Tham khảo tài liệu 'high temperature strain of metals and alloys part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Rafting 133 ble with increasing strain. Thus the morphological instability retards the evolution of creep strain especially at lower stress levels. This effect is attributed by the authors to the reduction in the number of vertical Y-channels which is caused by rafting. Stable networks of dislocations are formed and the passage of dislocations from the Y phase in y particles is hindered. Intersections of the dislocation networks become obstacles for mobile dislocations. Zhang and coworkers 49 50 have reported new data concerning the creep strength of the fourth generation superalloys. The base superalloy TMS-75 has been developed by increasing the concentration of molybdenum or ruthenium or both. The latter alloy was denoted TMS-138. The alloys were tested at 1373 K under a stress of 137MPa. The creep curves obtained by these authors are of the type shown in Fig. c and have a relative long steady-state stage. The steady-stage creep continues up to 3-4 strain and is followed by the tertiary creep of a very high trajectory. The TMS-138 superalloy which contains 3 Mo 5 Re and 2 Ru reveals the largest rupture life. This superalloy has significantly improved properties compared with the third generation superalloy TMS-75 . The low creep rate turned out to be due to a fine interfacial dislocation network and a solid solution strengthening. Transmission electron microscopy after creep testing reveals the typical regular dislocation networks Fig. . These networks are similar to those observed for the CMSX-4 superalloy and are shown in Fig. . The CMSX-4 material was crept at 1423 K under stress 100MPa while the TMS-138 superalloy was tested at 1373K under stress 137MPa. Thus the formation of the fine interfacial networks is not an accidental finding but is also found for the rafted crept superalloys at temperatures above 1323K. Zhang et al. 49 have measured the mean dislocation spacing in the dislocation networks. The relationship between the minimum creep
