G. 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.

Enhanced Nitinol Properties for Biomedical Applications

In recent years, Nitinol producers and medical products have experienced an exponential growth, driven by advanced manufacturing techniques and the use of progressively less invasive medical procedures. Concurrently, new processing techniques have been developed to further enhance the valuable properties of Nitinol used in medical devices; recent patents on these techniques include changing the composition of nickel and titanium, alloying the nickel-titanium with other elements, improving melting practices, heat-treating the alloy, and mechanical processing of the alloy. For example, alloying the nickel-titanium with ternary elements may widen the superelastic temperature operating window, maximize/minimize the stress-strain hysteresis, and improve the radiopacity of a Nitinol intraluminal device comparable to that of a stainless steel device of the same strut pattern coated with a thin layer of gold. Limiting to less than 30% the final cold work step (after a full anneal, and before the shape-setting step) may improve the Nitinol fatigue lifetime of about 37%, the fatigue lifetime being a primary factor limiting the performances of Nitinol endoluminal prosthetic implants. Local selective and differential thermo-mechanical treatments have also been devised to achieve different physical properties in different portions of a Nitinol medical device in order to improve its performance under expected operating conditions.

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.

Effect of H-Doping on Damping Capacity of Various NiTi-Based Alloys at kHz Frequencies

The internal friction Q -1 and the Youngs modulus E of NiTi based alloys have been measured as a function of temperature after various thermomechanical and hydrogen-doping treatments given to the materials. Hydrogen is found to play a major role introducing tall damping peaks associated with Snoek-type and H-twin boundary relaxations. Levels of Q -1 as high as 0.08 have been detected, which are among the highest to date measured in metal alloy systems. For appropriate alloy compositions, these peaks occur at around room temperature (for acoustical frequencies), thus providing a good opportunity to reduce machinery vibrations and noise pollution. In the paper, the conditions are highlighted under which maximum efficiency can be reached in the conversion of mechanical energy into heat.

Low-temperature Snoek-type relaxation of hydrogen interstitial atoms in Nb0.8Mo0.2

Abstract Neutron spectroscopy on H-doped b.c.c. Nb0.8Mo0.2 alloys demonstrated recently (i) a complete suppression of low-temperature H precipitation up to 5 at.% H, and (ii) an exclusive H occupation of tetrahedral interstitial sites, like pure b.c.c. Nb. We studied by mechanical spectroscopy Nb0.8Mo0.2 alloys containing 0.85 and 3 at.% H and found a H-induced low-temperature relaxation at 2 kHz around 80 K. Activation energy and reciprocal pre-exponential relaxation time are ∼0.054 eV and ∼2.8·107 s−1, respectively. The relaxation peak is about 80% broader than for an ideal Debye relaxation, which reveals a spectrum of relaxation times. The height of the relaxation (Q−1) peaks is approximately linear to H concentration, thus indicating a Snoek-type relaxation of single H atoms. The heights of the peaks yield a tetragonality |λ1–λ2| of about 0.04, which is an order of magnitude smaller than that of the Snoek effect of O in Nb. The results suggest that the observed relaxation reflects low-temperature diffusive jumps of single H atoms between tetrahedral interstitial sites, with a jump rate above ∼1.2·104 s−1 at 80 K.

Elastic constant softening and martensite nucleation in a CuZnAl single crystal

Abstract The ultrasonic attenuation A and the elastic constants C 44 , C L and C ′ have been measured during thermal cycles carried out above the martensite start-temperature M s ( M s =267 K) in a single crystal of the Cu 67.93 Zn 19.03 Al 13.04 alloy. The ultrasonic attenuation displays a sharp maximum at a temperature which is about 6 degrees higher than M s . This temperature difference can be accounted for either in terms of the Clausius–Clapeyron law for the martensite induced by the applied ultrasonic stress-field or in terms of premartensitic effects. Thermal hysteresis occurs for all the elastic constants C 44 , C L and C ′ and the data are consistent with those previously obtained for the Young’s modulus in a polycrystalline alloy of the same system. The thermal hysteresis is attributed to the formation of stable nuclei of martensite at strong defects.