Effect of solution aggressiveness on the crack - growth resistance and cracking mechanism of AA2024-T3

Aluminium alloy 2024-T3 was examined – using a range of microscopy techniques – at the early stages of corrosion attack to investigate the corrosion-induced cracking mechanism. Two different corrosive environments, exfoliation corrosion (EXCO) and 3.5 % wt. NaCl, were used for the exposure of tensile and pre-notched compact-tension C(T) specimens of AA2024-T3. Different embrittlement mechanisms are noticed for the two investigated corrosive environments. Significant intergranular corrosion (IGC) and grain boundary embrittlement is evident in the specimens exposed to EXCO solution, while this was not the case for the milder solution; comprising of 3.5 % wt. NaCl. With regards to the milder solution, corrosion attack is not restricted to the grain boundary, but evolves transgranularly to the neighbouring grains of the IGC attacked region and, consequently, the grain boundary strength in the direct vicinity is not notably affected. The extent of secondary cracks – after the exposure of C(T) specimens to EXCO solution and the subsequent crack-growth resistance evaluation – were found to correlate with the diameter of the plastically affected zone (≈ 3.78 ± 0.04 mm). Additionally, the depth of these cracks was found to correlate well with the thickness of the intergranular fracture surface, giving evidence that the secondary cracks form due to grain boundary embrittlement; probably attributed to hydrogen embrittlement phenomena.

Research Progress of the Effects of Trace S Element on the Microstructure of Cast Nickel-base Superalloys

In the process of alloy melting and vacuum pouring, the impurity S element will inevitably be introduced into the nickel-base superalloys, which will cause adverse effects on the microstructure and properties of the cast nickel-base superalloys. In this paper, the research progress of S element in cast nickel-base superalloys is summarized. The effects of S element on the microstructure are described in detail from experimental researches and first-principles. The reasons for the grain boundary embrittlement caused by S atom are discussed, and the authors wish to provide some references for the better development of cast nickel-base superalloys.