On the yield point of icosahedral AlCuFe quasicrystals

Nadine Baluc, Joël Bonneville
2003
Article

Résumé

Transient creep (1) tests have been performed at the yield point of icosahedral AlCuFe poly-quasicrystalline specimens in order to investigate the actual causes of this phenomenon. The experimental results strongly suggest that the yield point occurrence is essentially related to the lack of dislocation mobility and that a competition does exist between dislocation velocity and short range recovery. (C) 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

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Transient creep experiments have been performed on poly-quasicrystalline specimens of the icosahedral Al-Cu-Fe phase. Three temperatures (870, 960 and 1000 K) have been selected in the ductile regime for creep stresses that range between 30 and 320 MPa. In these experiments, the specimens are first loaded in constant strain-rare conditions, then the testing machine is switched to load control and the specimens are allowed to deform under creep conditions. One- or two-stage creep curves have been obtained depending on temperature, stress and on the deformation level of the stress-strain curve at which the creep tests were performed. When creep conditions are applied before the upper yield stress, at low stress, only a stage of continuous decrease in creep rate is observed, while at higher stress this primary stage is followed by a stage of accelerated creep rate. These two-stage creep curves are predicted within the framework of the strain dependent friction stress model. For creep tests beyond the upper yield stress, only the stage of accelerated creep rate is observed. No distinct steady-state stage has been observed for all investigated creep conditions. (C) 2000 Elsevier Science B.V. All rights reserved.

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On the constitutive behavior of the F82H ferritic/martensitic steel

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This paper focuses on the constitutive behavior of the F82H ferritic/martensitic steels internationally investigated in the frame of the fusion material development program. Tensile tests were carried out in the temperature range (80-723 K) at different strain rates. A decomposition of the how stress into a yield stress plus a hardening stress component is proposed. The temperature, strain and strain rate dependence of the stress is discussed based on the current available data. The strain-hardening is calculated and two strain-hardening stages are identified along the deformation curves between the yield stress and the necking instability. Finally, semi-empirical constitutive relations, which are partly based on dislocation mechanics, are derived. (C) 2001 Elsevier Science B.V. All rights reserved.

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Plastic behaviour of icosahedral Al-Cu-Fe quasicrystals: Experiment and modelling

Nadine Baluc, Joël Bonneville, Emmanuelle Giacometti

Constant strain-rate and creep tests have been performed on poly-quasicrystalline specimens of the icosahedral Al-Cu-Fe system. Three temperatures (870, 960 and 1000 K) have been investigated by creep for samples predeformed at constant strain-rate before and beyond the upper yield stress, over the stress range 30-320 MPa. The influence of the creep stress level and plastic strain amount on the stress-strain curve has also been studied when the samples are deformed again at constant strain rate. Depending on temperature, stress and initial predeformation states, single or two-stage creep curves have been observed. When performed before the upper yield stress, in the high work-hardening portion of the stress-strain curve, only a primary stage of continuous decreasing creep rate is observed. At higher stresses, close to the upper yield stress, the primary stage is followed by a second stage of work-softening leading to an increasing strain-rate. The shape of the constant strain-rate and creep curves, as well as the transients observed when creep is switched-back to constant strain-rate conditions, are in fair agreement with the predictions of the dislocation model recently proposed by Guyot and Canova on the basis of an irreversible strain dependent friction stress [P. Guyol, G. Canova, Phil. Mag. A 79 (1999) 28151]. (C) 2001 Elsevier Science B.V. All rights reserved.

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