Özet
Shape Memory Alloys (SMAs) are special materials due to their shape recovery behaviors. SMAs remember their original shape after being deformed in their low temperature martensite phase and then heated back to their high temperature austenite phase. Thus, SMAs can be utilized as actuators in aerospace industry. NiTi based SMAs are widely used ones due to their high shape recovery and work output ability against applied load. However, their transformation temperatures (TTs) are lower than 100 ˚C and this limits their application area. There is a strong desire to increase TTs of SMAs for making them suitable candidates for high temperature applications. Nevertheless, as their working temperatures increase with the increase in TTs, the cyclic stability of the alloys starts to decrease due to the decrease in the resistance to plastic deformation via dislocation formation. As the martensite-austenite transformation takes place, dislocations, which are formed with the thermal and/or mechanical cycles pin the martensite/austenite boundary. Therefore, SMA is not able to demonstrate full shape recovery due to plastic deformation, which also leads to the presence of retained martensite. There are several ways to raise TTs of NiTi binary alloys and to provide cyclic stability such as adding ternary element and applying heat treatments. The most promising additional element is Hf due to its lower cost and its effect in increasing the TTs to very high levels. Furthermore, it should be noted that NiTiHf ternary alloys are not only known as high temperature shape memory alloys (HTSMAs) but also high strength alloys.
In this study, Ni50.1Ti19.9Hf30 (at%) was used due to its very high TTs and strength. Although NiTiHf alloys have very good properties as mentioned before, they lose these properties at high temperatures. Therefore, thermo-mechanical heat treatments were applied to very Hf-rich Ni50.1Ti19.9Hf30 (at%) HTSMA to enhance its high temperature, functional and shape memory properties (SMPs). The alloy was first homogenized (H) and then warm rolled (WRed) at 3 different temperatures via following 2 different thickness reductions. Functional fatigue experiments (FFE) were conducted on homogenized and on each warm rolled sample. The effect of rolling temperature together with the percentage of thickness reduction on SMPs such as TTs (Austenite start (As) and finish (Af), martensite start (Ms) and finish (Mf) temperatures), actuation (εact) and irrecoverable strains(εirr) was revealed by comparing the WRed samples with the H one. The hot extruded alloy was homogenized at 1050 ˚C for 2 hours and then the slices, which were cut from the extruded billet, were WRed at 500°C with 2% thickness reduction, at 800°C and 900°C with 10% thickness reduction.
TTs of all samples were measured by Differential Scanning Calorimetry (DSC) to determine the effect of warm rolling. Then FFEs were conducted using dog bone shape tensile specimens. The samples were loaded to 200MPa constant stress level and thermally cycled between 250°C and 700°C. All results, which were gathered from DSC and FFE, were compared.
One of the most promising findings in this study was the effect of warm rolling on the stability of the functional properties of Ni50.1Ti19.9Hf30 (at%) alloy. Actuation strain (εact) values were found to be quite low but very stable throughout the FFE cycles. Moreover, TTs did not decrease significantly with warm rolling process.
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