Leo A.I. Kestens

Bio: Leo A.I. Kestens is an academic researcher from Ghent University. The author has contributed to research in topics: Recrystallization (metallurgy) & Electron backscatter diffraction. The author has an hindex of 38, co-authored 272 publications receiving 4773 citations. Previous affiliations of Leo A.I. Kestens include Delft University of Technology & MSE.

Effect of fresh martensite on the stability of retained austenite in quenching and partitioning steel

Abstract: Restrictions on fuel consumption and safety in the automotive industry have stimulated the development of quenching and partitioning (Q&P) steel. This steel is expected to have very high strength in combination with acceptable ductility owing to its microstructure consisting of martensite with a considerable amount of retained austenite. The effect of retained austenite on the mechanical properties and its transformation stability were determined by stepwise uniaxial micro-tensile testing and subsequent electron backscatter diffraction (EBSD) study of a pre-selected region. The austenite fraction evolution with increasing plastic deformation and the influence of fresh martensite on the local strain distribution were quantified based on the orientation data. The decrease of the retained austenite as a function of the applied strain was described by an exponential function with the pre-exponential and exponential factors related to the starting austenite fraction and its transformation stability respectively. It was proven that the presence of fresh martensite has a negative influence on this austenite transformation stability due to its constraining effect on the strain distribution. This effects the mechanical properties manifested by changes in the strain hardening behavior and total elongation. The results suggest that the ductility of the Q&P steels can be improved by an appropriate design of the heat treatment schedule in order to ensure high retained austenite fractions without the presence of fresh martensite in the final microstructure.

Microstructure and texture of a lightly deformed TRIP-assisted steel characterized by means of the EBSD technique

Abstract: The microstructural and textural changes after a tensile strain of 10% were observed by orientation contrast measurements in a TRIP-assisted steel. On the undeformed samples it was shown that the electron back scatter diffraction (EBSD) technique could be used successfully for determining the volume fraction of the microstructural constituents bainite, ferrite and austenite, whereas after deformation only the BCC and FCC phases could be separated. The results show that the tensile strain of 10% gave rise to a drop in residual austenite content from 10 to 4%, which was also confirmed by magnetic measurements. The texture data showed only minor orientation rotations after 10% tensile strain for the BCC ferrite and bainite grains, whereas the residual austenite did show a significant texture change. By meticulously monitoring the local intra-granular misorientations it was concluded that the BCC phases (ferrite and bainite) took up the larger part of the nominal strain whereas the residual austenite primarily responded to the mechanical load by a partial (stress-induced) martensite transformation. Hence, the texture change observed in the residual austenite could be attributed to the orientation selective character of the phase transformation.

Texture formation in metal alloys with cubic crystal structures

Abstract: The present paper gives a concise overview of a number of current issues in the literature on texture formation in alloys with cubic crystal structures, mainly steel and aluminium alloys. As crystallographic texture determines to a large extent the anisotropy of material properties, it is of paramount importance to understand and control the physical mechanisms by which the texture is formed in the subsequent stages of metals manufacturing processes. In the present overview three key solid-state transformation processes are considered: allotropic phase transformations, plastic deformation and recrystallisation. The intention is to highlight a number of key elements in the literature and some recent tendencies, which may provide some insight to scientists and engineers dealing with texture issues in daily practice.

Current issues in recrystallization: a review

Abstract: The current understanding of the fundamentals of recrystallization is summarized. This includes understanding the as-deformed state. Several aspects of recrystallization are described: nucleation and growth, the development of misorientation during deformation, continuous, dynamic, and geometric dynamic recrystallization, particle effects, and texture. This article is authored by the leading experts in these areas. The subjects are discussed individually and recommendations for further study are listed in the final section.

High manganese austenitic twinning induced plasticity steels: A review of the microstructure properties relationships

Abstract: A significant increase in the research activity dedicated to high manganese TWIP steels has occurred during the past five years, motivated by the breakthrough combination of strength and ductility possessed by these alloys. Here a review of the relations between microstructure and mechanical properties is presented focusing on plasticity mechanisms, strain-hardening, yield stress, texture, fracture and fatigue. This summarized knowledge explains why TWIP steel metallurgy is currently a topic of great practical interest and fundamental importance. Finally, this publication indicates some of the main avenues for future investigations required in order to sustain the quality and the dynamism in this field.

Structure–properties relationship in TRIP steels containing carbide-free bainite

Abstract: The purpose of the present contribution is to review the current knowledge about the relationship between the micro-structure of cold rolled intercritically annealed low alloy TRIP-aided sheet steels and their mechanical properties from a materials engineering point of view. The focus is on their production in existing industrial lines and on their application in the manufacture of passenger cars with a body-in-white which offers an improved passive safety. The review aims to make clear that although low alloy TRIP-aided sheet steel is by now starting to be an established structural material in BIW manufacturing, there is still room for the further optimization of the composition and the processing. In addition, there are still a number of problems related to their physical metallurgy that require a better fundamental understanding.