Neutron powder thermo-diffraction in mechanically alloyed Fe64Ni36 invar alloy

Abstract: Nanostructured Fe64Ni36 alloy has been obtained using high-energy ball milling for 35h of milling time, Fe64Ni36 MA-35h. The initial as-milled Fe64Ni36 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 300K and 1100K 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 ∼10nm and ∼0.7%, respectively. Moreover, after heating the Fe64Ni36 MA-35h powders up to 1100K, an increase of around 65K 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 Fe64Ni36 MA-35h sample subjected to the heating process exhibits invar behaviour. On heating up to 1100K thermal relaxation of the microstructure occurs giving rise to grain growth above 100nm, nearly vanishing values for the maximum strain, and the transformation of the bcc phase into the fcc one above 800K, being the latter stable in subsequent heating–cooling processes. [Copyright &y& Elsevier]