tailieunhanh - Modern Physical Metallurgy and Materials Engineering Part 10

Tham khảo tài liệu 'modern physical metallurgy and materials engineering part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Strengthening and toughening 261 Figure The ageing of aluminium-copper alloys at a 130 C and b at 190 C after Silcock Heal and Hardy 1953-4 . alloy becomes softer the temperature above which the nuclei or zones dissolve is known as the solvus temperature Figure shows the solvus temperatures for GP zones Q ff and 0. On prolonged ageing at the higher temperature larger nuclei characteristic of that temperature are formed and the alloy again hardens. Clearly the reversion process is reversible provided re-hardening at the higher ageing temperature is not allowed to occur. Structural changes during precipitation Early metallographic investigations showed that the microstructural changes which occur during the initial stages of ageing are on too fine a scale to be resolved by the light microscope yet it is in these early stages that the most profound changes in properties are found. Accordingly to study the process it is necessary to employ the more sensitive and refined techniques of X-ray diffraction and electron microscopy. The two basic X-ray techniques important in studying the regrouping of atoms during the early stages of ageing depend on the detection of radiation scattered away from the main diffraction lines or spots see Chapter 5 . In the first technique developed independently by Guinier and Preston in 1938 the Laue method is used. They found that the singlecrystal diffraction pattern of an aluminium-copper alloy developed streaks extending from an aluminium lattice reflection along 10 0 Al directions. This was attributed to the formation of copper-rich regions of plate-like shape on 10 0 planes of the aluminium matrix now called Guinier - Preston zones or GP zones . The net effect of the regrouping is to modify the scattering power of and spacing between very small groups of 10 0 planes throughout the crystal. However being only a few atomic planes thick the zones produce the diffraction effect typical of a twodimensional lattice . the .

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