Internal friction spectra of the Ni_(40)Ti_(50)Cu_(10) shape memory alloy charged with hydrogen

摘要

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 Ni_(40)Ti_(50)Cu_(10) alloy containing various amounts n_H of H (n_H = H/Me = 0; 0.004; 0.008; 0.013 and 0.018 at.). The Young's modulus exhibits softening when the start temperature M_s of the B2→B19 martensitic transition is approached on cooling and a much steeper modulus decrease between M_s and M_f. 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, P_H and P_(TWH), occur below M_s 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.