Ductility coaxing during creep and tensile deformation of high temperature alloys

Ductilities generated after four types of creep test are compared: (i) constant load, (ii) constant stress (calculated); (iii) constant stress (from direct observation of the minimum section) and (iv) constant strain rate. Supplementary information is obtained from a regularly interrupted plastic deformation test. The materials considered are 21/4Cr1Mo and 9Cr1Mo steels at 550 degrees C, type 304 and 316 alloys at 650 degrees C and 700 degrees C, respectively, and 20Cr25Ni/Nb steel at 750 degrees C. The principal parameter of interest is the 'uniform ductility' in the gauge section, that is, that part which has not severely necked. It is shown that uniform ductility is effectively described by the Monkman-Grant relation and this in turn can be used to sum creep damage via the ductility exhaustion method. It is further demonstrated that, assuming constant volume conditions, gauge length behaviour can be modelled by considering a truncated cone rather than a traditional cylindrical shape.