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Ti-6Al-4V alloy is widely used in superplastic forming process. The conventional conditions require high forming temperatures (T ≥ 900 • C) and low strain rates (ε ≤ 10 −3 s −1). In order to reduce the costs of the industrial process, recent investigations focus on the micro-structural refinement of the material. It allows an improvement of the forming conditions which makes lower forming temperatures and higher strain rates eligible whereas low strain rates (ε ≤ 10 −3 s −1) and high temperatures (T ≥ 800 • C) are particularly suitable for conventional superplastic forming conditions. However, the mechanical response of the Titanium alloy strongly depends on the starting micro-structure considered and on its evolution with the temperature and the deformation. The objective of the present investigation is to observe the micro-structural evolutions of Ti-6Al-4V alloy under thermal and mechanical loadings from different starting micro-structures. Hence, an internal grain growth variable is identified by the use of these observations. Then, it is introduced into the behavior model and its influence on the mechanical response of the material is analysed. The final constitutive equations are able to take into account viscosity, strain hardening and grain size evolution for a wide range of strain rates and forming temperatures (10 −4 s −1 ≤ε ≤ 10 −2 s −1 ; 650 • C ≤ T ≤ 870 • C). Moreover, the model is able to consider several starting microstructures (different initial grain sizes) and to predict their influence on the viscous flow and the strain hardening. At last, some model verifications are conducted to check the validity of the non-isothermal model formulation. Some predictions are also performed by considering several starting microstructures.

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In situ neutron diffraction was performed on Cu∕Nb nanocomposite wires composed of a multiscale Cu matrix embedding Nb nanofilaments with a diameter of 267nm and spacing of 45nm. The evolution of elastic strains and peak profiles versus applied stress evidenced the codeformation behavior with different elastic-plastic regimes: the Cu matrix exhibit size effect in the finest channels while the Nb nanowhiskers remain elastic up to the macroscopic failure, with a strong load transfer from the Cu matrix onto the Nb filaments. The measured yield stress in the finest Cu channels is in agreement with calculations based on a single dislocation regime.

Hardfacing Plastic strain High load tribological test Work-hardening Strain-induced phase transformation a b s t r a c t Aeronautic forging dies are subjected to very high loads and temperatures for a long contact time between the pre-heated parts and dies. Cobalt-based hardfacings are commonly deposited on dies and their main wear mechanism is large plastic deformation of the die radii.This paper deals with the wear damage mechanisms of three different cobalt-based hardfacings: Stellite 21 deposited by a MIG process, Stellite 21 and Stellite 6 deposited by a LASER process. The tribological tests are carried out on a high load Ring on Disc tribometer at room temperature. The postmortem investigations are undertaken by SEM observations, micro-hardness measurements as well as by X-ray diffraction analyses.Results show that the increase of the hardness, in order to improve the wear behaviour, can be achieved by a higher carbon content and by a lesser iron dilution that depends on the deposition process. A very important work-hardening, up to 90%, is also observed under sliding conditions and a relationship is established between the increase of the micro-hardness and the plastic strain level. Two different plastic strain mechanisms are observed. For high (MIG) or low (LASER) iron dilution levels, the plastic strain causes respectively a reorientation of grains or a FCC to HCP phase transformation; the latter being associated with a lower friction coefficient.

In situ multiple tensile load-unload cycles under synchrotron radiation are performed on nanocomposite Cu/ Nb wires. The phase specific lattice strains and peak widths demonstrate the dynamics of the load-sharing mechanism where the fine Cu channels and the Nb nanotubes store elastic energy, leading to a continuous buildup of internal stress. The in situ technique reveals the details of the macroscopically observed Bauschinger effect.

Copper-based high-strength nanofilamentary wires reinforced by tantalum nanofilaments were prepared by severe plastic deformation (repeated hot extrusion and cold drawing steps) to be used in the windings of high-pulsed magnets. This application requires a complete characterization of the microstructure and the strength and their relationship for further optimization: after heavy strain, the Cu matrix is nanostructured and the Ta nanofilaments develop a strong ribbon-like shape resulting in an early microstructural refinement. The macroscopic strength is greater than rule-of-mixture predictions as confirmed by nanohardness values. The observed size effect is related to the dislocation starvation in the nanostructured materials combined with the barrier role of Cu/Ta interfaces. The strengthening is lower, however, as expected, because of the distorted ribbon morphology of the Ta filaments preventing them from behaving as nanowhiskers, as Nb fibers do in Cu/Nb wires. This shows that size and geometry play key roles in the plasticity of nanomaterials.

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