M. Véron

Directional coarsening of Ni-based superalloys: Computer simulation at the mesoscopic level

Directional coarsening of nickel based superalloys is modeled taking into account the role of anisotropic misfit relaxation by dislocations generated during creep. The directional coarsening is the response to the gradient in elastic energy induced by this anisotropic relaxation. A 3-dimensional computer simulation has been developed to describe both the morphologies and the kinetics of the phenomenon. Results of the simulation are presented and compared to experimental observations on an AM1 superalloy. Morphology maps describing the expected rafting geometry for other superalloys, as a function of misfit and applied stress, are established and discussed.

Strain induced directional coarsening in Ni based superalloys

Directional coarsening (or rafting) in Ni-based single crystal superalloys occurs after short times under stress at high temperature. This phenomenon results in a strongly anisotropic evolution of the microstructure that needs to be understood since it can occur in superalloy turbine blades during service. As the strain induced during creep seems to be responsible for the rafting phenomenon, it is worth studying the effect of a strain gradient on coarsened structures. A simple way to do this is to investigate the coarsening morphologies developed around an indentation.

Orientation and Phase Mapping in TEM Microscopy (EBSD-TEM Like): Applications to Materials Science

EBSD is a well known technique that allows orientation and phase mapping using an SEM. Although the technique is very powerful, has serious limitations related with a) special resolution limited to 50 nm (SEM-FEG) and b) specimen preparation issues as is not possible to obtain EBSD signal from rough surfaces or strained materials , nanoparticles etc.. To address those difficulties , a novel technique has been developed recently (EBSD-TEM like) allowing automatic orientation and phase mapping using template matching analysis of acquired diffraction patterns in TEM. Electron beam is scanned through the sample area of interest ; the acquired electron diffraction patterns from several sample locations are compared via cross-correlation matching techniques with pre-calculted simulated templates to reveal local crystal orientation and phases. The dedicated device (ASTAR) allows orientation and phase identification of crystallographic orientation in a region of interest up to 10µm2, with a step size ranging from 1nm to 20nm depending on the transmission electron microscope setting (FEG or LaB6).

Characterisation of the Phase Transformations in a Metastable β Titanium Alloy

beta titanium alloys are more and more best choice materials for automotive and aerospace applications due to their high performance to density ratio. Among these alloys, the TIMETAL Ti-LCB is already used in the automotive industry since it presents excellent mechanical properties and a lower cost compared to other Ti alloys. The present study deals with the characterisation of the nucleation and growth of the alpha phase during several thermomechanical processes since the distribution and size of the alpha grains strongly influence the mechanical properties of the resulting microstructures. Several heat treatments were conducted after annealing. The resulting microstructures were characterised by SEM, TEM, XRD or EBSD. It is observed that the morphology and the volume fraction of the alpha phase is strongly dependent on the holding temperature, on the heating or cooling rate and on the beta grain size.

Duplex Stainless Steel Microstructural Developments as Model Microstructures for Hot Ductility Investigations

Duplex stainless steels (DSS) are alloys made of ferrite and austenite, with a proportion of each phase around 50%. Their main advantage in comparison with other austenitic and ferritic stainless steels is the attractive combination of high strength and corrosion resistance together with good formability and weldability. Unfortunately, DSS often present a poor hot workability. This phenomenon can stem from different factors associated to the balance of the phases, the nature of the interface, the distribution, size and shape of the second phase, and possibly also from difference in rheology between ferrite and austenite. In order to determine the specific influence of phase morphology on the hot-workability of DSS, two austenite morphologies (E: Equiaxed and W: Widmanstätten) with very similar phase ratio have been generated using appropriate heat treatments. It was checked that the latter treatments generate stable microstructures so that subsequent hot mechanical tests are performed on the microstructures of interest. One microstructure consists of a ferritic matrix with austenitic equiaxed islands while the other microstructure is composed of a ferritic matrix with Widmanstätten austenite. The latter morphology corresponds to the morphology observed in as-cast slabs.

Characterization of the hot cracking resistance using the Essential Work of Fracture (EWF): application to duplex stainless steels

Duplex stainless steels (DSS) involve two ductile phases, i.e. ferrite and austenite, with a proportion of each phase around 50%. The main advantage in comparison with other austenitic and ferritic stainless steels is the excellent combination of high strength and corrosion resistance together with good formability and weldability. Unfortunately, DSS present in general a poor hot workability. Standard hot ductility tests like hot tensile or hot torsion tests are always helpful to compare the fracture resistance of two very ductile materials. A new method based on the essential work of fracture (EWF) concept has been used in order to determine the hot cracking resistance. The EWF concept was introduced to address ductile fracture based on the entire load-displacement response up to the complete fracture of a specimen and not from the initiation measurements such as in classical fracture mechanics concepts. The aim of the method consists in separating, based on dimensional considerations, the work performed within the plastic zone from the total work of fracture in order to provide an estimate of the work spent per unit area within the fracture process zone to break the material. This method proved to be very well adapted to high temperature cracking. Two different duplex stainless steels have been characterized by the essential work of fracture method. Examination of the fracture micrographs and profiles match the EWF results. This method turns out to be a discriminating tool for quantifying hot cracking and to generate a physically relevant fracture index to guide the optimization of microstructures towards successful forming operations.

Influence of Nb stabilization on the recovery and recrystallization kinetics of a ferritic stainless steel with soft magnetic properties for automotive applications

Usually niobium is added in ferritic stainless steels to avoid chromium carbides precipitation and then to improve corrosion resistance and to avoid embrittlemet. This study shows that a low Nb stabilization makes recrystallization nucleation much faster and prevents incomplete recrystallization. A qualitative interpretation, based on interaction with precipitates, is proposed and explains the main features of the softening kinetics as well as the microstructures obtained. Above a specific magnetizing frequency, the deformed state led to smaller losses than the recrystallized state. These results are believed to be attributed to a grain size effect. This leads to soft magnetic properties that makes 17%CrNb ferritic stainless steels a very interesting solution for the market of electromagnetic injection. Improving response-time of fuel injection valves is a great challenge for automotive industry in order to enhance car engine efficiency and to limit noxious gas emission.

Influence of Nb Stabilization on the Recovery and Recrystallization Kinetics of a Ferritic Stainless Steel: Consequences on Magnetic Losses

Softening kinetics of two 17% chromium (Cr) stainless steel grades that differ in niobium (Nb) content are compared. In the experiments, we observed that a low Nb stabilization makes recrystallization nucleation much faster and prevents incomplete recrystallization. A qualitative interpretation, based on interaction with precipitates, is proposed and explains the main features of the softening kinetics as well as the microstructures obtained. For the Nb stabilized grade, magnetic losses were measured in the deformed state and after recrystallization. Above a specific magnetizing frequency, the deformed state led to smaller losses than the recrystallized state. These results are believed to be attributed to a grain size effect.