Study For Interfacial Design And Damping Mechanisms Of Sn-Bi Coated Aluminum Borate Whisker Reinforced Aluminum Matrix Composite

Alumina borate whisker (Al₁₈B₄O₃₃w)-reinforced aluminum matrix composites were the object of investigation. Bi₂O₃ and SnO₂?Bi₂O₃ were coated on the surface of alumina borate whiskers using a chemical method. Metal interfacial phase with a low melting point could be introduced into the whisker/matrix interface of the composite by the reaction between coatings and aluminum matrix. In this case, the interfacial states of the composites can be adjusted and controlled. The damping capacities of the coated composites can be improved markedly and the mechanical properties of the coated composites were decreased faintly.The thermal decomposition of and the thermal stability whisker with coatings were described by means of TG-DSC. The melting and the phase transformation of the interfacial phases were also studied in detail during the heating process of the coated composites by means of XRD analysis. The microstructure evolution of the coated composites and the interfacial phases with the low melting point were investigated by TEM and HRTEM. Particular attention was paid to the morphologies, the structure and the distribution of interfacial phases. Some of the characteristics of dislocation status near the interface and the grain boundary were also investigated. The effects of coating contents on the damping capacities of the coated Al₁₈B₄O₃₃w/Al composites at various strain amplitudes and vibration frequencies were examined and the damping mechanisms of the coated composites were also discussed.The results of DSC and XRD analysis indicated that the presence of the whiskers not only accelerated the decomposition of Bi(OH)3, but also changed the transition temperature of Bi₂O₃. When the temperature was 650℃, Al₁₈B₄O₃₃ whisker can react with Bi₂O₃ to form Al₄Bi₂O₉. The presence of whisker did not affect the decomposition behavior of the mixed Bi(OH)₃ and Sn(OH)₄. When the temperature was beyond 605℃, a reaction between SnO₂and Bi₂O₃ can be occurred to form Sn2Bi2O7 on the surface of whiskers.The results of TEM observation suggested that Bi or Sn and Bi could be introduced into at the interface in ABOw/Al composite during squeeze casting process. The existence of the metal phases with a low melting point changed not only the interfacial structure of the coated composites, but also the dislocation states of the matrix. The dislocation tangles lessen and the dislocation density reduced in the coated composites. The sintering temperature of the coated-whisker preform affected not only the coating structure on the whisker surface, the interfacial structure of the coated composites, but also the damping behavior and damping mechanism of the coated composites. The existence of two damping peaks in the Bi₂O₃-coated composites at around 80℃(P1) and 285℃(P2) has been observed for the first time. The damping capacity of the coated composites strongly depended on not only the content of coatings, but also the strain amplitude.The damping mechanisms of P1 in the coated composites are attributed to the dislocation motion and interfacial slip between whisker/Sn because the damping capacities of peak 1 depend not only on the strain amplitudes but also on the coating contents. The damping mechanisms of P2 in the coated composites at elevated temperatures strongly depend on the strain amplitude. The dominant damping mechanisms of the coated composites are transferred from Bi melting damping into interface damping. When temperature exceeds 330°C, the grain-boundary damping becomes the main damping mechanism for the coated composites.Only one damping peak can be observed in the SnO₂?Bi₂O₃-coated composites. The damping capacities of the coated composites strongly depend on not only the coating contents but also the strain amplitude and the damping mechanism is attributed to the interfacial slip between whisker/coatings along with the melting of Sn in the coatings.The measurement frequency has a dramatic influence on the damping capacities of the coated composites. The presence of a critical temperature in the damping temperature curves of the coated composites has been found first when the vibration frequencies are 0.5 Hz and 70 Hz respectively. The critical temperature decreases with the increasing of coating contents. The dominant damping mechanisms are transferred from dislocation damping into interfacial damping below and beyond the critical temperature point.When the measurement frequency is below 40 Hz, the damping peak was found to shift towards higher temperatures with an increase in frequency, exhibiting the characteristics of a relaxation type. When the measurement frequency is beyond 40 Hz, the damping value increases with increasing frequency, thermoelastic damping becomes the dominant damping mechanism. There is a strain peak in damping-strain curves of the coated composites as vibration frequency is 50 Hz, which could be attributed to the dislocation unpinned process.