Hi,
I interviewed Bill or Andy Zildjian (cannot remember which, though I do recall the inimitable Nort Hargrove vividly) in the early days of Sabian for a mag called Canadian Music Trade.
This would be Spring 1983:
He said that in the past they never knew exactly what was in the metal they purchased and there were lots of impurities and that they had much better control now (1983) than in the past.
He never commented on metal fatigue vis a vis impurities, just sound and quality control in general.