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Advances in Gas Turbine Technology Part 14

Tham khảo tài liệu 'advances in gas turbine technology part 14', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 380 Advances in Gas Turbine Technology ẽ Aơnexp -Q RT Here A and n are dimensionless coefficients Ơ is the creep stress Q is the activation energy for the creep T is the absolute temperature while R is the gas constant. In Fig. 9 the value of n is around 1-2 for sintered composites and 5 6 for Al2O3 YAG single crystal composites. In sintered composites it can be assumed that the creep deformation mechanism follows the Nabarro-Herring or Coble creep models while in Al2O3 YAG single crystal composites the creep deformation mechanism can be assumed to follow the dislocation creep models corresponding to the dislocation structure in Fig. 10. The activation energy Q is estimated to be about 700 kJ mol from an Arrhenius plot which is not so different from the values estimated from the high temperature creep in Al2O3 single crystal compression axis is 110 and YAG single crystal compression axis is 110 . It is also reported that the activation energy for oxygen diffusion in Al2O3 is about 665 kJ mol which is not so far from the activation energy of Al2O3 single crystal for plastic flow even though that of A diffusion is about 476 kJ mol. This fact means that the deformation mechanism of the Al2O3 single crystal is the diffusion controlled dislocation creep. On the other hand the activation energy for oxygen diffusion in YAG is about 310 kJ mol which differs significantly from the activation energy of YAG single crystal for plastic now. However dislocation is always observed in both Al2O3 phase and YAG phase of compressively deformed specimens at 1773 K-1973 K and at strain rate of 10-4 s - 10-6 s. Therefore the compressive deformation mechanism of the Al2O3 YAG single crystal composite must follow the dislocation creep models Wake Sakuma 2000 Waku et al. 2002 . Log Strain Rate 11 s Fig. 9. Relationship between compressive flow stress and strain rate for an Al2O3 YAG single crystal composite a sintered composite and an a-axis sapphire. 381 Unidirectionally Solidified .