Comparison of the Effect of Individual and Combined Zr and Mn Additions on the Fracture Behavior of Al-Cu-Li Alloy AA219
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THE fracture behavior of the first two generations of Al-Li alloys has been studied over many years.[1–6] A main conclusion from this research is that there is a detrimental effect of the metastable d¢ phase, because it causes intense slip localization. This results in stress concentration at grain boundaries which, when combined with the presence of grain boundary phases that have poor matrix cohesion (e.g., d and T2) and their associated precipitate-free zones (PFZs), readily leads to intergranular fracture and a low toughness.[7,8] As a result, such alloys have only found limited application in aircraft design, despite the potential they offer for mass reduction.[1–4] More recently, there has been renewed interest in the, so called, ‘‘3rd generation’’ of Al-Li alloys because of the improved mechanical properties they can provide, of high strength combined with significantly better damage tolerance.[5,9] The main reason for the improved performance of these newer materials lies in their lower lithium concentration (1.0 to 1.8 wt pct),[3] which does not exceed the metastable d¢ phase solvus.[4,10] In peak aged tempers suppression of d¢ causes dominance of the T1 phase (together with some S and h¢) which leads to a reduction in slip localization and, when combined with less detrimental grain boundary precipitation, virtually eliminates the intergranular DIMITRIOS TSIVOULAS, Research Associate, and PHILIP B. PRANGNELL, Professor, are with the School of Materials, University of Manchester, Manchester, M13 9PL, U.K. Contact e-mail: [email protected] Manuscript submitted May 17, 2013. Article published online November 8, 2013 1338—VOLUME 45A, MARCH 2014
fracture issues associated with earlier alloy generations.[11,12] Microstructural factors that have a strong influence on the fracture behavior of Al alloys include: the state of matrix and grain boundary precipitation, which is related to the aging condition, the presence of coarse constituent particles, trace impurities, PFZs, the grain size and structure, level of recrystallization, the dispersoids present within the material, and crystallographic texture.[7,8,12–20] In older, d¢-containing Al-Li alloys the main fracture mode is intergranular ductile fracture, which is exacerbated by poor grain boundary cohesive strength and the concentration of slip in shear bands and within grain boundary PFZs.[7,8] In comparison, fracture in more recently developed Al-Li alloys has been reported to involve several synergistic factors including: (i) initiation due to cracking at coarse Fe- and Sicontaining constituent particles at grain boundaries, and the subsequent growth of associated voids, (ii) the spread of intergranular fracture by microvoid formation at finer grain boundary particles, such as dispersoids and equilibrium precipitates formed during age hardening, and (iii) transgranular linkage in void sheets nucleated at matrix dispersoids, which coalesce within shear bands.[8] This is frequently still associated with strain localization, the intensity of w
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