Description of martensitic transformation kinetics in Fe-C-X(X = Ni,Cr,Mn,Si) system by a modified model

Controlling the content of athermal martensite and retained austenite is important to improving the mechanical properties of high-strength steels,but a mechanism for the accurate description of martensitic transformation during the cooling process must be addressed.At present,frequently used semi-empirical kinetics models suffer from huge errors at the beginning of transformation,and most of them fail to exhibit the sigmoidal shape characteristic of transformation curves.To describe the martensitic transformation process accurately,based on the Magee model,we introduced the changes in the nucleation activation energy of martensite with temperature,which led to the varying nucleation rates of this model during martensitic transformation.According to the calculation results,the relative error of the modified model for the martensitic transformation kinetics curves of Fe-C-X(X = Ni,Cr,Mn,Si) alloys reached 9.5% compared with those measured via the thermal expansion method.The relative error was approximately reduced by two-thirds compared with that of the Magee model.The incorporation of nucleation activation energy into the kinetics model contributes to the improvement of its precision.

Experimental Study on the Dependency of Ice Nucleation Active Surface Site Density on ATD Aerosol Size

In light of the percentage of Earth’s cloud coverage, heterogeneous ice nucleation in clouds is the most important global-scale pathway. More recent parameterizations of ice nucleation processes in the atmosphere are based on the concept of ice nucleation active surface site density (ns). It is usually assumed that ns is independent of time and aerosol size distribution, i.e. that the surface properties of aerosols of the same species do not vary with size. However, the independence of ns on aerosol size for every species has been questioned. This study presents the results of ice nucleation processes of ATD laboratory-generated aerosol (particle diameters of 0 - 3 μm). Ice nucleation in the condensation mode was performed in a Dynamic Filter Processing Cham- ber at temperatures of -18°C and -22°C, with a saturation ratio with respect to water of 1.02. Results show that ns increased by lowering the nucleation temperature, and was also dependent on the particle size. The ns of particles collected on the filters, after a 0.5 μm D50 cut-off cyclone, resulted statistically higher with respect to the values obtained from the particles collected on total filters. The results obtained suggest the need for further investigation of ns dependence of same composition aerosol particles with a view to support weather and climate predictions.

Effects of Low Temperature Stress and INA Bacteriaon Chlorophyll a Fluorescence Induction Kineticsin Young Fruit of Two Apricot Cultivars

Effects of low temperature and INA bacteria on the change of chlorophyll a fluorescence in young fruit from two apricot cultivars were investigated. Low temperature decreased the potential activity (Fv/ Fo), conversion efficiency of primary light energy (Fv/Fm)of PS II and photochemical quenching (qP) in young fruit of two apricot cultivars. Low temperature enhanced non-photochemical quenching qN, decreasing the quantum yield of photosynthetic electron transfer. The presence of ice nucleating active (INA) bacteria intensified the effects of low temperature, raised the injury temperature threshold from - 4℃ to - 2 - - 3℃. INA bacteria can be a factor to induce frost susceptibility of apricot fruit. The amount of damaged PS I activity center was related to apricot fruit size and cultivar.