Tracer diffusion of titanium in α-iron

The tracer diffusion coefficient of 44Ti in α-Fe is determined by the serial sectioning method in the temperature range of 948 to 1174 K. In paramagnetic α-Fe the diffusion coefficient of Ti shows a linear Arrhenius relationship from the α-γ transformation temperature to about 30 K above the Curie temperature TC = 1043 K. Below 1076 K, the temperature dependence of the diffusion coefficient deviates from the linear Arrhenius relationship due to the magnetic spin ordering. The temperature dependence of the diffusion coefficient D in the whole temperature range of α-Fe can be expressed by where M is the magnetic long-range order parameter. The frequency factor D0 = (0.21 m2 S−1 and the activation enthalpy Q = (293.2 ± 8.8) kJ mol−1 describe Ti diffusion in paramagnetic α-Fe and correspond to a normal vacancy diffusion mechanism. Ti diffusion is faster by a factor of about 5 when compared to Fe self-diffusion. This can be understood by a vacancy-solute binding due to the larger atomic radius of Ti. A comparison with the diffusion behaviour of other transition elements in α-Fe confirms the correlation between diffusion velocity and solute atomic radius.