In order to investigate the structural degradation during creep, interrupted creep tests were conducted of a Mod.9Cr-1Mo steel in the range of stress and temperature from 71 to 167 MPa and 873 to 923 K. The change of hardness and tempered martensitic lath width were measured in grip and gauge parts of interrupted specimens. The lath structure was thermally stable in static conditions, however, it was not stable during creep and the structural change was enhanced by creep strain. The relation between the change in lath width and strain was described quantitatively. The change in Vickers hardness was expressed by a single valued function of creep life consumption ratio. Based on the empirical relation between strain and lath width, a model was proposed to describe the relation between change in hardness and creep life consumption ratio. The comparison of the model with the empirical relation suggests that about 65% of hardness loss is due to the decrease of dislocation density accompanied by the movement of lath boundaries. The role of precipitates on subboundaries was discussed in connection with the abnormal subgrain growth appearing in low stress regime.
Precipitation behavior of metastable phases in an Al 1.94atCu alloy during isothermal aging at 373 K was investigated by means of Vickers microhardness tests, DSC measurements and TEM observations. The size distribution of the precipitates was quantitatively investigated based on the TEM, HRTEM and HAADF STEM images, and statistical parameters that fit the precipitate size distribution were determined under a log normal distribution approximation. We have successfully estimated the volume fraction of copper in precipitates, and found that the G.P.(II) formation results in increases of volume fraction of metastable particles, mean size and hardness.
Precipitation behavior of metastable phases in an Al-1.94 at%Cu alloy during isothermal aging at 373 K was investigated by means of Vickers microhardness tests, DSC measurements and TEM observations. The size distribution of the precipitates was quantitatively investigated based on the TEM, HRTEM and HAADF-STEM images, and statistical parameters that fit the precipitate size distribution were determined under a log-normal distribution approximation. We have successfully estimated the volume fraction of copper in precipitates, and found that the G.P.(II) formation results in increases of volume fraction of metastable particles, mean size and hardness.
Recently, high strength tungsten (W) alloyed steels have been developed for use in power plants with higher steam conditions for environmental reasons as well as the improvement of thermal efficiency resulting in lower fuel costs. In order to establish a creep modeling of high strength martensitic steel and to understand the basic role of W in tungsten alloyed 9-12Cr steels, conventional martensitic steels (X20CrMoV121, X20CrMoWV121, and Mod.9Cr-1Mo) and tungsten alloyed steels (NF616 and HCM12A) were employed for creep tests and creep behavior analyses by the : method. The proposed creep model, which takes into account both primary and tertiary creep, satisfactorily described the creep curves and accurately predicted creep life, as martensitic steel undergoes a relatively large amount of primary creep, up to nearly 30 %, over its normal life. The tungsten alloyed steels exhibited a smaller minimum creep rate and a larger stress exponent compared to the conventional steels. In addition, in tungsten alloyed steel, the : value features strong stress dependence such that creep life is prolonged at lower stresses due to high : values. The importance of the : value from the standpoint of creep strengthening in primary and tertiary creep is discussed.
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