2
M. P. Echlin et al.
Fig. 1 Turbine disks (left) are often made from supersolvus nickel superalloys, such as René
88DT. This class of polycrystalline superalloys used for disks have microstructure at various length
scales from precipitate structure (10’s–100’s nm as shown on the right) to twin related domain
structure (10’s–100 μm as shown in the center) with grains containing multiple, fine twinned
structures of varying sizes that are crystallographically related to the parent material. They are
strengthened by L1 2 γ precipitates, which can exist over a range of length scales, depending on
the processing conditions. Populations of secondary and tertiary γ particles exist within the γ
matrix phase (right), with the secondary particles typically being around 100 nm in diameter in
René 88DT
exist. The fraction of 3 annealing twin boundaries, a product of the processing
path, can approach 46% by 2D measurement [8] or 70% by 3D measurement of
the total boundary length fraction [9]. There is relatively limited crystallographic
texture in these materials as a result of powder metallurgy processing or forging
under nominally superplastic conditions. Depending on the alloy composition and
processing route, populations of secondary and tertiary γ particles exist within the
γ matrix phase. The γ precipitates inhibit the passage of dislocations through
the γ matrix [5, 10], by requiring them to either shear through or bow around
the ordered precipitates or cross-slip to continue to glide, effectively strengthening
the material up to the solvus temperature of the precipitates. Typically, powder
metallurgy consolidated components are oil quenched from near 1150 ◦ C and then
aged at 760 ◦ C to produce a volume fraction near 40% of secondary and tertiary γ
precipitates [3].
In powder metallurgy superalloys such as René 88DT [1–3] cracks initiate in
large grains that are in the tail of the size distribution and contain favorably oriented
annealing twin boundaries [8, 11, 12] or at nonmetallic inclusions [7, 13, 14].
Though the annealing twin boundaries form during thermomechanical processing,
the mechanisms by which they form are still not understood well enough to fully
control their size and distribution. The relatively small grain size combined with
moderate levels of L1 2 ordered precipitates imparts yield strengths above 1 GPa [4].
The relatively small grain size also limits the maximum length over which strain
localization and slip events can occur over [6, 15, 16], before impinging on the
adjacent high angle grain boundary, causing dislocation pileups.
M. P. Echlin et al.
Fig. 1 Turbine disks (left) are often made from supersolvus nickel superalloys, such as René
88DT. This class of polycrystalline superalloys used for disks have microstructure at various length
scales from precipitate structure (10’s–100’s nm as shown on the right) to twin related domain
structure (10’s–100 μm as shown in the center) with grains containing multiple, fine twinned
structures of varying sizes that are crystallographically related to the parent material. They are
strengthened by L1 2 γ precipitates, which can exist over a range of length scales, depending on
the processing conditions. Populations of secondary and tertiary γ particles exist within the γ
matrix phase (right), with the secondary particles typically being around 100 nm in diameter in
René 88DT
exist. The fraction of 3 annealing twin boundaries, a product of the processing
path, can approach 46% by 2D measurement [8] or 70% by 3D measurement of
the total boundary length fraction [9]. There is relatively limited crystallographic
texture in these materials as a result of powder metallurgy processing or forging
under nominally superplastic conditions. Depending on the alloy composition and
processing route, populations of secondary and tertiary γ particles exist within the
γ matrix phase. The γ precipitates inhibit the passage of dislocations through
the γ matrix [5, 10], by requiring them to either shear through or bow around
the ordered precipitates or cross-slip to continue to glide, effectively strengthening
the material up to the solvus temperature of the precipitates. Typically, powder
metallurgy consolidated components are oil quenched from near 1150 ◦ C and then
aged at 760 ◦ C to produce a volume fraction near 40% of secondary and tertiary γ
precipitates [3].
In powder metallurgy superalloys such as René 88DT [1–3] cracks initiate in
large grains that are in the tail of the size distribution and contain favorably oriented
annealing twin boundaries [8, 11, 12] or at nonmetallic inclusions [7, 13, 14].
Though the annealing twin boundaries form during thermomechanical processing,
the mechanisms by which they form are still not understood well enough to fully
control their size and distribution. The relatively small grain size combined with
moderate levels of L1 2 ordered precipitates imparts yield strengths above 1 GPa [4].
The relatively small grain size also limits the maximum length over which strain
localization and slip events can occur over [6, 15, 16], before impinging on the
adjacent high angle grain boundary, causing dislocation pileups.