Fatigue crack propagation and cryogenic fracture toughness behavior in powder metallurgy aluminum-lithium alloys

Abstract

Fatigue crack propagation and cryogenic fracture toughness properties of powder metallurgy (P/M) aluminum-lithium alloys have been examined by studying the behavior in mechanically alloyed (MA) Al−4.0Mg−1.5Li−1.1C−0.8O2 (IN-905XL) and rapid solidification processed (RSP) Al−2.6Li−1.0Cu−0.5Mg−0.5Zr (Allied 644-B) extrusions. Results are presented as a function of microstructure, mean stress, and specimen orientation and are compared with previous data on equivalent high-strength aluminum alloys fabricated by both ingot metallurgy (I/M) and P/M methods. It is found that the fatigue crack propagation resistance of the RSP Al−Li alloy is superior to traditional RSP aluminum alloys without lithium and even comparable to I/M Al−Li alloys, particularly at near-threshold and intermediate stress intensity levels. In contrast, crack growth rates in MA 905XL P/M extrusions are nearly three orders of magnitude faster and do not show benefits of alloying with lithium. Growth rate behavior in both alloys, however, is anisotropic; for example, crack growth rates in RSP 644-B alloy are up to three orders of magnitude faster in theT-L, compared toL-T, orientation. However, when characterized in terms of a closure-corrected near-tip “driving force,” ΔKff such differences are reduced. With respect to toughness, plane strainKIc values (L-T orientation) in the RSP alloy are observed to increase with decrease in temperature from 298 to 77 K; conversely, the MA alloy shows a small decrease inKIc at 77 K. Such results are interpreted in terms of the micromechanisms influencing fatigue and fracture behavior in Al−Li alloys, specifically involving the microstructural role of hardening mechanism, slip mode, grain structure, and texture on the development of crack tip shielding (crack path deflection and crack closure) and short-transverse delamination cracking.