Based on the M5 ® alloy metallurgy, the Q12™ alloy (Zr-1Nb-0.5Sn-0.1Fe) was developed by AREVA NP for structural components, with ultra-low Sn addition and slightly increased Fe content. The behavior of this alloy was tested under irradiation in PWR and has shown, in comparison to M5 ® , improvement in irradiation creep strength and similar free growth. The present paper provides results on dimensional stability and details the microstructural evolution of Q12™ alloy under neutron irradiation in PWR. The microstructural evolution under irradiation of Q12™ was studied for fast neutron fluences up to 13x10 25 n/m 2 (E>1 MeV) with Analytical Transmission Electron Microscopy observations, focused on radiation-enhanced needle-like particles, Laves Phases, and linear density of -component loops. These results are compared with other quaternary Zr-Nb-Sn-Fe alloys and with M5 ®. All these results allow a general discussion about microstructural evolution and behavior under irradiation of quaternary type alloys compared to M5 ® alloy. This study, in agreement with previous works on Zr1Nb and quaternary alloys, seems to show that increasing the iron content with the presence of niobium and tin will decrease the component loop linear density and delay the growth breakaway.