F.S. Han

Effects of macroscopic graphite particulates on the damping behavior of commercially pure aluminum

Abstract The objective of present work is to investigate the effect of macroscopic graphite (Gr) particulates on the damping behavior of commercially pure aluminum (Al). Macroscopic defects are graphite particulates with sizes of the order of a millimeter (0.5–1.5 mm) and in large proportions, typically 70 vol.%. Macroscopic graphite particulate-reinforced commercially pure aluminum metal matrix composites (MMCs) were prepared by pressure infiltration process. The damping characterization was conducted on a multifunction internal friction apparatus (MFIFA). The internal friction (IF), as well as the relative dynamic modulus, was measured at frequencies of 0.5, 1.0 and 3.0 Hz over the temperature range of 25–400xa0°C. The microstructural analysis was performed using transmission electron microscopy (TEM). The damping capacity of the Al/Gr MMCs, with three different volume fractions of macroscopic graphite reinforcements, was compared with that of unreinforced commercially pure aluminum specimens. The damping capacity of the materials is shown to increase with increasing volume fraction of macroscopic graphite particulates. Finally, the operative damping mechanisms in the macroscopic graphite particulate-reinforced MMCs were discussed in light of internal friction measurements and microstructural studies.

Low-frequency damping behavior of foamed commercially pure aluminum

Abstract Experiments have been carried out to investigate the damping behavior of foamed commercially pure aluminum (FA). The FA specimens were prepared using pressure infiltration process. The damping characterization was conducted on a multifunction internal friction apparatus (MFIFA). The internal friction (IF), as well as the relative dynamic modulus, was measured at frequencies of 0.5, 1.0 and 3.0 Hz over the temperature range of 20–400xa0°C. The size of macroscopic pore is on the order of a millimeter (1.0 mm) and in large proportions, typically up to 69 vol.%. The measured IF shows that FA has a damping capacity that is enhanced in comparison with bulk commercially pure aluminum. Especially, an IF peak was found and corresponding to a rapid drop of relative dynamic modulus (vs. temperature) in the FA specimen. The average values for the activation energy of the IF peak is approximately 1.39±0.03 eV. The microstructural analysis was performed using transmission electron microscopy (TEM). TEM observations showed that dislocation substructures exist near the grain boundaries. Accordingly, one can propose that such substructures intersecting and interacting with the grain boundaries are dragged along with the grain boundary during the viscous sliding of the boundary, so that the sliding process is limited with the appearance of this IF peak. Finally, a four-parameter mechanical model used for describing the operative damping mechanism of the IF peak in the FA were discussed in light of IF measurements and microstructural studies. An approximate expression for IF is derived, which is based on the four-parameter mechanical model in FA specimen and the experimental results have been better explained.

Influences of Heat Treatment and Grain Size on the Damping Capacity of an Fe–Cr–Al Alloy

The effect of heat treatment and grain size on the damping capacity of an Fe-Cr-Al alloy with composition of (wt%) Fe-25Cr-5Al has been investigated. It has been shown that annealing temperature and grain size have a significant influence on the damping capacity and strain amplitude dependence of the alloy. Moderate annealing and grain size are necessary for a higher damping capacity, although if annealing does not yield relatively large grains, it has little effect on the damping capacity. The alloy has a rather low damping capacity after being water quenched or cold worked due to a high internal stress in the structure. It has been found that a relaxation maximum appears in the internal friction-temperature plot at about 550 °C and its activation energy is 2.6 (±0.3) eV. It is proposed that the peak originates from the Zener relaxation.

Internal friction peak associated with the interface motion in the martensitic transformation of CuAlNiMnTi shape memory alloy

Two internal friction peaks were observed in a CuAlNiMnTi polycrystalline shape memory alloy during the martensitic transformation through an incomplete phase transformation method, of which the high-temperature peak PH is discussed in the present study. It has been found that the PH peak is discernible only at relatively low frequencies and its maximum corresponds to the inflection point of the relative dynamic modulus rather than its minimum, i.e., this peak is related to a process without soft mode effect. An internal friction model is proposed to describe the PH peak based on the theory of phase nucleation and growth in the thermoelastic martensitic transformation and is verified by the experimental results.

Considerable knock-on displacement of metal atoms under a low energy electron beam

Under electron beam irradiation, knock-on atomic displacement is commonly thought to occur only when the incident electron energy is above the incident-energy threshold of the material in question. However, we report that when exposed to intense electrons at room temperature at a low incident energy of 30u2009keV, which is far below the theoretically predicted incident-energy threshold of zirconium, Zircaloy-4 (Zr-1.50Sn-0.25Fe-0.15Cr (wt.%)) surfaces can undergo considerable displacement damage. We demonstrate that electron beam irradiation of the bulk Zircaloy-4 surface resulted in a striking radiation effect that nanoscale precipitates within the surface layer gradually emerged and became clearly visible with increasing the irradiation time. Our transmission electron microscope (TEM) observations further reveal that electron beam irradiation of the thin-film Zircaly-4 surface caused the sputtering of surface α-Zr atoms, the nanoscale atomic restructuring in the α-Zr matrix, and the amorphization of precipitates. These results are the first direct evidences suggesting that displacement of metal atoms can be induced by a low incident electron energy below threshold. The presented way to irradiate may be extended to other materials aiming at producing appealing properties for applications in fields of nanotechnology, surface technology, and others.

Internal friction related to viscous motion of phase interfaces during thermoelastic martensitic transformation

Two internal friction peaks were observed in CuAlNiMnTi polycrystalline shape memory alloy by a partial phase transition method in non-isothermal measurements. The low-temperature internal friction peak arising from the soft mode effect caused by the viscous movement of atoms along phase interfaces was studied in the present study in terms of internal friction fundamentals. An internal friction model related to the peak was established, which was shown to be in good agreement with experimental results.