Numerical Calculations on Thermodynamic Parameters in Solidification and Average Grain Sizes of Bulk Nanocrystalline Fe_3Al Based Alloys Prepared by Aluminothermic Reaction Casting

摘要

Numerical calculations on thermodynamic parameters in solidification of homogeneous nucleation and the effects of alloying elements on average grain sizes in bulk nanocaystalline Fe_3Al based alloys prepared by aluminothermic reaction casting were conducted. As the undercooling increases from 100 K to 400 K, the critical activation energy of solidification drops dramatically and the critical radius of nucleus decreases from 2.3 nm to 0.5 nm. The thermodynamic driving force of nucleation increases from 100 MJ·m~(-3) to 600 MJ·m~(-3), but the nucleation rate changes little until the undercooling comes to 300 K and subsequently shows a significant change, increasing rapidly to 9.6 × 10~(21) m~(-3)·s~(-1). Calculated average grain sizes decrease gradually from 568 nm to 68 nm as the undercooling increases from 300 K to 500 K for the alloy with 5 wt.%Cr, while decrease first and then increase for the alloys with 10 wt.% and 15 wt.%Cr, and increase continuously for the alloy with 20 wt.%Cr. Calculated average grain sizes decrease through the sequence of Mn, Cr, Mo, and Ni alloying elements, respectively, and fall into the scope of 9 to 73 nm, which are well in accordance with the experimental data.