Pascal Jacques

On the measurement of the nanohardness of the constitutive phases of TRIP-assisted multiphase steels

The nanohardness of the phases present in the microstructure of two TRIP (for TRansformation Induced Plasticity)-assisted multiphase steels differing by their silicon content was measured by nanoindentation in an atomic force microscope. It is observed that the softest phase in both steels is the ferritic matrix, followed by bainite, austenite and martensite. It is also shown that the silicon content of the steel grades is responsible for an increase of the hardness of the ferritic matrix due to solid solution strengthening. Finally, the influence of the preparation mode of the surface prior to the nanoindentation measurements has been investigated. An electropolishing stage after mechanical polishing is acceptable to allow valuable nanohardness measurements

Comparison of the effects of silicon and aluminium on the tensile behaviour of multiphase TRIP-assisted steels

Ghent University,Laboratory for Iron and Steelmaking, Technologiepark 9, B-9052 Ghent, Belgium(Received July 12, 2000)(Accepted in revised form November 21, 2000)Keywords: TRIP-steels; Microstructure; Phase transformations; Mechanical propertiesIntroductionMultiphase TRIP-assisted steels are a new generation of low alloy high strength steels that exhibitexceptional formability [1]. The remarkable strength to ductility balance results from the occurrenceduring testing of the Transformation Induced Plasticity (TRIP) phenomenon [2], which involves thestrain-induced transformation of austenite to martensite. The presence of austenite in the initialmicrostructure appears to be critical to the achievement of the desired properties. The retention ofaustenite is usually obtained by the combined effect of an appropriate chemistry and a typicalheat-treatment. In this respect, it is known that silicon and aluminium may both retard the kinetics ofcarbide formation and thus favour the austenite stabilisation by a bainitic holding stage [3]. Despite thisqualitative knowledge, very little literature can be found that rigorously compares the effect of siliconand aluminium on the austenite retention, on the extent of the TRIP effect, and on the resulting tensilebehaviour, all other chemical constituents have been kept constant [4]. The objective of this paper is toquantitatively assess the influence of aluminium and silicon contents, in view of the development ofmultiphase TRIP-assisted steels.Materials and Experimental ProcedureThe chemical compositions of the steels studied in this work are given in Table 1. Specific care wastaken to keep the same carbon content for each alloy. The slabs were initially hot and cold-rolled tothicknesses between 0.8mm and 1.0mm, following classical processing routes.The desired multiphase microstructure was obtained as displayed in Figure 1. The cold-rolledmaterial was first annealed for 4 minutes in the (a1g) region at a temperature 25°C above its Ac1temperature. It was then rapidly cooled and held at an intermediate temperature (i.e. between 375°C and450°C), where bainite formation takes place and contributes to the stabilisation of the austenite. Theheat-treatment was eventually interrupted by quenching the samples to room temperature. After the

Schottky barrier lowering with the formation of crystalline Er silicide on n-Si upon thermal annealing

The evolution of the Schottky barrier height (SBH) of Er silicide contacts to n-Si is investigated as a function of the annealing temperature. The SBH is found to drop substantially from 0.43 eV for the as-deposited sample to reach 0.28 eV, its lowest value, at 450 degrees C. By x-ray diffraction, high resolution transmission electron microscopy, and x-ray photoelectron spectroscopy, the decrease in the SBH is shown to be associated with the progressive formation of crystalline ErSi2-x.

Improvement of the Mechanical Properties of High Manganese Steels by Combination of Precipitation Hardening and Mechanical Twinning

Twinning-Induced Plasticity steels (TWIP steels) are extensively studied due to their ultra-high strain-hardening rate, that brings about a remarkable combination of ductility and strength. Twinning can be observed in high manganese-carbon steels. This paper considers hardening by combination of mechanical twinning with carbide precipitation. The kinetics of precipitation and the morphological evolution of carbides with annealing time were studied for two different TWIP steels with high manganese and carbon contents. The steels are first cold-rolled and then annealed at 800°C for recrystallization and carbide precipitation. Depending on the steel composition, the kinetics of precipitation and the morphology of the carbides are quite different. The influence of the annealing cycle on the mechanical properties has also been assessed. The results are used to discuss the influence of composition, stacking fault energy (SFE) and carbide precipitation on twinning. We show that the usual criteria based on the SFE only are not sufficient to characterize the twinning ability of a steel.

Interactions between recrystallisation and phase transformations during annealing of cold rolled Nb-added TRIP-aided steels

Fe and Fe-C based alloys present the exceptional feature that the processing route can be adapted to lead to various phases that present antagonist mechanical properties ranging from soft ferrite to high strength martensite. Among the different deformation mechanisms that can be exhibited by these phases, the TRIP effect brings about large enhancements of the work-hardening rate. The current TRIP-assisted multiphase steels present a ferrite-based matrix with a distribution of islands of bainite and retained austenite obtained at the end of specific thermal or thermomechanical treatments. The present study aims at characterising the interactions occurring between ferrite recrystallisation and austenite formation during the intercritical annealing of cold rolled Nb-added TRIP-aided steels. It is shown that the addition of niobium retards the ferrite recrystallisation during heating. As a consequence, ferrite may not be completely recrystallised before the nucleation and growth of the austenite grains. Strong interactions between these phenomena can then be observed, i.e. a strong hindering of the ferrite recrystallisation due to the austenite formation. Furthermore, the heating rate from room temperature to the intercritical temperature range influences the thermodynamic conditions prevailing at the ferrite / austenite interface and dictates the phase proportions.

Characterization of the alpha phase nucleation in a two-phase metastable beta titanium alloy

Beta titanium alloys are increasingly the best choice 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 because it presents excellent mechanical properties and a lower cost compared with other Ti alloys. The current study deals with the characterization of the nucleation and growth of the α phase in several thermomechanical processes, because the distribution and size of the α phase strongly influence the mechanical properties of the resulting microstructures. Several heat treatments were conducted after either cold rolling or annealing. The resulting microstructures were characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, or electron backscatter diffraction. It was observed that the morphology and the volume fraction of the α phase are strongly dependent on the holding temperature, on the heating or cooling rate, and on the β grain size.

Mechanism of Austenite Formation from Spheroidized Microstructure in an Intermediate Fe-0.1C-3.5Mn Steel

The austenitization from a spheroidized microstructure during intercritical annealing was studied in a Fe-0.1C-3.5Mn alloy. The austenite grains preferentially nucleate and grow from intergranular cementite. The nucleation at intragranular cementite is significantly retarded or even suppressed. The DICTRA software, assuming local equilibrium conditions, was used to simulate the austenite growth kinetics at various temperatures and for analyzing the austenite growth mechanism. The results indicate that both the mode and the kinetics of austenite growth strongly depend on cementite composition. With sufficiently high cementite Mn content, the austenite growth is essentially composed of two stages, involving the partitioning growth controlled by Mn diffusion inside ferrite, followed by a stage controlled by Mn diffusion within austenite for final equilibration. The partitioning growth results in a homogeneous distribution of carbon within austenite, which is supported by NanoSIMS carbon mapping.

In-situ measurements of load partitioning in a metastable austenitic stainless steel: Neutron and magnetomechanical measurements

In order to construct physically based models of the mechanical response of metastable austenitic steels, one must know the load partitioning between the austenite and the strain-induced martensitic phases. While diffraction-based techniques have become common for such measurements, they often require access to large facilities. In this work, we have explored a simple magnetic technique capable of providing a measure of the stresses in an embedded ferromagnetic phase. This technique makes use of the coupling between the elastic strain and the magnetic response of the

Improved simulation based HR-EBSD procedure using image gradient based DIC techniques

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