Observed dependencies of the large thermal-compressive response of a NiTi shape-memory alloy fiber aluminum metal matrix
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THIRUMALESH BANNURU, Resemch Assistant, is with the Depart ment of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015. WILLIAM D. ARMSTRONG, Assistant Professor, is with the Department of Mechanical Engineering, University of Wyoming, Laramie, WY 82071. Contact e mail: [email protected] Manuscript submitted January 24, 2003. METALLURGICALAND MATERIALSTRANSACTIONSA
(approximately 2.2 pct) longitudinal contraction during the external stress-free heating process, a response clearly indicaring large plastic compressive flow in the composite aluminum metal matrix driven by the shape-memory response of the composite NiTi fiber actuators. The development of large plastic flows in this study was clue to an increase in NiTi fiber volume fraction and the selection of a softer matrix heat-treahnent condition relative to the earlier works. The present article extends this earlier work by examining how the characteristics of the large thermal-compressive response of a 20 vol pct NiTi fiber 6082-T0 composite change with variations in the value of maximum tensile strain imposed during the preceding room-temperature tensile process. Six 50.7 at. pct NiTi fiber 6082 aluminum metal matrix composite and two homogeneous 6082 aluminum specimens were produced from a single hot pressmg treatment. The 50.7 at. pct NiTi material was received as hot-drawn, 190-p,mdiameter wire. The wire was longitudinally laced onto 0.3-mmthick, end-notched, 45-mm-wide by 75-mm-long 6082-T6 sheets, and then chemically cleaned with a 16 pct HNO3 + 1 pct HF solution. The fiber-laced sheets were loaded into a welded aluminum vacuum canister made from the composite matrix aluminum alloy. The loaded and sealed hot-press canister was then continuously evacuated to better than 0.05 mbar and hot-press consolidated under a 25 metric ton maximum load at approximately 520 °C for 5 minutes. At the end of the hot pressing treatment, the hot pressing die containing the test material was removed from the press and rapidly water quenched. Composite test specimens were machined with longitudinally oriented fibers by diamond sawing specimen blanks from the pressing billet, which were then conventionally milled to specification. A 20 pct NiTi volume fraction was obtained from mass-density measurements. Homogeneous aluminum alloy control specimens were machined from the walls of the hot-press billet; this ensured that homogeneous control specimens experienced as near as possible the same time-temperature conditions as the composite specimens. Each sample was subjected to a 427 °C for 2 hours heat process 48 hours before testing. This ensured that the aluminum matrix was in a fully annealed condition. A 1.24-mm-thick central cross-section specimen was diamond sawed from the center of an untested annealed composite sample. This resulted in the contact of numerous vertically oriented NiTi fibers with the bottom of the DSC pan. The aspect ratio of the NiTi fibers was approximately 8 to 10. A large aspect ratio was selected so that the internal stress state in the
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