Formation of bainite in ferrous and nonferrous alloys through sympathetic nucleation and ledgewise growth mechanism
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INTRODUCTION
MUCHwork has been done on the morphology of bainite in steels, by Mehl, m Aaronson and Wells, t21 Oblak and Hehemann,E3] Pickering,[4] Bhadeshia and Christian,tSJ and Hehemarm et al., [6] especially regarding the bainitic structure in Si-containing steels. LT,sjSome of these works concludedt3,5,6,8j that bainitic ferrite is formed via a shear mechanism. But so far, the observed smallest structural unit of bainite observed by transmission electron microscopy (TEM)t81 is a subunit. Herein, we will present more information on the internal structure of bainitic subunits in steels and of bainitic plates in Cu-Zn-A1 alloys, which is called the ultrafine structure. The study of the ultrafine structure in bainite should present more evidence, helping to clarify the transformation mechanism of bainite. For convenience, "microstructure" will refer to that observed by light optical microscopy (LOM) and "fine structure" will refer to that observed by TEM. The detailed structure, which has been discovered by means of scanning tunneling microscopy (STM) (but not by TEM), is called the ultrafine structure. As to the surface relief accompanying the bainite reaction, Ko and Cottrellt9] reported in their earlier works that it is almost identical with that associated with a martensitic transformation. But Aaronson and co-workers[1~ pointed HONG-SHENG FANG and Y.K. ZHENG, Professor, JIA-JUN WANG, Associate Professor, ZHI-GANG YANG and C.X. LI, Postdoctoral Students, and C.M. LI, Lecturer, are with the Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People's Republic of China. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled "Atomistic Mechanisms of Nucleation and Growth in Solids," organized in honor of H.I. Aaronson's 70th Anniversary and given October 3-5, 1994, in Rosemont, Illinois. METALLURGICALAND MATERIALSTRANSACTIONSA
out that the two kinds of surface relief have different shapes, e.g., bainitic relief is often tendike, while martensitic relief is an invariant plane strain (IPS) type. BhadeshiaV4] proposed that tentlike reliefs arise from plastic accommodation (by slip) of austenite adjacent to the broad faces of bainite plates. If the accommodation is neglected, however, the reliefs will appear as IPS type. There is no doubt that the nature of surface reliefs remains a key in studying the bainitic transformation mechanism. Nevertheless, most of the previous theories as to the nature of bainitic relief are based on the results obtained through light optical differential interference microscopy (LODM), whose maximum vertical resolution is approximately only 0.01 /xm. However, in sharp contrast, STM has a vertical resolution of approximately 0.01 nm and a lateral resolution of approximately 0.1 nm.t~s] Hence, STM can be used to study the fine or ultrafine structure of surface relieW 61 even to an angstr6m scale. In the present investigation, surface relief effects during the bainite reaction in steel and in CuZn-AI alloys
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