Key Engineering Materials Vol. 319

Abstract: The internal friction (IF) and the Young’s modulus (E) of NiTi based alloys have been investigated at 1 Hz and 1 kHz frequencies after various sequences of thermo-mechanical treatments and hydrogen-doping given to the materials. Differential scanning calorimetry (DSC) has also been used as a complementary investigation tool. Apart from the transient effects, only occurring at 1 Hz frequencies, the data indicate a substantial insensitivity of damping to frequency. The results show that the H-Snoek and the H-twin boundary relaxations get their maximum height for H contents nH (nH=H/Me) equal to about 0.025 and 0.008, respectively. At kHz frequencies the IF peaks associated with these relaxations occur at around room temperature in the Ni49Ti51 and Ni30Ti50Cu20 alloys. Thus, these appear to be the most promising materials for applications aimed at the reduction of vibrations.

Abstract: Damping capacity and storage modulus of cold-rolled and annealed Ti50Ni50 SMA are systematically investigated by DSC and DMA tests. The specimens annealed at 500 oC for 1 ~ 72 h and annealed at 650 oC for short time can all reach a high tan δ value and a deep storage modulus minimum during B2→R transformation. This reveals that the occurrence of R-phase can strongly soften the storage modulus and thus promote the damping capacity of Ti50Ni50 SMA. The specimens anneaed at 650 oC for longer time have a lower tan δ value due to the diminishing of R-phase formation. Therefore, a good damping capacity associated with a significant softening storage modulus can be acquired by suitably control the annealing conditions of cold-rolled Ti50Ni50 to generate adequate of R-phase premartensite.

Abstract: The shape memory effect and the high damping in shape memory alloys are based on the martensitic phase transformation, which takes place essentially without diffusion and any change of order have an influence on its side effects: the memory effect, the superelasticity and the high damping capacity of the martensitic phase. A new method to control the performance of shape memory alloys is presented, which is based on selective modification of specified parts of working components. In this research, ion irradiation has been used to introduce locally disorder into a crystal or even amorphise it. A pre-deformed Ni-Ti, 6μm thin film in its martensitic state has been irradiated with Ni-ions of energy of 5 MeV up to a dose of 1016 ions/cm2. By this treatment, a 2μm thin surface layer has been finally transformed into an amorphous state, in which the martensitic transformation is suppressed. During heating the underlying non-modified layer is contracting and an out-of-plane movement is observed. The amorphous layer is elastically deformed and its energy is used during cooling to bring the film in its original shape. In this way, a reversible movement of the film is created. This new technique not only allows us to design new types of micro-actuators, but also to influence locally the high damping, which can be of great importance for micro-engineering applications.

Abstract: Low frequency internal friction of Ti49Ni51 binary and Ti50Ni40Cu10 ternary shape memory alloys has been measured. The effect of solution and aging heat treatments on the damping property was examined. The temperature spectrum of internal friction for TiNi binary alloy consists, in general, of two peaks; one is a transition peak which is associated with the parent-martensite transformation and is rather unstable in a sense that it strongly depends on the frequency and decreases considerably when held at a constant temperature. The other one is a very high peak of the order of 10-2, which appears at around 200K. It appears both on cooling and on heating with no temperature hysteresis, and is very stable. The behavior of the peak is strongly influenced by the heat treatments. The trial of two-stage aging with a purpose of improving the damping capacity has been proved unsatisfactory. TiNiCu has a very high damping, the highest internal friction reaching 0.2, but by quenching from very high temperature, say 1373K, the damping is remarkably lowered. For the realization of high damping the quenching from a certain temperature range around 1173K seems the most preferable condition.

Abstract: Effects of hydrogen doping on the internal friction (IF) of ternary Ti50Ni50-xCux (x=15, 20, 25)
shape memory alloys, prepared by rolling and annealing laminating Ti and Ni-Cu alloy sheets, have been measured with a damping mechanical analyzer in a forced bending oscillation mode at temperatures from 173K to 423K at three frequencies, 0.1, 1 and 5Hz. The effects of hydrogen doping on IF are common to the three alloys: a hydrogen IF peak appears at around 260K; the IF peak value (tanφ) increases rapidly with increasing hydrogen concentration up to tanφ=0.03 at 0.5at% and then decreases; the peak temperature also increases rapidly and then gradually decreases. From the frequency dependence of the peak temperature, the activation energy E and the pre-exponential factor ω0 have been analyzed to be E=0.6-0.7eV and ω0=1013-14s-1. The origin of the hydrogen IF is interpreted to be the Snoek-Koester effect due to interaction of twin boundary dislocations and segregated hydrogen atoms. Effects of hydrogen on mechanical properties of the alloys have also been studied.

Abstract: The strain amplitude dependent internal friction at room temperature and the transition temperatures of CuAlMn-shape memory alloys with Al contents from 8.9 wt.% to 12.7 wt.% and Mn contents from 4.7 wt.% to 9.3 wt.% were investigated. The investigated strain range was 10-6 - 10-3. Rods of various compositions were die cast and machined to single clamped damping bars. Their transition temperatures and amplitude dependent damping was determined in as cast and homogenized state. The damping in the investigated shape memory alloys was found to be generally much higher than in metals without martensitic transition. In as cast state some alloys exceeded the damping of a Sonoston type alloy measured in comparison for strains higher than 3 x 10-5. The influence of grain size on damping was investigated by additional sand casting and the use of Boron for grain refinement. It was found that only the material with the biggest grains had a noticeable higher damping over the whole measured strain range. Homogenization heat treatment can still extremely increase the damping of CuAlMn alloys. After homogenization this extremely high damping decreases slowly to medium values in the order of as cast alloys.

Abstract: The damping behavior of an Fe-28Mn-6Si-5Cr-0.5NbC (mass%) shape memory alloy was measured by low cycle fatigue tests during tension-compression loadings. A remarkable damping capacity was observed above the strain amplitude of 0.1%, and the specific damping capacity (SDC) parameter reached saturation at ~ 80% above 0.4%. The reversible motion of the γ/ε interfaces is considered to dominate the cyclic deformation behavior, while the work hardening during tension-compression loading is negligible. These characteristics are favorable for seismic damping devices that protect civil structures from earthquakes.