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The microstructure of the Ni-base superalloy IN617 that had undergone prolonged aging (approximately 65,000 hours) at a series of temperatures from 482°C to 871°C has been characterized by microhardness measurements, optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Cr 23 C 6 , Mo-rich eta-M 6 C, and Ti(C,N) constitute the major primary coarse precipitates both within the grains and along the grain boundaries. The secondary carbides were mostly fine Cr 23 C 6 , which had a cube-on-cube orientation relationship (OR) with the fcc matrix, and at long times were present in cuboidal and plate-shape forms within the grains and as films along the grain boundaries. Fine, eta-M 6 C carbides were also observed at low to intermediate temperatures with an OR given by [011] carbide//[011] matrix, " 1 " 11 À Á carbide// " 1 " 11 À Á matrix. The coarse eta-M 6 C carbides increased in extent at 871°C, whereas the counterpart fine carbides were absent. The c¢ phase was found to be present at all aging temperatures up to 871°C, with a volume fraction ranging from very low to approximately 5 pct at 593°C, where the peak in microhardness occurs. The observations have also suggested that the presence of a very small amount of c¢ at temperatures as high as 871°C at long times may be associated with a reaction between the fine eta-carbides and the c matrix. Ultrafine precipitates of the intermetallic phase Ni 2 (Cr,Mo) with the Pt 2 Mo-type structure was observed in addition to c¢ in samples aged for 28,300 hours at the lowest aging temperature of 482°C. These precipitates were absent in samples aged at higher temperatures. The various observations made have suggested that the long-term thermal stability of the IN617 alloy is reasonably good over a wide temperature range of 538°C to 704°C, whereas at higher temperatures (871°C), the substantial decrease in the volume fraction of c¢ and coarsening and clustering of the carbides lead to a large drop in the microhardness. A modified time-temperature-transformation (TTT) diagram was constructed based on the results of this study and comparison with previous reports.

In many situations where the characterisation of the mechanical behaviour of a specific material is required, source material for manufacture of conventional test specimens may be at a premium. Examples include the validation of new alloys for use in the power industry, the description of the heat affected zone (HAZ) of weldments 1 or performing a remnant life study on an in service component (such as steam pipe work used extensively in the power generation industry). The potential for a limit in sample material has necessitated the development of small specimen designs and associated test methods, particularly for the determination of the creep behaviour of a sample material. The small punch creep test (SPCT) has the potential to characterise the full uniaxial creep curve (as the specimen is taken to fracture). It is for this reason that the small punch creep test has attracted much interest from the research community. Owing to the complex deformation mechanism interactions experienced in the small punch creep test, interpretation of the results has received attention from many authors since its application was proposed by Parker et al. in the 1990s 2 (based on small punch plasticity test by Manahan et al. in the 1980s 3 – 5 ). In this review paper, several methods for the interpretation of small punch creep test (SPCT) data are reported and compared, together with examples of their application. Considerations for finite element (FE) modelling of small punch creep tests are highlighted and critiqued. Recommendations for potential areas of future research are also presented based on the authors’ investigation into published literature and research.

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