While the alloys used are less ductile than pure aluminum, they still have similar properties and are more prone to tearing than shearing. Even the metal matrix (M2) alloy with boron fibers in it that Specialized used on mountain bikes in the late 90s didn't crack and fail suddenly. I know because I broke one. It creaked quite a bit before I noticed that it had cracked.

Even rims...of which I've broken many including one within the last month...don't fail suddenly. Cracks develop and propagate but the process has always been very slow and with lots of warnings...i.e. creaking. There is not "ping" involved as there is in steel failures.

I think a lot of the "sudden" failure of aluminum can be attributed to people not seeing the crack nor paying heed to the creaking and groaning of the frame. They ride for a long ways with a creaking bike and are suddenly surprised when the failure occurs but the damage has been there for a long time. That's been my experience even with my recent rim failure. The sidewall cracked and the wheel felt "weird" for a while and the tube blewout unexpectedly before I noticed the cracked sidewall. I probably would have noticed it sooner on a rim brake equipped bike because the brakes would have pulsed.