F.M. Mazzolai

Internal friction spectra of the Ni40Ti50Cu10 shape memory alloy charged with hydrogen

Abstract The temperature dependence of the dynamic Young’s modulus E, the elastic energy dissipation coefficient Q−1 and the heat flow (DSC) has been studied between 90 and 370 K in an Ni40Ti50Cu10 alloy containing various amounts nH of H (nH=H/Me=0; 0.004; 0.008; 0.013 and 0.018 at.). The Young’s modulus exhibits softening when the start temperature Ms of the B2→B19 martensitic transition is approached on cooling and a much steeper modulus decrease between Ms and Mf. This steep decrease appears to be associated with stress-induced motions of twin boundaries within the B19 martensite as it is drastically reduced by H pinning of these boundaries. No internal friction (IF) peak occurs at the B2→B19 transition and the values of Q−1 are high in the B19 martensite (≅100×10−4). Two IF peaks, PH and PTWH, occur below Ms in the H-doped material; the first is likely due to stress-assisted reordering of H elastic dipoles within a hydride phase, the second to H dragging processes by twin boundaries.

Extraordinary high damping of hydrogen-doped NiTi and NiTiCu shape memory alloys

Abstract The internal friction, Q −1 , and the Young’s modulus, E , have been investigated as a function of temperature at kHz frequencies in the Ni 30 Ti 50 Cu 20 alloy containing various amounts n H ( n H =H/Me at.) of H. Several dissipation processes have been observed which are associated with stress-induced motions either of isolated H atoms, or of twin boundaries interacting with H. Values of Q −1 as high as 0.075 have been measured in the presence of H impurities over extended temperature regions at around the B2–B19 and B2–B19′ martensitic transitions. The observed damping is not a transient effect as those usually reported at low frequencies in H-free materials, thus, it does not depend on the rate of temperature change. No appreciable dependence of the damping on the frequency and strain-amplitude are observed between 0.48 and 1.5 kHz and between 1×10 −7 and 3×10 −5 , respectively. A brief review of previous results obtained with other alloy compositions and relevant comparisons is also included.

Hydrogen diffusion and interpretation of the 200K anelastic relaxation in NiTi alloys

The internal friction, the Young’s modulus, and the heat flow have been measured as a function of temperature (20–360K) at kilohertz frequencies in a H-free and H-doped Ni50.8Ti49.2 alloy, solubilized under vacuum and rapidly furnace cooled. The chemical H-diffusion coefficient DC has been deduced from absorption experiments (323–1063K) and has been compared with the Einstein diffusion coefficient DE derived from a Snoek (or Zener)-type relaxation PH. The comparison has allowed the interpretation of the so-called 200K relaxation as a Snoek (or Zener)-type relaxation due to hydrogen.

Mechanical spectroscopy of the H-free and H-doped Ni30Ti50Cu20 shape memory alloy

Abstract The temperature dependence of the dynamic Young’s modulus E , the elastic energy dissipation coefficient Q −1 and the heat flow (DSC) has been studied between 20 and 370 K in a H-free and H-doped ( n H =H/Me=0.006 and 0.01 at.) Ni 30 Ti 50 Cu 20 alloy. The Young’s modulus exhibits softening when the start temperature M s of the B2→B19 martensitic transition is approached on cooling and a steep modulus decrease between M s and M f . This steep decrease is associated with stress-induced motions of twin boundaries within the B19 martensite. Hydrogen reduces background damping of the martensite and dramatically enhances the dissipation in the temperature region of the transformation. These observations suggest that hydrogen (a) forms fixed pinning points for twin boundaries at low temperature and (b) gives rise to an anelastic relaxation P H associated with H dipoles and to a peak P AM due to H-twin boundary interactions.

Damping Properties of Vacuum Annealed and H-Doped NiTi Based Alloys at Low Stress Amplitudes

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.

