Estudo da viabilidade de crescimento anormal de grãos em uma liga de memória de forma Cu-­Al-­Ni-­Mn-­Nb

Abstract

Shape memory alloys (SMA’s) are smart materials that present shape memory effect, which consists on the recovery of the deformed state after a thermal cycle, and the superelasticity, which is the recovery from large deformations after mechanical unloading. These phenomena are the result of a reversible phase transformation of the austenite/martensite. Being a copper-based alloy, the Cu-Al-Ni shape memory alloy has low elemental cost compared to Ni-Ti alloys and has a potential for application at temperatures above 100°C. However, brittle intergranular fracture is a limitation for its application. In this sense, obtaining an abnormal grain growth in this material is a way to overcome the limitation imposed by intergranular fracture in this alloy. The aim of this master thesis is to obtain a Cu-Al-Ni-Mn-Nb with large grains through a combination of thermal cycling and hot rolling. The SMA studied was Cu-11.76Al-3.22Ni-2.78Mn-0.02Nb (wt %), processed by spray forming. It was submitted to isothermal treatments at 300°C, 400°C, 500°C, 600°C and 700°C for 24h in order to determine the formed phases at these temperatures. After studying the isothermal treatments, the spray formed sample was thermally cycled at 400°C and 600°C. In addition, a rolling at 950°C was performed on the spray formed sample and part of it was cycled at 400°C. In view of this, it was found that the most optimized route for grain growth was the rolling at 950°C and cycled at 400°C. The sample spray formed cycled at 400°C and the sample rolled at 950°C showed abnormal grain growth (AGG), but with a lower efficiency. It is suggested that the mechanism involved in the AGG applied to the Cu-11.76Al-3.22Ni-2.78Mn-0.02Nb (wt %) SMA is associated with subgrains formation.