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The effect of H doping in combination with various thermomechanical treatments given to materials, in order to identify conditions for maximum efficiency in the conversion of mechanical energy into heat at around room temperature was studied. It was observed that for appropriate alloy composition, the peaks occur at around room temperature (for acoustical frequencies), providing an opportunity to reduce machinery vibrations and noise pollutions.

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. [Copyright &y& Elsevier]

Abstract: The lattice constants a and c of the as cast hexagonal (C14) Laves phase TiCr1.78−x Mn x (x =0, 0.4, 0.6 and 0.8) have been measured by X-ray diffraction at room temperature as a function of the Mn content. It has been found that a decreases with increasing x while c remains almost unchanged. Being this alloy interesting as material for solid state hydrogen storage, pressure–composition isotherms have been traced on desorption mode for the H2 gas pressure and temperature varying from 10 to 0.01MPa and from 209 to 307K, respectively. From the temperature dependence of the plateau pressure the molar quantities and associated with the hydride decomposition have been determined as a function of the Mn content. With increasing x, both and progressively decrease, and the desorption pressure initially decreases (for x ≤0.4) then slightly increases. At the same time, the pseudo-plateaus become flatter. [Copyright &y& Elsevier]

Abstract: The H chemical diffusion coefficient has been determined in the Ni47Ti40Hf10Cu3 alloy from the kinetics of the gas/solid-surface reaction in the temperature range 350–1000 K. The mobility of H was also investigated by mechanical spectroscopy at temperatures included between 240 and 260 K, where a Snoek internal friction peak occurs for frequencies in the kHz range. The results on the Einstein diffusion coefficient obtained from the relaxation time of the Snoek effect were correlated to the high temperature absorption results. Combined Arrhenius plots of and data allow to determine the H diffusion parameters to:  eV,  cm2/s. [Copyright &y& Elsevier]

The elastic constants C′, C44 and CL of a single crystal of the Ni40Ti50Cu10 (numbers indicate at.%) alloy have been measured in the premartensitic region (320–350 K) and have been found to exhibit softening on approaching the B2→B19 transition from high temperature. This means that these elastic constants decrease with decreasing T. The anisotropy factor A=C44/C′ is relatively small (≅2.2) over all the temperature range and slightly increases with decreasing temperature down to the transition start-temperature Ms. Thus, its temperature coefficient ∂A/∂T has opposite sign of that exhibited by the binary Ni50.8Ti49.2 alloy prior to the B2→B19′ transition. The present results support the view recently expressed by some authors that the B2→B19′ transition occurring in binary Ni–Ti alloys is a consequence of concomitant lattice instabilities against {1 1 0}〈1 1¯ 0〉B2 and {0 0 1}〈 1 1¯ 0〉B2 shear strains respectively corresponding to the C′ and C44 ultrasonic propagation modes. [Copyright &y& Elsevier]

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 (nH=H/Me=0.006 and 0.01 at.) Ni30Ti50Cu20 alloy. The Young’s modulus exhibits softening when the start temperature Ms of the B2→B19 martensitic transition is approached on cooling and a steep modulus decrease between Ms and Mf. 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 PH associated with H dipoles and to a peak PAM due to H-twin boundary interactions. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

Abstract: Low frequency (0.1–50Hz) internal friction (IF) measurements have been made as a function of temperature (100–300K) in the H-free and H-doped Ni50Ti50 and Ni52Ti48 alloys. In the H-free equiatomic alloy no IF peak occurs in the martensitic state, while hydrogen doping gives rise to a well developed Snoek-type relaxation within the martensite (B19′). In the Ni-rich alloy not undergoing martensitic transition, a well evident peak (P TWM) appears at around 150K. H-doping cancels this peak and introduces an H Snoek relaxation in the B2 austenite. A recently reported peak associated with a sort of strain–glass transition occurring in Ni-rich NiTi alloys is shown to coincide with P TWM, while the “200K peak”, earlier reported by many authors with non-intentionally H loaded NiTi alloys is confirmed to be a H-twin boundary interaction effect (P TWH). [Copyright &y& Elsevier]