Single crystal Ni-based super alloys were introduced in the early 1980s and since then have been widely used in turbine aerofoils in jet engines. The desire for weight reduction and the use of advanced metal cooling schemes tends to drive designs toward thinner airfoil walls. Creep tests on Ni-based super alloy specimens have shown greater creep strain rates and/or reduced creep rupture times for thinner specimens than is predicted by current theories. This is termed the thickness debit effect. To investigate the mechanism of the thickness debit effect, isothermal, constant nominal stress creep tests were performed on uncoated PWA1484 Ni-based single crystal super alloy sheet specimens with two thicknesses and under two test conditions: 760 deg. C/758MPa and 982 deg. C/248MPa. The specimens contained initial micro-voids formed during the solidification and homogenization processes. The experiments showed that porosity evolution could play a significant role in the thickness debit effect.

 

This motivated basic mechanics studies of porosity evolution in single crystals subject to creep loading. Three-dimensional finite deformation finite element analyses of unit cells containing a single initially spherical void were carried out for various values of stress triaxiality and various values of the Lode parameter. At low values of the stress triaxiality, well separated voids can collapse into crack-like or needle-like shapes. On the other hand, if the voids are sufficiently close the voids can coalesce.  Depending on void spacing, there is a transition between void collapse and void coalescence.  Implications for the thickness debit effect will be discussed.