Alain Hazotte

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.

Microstructural analysis of alumina chromium composites by X-ray tomography and 3-D finite element simulation of thermal stresses

Abstract X-ray microtomography is used to measure volume fraction and connectivity of the metallic phase in an alumina–chromium composite. Reconstructed images are used as input data for a finite element calculation of the residual thermal stresses. Results confirm the main trends shown by similar calculations previously performed on less-realistic finite element models.

Interactions between ferrite recrystallization and austenite formation in high-strength steels

Using both experimental and modeling approaches, we give some clarifications regarding the mechanisms of interaction between ferrite recrystallization and austenite formation in cold-rolled high-strength steels. Using different thermal paths, we show that ferrite recrystallization and austenite formation can be strongly interdependent. The nature of the interaction (weak or strong) affects significantly the austenite formation and the resulting microstructure. We show that the kinetics of austenite formation depends intrinsically on both heating rates and the extent of ferrite recrystallization. An unexpected behavior of austenite growth was also seen at high heating rates. A possible explanation is presented based on the nature of the local equilibrium at the ferrite–austenite interface. The microstructure is more heterogeneous and anisotropic when both austenite formation and ferrite recrystallization overlap. A mechanism of microstructural formation is proposed, and this is supported by 2D simulations’ images.

Multiscale γ variant selection in a quaternary near-γ Ti–Al alloy

Classical electron back-scattered diffraction in a scanning electron microscope (EBSD/SEM) technique and a novel approach based on computer-aided collection and indexing of TEM Kikuchi patterns were used to study the γ variant and γ|γ misorientation distributions in lamellar grains of a near-γ TiAl alloy. Coarse and fine lamellar structures obtained by continuous cooling at two different rates were investigated. The two methods were complementary and reveal the presence of one-twin-dominant zones (OTDZ) in all grains of both samples. In agreement with previous works, these OTDZ were proposed to result from a mechanism of self-accommodation through twin-variant selection at the nucleation stage. Moreover, it has been pointed out that one γ -order variant often appears to be dominant in a given OTDZ. As the previous mechanism can hardly explain this experimental evidence, the presence of back-stresses due to grain confinement is proposed as a possible explanation. The statistical analysis of γ|γ misorientation distributions, de-correlated from the effect of the non-random distribution of γ variants, reveals that γ|γ interfaces in true-twin relationship are clearly predominant in the coarse lamellar structure, but not in the fine one. This difference is discussed on the basis of different growth mechanisms recently underlined and whose respective influences depend on the transformation driving force, i.e. on the cooling rate.

DIRECTIONALLY SOLIDIFIED MATERIALS: NICKEL-BASE SUPERALLOYS FOR GAS TURBINES

From the first forged turbine blades made of iron base alloys to the present nickel base single-grain turbine blades and vanes manufactured by directional solidification, an enormous amount of research has been directed to attaining the hottest possible combustion chamber temperatures in jet engines. Temperature has been increased by about 15 K each year for the last two decades, improving the thermodynamic efficiency of the engines. The more recent developments concern the manufacturing of single-grain parts made of nickel base superalloys with large amount of the γ′ hardening phase.

Initial stage of isothermal decomposition of austenite to ferrite and graphite in spheroidal graphite cast iron

AbstractThe isothermal decomposition of austenite to ferrite and graphite in a spheroidal graphite cast iron was investigated by dilatometry, metallography, and image analysis. The results gave quantitative, although indirect, information on the kinetics of carbon transfer from austenite to graphite, as well as on the kinetics of the austenite to ferrite and graphite transformation. The relationship between the kinetics and the graphite nodule count was highlighted. The incubation period before ferrite growth was found to be associated with a global contraction of the material, in spite of the related increase in the graphite volume fraction. Calculations showed good agreement with experimental results, except for the rate of carbon transfer, which was found to be much slower experimentally than predicted by calculations based on the volume diffusion of carbon. Possible reasons for this discrepancy are discussed.

A 10μm pitch interconnection technology using micro tube insertion into Al-Cu for 3D applications

Future 3-D applications require a very low pitch for inter-strata vertical interconnection. The last International Technology Roadmap for Semiconductors (ITRS) assessment for vertical interconnection predicts a need for decreasing the interconnection pitch to 10μm [1]. The room-temperature insertion technology has been proposed and developed using micro tubes as inserts [3] to address many assembling difficulties of industrial process. In the present work, we study the mechanical and electrical behavior of a single micro tube insertion into Al-0.5Cu pads. A modified nanoindenter with a very accurate load and displacement control is used, coupled with an electrical measurement device to qualify the insertion process. Finally, the best Die To Wafer (D2W) parameters are determined thanks to a composed experimental design.

Micro tube insertion into indium, copper and other materials for 3D applications

The viability of inter-strata low pitch vertical interconnection technologies is mandatory for 3D applications. In order to increase the connection complexity and to reach a 10μm or even smaller interconnection pitch, a new room-temperature insertion technology has been proposed and developed using micro tubes as inserts [1]. In the present work, we study the load required to obtain a full and efficient insertion of a single micro tube into indium, copper or other materials. We use a modified nanoindenter with a very accurate load and displacement control, to qualify the insertion process. The nature of the couple constituted by the insert and the soft metal is of major importance. Our study focuses on two specific parameters: Mechanical properties of the soft metal; Combined geometries of the soft metal and the insert. Thanks to this study, we can give the best conditions to improve insertion load and time for different couple. Finally, some first chips hybridizations are performed and tested mechanically and electrically to validate our work.

Effect of Intercritical Annealing Time on Microstructure and Mechanical Behavior of Advanced Medium Mn Steels

The study about the influence of intercritical annealing time on a cold rolled 0.1%C – 4.6%Mn (wt.%) steel was performed. The tensile tests show an interesting balance between strength and ductility especially after 7 hours annealing at 670°C. A part of this good result can be explained by the presence of rather high fraction of metastable austenite at room temperature. On the other hand a very complex microstructure combining lath-like and polygonal features was observed making the interpretation complicated.

Banded structures in dual-phase steels ― A novel characterization method

Abstract A simple to implement and original automated method is presented to quantify the intensity and wavelength of “banded structures”, i. e. microstructures presenting irregular and parallel bands enriched in a given phase. This method is based on the analysis of the covariance function of binary images. It is firstly tested on model representative images. It is compared with a conventional method derived from the ASTM E-1268 standard and appears to present a higher robustness with regard to preparation artifacts. Then, it is applied to real microstructures of dual-phase steels, which show more or less regular martensite-enriched bands in a ferrite matrix. The method is capable of efficiently discriminating dual-phase microstructures resulting from different thermo-mechanical routes.