The effects of prolonged thermal exposure on the fracture and fatigue behavior of aluminum-lithium alloy 8090
- Univ. of California, Berkeley, CA (United States)
- Univ. of California, Santa Barbara (United States). Materials Dept.
Aluminum-lithium alloys are currently being considered for applications at moderately elevated temperatures; accordingly, a study has been made on the effects of prolonged (100 and 1,000 hours overaging) thermal exposure at 149 C and 260 C on the mechanical properties of a peak-aged Al-Li-Cu-Mg-Zr alloy 8090-T8771. In the as-received T8771 temper, the alloy exhibits an excellent combination of strength ([approximately]500 MPa) and toughness (35 MPa[radical]m) with moderate tensile elongation (4 pct). Overaging at 149 C results in a [approximately]50 pct reduction in ductility and toughness, primarily associated with the growth of equilibrium phases along grain/subgrain boundaries, resulting in formation of solute-depleted precipitate-free zones and coarsening of matrix [delta][prime] and S precipitates; strength levels and fatigue-crack growth rates, however, remain largely unchanged. Thermal exposures at 260 C, conversely, lead to dramatic reductions in strength (by [approximately]50 to 80 pct), toughness (by [approximately]30 pct) and fatigue-crack propagation resistance; crack-growth rates at all [Delta]K levels above [approximately]5 MPa[radical]m are 2 to 3 orders of magnitude faster. Microstructurally, this was associated with complete dissolution of [delta][prime], severe coarsening of S and T[sub 2] precipitates in the matrix, and formation of equilibrium Cu and Mg-rich intermetallic phases in the matrix and along grain boundaries. The resulting lack of planar-slip deformation and low yield strength of 8090 following overaging exposures at 260 C increase the cumulative crack-tip damage per cycle and reduce the tendency for crack-path deflection, thereby accelerating fatigue-crack growth rates. Despite this degradation in properties, the 8090-T8771 alloy has better strength retention and generally superior fatigue-crack growth properties compared to similarly overaged Al-Li-Cu-Zr 2090 and Al-Cu-Zn-Mg 7,150 alloys.
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 5858454
- Journal Information:
- Metallurgical Transactions, A (Physical Metallurgy and Materials Science); (United States), Vol. 24A:10; ISSN 0360-2133
- Country of Publication:
- United States
- Language:
- English
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