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Study of Martensitic Transformations in Shape Memory Alloys by Real-time Measurement of Surface Work Function Change


EMSL Project ID
21613

Abstract

Shape memory alloys (SMA) belong to the advanced intelligent materials, the properties of which can be programmed to carry out a required action. The alloys exhibit reversible shape changes (i.e. shape memory effect, SME) upon changes in external stress and/or temperature. This unique property is associated with martensitic transformation (MT) in solid state. Engineering applications of SMA's are steadily growing and development of new SMA's is being driven by market demands (esp. Microelectromechanical System, MEMS). The aim of the proposed project is to develop new experimental techniques of nondestructive identification of structural transitions taking place in the bulk of the transforming SMA and their modeling. The in-situ surface work function measurement technique to be developed shall provide 2D ("thin filmed surface") information concerning the onset and fractions of transforming austenite/martensite phases developing in the transforming shape memory alloy prepared in single- as well as polycrystalline form. These data are generally of high academic interests and important for the development of SMA and comparable to other experimental method.
According to Wigner and Bardeen's double-layer theory, the change in lattice structure influences interlayer spacing, the charge and distribution of the electron density and therefore the surface work function. It has been reported that the difference in work function upon surface orientation is in the order of 0.1 eV to 0.5 eV. Recent study shows that the total electronic density of state at the Fermi energy of NiTi decreases as the successive austenite ? R phase ? martensite transformation proceeds, which indicates the change in work function associated with MT.
Two types of SMAs will be examined with UPS system at PNNL to work out the surface work function change with temperature: Cu-based and NiTi-based alloys. Both alloys have phase transformation temperature at around 50 ?C.

Project Details

Project type
Exploratory Research
Start Date
2006-09-13
End Date
2007-09-13
Status
Closed

Team

Principal Investigator

J. Dickinson
Institution
Washington State University

Team Members

Mingdong Cai
Institution
Washington State University

Kenneth Beck
Institution
Environmental Molecular Sciences Laboratory

Wayne Hess
Institution
Pacific Northwest National Laboratory

Alan Joly
Institution
Pacific Northwest National Laboratory

Related Publications

Mingdong Cai, Stephen C. Langford, Mingjie Wu, Weimin Huang, Gang Xiong, Timothy C. Droubay, Alan G. Joly, Kenneth M. Beck, Wayne P. Hess, and J. Thomas Dickinson. 2007 "Study of Martensitic Phase Transformation in a NiTiCu Thin-Film Shape-Memory Alloy Using Photoelectron Emission Microscopy", Adv. Funct. Mater. 2007, 17, 161–167