Solid State Phenomena Vols. 172-174

Abstract: New grades have been developed in high alloy tool steels, enabling improved mechanical properties by modifications on Si, Cr or Mo contents. The correlation between alloy modifications and mechanical properties were successively correlated to microstructural aspects in previous work, especially in terms of secondary hardening carbides. In the present paper, thermodynamic and kinetic simulations were carried out and correlated to these data, especially in terms of Si effect on M₃C carbide precipitation. Free energy evaluation points to easier formation of M₇C₃ carbide in high Si steels, especially at high tempering temperatures, which are typical for hot work tool steels. Kinetic evaluations, based on the present simulations and literature, also show that cementite formation is retarded in high Si steels, which also lead to direct formation of M₇C₃. Regarding the low Si steels, cementite enrichment by Cr, Mo and V was also evaluated in terms of simulation. The results are in good agreement and help the explanation on the slower dissolution of cementite particles in low silicon steels, leading to important implications to the higher tempering resistance of these grades. Therefore, tempering simulation on the new hot work tool steel compositions aided the understanding of the microstructure modifications found experimentally.

Abstract: In order to predict microstructures during vacuum carburizing, the model which simulates not only the carbon(C) diffusion but also growth/dissolution of cementite(θ) is required. For development of a new model we applied vacuum carburizing to low alloy steels and analyzed the distribution of C and θ by GD-OES and image analysis of microstructures. The C in retained austenite(γ) phase after carburizing was also measured by lattice constants obtained from XRD. We also simulated multi-component diffusion with γ matrix and θ layer to analyze a velocity of the moving interface. The new carburizing model was proposed based on the findings, which suggest that C in γ phase at the carburizing surface is supersaturated and corresponds to C concentration for metastable equilibrium condition to graphite. The growth and dissolution of the θ follow a square root of time with the coefficients controlled by diffusion of Si in γ and Cr in θ respectively. The required parameters such as diffusivity coefficients are obtained by the CALPHAD method. The calculated C distributions and volume fractions of θ represent the experimental results.

Abstract: Bainite is an essential constituent in the microstructure of many advanced high strength steels, e.g. ferrite-bainite dual-phase, transformation induced-plasticity (TRIP) and complex phase (CP) steels. A complex thermo-mechanical processing is employed in industry such that following ferrite formation a desired fraction of bainite can be obtained during austenite decomposition. In order to evaluate robust processing routes it would be very useful to have a bainite transformation model with predictive capabilities. In this work a transformation start criterion for bainite is proposed by defining a critical driving pressure concept. Subsequent bainite formation kinetics from a mixture of ferrite-austenite is described using phenomenological modelling methodologies. In particular, the predictive capabilities of two approaches will be critically discussed, i.e. (i) the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model in conjunction with Rios treatment of the additivity rule and (ii) a nucleation-growth based model that describes simultaneous formation of bainitic ferrite and carbides. Using experimental transformation data for TRIP and CP steels, status and limitations of these models will be delineated.

Abstract: A simple model of dissipative processes originally developed for magnetic materials is extended to ferroelastic materials, in particular to shape memory alloys that display a pseudoelastic behaviour. The model based on non-equilibrium dissipative dynamics leads to kinetic equations. The characteristic feature of the model is a three-level response to the acting force field describing the transition between two opposite states of saturation through the intermediate parent structural state.

Abstract: Growth and formation of pearlite colonies are simulated for some simple models of alloysiron-carbon. The steady-state growth is shown to be possible only via the interfacial but not the volumecarbon diffusion mechanism. The model of formation of pearlite colonies based on assumptions ofstrong enhancement of carbon diffusion near grain boundaries is suggested.

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 formation of ferrite (α) from austenite (γ) and vice versa, upon thermo-mechanical processing of steels, are phase transformations of great technological importance. Often these transformations occur in the presence of externally or internally imposed stress. This paper provides an overview of recent research on the quantitative analysis of the transformation kinetics of the γ®a and a®g transformations subjected to uniaxial compressive stress below the yield stress of g and a, based on the application of the high-resolution differential dilatometry and the modular model of transformation kinetics. The application of uniaxially compressive stresses leads to antagonistic effects on the transformation kinetics: the stress applied upon the γ®a transformation prompts the transformation, while it obstructs the a®g transformation. These results can be quantitatively discussed in terms of chemical driving forces and transformation-induced deformation energies.

Abstract: We have adapted the Quickhull algorithm with the general dimension Beneath-Beyondalgorithm [6] for computing the convex hull of the Gibbs energy hypersurface of multicomponenttwo-phase alloys. We illustrate the salient features of our method with calculations of isothermalferrite-austenite equilibria in Fe-C-Cr. Finally, successive equilibrium calculations in a Fe-C-Cr-Mosteel over a large temperature range show the benefit of computing the convex hull before performingthe conventional Newton-Raphson search.

Abstract: Beyond 100 GPa at ambient temperature, β-boron exhibits an amorphization [1]. This paper presents Quantum Molecular Dynamics simulations of the equation of state (EoS) of amorphous boron under pressure.

Abstract: Our recent model has been used to evaluate the point defect characteristics including those determining the effect of pressure on the concentration of vacancies, di-vacancies, interstitials and their diffusion mobility in set of BCC and FCC metals. Our model has been developed to calculate temperature dependences of mentioned features. In contrast to other studies, the vacancy migration volumes have been found for all the metals studied.