Các chuyên ngành cơ bản của cơ khí bao gồm: động học, tĩnh học, sức bền vật liệu, truyền nhiệt, động lực dòng chảy, cơ học vật rắn, điều khiển học, khí động học, thủy lực, chuyển động học và các ứng dụng nhiệt động lực học. Các kỹ sư cơ khí cũng đòi hỏi phải có kiến thức và năng lực áp dụng những khái niệm trong môi trường kỹ thuật điện và hóa học. Với một mức độ nhỏ, cơ khí còn trở thành kỹ thuật phân tử - một mục tiêu viễn cảnh của nó là tạo. | 400 The Coming of Materials Science The most intriguing aspect of nanostructured metals and especially ceramics such as titania is that the very small grain size encourages Herring-Nabarro creep which in turn is the precondition of superplastic forming under stress. The essential facts concerning this process are laid out in Section . Nanostructured ceramics can be plastically formed in spite of extreme resistance to dislocation motion and this has been plentifully documented in many studies. Examples are set out in Gleiter s own 1996 overview of nanostructured materials. The ability to form nano-ceramics to near net shapes looks to have very promising industrial potential. The exploitation of easy superplastic forming of nanostructured ceramics is hindered by one major flaw the heat treatment needed to sinter a green solid to 100 density also leads to grain growth so that by the time the material is fully dense it is no longer nanocrystalline. Very recently a way has been found round this difficulty. Chen and Wang 2000 studying Y2O3 have found that a two-stage sintering process allows full density to be attained while grain growth is arrested during the second stage. Typically the compact is briefly heated to 1310 C and the temperature is then lowered to 1150 C if that lower temperature were applied from the start complete densification would not be possible. The paper analyses various conceivable explanations but it is not at present clear why a brief high-temperature anneal inhibits grain growth at a subsequent lower temperature this valuable finding is likely to engender much consequential research. A number of functional properties can also be affected by nanocrystallinity. The most interesting of these is soft ferromagnetism. Yoshizawa et al. 1988 discovered that a bulk metallic glass trade-named Finemet of composition Nbs on partial crystallization assumes a structure with nanometresized 5-20 nm crystallites embedded in a residual glassy
