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Viscoplasticité et microstructures d'un alliage de titane : effets de la température et de la vitesse de sollicitation

Abstract : The main goal of this research work was to analyze and characterize the mechanical behavior of Ti 6242-Si in a wide range of temperatures, of strain rates and of loading paths. Actually, a more important viscoplastic flow at room temperature than at higher temperatures stands as an intrinsic characteristic of most titanium and zirconium alloys. This unusual behavior is related to interactions phenomena taking place between dislocations and interstitial atoms (O, C, N, H), generally described by expressions such as "dynamic strain ageing" (DSA) or "static strain ageing" (SSA). Dislocation microstructures observed by transmission electron microscopy on a number of samples appropriately strained and carefully chosen across the entire temperature domain, provide instructive insights regarding possible physical basis for a sound interpretation on the microscopic level of phenomena observed and measured on the macroscopic level. Mechanical test results are then interpreted in terms of the relative mobility of dislocations or dislocations groups in relation, either with their interactions with supersaturated solute atoms (edge dislocations), or with their peculiar core structure at low temperature (screw dislocations). Due to the large range of strain rates covered by relaxation tests, it was possible to estimate in which proportion various viscoplastic regimes were mixing, within the tested structure, at each temperature. Indeed, two drastically different plastic modes are combining in order to control the macroscopic viscoplastic flow : - at high temperatures (or low strain rates), the solute drag mode is dominant : edge dislocations are dragging their solute atmospheres. - at low temperatures (or high strain rates), on the other hand, the friction mode is prevailing : dislocations move rapidly, solute atoms remain quasi-immobile and simply contribute to solid solution hardening. In the region where these two plastic eigenmodes overlap - PLC domain - sudden changes in the viscoplastic behavior can appear, since the friction mode tends naturally to localize plasticity into "speed bands", while the rest of the structure or of the specimen continues to strain slowly in the solute drag mode, until it may even trigger a complete arrest of the plastic flow at low temperatures. Viscoplastic flow in this material is intrinsically heterogeneous. A detailed study of the data collected during relaxation tests reveals the presence of "strain bursts" strictly localized in time and space on a mesoscopic scale. The level of correlation between these elementary events determines the amplitude and the features of the microscopic events (speed bands, serrations, stress peaks...). The boundaries separating the various domains of viscoplastic behavior were determined for this alloy by way of mechanical testing. Various relevant macroscopic parameters were measured, such as apparent activation energies and apparent activation volumes corresponding to each of these viscoplastic modes. At high temperature (600°C - 450°C), the solute drag mode is easy and spreads out. At the center of the viscoplastic anomaly domain, around 400°C, dynamic strain aging prevails. At this temperature, the restoration processes of the material are very limited : a short and ineffective relaxation period is followed by a complete arrest of plastic flow. At intermediate temperatures, between 300°C and 200°C, strict arrest of plasticity is quasi-instantaneous. In the low-temperature regime, around 150°C and below, viscoplasticity is restored with the full activity of the friction mode and strain hardening and age hardening processes are cumulating. Finally, at room temperature or near the lower boundary of the domain, static strain ageing starts operating.
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Submitted on : Friday, October 30, 2009 - 10:55:18 AM
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  • HAL Id : tel-00428968, version 1

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Hélène Jousset. Viscoplasticité et microstructures d'un alliage de titane : effets de la température et de la vitesse de sollicitation. Mécanique [physics.med-ph]. École Nationale Supérieure des Mines de Paris, 2008. Français. ⟨NNT : 2008ENMP0004⟩. ⟨tel-00428968⟩

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