N. Bergeon

Atomic force microscope study of stress-induced martensite formation and its reverse transformation in a thermomechanically treated Fe–Mn–Si–Cr–Ni alloy

Abstract Consecutive observations of the stress-induced martensite formation and its reversion by atomic force microscopy have been carried out for the fcc/hcp transformation in the thermomechanically treated sample of an Fe–Mn–Si–Cr–Ni shape memory alloy. It is found that thin martensite plates of 0.1–0.2 μm thickness, which are the same martensite variant on the same habit plane, are formed one after another at the immediate neighbor of the existing martensite plate. These martensite plates make a group of several plates within the width of 1–2 μm. This formation mode of martensite is compared with those martensite plates observed by high resolution microscopy and optical microscopy and it is concluded that the basic mode of the stress-induced transformation is that each martensite plate is induced to relieve the shape strain of the existing martensite plate for all the observed magnifications ranging from several hundreds to several millions. The first martensite plate formation is presumed to occur at the pre-existing stacking fault in austenite. In the reverse transformation on heating, it is likely that each martensite plate is reverse-transformed one after another by reverse movement of the Shockley partial dislocations residing at the tip of the plate. This seems to be true for every range of the observable magnification from namometres to microns. Such a reverse transformation mode ensures a good shape memory effect in Fe–Mn–Si-based shape memory alloys.

Fragmentation in an Al–7 wt-%Si alloy studied in real time by X-ray synchrotron techniques

Abstract One mechanism for the formation of equiaxed grains is the detachment of dendrite fragments which is believed to be at the origin of the central equiaxed core region in casting processes. Unfortunately, the dynamics of the fragmentation phenomena cannot be revealed by classical methods. Investigation of a unrefined Al–7 wt-%Si alloy using in situ and real time synchrotron X-radiography and X-ray topography at the European Synchrotron Radiation Facility, has allowed verification of the existence of dendrite fragmentation and of cascade fragmentation during directional solidification, and to study the evolution of the growth and sedimentation of the equiaxed grains formed from these fragments. An examination of the crystallographic misorientation of dendrites as fragmentation is ongoing. These results contribute to the understanding of the characteristics of the columnar to equiaxed transition and to knowledge of the origin of new equiaxed grains in unrefined alloys.

Measurement of solute profiles by means of synchrotron X-ray radiography during directional solidification of Al - 4 wt% Cu alloys

In the present study, we report on an image analysis procedure, which enables to extract from synchrotron radiographs the long range solute profiles in the whole sample and in both phases (solid and liquid). This image analysis is based on the measurement of local density differences, and is applied to study the directional solidification of Al - 4wt% Cu alloy, from planar to onset of the initial instability. Dedicated experiments were carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France). In order to validate this analysis the value of a key solidification parameter, namely the partition coefficient, was experimentally determined during the planar solidification, and a very good agreement was found with value found usually in the literature. On a further step, the evolution of the microstructure and solute profile during the initial transient of solidification was analysed in detail.

Columnar-to-equiaxed transition in solidification processing of ALSi7 alloys in microgravity-the CETSOL project

This paper gives an overview of the experiments on-board the International Space Station (ISS) performed so far by the CETSOL team. Al-7 wt% Si alloys with and without grain refiners were solidified in microgravity. Detailed grain structure analysis showed columnar growth in case of non-refined alloy, but the existence of a columnar to equiaxed transition (CET) in refined alloy. One main result is a sharp CET when increasing the solidification velocity and a progressive CET for lowering the temperature gradient. Applying a front tracking model this behavior was confirmed numerically for sharp CET. Using a CAFE model both segregation and grain structures were numerically modeled and show a fair agreement with the experimental findings.

CET by Fragmentation during the Solidification under Natural and Forced Convection of Non-Refined Al-Based Alloys

The columnar to equiaxed transition (CET) has been widely studied for many years [1] because this phenomenon is observed in metallurgical applications like castings. In non refined alloys, detachment of dendrite fragments is the most probable mechanism responsible for the formation of an equiaxed microstructure [1]. In this frame, melt convection influences the grain structure evolution by playing a role in the fragmentation phenomena [2].

In situ and real-time analysis of the growth and interaction of equiaxed grains by synchrotron X- ray radiography

The phenomena involved during equiaxed growth are dynamic, so that in situ and real-time investigation by X-ray imaging is compulsory to fully analyse the microstructure formation. The experiments on Al - 10 wt% Cu alloy of this paper are carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France). Equiaxed growth was achieved in nearly isothermal conditions and continuously monitored from the very early stages of solidification to an asymptotic state. First, measurements of dendrite arms velocity for a same grain showed slight differences in the early stages of the growth. This effect is attributed to a gravity-related self - poisoning of the grain. Then, the propagation of primary dendrite arms was analysed and two successive growth regimes were observed. First, due to the relative distance with neighbour grains, each grain could be considered as isolated (i.e. growing freely) and tip growth rate gradually increased. In a subsequent phase, tip growth rate slowly decreased towards zero, due to the proximity of neighbouring grains. Using an image analysis technique, we were able to measure the solute profiles in the liquid phase between interacting arms. These measurements confirmed that solutal impingement is responsible for stopping the grain growth.

Effects of the Interface Curvature and Dendrite Orientation in Directional Solidification of Bulk Transparent Alloys

The properties of structural materials are to a large extent determined by the solid microstructure so that the understanding of the fundamental physics of microstructure formation is critical in the field of materials engineering. A directional solidification facility dedicated to the characterization of solid-liquid interface morphology by means of optical methods has been developed by CNES in the frame of the DECLIC project. This device enables in situ and real time studies on bulk transparent materials. The aim of the project is to perform experiments in microgravity to eliminate the complex couplings between solidification and convection and to get reliable benchmark data to validate and calibrate theoretical modeling and numerical simulations. Presently, ground experiments are performed to finalize the design and the experimental procedures and to guarantee the accuracy of the measurements. These experiments also provide reference data for the study of solidification microstructure dynamics in the presence of buoyancy-driven natural convection. Recent progress is presented concerning the control of the interface shape (critical for pattern analysis), the selection of single crystal of defined orientation (critical for dendritic growth) and the analysis of the dendrite shape.

In situ characterization of interface-microstructure dynamics in 3D-Directional Solidification of model transparent alloys

Microstructure plays a central role in determining properties of materials so that the fundamental understanding of the physics of microstructure selection is critical in the design of materials. Under terrestrial conditions fluid flow effects are dominant in bulk samples which preclude precise characterization of fundamental physics of microstructure selection. Experiments in thin samples, carried out to obtain diffusive growth, give microstructures that are neither 2D nor 3D. Rigorous theoretical models, using the phase-field method, have shown that the fundamental physics of pattern selection in 2D and 3D is significantly different. A benchmark experimental study is required in bulk samples under low gravity conditions. Also, the selection of microstructure occurs during the dynamical growth process so that in situ observations of spatio-temporal evolution of the interface shapes are required. Microgravity experiments on ISS are thus planned in a model transparent system by using a new Directional Solidification Insert (DSI), designed for use in the DECLIC facility of CNES and to be adapted to also fit ESA experiments. The critical aspects of hardware design, the key fundamental issues identified through 1g-experiments, the proposed experimental study on ISS, and the results of rigorous theoretical modeling are presented.