Simple and Precise Description of the Transformation Kinetics and Final Structure of Dual Phase Steels

The kinetics of diffusion-dependent phase transformations (including austenitisation of ferrite in dual steels or ferritic nodular cast irons) is very often described by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. This description is not complete when the conversion is only partial due to insufficient overheating, as the equilibrium fraction of ferrite transformed into austenite cannot be determined directly from the JMAK equation. Experimental kinetic curves of partial austenitisation at various temperatures can be fitted using the JMAK equation, but the equilibrium fraction of the newly formed phase for each temperature has to be calculated as a regression parameter. In addition, the temperature dependence of the kinetic exponent in the JMAK equation is quite complicated and cannot be expressed by a simple general function. On the contrary, the equation of autoinhibition used for the description of austenitisation kinetics in present work directly gives the equilibrium fraction at partial conversion. It describes transformation kinetics at various temperatures independently of whether the conversion is complete or partial. Rate constants of the equation of autoinhibition depend on temperature according to the Arrhenius equation. In addition, the equation of autoinhibition has no weakness as the JMAK equation has, which consists in questionable temperature dependence of kinetic exponent.

PRE-EQUILIBRIUM FRACTION IN ALPHA INDUCED REACTIONS FROM THRESHOLD TO 40 MeV

α-particle induced excitation functions for the reactions 55Mn (α, n), 63Cu (α, n), 65Cu (α, n), 107Ag (α, n), 121Sb (α, n), 123Sb (α, n), 165Ho (α, n), 197Au (α, n), in a number of target nuclei from A=55 to 197(Z=25 to 83) have been calculated using the Statistical Model with and without inclusion of pre-equilibrium particle emission in the energy from threshold to 40 MeV. It is found that without consideration of pre-equilibrium decay, the data are not reproducible. As expected, inclusion of pre-equilibrium emission in compound nucleus calculation agree well with the experimental excitation functions. The pre-equilibrium fraction (FR) is found to be dependent on excitation energy, target mass number, neutron number, atomic number and, asymmetry parameter (N−Z).

ALPHA-INDUCED REACTIONS IN ANTIMONY

The excitation function of α-induced reactions on 121Sb and 123Sb has been measured. The α-beam energy ranges from 55.0±0.5 MeV to 21.9±1.2 MeV. In these experiments, the stacked foil activation technique was used. The reaction yield was measured by counting the gamma ray activity produced by the alpha-induced reactions. Results so obtained were compared with the calculations from the geometry-dependent hybrid (GDH) model. The assumption of initial exciton number n0=4 with n=2, p=2 and h=0 best satisfies the results measured in the present work. The model calculations were done using the ALICE/LIVERMORE-82 computer code. The pre-equilibrium fraction has also been calculated.