Thierry Grosdidier

Evolution of recrystallisation texture and microstructure in low alloyed titanium sheets

The evolution of microstructure and crystallographic texture in low alloyed titanium sheets, initially deformed by 80% cold rolling, are investigated at different stages of the recrystallisation process. Optical and transmission electron microscopies, as well as X-ray diffraction and EBSD are used to provide information about recrystallisation mechanisms and kinetics. Orientation Density Function (ODF) differences are used to quantitatively compare recrystallised and deformed states. The main texture features of the deformed state evolve only slightly during the primary recrystallisation. The major changes in texture result from secondary recrystallisation or grain growth. Primary recrystallisation can be roughly separated into two stages. The first one is very fast and corresponds to the appearance of new grains in about 80% of the material volume. The second stage is more sluggish. It corresponds to the disappearance of the so-called “white grains”, which did not twin during deformation due to their stable orientation near {ϕ1=0°, φ=45°, ϕ2=0°}. Recovery is an important mechanism throughout the process and deformation heterogeneities must be taken into account for a good understanding of the recrystallisation in titanium.

Precipitation and dissolution processes in γ/γ' single crystal nickel-based superalloys

Abstract An experimental analysis of the γ ↔ γ + γ ′ phase transformations in two single-crystal Nickel-based superalloys is presented. The transformations are characterised macroscopically by differential thermal analysis and dilatometry measurements and microscopically by qualitative and quantitative metallography of samples quenched during various thermal cycles. This fully covers the large changes in precipitate morphology and volume fraction encountered in the upper temperature transformation domain (between 1100 and 1300°C). It is shown that dissolution and precipitation processes are strongly influenced by the local elastic distortions resulting from the lattice mismatch between the two phases. The complex microstructure evolution observed is explained by considering the successive equilibrium shapes of a coherent precipitate growing in more or less strong interaction with its neighbors.

Selective surface purification via crater eruption under pulsed electron beam irradiation

This letter reports an interesting phenomenon associated with the high-current pulsed electron beam treatment:selective surface purification. The treatment induces crater eruptions that preferentially occur at irregular composition and structure sites. The eruptions of second phase inclusions naturally lead to the purification and homogenization of the melted surface layer. This improves significantly the corrosion resistance of NiTi and 316L alloys.

The deformation mechanisms in the β-metastable β-cez titanium alloy

{beta} metastable alloys have, potentially, better cold formability and age-hardening response than {alpha}-{beta} titanium alloys. In the {beta} metastable alloys, the Ms temperature is below room temperature and the {beta} phase can be retained in a metastable state. {beta}-Cez is an alloy initially developed for turbine compressor applications. Most of the research on this alloy has concentrated on its structural stability, its high temperature deformation microstructures, its phase transformation kinetics and the structure/mechanical properties relationships. Little information is however available concerning the room temperature deformation mechanisms that actually control the mechanical properties of the alloy. Production of wires or thin sheets of the {beta}-Cez alloy that are obtained by cold deformation and subsequent heat treatments is currently under development. The optimization of such manufacturing processes has made it necessary to examine more precisely the deformation mechanisms of {beta} rich microstructures in this alloy. The aim of this work was therefore to clarify which are the deformation mechanisms and to what extent they affect the mechanical properties.

Nanostructures in thermal spray coatings

The nature of the nanograins formed by high velocity oxy-fuel thermal spraying of (FeAl) milled powder has been investigated using transmission electron microscopy on cross-sectional thin foils. Equiaxed 3D nanometer crystallites are formed by recrystallization in the unmelted powder particles while 2D nanometer columnar grains are produced by rapid solidification within the fully molten splats.

