The role of minor elements on mechanical properties of high toughness aluminum alloys

Tensile properties, fracture toughness, fatigue crack propagation properties, quench sensitivity and resistance to stress-corrosion cracking were compared between 7150 and 7475 high toughness aluminum alloys. The effect of impurity level (iron and silicon) on these properties was also investigated. Further the role of minor additional elements (zirconium in 7150 and chromium in 7475) was discussed. As a result, tensile and yield strength of 7150 plates were higher than those of 7475 ones, however elongation and fracture toughness of the former were lower than those of the latter. In particular, fracture toughness of T651-tempered 7150 plates was not improved even if the content of iron and silicon were diminished. Many fine dimples (0.1 to 0.5 μm in spacing)were observed in the fracture surface of 7475 plates. On the other hand, there were few dimples in that of 7150 ones. It is considered that the difference of fracture surface was caused by minor additional elements. Zirconium in 7150 precipitates in the meta-stable phase, Al3Zr, which has a Ll2 ordered structure and has coherency with the matrix. On the other hand, chromium in 7475 precipitates in the stable phase which has no coherency with the matrix. Therefore plastic deformation around chromium-bearing compounds occurs more easily than around zirconium-bearing ones, since equilibrium phase particles (MgZn2, etc) precipitate at the misfit surface of chromium-bearing ones and precipitate-free zone occurs around them more easily compared with zirconium-bearing ones. Thus the plasticity of 7475 is superior to that of 7150.