Papers by Keyword: Anelastic Relaxation

Abstract: The β-Ti alloys exhibit excellent shape memory effect and superelastic properties. The interstitial atoms in the alloys have important effect on their physical and mechanical properties. For the interstitial atoms, the internal friction technique can be used to detect their distributions and status in the alloys. The anelastic relaxation in β-Ti alloys is discussed in this paper. β-Ti alloys possesses bcc (body center body) structure. The oxygen (O) atoms in in the alloys is difficult to be removed. The O atoms located at the octahedral sites in the alloys will produce relaxation under cycle stress. In addition, the interaction between the interstitial atoms and substitute atoms, e.g., Nb-O,Ti-O can also produce relaxation. Therefore, the observed relaxational internal friction peak during the measuring of internal friction is widened. The widened multiple relaxation peak can be revolved into Debye^,s elemental peaks in Ti-based alloys. The relaxation peak is associated with oxygen movements in lattices under the application of cycle stress and the interactions of oxygen-substitute atoms in metastable β phase (β_M) phase for the water-cooled specimens and in the stable β (β_S) phase for the as-sintered specimens. The damping peak height is not only associated with the interstitial oxygen, but also the stability and number of β_M in the alloys.

Abstract: Anelastic properties of Bulk Metallic Glasses (BMG) were studied by mechanical spectroscopy using a flexural vibration apparatus. BMG’s samples, with nominal composition Cu₄₈Zr₄₃Al₉ and Cu₅₄Zr₄₀Al₆, were produced by skull push-pull casting technique in rectangular cavity cooper mold. In both samples, the differential scanning calorimeter patterns have evidenced the presence of amorphous structure, although the X-ray diffraction for Cu₄₈Zr₄₃Al₉ composition has shown a heterogeneous microstructure embedded in the amorphous matrix. Anelastic relaxation spectra were obtained using an acoustic elastometer system with vibration frequency in the kilohertz bandwidth, a heating rate of 1 K/min, vacuum greater than 10^-5 mBar in the temperature range of 300 K to 620 K. In the flexural apparatus, an acoustic elastometer system, the internal friction (energy loss) and the elastic modulus were obtained by free decay of vibrations and by the squared of the oscilation frequency, respectively. Internal friction spectra were not reproducible among the measurements, which may imply atomic rearrangement in the samples due to consecutive heating. Normalized elastic modulus data showed distinct behavior from the first to the other measurements evidencing irreversible microstructural alterations in the samples possibly associated with mechanical relaxation due to the motion of atoms or clusters in the glassy state.

Abstract: To verify the assumption that the anelastic relaxation effect observed in Ni₃Al is due to stress-induced reorientation of antisite Al atoms [Numakura and Nishi, Mater. Sci. Eng. A 442 (2006) 59-62], the magnitudes of the anisotropic distortion produced by the intrinsic point defects have been evaluated by ab initio calculations. The anisotropy of the λ tensor (the strain per unit concentration of a particular defect) for the two candidate defect species, namely a Ni vacancy and an antisite Al atom, has been computed by full structure optimization of a supercell containing a single point defect: the difference in the principal values is +0.46 and −1.12, respectively. The relaxation strength estimated for antisite Al atoms agrees fairly well with experiment, while that for Ni vacancies is far too small because of their much lower concentration. The relaxation is, therefore, conclusively attributed to antisite Al atoms.

Abstract: Hydrogen mobility has been studied at high temperature by absorption experiments in the Ni₅₂Ti₄₈ alloy, which does not transform martensitically but rather behaves like a so-called strain glass. The results obtained have been compared with those deduced from an anelastic relaxation occurring in this alloy below the strain-glass transition temperatures. An accurate analysis of the anelastic data has confirmed the conclusion that the relaxation is related to H rather than to the glass transition. Its relaxation time obeyed a Voogel-Fulcher type of temperature dependence. Combining absorption and anelastic results, the H diffusion coefficient in the B2 lattice structure of this alloy could be studied from 1200 K down to 170 K. The agreement between the absorption and mechanical spectroscopy data was satisfactory. The activation energy (0.33 eV) deduced from a Vogel-Fulcher representation of the H diffusion coefficient D was sensibly lower than earlier determinations (0.44-0.50 eV) from Arrhenius plots. The high temperature data of Ni₅₂Ti₄₈ alloy, compared with the ones available in the literature for other NiTi SMA in their B2 structure, show a substantial independence of D on the alloy composition.

Abstract: Measurements of anelastic relaxation (internal friction and frequency) as a function of temperature were carried out in samples of Ti-13Nb-13Zr using two experimental apparatus: Flexural Vibration of the first tone of samples in Acoustic Elastometer System (Vibran Technology®) operating in a kilohertz bandwidth, and Torsional Vibration of the samples in Kê-type Torsion Pendulum operating in a hertz bandwidth. Experimental spectra of anelastic relaxation were determined in the temperature range from 300 K to 450 K for a heating rate of 1K/min under pressure of 10-5 Torr, in both apparatus. The results show a relaxation structure strongly dependent on the microstructure of the material. The dynamical elastic modulus (E) of Ti-13Nb-13Zr alloy can be determined by flexural vibrations by frequency (f) measurements (f  E1/2). The anelastic relaxation spectrum of Ti-13Nb-13Zr alloy was a function of temperature obtained by torsional vibrations, not revealing the presence of interstitial solutes in solid solution in the temperature range of measurements.

Abstract: The Snoek relaxation is associated with the redistribution of interstitial atoms in the bcc lattice under the application of the oscillatory stress. Addition of substitutional solutes introduces new peaks or broadening of the normal Snoek peak. The experimental and theoretical investigations of the effect are briefly reviewed.