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In metal matrix composites (MMCs) thermal stress relaxation can be achieved either by interface debonding, crack propagation or by dislocation motion. The present paper shows that in the case of magnesium matrix, interface thermal stresses are relaxed by dislocation motion. Moreover the results obtained by mechanical spectroscopy prove that this dislocation motion is controlled by a solid friction mechanism, which is not thermally activated. This point is very interesting for the development of MMCs, which exhibit a high damping capacity over a wide frequency range. Dislocation hysteretic motion in the magnesium matrix is evidenced by the dependence of the mechanical loss on the stress amplitude. The obtained relationship obeys perfectly to the Granato-Lucke model for dislocation breakaway. (C) 2009 Elsevier Ltd. All rights reserved.

Polycrystalline zirconia (3Y-TZP grade) specimens unreinforced and reinforced with multiwalled carbon nanotubes (MWCNTs) or silicon carbide whiskers (SiCws) were processed by powder metallurgy and studied by mechanical spectroscopy. The high-temperature mechanical-loss spectrum of pure 3Y-TZP presents mainly an exponential background. Doping 3Y-TZP with nano-sized reinforcements such as MWCNT or SiCw results in a decrease in the high-temperature background with respect to pure zirconia. It has been shown that these spectrum modifications by doping are associated with a better resistance to creep. (C) 2009 Elsevier B.V. All rights reserved.

Mechanical-loss measurements were performed in three grades of martensitic high-carbon tool steels. These steels were treated under carburizing gas before quenching, in order to vary the concentration of interstitial carbon in the martensite, as assessed by thermoelectric-power measurements. The mechanical-loss spectra present a relaxation peak due to interaction between dislocations and carbon interstitials, similar to the Snoek-Koster peak found in ferrite. The amplitude of the Snoek-Koster peak is expected to be independent of the concentration of interstitials. However, the amplitude of the corresponding peak in martensite is found to increase with the concentration of interstitial carbon. This effect is attributed to an increase of the dislocation density with the carbon content. (C) 2009 Elsevier B.V. All rights reserved.

We consider dislocations as vortices in elastic strain field and show that an ordered system of partial dislocations in faulted martensites is analogous to a system of flux lines (supercurrent vortices) in type-II superconductors. The network of interacting partial dislocations can form vortex matter, whose dynamics is controlled by applied stress amplitude and by a degree of quenched and thermal disorder. Experimental results on elasticity and ultrasonic anelasticity of ternary faulted martensites from Cu-Al, Cu-Zn families and from binary Cu-Al system, obtained for temperatures between 6 and 300 K, point to the existence of glassy and liquid states of vortex matter in martensites. We show that the liquid and glassy dynamics of vortex matter and reversible transitions between these states account for a large number of unusual low-temperature anelastic effects in faulted martensites. (C) 2007 Elsevier B.V. All rights reserved.

The mechanical loss and dynamic modulus temperature spectra of two 18-carat white gold alloys, Au58%Cu24%Pd18% without (grade A) and with alloying additions (grade B), were measured in the 400-1000 K temperature range. A peak interpreted as a Zener relaxation peak is found at about 630 K in both alloys. In the as-quenched state the peak shifts to lower temperatures due to excess vacancies, creating a transient peak at 520 K visible only upon the first heating. In grade A, a relaxation peak is observed at 750 K and is attributed to grain boundary relaxation, since it is absent in monocrystalline sample. in the same temperature region, the grade B features a peak with a large hysteresis between heating and cooling. A sharp peak observed in heating at 830 K is accompanied by a strong modulus decrease. In cooling, we observe a peak at 750 K, below which the mechanical loss abruptly decreases, accompanied by a modulus recovery. The peak hysteresis is interpreted as being due to dissolution or precipitation, respectively. The internal friction spectrum is correlated with the alloy's mechanical properties, namely the hardening of the grade B upon tempering at a temperature below the dissolution peak. (C) 2009 Elsevier B.V. All rights reserved.