Thin section size and grain size dependent Monkman–Grant ductility of a nicltel base superalloy

Abstract

The combined effects of thin section size D and grain size d_G (or number of grains across the gauge section n_G) on the creep behaviour of conventionally cast MAR-M002 were studied for creep conditions of 900°C and 300 MPa. It was observed that there was a linear correlation between the t_R/d_G and <disp-formula><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="splitsection9-m2.tif"/></disp-formula> ratios on log-log scales (the modified Monkman–Grant relationship), where t_R is the creep rupture life and <disp-formula><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="splitsection9-m3.tif"/></disp-formula> is the minimum creep rate. The Monkman–Grant ductility <disp-formula><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="splitsection9-m4.tif"/></disp-formula> showed a rapid improvement with increasing D²/n_G ratio. It is concluded that the existence of the modified Monkman–Grant relationship is due to the coupling of the creep deformation in the secondary region through grain boundary sliding with the creep crack growth behaviour in the tertiary region.