Room-Temperature Short-Range Ordering in Fe-Si Alloys Observed by Internal Friction

Research performed at Ghent University, regarding new production methods for electrical steel, has shown that high silicon steel suffers an ageing phenomenon at room temperature. Recent studies carried out by the same group using different analysis techniques (Mossbauer spectroscopy, neutron diffraction, etc) brought to light a probable process of ordering towards the D03-structure, which is responsible for the observed low ductility during cold rolling and makes the processing of steel extremely difficult. In addition, the Si-steels become more brittle as the delay time between hot and cold rolling is increased. Frequency dependent internal friction (FDIF) studies were performed on different Fe - Si alloys with a Si content varying from 3.73 at. % to 8.7 at. % immediately after several thermal treatments and compared with ultra-low carbon steel. The evolution of relaxation peaks during the IF measurements, performed at constant room temperature, helps to understand the ageing mechanisms. Three processes have been observed: firstly, as expected, addition of Si reduces the carbon Snoek peak. Secondly, a peak associated to C - Si is formed. Thirdly, a low frequency peak associated with Zener relaxation (Si atom pairs) appears for a content of approximately 3.77 wt. % Si. The two latter peaks decrease with ageing time and in the case of the Zener peak there is a notable displacement to higher frequencies with a small increase of the Si content. The reduction of the peaks during the ageing after annealing is more noticeable in quenched specimens than in air cooled ones, and in furnace cooled specimens the reduction is even smaller, indicating that the process is really an ageing phenomenon. Room temperature short-range ordering might explain both the lowering of the Zener peak and the observed macroscopic embrittlement.