In-Situ Observations Of Hot Compression Of Gold-Tin Alloys By Synchrotron Radiation X-Ray Diffraction

Gold-tin eutectic alloys(Au-20.4 wt.%Sn)have a low melting point(551 K),good electrical and thermal conductivity,good wettability and high fatigue resistance.For such merits,they have been widely used for microelectronics packaging and integration.They consist of two intermetallic compounds of a hexagonal structure,AuSn and Au5Sn.Both compounds are brittle phases and are hard to be deformed at room temperature.Research on the deformation process of alloys containing two intermetallic compounds has rarely been reported in the literature.In this thesis,their hot deformation mechanism was investigated using in-situ and ex-situ methods.The heating processes of two constituent compounds and gold-tin eutectic alloys in were investigated first using differential scanning calorimetry(DSC)and the high-energy X-ray diffraction(HEXRD)technique.Then their uniaxial hot compression deformation processes were investigated using in-situ HEXRD and high resolution transmission electron microscopy(HRTEM)to determine the deformation mechanism.Main conclusions of the present thesis work are drawn as follows:(1)DSC and HEXRD studies showed theat the AuSn compound undergoes a diffused phase transition during heating.This phase transition is caused by stacking faults.HRTEM studies revealed a large number of insertion-type stacking faults in as-cast samples of AuSn.The Au5Sn compound undergoes an ordered-disorder phase transition during heating.This transition brings about many subgrains in samples of Au5Sn.Such transitions were also observed in heating of Au-20.4Sn eutectic alloys.(2)In-situ HEXRD studies showed that Au5Sn sample can be deformed by an amount of 35.5%.Such deformability was attributed to dynamic recrystallization during hot compression.HRTEM studies confirmed that the hot deformation of the sample is enabled by slipping of dislocations,especially by basal slip.(3)In-situ HEXRD studies showed that AuSn sample can be deformed by an amount of 26.9%.It is less than that of Au5Sn.The reason is that work hardening in AuSn is more severe,while dynamic recrystallization is less.Ex-situ HRTEM studies showed that many dislocations and stacking faults exist in the deformed AuSn compound.This result showed fully that the AuSn compound has a low stacking faults energy.For this reason,deformation proceeds by slip of discloations,especially by prismatic slip.(4)Gold-tin eutectic alloys are easier to deform than either compound,which can be deformed by an amount of 76.2%.The reason is that Au5Sn is easier to deform than AuSn.When dislocations pile up at the phase interface,they provide a local stress field and promote the deformation process of AuSn under action of an external load.It is concluded that cooperative deformation of the two compounds increases deformability of the eutectic alloys.(5)In-situ studies of the effect of strain rates showed that twinning and dynamic recrystallization depend on the strain rates.The higher the strain rate,the easier the twinning-mediated deformation.The stress concentration caused by the dislocation slip is released by twinning,and the dislocation slip continues to slip on the reoriented crystal plane.The smaller the strain rate,the more significant dynamic recrystallization.This is the reason why the total amount of deformation of the eutectic alloys is greater at low strain rate.(6)The effect of annealing on the deformation process is mainly achieved by changing the microstructure of the gold-tin eutectic alloys.After undercooling and annealing,the coarse dendrites disappear and the lamellar eutectic is ripened.These changes bring about fine grains,which are beneficial to the deformation of the alloys.