We investigated the tensile properties of Fe-30Mn-(6 À x)Si-xAl (in wt.%, x = 0, 2, 3, 6) austenitic alloys precharged with hydrogen gas at 10 MPa. The Fe-30Mn-6Al alloy showed hydrogen-improved elongation through enhancement of work hardening capacity in the plastic instability condition, although the other alloys did not. This improvement in elongation is considered to be a result of a reduction in stacking fault energy because deformation twinning appeared only in the hydrogen-precharged condition.Hydrogen is recognized as a negative solute element that deteriorates mechanical properties of advanced high strength steels. For instance, martensitic steels [1,2], dual-phase steels [3], transformation-induced plasticity (TRIP) steels [4,5], and twinning-induced plasticity (TWIP) steels [6][7][8] have been reported to show hydrogen-assisted cracking and its associated embrittlement.Although hydrogen negatively affects mechanical properties in most cases, embrittlement is caused by various factors: enhancement of dislocation motion [9], reduction in surface energy and increase in interface energy [10], promotion of vacancy formation [11], reduction in stacking fault energy, etc. [12]. When hydrogen embrittlement occurs, all of these factors are noticed only as negative effects.However, a reduction in stacking fault energy of austenitic steels is generally known to enhance work hardening [13,14], which improves tensile elongation [13]. For instance, TWIP steels were recently designed by decreasing stacking fault energy to promote deformation twinning in austenitic steels [15,16], which have drawn attention as a new class of high strength steels with excellent workability. Therefore, the hydrogen effect decreasing the stacking fault energy is expected to enhance work hardening through promotion of deformation twinning or suppression of dislocation cross-slip. Furthermore, hydrogen would segregate to partials and stacking faults due to strain aging and Suzuki effect, enhancing the effect of hydrogen on work hardening. Accordingly, the tensile elongation is also expected to be improved by hydrogen.Astafurova et al. [17] reported that hydrogen could promote deformation twinning in a single-crystalline Hadfield steel, and accordingly, elongation and strength were improved. However, a similar strategy for positively utilizing hydrogen in polycrystalline austenitic steels has never been reported. In this paper, we report an example of positive effect of hydrogen on tensile elongation in terms of a reduction in stacking fault energy.Fe-30Mn-(6 À x)Si-xAl (x = 0, 2, 3, 6) austenitic alloys were prepared using vacuum induction melting with high-purity metals. Chemical compositions of the Fe-30Mn-(6 À x)Si-xAl (x = 0, 2, 3, 6) alloys are listed in Table 1. The Fe-30Mn-6Si alloy is known to be a shape memory alloy with deformation-induced e-martensitic transformation [18,19]. In this study, Si was substituted by Al to increase stacking fault energy [15]. The stacking fault energies of the Fe-30Mn-6Si, Fe-30Mn-4Si-2Al, Fe-30Mn-3S...
