Disc/Color/Surface.
The Discs are highly polished prior to install, the final rub is Colloidal Silica. Similar to jeweler's rouge. Now this Wheel had seen 200plus cycles, and heat debris and friction, Gas components can work their mischief on the shine. In the ATSB picture, my guess would be that the Disc had been cleaned and inspected, but the image is prior to destructive sampling of the metal. The fracture at Drive Arm appears to be "Puddled/Smeared", as if autonomously and randomly 'forged' prior to parting the Shaft remnants of Drive Arm. It wouldn't be a wild guess that the Disc had experienced a circumferential fracture. Note the Drive Arm 'fracture portion of the image'. It is beaten around, and it lacks sharp fracture delineation that the Disc separations show. So lost from the Shaft through circular fracture, a small hesitancy while describing ever more elliptical orbits of the Drive Arm (with consequent 'folding over'), then separation into three large pieces, and immediate exit. The remnants of Blades are suggestive of a gradual loss (tenths of a second?), and an inconsistent failure mode, the blades/roots which are missing may be more indicative of a 'vibratory/centrifugal/non planar' loss.
Turbine D, old engineer, annex14, mm43, first floor.......all
http://www.atsb.gov.au/media/2891297/vh-oqa-fig8.jpg
Take a big look at the severed torque links, and a peek inside the IPLP cave. Does one see the remnant of NGV1LP? Or is that a remnant of the
Intermediate Turbine? Also beneath the centrifugal Oil Breather, see the burned Stator ring hanging down?? displaced Much?? Without the Torque tubes (which look like they snapped in tension!!) the engine can swing to its heart's content, not good in Yaw, and one step away from losing the powerplant off the pylon. Thanks to the suspension cleats, no worries!!