GTD111, a creep resistant Ni-based superalloy developed by GE, is widely used in land-based
gas turbine first stage blades. However, there is little published information on its creep
properties and microstructure. The European Creep Collaborative Committee (ECCC) Working
Group 3C consequently selected GTD111 as a model material for testing and complementary
data assessment. The aim of this paper is to present the results from the ECCC test program and
data assessment, and to compare equiaxed (EA) and directionally solidified (DS) material
performance. Testing and metallographic laboratories from six European nations collaborated to
produce strain monitored creep rupture data on four EA and DS materials out to beyond 10,000
hours within a wide range of temperatures, 850-950°C, and stresses, 293-99 MPa. Available
(generally short term) results from other sources were also included in the compiled, small but
viable, 51-test data set. Assessment was carried out by three different assessors using different
tools and adopting different prediction models. Conventional ECCC post-assessment techniques
and novel “back-fitting” methods were used to identify a preferred model. It was shown that
assessing all the EA and DS data together can lead to non-conservative predictions for EA
materials, but separating the two classes creates small data subsets which cannot be modelled
effectively. As a pragmatic compromise, the DS data and those EA data which also showed good
ductility were included in a final "ductile GTD111" assessment. The resulting creep rupture
material models and rupture strength predictions are presented up to 3 times the longest test
duration. It was then shown that the performance of lower ductility EA materials can also be
predicted effectively with the "ductile" model by truncating the rupture time at the measured
fracture strain. For this exercise, a creep strain model based on rupture and time to strain data was
fitted. In parallel, microstructural examination was performed to characterize the damage modes
involved in the low ductility failures. It was thereby shown that the creep rupture strength
shortfall of an EA material compared to its DS equivalent is not a constant factor, but is primarily
governed by the reduced creep ductility. Hence, the shortfall varies between different EA casts,
and tends to become greater in the longer term.JRC.F.4-Innovative Technologies for Nuclear Reactor Safet
