Papers by Keyword: Phase Transformation

Abstract: We propose to reconstruct the g orientation maps from the α maps inherited by the bainitic or martensitic γ→α phase transformation. Our approach comprises two main steps (1) identification of reliable fractions of parent grains – each γ fraction is determined from neighbouring variants related to a unique γ orientation with a low tolerance angle; (2) expansion of these fractions by collecting adjacent variants being in orientation relation (OR) with the calculated g orientation - using a higher tolerance around the strict OR. Moreover, this fully automated g map reconstruction is further improved by an additional semi-automated analysis of α/γ maps. This reconstruction procedure was applied to the α microtexture of bainitic steels. The reliability of the calculations was checked by comparing each γ grain orientation and their corresponding α variants. The results show that even with a large spread around OR, the shape and orientation of most of the γ grains are accurately calculated.

Abstract: In the present study we focus on the precipitation processes during heating and ageing of β-metastable phase in the near β Ti-5553 alloy. Transformation processes have been studied using continuous high energy X-Ray Diffraction (XRD) and electrical resistivity for two different states of the β-metastable phase. Microstructures have been observed by electron microscopy. Different transformation sequences are highlighted depending on both heating rate and chemical composition of the β-metastable phase. At low temperatures and low heating rates, the hexagonal ω_iso phase is first formed as generally mentioned in the literature. Increasing the temperature, XRD evidences the formation of an orthorhombic phase (α’’), which evolves toward the hexagonal pseudo compact α phase. For higher heating rates or for richer composition in β-stabilizing elements of the β-metastable phase, ω phase may not form and α’’ forms directly and again transforms into α phase. A direct transformation from β-metastable to a phase is observed for the highest heating rate. The formation of the metastable ω_iso and α’’ phases clearly influences the final morphology of α.

Abstract: A finite element approach is used to simulate the precipitation of Ni₃(Al,Ti) intermetallics in nickel-based superalloys containing a low volume fraction of spherical g’ precipitates, in which precipitation occurs following nucleation and growth mechanisms. Classical differential equations of nucleation and growth are implemented in the software Comsol (formerly Femlab), to compute the number of precipitates per unit volume and their mean size. Another originality of the model is the use of thermodynamic quantities coming from phase diagram computations (Thermo-Calc), like the temperature variation of the equilibrium g’ volume fraction, and the evolution of the concentration of g’ forming elements (Al, Ti) in the matrix with the volume fraction of precipitates. Once adjusted to experiment in the case of isothermal ageings, the model can be used to simulate precipitation during complex thermal histories. Finally, automatic heat treatment optimisation procedures are proposed and tested, which can reduce heat treatment times by a factor of more than five.

Abstract: Based on an analytical one-dimensional model, austenite growth into pearlite lamella and the corresponding phase evolution during isothermal reverse transformation to austenite at 1000-1183 K in Fe-C fully pearlitic steels containing 0.6-1.0 mass% C (in the austenite single phase field of Fe-C phase diagram) were simulated. It was found that the rate of austenite growth into ferrite increases faster with increasing reversion temperature than into cementite. Three types of phase evolution dependent on reversion temperature and carbon content were classified: 1) cementite rather than ferrite disappears first; 2) ferrite and cementite simultaneously disappear; 3) ferrite rather than cementite disappears first. The type of phase evolution in a hypoeutectoid steel heated above its Ae₃ temperature possibly changes in the order of 1), 2) and 3) as the reversion temperature increases. For eutectoid and hypereutectoid steels, the phase evolution during isothermal reversion always obeys the type 3).

Abstract: The LuB₁₂ ® LuB₄ ® LuB₂ phase transformations on annealing in vacuum (T=(1400÷1800) K, p < 10^-2 Pa) are investigated with use of the parent single crystals. SEM and X-ray researches of the corresponding lutetium boride single crystals were carried out before and after their annealing. It is shown that the LuB₁₂ → LuB₄ phase transformation takes place in surface layer and transition region spreads inwards the single crystal bulk with time. According to the assessed Lu-B phase diagram the LuB₄ phase transformation into other individual phases is impossible, and at first it is shown that under corresponding conditions the LuB₄ → LuB₂ spontaneous phase transformation takes place both on the surface and in the LuB₄ single crystal bulk.

Abstract: Nowadays more and more hot stamped steel sheets dedicated for the automotive body-in-white structure are pre-coated to prevent the steel surface against iron oxidation and decarburization during the austenitization step. For these applications, the coating is deposited by continuous hot-dipping the steel in an Al-Si bath. The Al-Si coating, at the delivery state, contains Al-grains, Al-Fe-Si ternary phases, Al-Fe binary phases. During the austenitization, the Al-Si coating transforms completely by inter-diffusion and solidification reactions. The mechanisms of Al-Fe-Si phase transformations at high temperature are almost unknown. The phase transformations occurring during austenitization define the final coating microstructure responsible for the in use properties of the product like spot welding, painting adherence or corrosion behavior. It is the aim of this paper to propose a new way of understanding the mechanisms of phase transformation in the Al-Si coating during the austenitization step (between 900 and 930°C) before the transfer into the hot-stamping press.

Abstract: Use of hydrogen as a temporary alloying element in Ti alloys is an attractive approach to improve the mechanical properties of the materials, enhance processability and thereby reduce manufacturing costs. In this paper, the hydrogen diffusion process and the phase transformation both between Ti particles and in Ti sheets were simulated to analyze the mechanism of hydrogen diffusion in different phases (α-Ti, β-Ti and TiHx). With the simulation based on the kinetics and thermodynamics, quantitative behaviors of the hydrogen diffusion and the phase transformation were analyzed. The simulation results provide an insight into the diffusion process and improve the fundamental understanding of the mechanism of diffusion and phase transformation.

Abstract: To confirm the constitutive model developed by the present authors, example calculations are conducted for the transformation behavior of shape memory alloys. The constitutive model describes the transformation behavior of the material by calculating the transformation stress and strain of transformation systems of micro structures in the material. In the calculating, the accommodation mechanism acting on microstructures of the material is also taken into consideration. Computational results by the constitutive model for these loading paths are revealed to be reasonable in comparing with experimental observations.

Abstract: In this paper, based on the experimental measured cooling curves, it was solved that the inverse problem of non-linear heat conduct equation for Steel 1045 quenched by water using finite difference method and nonlinear estimate method. The volume fraction of constituent of austenite, pearlite, bainite, martensite and the non-linear surface heat-transfer coefficients in continuous cooling during quenching were calculated and simulated by mathematical transformation. The volume fraction of phase constituents in the quenched work piece of 1045 steel was tested subsequently. Then the temperature field and thermal stress field of Steel 1045 cylinder during water quenching was simulated by ANSYS. The study results show that the 2D transient temperature field of the cylinder specimen calculated by ANSYS during quenching coincides with the practical quenching process, and the results of thermal stress field simulated by ANSYS can guide quenching process.

Abstract: First-principles simulation was carried out to study the phase transformation of vanadium doped TiO₂ from anatase to rutile based on Ti5O10 cluster model. The Ti5O10 cluster models of anatase and rutile and corresponding V⁴⁺ isomorphous replaced cluster models were calculated to study the change of total energy, the ways of angular distortion, the change of frontier orbitals and the Ti⁴⁺ in the model preferentially substituted by V⁴⁺. The calculation results reflect that the V₂O₄ species, which has a rutile-like structure, can promote the phase transition from anatase to rutile. The single α electron in Ti4VO10 cluster model makes change to the frontier orbitals and the uneven contribution of electrons in valence shell may be the force driving the angular distortion in anatase lattice to rutile-like structure.