The interstitial alpha-Fe-C solid solution has an only narrow homogeneity range in the metastable Fe-Fe3C system. More C can be incorporated by forming disordered or Zener-ordered tetragonal alpha’ martensite. At constant C content strain-induced interactions between the C atoms on the octahedral sites can lead, at a given composition, to spontaneous order of C, thus preferentially one of the three octahedral sublattices with the short-octahedral axis along ,  or  . According to Xiao et al.  this happens for mass fractions of C above 0.18 % at room temperature. The thermodynamic model for Zener ordering  has recently [3,4] been incorporated into a Compound Energy Formalism (CEF) description of the alpha/alpha’-Fe-C for use within the CALPHAD approach.
Recently, reports appeared about Fe-C alloys (or of steels) containing considerably less carbon than the above mentioned 0.18 wt.% in the alpha/alpha’ phase [5,6], which exhibited diffraction patterns which were interpreted in terms of tetragonality, implying presence of Zener ordering for these low C contents. In Ref.  this finding was explicitly related with the presence of stress.
In this presentation the CEF description of the alpha/alpha’-Fe-C solid solution with potential ordering on the octahedral sublattices is extended by incorporating possibility of non-hydrostatic stress, motivated by previous treatments of the Snoek effect. Thereby, partially use is made of concepts developed to describe anelastic relaxation of alpha-Fe-C, leading to formulations of the Gibbs and Helmholtz energies. Based on these model descriptions, the effect of external stress on occurrence of true or apparent tetragonality will thus be discussed. Moreover, alternative approaches to interpret diffraction patterns apparently tetragonal alpha/alpha’-Fe-C will be discussed.
 A. G. Khachaturyan, Theory of Structural Transformations in Solids, Wiley, New York, 1983.
 L. Xiao et al., Phys. Rev. B 52 (1995) 9970.
 R. Naraghi et al., in Proceedings of the 1st Word Congress on Integrated Computational Materials Engineering, TMS, 2011, p. 235.
 R. Naraghi et al., Calphad 46 (2014) 148.
 C.N. Hulme-Smith et al., Scr. Mater. 69 (2013) 409.
 S. Djaziri et al., Adv. Mater. 28 (2016) 7753.