José San Juan

Nanoscale shape-memory alloys for ultrahigh mechanical damping

Shape memory alloys undergo reversible transformations between two distinct phases in response to changes in temperature or applied stress. The creation and motion of the internal interfaces between these phases during such transformations dissipates energy, making these alloys effective mechanical damping materials. Although it has been shown that reversible phase transformations can occur in nanoscale volumes, it is not known whether these transformations have a sample size dependence. Here, we demonstrate that the two phases responsible for shape memory in Cu-Al-Ni alloys are more stable in nanoscale pillars than they are in the bulk. As a result, the pillars show a damping figure of merit that is substantially higher than any previously reported value for a bulk material, making them attractive for damping applications in nanoscale and microscale devices.

High-Temperature Mechanical Spectrometer for Internal Friction Measurements

A new high temperature mechanical spectrometer, based on an inverted torsion pendulum, has been constructed for the measurement of the internal friction and the dynamic shear elastic modulus in two different working modes: (a) as a function of temperature (300 – 1800 K) at imposed frequency, during heating or cooling; and (b) as a function of frequency (10-3 – 10 Hz) in isothermal conditions. The whole installation is computer controlled by a dedicated software specifically developed. We describe the different parts of this new installation, as well as its performances in both temperature and frequency through an original example study on a high temperature structural intermetallic of Fe-Al.

High Temperature Internal Friction in Fine Grain and Nano-Crystalline Zirconia

Engineering ceramics are being developed to improve their high-temperature mechanical properties and in particular creep resistance. Recently the production of fine grain ceramics undergoes another step-forward with the development of new technologies to produce nanocrystalline materials. The question is whether the properties depending on the grain size can be extrapolated at nanoscale or, on the contrary, new microscopic mechanisms could appear to be dominant at this nanometer grain size. In the present work we study, by mechanical spectroscopy, the high temperature behavior up to 1350°C of a fine grain Zirconia and a nanocrystalline Zirconia sintered in a conventional way. A new forced torsion pendulum, recently built, has been used for the mechanical spectroscopy measurements. The high temperature background (HTB) of internal friction has been measured as a function of temperature for different frequencies in both materials. The analysis of the HTB shows that the fine grain Zirconia exhibits a single process of defects mobility, with an apparent activation enthalpy similar to the one measured by creep. On the contrary, the HTB of the nanocrystalline sample becomes more complex, showing a much higher energy loss, which will be discussed at the light of the internal friction spectra analysis.

Crystal structure determination of a ternary Cu(In,Sn)2 intermetallic phase by electron diffraction

Small grains of an intermetallic phase with an approximate composition Cu(In,Sn)2 were observed in a metal matrix composite obtained from powders of a Cu–Al–Ni shape-memory alloy and an In–Sn matrix alloy. Samples of this composite were prepared for transmission electron microscopy and the crystal structure of the intermetallic phase was carefully investigated by applying electron diffraction techniques (microdiffraction, convergent-beam electron diffraction and precession), based on the analysis of the symmetry and the relative positions of reflections in the zero- and high-order Laue zones. It was found that the intermetallic phase has a body-centred tetragonal unit cell with lattice parameters a = 0.70 (3) nm and c = 0.56 (2) nm. Its crystal symmetry can be described by the I4/mcm (No. 140) space group.