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Type Li, Yong; Langdon, Terence G.
  Publication (up) Creep behavior of an Al-6061 metal matrix composite reinforced with alumina particulates Volume Journal Article
Pages 1997
  Abstract Acta Materialia  
  Corporate Author Acta Mater.  
Publisher 45  
Editor 11
  Summary Language 4797-4806 Series Editor  
Abbreviated Series Title Creep tests were conducted on an Al-6061 matrix alloy reinforced with 20 vol.% of irregularly shaped Al2O3 particulates. The composite was fabricated using an ingot metallurgy technique and the creep properties were determined at temperatures from 623 to 773 K. The results show high values for both the apparent stress exponent (up to > 10) and the apparent activation energy for creep (~ 200–275 kJ/mol) but it is demonstrated, by incorporating a threshold stress into the analysis, that the true stress exponent is close to 3 and the true activation energy is close to the value for diffusion of Mg in the Al matrix. The results suggest that creep is controlled by the viscous glide of dislocations in the Al-6061 matrix alloy. Very fast creep rates are observed at the highest stress levels owing to the breakaway of dislocations from their solute atom atmospheres. Direct comparison shows that the creep resistance of this composite is less than in an Al-6061 alloy reinforced with 20 vol.% of Al2O3 microspheres. This difference is attributed to the creation of additional precipitates in the microsphere-reinforced composite because of an interfacial reaction between the matrix alloy and the reinforcement.
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no NU @ karnesky @ 562
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Thornton, K.; Agren, J.; Voorhees, P. W. Modelling the Evolution of Phase Boundaries in Solids at the Meso- and Nano-Scales Journal Article 2003 Acta Materialia Acta Mater. 51 19 5675-5710 refbase @ user 1629
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Blobaum, K.J.M.; Krenn, C.R.; Wall, M.A.; Massalski, T.B.; Schwartz, A.J. Nucleation and growth of the [alpha]' martensitic phase in Pu-Ga alloys Journal Article 2006 Acta Materialia Acta Mater. 54 15 4001-4011 Phase transformations; Martensitic phase transformation; Nucleation of phase transformations In a Pu-2.0 at.% Ga alloy, it is observed experimentally that the amount of the martensitic [alpha]' product formed upon cooling the metastable [delta] phase below the martensite burst temperature (Mb) is a function of the holding temperature and holding time of a prior conditioning ("annealing") treatment. Before subjecting a sample to a cooling and heating cycle to form and revert the [alpha]' phase, it was first homogenized for 8 h at 375 [degree sign]C to remove any microstructural memory of prior transformations. Subsequently, conditioning was carried out in a differential scanning calorimeter apparatus at temperatures in the range between -50 and 370 [degree sign]C for periods of up to 70 h to determine the holding time and temperature that produced the largest volume fraction of [alpha]' upon subsequent cooling. Using transformation peak areas (i.e., the heats of transformation) as a measure of the amount of [alpha]' formed, the largest amount of [alpha]' was obtained following holding at 25 [degree sign]C for at least 6 h. Additional time at 25 [degree sign]C, up to 70 h, did not increase the amount of subsequent [alpha]' formation. At 25 [degree sign]C, the Pu-2.0 at.% Ga alloy is below the eutectoid transformation temperature in the phase diagram and the expected equilibrium phases are [alpha] and Pu3Ga, although a complete eutectoid decomposition of [delta] to these phases is expected to be extremely slow. It is proposed here that the influence of the conditioning treatment can be attributed to the activation of [alpha]-phase embryos in the matrix as a beginning step toward the eutectoid decomposition, and we discuss the effects of spontaneous self-irradiation accompanying the Pu radioactive decay on the activation process. Subsequently, upon cooling, certain embryos appear to be active as sites for the burst growth of martensitic [alpha]' particles, and their amount, distribution, and potency appear to contribute to the total amount of martensitic product formed. A modeling approach based on classic nucleation theory is presented to describe the formation of [alpha]-phase embryos during conditioning. The reasons why the holding times during conditioning become eventually ineffective in promoting more [alpha]' formation on cooling are discussed in terms of the differences in the potency of the embryos created in the [delta] matrix during conditioning and in terms of growth-impeding volume strains in the matrix resulting from an increasing number of martensite particles, thus opposing further growth. It is suggested that the disparate amounts of the [alpha]' formation reported in the literature following various studies may be in part a consequence of the fact that conditioning times at ambient temperatures are inevitably involved in any handling of radioactive samples prior to testing. no NU @ karnesky @ 1916
