Study on the formation mechanism of nanocrystalline structure by severe plastic deformation and analysis of its structure in atomic scale

在过去的十年里，纳米晶材料引起了人们极大的科学兴趣。研究表明，颗粒冲击变形、喷丸、磨损和钻削加工可获得纳米晶结构。通过这些工艺成功地制备出了表面和内部厚度均为数μm的纳米晶层。由这些工艺形成的钢中的纳米晶结构为随机取向的等轴纳米晶，直径约为20 nm。纳米晶层具有均匀的纳米结构，加工硬化区边界清晰，未观察到纳米晶态和加工硬化态之间的中间状态。纳米晶化后，球状、片状渗碳体等组织形态消失。纳米晶层的硬度极高(8~14 Gpa)，高于加工硬化区。纳米晶层的退火表现出明显的缓慢的晶粒长大。这种退火行为与加工硬化区有很大不同，加工硬化区的特点是再结晶和快速的晶粒长大。这些特性与在纳米晶体中观察到的特性相似。球磨和球滴变形加工产生的结构通过变形产生纳米晶材料的必要条件可能是大应变，其真应变约为7。钢中的纳米晶组织可能是由形变诱导的非晶化和随后的机械诱导纳米化形成的。

Nanocrystalline materials have attracted considerable scientific interests in the past decade. Particle impact deformation, shot peening, wear and drilling processing were demonstrated to produce nanocrystalline structure. Nanocrystalline layers with several μm thick at surface and interior of specimens were successfully produced by these processes. The nanograined structure in steels formed by these processes was randomly oriented equiaxed nanograins with about 20 nm: in diameter. The nanocrystalline layer has the homogeneous nanostructure with sharp boundary between work-hardened region: The intermediate state between the nanocrystalline and work-hardened states was not observed. The morphologies, such as spheroidal and lamellar cementites, disappeared by nanocrystallization. The hardness of the nanocrystalline layer is extremely higher (8〜14 GPa) than that of work-hardened region. The annealing of nanocrystalline layer shows substantially, slow grain growth. This annealing behavior is quite different from the work-hardened region which is characterized by recrystallization and fast grain growth. These characteristics are similar to those observed in the nanocrystalline. structures produced by ball milling and ball drop deformation processing The necessary condition to produce nanocrystalline materiall by deformation may be a large strain of more than about 7 in true strain. Nanograined structure in steels f is probably formed by the deformation induced amorphization and following mechanically induced nanocrystallization.