Mechanistic modelling of deformation and void growth behaviour in superalloy single crystals
In this work the constitutive behaviour and the influence of casting defects on
the failure of Ni-base single crystal (SC) superalloy components is investigated. It is
well known that the presence of casting-related porosities can lead to the nucleation
of microcracks under both creep and fatigue conditions and thus, ultimately produce
A rate dependent crystallographic formulation is introduced to describe the inelastic
deformation behaviour of the latest generation of single crystal superalloys.
The evolution of the current dislocation and obstacle network is described through
appropriate slip resistance and internal or back stress variables for each slip system.
Good correlations are obtained between experimental data and numerical predictions
within the 750◦C .- 950◦C temperature range and for < 001 > and < 111 >
The formulation is then numerically implemented into the FE method and used
to investigate the deformation of a representative material volume containing a
spherical void of approximately 20 micrometers diameter. The functional dependence
of the void growth rates in terms of material anisotropy, stress state, temperature
and interaction with a free surface is determined. It is shown that the rate of
growth of casting defects in an infinite single crystal medium is strongly dependent
on the applied triaxiality and relative orientation between the crystallographic axes
and the applied stresses. Furthermore, it has been found that, for the acceleration
of the defect growth rate as the result of its proximity to a free-surface to be
non-negligible, the void needs to be within two diameters of the free surface.
Based on the above results, a framework is proposed to describe the growth of
embedded casting defects within superalloy single crystals under a given applied
multiaxial stress state. The framework provides an explicit link between the mesoscopic
(at the level of the voids) and the macroscopic length scales.
A number of blunt notch bar creep specimens and thermo-mechanical cyclic
stress-strain specimens were tested to validate both the rate dependent crystallographic
formulation and the micro-mechanics void growth model. In addition
microstructural analysis of the blunt notch bars provided necessary data for the
development of a micro-crack initiation criterion.
A life assessment methodology combining these models is developed and applied
in the analysis of an actual gas turbine blade. It is expected that the understanding
of defect growth kinetics and the corresponding damage accumulation will be
beneficial in the design and life prediction of superalloy components.