High current pulsed electron beam treatment of AZ31 Mg alloy

This paper reports, for the first time, an analysis of the effect of High Current Pulsed Electron Beam (HCPEB) on a Mg alloy. The AZ31 alloy was HCPEB treated in order to see the potential of this fairly recent technique in modifying its wear resistance. For the 2.5J∕cm2 beam energy density used in the present work, the evaporation mode was operative and led to the formation of a “wavy” surface and the absence of eruptive microcraters. The selective evaporation of Mg over Al led to an Al-rich melted surface layer and precipitation hardening from the over saturated solid solution. Due to the increase in hardness of the top surface layer, the friction coefficient values were lowered by more than 20% after the HCPEB treatments, and the wear resistance was drastically (by a factor of 6) improved. The microhardness of the HCPEB samples was also increased significantly down to a depth of about 500μm, far exceeding the heat-affected zone (about 40μm). This is due to the effect of the propagation of the shockwave associated with this HCPEB treatment.This paper reports, for the first time, an analysis of the effect of High Current Pulsed Electron Beam (HCPEB) on a Mg alloy. The AZ31 alloy was HCPEB treated in order to see the potential of this fairly recent technique in modifying its wear resistance. For the 2.5J∕cm2 beam energy density used in the present work, the evaporation mode was operative and led to the formation of a “wavy” surface and the absence of eruptive microcraters. The selective evaporation of Mg over Al led to an Al-rich melted surface layer and precipitation hardening from the over saturated solid solution. Due to the increase in hardness of the top surface layer, the friction coefficient values were lowered by more than 20% after the HCPEB treatments, and the wear resistance was drastically (by a factor of 6) improved. The microhardness of the HCPEB samples was also increased significantly down to a depth of about 500μm, far exceeding the heat-affected zone (about 40μm). This is due to the effect of the propagation of the shockwave...

Mechanisms of structural evolutions associated with the high current pulsed electron beam treatment of a NiTi shape memory alloy

The aim of this study was to investigate, for the first time, the surface modifications associated with the use the recently developed high current pulsed electron beam technique for modifying the surface of an intermetallic NiTi alloy. Samples were treated with the same electron beam parameters but different numbers of pulses (i.e., five and ten pulses) and the present article concentrates on a detailed characterization of their texture and microstructure modifications. The observation of surface features such as craters, wavy surfaces with protrusions, chemistry modifications, and the development of specific texture components are discussed as the consequence of the combination of surface melting and evaporation mechanisms. It is also shown that in the subsurface, below the melted layer, the martensitic transformation was triggered due to the effects of the thermal stresses and shock waves propagating in the material.

Formation and evolution of craters in carbon steels during low-energy high-current pulsed electron-beam treatment

The authors investigated in detail the formation and evolution of microcraters induced by low-energy high-current pulsed electron-beam treatment on several quenched and tempered carbon steels. They have shown that the crater formation mechanism is the same for the three selected steels regardless of the carbon content and original microstructure state. Melting starts at the subsurface layer during treatment, resulting in the nucleation of small droplets preferentially at grain or phase boundaries. Under further heating, the boiling droplets erupt through the surface. The liquid around these craters shrinks to supply the lost part and, during the cooling process, leads to the formation of the funnel-like crater morphology. Microirregularities help retain locally the heat flux and, consequently, serve as nucleation sites for crater formations. By increasing the number of pulses, microirregularities were gradually removed and melted layer depth increased. As a result, crater formation became less effective. ...

Deep Modification of materials by thermal stress wave generated by irradiation of high-current pulsed electron beams

High-current pulsed electron beams generate modifications in materials over a depth range of a few hundred micrometers, far beyond the heat-affected zone. In this article, the authors presented relevant experimental results in a stainless steel substrate and described the associated transient thermal and stress processes. They attributed the deep modification to an intense stress wave arising from drastic heating of the materials about 2∼3μm below the surface as a result of the deep penetration of electrons. Thanks to the large amplitude at several gigapscals the stress wave exerted intense impacts over a long distance. They also revealed that the interactions were dependent on the grain orientation.

Ti surface alloying of an AISI 316L stainless steel by low energy high current pulsed electron beam treatment

Ti has been added into the surface layer of an AISI 316L stainless steel—by means of rapid surface alloying of Ti powder—using the low energy, high current pulsed electron beam (LEHCPEB) technique with the aim of enhancing its corrosion resistance. The alloyed layer contained a mixture of the α and γ phases and its depth increased with the number of pulses. Compared to the samples treated under the conventional LEHCPEB treatment (i.e., without surface alloying), Ti addition improved further the corrosion resistance. This can be attributed to the addition of Ti into the surface layer together with the removal of MnS inclusions. However, the homogeneity of the Ti distribution decreased when the number of pulses increased. Therefore, contrary to the conventional LEHCPEB treatment, as a result of this decreased homogeneity, the corrosion resistance decreased when the number of LEHCPEB was too high. Therefore, a good control of the processing parameters is required to take the full advantage of the surface Ti ...