Influence of Heat Treatment on Magnetic and Damping Properties of Fe-11 at.% Al Alloys

The influence of heat treatment on the amplitude dependence of internal friction in Fe - 11 at. % Al alloys with carbon contents in the range 0.005 - 0.2 at. % has been studied using an inverted torsion pendulum in the temperature range 300 – 950 K and a vibrating reed apparatus at room temperature. The specimens were annealed at 1273 K in vacuum and cooled down with different cooling rates in order to obtain different degrees of order. It was found that ordering is hardly avoidable in Fe - Al alloys with Al contents > 11 at. %. Ordered alloys are characterised by lower damping capacity due to higher coercivity caused by additional pinning of magnetic domain walls by antiphase boundaries. X-ray diffraction investigations indicate that water-cooling suppresses ordering in Fe - 11 at. % Al alloys while cooling in air or in furnace provokes D03–type ordering. Slowly cooled specimens are characterised by higher damping capacity due to lower coercivity than water cooled or plastically deformed specimens. The amplitude dependent magneto-mechanical damping was determined as the difference between amplitude dependent damping without and with saturating magnetic field (~ 20 kA/m). Magneto-mechanical damping was found to be proportional to the strain where the amplitude dependent damping is maximum and reciprocal to the coercivity and saturation polarisation. Cold rolling increases the coercivity and therefore decreases the magneto-mechanical damping. An increase of the grain size in the investigated samples by heat treatment leads to a qualitatively expected decrease of coercivity and therefore to an increase of magneto-mechanical damping.