TEM STUDY OF PRECIPITATION IN HIGH-STRENGTH 1987 ALLOY SHEETS AFTER SUPERPLASTIC FORMING.

摘要

Superplastic high-strength Al-Zn-Mg-Cu alloys (1987 a.o.) have promising potential for application in aerospace industry. However the development of the manufacturing process for superplastic forming (SPF) of the alloys requires investigation concerning the influence of SPF on microstructure, which determines in its turn the properties. Using high resolution transmission and scanning electron microscopy (TEM, SEM) and optical microscopy, the microstructural features such as precipitates, grain morphology, cavity were investigated for the 1987 experimental components deformed to e_t ～0,8 (equivalent elongation ～125%) at T = 475°C, ε = 10～(-3) s～(-1) and then heat treated to T2 (T76) temper. The components were made of 2 mm sheet with initial unrecrystallized structure. It was found that the matrix and the grain boundary precipitation of the metastable and stable η (MgZn_2Cu) and η-phases was essentially identical in the initial sheet and SP formed component. SPF had not influenced on the precipitation kinetics during two-step T2 (T76) ageing. It was observed the uniform distribution of fine-dispersed precipitates in the grain body. Starting from sheet strain of e-t ～ 0,25 (elongation ～30 %) the grain structure is mainly recrystallized, fine (2-7 μm in diameter) due to dynamic recrystallization.