The formation of G-phase in 20/25 Nb stainless steel AGR fuel cladding alloy and its effect on creep properties

Precipitation within localised chromium-enriched regions in a Type 316H austenitic stainless steel

“…• Faceted G phase precipitates with a composition of (Fe, Ni, Mo) 16 (Cr) 6 Si 2.3 have been observed at austenite-chi phase boundaries. Phase identification has been achieved using fast Fourier transform-generated diffraction patterns from atomic resolution images of the phase.…”

“…If the G phase precipitates in the ex-service steel are normalised for Fe, Ni and Mo, an average measured composition becomes (Fe, Ni, Mo) 16 (Cr) 6 Si 2 . A theoretical composition for G phase has been given as (Ni, Fe, Cr) 16 (Nb, Ti) 6 Si 6 [12]; however, substitution of several alloying elements has previously been observed such that a more representative combination would be (Ni, Fe, Mo, Cr) 16 (Nb, Mn, Cr, Ti) 6 Si 6 or 7 [14,19,23] or as generalised by Sourmail to be A 16 D 6 C 7 where A and D are transition elements and C is a group IV element [24]. Although the ratio of alloying elements shown in Table 3 is comparable to that observed in G phase, the silicon level is significantly lower.…”

“…The composition corresponds to (Fe,Ni,Mo) 16 (Cr) 6 Si 2 . Previous workers have reported various compositions of the G phase in a range of austenitic stainless steels: a typical example for the compositions is (Ni, Fe, Mo, Cr) 16 (Nb, Mn, Cr, Ti) 6 Si 6 or 7 [12,14,19,23]. The crystal structure of the G phase detected in the current specimen is consistent with previous observations, although the lattice parameter is slightly smaller (a = 1.115-1.120 nm [12]).…”

“…G phase precipitates form within the ferrite regions of duplex [21,22] and nominally austenitic stainless steels (e.g. Type 300 series austenitic steels) [13,15,16,19] during ageing within the temperature range of 250-500°C [12,14,22]. This phase typically nucleates and grows at austenite grain boundaries [21,24], and/or at austenite-a-ferrite phase boundaries but in the latter case it has a cube-on-cube orientation relationship with the ferrite [19,21].…”

“…One of the less encountered precipitates is G phase, a silicon containing FCC intermetallic phase (a = 1.115-1.120 nm [12]) with a nominal composition observed in austenitic stainless steels [12][13][14][15][16][17][18][19][20] of (Ni/Fe/Cr) 16 (Nb/ Ti) 6 Si 6 [12]. By comparison, in duplex stainless steels [12,21,22] the composition is (Fe/Ni) 16 (Mn/Cr) 6 Si 7 [19]. Other studies have found that substitution of Mo for Ni is possible [14,23].…”