tailieunhanh - Diffusion Solids Fundamentals Diffusion Controlled Solid State Episode 2 Part 6

Tham khảo tài liệu 'diffusion solids fundamentals diffusion controlled solid state episode 2 part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 374 21 Diffusion in Quasicrystalline Alloys is because natural Al is monoisotopic which forbids SIMS studies and a suitable radioisotope for tracer studies is missing2 . Instead diffusion of several solutes was investigated especially in icosahedral Al-Pd-Mn. Single crystals should be used for basic studies to avoid complications due to diffusion along grain boundaries see Chap. 32 . The diffusion coefficient in a quasicrystal like in a crystal is a symmetric second rank tensor. Icosahedral quasicrystals are highly symmetric diffusion is isotropic and the diffusion coefficient is a scalar quantity. Decagonal quasicrystals are uniaxial and the diffusion coefficient has two principal components like in uniaxial crystals. One component corresponds to diffusion parallel to the decagonal axis and the other one to diffusion perpendicular to the axis. A thorough diffusion study on decagonal quasicrystals requires diffusivity measurements on monocrystalline samples with two different orientations. Diffusion in quasicrystalline alloys has been reviewed by Nakajima and ZuMKLEy 13 and by Mehrer ET al. 14 . In this chapter we summarise the state-of-the-art in this area compare diffusion in quasicrystals with diffusion in related crystalline metals and discuss possible diffusion mechanisms. Icosahedral Quasicrystals Icosahedral Al-Pd-Mn The information on the phase diagram for the ternary Al-Pd-Mn system is rather complete and has been reviewed by Luck 21 . Al-Pd-Mn was the first system for which stable icosahedral as well as decagonal quasicrystals were detected 22 . The formation of the icosahe-dral phase occurs in a ternary peritectic reaction from the melt. Conventional procedures permit the growth of single crystals 15 . The formation of the decagonal phase is sluggish and the determination of the phase diagram in the decagonal region is difficult. Icosahedral Al-Pd-Mn contains about 70at. Al see Table . The contents of transition elements Pd and Mn are about .

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