Effect of Hot Extrusion, Annealing and Warm Rolling on the Functional Fatigue Behaviors of Nitihf High Temperature Shape Memory Alloys

Abstract

High temperature shape memory alloys (HTSMAs) have a wide range of application in aerospace, automotive and petroleum industries as high temperature actuators. NiTiHf HTSMAs are the most potential alloys compared to the other HTSMAs due to being cost friendly, having very high transformation temperatures and their ability to show high work output. Since exposure time of the alloy to high temperatures is dictated by heating-cooling rate of thermo-mechanical cycling, clarifying the effect of this rate to the functional fatigue life and the stability of the shape memory properties with the number of thermal cycles is crucial to offer a solution for enhancing these properties. The time spent at high temperatures may lead to observe higher plastic deformation since the strength of the metal alloys decreases and dislocation formation during phase transformation becomes easier. The cyclic instability of shape memory behaviors such as the shift of transformation temperatures, the increase or decrease of thermal hysteresis, actuation and accumulated irrecoverable strains stemming from lattice incompatibility between low temperature and high temperature phases and the phase transformation induced dislocations is also a substantial problem, which needs to tackle with. Besides the precipitation and deformation strengthening treatments that are conducted to increase the strength of these alloys, medium temperature rolling operation is another possible thermo-mechanical processing method in equiatomic NiTiHf alloys since they are known as hard-to-deform materials and precipitation formation is not possible in these materials. In the light of the information presented above, the aims of the present study can be summarized as to characterize the actuation fatigue properties till failure and the effect of heating-cooling rate during thermal cycling to the actuation/functional fatigue life and the shape memory properties with the number of cycles. Moreover, enhancing the cyclic stability of the alloy by increasing the critical shear stress for slip and decreasing the plastic deformation during phase transformation under stress for equiatomic Ni50Ti30Hf20 (at.%) HTSMAs are among the desired aims of this study as well. Actuation fatigue behaviors of the hot extruded material, the influences of annealing heat treatment and warm rolling at different temperatures on the thermal cycling stability and functional fatigue properties of the equiatomic Ni50Ti30Hf20 (at.%) HTSMAs were revealed in the present study. First of all, thermal cycling under constant stress experiments were performed on the randomly selected samples from the hot extruded billet with different heating and cooling rates such as 5, 10, 15, 20 and 25°C/s. Not only the different rates but also the same rates displayed variation in the shape memory properties and fatigue lives. For instance, the specimens tested with 15°C/s rate exhibited 580, 754 and 1738 thermal fatigue cycles under 200 MPa constant stress level via heating the samples to an identical upper cycle temperature. Temperature distribution through the length of the test samples during heating-cooling cycles were analyzed and almost similar temperature gradient was observed for the different rates. To investigate the reason of the discrepancy in the shape memory properties, six fatigue test specimens were cut along the cross section of another virgin piece of the hot extruded billet and enumerated for futher fatigue experiments. Additionally, edges through the gage sections of the enumerated samples were ground and in order to mitigate the surface roughness and micro surface cracks for test standardization. The 15°C/s heating-cooling rate was utilized on all enumerated and edge ground samples during functional fatigue experiments as an average rate. Even though the actuation fatigue life were increased to almost 8500 thermal cycles for hot extruded samples after edge grinding, the discrepancy in the shape memory behaviors were still oberved. This different behavior was attributed to microstructural nonuniformity and deformation variation induced by hot extrusion process. Annealing heat treatment at 550°C-3h was utilized following hot extrusion on the samples obtained from different regions of bulk material in order to overcome deformation variation problem. The similar transformation temperatures, thermal hysteresis, actuation and irrecoverable strains magnitudes were observed for both of the annealed samples due to relieving the internal stress and removing the microstructural nonuniformity via annealing treatment. More importantly, the stable transformation temperatures were measured and actuation fatigue life time was reached to about 12000 cycles. To increase thermal cycling stability, warm rolling at 600°C and 700°C with 5% thickness reduction were conducted on hot extruded-annealed alloy. The stability of all shape memory properties with the increase in the number of cycles was almost attained with the warm rolling at 600°C process, however, less fatigue lives were obtained for both warm rolled samples compared to annealed ones due to possible microcrack formations, which were induced during rolling operations.