tailieunhanh - The Coming of Materials Science Part 6

Tham khảo tài liệu 'the coming of materials science part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 180 The Coming of Materials Science so a much higher stress would be needed to sustain the formation of the neck. In a glass very rapid drawing out is feasible for instance many years ago it was found that a blob of amorphous silica can be drawn into a very fine fibre for instrument suspensions by shooting the hot blob out like an arrow from a stretched bow. In alloys the mechanism of deformation is quite different it involves Nabarro-Herring creep in which dislocations are not involved under tension strain results from the stress-biased diffusion of vacancies from grain boundaries transverse to the stress to other boundaries lying parallel to the stress. The operation of this important mechanism which is the key to superplasticity can be deduced from the mathematical form of the grain-size dependence of the process Nabarro 1948 Herring 1950 it plays a major part in the deformation-mechanism maps outlined in the next chapter Section . For large superplastic strains to be feasible very fine grains a few micrometres in diameter and relatively slow strain rates typically second are requisite so that the diffusion of vacancies can keep pace with the imposed strain rate. Sliding at grain boundaries is also involved. Practical superplastic alloys are always two-phase in nature because a second phase is needed to impede the growth of grains when the sample is held at high temperature and a high temperature is essential to accelerate vacancy diffusion. The feasibility of superplastic forming for industrial purposes was first demonstrated half a century after the first observation by a team led by Backofen at MIT in 1964 until then the phenomenon was treated as a scientific curiosity. a parepisteme in fact. In 1970 the first patent was issued with reference to superplastic nickel alloys and in a book on ultra-fine-grained metals published in the same year Headley et al. 1970 gave an account of the current status of applied superplasticity . In 1976 the first .