The Determination of The Functional Fatigue Life of High Temperature Shape Memory Alloys After Cold Rolling Process

Abstract

Shape memory alloys (SMA) are unique materials due to their shape recover characteristics. These materials can work against load with their shape recover ability, which lead them to be used as actuators in aerospace applications. Most applications of SMAs request a large number of cycles and the transformation temperatures which are above 100 ˚C. For Ni-rich NiTi alloys, aging heat treatment, ternary element addition and plastic deformation are the known methods to adjust the transformation temperatures and to improve the cyclic stability. Achieving the cyclic stability in terms of the transformation temperatures and the actuation strain is an important objective for the actuation applications of shape memory alloys. In this study, Ni50,3Ti29,7Hf20 high temperature alloy was used due to its high transformation temperatures, high strength and promising cylic stability. The functional fatigue properties of the cold-rolled and aged Ni50,3Ti29,7Hf20 high temperature shape memory alloy were investigated to exhibit the effect of cold rolling together with aging on functional fatigue behaviour. The material was produced with high-purity Ni, Ti and Hf elements via vacuum induction melting under high purity argon atmosphere. One batch of the material was only solutionized at 1050˚C for 2 hours. Another batch was solutionized and aged at 550˚C for 3 hours. The other two batches are solutionized, cold rolled by 5 and 10% and aged, respectively. The solutionizing and the aging heat treatments were accomplished in a vertical cylindrical furnace under high purity argon atmosphere. The samples were rolled with a laboratory-type rolling mill at room temperature. The samples were cut to flat dog bone geometry to perform the load-biased heating-cooling experiments and the functional fatigue experiments. Stress-free transformation temperatures were measured by using Differential Scanning Calorimetry (Perkin Elmer 8000). The load-biased heating-cooling experiments were done by UTEST 50 kN servo-mechanical test instrument. These experiments were performed to determine the transformation temperatures and to observe the shape memory behaviour of the samples under different stress magnitudes. The functional fatigue experiments were conducted with a custom-built functional fatigue test setup to determine the functional fatigue life of all the samples. Load-biased heating-cooling experiments showed that the actuation strain decreases with the increase of the percentage of cold rolling due to the texture formation or the decrease in the transforming volume. The cold rolling induces high amount of dislocations to the matrix such that these dislocations increase the strength of the alloy and inhibit the martensite-austenite boundary movement. Additionally, the critical shear stress for slip increases and thus the irrecoverable strain decreases. The results of the functional fatigue experiments proved that the cold rolling improves the cyclic stability in terms of actuation strain, dimensional stability and transformation temperatures. However, the fatigue life of the alloy was diminished due to the increase in the crack formation with the increase in cold rolling percentage. NiTiHf alloys are high strength alloys such that it is very difficult to deform them. Cold rolling process may form more cracks and these cracks can easily propagate during the fatigue cycles. Therefore, the functional fatigue life decreases with the increase in cold rolling percentage.