Papers by Keyword: Martensite

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Authors: Fujio Abe
Abstract: The effect of fine precipitates, excess dislocations and sub-boundary hardening on creep strain behavior in the transient region has been investigated for tempered martensitic 9%Cr steel at 600 and 650oC. The fine precipitates that form during tempering or during creep decrease the creep rate in the transient region, while excess dislocations produced by cold rolling promote the recovery of dislocations during creep, resulting in higher creep rates. The sub-boundary hardening is enhanced by fine precipitates along lath and block boundaries, which retards the onset of acceleration creep. The movement and annihilation process of dislocations in the transient region is controlled by not only the movement of dislocations in the matrix but also the absorption of dislocations at boundaries. The minimum creep rate is basically determined by the time to minimum creep rate.
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Authors: Marton Benke, Valéria Mertinger, Ferenc Tranta
Abstract: A group of austenitic steels exhibit high deformability and strength due to TRansformation Induced Plasticity (TRIP) and/or TWinning Induced Plasticity (TWIP). The phase transformations of the TRIP and TWIP steels have been examined in details in many FeMnX alloy systems (X: Ni, Al, Si). However, less attention was given to the FeMn(Cr) alloys. The γ ↔ ε transformations in the austenitic FeMn(Cr) alloys have been examined during heat cycling by in situ optical microscopy and DSC measurements.
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Authors: Le Zhou, Anit Giri, Kyu Cho, Yong Ho Sohn
Abstract: The ferromagnetic shape memory and magnetocaloric properties of NiMnGa alloys are closely related to the martensitic transformation from high temperature austenitic phase to low temperature martensitic phase. The transformation temperature and the resulting microstructure and crystallography of the martensites can be very complex, but are crucial to the optimization of the material performance. A combinatorial study with a series of solid-to-solid diffusion couples and various characterization techniques, including scanning electron microscopy, focused ion beam, transmission electron microscopy, electron probe microanalysis, and nanoindentation, was carried out to investigate the microstructural and crystallographic development, and mechanical properties in NiMnGa alloys. Both austenitic and martensitic phases were found at room temperature in each diffusion couple with a clear interphase boundary. Crystallographic variations in martensitic phase, including non-modulated (NM) martensite and modulated (5M or 7M) martensite, were found in the diffusion couples. All martensitic microstructure consists of variants with different orientations and the twinning relationship. A decrease of reduced elastic modulus (Er) was observed with Ni substituting for Ga in the austenitic phase. However, an opposite trend of an increase in Er was found in the martensitic phase. The softening of the elastic constants near the vicinity of martensitic transformation contributed to a sharp decrease in Er near the interphase boundary. The measured Er had a larger scatter for the martensitic phase than that for the austenitic phase.
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Authors: Dong Ying Ju, Ryuji Mukai, Nobuaki Minakawa, Yukio Morii, Atsushi Moriai
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Authors: Richard G. Thiessen, Jilt Sietsma, I.M. Richardson
Abstract: This work presents a unique approach for the modelling of the austenitisation of martensite in dual-phase steels within the phase-field method. Driving forces for nucleation and growth are derived from thermodynamic databases. Routines for nucleation are based on a discretisation of the classical nucleation theory. Validation is given via dilatometric experiments.
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Authors: Tansel T. Arif, Rong Shan Qin
Abstract: The phase field method is rapidly becoming the method of choice for simulating the evolution of solid state phase transformations in materials science. Within this area there are transformations primarily concerned with diffusion and those that have a displacive nature. There has been extensive work focussed upon applying the phase field method to diffusive transformations leaving much desired for models that can incorporate displacive transformations. Using the current model, the formation of martensite, which is formed via a displacive transformation, is simulated. The existence of a transformation matrix in the free energy expression along with cubic symmetry operations enables the reproduction of the 24 grain variants of martensite. Furthermore, upon consideration of the chemical free energy term, the model is able to utilise both the displacive and diffusive aspects of bainite formation, reproducing the autocatalytic nucleation process for multiple sheaves using a single phase field variable. Transformation matrices are available for many steels, one of which is used within the model.
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Authors: Christian Lexcellent, Elie Gibeau
Abstract: At first, some comments are made concerning the capacity of prediction of the microstructure for shape memory alloys by the Crystallographical Theory of Martensite. Secondly, the basic foundations of the phenomenological modeling of shape memory alloys behavior at the macroscopic scale are given. A special attention is devoted to the yield surface of phase transformation initiation in the stress space and its convex dual: the set of effective transformation strains in the strain space.
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Authors: T.Y. Hsu
Abstract: In order to diminish the industrial pollution to maintain the sustainable development and to reduce the cost of the steel production, a unified technology combining plastic forming and heat treatment for some steel parts production is suggested. This article mainly concerns part theoretical foundation of such technology, i.e. the thermodynamic and kinetic models of the ferrite and pearlite transformations under external stress. Simulation of the ferrite fraction after continuous cooling under stress in a low-alloyed steel is presented. The effects of stresses on bainitic and martensitic transformations are also briefly introduced. The unified technology seems favorable to be realized in manufacturing practice.
31
Authors: Vyacheslav Gundyrev, Vitaly Zel'dovich
Abstract: The criterion of an estimation of a reality of the mechanism of deformation of a lattice at martensite transformation is proposed. The most real mechanism of deformation of a lattice in FeNi alloy with the twinned martensite is determined. The Kurdjumov-Sachs shear on a plane (111) fcc in a direction [11-2] fcc is the basic component of deformation of a lattice in the given mechanism.
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Authors: D.V. Edmonds
Abstract: Recent decades have witnessed some remarkable advances in engineering steels driven by the need to respond to challenges posed, for example, by recovery and transmission of oil and gas, or enhanced vehicle safety and fuel economy. Foremost amongst these must surely be the extended application of carbon steels, achieved principally through ferrite grain refinement by the practice of microalloying coupled with controlled thermomechanical processing. Limitations to strengthening ferrite/pearlite structures further by grain refinement or precipitation, however, has focused attention back to acicular forms of microstructure. One of the most interesting advances in this area has been the development of bainitic steels, which have been almost dormant since the mid-20th century. This resurgence may partly be attributed to a better appreciation of the bainite transformation mechanism, and the experimental work for this which unexpectedly spawned some interesting bainitic microstructures which have seen further development and application. These are the so-called ‘carbide-free’ bainites, which employ alloying to replace carbides, principally cementite, with carbon-stabilized retained austenite. Particularly noteworthy has been the emergence of the transformation induced plasticity (TRIP) sheet steels with enhanced properties principally targeted for automotive use. It is worth mentioning also that a parallel development has produced similar microstructure in austempered ductile irons (ADI), another important ferrous alloy which has seen recent expanding interest in its application. Even more recently, as we proceed into the 21st century, the concept of employing steel microstructures containing carbon-enriched retained austenite, has been developed further by combining both alloying and novel heat treatment procedures to exchange ‘bainitic’ ferrite with ‘martensitic’ ferrite. Interestingly, this non-equilibrium ‘quenching and partitioning’ process route also offers the possibility to increase the retained austenite carbon concentration to very high levels, potentially revealing new and previously unobtainable properties.
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