Environmentally assisted cracking of aluminum-copper-lithium alloys.

摘要

The Al-Cu-Li-Ag-Mg alloys X2095 and X2094 were developed as ultra-high strength replacements for Al-Cu alloys currently used for launch systems. The Ag + Mg addition in these alloys acts as a nucleation aid for the precipitation of strengthening precipitates; particularly, the platelike T{dollar}sb1{dollar}-phase (Al{dollar}sb2{dollar}CuLi). Ultra-high strengths ({dollar}>{dollar}700 MPa) were observed in these alloys for peak-aged tempers. As with other aluminum alloys, the ductility decreases with artificial aging and the highest ductilities are observed in the under-aged tempers. Since other Al-Cu alloys suffer from stress corrosion cracking (SCC) in the under-aged tempers, it was necessary to determine the effect of aging on SCC resistance of X2094 and X2095. In the present study, SCC resistance was determined for both plate and extrusions as a function of artificial aging conditions, using initiation and propagation dependent tests. In addition, the grain boundary and matrix microstructures were investigated using transmission electron microscopy.; Under-aged tempers of X2095 with and without the Ag + Mg addition suffer from intersubgranular corrosion, which results from the electrochemical potential difference between the grain interiors and the boundaries. The active nature of the subgrain boundaries is due in part to the precipitation of T{dollar}sb1{dollar}-phase precipitates, with a possible additional contribution from copper depletion adjacent to the boundaries. This intersubgranular attack does not require stress and results in failures in stressed specimens and reduced mechanical properties in unstressed specimens exposed to alternate immersion in a 3.5% NaCl solution.; Peak-aged X2095 and X2094 suffer from directional pitting associated with the primary-phase precipitates present in their microstructures. This pitting, which does not require stress, results in a degradation in the mechanical properties in X2095 and reduced ductility ratios for CERT of X2094.; Although localized corrosion was present for X2094, X2095 and X2095 (0Ag + 0Mg) exposed to 3.5% NaCl solutions, no SCC was observed in any temper tested. This localized attack, which did not require stress, resulted in decreases in the mechanical properties. Thus, the alloys can be used in damage tolerant tempers with no SCC.