Mathematical Model of Bainitic Transformation in Austempered Ductile Iron

A mathematical model of austenite - bainite transformation in austempered ductile cast iron has been presented. The model is based on a model developed by Bhadeshia [1, 2] for modelling the bainitic transformation in high-silicon steels with inhibited carbide precipitation. A computer program has been developed that calculates the incubation time, the transformation time at a preset temperature, the TTT diagram and carbon content in unreacted austenite as a function of temperature. Additionally, the program has been provided with a module calculating the free energy of austenite and ferrite as well as the maximum driving force of transformation. Model validation was based on the experimental research and literature data. Experimental studies included the determination of austenite grain size, plotting the TTT diagram and analysis of the effect of heat treatment parameters on the microstructure of ductile iron. The obtained results show a relatively good compatibility between the theoretical calculations and experimental studies. Using the developed program it was possible to examine the effect of austenite grain size on the rate of transformation.

TTT-Diagram of Intermetallic Phases Formation in Heat-Resistant Titanium Alloys

The silicides and aluminides particles precipitation in near α-and two-phase (α + β) titanium – alloys has been investigated. It is shown that in Ti-Al-Si-Zr alloys three different types of particles are precipitated: (Ti,Zr)5Si3 (S1), (Ti,Zr)6Si3 (S2), (Ti,Zr)2Si (S3). The types of precipitated particles depend on the heat treatment modes and an alloying elements ratio. The investigation of silicide particles influence on the mechanical properties of the alloys has shown that silicides S1 reduced viscosity characteristics. While silicides S2 and S3 were found to reduce the heat resistance due to the depletion of solid solution by Al and Si and to impede the formation of α2 - phase particles. Microstructural investigation has been performed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The orientation relationships between silicides and matrix phase were identified by electron diffraction. Based on obtained results the schematic TTT-diagram of intermetallic phases formation has been suggested.

Neutron and hard X-ray diffraction studies of the isothermal transformation kinetics in the research reactor fuel candidate U–8 wt%Mo

Exposing uranium–molybdenum alloys (UMo) retained in the γ phase to elevated temperatures leads to transformation reactions during which the γ-UMo phase decomposes into the thermal equilibrium phases,i.e.U2Mo and α-U. Since α-U is not suitable for a nuclear fuel exposed to high burn-up, it is necessary to retain the γ-UMo phase during the production process of the fuel elements for modern high-performance research reactors. The present work deals with the isothermal transformation kinetics in U–8 wt%Mo alloys for temperatures between 673 and 798 K and annealing durations of up to 48 h. Annealed samples were examined at room temperature using either X-ray or neutron diffraction to determine the phase composition after thermal treatment, andin situannealing studies disclosed the onset of phase decomposition. While for temperatures of 698 and 673 K the start of decomposition is delayed, for higher temperatures the first signs of transformation are already observable within 3 h of annealing. The typical C-shaped curves in a time–temperature–transformation (TTT) diagram for both the start and the end of phase decomposition could be determined in the observed temperature regime. Therefore, a revised TTT diagram for U–8 wt%Mo between 673 and 798 K and annealing durations of up to 48 h is proposed.