R.J. Perez

Effects of secondary phases on the damping behaviour of metals, alloys and metal matrix composites

An understanding of the precise correlation between the presence of secondary phases and material damping has eluded investigators, partly as a result of the fact that often there are various mechanisms involved. As a step towards the clarification of damping phenomena in metals and alloys, this paper provides a systematic review of the studies that have been completed on the damping mechanisms present in metals and alloys, with particular emphasis on precipitation. The damping mechanisms associated with secondary phases in metals and alloys have been subdivided into four categories, namely interface damping theory, thermal mismatch-induced dislocation damping theory, interaction damping theory and the rule of mixtures damping theory. A number of alloy systems are discussed to demonstrate the applicability of the four types of theory and the level of understanding of these complex mechanisms. As an extension of precipitation damping theory, the damping behaviour and mechanisms in particle-reinforced metal matrix composites are extensively discussed.

Documentation of damping capacity of metallic, ceramic and metal-matrix composite materials

High-damping materials allow undesirable mechanical vibration and wave propagation to be passively suppressed. This proves valuable in the control of noise and the enhancement of vehicle and instrument stability. Accordingly, metallurgists are continually working toward the development of high-damping metals (hidamets) and high-damping metal-matrix composites (MMCs). MMCs become particularly attractive in weight-critical applications when the matrix and reinforcement phases are combined to provide high-damping and low-density characteristics. In selecting the constituents for an MMC, one would like to have damping capacity data for several prospective component materials. Based upon data which have been published in the scientific literature, a concise documentation is given of the damping capacity of materials by three categories: (a) metals and alloys, (b) ceramic materials, and (c) MMCs.

Damping behavior of discontinuously reinforced ai alloy metal-matrix composites

High damping materials allow undesirable mechanical vibration and wave propagation to be passively suppressed. This proves valuable in the control of noise and the enhancement of vehicle and instrument stability. Accordingly, the scientific community is continually working toward the development of high damping metals (hidamets) and high damping metal-matrix composites (MMCs). The MMCs are particularly attractive in weight-critical applications when the matrix and reinforcement phases are combined to provide desirable property combinations, such as high damping and low density. Inspection of the available scientific literature, however, reveals that an understanding of the precise correlation between the presence of secondary phases (either reinforcements or precipitates) and material damping has eluded investigators, partly as a result of the superposition of multiple mechanisms. As a step toward the clarification of damping phenomena in discontinuously reinforced MMCs, this article describes the damping behavior and mechanisms that are present in discontinuously reinforced MMCs, with particular emphasis on particulate-reinforced Al alloy MMCs processed using spray atomization and deposition. The operative damping mechanisms in the particulate-rein-forced MMCs are discussed in light of the data obtained from microstructural studies and damping capacity measurements.

Thermal stability of nanocrystalline Fe-10 wt.% Al produced by cryogenic mechanical alloying

Abstract Nanocrystalline Fe-10 wt.% Al material is synthesized using cryogenic high energy ball milling (cryomilling). The resultant powders are compacted in a rigid die at 350 MPa and 823 K in an argon atmosphere. The grain size of the powder compacts, determined using transmission electron microscopy, is found to be 11 ±5 nm. Subsequent heat treatment for 1 hour at temperatures of 1073 K and 1223 K reveals that fine grain sizes of 13 ± 6 nm and 16 ± 7 nm, respectively, are maintained. Only when the heat treatment temperature is increased to 1373 K, or over 75% of the melting point of Fe, do relatively large grainsform, approaching 100 nm. This level of thermal stability is shown to exceed that of pure Fe processed under identical conditions, by a significant margin. The present results indicate that the increase in thermal stability may originate from the pinning effect offine dispersoids formed during cryomilling and subsequent heat treatment.

Dislocation-induced damping in metal matrix composites

The damping response of crystalline metals and alloys is generally associated with the presence of defects in the crystal lattice. The disturbance of these defects, usually in response to an applied cyclic load, dissipates energy, a mechanism known as “internal friction”. The various defects commonly found in crystalline materials include point defects (e.g. vacancies), line defects (e.g. dislocations), surface defects (e.g. grain boundaries) and volume defects (e.g. inclusions). Among these, dislocations are noteworthy because they play a critical role, not only in the damping response of crystalline materials, but also in the overall mechanical behaviour of the materials. Among the various structural materials actively being developed, metal matrix composites (MMCs) have received considerable attention as a result of their potential to combine reinforcement properties of strength and environmental resistance, with matrix properties of ductility and toughness. Of interest is the generally observed phenomenon that MMCs exhibit unusually high concentrations of dislocations, an observation typically attributed to the difference in coefficient of thermal expansion between matrix and reinforcement. The objectives of the present paper are to provide an overview of the sources of dislocation generation in MMCs, and to provide insight into the effects that dislocations have on the damping response of MMCs. The presence of dislocations in MMCs is highlighted on the basis of transmission electron microscopy studies, and the dislocation damping mechanisms are discussed in light of the Granato-Lücke theory.

