珠光体、贝氏体、马氏体相变的形核*

Nucleation Mechanism of Pearlite, Bainite and Martensite*

摘要: 研究过冷奥氏体转变产物的形核规律具有重要理论价值。采用20MnCrMo、60Si2CrV等材料，奥氏体化后在不同温度进行等温处理，得珠光体、贝氏体、马氏体等产物，应用QUANTA-400型扫描电镜、JEM-2100透射电镜等观察各种相变的形核。发现：珠光体、贝氏体、马氏体均优先在奥氏体晶界处形核，下贝氏体、马氏体也在晶内形核。珠光体晶核由共析铁素体+共析渗碳体两相组成，共析共生，在700℃~650℃，珠光体临界晶核尺寸r* = 150~70 nm；临界形核功 155~292 J/mol。贝氏体在奥氏体的贫碳区形核，晶核是单相(BF)，其临界尺度a* = 16.7~25 nm，临界形核功 2.7 × 102 J/mol。马氏体的临界晶核尺寸约为17~20 nm，形核功约为200~600 J/mol。表明随着温度的降低，临界晶核尺寸越来越小，而形核功越来越大。过冷奥氏体转变产物的形核是一个逐渐演化的过程，符合相变形核的一般规律。

Abstract: It is significant theoretically to study the nucleation mechanism of the phase transformation products of the supercooled austenite. 20MnCrMo, 60Si2CrV and other material were respectively austenized and isothermal quenched at the different temperature to obtain pearlite, bainite and martensite. The nucleation of the phase transformation products was observed by QUANTA-400 environmental scanning electron microscope and JEM-2100 transmission electron microscope. It is found that pearlite, bainite and martensite can all nucleate at the crystal grain boundary of austenite preferentially, moreover, lower bainite and martensite can nucleate in the austenite crystal grain interior. The pearlite crystal nucleus consists of two phases of eutectoid ferrite and eutectoid cementite, which can coexist. At the temperature range of 700˚C - 650˚C, the dimension of the pearlite critical nucleus is about 150 - 70 nanometer and the critical nucleation energy is 155 - 292 J/mol. Bainite nucleates in carbon-poor region and the bainite crystal nucleus is single phase (BF). The dimension of the bainite ferrite critical nucleus is 16.7 - 25 nanometer and the critical nucleation energy is 2.7 × 102 J/mol. The dimension of the martensite critical nucleus is 17 - 20 nanometer and the critical nucleation energy is 200 - 600 J/mol. The results show that, with the decrease of the transformation temperature, the dimension of the critical nucleus is becoming smaller and smaller and the critical nucleation energy is getting larger and larger. The nucleation of the phase transformation products of the supercooled austenite is a gradual evolutionary process, which accords with the general rule of the phase transformation nucleation.