AbstractIn order to find a way to obtain stable α″‐Fe16X2 phase, the formability and thermal stability of α′ (bct) phase were discussed. According to a rigid sphere model, we concluded that the less formability of B for the α′ phase is due to its large atomic radius. We elucidated the difference of thermal stability of α′‐Fe–X, taking into account their decomposition process. While, the decomposition of α′‐Fe–N progresses only by the migration of N, without changing the bone structure of Fe lattice, the additional energy is needed to break the original α‐Fe lattice in the cases of α′‐Fe–B and α′‐Fe–C. Therefore thermal stability of α′‐Fe–B and α′‐Fe–C is higher than that of α′‐Fe–N. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)