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Zackrisson, J.; Rolander, U.; Jansson, B.; Andren, H. - O. Microstructure and performance of a cermet material heat-treated in nitrogen Journal Article 2000 Acta Materialia Acta Mater. 48 4281-4291 no 9022
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Sauvage, X.; Renaud, L.; Deconihout, B.; Blavette, D.; Ping, D. H.; Hono, K. Solid state amorphization in cold drawn Cu/Nb wires Journal Article 2001 Acta Materialia Acta Mater. 49 389-394 no 9027
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Hattestrand, M.; Andren, H. O. Influence of strain on precipitation reactions during creep of an advanced 9% chromium steel Journal Article 2001 Acta Materialia Acta Mater. 49 2123-2128 no 9062
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Erneman, J.; Schwind, M.; Liu, P.; Nilsson, J. O.; Andren, H. O.; Agren, J. Precipitation reactions caused by nitrogen uptake during service at high temperatures of a niobium stabilised austenitic steel Journal Article 2004 Acta Materialia Acta Mater. 52 4337-4350 no 9326
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Ivanisenko, Y.; MacLaren, I.; Sauvage, X.; Valiev, R. Z.; Fecht, H. J. Shear-induced alpha ->gamma transformation in nanoscale Fe-C composite Journal Article 2006 Acta Materialia Acta Mater. 54 1659-1669 no 9414
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Rupert, Timothy J.; Trenkle, Jonathan C.; Schuh, Christopher A. Enhanced solid solution effects on the strength of nanocrystalline alloys Journal Article 2011 Acta Materialia 59 4 1619-1631 Solute strengthening; Nanocrystalline metals; Mechanical behavior; Nanoindentation; Thin films Solid solution strengthening in nanocrystalline alloys is studied using sputtered Ni-W as a model system. In the composition range of 0-20 at.% W, the sputtered alloys have a nanocrystalline structure with a grain size that is independent of composition. Nanoindentation of these alloys shows that solute addition increases strength to very high levels, almost in proportion to the solute content. This behavior is not expected based on traditional solid solution strengthening mechanistic models of local dislocation pinning at solute atoms, but can be explained by further considering a global effect of solute on the average properties of the Ni lattice. The new strengthening term arises by considering grain boundaries as pinning points for dislocation motion in nanocrystalline materials and incorporating the effect of solutes on such a mechanism. Our discussion surrounding Ni-W also provides insights into other solid solution nanocrystalline systems, a variety of which we show can be accurately described using the same concept. These developments also explain the origin of solid solution softening in some nanocrystalline alloys. 1359-6454 no NU @ karnesky @ 11000
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Caballero, F.G.; Yen, Hung-Wei; Miller, M.K.; Yang, Jer-Ren; Cornide, J.; Garcia-Mateo, C. Complementary use of transmission electron microscopy and atom probe tomography for the examination of plastic accommodation in nanocrystalline bainitic steels Journal Article 2011 Acta Materialia In Press Corrected Proof Bainite; Steels; Three-dimensional atom probe; Transmission electron microscopy A displacive transformation involves the motion of a glissile interface. As in work hardening, its motion can be halted by defects such as dislocations, stacking faults or twins in the austenite. The defects are created when the shape deformation accompanying bainite growth is accommodated by plastic relaxation of the surrounding austenite. The growing plate stops when it collides with the austenite grain boundary. Because transformation from strong austenite leads to fine plates, alloys can be designed such that the bainite transformation is suppressed to low temperatures (125350 °C), leading to a nanoscale bainitic microstructure. Complementary high-resolution transmission electron microscopy and atom probe tomography have provided new experimental evidence on the accommodation of transformation strain, a subject critically relevant to understanding the atomic mechanisms controlling bainitic ferrite growth. 1359-6454 <p><br/>A displacive transformation involves the motion of a glissile interface. As in work hardening, its motion can be halted by defects such as dislocations, stacking faults or twins in the austenite. The defects are created when the shape deformation accompanying bainite growth is accommodated by plastic relaxation of the surrounding austenite. The growing plate stops when it collides with the austenite grain boundary. Because transformation from strong austenite leads to fine plates, alloys can be designed such that the bainite transformation is suppressed to low temperatures (125-350��C), leading to a nanoscale bainitic microstructure. Complementary high-resolution transmission electron microscopy and atom probe tomography have provided new experimental evidence on the accommodation of transformation strain, a subject critically relevant to understanding the atomic mechanisms controlling bainitic ferrite growth.</p> no NU @ karnesky @ 5 11143
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