A method for correlating mechanical property changes and microstructural changes as a function of depth in ion irradiated specimens has been applied to a series of model pressure vessel steels. The technique employs nanoindentation with very small indent depths, on cross sectional specimens, to measure changes in hardness with sub-micron spatial resolution. Conventional TEM imaging of the cross sectional specimens allows the investigation of the defect microstructure as a function of depth below the original irradiated surface. For ion irradiations (implantations) the dose varies as a function of depth in the specimen and can be calculated with reasonable accuracy using TRIM calculations. Thus changes in both hardness and defect microstructure can be measured and correlated, over a range of doses, from a single specimen.The technique has been applied to a set of model ferritic pressure vessel alloys to study the effect of various solutes on embrittlement. 2.5 MeV He ion irradiation was used to produce qualitatively similar microstructural features to those created in a nuclear reactor environment allowing the study to be carried out without the added complications associated with radioactive specimens. Figure la is an SEM image showing a set of nano-indents that starts on the left, beyond the end of range of the ions which is easily seen as the dark band. The indents eventually cross the thin oxide that marks the original irradiated surface (the white line) and enter the Fe electroplating seen here as the fine grained material at the bottom of the images. Figure lb is a higher magnification SEM image of the same indents showing the three-sided pyramid shaped indents reflective of the Berkovich diamond used in nanoindenting. Each of these residual indents is the result of a multipule indent process. The first indent is 50nm deep to acquire the sub-micron resolution hardness data, and the second is twice as deep (thus twice as wide) to clearly mark the position of the shallower indent before moving to the next position. Figure 2a is a bright-field TEM micrograph showing the defect microstructure of the entire irradiated region of an Fe + low N alloy, taken with g = {330} such that the dislocation loops appear dark. Figure 2b is a plot of the actual change in hardness, AH, as a function of depth below the original irradiated surface at the same scale, and for the same alloy shown in figure 2a.
The effects of cracks on the quantification of the cancellous bone fabric tensor in fossil and archaeological specimens: a simulation study
