Not understanding the metallurgy bit, but if it was a matter of age and not related to flying hours or cycles, I must ask, how was XD816 kept flying so long after the rest of the Valiant's were scrapped?
My understanding, until this forum came up, was that XD816 had been a hangar queen while it was at Boscome Down, and used as the model for the fitting of HDU's. And as such it had way below the number of flying hours of the rest of the Valiant fleet.
It definitely spent at least 6 years on 214 Sqdn from Feb 59 until Feb 65.