Diffusion Behavior And Microstructural Evolution On Interfaces In Al/Cu Bimetal

Bimetals are an important type of composite materials that are prepared using advanced composite preparation techniques,achieving a strong metallurgical bonding of two or more metals with different physical,chemical and mechanical properties.Al/Cu bimetal combines the high corrosion resistance,lightweight,economic and aesthetic of aluminum,and the low contact resistance,high thermal and electrical conductivity of copper.Thus this type of material has been widely applied in many industrial fields,such as automobile,power communication,electrics and electronics fields.Up to now,many preparation methods have been developed for bimetals,which can be classified into three major categories:liquid-liquid bonding,liquid-solid bonding and solid-state bonding.Most of the researchers have mainly focused on the influence of different preparation process conditions on the properties of bimetals and on improving and optimizing the composite preparation techniques.Although the related researches on the interfacial bonding mechanisms in various preparation techniques have been carried out,there is still no systemic and complete understanding of the interfacial diffusion reaction or dissolution,the formation and evolution of intermetallic compounds(IMCs),the growth kinetics and the non-equilibrium solidification of interfacial melt,etc.,which all require further in-depth studies.In recent years,with the development of third-generation high performance synchrotron radiation light source,the synchrotron radiation two-dimensional(2D)/three-dimensional(3D)imaging techniques have been applied and developed unprecedentedly in the field of metallic material studies.Based on this,this thesis is proposed to systemically investigate the interfacial diffusion behavior and microstructural evolution during the liquid-solid reaction,semisolid-solid reaction and solid-state reaction of Al/Cu bimetal by synchrotron radiation imaging technique and static diffusion couple technique.First,the synchrotron X-ray radiography was used to in situ study the interfacial diffusion behavior during the liquid-solid reaction and semisolid-solid reaction of Al/Cu bimetal.The imaging results show that the A1 and Cu elements diffuse mutually to each other at the interface and form a clear diffusion front during heating and holding.It is also found that the gravity has a significant effect on the morphology of the diffusion front in up/down structural type and left/right structural type Cu/Al bimetal,resulting in the instability of diffusion front.Based on the quantitative data analysis,the migration distance of the diffusion front gradually increases with increasing diffusion time and temperature.Meanwhile,the migration distance towards Al side(L1)is much longer than that towards Cu side(L2),and the effect of diffusion temperature on L1 is greater than that on L2.Besides,based on the extraction of gray level from the sequenced radiographs and EPMA quantitative composition analysis,the relationship between the image gray level and the concentration of Cu element around the Al/Cu interface was established.Then,the temperature dependent diffusion coefficients of Cu in liquid A1 were calculated from the known concentration variations by an inverse method according to Fick's second law.The diffusion coefficients of Cu in A1 can be described by a straight Arrhenius equation expressed as D=2.83×10-5exp(-96.0/RT)m2/s with the temperature range from 620 to 693 ?.Then,the formation and evolution of interfacial microstructure during the liquid-solid reaction and semisolid-solid reaction of Al/Cu bimetal were directly observed by synchrotron X-ray radiography.The results show that the interfacial transition zone of Al/Cu bimetal can be identified as two different zones:"solidification zone" and "solid-state diffusion zone".Under different reaction temperatures and reaction time conditions,the solidification zonemay contain ?-Al dendrite,eutectic structure(a-Al+Al2Cu),needle-like primary Al2Cu phase or petal-like primary Al3Cu4 phase and peritectic AlCu phase,while the solid-state diffusion zone may consist of continuous layered Al2Cu?AlCu?Al3Cu4?Al2Cu3?Al4Cu9 and AlCu3 phases.When Cu is diffused to A1 side to a moderate degree(Ccu<CE),the ?-Al dendrites appear from the top of the Al-rich side first and grow in a direction towards the initial interface.The growth behavior of a-Al dendrite is mainly dominated by the thermal field and the variation of Cu concentration which is caused by interfacial diffusion and solute accumulation.When Cu is excessively diffused to Al side(Ccu>CAl2Cu),petal-like primary A13Cu4 phase and peritectic AlCu phase with higher content of Cu are formed instead.The growth of AlCu3 and(Al4Cu9+Al2Cu3)phases produced near Cu-rich side is mainly controlled by the interfacial reaction and volume diffusion,while the growth of layered A13Cu4 phase adjacent to the melt was mainly controlled by the interfacial reaction.In this study,the Al2Cu phase can be formed in two different ways:"solidification" and "solid diffusion".The needle-like primary Al2Cu phase and lamellar eutectic Al2Cu phase are produced through solidification,and the layered/cellular Al2Cu phase is formed through solid diffusion.Among them,the 2D/3D morphologies of primary Al2Cu phase mainly present a typical faceted growth feature,and its growth can be divided into "longitudinal growth" and "lateral growth".In addition,the morphology,types and growth pattern of interfacial microstructure can be modified by controlling the degree of interdiffusion of A1 and Cu elements.Finally,the diffusion couple technique was used to study the formation and growth of IMCs statically in Al/Cu during isothermal diffusion at various temperatures of 480,500,520 and 540 ? for different diffusion time up to 40 h.The results show that four continuous layered IMCs phases,including Al2Cu?AlCu?Al2Cu3 and Al4Cu9,are formed at the initial sample interface and then grow by phase interface migration.The thickness of all IMCs layers increases with increasing diffusion time and temperature.Meanwhile,the thickness of IMCs layers can be expressed as a function of the square root of time for each diffusion temperature,i.e.L = kt1/2,indicating that the growth of IMCs is controlled by volume diffusion.According to Arrhenius relationship,the activation energy calculated for the growth of A12Cu,AlCu,A12Cu3 and Al4Cu9 is 120.73 kJ/mol,79.74 kJ/mol,285.01 kJ/mol and 149.78 kJ/mol,respectively.Besides,the A12Cu phase is predicted to form first at the interface and the formation sequence of the four IMCs is AI2Cu?Al4Cu9?AlCu and Al2Cu3 phase based on the effective heat of formation(EHF)model,Gibbs free energy and the growth kinetics of IMCs.