Characterization of a new precipitate phase in the high temperature shape memory alloys NiTiHf and NiTiHfPd
EMSL Project ID
44898
Abstract
The development of high temperature shape memory alloys (HTSMAs) could open up many applications for these materials as energy efficient actuators. Recent work indicates that ternary alloys based on the NiTi system, and specifically NiTiHf alloys, are the most promising of the HTSMAs. During aging, nanoscale precipitates form and lead to improved shape memory response, in terms of increased transformation temperature and strain, and reduced permanent deformation. It is the structure and composition of these nano-scale precipitates, and their coherency with respect to the matrix, that are key issues we propose to determine in this proposed study. We have already performed extensive HAADF analysis of the precipitate structures for two alloys: Ni45.3Pd5Ti29.7Hf20 (B2 matrix at room temperature) and Ni50.3Ti29.7Hf20 (B19 martensite at room temperature), after aging at 500 degrees C and 600 degrees C. The HAADF imaging clearly indicates that these nanoscale precipitates are not consistent with any of the known precipitate phases in the TiNi binary are ternary alloys analyzed previously in the literature. A tentative structure model has been proposed based on this HAADF data. While the present model can explain the strong intensities observed in the HAADF images and the basic symmetry of the diffraction information, the composition of the present structural model does not agree with our coarse-scale EDS measurement. Therefore, significant refinement of the structure model is required. To this end, both experimental and computational efforts are proposed, consistent with EMSL's distinctive facilities and expertise. Electron energy loss spectroscopy (EELS) with the aberration-corrected and monochromated Titan 80-300 will provide essential composition information at an atomic level. Analysis using Atom Probe Tomography (APT) will enable accurate 3D chemical imaging, which would provide critical morphology and composition information of both the precipitate phase and its surrounding matrix. All the composition information gathered from these experiments will be incorporated into the proposed structural model. In order to clarify uncertainties in terms of the exact atom locations of certain elements; VASP calculations will be employed to allow local relaxations of the atoms and determination of minimum enthalpy structure. These calculations will also enable determination of the unconstrained lattice parameter -- a key for understanding the coherency stresses and strains generated by these nanoscale particles, as well as their influence on the martensitic transformation. This research is supported by a grant from DOE Office of Basic Energy Sciences (#DE-SC0001258 with a project period of 07/01/2009 - 06/30/2012), and from Los Alamos National Laboratory (Subcontract No. 78107-001-09). The results sought in this proposal would be key to achieving a successful renewal of our DOE/OBES grant. The results of the proposed study, when combined with the HAADF structure information already obtained, will provide for seminal journal articles on the fundamental role of these nanoscale precipitates on HTSMA properties.
Project Details
Project type
Exploratory Research
Start Date
2011-07-25
End Date
2012-07-29
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members