Crack Propagation During Actuation Cycles of The Nitihf High Temperature Shape Memory Alloys
Özet
Shape Memory Alloys (SMAs) are very special materials with their distinctive ability to recover their shape change by phase transformation via mechanical and/or thermal stimulations. When the alloy heated above a certain temperature, shape change recovery under load/stress could be accomplished. By using this shape change mechanism, SMAs could produce work against loads and they could be used as actuators due to this property in several industries.
NiTi alloys are the most widely used SMAs in commercial applications. However, they have limited TTs below 100°C which inhibits their usage as actuators at elevated temperatures. To achieve higher TTs, NiTiHf alloys are the most promising among other SMAs.
There is an increasing demand for using these alloys in actuator type of applications at high operating temperatures. In this demand, NiTiHf High Temperature Shape Memory Alloys (HTSMAs) could be good candidates since they have been known as not only HTSMAs but also SMAs having high strength. Although they have relatively better mechanical and shape memory properties at high temperatures they still suffer from thermal and thermomechanical cyclic instability problem due to the decrease of their strength with the increase of operating temperatures. Thus, knowing their fatigue and crack growth properties could be useful by considering their reliability and robustness in their usage. However, there are limited number of studies in the literature on fatigue, crack initiation and growth behavior that leads to fracture by running actuation thermal cycles on these alloys under constant load. Therefore, it is required to know and understand the actuation fatigue properties together with the crack growth behavior at elevated temperatures. Additionally, creep based viscoplasticity could have an effect on crack formation and propagation, since these materials work at elevated temperatures as mentioned above. Thus, knowing the crack growth behavior under creep formation could be also very beneficial for utilizing these alloys in especially aerospace industry.
Within the scope of the mentioned context, actuation fatigue experiments under load were conducted for different conditions then crack growth behavior and actuation fatigue properties were investigated.
Firstly, dog-bone shaped hot-extruded Ni50Ti30Hf20 (at.%) HTSMA tensile test sample without a pre-notch was thermally cycled under constant load magnitude. Actuation fatigue behavior, surface crack formations and crack growth behavior were investigated at intermittent cycles. The thermal cycles were stopped after running certain amount of cycles, the sample was detached from the test set up and the crack formations and the changes in the crack lengths were determined by using Optical Microscope (OM).
Secondly, hot extruded and pre-notched (HE-PN) Ni50Ti30Hf20 (at.%) test samples were used to investigate the crack growth and actuation fatigue properties during heating-cooling cycles under load. However, different type of crack growth behavior and shape memory properties were observed from each sample.
Thirdly, to achieve more stable crack growth and actuation fatigue properties, hot-extruded, pre-notched and subsequently annealed (HE-PN-AN) samples were tested via running actuation fatigue tests on Ni50Ti30Hf20 (at.%) alloys. Slower crack growth was determined in HE-PN-AN sample than that of in HE-PN sample. In addition, similar crack growth rates and more stable actuation fatigue performance were observed in annealed samples. On the other hand, there were variations in the crack growth rate and the fatigue properties of HE-PN samples. Lastly, minor cracks were formed and propagated together in annealed samples due to relieving the induced stress during hot-extrusion and relatively easier new surface generation with the same amount of energy.
Moreover, actuation fatigue tests were conducted on 3 different samples to see the effect of Upper Cycle Temperature (UCT), applied stress magnitude and Hf content on crack formation, crack growth and actuation fatigue properties. It is important to note that UCT is the limit temperature to which the samples are heated when conducting actuation fatigue tests. HE-PN-AN coded Ni50Ti30Hf20 (at.%) samples were tested using different test parameters. While testing one of the HE-PN-AN Ni50Ti30Hf20 (at.%) samples UCT value was set to 600°C and the stress was kept constant at 200 MPa stress value. Another HE-PN-AN sample having the same aforementioned composition was tested under
300 MPa stress magnitude by setting the UCT to 440°C. Actuation properties were in similar trends with the previous tests conducted on the other annealed samples with the application of 200 MPa, but increase in the stress magnitude led a shorter actuation fatigue life through which crack propagation could not be followed due to the sudden fracture. To see the effect of Hf content to the actuation fatigue properties of NiTiHf HTSMA, Ni50Ti25Hf25 (at.%) was tested under 200 MPa stress and by setting the UCT to 600°C. UCT value was fixed as 600°C to observe the complete austenitic transformation in cycles since this alloy has higher Hf content and hence higher TTs. Actuation strain values of Ni50Ti30Hf20 (at.%) were found to be higher than that of Ni50Ti25Hf25 (at.%) alloy throughout the actuation cycles which were conducted under 200 MPa and by using 600°C as UCT. Higher UCT resulted higher actuation strains due to the increase in transforming volume but high amount surface cracks due to severe oxidation were formed around pre-notch area. High amount of oxidation-based crack formation and propagation also increased plastic strain accumulation over the actuation cycles. Although higher actuation strain values could be achieved by higher UCT application, which is one of the important parameters in actuator applications achieving higher actuation strain values did not provide longer fatigue lifetimes, reliable and stable shape memory properties.
Lastly, since these SMAs were heated to higher temperatures during actuation fatigue cycles, creep effect on crack formation at 600°C temperature under different stress levels were investigated. Ni50Ti30Hf20 (at.%) HE-PN-AN samples were loaded to 300 MPa,
400 MPa and 500 MPa at 600°C and the creep experiments were run until failure. It was observed that higher applied stress magnitudes and high UCT led to observe very high creep rates.
