Guo X L, Su H Q, Wu B Y, Liu Z G
National Laboratory of Solid State Microstructures, Nanjing University, P.R. China.
Microsc Res Tech. 1998 Feb 15;40(4):336-40. doi: 10.1002/(SICI)1097-0029(19980215)40:4<336::AID-JEMT7>3.0.CO;2-P.
Mechanical properties of austempered ductile iron (ADI) are mainly controlled by its unique microstructure. The objectives of this paper are to characterize the microstructural morphology and the phase distribution of ADI using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and to determine the mechanism of strengthening and toughening of ADI. The experimental results show that, in the microstructure of ADI composing of upper bainite, retained austenite, graphitic nodule, and a small amount of martensite, the upper bainite is composed of sub-units of ferrite in the shape of "wheat ears" on which the "wheat grains" grow at an angle of about 60 degrees to the long axis of the "wheat ears." The retained austenite is connected with each other in the shape of a continuous net. The wheat-ear like bainite with a homogeneous distribution in the continuous austenite net plays an important role to the strengthening and toughening of ADI. The metastable austenite appears in the shape of a large plate in which the martensite is preferentially formed. The appearance of martensite can be suppressed at the time when retained austenite remains stable, which is of benefit to the continuity and homogeneity of austenite net.
等温淬火球墨铸铁（ADI）的力学性能主要由其独特的微观结构控制。本文的目的是利用扫描电子显微镜（SEM）和透射电子显微镜（TEM）来表征ADI的微观结构形态和相分布，并确定ADI的强化增韧机制。实验结果表明，在由上贝氏体、残余奥氏体、石墨球和少量马氏体组成的ADI微观结构中，上贝氏体由“麦穗”状铁素体亚单元组成，“麦粒”在其上以约60度角相对于“麦穗”的长轴生长。残余奥氏体以连续网络的形式相互连接。在连续奥氏体网络中均匀分布的类麦穗状贝氏体对ADI的强化增韧起着重要作用。亚稳奥氏体呈大板状，其中优先形成马氏体。当残余奥氏体保持稳定时，可以抑制马氏体的出现，这有利于奥氏体网络的连续性和均匀性。
