Papers by Keyword: Phase Transformation

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Authors: Davi Munhoz Benati, Kazuhiro Ito, Kazuyuki Kohama, Hajime Yamamoto, Eugênio José Zoqui
Abstract: Fe-2.5C-1.5Si gray cast iron evaluated in previous works exhibited promising potential as semisolid raw material presenting low levels of maximum stress and viscosity, similar to Al-Si alloys. This work is intended to investigate phase transformations and liquid phase formation for the Fe-2.5C-1.5Si gray cast iron in order to understand the performance of the alloy during the semisolid processing. Thus in situ heating experiments via high temperature laser scanning confocal microscopy were performed to analyze the solid-to-liquid transition. At room temperature alloy presented a matrix of pearlite and ferrite with type D flake graphite. During the heating process the main transformations observed were graphite precipitation on the austenite grain boundaries, graphite precipitates and flakes graphite growing and coarsening with the increasing of temperature and the beginning of melt around 1140°C. Coarsened flakes at high temperatures resulted in a liquid continuous network after melting, thereby the liquid phase was formed surrounding and wetting homogeneously the solid phase. This favors the detachment of grains from each other and leads to the intended solid globules immersed in liquid.
Authors: Shan Dai, Jin Gang Qi, Zuo Fu Zhao, Jian Zhong Wang
Abstract: In the unmodified and EP-modified melt of Q235 steel, the solidification behavior including the nucleation and growth of crystals has been observed in situ by use of a confocal scanning laser microscope combined with an infrared image furnace, and meanwhile, described the characteristic of phase transformation. The result shows that the every stage of phase transformation is advanced; the phase transformation time is shortened, and temperature is reduced during heating proceeds. On the other hand, it can be observed that the nucleation density of solid phase increases and has an even distribution among the liquid phase.
Authors: Guillaume Geandier, Moukrane Dehmas, Mickael Mourot, Elisabeth Aeby-Gautier, Sabine Denis, Olivier Martin, Nikhil Karnatak
Abstract: In situ high energy X-ray diffraction synchrotron was used to provide direct analysis of the transformation sequences in steel-based matrix composite (MMC) reinforced with TiC particles. Evolution of the phase fractions of the matrix and TiC particles as well as the mean cell parameters of each phase were determined by Rietveld refinement from high energy X-ray diffraction (ID15B, ESRF, Grenoble, France). In addition, some peaks were further analysed in order to obtain the X-ray strain during the cooling step. Non-linear strain evolutions of each phase are evidenced, which are either associated with differences in the coefficient of thermal expansion (CTE) between matrix and TiC particle or to the occurrence of phase transformation. Micromechanical calculations were performed through the finite element method to estimate the stress state in each phase and outline the effects of differences in CTE and of volume change associated with the matrix phase transformation. The calculated results led to a final compressive hydrostatic stress in the TiC reinforcement and tensile hydrostatic stress in the matrix area around the TiC particles. Besides, the tendencies measured from in situ synchrotron diffraction (mean cell parameters) matched with the numerical estimates.
Authors: Anja Weidner, Harry Berek, Christian Segel, Christos G. Aneziris, Horst Biermann
Abstract: Composite material on the basis of a TRIP (transformation induced plasticity) steel with zirconia particles as reinforcement was produced by powder metallurgical technology and conventional sinter process. The goal of such type of material is to obtain exceptional mechanical properties like high deformation energy absorption due to the combination of martensitic phase transformations both in steel and ceramic. The steel matrix was made of the commercial steel AISI 304, which shows a deformation-induced martensitic phase transformation from the austenitic phase (fcc) into the α’-martensite (bcc). The zirconia particles were partially stabilized with MgO and show a stress-assisted martensitic phase transformation from the tetragonal to the monocline phase. Flat specimens were tensile deformed in-situ in a scanning electron microscope in order to follow the damage behaviour of the material. Some zirconia particles were characterized before and after tensile testing both by backscattered electron contrast as well as by electron backscatter diffraction (EBSD) in combination with energy dispersive X-ray spectroscopy (EDS).
