College Park, Maryland      June 6 - 10 , 2004

TP12: Neutron Diffraction Investigation of Shape-Memory Alloys during Mechanical Loading at Cryogenic Temperatures

S. Shmalo, T. Woodruff, C.R. Rathod (AMPAC/MMAE, University of Central Florida), V. Livescu, M.A.M. Bourke (Los Alamos National Laboratory), W. Notardonato (NASA Kennedy Space Center), R. Vaidyanathan (AMPAC/MMAE, University of Central Florida)

The shape-memory effect refers to a phenomenon wherein a material on being heated, "remembers" and returns to a preset shape due to a phase transformation. In the process of returning to the preset shape, the phase transformation can work against large forces, resulting in their use as actuators. While shape-memory alloys have been commercially used at cryogenic temperatures in single-use applications (e.g., couplings), their use at cryogenic temperatures in cyclic, switch-type applications has been limited by a lack of understanding of the underlying deformation mechanisms (i.e., phase transformation and twinning) at cryogenic temperatures. Such an understanding could extend the range of use of shape-memory alloys to, among others, spaceport related technologies such as latch and/or release mechanisms, thermal switches for cryogenic liquefaction and storage systems, fluid-line repair, valves, seals and self-healing gaskets.

At Los Alamos National Laboratory, in situ diffraction measurements at stress and elevated temperature have been routinely used to provide quantitative, phase specific information on the evolution of strains, texture and phase volume fractions. However, a capability for simultaneous loading and neutron diffraction spectra acquisition at cryogenic temperatures was unavailable. In order to investigate deformation in shape-memory alloys at cryogenic temperatures, the design of a low temperature loading capability for in situ neutron diffraction measurements has been analyzed and implemented on the Spectrometer for Materials Research at Temperature and Stress (SMARTS). An aluminum vacuum chamber with liquid nitrogen circulating to cool platens in contact with compression specimens was used. Preliminary results are reported here of quantitative, in situ measurements of evolving internal strains, texture, phase and twin volume fractions in the rhombohedral and monoclinic phases of shape-memory NiTiFe at cryogenic temperatures. This work is supported by grants from NASA and NSF (CAREER DMR-0239512).

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