Effects of frequency, composition, hydrogen and twin boundary density on the internal friction of Ti₅₀Ni_50-xCu_x shape memory alloys

The internal friction (IF) of Ti₅₀Ni_50-xCu_x polycrystals, which were normally solution-treated, was systematically studied using dynamic mechanical analysis. As a result, the broad peak appearing in B19 martensite was confirmed to be a relaxation peak with an activation energy of 0.67 and 0.76 eV for x = 20 and 16, respectively. Then the broad peak was shown to disappear in a single-crystal experiment, if twin boundaries in martensite are largely eliminated. This is direct evidence to show twin boundaries are indispensable for the broad peak. The effect of hydrogen on IF was also examined using a vacuum system with a mass spectrometer, and the broad peak at 250 K was observed to disappear, while a new small relaxation peak was found at 178 K, when hydrogen was eliminated. These experiments clearly show that there are two essential ingredients to observe the high broad peak: twin boundaries and hydrogen. Based on these results a possible origin for the broad peak is discussed.