Changes in the Thermoelastic Properties Caused by Metal Forming Processes in Ni-Ti Shape Memory Alloys
Cezar Henrique Gonzalez1; Carlos Augusto do Nascimento Oliveira1; Niédson José da Silva2; Oscar Olímpio de Araújo Filho1; Carlos José de Araújo3; Karla Carolina Alves da Silva1; Luiz Carlos Sekitani da Silva1
1 Universidade Federal de Pernambuco; 2 Instituto Federal de Pernambuco; 3 Universidade Federal de Campina Grande
doi:10.20906/CPS/COB-2015-2516
Resumo
Plastic forming processes have been applied in the shape memory alloys (SMA) to improve their mechanical properties. However, these processes lead to changes in its thermoelastic properties (temperatures and enthalpies of transformation, critical stress induced martensitic, reversible thermoelastic deformation and others). For technological applications of these alloys such as thermomechanical actuators, they should keep their constant characteristics throughout its use (thermoelastic properties). In this work are analysed these properties in the Ni-Ti shape memory alloys that have been processed mechanically by conventional rolling and Equal Channel Angular Extrusion (ECAE). The analyses were performed by differential scanning calorimetry, optical and electronic microscopy, X-rays diffraction, microhardness and tensile tests under constant load (simulation of the memory effect in order to determine reversible thermoelastic strain). The results showed that the plastic forming processes drastically alter the properties, especially for those samples subjected to the ECAE that can reach block the martensitic transformation. Heat treatments were applied in order to improve the shape memory effect properties. Results of X-rays diffraction and micrographs indicate that each type of forming process and the presence of precipitates has great influence in the elastic property behaviours. These analyses are relevant for applications of these SMA which require large mechanical strength combined with functional properties of the shape memory phenomena.
Palavras-chave: Shape Memory Alloys; Metal Forming Processes; Equal Channel Angular Extrusion - ECAE; Thermoelastic Properties