  The experimental transport scattering rate was determined for a wide range of
optimally doped transition metal-substituted FeAs-based compounds with the
ThCr2Si2 (122) crystal structure. The maximum transition temperature Tc for
several Ba-, Sr-, and Ca-based 122 systems follows a universal rate of
suppression with increasing scattering rate indicative of a common
pair-breaking mechanism. Extraction of standard pair-breaking parameters puts a
limit of \sim26 K on the maximum Tc for all transition metal-substituted 122
systems, in agreement with experimental observations, and sets a critical
scattering rate of 1.5x10^14 s^-1 for the suppression of the superconducting
phase. The observed critical scattering rate is much weaker than that expected
for a sign-changing order parameter, providing important constraints on the
nature of the superconducting gap in the 122 family of iron-based
superconductors.
