I've always been of the opinion that since Antarctic Vibranium can use some kind of weird vibrations to destroy adamantium, Flash's vibration powers could do something similar.
Just mix a bunch of frequencies and go with what works. I'm sure that there must be some kind of vibration hax besides the precise frequency of antarctic vibranium that will work, after all adamantium is vulnerable to molecular alteration/transmutation and Flash's vibrations could make molecules dance the hula if he wanted them to.
I guess any freq. past antarctic vibranium should work, but frequencies don't act as a linear infinite loop thus without prior knowledge it can't be brought up as a viable tactic in combat...
Though it is a slim possibility
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One problem is that frequency is a measure per unit time, so that strategy may be kind of difficult. A million different frequencies in a second means a millionth of a second for each test. So he can't really be testing frequencies like a million per second or less in 1/millionth of a second. The destructive frequency might be 300,000 per second.
And even if the key frequency is more than a million per second, say 10 or 100 million per second (10 or 100 cycles in 1 the one millionth of a second) then it probably is not going to have an effect in one millionth of a second. It might take a couple of seconds and hundreds of cycles to even notice any effect. It also requires sufficient energy to have an effect. It could take days to test millions of frequencies.
And yeah, there are infinite frequencies, it could be something like exactly 1,009,855.7555555555558 per second.
Now this I can see, comic book physics FTW. Flash must have some way of measuring and reproducing all these frequencies he uses. I guess the SF allows him to sense this stuff. It might work directly or he might be able to sense the resonant frequency of adamantium by doing a test punch on Ultron and checking the vibrations it produces. When he has that he may be able to cut through adamantium vibrationally.
I could see it working on Destroyer. I'd love to see Shaw take a hit like that. I've always thought he has great potential, but just needs a decent writer. Kinda like Darkseid.
Ultron and Destroyer aren't technically alive so the is no more immoral than breaking a toaster. That beong said I could see it working on Destroyer before Ultron. Ultron seems to have a more consistent durability IMO.