Grain growth of nanocrystalline Fe–Al alloys produced by cryomilling in liquid argon and nitrogen

Abstract Cryomilling of Fe–10 wt.%Al powders in liquid argon as well as in liquid nitrogen resulted in nanocrystalline structures which were thermally stable at least up to 1223 K, or 67% of the melting temperature of Fe. In contrast, cryomilling of elemental Fe resulted in a nanocrystalline structure which grew to a sub-micron scale following annealing at 1223 K. The enhanced thermal stability of the cryomilled Fe–10 wt.%Al powders in liquid argon was attributed to the formation of γ -Al 2 O 3 due to the moisture condensation. The thermal stability of the Fe–10 wt.%Al powders milled in liquid nitrogen was attributed to the formation of oxynitrides, or γ -Al 2 O 3 and AlN particles during cryomilling in liquid nitrogen. The formation of Fe 3 O 4 particles did not result in enhanced thermal stability. The presence of Al is essential in achieving thermal stability of nanocrystalline structures in cryomilled Fe powders.

Damping behavior of 6061Al/Gr metal matrix composites

The damping behavior of graphite particulate-reinforced 6061A1 alloy metal matrix composites (MMCs) processed by spray atomization and codeposition is studied. Four spray deposition experiments are made, yielding materials with graphite volume fractions of 0, 0.05, 0.07, and 0.10. A dynamic mechanical thermal analyzer is used to measure the damping capacity and elastic modulus at 0.1, 1, and 10 Hz over the temperature range of 30 °C to 250 °C. The damping capacity of the materials is shown to increase with increasing volume fraction of graphite. Hot extrusion of the spray-deposited MMCs is shown to further increase the damping capacity. The elastic moduli of the spray-deposited MMCs are reduced with the addition of graphite but are improved by hot extrusion. At low temperatures (below 150 °C), the high damping capacity of the MMCs is attributed primarily to thermal expansion mismatch-induced dislocations and the high intrinsic damping of graphite. At high temperatures (above approximately 200 °C), the damping capacity is attributed to Al/graphite interface viscosity, preferred orientation of the graphite, and the presence of dislocations.

Mechanically induced crystallization of metglas Fe78B13Si9 during cryogenic high energy ball milling

Nanocrystalline structures with crystallite size of approximately 2 nm were synthesized during cryogenic ball milling in liquid nitrogen. The crystallization kinetics, studied using differential scanning calorimetry (DSC), revealed a sigmoidal transformation curve, which may be described by a Johnson-Mehl-Avrami relation. Mechanisms of the mechanical crystallization during cryogenic ball milling were found to be related to bending and wear-like events during milling.

Formation of supersaturated solid solutions by mechanical alloying

Abstract Results obtained from cryogenic mechanical alloying of Ni-50 at. %Al powders indicate that plastic deformation produces a nanoscale mixture of Ni and Al. This nanoscale mixture transforms to ordered NiAl phase without forming a supersaturated solid solution as the intermediate phase. The observed disappearance of the X-ray peaks of constituent elements is associated with the lower atomic scattering factor and grain refinement rather than the actual dissolution of the element. The present study also addresses the effects of thermal exposure accompanying the ball milling process. The study of Spex milling of Fe-13 at.%B-7 at.%Si demonstrates that prolonged mechanical alloying at room temperature results in partial dissolution of Si and B in Fe by enhanced diffusion. A supersaturated solid solution of 4 at.% B in Fe is observed.

Synthesis of nanocrystalline M50 steel powders by cryomilling

Abstract The present paper reports on a study of the synthesis of nanocrystalline high speed tool steel M50 powders (4.5% Mo, 4.0% Cr, 1.0% V, 0.8% C, balance Fe) by cryogenic high energy ball milling (cryomilling). Pre-alloyed M50 steel is spray atomized, and subsequently cryomilled in liquid nitrogen for 25 hours. Elemental Al powder is added prior to cryomilling to promote the formation of nanoscale Al2O3 and AlN dispersoids to improve the thermal stability of the nanocrystalline M50 steel. High resolution transmission electron microscopy (HRTEM) reveals the formation of various carbides (V8C7, Fe3C, and FeC), oxides (Al2O3, MoO3, and V3O7), and a nitride phase (AlN) during cryomilling. Following one hour of heat treatment at 1373 K (0.77 Tm), an average grain size of 70 nm was retained for the M50 steel powders.