Yüksek Sıcaklık Şekil Hafızalı Alaşımların Fonksiyonel Yorulma Ömürlerini İncelemek Üzere Fonksiyonel Yorulma Test Cihazı Geliştirilmesi ve Bu Alaşımların Fonksiyonel Yorulma Ömürlerinin Tayini

Abstract

Shape memory alloys are special materials which can be utilized as actuators for several applications. Their high power to weight ratio draws attention to many applications, especially in aerospace industry. Most of the applications require multiple actuation and transformation temperatures which are above the 100oC. Thus, stability of shape memory alloys in terms of actuation strain and transformation temperatures are very important in order to define the feasibility of using shape memory alloys for industrial applications. Aging is a one of the methods for Nickel rich NiTi alloys that adjust the transformation temperatures and enhance the cyclic stability of the shape memory alloys. In this study, Ni50.3Ti29.7Hf20 high temperature shape memory alloy was used since it is one of the most promising high temperature shape memory alloy in terms of high transformation temperatures as well as high strength. The functional fatigue properties of the extruded and aged Ni50.3Ti29.7Hf20 high temperature shape memory alloy were investigated in order to reveal the effect of aging on functional fatigue behavior. Material was produced using high purity Ni, Ti and Hf elements via vacuum induction melting under high purity argon atmosphere, then placed in a mild steel can for hot extrusion at 900oC with an area reduction of 4:1. One batch of the material was aged at 550oC for 3 hours and one batch of material was used in the experiments in the extruded condition. Same experiments were conducted on both of the materials for comparison. Stress-free transformation temperatures were measured using Perkin Elmer Differential Scanning Calorimetry (DSC) 800. Load-biased heating and cooling experiments were conducted by UTEST 50 kN servo-mechanical test machine with embedded heating and cooling systems. These experiments were done to determine the stress magnitude under which the samples showed the first irrecoverable strain. Functional fatigue experiments were performed on custom-built functional fatigue test setup. Post-mortem optical microscope investigations after functional fatigue tests were performed to investigate the crack propagation during the fatigue experiments. Transmission electron microscopy was also conducted before functional fatigue tests to determine the twin and precipitate structures and the oxide particles in the matrix. Stress-free transformation temperatures were increased above 100oC after aging heat treatment at 550oC for 3 hours, as expected. It was also found that aging at 550oC for 3 hours led to an increase in critical stress for slip such that the first irrecoverable strain was determined under 600 MPa stress level while extruded sample showed irrecoverable strain under 300 MPa in load-biased heating-cooling experiments. Therefore, 200 MPa stress magnitude was chosen as the threshold stress level since no irrecoverable strain was detected for extruded and aged samples and the functional fatigue experiments were conducted under 200 MPa constant stress level. The sample aged at 550oC for 3 hours exhibited considerably better stability during functional fatigue experiments with respect to the extruded one. The decrease in actuation strain values of the aged sample through 16534th cycles was smaller than that of the extruded sample. The test was stopped at this point because fracture was experienced. On the other hand, extruded sample lost its actuation strain before 5000th cycle and therefore, test was ceased. Transmission electron microscopy investigations revealed that oxide particles are in the matrix, thus these oxide particles might be one the reasons of fracture since they may act as the crack initiation points at the beginning of the functional fatigue experiments.