Session: 01-03 Shape Memory Alloys 2

Paper Number: 171955

171955 - Work Output in Runb-Based Ultra-High Temperature Shape Memory Alloys 

Ultra high temperature shape memory alloys (UHTSMAs) have been sought for a multitude of applications, including actuators for jet engines, on landers for Venus, and for hypersonics.  Several UHTSMA compositions with transformation temperatures ranging from 500 ºC to over 1000 ºC have been discovered, such as NiHf, HfPd, TiPt, or multi-component alloys based on NiPt-TiHfZr alloys.  The work output of Ni-based and Ti-based alloys can be quite high in the lower transformation temperature alloys, due to transformation strains reaching 10%.  In the HTSMA formulations, however, as the alloying elements increase the transformation temperatures, the work output decreases, often reaching low or zero value.  This reduction in work output may stem from several factors, including changes in the lattice parameter due to alloying, the high relative temperature (T/Tmelt) during the transformation, creep processes, and/or the transformation temperature exceeding the alloy's Md. Additionally, the transformation may occur at a temperature where slip is more favorable than twinning or martensitic recovery processes.

One candidate UHTSMA is the RuNb/RuTa system.  This alloy family has been studied for several decades, and thermal results indicate great high temperature behavior, with transformation temperatures up to 1100 ºC, strong transformation peaks, and narrow hysteresis, but the work output and other mechanical behavior of these alloys has not been studied.   Additionally, phase transitions in RuNb and RuTa are completely different from those of NiTi-like SMAs. Ru-based UHTSMAs go through two crystallographic transitions: the first at temperatures generally between 600ºC and 800ºC from monoclinic (𝑃 21/𝑚) to tetragonal (𝑃 4∕𝑚𝑚𝑚) phase and the second generally at temperatures between 800ºC and 1000ºC from tetragonal to cubic (𝑃𝑚̄3𝑚). While the second transition is at a higher temperature, the first transition comes with a higher transformation strain capability and a greater tendency for dimensionally stable behavior.  Therefore, one target is to separate the two-phase transitions such that the first can be used for actuation without triggering the second transformation during heating.  

In this study, RuNbTa alloys were designed to establish a clear boundary between the two transformation regions, aiming to maximize the benefits of high work output during the transition from tetragonal to monoclinic phases, while suppressing higher phase transitions. Several compositions were vacuum arc melted and suction cast into 5 mm diameter rods. Sample screening was performed in compression following a modified ASTM E3097 standard test method for mechanical uniaxial constant force thermal cycling of shape memory alloys. It was found that the addition of Ta could be used to modify both the transformation temperature and the separation range of the two phase transitions. Work output on the order of 6 J/cm3 can be achieved with relatively good dimensional stability if the higher temperature phase transition is avoided. On the other hand, transitioning from tetragonal to cubic phase results in a response dominantly characterized by creep.

Presenting Author: Glen Bigelow NASA Glenn Research Center

Presenting Author Biography: Glen Bigelow is a materials research engineer at the NASA Glenn Research Center in Cleveland, OH. He received his BS in Mechanical Engineering and his BS and MS in Metallurgical and Materials Engineering from Colorado School of Mines. His research focus is on shape memory alloy material design, processing, and applications in aeronautics and space.