Mohamed Gouné

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.

Additive manufacturing of metals: a brief review of the characteristic microstructures and properties of steels, Ti-6Al-4V and high-entropy alloys

Abstract We present a brief review of the microstructures and mechanical properties of selected metallic alloys processed by additive manufacturing (AM). Three different alloys, covering a large range of technology readiness levels, are selected to illustrate particular microstructural features developed by AM and clarify the engineering paradigm relating process–microstructure–property. With Ti-6Al-4V the emphasis is placed on the formation of metallurgical defects and microstructures induced by AM and their role on mechanical properties. The effects of the large in-built dislocation density, surface roughness and build atmosphere on mechanical and damage properties are discussed using steels. The impact of rapid solidification inherent to AM on phase selection is highlighted for high-entropy alloys. Using property maps, published mechanical properties of additive manufactured alloys are graphically summarized and compared to conventionally processed counterparts.

Atom probe tomography evidence of nitrogen excess in the matrix of nitrided Fe–Cr

Excess nitrogen uptake is a key point in the understanding of nitriding process. The increased nitrogen solubility in the matrix of nitrided Fe–Cr alloys is recognised as one possible source of nitrogen excess, and investigated by means of atom probe tomography. The nitrogen concentration in the matrix of as-nitrided specimens, containing a high density of nanometre scale chromium nitride platelets, is measured significantly higher than the one in re annealed samples, and than in the calculated equilibrium values. These results bring direct experimental evidence that the additional amount of nitrogen dissolved in the matrix of nitrided alloys is a significant part of the so-called excess nitrogen.

A criterion for the change from fast to slow regime of cementite dissolution in Fe - C- Mn steels

The present study clarifies the role of Mn in cementite on the driving force of cementite dissolution and the growth of austenite. From an experimental study, the effects of manganese composition and temperature on the cementite dissolution were shown. From a theoretical analysis based on thermodynamic and kinetics considerations, a criterion for the change from fast to slow regime of cementite dissolution was proposed. This criterion is in good agreement with the experimental results. It can be easily calculated and can define the composition and temperature ranges where the cementite dissolution is slow or fast.

Application of the stagnant stage concept for monitoring Mn partitioning at the austenite-ferrite interface in the intercritical region for Fe–Mn–C alloys

The length of the stagnant stage during the new ferrite growth starting from a mixture of austenite and ferrite has been investigated for a Fe-0.17Mn-0.023C (wt%) alloy. It was found that the stagnant stage depends on the thermal path followed to create the mixture, and deduce that the tie-lines for austenite to ferrite transformation are quite different from those for ferrite to austenite transformation. The length of the stagnant stage is determined by the very local partitioning effect at the interface, and it can be used as a tool to monitor the Mn partitioning.

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.

Static and dynamical ageing processes at room temperature in a Fe25Ni0.4C virgin martensite: effect of C redistribution at the nanoscale

The work-hardening behaviour of virgin martensitic steel has been investigated in a strictly un-aged state and after various ageing conditions. At room temperature (RT), the un-aged alloy shows astonishing tensile performances (ultimate tensile stress = 1600 MPa/uniform elongation = 15%) but unexpected serrations. These serrations can be suppressed by static ageing (at RT or higher) while maintaining the initial work-hardening rate (ageing at RT). Parallel investigations using atom probe tomography reveal that the distribution of carbon at the atomic scale evolves from purely homogeneous for virgin martensite to partly segregated at a very fine scale (5–10 nm) after static ageing. This particular mechanical behaviour can therefore be associated with a very local decrease in available carbon in solid solution due to redistribution and segregations on defects (nanotwins) that occurs rapidly, even after few days at RT.

Superledge Model for Interphase Precipitation During Austenite-to-Ferrite Transformation

A model for interphase precipitation has been developed based on the ledge mechanism of austenite-to-ferrite transformation. Carbide precipitation is considered on the migrating ferrite/austenite interface as an interaction of transformation and precipitation kinetics. The derived equations describe sheet spacing and particle spacing of interphase-precipitated carbides as well as the overall interface velocity which are related to the nucleation rates of carbides and ferrite ledges, respectively. The microstructure characteristics of interphase precipitation are predicted as a function of transformation temperature and steel composition and replicate trends observed experimentally.

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.

On the Origin of the Enhanced Creep Resistance in Underaged Al-Cu Based Alloys

It has recently been observed that the creep resistance of Al-Cu based precipitation hardened alloys may be enhanced through use of the underaged temper. We have treated this problem theoretically by considering the motion of a dislocation through a precipitation hardened structure and discuss the physical origin of the enhanced creep resistance in the underaged condition. The variation in expected creep resistance as a function of aging treatment is calculated and the possible generality of the experimental observations is considered.