1 Neutron powder thermo-diffraction in mechanically alloyed Fe 64 Ni 36 invar alloy Pedro Gorria 1 , David Martínez-Blanco 2 , Jesús A. Blanco 1 , Ronald I. Smith 3 1 Departamento de Física, Universidad de Oviedo, Calvo Sotelo s/n, Oviedo 33007, Spain 2 Unidad de Magnetometría, SCT’s, Universidad de Oviedo, Julián Clavería, 8, 33006 Oviedo, Spain 3 ISIS facility,RAL,Chilton, Didcot, Oxon OX11 0QX, United Kingdom Abstract Nanostructured Fe 64 Ni 36 alloy has been obtained using high-energy ball milling for 35 hours of milling time, Fe 64 Ni 36 MA-35h. The initial as-milled Fe 64 Ni 36 MA-35h powders are inhomogeneous, showing a majority phase with a face-centred cubic (fcc) crystal structure [88(2)%] and a minority phase with body-centred cubic (bcc) crystal structure [7(2)%]. The evolution of the microstructure with temperature between 300 K and 1100 K has been followed by means of in situ neutron powder thermo-diffraction experiments. The room temperature values for the mean crystalline size and the mechanical-induced microstrain of the fcc phase in the as-milled sample are ∼10 nm and ∼0.7%, respectively. Moreover, after heating the Fe 64 Ni 36 MA-35h powders up to 1100 K, an increase of around 65 K in the Curie temperature respect to that of the commercial coarse-grained alloy of the same composition is observed. The latter together with the observed temperature dependence of the lattice parameter suggests that the Fe 64 Ni 36 MA-35h sample subjected to the heating process exhibits invar behaviour. On heating up to 1100 K thermal relaxation of the microstucture occurs giving rise to grain growth above 100 nm, nearly vanishing values for the maximum strain, and the transformation of the bcc phase into the fcc one above 800 K, being the latter stable in subsequent heating-cooling processes. Keywords: nanostructured materials; mechanical alloying; neutron diffraction, invar alloys doi:10.1016/j.jallcom.2009.10.112 Corresponding author: Dr. P. Gorria Address: Departamento de Física, Universidad de Oviedo, Calvo Sotelo s/n, Oviedo 33007, Spain Phone: +34 985102899 Fax: +34 985103324 e-mail: pgorria@uniovi.es