Studies on Damping Capacities and Properties Evolution of Early-Aged TiNi-based Shape Memory Alloys
Date Issued
2015
Date
2015
Author(s)
Chien, Chen
Abstract
Damping properties of Ti50Ni50−xCux (x = 0~30 at.%) and Ti50Ni50−xFex (x = 1~4 at.%) shape memory alloys (SMAs) are characterized under temperature, strain and frequency sweep tests by dynamic mechanical analyzer. Experimental results indicate that, for Ti50Ni50−xCux SMAs, the magnitudes of tan δ value and storage modulus (E0) softening/hardening exhibited in B2↔B19 transformation are all higher than those in B2↔B19’ one. It is suggested that the greater mobility of the twin boundaries and the larger magnitude of the strain variation are both to cause the higher tan δ value. In addition, the relaxation peaks are observed in B19’ martensite, but not in B19 martensite, because the latter has intrinsically rare twinned variants. In the strain sweep test, the tan δ value decreases as the frequency increases, but it increases as the applied strain increases. The tan δ curves of the strain sweep tests in Ti50Ni50-xCux SMAs (x=0~20 at.%) can be fitted by tan δ = K εn with n values of 0.41-0.60 which are close to the friction type model (n=0.5). For Ti50Ni50−xFex SMAs, the internal friction (IF) contribution of hydrogen atoms at R→B19’ transformation is more than that at B2→R one because the former has abundant twin boundaries in both R-phase and B19’ martensite, which can interact with the hydrogen atoms to dissipate energy. The tan δ values of (IFPT+IFI)R→B19’ are higher than those of (IFPT+IFI)B2→R peak. This feature comes from the facts that the twin boundaries of R→B19’ transformation is more than those of the B2→R one. By the SAXS technique, the nanodomain evolution corresponding to strain glass in quenched and 250oC early-aged Ti48.7Ni51.3 SMA can be revealed. The 2D SAXS patterns provide the direct evidence that the shape of strain glass nanodomains is plate-/disk-like, and these nanodomains lie on the habit lattice planes. The nanodomains are nucleated heterogeneously in different grains, and the model analysis of 1D SAXS profiles quantitatively determines the strain glass nanodomains’ radii and thicknesses which the thickness of 3hr aged specimen are larger than that of as-quenched one. The results of thermal physical properties indicate that the ρ decreases but the �` and S increase as the temperature increases which demonstrate the characteristics of the strain glass exhibited in 250oC early aged Ti48.7Ni51.3 SMA.
Subjects
TiNi shape memory alloys
damping properties
martensitic transformation
strain/frequency sweep
small X-ray scattering
strain glass
Type
thesis