Texas A & M University (TAMU) is planning to establish a site for the NSF I/UCRC program on shape memory alloys (SMA) and actuation technologies as a new thrust area of the recently established I/UCRC Smart Vehicles Concepts Center (SVC) at Ohio State University. The proposed research site will focus on development, processing, characterization, design and analysis of SMAs and SMA actuators with high actuation work outputs and operating temperatures spanning from subzero temperatures to 500C. In particular, the proposed research will establish the relationship amongst processing, microstructure and properties of SMAs. Emphasis will also be placed on studying the complicated thermo-mechanical response of SMAs and SMA actuator designs and analysis.
The new SMA materials, actuators, design and analysis tools developed at TAMU will benefit a wide range of industries and especially the design of new vehicles and mechanisms. The proposed site will contribute to and support the smart technology activities that are currently under development at OSU. TAMU will help disseminate the SMA knowledge to the interested US industries and help maintain or improve their global competitiveness. The researchers at the proposed site intend to recruit women and minority undergraduate and graduate students to participate in the SVC-TAMU research work.
Intellectual Merit This research focused on two topics: (1) Modeling and experimental characterization of the fatigue of Ni-rich shape memory alloys (SMAs): Our fatigue studies demonstrated the extent to which the fatigue strength and life depends on the environment, surface finish, precipitate orientation, and precipitate size. Specimens that are cycled at different stresses have different response curves on the strain vs cycles to failure plot. However, when these tests are normalized by the amount of transformation work, the different lines collapse to a common line. Furthermore, we have shown that the phase transformation work per unit cycle can be used to collapse all fatigue curves on to a single plot. We consider this to be a significant contribution, one that should be useful to structural designers. In addition, we have developed improved, unique equipment for fatigue testing of SMAs in which vortex tubes are used to cool specimens that were heated by electrical resistance heating. (2) Thermomechanical processing and characterization of the high-temperature actuation capabilities of NiTiHf and NiTiPd and NiTiPd-X SMAs. We investigated the effect of novel processing tychniques including Equal Channel Angular Extrusion (ECAE) on the behavior of conventional SMAs and high temperature (HTSMAs) for improving dimensional stability. ECAE refined the grain size and helped control morphology. Relative to conventionally processed HTSMAs, ECAE-processed alloys had less irrecoverable strain (which is good in SMAs) and less thermal hysteresis. In addition, the thermal hysteresis was more stable, emaning that it was more consistent. The transformation strain (recoverable strain) increased without an increase in the irrecoverable strain, which is desirable. Ni50.7Ti49.3 (at%) and Ni50.3Ti34.7Hfl5 (at%) materials show good thermal and dimensional stability. A maximum of 5% fully recoverable strain in Ni50.3Ti34.7Hfl5 (at%) aged at 450oC for 10h under 300MPa were obtained. When precipitates are coherent with the matrix and smaller in size, shape recovery is better. However, the matrix loses its strength when the particles coarsen and samples begin to develop irrecoverable strains. Smaller precipitate size results in an initial decrease in Ms due to smaller inter-particle spacing. There is a good correlation between the effect of precipitate size, volume fraction and Ms temperatures and shape memory response. Compositional changes are more dominate during high temperature aging and longer aging times. TiC impurities have a detrimental effect on fatigue life. Ni50.1Ti24.9Hf25 (at%) systems show high transformation temperatures and near perfect thermal stability. It is possible to reach Ms temperatures as high as 240oC with proper aging. We studied the effect of microstructure on extending the thermo mechanical fatigue life of Ni50.7Ti49.3 to obtain an extensive database of fatigue life and mechanical properties. The temperature was cycled under constant stress such that forward and reverse transformation occurs to get actuation. Five different heat treatments for Ni50.7Ti49.3 were investigated. The solutionized case gives baseline fatigue life around 1000 cycles. Although nano precipitates increase the deminsional stability, they reduce the fatigue life significantly to 100 to 300 cycles. With over-aging fatigue life increases back to 1000 cycles. The heat treatment also changes the actuation strain. When the work density, the product of actuation strain and applied stress, is plotted with the fatigue life, it is found that there is a trend line which is obeyed by Ni55Ti45, Ni50.7Ti49.3 and Ni50Ti50. In addition to thermo mechanical fatigue, evaluation of Charpy impact energies with microstructure and temperature is investigated. It was found that the Charpy impact energy has a minima in temperature around Af for all heat treatments. Broader Impacts Fatigue is a limiting factor in the use of shape memory alloys in aircraft. Our improved understanding of fatigue will help our sponsor, The Boeing Company, certify the materials for flight. In addition, our improved experimental fatigue equipment should benefit other SMA researchers. High temperature SMAs are promising candidates for actuators int he hot sections of engines, which are too hot for conventional actuators such as solenoids, electric motors, magnetic and hydraulic actuators. The ability to use actuators in the hot section of engines can improve engine efficiency.
