Papers by Keyword: Martensitic Transformation (MT)

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Authors: Thomas Rieger, Klaus Herrmann, Dagmar Carmele, Stephan Meyer, Thomas Lippmann, Andreas Stark, Wolfgang Bleck, Uwe Klemradt
Abstract: The ‘Quenching and Partitioning’ (Q&P) concept aims to increase the strength level of conventional TRIP-assisted advanced high strength steel (AHSS) by replacing ferritic constituents by tempered martensite. The Q&P heat treatment process involves austenitization and interrupted quenching followed by carbon partitioning from martensite to austenite at elevated temperatures. The final microstructure is traditionally investigated at room temperature after metallographic preparation by microscopy and x-ray analysis with laboratory tubes. Besides other disadvantages the established characterization methods are not adequate to observe the development of the microstructure during Q&P treatment. In the present work the microstructural evolution during Q&P processing was monitored by in-situ diffraction experiments using very hard (100 keV) synchrotron x-ray radiation. Debye-Scherrer rings were recorded as a function of time and temperature during the heat treatment in a state-of-the-art dilatometer (type Bähr DIL805AD) at the Engineering Materials Science beamline HARWI-II (HZG outstation at Deutsches Elektronensynchrotron (DESY), Hamburg). The diffraction patterns contain quantitative information on the phases present in the sample (for more details cf. Abstract Carmele et al, this conference). The evolution of the austenite phase fraction during the partitioning treatment at the quench temperature (1-step Q&P) is discussed exemplarily for a Si-based TRIP steel with additions of Ni.
713
Authors: Howard J. Stone, Harshad K.D.H. Bhadeshia, Philip J. Withers
Abstract: The level of residual stresses generated in fusion welds has been a major area of interest for many years. For steels, a major influence on the final state of stress is through martensitic transformation. This is because the martensitic transformation is accompanied by significant shear and volume strains. One way to mitigate the development of residual stress is by controlling the onset of the transformation such that the associated strain is able to compensate for thermal contraction all the way down to ambient temperatures. In the past it has only been possible to follow the evolution of the phase transformation during cooling of the weld metal using indirect methods such as dilatometry and differential scanning calorimetry. This paper describes the first work in which the phases present are characterized directly during the cooling of reheated weld metal at conditions typical of those encountered during welding by installing a thermomechanical simulator on a synchrotron diffraction beam line at ESRF.
393
Authors: Hong Tao Liu, Guang Ai Sun, Yan Dong Wang, Bo Chen, Xiao Lin Wang, Qiang Tian
Abstract: The stress-induced martensitic transformation in Ni47Ti44Nb9 was examined using X-ray diffraction (XRD) during in situ uniaxial loading and unloading. A new martensitic (020) peak in XRD patterns is observed under strain from 10% to 12%. It indicates that the martensitic texture has reached the optimum orientation. After unloading, approximately 8% irreversible strain still remains. It is associated with the reorientation of martensites and the plastic deformation of the B2-phase. In addition, the deformed β-Nb particles would also reduce the driving force for the reverse transformation. The details of lattice-strain and shared applied stress (SAS) in the B2-phase and β-Nb phase are also discussed in this work.
1096
Authors: Hemantha Kumar Yeddu, John Ågren, Annika Borgenstam
Abstract: Complex martensitic microstructure evolution in steels generates enormous curiosity among the materials scientists and especially among the Phase Field (PF) modeling enthusiasts. In the present work PF Microelasticity theory proposed by A.G. Khachaturyan coupled with plasticity is applied for modeling the Martensitic Transformation (MT) by using Finite Element Method (FEM). PF simulations in 3D are performed by considering different cases of MT occurring in a clamped system, i.e. simulation domain with fixed boundaries, of (a) pure elastic material with dilatation (b) pure elastic material without dilatation (c) elastic perfectly plastic material with dilatation having (i) isotropic as well as (ii) anisotropic elastic properties. As input data for the simulations the thermodynamic parameters corresponding to Fe - 0.3% C alloy as well as the physical parameters corresponding to steels acquired from experimental results are considered. The results indicate that elastic strain energy, dilatation and plasticity affect MT whereas anisotropy affects the microstructure.
1066
Authors: Ti Kun Shan, Li Liu
Abstract: An enhanced elastic-plastic constitutive equation taking into account strain induced transformation and its effect on work hardening of TRIP steel during deformation are investigated. The transformation rate relies on the stress triaxiality. The strain hardening of the TRIP steel takes on parabola shape because of the austenite changed to the martensite during straining. The physical model is verified by comparing with the stress-strain relation of the uniaxial tension experiment. The results showed that the steel keeps a high hardening potential which retards the onset of necking and a good formability thanks to the martensitic strain-induced transformation and the subsequent austenite hardening.
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Authors: Julia Ivanisenko, Ian MacLaren, Ruslan Valiev, Hans Jorg Fecht
Abstract: Recent studies of nanocrystalline materials have often found that the deformation mechanisms are radically different to those in coarse-grained materials, resulting in quite different mechanical properties for such materials. The use of pearlitic steels for the study of the deformation mechanisms in bcc materials with ultrafine grain sizes is quite convenient, because it is relatively straightforward to obtain a homogenous nanocrystalline structure with a mean grain size as small as 10 nm using various modes of severe plastic deformation (SPD). In this paper we show that highpressure torsion of an initially pearlitic steel results in a nanostructured steel in which austenite has been formed at or close to room temperature. The orientation relationship between neighboring ferrite and austenite grains is the well-known Kurdjumov-Sachs orientation relationship, i.e. the same observed in temperature-induced martensitic transformation of iron and steels. It is shown that this must have resulted from a reverse martensitic transformation promoted by the high shear strains experienced by the material during severe plastic deformation of the nanocrystalline structure. This transformation represents an alternative deformation mechanism that can be activated when conventional deformation mechanisms such as slip of lattice dislocations become exhausted.
439
Authors: Xue Jun Jin, M. J. Jin, J. Y. Liu, G. L. Fan
Abstract: Strain glass is a new type of glass state discovered recently in Ni-rich Ti-Ni ferroelastic alloys. It is formed by doping sufficient point defects such as solute atoms or alloying elements into a martensitic alloy to destroy the long range strain order generated by the diffusionless martensitic phase transformation. The strain glass was observed in an off-stoichiometric Heusler Au7Cu5Al4 alloy by the mechanical spectrum analysis. The martensitic transformation was experimentally confirmed following the occurrence of strain glass transition during the cooling. It is different from the model developed mainly in Ni-Ti based alloys among which the martensitic transformation was totally suppressed once the strain glass transition takes place. We conclude that newly observed strain glass in an off-stoichiometric Au7Cu5Al4 alloy belongs to one type of precursor phenomena.
361
Authors: M. Inami, Takahiro Ishii, Xiao Bing Ren, Kazuhiro Otsuka
397
Authors: Jian Jun Xu, Jin Fang Guo, De Guo Wang, Li Shan Cui
357
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