Stefano Amadori

Anelasticity and Structural Stability of ECAP Processed Al-Mg-Si Alloys Investigated by Mechanical Spectroscopy

Mechanical spectroscopy was employed to investigate the microstructure evolution of a Zr-modified 6082 Al-Mg-Si alloy and the same alloy with Sc addition after ageing and following severe plastic deformation through equal channel angular pressing. Measurements of the internal friction and dynamic young modulus have been performed in isothermal and isochronal runs in the frequency range 0.1 - 104 Hz. The anelasticity spectra reveal in the temperature range 470-870 K both structural and anelastic relaxation processes. Two structural damping maxima connected with inverse temperature trend of the modulus occur in the alloys submitted to equal channel angular pressing, the first one is strongly suppresed by Sc and Zr addition. An anelastic relaxation peak whose strength depends on the nature and morphology of precipitates and dispersoids and on the deformation and ageing condition was observed in all samples investigated. The high background damping occurring before the first structural damping maximum is analyzed with reference to a superplastic behavior of the equal channel angular pressing processed alloys.

Atomic ordering in CuZnAl shape memory alloys investigated via x-ray absorption and diffraction

We investigate the structure of the austenite phase in CuZnAl shape memory alloys by a combined x-ray absorption and diffraction analysis. Ab initio simulations of the near Zn-edge x-ray absorption coefficient allow us to directly discard the hypothesis of a DO3 superstructure. At the same time, we give evidence of the existence of an ordered structure (B2-like) different from the L21 one recently proposed by neutron diffraction. However, some partial L21 ordering is present at room temperature. This superstructure develops and recovers order when increasing the temperature above 400K.

Anelastic phenomena associated to water loss and collagen degradation in human dentin

This work describes the anelastic and dynamic Young modulus behaviour of human dentin from room temperature up to 673 K. Human molars, extracted from individuals (males 55-70 years old) as part of their dental treatment, were cut to obtain bar-shaped samples subsequently used for mechanical spectroscopy experiments. In addition, thermo-gravimetric analysis (TGA) has been performed to assess a possible weight loss occurring in the same temperature range of mechanical spectroscopy tests. A broad and asymmetric internal friction (Q(-1)) maximum at 500 K has been observed during the heating of the as-prepared samples. This maximum is absent during the following cooling down to room temperature. It is therefore due to the occurrence of an irreversible transformation in the sample. TGA shows a remarkable weight loss in the same temperature range. This effect has been related to loss of fluids and degradation of collagen. Another set of samples, previously kept for 36 h under a vacuum of 10(-2)Pa, were submitted at room temperature to test at increasing strain from 6×10(-6) to 7×10(-4). The results show transient and fully recoverable Q(-1) increase and dynamic modulus (E) decrease. The phenomenon has been ascribed to the breaking of weak H-bonds between polypeptide chains forming the triple-helix with consequent increase of the mean length of vibrating chain segments.

Anelastic Phenomena in Human Dentin below Room Temperature

The work describes the anelastic behaviour of human dentin below room temperature. Human molars, extracted from individuals as part of their dental treatment, were cut to obtain bar-shaped samples for mechanical spectroscopy (MS) experiments. Repeated cooling-heating cycles in the range 300-100 K have been carried out on the same samples. In the cooling stage of the first cycle Q-1 exhibits a very broad maximum due to a series of phase transformations involving water present in the pores, in the interstices between fibres, between fibrils and inside collagen triple helix. The formation of ice Ih produces permanent damages to the dentin structure (rupture of fibres and fibrils) leading to a decrease of maximum intensity in the following cycles. In the heating stage of all the cycles two maxima, M1 and M2, have been observed around 155 K and 178 K. M1 is due to the transformation of low-density amorphous (LDA) ice into ice IC while M2 to that of ice IC to ice Ih. Above 200 K, Q-1 progressively increases with lower damping values in the cycles after the first one. Dehydrated samples do not exhibit the aforesaid anelastic phenomena confirming that their origin is connected to water and its transformations.

Production and Characterization of Aluminum Iron Powder Composites with Ferromagnetic Properties

Composites made from compacted powders blends of Al with different Fe contents were produced and characterised with respect to mechanical and induction heating properties. Mechanical spectroscopy and hardness measurements were employed to follow the evolution of Young modulus and internal friction after ageing. It was found that above a critical iron content (>30% of the volume) a percolation network of Fe grains is obtained inside the specimen and the induction heating characteristics become comparable with those of ferritic steel samples.

Non parametric identification of the constitutive model of non-standard viscoelastic materials by means of dynamical measurements

A procedure for the identification of the constitutive model of non-standard materials is presented. The material constitutive equation is modeled with a generalized Kelvin model of n-order expressed in the form of a rational polynomial function in the frequency domain. A least square error formulation that makes use of orthogonal polynomials is employed to identify the model parameters from dynamical measurements made on test specimens. Dynamical mechanical measurements made on Polytetrafluoroethylene samples at variable excitation fixed frequency are used to test the devised fitting procedure. Fit results obtained by means of different model order are compared and discussed.

Mechanical Spectroscopy Examination of Human Dentin

This work describes the anelastic behaviour of human dentin in the temperature range from 100 K to 673 K. Human molars, extracted from individuals (males 55-70 years old) as part of their dental treatment, were cut to obtain bar-shaped samples for mechanical spectroscopy (MS) experiments. The results are presented and discussed in two parts referring to experiments above and below room temperature.