The failure the control alternator gearshaft, which disconnected the accessory gearbox from the rest of the engine and resulted in an uncommanded in-flight shutdown of the left engine and subsequent flight diversion. The control alternator gearshaft failed because of high cycle fatigue intergranular corrosion cracking. Contributing to the failure of the control alternator gearshaft were additional manufacturing processing steps to address part non-conformances that reduced its high cycle fatigue capability and potential change in vibratory environment since the last shop visit.