Síntese e caracterização de ligas Fe-Mn-Si com efeito memória de forma obtidas por moagem mecânica de alta energia

Abstract

This thesis presents studies on Fe-30Mn-6Si shape memory alloys produced by high energy mechanical milling. Elemental powders of iron, manganese and silicon were processed in a planetary mill with a rotational speed of 500 rpm with milling times of 1 to 10 h. X-ray diffractograms showed the evolution of the various crystallographic phases present in the material. A single FeMnSi austenitic phase with nanometric crystalline size is found at 4 h of milling time. This phase is stable up to 10 h of milling. Mössbauer spectra showed the existence of a ferromagnetic sextet and a paramagnetic singlet at the beginning of the milling process. During the course of grinding the sextet is replaced by the singlet. Only the paramagnetic singlet associated to the FeMnSi austenitic phase is seen from 4 h to 10 h. Scanning electron microscopy and granulometry results showed that the samples are not homogeneous and have particle sizes in the micrometer range. The powder sample milling for 4 h was compacted and sintered to produce cylinders with a height of 11 mm and a diameter of 8 mm. The sintered cylinders were subjected to compression deformation and subsequent heat treatment for shape recovery. X-ray diffraction results showed the presence of γ austenite phase and of a small fraction of ε martensite with HCP structure in the sintered cylinder. The diffractogram of the deformed sample showed that a large fraction of austenite was transformed into martensite. After heat treatment it was observed that part of the martensite phase was transformed back into austenite. The Mössbauer spectra of these samples showed that the Fe atoms are found in more than one paramagnetic environment, which is consistent with the results of X-ray diffraction. Scanning electron microscopy revealed the presence of pores created by the cylinder manufacturing process. Values of hardness can be associated to the porosity of the samples. Corrosion tests showed that the deformed sample showed the best corrosion performance, which is attributed to the higher presence of martensite in this sample. Deformation and shape recovery tests led to an elastic recovery of 5.3% after removal of the compressive load and a plastic recovery of 0.9% after the heat treatment, resulting in a total recovery of 6.2%. These results provide evidence that the shape memory effect occurs both after mechanical relief as well as after heat treatment. The plastic recovery effect is associated with ε − γ transformation observed by X-ray diffraction. The plastic recovery around 0.9% after the heat treatment obtained in this study is consistent with results previously reported for FeMnSi alloys with similar chemical composition.