle plasma sintering and large current electric field sintering methods,there are also two methods which contain liquid phase sintering and infiltration sintering method utilized by other research institutes.The field-assisted sintering methods have the advantages of the faster temperature rise and fall,lower sintering temperature,shorter sintering time and finer grains.Our group has successfully prepared W-Mo-Cu alloys by large current electric field sintering,and has conducted systematic research on the W-Mo-Cu ternary system and achieved certain milestones.However,research on the sintering of W-Mo-Cu alloy by large current electric field sintering method is not sufficiently advanced,e.g.,how new phases are generated between the insoluble W,Cu and Mo and Cu elements,the densification mechanism involved in this alloy during the sintering process,the evolution of the phases and the existence of defects have not yet been explored.Therefore,in this paper,based on our group’s previous researches,the thermodynamic calculation of the formation of the physical phase in W-Mo-Cu alloy is carried out from the perspective that the phases can be divided into crystalline phases and non-crystalline phases;the densification data of the W-Mo-Cu alloy collected during the large current electric field sintering process is calculated using the Su&Johnson kinetic model,thus establishing the master sintering curve（MSC）of W-Mo-Cu alloy;the transformation periods of the generated phases and the densification mechanism of the alloy are also discussed.In addition,in order to improve the denseness and properties of the alloy,we also attempt to add Ni,Fe and Co activating elements to study the effect of activating elements on W-Mo-Cu alloy and the related activation mechanism,etc.The main results achieved are as follows.（1）The variation of Gibbs free energy of phase formation for the W-Mo-Cu ternary system was calculated by the mathematical extrapolation,which was based on the calculation of the variation of Gibbs free energy of phase formation for the binary systems of W-Mo,W-Cu and Mo-Cu from the Miedema’s model.Meanwhile,the discussion of the factors in the system that affect the formation of new phases was also carried out.The results show that,the variation of Gibbs free energy of phase formation for the W-Mo-Cu ternary system is a positive value,meaning that it is unable to form new phases spontaneously under equilibrium conditions for the ternary system.The value of the variation of Gibbs free energy change for the formation of crystals in the system is smaller than that for the formation of amorphous phases,indicating that crystals are more likely to form in the system.The variation of Gibbs free energy of phase formation is much higher when the proportion of Cu is in the range of 30-60 at.%or when the system is at a lower temperature.The phases in the W-Mo-Cu alloy are formed in the following order:（W,Mo）crystalline phase,（W,Mo）amorphous phase,（Mo,Cu）crystalline phase,（Mo,Cu）amorphous phase,Cu_xW_1-x crystalline phase,Cu_xW_1-x amorphous phase.Under the same conditions,Cu has the greatest influence on the Gibbs free energy change of the W-Mo-Cu system compared to the W and Mo:for each 10%increase in Cu content,the variation of Gibbs free energy of the crystalline and non-crystalline formation in the system can be increased by up to 80%and 25%respectively.（2）The W-Mo-Cu alloy was prepared by large current electric field sintering method and the phase and microstructure of the alloy were investigated.It was shown that the alloy produced the field emission under the action of high current and the Cu phase melted to form local micro-areas of liquid phase which promoted the sintering densification.Due to inter-atomic diffusion,（W,Mo）solid solution,（W,Mo）amorphous phase,（Mo,Cu）solid solution and Cu_0.4W_0.6intermetallic compound were formed at the phase interfaces.The formation of new phases improved the bond strength of the alloy phase interface and contributed to the overall properties of the alloy.（3）The evolution of the phase and microstructure during the electric field sintering of W-Mo-Cu alloys was investigated using the stepwise sintering experiments.The results showed that the rapid heating process of the large current electric field sintering could effectively activate W,Mo and Cu atoms,promote the formation of sintering necks among powders particles and increase the alloys’density.The electro-thermal cycling process makes full use of the high current to promote densification.The（W,Mo）solid solution and Cu_0.4W_0.6 intermetallic compound were formed in this process and the density and physical properties of the alloy increase rapidly.Meanwhile,the master sintering curves of W-Mo-Cu alloys were established using the time,temperature,size and strain data collected during the sintering process combined with Su&Johnson kinetic model calculations,and the experimental results showed that the master sintering curves can be effectively used for densification prediction of W-Mo-Cu alloys.（4）The densification mechanism of W-Mo-Cu alloy was analyzed by calculating the shrinkage data collected during the large current electric field sintering.The results showed that the main densification mechanism during the large current electric field sintering of W-Mo-Cu alloys changed from plastic deformation to grain boundary diffusion,and the shrinkage caused by plastic deformation accounts for more than 20%of the total shrinkage of the alloy,indicating that pressure is critical to the densification of the alloy during large current electric field sintering.Comparing the densification results of the two heating methods,electro-thermal cyclic sintering and constant temperature sintering,the shrinkage of the samples sintered by electro-thermal cyclic sintering was approximately 27%higher than that of constant temperature sintering,indicating that electro-thermal cyclic sintering can make full use of the high current to improve the sintering densification of the W-Mo-Cu alloy.The sintering activation energies of 37 k J/mol and 45 k J/mol for the electrothermal cyclic sintering and the constant temperature sintering were respectively,and the reduction of the sintering activation energy was beneficial to the improvement of the densification of the W-Mo-Cu alloy.（5）Appropriate amounts of Ni,Fe and Co elements can reduce the sintering activation energy of W-Mo-Cu alloy,increase the phase interface bonding strength and improve the tissue distribution uniformity,thus increasing the relative density and hardness,but compromising the electrical conductivity.It was found that when 3 wt.%Ni,Fe and Co were added,the sintering activation energy of the alloy was reduced by35.8%,66.7%and 65.4%respectively compared to the non-added sample.The activation ability of each element and its effect on the overall performance of the W-Mo-Cu alloy were ranked from highest to lowest as Fe>Co>Ni.The electric field sintering process generated the Cu_0.81Ni_0.19 solid solution because of the diffusion reaction between Ni and Cu elements,while Fe and Co elements were deviated to the W and Mo phase interface,promoting the movement and rearrangement of W and Mo particles.The activation principle of the three elements is explained by the valence electron configuration theory,which shows that Ni,Fe and Co atoms have more electrons outside the nucleus than W,Mo and Cu atoms.The electron exchange movement under the action of the electric field is dominated by the atoms of the activating elements getting electrons,which manifests itself in the macroscopic diffusion process as the atoms of the activating elements entering the W and Mo atoms.