Heat treatments of Fe-Mn-Si based alloys and related shape memory phenomena

A. DRUKER; A. PEROTTI; A. BARUJ; J. MALARRÍA

Rio de Janeiro

Congreso; 18th International Federation for Heat Treatment and Surface Engineering IFTHSE 2010; 2010

AMB Brasil

Purpose:Heat treatments are a normal means for modifying alloy properties, through changes in the microstructure. Our goal was to develop new processes for the Fe-Mn-Si shape memory alloys. The SM effect is due to a stress induced martensitic transformation.Different processing and heat treatments generate or annihilate defects, such as stacking faults, nuclei for martensite, and dislocation density, which affects the plastic deformation of the austenite.Methodology:After rolling at 1000oC to 1.7 mm, a Fe-15Mn-5Si-9Cr-5Ni alloy was deformed to 1 mm by rolling at different temperatures. Then the sheets were annealed at recovery and recrystallization temperatures. The SM properties were evaluated by tensile and bending tests conducted in an Instron 1362. The reverse transformation was obtained by heating at 600oC.Specimens for TEM were chemically thinned and then electropolished by using the double-jet technique in a 90/10 (vol. %) acetic/perchloric solution. Metallographic observations were performed with an Olympus PME3, Philips EM300 and JEOL JEM-2000FX microscopes. Results:Observations show the microstructure resulting from the thermomechanical treatments. The matrix annealed at high temperature contains a low density of defects, reducing the nucleation sites for martensite. In this case, the transformation proceeds by the growth of pre-existing plates. Annealing at intermediate temperatures produces an ordered structure containing a great number of defects, such as stacking faults. The consequences are higher degrees of shape recovery. Our investigations of the SM effect allowed us to determine the best thermomechanical treatment.