**1. Introduction**

Intermetallic compounds with coarse particle size are usually avoided in the alloy designing, because their brittleness often leads to poor deformability at room temperature. However, recent studies showed that some steels and alloys containing intermetallic compounds as second phase exhibited excellent combination of tensile strength and ductility, even at room temperature. Furuta et al. [1] reported that a heavily deformed Fe-Ni-Al-C alloy containing NiAl-type B2 intermetallic compound particles showed a yield strength of 2.2 GPa, whilst still keeping a 25% tensile elongation. Kim et al. [2] developed an Fe-Mn-Al-Ni-C light-weight steel composed of ultrafine-grained austenite and B2 intermetallic compounds phase that exhibited quite high specific strength and good tensile elongation. In addition, B2 phase has also been frequently observed in high entropy or multi-component alloys having high strength and good tensile elongation [2–6]. It was believed that the B2 phase played an important role in the excellent mechanical properties of those materials. Yang et al. [7] studied

the strain hardening behavior of Fe-Mn-Al-Ni-C steel and suggested that the high back stress, rising from the incompatibility between the matrix and B2 phase, accounted for the high strain hardening rate and the excellent tensile properties of the material. The present author studied tensile properties of an ultrafine-grained dual-phase Fe-24Ni-6Al-0.4C alloy composed of ultra-fine grained (UFG) austenite and B2 phase and suggested that B2 phase was somehow important to the high strength of the material [8]. More recently, Kim et al. [9] argued that at least in the Fe-Mn-Al-Ni-C system, the high strain hardening rate was largely attributed to the intensive planar slip of dislocations in the austenite matrix that was enhanced by the short-range ordering of the Mn-C, rather than the existence of B2 particles. Those studies, mostly using post-mortem microstructure characterization, are either qualitative or indirect. The role of B2 phase in the tensile properties of the materials has not yet been dynamically evaluated during deformation. In situ diffraction measurement has been proven as an appropriate tool to study the deformation behavior of alloys consisting of multiple phases for its capability to follow and distinguish the evolution of the stress state and phase volume fraction of different constituent phases during mechanical test [8–15]. In the present study, a tensile test with in situ synchrotron radiation X-ray diffraction measurement was carried out on an ultrafine-grained Fe-Mn-Al-Ni-C alloy containing B2 particles, in order to elucidate the contribution of B2 phases to the tensile properties of the material.
