Nucleation of proeutectoid ferrite at ceramic/steel interfaces

钢中非金属夹杂物的晶内铁素体形核现象作为一种控制钢组织和性能的新技术而引起人们的广泛关注。为了阐明铁素体晶内形核的机理和确定控制铁素体晶内形核的最重要参数，在高温下将许多陶瓷（商业上可买到的烧结多晶体）与Fe-C-Mn合金结合，并在低于Ae_3温度下等温反应不同时间。在所研究的陶瓷中，TiO_2和MnS的结合不完全（发生了特殊的反应或界面区熔化）。VN/奥氏体界面处的铁素体数量远大于奥氏体晶界处的铁素体数量，这可能表明VN起到了铁素体形核的孕育剂的作用。Al_2O_3也有类似的趋势，但程度较轻。SEM-EDX分析表明，这些陶瓷没有发生明显的界面反应或溶质扩散穿过界面。用简单的理论计算了热力学驱动力、陶瓷与铁之间热膨胀差异可能引起的应变能、陶瓷与铁之间的界面能，并根据经典成核理论计算了各陶瓷的相对成核能力。结果表明，控制铁素体形核的最重要的参数似乎是陶瓷/奥氏体和陶瓷/铁素体之间的界面能的差异。然而，可能需要进一步详细研究陶瓷/钢界面的原子结构和能量，夹杂物附近和夹杂物内的溶质分布，etc.to利用这种有趣的现象来控制钢的微观结构。

Intragranular ferrite nucleation at non-metallic inclusions in steel has drawn a considerable amount of attention because the phenomena may be used as a novel technique of the control of steel microstructure and properties. In order to elucidate the mechanism and identify the most important parameters controling the intragranular nucleation of ferrite, a number of ceramics (sintered polycrystals available commercially) were bonded to an Fe-C-Mn alloy at high temperatures and isothermally reacted for varying times below Ae_3 temperature. Among the ceramics studied either the bonding appaeared to be incomplete with TiO_2 and MnS (either a peculiar reaction occurred or the interface region was melted). The number of ferrite at VN/austenite interfaces was considerably larger with that at austenite grain boundaries, which may indicate that VN served as an innoculant for ferrite nucleation. A similar tendency was observed with Al_2O_3, but to a lesser extent. A SEM-EDX analysis has shown that an interfacial reaction or solute diffusion across the interface did not occur significantly with these ceramics. The thermodynamic driving force, the strain energy possibly caused by the difference in thermal expansion between iron and ceramics, the interfacial energy between ceramics and iron were calculated using a simple theory and the relative nucleation potency of each ceramic was evaluated on the basis of classical nuleation theory. As a result, the most important parameter governing the ferrite nucleation appears to be the difference in the interfacial energy between ceramics/austenite and ceramics/ferrite. However, further detailed studies may be necessary of atomic structure and energies of ceramics/steel interfaces, solute distribution near and within inclusions, etc.to utilize this interesting phenomena for the microstructure control of steels.