In-Situ Synchrotron Characterization of Transformation Sequences in TiNi-Based Shape Memory Alloys during Thermal Cyclin
- PDF / 599,566 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 59 Downloads / 249 Views
N
CURRENTLY, TiNi-based shape memory alloys (SMAs) are of interest to the aerospace industry for potential use in biased spring replacements for mechanical actuators. Existing components require bulky hydraulic systems to provide movement, whereas a component manufactured from an SMA would be selfactuating. The movement is based on a fully reversible martensitic transformation, which can be induced by stress, temperature, or combinations of the two. If this phase transformation can be tailored to suit the application, then the need for hydraulic actuators would be eliminated, reducing weight and leading to lower emissions. However, instability of the shape memory behavior during thermal and mechanical cycling has been widely reported, and therefore, before SMA actuators can become feasible in aerospace applications, their response to cyclic regimes of temperature, stress, and the two combined must be fully understood. In binary TiNi alloys, the high-temperature cubic austenitic phase (B2) transforms to a monoclinic martensite (B19¢) with an orientation relationship of {110}B2k(002)B19¢.[1] The transformation exhibits a thermal hysteresis, which can be defined by four characteristic temperatures: As, Af, Ms, and Mf, representing the start and finish temperatures of the austenitic and martensitic transformations, respectively. Typical N.G. JONES and S.L. RAGHUNATHAN, Research Associates, and D. DYE, Senior Lecturer, are with the Department of Materials, Imperial College London, London, SW7 2AZ, U.K. Contact e-mail: [email protected] Manuscript submitted April 29, 2009. Article published online February 6, 2010 912—VOLUME 41A, APRIL 2010
thermal hysteresis of binary TiNi alloys is ~20 C.[2] The addition of Cu, at the expense of Ni, to TiNi alloys is of current interest to the aerospace industry as the characteristic transformation temperatures are influenced, narrowing the observed thermal hysteresis and stabilizing Ms with respect to compositional variations.[3–5] Additions of 75 pct volume fraction) is B19¢. Upon heating, the volume fraction of B19 begins to increase while B19¢ decreases, suggesting that the B19¢ is transforming to B19. The B19 + B19¢ fi B19 transformation reaches completion (B19f) at ~48 C, and the microstructure remains 100 pct B19 until reaching B2s at ~63 C. The material then undergoes a B19 fi B2 transformation, which finishes at ~68 C (B2f), being entirely austenitic. Cooling from this region reverses the process through the exact same sequence B2 fi B19 fi B19¢ + B19, with the following critical temperatures: B19s ~ 56 C, B19f ~ 50 C, and B19¢s ~ 38 C. Again, the material transforms entirely to B19 from B2 prior to the formation of any B19¢, thus suggesting that for an alloy of this composition, the B2 fi B19¢ reaction is not possible. The data presented offer a greater level of detail of this transformation sequence than has been reported before. These data are also in good agreement with previous observations that the intermediate B19 phase forms over a small thermal range[38] and
Data Loading...
