Investıgatıng the Effect of Low Heatıng Coolıng Rate and the Thermomechanıcal Treatments on The Shape Memory Behavıor of Nıtıhf Alloys

Abstract

Shape memory alloys (SMAs) have an extraordinary ability to recover their shape via martensitic transformation. That ability provides SMAs to be used as actuators in many applications as a result of doing work against load. Superelasticity and shape memory are two key phenomena observed in SMAs as a result of the martensitic transformation. Superleasticity takes place when SMA is in parent phase. Under the applied load, parent phase transforms to martensite phase and elongates via martensite reorientation and detwinning and martensite transforms back to austenite and the alloy remembers its original shape via unloading. On the other hand, shape memory effect is triggered via heating the alloy above Austenite Finish (Af) and cooling to Martensite Finish (Mf) temperatures.

Nickel Titanium-based shape memory alloys are the most popular SMAs due to their excellent shape memory characteristics however their transformation temperatures are limited to 100-120 ˚C if the alloy is Ni-Lean. As Nickel content increases above 50 at. %, transformation temperatures decrease. However, many applications require higher transformation temperatures. Addition of Hf element effectively strengthens the alloy and increases transformation temperatures of NiTi-Based SMAs. However, as service temperature increases, cyclic stability of SMAs decreases due to the softening at elevated temperatures thus it is crucial to have knowledge about shape memory behavior at these temperatures.

In the first part of this thesis, Ni50Ti30Hf20 at. % shape memory alloy was used to investigate the heating/cooling rate effect on the shape memory properties. Material was purchased from Sophisticated Alloys Inc. and produced with high purity Ni, Ti and Hf elements via vacuum induction melting. Melting process was conducted under high purity argon atmosphere. Material was sealed within mild steel can and extrusion process with an area reduction of 4:1 was conducted. DSC experiments were conducted via Perkin Elmer Differential Scanning Calorimetry 800 on extruded specimens to reveal heating/cooling rate effect on equiatomic Ni50Ti30Hf20 at. % shape memory alloy. Isobaric heating/cooling experiments were run to investigate shape memory effect via following different heating/cooling rates which were the same rates used in DSC experiments. Thermal stability was investigated via DSC experiments while mechanical stability was investigated via isobaric experiments.

Stress-free DSC experiments showed that transformation temperatures were not affected by different heating/cooling rates while transformation enthalpy increased as scanning rate was increased. Isobaric experiments, which were done under 200 MPa demonstrated no variety in terms of shape memory properties such as the transforming volume and transformation temperatures with different heating/cooling rates.

In the second part of this thesis, cyclic stability and shape memory characteristics were investigated on equiatomic Ni50Ti25Hf25 at. % SMA. Material was produced with the same procedure as defined above and purchased from Sophisticated Alloys Inc. Addition to the extrusion process, material was subjected to homogenizing heat treatment at 1050 °C for 2 hours. High Hf content of NiTiHf alloy has been admitted as a great candidate for very high temperature (up to 600 °C) actuation applications. DSC studies were conducted to characterize transformation behavior of the alloy under no load. After determining transformation characteristics, functional fatigue experiments were conducted under pre-determined load on homogenized sample to investigate shape memory behavior of Ni50Ti25Hf25 at. % shape memory alloy. It is known that plastic deformation methods are effective way to improve shape memory characteristics of SMAs. Cold rolling was applied to homogenized sample with thickness reduction of 5% then cold rolled sample was subjected to annealing heat treatment at 500 °C for 30 minutes to further improve SME behavior. Warm rolling study was also conducted at 500 °C with same thickness reduction as it was achieved with cold rolling operation.

Recoverable strain, transformation temperatures, hysteresis, irrecoverable strain values were gathered from the experiments. It was observed that cyclic stability of Ni50Ti25Hf25 at. % HTSMA was increased via both cold and warm rolling processes although the operating temperatures were very high.