Investigation of the two martensitic phase transitions hcp-dhcp and dhcp-fcc in Co-0.85at%Fe by neutron scattering

Upon heating, Co-0.85at%Fe undergoes the two martensitic phase transitions hcp→dhcp (at ∼590 K) and dhcp→fcc (at ∼690K). The transitions are characterized by a large hysteresis permitting to investigate properties of two phases at the same temperature, respectively. The distance between neighboring close-packed planes remains virtually constant during the hcp-dhcp transition while a change of ∼0.3% is observed at the dhcp-fcc transformation. The lattice dynamics of Co-0.85at%Fe was investigated by means of inelastic neutron scattering at a series of temperatures up to 750 K in order to look for precursor effects in the phonon dispersion curves near the two martensitic transformations. In all of the phonon branches investigated, anomalies were neither found near the hcp-dhcp nor the dhcp-fcc transition. Both transformations in this system occur slowly and with a continuous change of the volume fractions of the phases involved over periods of several hours.