R. Campanella

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.

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.

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.

Reorientation of the Ce–D pairs in a Pd95Ce5Dn alloy

Abstract Internal friction measurements have been carried out in a Pd 95 Ce 5 alloy containing different amounts of D (0.003 n D =D/(Pd+Ce) 1 , P 2 and P 3 have been observed at around 160, 115 and 103 K, respectively. The highest temperature peak P 1 is due to D-dislocation interactions (Snoek-Koester type of relaxation), the peak P 2 to stress-induced reorientation of Ce–D pairs and the peak P 3 is due to stress-induced changes in the short-range order of D within the sublattice of the octahedral interstitial sites (Zener type of relaxation). The relaxation parameters of peak P 2 are: τ 0 −1 =7×10 12 s −1 and W =0.22 eV, while an estimate of the anisotropy factor for the Ce–D pairs gives a value of 0.038.

Nucleation of martensite and thermal hysteresis of the young's modulus in a Ni50.8Ti49.2 shape memory alloy

Among the most interesting aspects of martensitic transitions (MT) are pre-transitional and nucleation-growth processes to which much attention has been paid during the last few decades. The main objective of the present investigation, which is part of a more extensive research program on the NiTi system, was to see whether the Youngs modulus of the nearly equi-atomic Ni{sub 50.8}Ti{sub 49.2} alloy displayed thermal hysteresis above the austenite/martensite start-temperature ascribable to the formation of stable embryos of martensite. In the present note results concerning the solubilized material are presented.

Stress-induced reorientation of oxygen-hydrogen pairs in a Ta75Nb25 alloy

Abstract Anelastic relaxation techniques have been applied to investigate at kHz frequencies the mobility of H and D trapped by O impurities in a Ta 75 Nb 25 alloy. An internal friction peak has been detected at liquid nitrogen temperatures, the height of which, for a given O content n O (n O =O/(Ta + Nb)), linearly increases with increasing n H(D) (n H(D) =H(D)/(Ta + Nb)), indicating that more than one H(D) atom can be trapped by a single O impurity. The peak is associated with a wide relaxation time spectrum, which is due to the intrinsic compositional disorder of the alloy and to interactions among the O H(D) complexes. These interactions give rise to a temperature dependence of Curie-Weiss type for the relaxation strength, as well as to a decrease of the peak temperature with an increase of the H(D) content.