On the functional degradation of binary titanium–tantalum high-temperature shape memory alloys — A new concept for fatigue life extension

Abstract

High-temperature shape memory alloys are promising candidates for actuator applications at elevated temperatures. Ternary nickeltitanium-based alloys either contain noble metals which are very expensive, or suffer from poor workability. Titanium-tantalum shape memory alloys represent a promising alternative if one can avoid the cyclic degradation due to the formation of the omega phase. The current study investigates the functional fatigue behavior of Ti-Ta and introduces a new concept providing for pronounced fatigue life extension.Keywords: Omega phase; martensite; phase transformation; microstructure; shape memory effect.Conventional shape memory alloys (SMAs) have been intensely investigated over the last decades due to their extraordinary properties. 1-6 Different metallic alloys can show a shape memory effect, e.g. Cu-based 5 and Fe-based systems, 6 but Ni-Ti is still the most widely used conventional SMA 3 due to good functional and structural properties. Unfortunately, the transformation temperatures of Ni-Ti are limited to below 100 ○ C, thus hindering applications in elevated and high-temperature regimes. 7 As shape memory actuators allow for designing devices characterized by high compactness and efficiency, a large effort has been spent to develop high-temperature shape memory alloys (HTSMAs). For actuation in a medium temperature range of about 100-400 ○ C, mainly Ni-Ti based ternary (Ni-Ti-X) systems have been proposed. [8][9][10] These alloys show promising shape memory characteristics, i.e. actuation strain and microstructural stability, but at the same time suffer from two main drawbacks, i.e. poor workability and/or high amounts of noble metals. 8,9,11 Titanium-tantalum SMAs represent a promising alternative to overcome all these issues if one can avoid the cyclic degradation due to the formation of the !-phase. The current study investigates the functional fatigue behavior of Ti-Ta and introduces a new concept providing pronounced fatigue life extension. The cyclic deformation behavior of a polycrystalline titanium-tantalum (Ti-Ta) alloy containing 30 at.% Ta under iso-stress loading during thermal cycling employing temperatures ranging from 50 ○ C to 600 ○ C was investigated. In order to reveal the most detrimental factor leading to a change of transformation temperatures and a loss of transformation strain, fatigue tests were conducted with differing heating and cooling rates and aging treatments. Using X-ray diffraction (XRD) it could be clearly shown that stabilization of the high-temperature parent phase (β-phase) and the concomitant evolution of the !-phase led to functional degradation. The results of the current study suggest that both phases are mainly induced through aging during high temperature exposure and not by martensitic transformation events during cycling. A new concept employing single-step short-time annealing for regeneration of the initial transformation behavior is introduced.As shown and intensively discussed in a series of studies published by Buenconsejio et al.,[12]...

Cyclic degradation of titanium–tantalum high-temperature shape memory alloys — the role of dislocation activity and chemical decomposition

“…[8][9][10][11] Ti-27Ta-5Al was shown to be stable for up to 20 thermal cycles, where the maximum test temperature was about 250 ○ C. 10 Experiments conducted on Ti-30Ta and Ti-30Ta-3Al in related studies revealed that changes in test procedure like employing a higher number of cycles at higher temperatures or long-term creep testing can lead to severe functional degradation. 12,13,19 In case of cyclic thermo-mechanical loading, the rate of degradation was strongly reduced by Al additions, but qualitatively a similar degradation behavior was found. 13 In addition, it was shown that transformation strain capability can be restored by short-time annealing treatments.…”

“…2 depicts the evolution of A s , A f and M s as well as the evolution of transformation strain as a function of cycle number. In previous work a strong decrease in transformation temperatures occurred after each cycle, 12,13 which was triggered by the nucleation and growth of !-phase particles. In contrast, the transformation temperatures for the current Ti-30Ta-3Al sample increase up to the 100th cycle.…”

“…Based on the transformation temperatures of the SMA under consideration, i.e., start and finish temperatures for austenite and martensite (A s , A f , M s , M f ), actuation can be triggered at different temperatures. 6,7 For Ti-Ta-X (e.g., X ¼ Al) high-temperature (HT) SMAs, M s is adjusted to allow for actuation response under superimposed stresses in a temperature window between 150 ○ C and 300 ○ C. [8][9][10][11][12][13] Actuation strains for Ti-30 Ta have been shown to be highly dependent on the crystal orientation, providing exploitable transformation strains of up to 3.6% for the h011i orientation. 13 Ternary elements were shown to affect the absolute value of strains, but they do not alter the orientation dependence.…”

“…13 In addition, it was shown that transformation strain capability can be restored by short-time annealing treatments. 12,13 However, the microstructural evolution in Ti-Ta-X alloys associated with thermo-mechanical cycling and thermal recovery treatments was not addressed. The objective of the present study was to investigate the elementary microstructural mechanisms that are responsible for the degradation of Ti-Ta-X HT SMAs subjected to different loading conditions.…”

“…12,13 In order to evaluate the effect of a short annealing period after each cycle, a Ti-30Ta-3Al sample was subjected to 240 thermal cycles. Thermal cycling was conducted in between 30 ○ C and 400 ○ C with a heating/cooling rate of 0.66 ○ C s À1 .…”