Base the radius on the weak link, i.e., the material with the lowest elongation. Don't forget to consider the properties of the filler metal as well as the two base metals being welded.

Best regards - Al

As Al said, the bend radius must be based on the material with the 18% elongation.  Note B in QW-466.1 states "The dimensions of the test jig shall be such as to give the bend test specimen a calculated percent outer fiber elongation equal to at least that of the base material with the lower minimum elongation as specified in the base material specification."

percent outer fiber elongation = 100t/(A + t)

Since the note says "at least", a smaller bend radius would be permitted.