Papers by Keyword: Room Temperature Ageing

Abstract: In this comparative study, the structural and superelastic characteristics of two thermomechanically treated metastable Ti-Nb based (Ti-22Nb-6Zr) and Ti-Zr based (Ti-18Zr-14Nb and Ti-18Zr-13Nb-2Ta (at. %)) alloy systems were studied. To study the influence of room temperature storage on the functional properties of these two alloy systems, the alloys were subjected to a multistage testing routine consisting of four ten-cycle loading-unloading testing series alternated with three room temperature ageing periods (1, 5 and 20 days). Based on microstructure-properties relationships, it was shown that for each alloy system, the forward stress-induced martensitic transformation was essentially dependent on the material microstructure, whereas the subsequent reverse martenstic transformation was controlled by the material composition. The Ti-Zr based alloys demonstrated more stable functional behavior than their Ti-Nb based counterparts. More specifically Ti-18Zr-13Nb-2Ta, subjected to a combination of cyclic training alternated with room temperature ageing showed a significant improvement in superelastic behavior with small accumulated strains and narrow stress hysteresis.

Abstract: 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.