A neutron-spectroscopy study of the local vibrations, the interstitial sites and the solubility limit of hydrogen in niobium-molybdenum alloys

Abstract We studied by neutron spectroscopy, and for temperatures from 10 to 300 K, the local vibrations of hydrogen interstitials in two niobium-based alloys containing 5 and 20 at.% molybdenum (Nb 0.95 Mo 0.05 H 0.03 and Nb 0.8 Mo 0.2 H 0.05 ). Within experimental accuracy, the vibrational energies agree with those of hydrogen in pure niobium. This indicates that the hydrogen in the two alloys occupies tetrahedral interstitial sites. At 10 K, the values of the vibrational energies show a complete hydrogen precipitation in the Nb 0.95 Mo 0.05 H 0.03 alloy and no hydrogen precipitation in the Nb 0.8 Mo 0.2 H 0.05 alloy. The fact that the hydrogen in the latter alloy was still in solid solution means that the molybdenum alloy components lead to an increase of the solubility limit of the hydrogen.

Phase transition features of NiTi orthodontic wires subjected to constant bending strains

Abstract A resistometric investigation of premartensitic and martensitic phase transitions in NiTi orthodontic wires has been carried out as a function of temperature between −20 and +60°C, during cooling and heating. Transformation features of two types of commercial wires were examined in either their undeformed state or under application of a constant bending strain. For the undeformed case the transformation behavior was also characterized by differential scanning calorimetry (DSC). The start temperature ( M s ) for the formation of martensite was appreciably different for the two types of material, while that ( R s ) for the formation of the R-phase was about the same. For one material (type 1) the enthalpies Δ H ∗ for the A → R and R → M transitions were deduced from DSC data and used for calculating the Clausius-Clapeyron relationships R s ( σ ) and M s ( σ ). For the other material (type 2), due to the exceedingly low value of M s , the M s ( σ ) relationship could not be determined. Above room temperature electrical resistivity data taken under bending did not give clear indications of stress-induced martensite for material of type 2, which, however, displayed good superelastic behavior. This lack of clarity arises from complex alterations taking place in the structure (lamellar) and proportions of the different phases (A, R and M) present under inhomogeneous bending strains when the temperature is changed. To fully clarify this point stress-strain tensile experiments were carried out, preliminary results of which are also presented in this paper.

A study of diffusion of deuterium in α'-Pd deuteride by Gorsky relaxation

Abstract Long-range diffusion of deuterium in a α′-Pd deuteride at deuterium contents, n D ( n D = D/Pd at.), ranging from 0.65 to 0.72 has been investigated by Gorksy relaxation between 90 and 380 K. The relaxation strength was found to depend on T according to a Curie-Weiss type of law with a negative value of the spinodal temperature, indicating repulsive deuterium-deuterium interactions. The macroscopic diffusion coefficient shows non-exponential temperature dependence below about 140 K. It seems likely that this behaviour arises from transitions of deuterium by tunnelling between a series of excited vibrational states localized at octahedral sites and a single state localized at adjacent tetrahedral sites.

Computer‐controlled apparatus for internal friction and Young’s modulus measurements as a function of temperature

A mechanical resonance apparatus has been developed which allows automatic recording of internal friction Q−1and dynamic Young’s modulus E as a function of temperature. The attainable accuracies ΔQ−1/Q−1 and ΔE/E are approximately equal to 3.5×10−2 and 1×10−5, respectively. The vibrating sample is kept at resonance, during heating and cooling, by continuously readjusting the frequency of the driving signal supplied by a computer‐controlled frequency synthesizer. The frequency regulation is obtained by keeping either the amplitude of the response signal at its maximum level or the phase difference between the excitation and vibration signals at a fixed, selected value. In comparison with previous automatic control systems designed for similar applications, the one described here is more versatile, being able to measure internal friction from the width of the resonance curve as well as from the logarithmic decrement of freely decaying vibrations, and being able to regulate the vibration frequency by either ...

Diffusion of H in a TaNb Alloy

Hydrogen mobility has been investigated between 125 and 415 in a Ta75Nb25 alloy with the use of the Gorsky relaxation technique. The macroscopic diffusion coefficient, D(T) shows an exponential temperature dependence, contrary to what happens in Ta and Nb. This different behaviour of D(T) in the alloy is attributed to a reduced tunnelling rate between ground states, resulting from inequalities among the potential wells at interstitial sites.