Authors: Julietta V. Rau, Marco Fosca, Vladimir S. Komlev
Abstract: In situ monitoring of structural changes, taking place upon calcium phosphate bone cements hardening process was carried out by means of the Energy Dispersive X-Ray Diffraction method. Two different cement systems were studied, one of them based on the octacalcium phosphate and another - on the dicalcium phosphate dehydrate. Both systems contained natural biopolymer chitosan and were soaked in Simulated Body Fluid. The obtained experimental results evidence that during the hardening of the cement containing octacalcium phosphate its partial transformation into hydroxyapatite takes place, whereas no significant changes were detected during the hardening process of cement containing the dicalcium phosphate dehydrate.
Authors: Ru Lin Peng, Xiao Peng Liu, Yan Dong Wang, Shu Yan Zhang, Yong Feng Shen, Sten Johansson
Abstract: In-situ neutron diffraction experiments under tensile loading were carried out to study the micromechanical behaviour of two iron-manganese based steels, a TWIP (twinning induced plasticity) steel with 30 wt% Mn and a TRIP steel (transformation induced plasticity) with 20 wt% Mn. The former was loaded to 31.3% strain and the latter to 20% strain. The 30 wt.% Mn steel had a fully austenitic microstructure which remained stable over the loading range studied, while stress induced austenite to α´- and ε-martensite transformations occur in the 20 wt.% Mn steel which initially contained an α´-martensite in addition to the austenite. The evolution of lattice strains under tensile loading differs between the two steels, reflected their different plastic deformation mechanisms. A stronger grain-orientation dependent behaviour is observed during deformation for the 20 wt.% Mn in contrast to the 30wt.% Mn steel.
Authors: Yi Gil Cho, Young Roc Im, Gyo Sung Kim, Heung Nam Han
Abstract: A finite element model was developed to simulate the deformation, temperature and phase transformation behavior in high carbon steels. The heat capacity of each phase and the heat evolution due to phase transformation were obtained from the thermodynamic analysis of S45C, 50CrV4 and SK85 steels. Phase transformation kinetics of the steels were derived from continuous cooling experiments. An additivity technique was applied to a modified Johnson-Mehl-Avrami equation to analyze continuous cooling curve. To predict the strain due to TRansformation Induced Plasticity (TRIP), a variant selection model for diffusionless transformation and an accelerative creep model for diffusional transformation were adopted. In order to calculate the deformation behavior, the elastic strain, the volumetric strain due to thermal contraction and phase transformation, the plastic strain and the TRIP strain were taken into account. Using the finite element model developed in this study, the temperature-phase-deformation behavior of the high carbon steels was calculated.
Authors: De Feng Zhang, Yin Liang, Zhong Lin Du, Xiao Huang Mao
Abstract: The time-dependent inhomogeneous temperature distribution in steel plates during the cooling increases thermal strains which, in turn, generate plastification and thus residual stresses. Moreover, phase transformation from the parent austenite phases into a product phase typically entails not only metallurgical strains but also accounts for transformation induced plasticity (TRIP), which again generates transformation related residual stresses. A unified material model that consisting of all relevant contributions to the total strain rate, i.e., elastic, plastic, thermal, metallurgical and TRIP strain contributions could be built in this paper. Transformation also released a significant amount of latent heat which naturally affected the temperature field that governed the evolution of the product phase. For the latter kinetic relationship about difference between diffusive and displacive transformation mechanisms depending on the local cooling rate is presented. Using an Avrami-like approach, the transformation kinetics is set up especially for complex cooling histories. The material model simulated the evolution of the residual stresses as well as the warping of the steel plates after completely cooling to room temperature.
Authors: Dong Ying Ju, Ryuji Mukai, Nobuaki Minakawa, Yukio Morii, Atsushi Moriai
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