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Autori: GURÃU, Gheorghe, GURÃU, Marlen, POTECAŞU, Octavian, ALEXANDRU, Petricã, Bujoreanu, Leandru
Editorial: Journal of Materials Engineering and Performance, 23(7) , p.2396-2402, 2014.
This paper introduces an adapted high-speed high pressure torsion (HS-HPT) method of severe plastic
deformation applied for obtaining shape memory alloy (SMA) active elements with revolution symmetry,
able to develop axial displacement/force. Billets with circular crown forms were cut from Fe-28Mn-6Si-5Cr
(mass%) SMA ingots and, by means of HS-HPT technology, were directly turned into modules, with
truncated cone shell configurations. This process was performed, during time intervals of seconds, under
the effect of high pressure (up to 1 GPa) cumulated with high rotation speed (hundreds of rotations per
minute) applied on the active surfaces of sintered-carbide anvils, specially designed for this purpose. Due to
pressure and friction, generated by rotation, the entire sample volume is heated and simultaneously deformed
to final shape. During the process, microstructure fragmentation occurred enabling to obtain
(ultra)fine grains and nanocrystalline areas, in spite of the heat developed by friction, which was removed
by conduction at the contact surface between sample and anvils, before the occurrence of any recrystallization
phenomena. When compressed between flat surfaces, the truncated cone modules developed a
superelastic-like response, unique among Fe-Mn-Si base SMAs and, when heated in compressed state, they
were able to develop either axial strokes or recovery forces by either free or constrained recovery shape
memory effect (SME), respectively. By means of optical (OM) and scanning electron microscopy (SEM)
marked structural changes caused by HT-HPT were revealed, along with fine and ultrafine crystalline
grains. The presence of stress-induced e-hexagonal close-packed (hcp) martensite, together with nanocrystalline
areas were confirmed by x-ray diffraction.
Cuvinte cheie: actuator, Fe-Mn-Si-Cr shape memory alloy, high pressure torsion, martensite, severe plastic deformation