Jean Ladriere

Bio: Jean Ladriere is an academic researcher from Catholic University of Leuven. The author has contributed to research in topics: Martensite & Ferrous. The author has an hindex of 3, co-authored 3 publications receiving 321 citations.

On the Influence of Interactions between Phases on the Mechanical Stability of Retained Austenite in Transformation-Induced Plasticity Multiphase Steels

Abstract: The mechanical stability of dispersed retained austenite, i.e., the resistance of this austenite to mechanically induced martensitic transformation, was characterized at room temperature on two steels which differed by their silicon content. The steels had been heat treated in such a way that each specimen presented the same initial volume fraction of austenite and the same austenite grain size. Nevertheless, depending on the specimen, the retained austenite contained different amounts of carbon and was surrounded by different phases. Measurements of the variation of the volume fraction of untransformed austenite as a function of uniaxial plastic strain revealed that, besides the carbon content of retained austenite, the strength of the other phases surrounding austenite grains also influences the austenite resistance to martensitic transformation. The presence of thermal martensite together with the silicon solid-solution strengthening of the intercritical ferrite matrix can “shield” austenite from the externally applied load. As a consequence, the increase of the mechanical stability of retained austenite is not solely related to the decrease of the M s temperature induced by carbon enrichment.

Relatively unoxidized vivianite in limnic coal from Capeni, Baraolt Basin, Romania

Abstract: Vivianite in limnic coal from Capeni, Baraolt Basin, Romania, is partially oxidized, despite the strongly reducing environment. The main reflections of its X-ray powder pattern may be indexed on a monoclinic cell with a 10.037(10), b 13.464(9), c 4.723(5) Angstrom and beta 102.55(4)degrees (space group I2/m) or a 10.113(14), b 13.464(9), c 4.723(5) Angstrom and beta 104.38(3)degrees (space group C2/m). The thermal analyses, taken in air, show effects attributable to the oxidation of Fe2+ to Fe3+ ions, i.e., the splitting of the first endothermic effect at about 190 degrees C, the presence of a supplementary exothermic peak at 270 degrees C on the DTA curve, and a gradual dehydration on the TGA curve. The Mossbauer spectrum consists, however, of four quadrupole doublets associated with two sites occupied by ferrous iron and two occupied by ferric iron. Approximately 13% of Fe(2) and 15% of Fe(1) are oxidized to Fe3+. The infrared absorption spectrum shows a splitting of the fundamental H-O-H stretching at 3000-3500 cm(-1), as well as the absence of an (OH) band at about 3370 cm(-1), confirming a slight oxidation of the sample analyzed. Chemical analyses show that only 18 to 22% of the iron is oxidized to Fe3+ and that less than 7.2% of the octahedra are occupied by cations other than iron. On the basis of the geological setting and trace-element chemistry, diagenetic formation in anoxic low-sulfide sediments is indicated. Partial oxidation is due to exposure to air following collection.

High dislocation density–induced large ductility in deformed and partitioned steels

Abstract: A wide variety of industrial applications require materials with high strength and ductility. Unfortunately, the strategies for increasing material strength, such as processing to create line defects (dislocations), tend to decrease ductility. We developed a strategy to circumvent this in inexpensive, medium manganese steel. Cold rolling followed by low-temperature tempering developed steel with metastable austenite grains embedded in a highly dislocated martensite matrix. This deformed and partitioned (D and P) process produced dislocation hardening but retained high ductility, both through the glide of intensive mobile dislocations and by allowing us to control martensitic transformation. The D and P strategy should apply to any other alloy with deformation-induced martensitic transformation and provides a pathway for the development of high-strength, high-ductility materials.

Effect of microstructure on the stability of retained austenite in transformation-induced-plasticity steels

Abstract: Two Fe-0.2C-1.55Mn-1.5Si (in wt pct) steels, with and without the addition of 0.039Nb (in wt pct), were studied using laboratory rolling-mill simulations of controlled thermomechanical processing. The microstructures of all samples were characterized by optical metallography, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The microstructural behavior of phases under applied strain was studied using a heat-tinting technique. Despite the similarity in the microstructures of the two steels (equal amounts of polygonal ferrite, carbide-free bainite, and retained austenite), the mechanical properties were different. The mechanical properties of these transformation-induced-plasticity (TRIP) steels depended not only on the individual behavior of all these phases, but also on the interaction between the phases during deformation. The polygonal ferrite and bainite of the C-Mn-Si steel contributed to the elongation more than these phases in the C-Mn-Si-Nb-steel. The stability of retained austenite depends on its location within the microstructure, the morphology of the bainite, and its interaction with other phases during straining. Granular bainite was the bainite morphology that provided the optimum stability of the retained austenite.