Development of a novel microstructure controlling process for high performance materials using severe -plastic-deformation

研究了5U5316L奥氏体不锈钢和镍等剧烈塑性变形材料的形成机理和显微组织。机械铣削（MM）工艺是一种严重塑性变形（SPD）工艺，可以产生超细晶粒结构。透射电镜观察到晶粒形成了位错密度高、晶粒尺寸小于100 nm的等轴晶粒结构。SADP表明其微观结构由FCC和BCC两种晶体结构组成。FCC和BCC晶粒的晶粒尺寸基本一致，但BCC晶粒的图像比FCC晶粒更圆，更清晰。换句话说，与BCC晶粒相比，FCC晶粒具有更不规则的晶界和更复杂的应变对比。均质化、研磨和退火（在358k（85℃）下退火5 mm或1小时）粉末的维氏硬度测试表明，在358k退火后硬度大幅下降。结果强烈表明，必须存在大量的空位才能引起如此低的温度恢复。这些结果还表明，BCC相是通过无扩散转变出现的。虽然5U5316L钢的奥氏体相在室温下是亚稳定的，但即使大变形也很难转变为马氏体相。因此，BCC相被认为是由块状铁氧体相变形成的。铁素体晶粒呈球形是由块状铁素体晶粒形核长大所致。晶格缺陷的增加导致低温恢复，不规则晶界组织的形成导致奥氏体向铁素体转变的驱动力增加。

Formation mechanism and microstructure of severe plastic deformed materials such as 5U5316L austenitic stainless steels and Nickel are studied. Mechanical Milling (MM) process is one of the severe plastic deformation (SPD) process which enables to produce an ultra fine grain structure. By the TEM, formation of an equiaxed grain structure with high dislocation density and its grain size of less than 100 nm are observed. The SADP demonstrates that the microstructure consists of FCC and BCC crystal structures. These FCC and BCC grains show almost the same grain size, however, the image of the BCC grain is more spherical and more clear than that of the FCC grain. In other words, compared to the BCC grain the FCC grain has more irregular grain boundary and more complex strain contrast. The Vickers hardness tests of as homogenized, as milled and annealed (at 358 K (85 degree C) for 5 mm or 1 hr) powders indicates a large decrease in the hardness after annealing at 358 K. The results strongly suggest that there must exist of a huge number of vacancies to cause such a low temperature recovery. These results also imply that the BCC phase appears by means of a diffusionless transformation. Although the austenite phase in the 5U5316L steel is meta-stable at the room temperature, it hardly transforms to martensite phase even by a heavy deformation. Thus, the BCC phase is considered to form by a massive-like ferrite transformation. The spherical shape of the ferrite grain is attributed by the nucleation and growth of the massive ferrite grain. The increase of lattice defects, which leads to low temperature recovery, and formation of the irregular grain boundary structure result in increase of driving force for the transformation of austenite to ferrite.