Effects of Different Dissolution and Aging Heat Treatments on Metallographic and Mechanical Properties of NiTi Based Sheets

Abstract

In this study; 1-mm-thick NiTi sheets showing phase transformation at 45 degrees C and 80 degrees C were used. First, three-point bending tests were performed to investigate the bendability. Later, solution heat treatments were applied to the samples taken from the bent sheets for 1 hour at 750 degrees C, 800 degrees C, 850 degrees C and 900 degrees C respectively. Aging heat treatment was applied for 15, 30, 45, 60, 75 and 90 minutes (min.) at 300 degrees C to these samples. Optical Microscope, Field Emission Scanning Electron Microscopy (FESEM), Element Dispersion Spectroscopy (EDS) and MAPPING (MAP) analyses were carried out to examine the effects of the heat treatment parameters. Macro hardness data were taken to examine the mechanical effects after each heat treatment. Moreover, the mechanical behavior of the material under load was investigated by Dynamic Mechanical Analyses (DMA). As a result, mechanical properties (hardness and storage modulus) have been improved by aging heat treatments. The highest hardness value was measured as 414HV after solution heat treatment at 900 degrees C and aging heat treatment at 300 degrees C for 75 minutes on the sample showing phase transformation at 80 degrees C. Furthermore, an increase in storage modulus and a decrease in tan delta value occurred. To benefit from super elastic properties of NiTi alloys, the necessity of using temperatures above the transformation temperatures was observed with the storage modulus measurements. Storage modulus value under transformation temperature was similar to 21500MPa while it was measured as similar to 28500MPa after the transformation was completed for the sample with 45 degrees C transformation temperature.