Effect of Cu and Li Contents on the Serrated Flow Behavior of Al-Cu-Li Based Alloys (original) (raw)
Related papers
Materialia, 2018
Al-Cu-Li alloys are known to exhibit serrated flow behavior, a typical characteristic feature of the dynamic strain aging and the Portevin-Le Chatelier (PLC) effect. In present study, the effect of microstructure and texture on serrated tensile flow behavior in a thick plate of Al-Cu-Li-Mg alloy was investigated. The microstructure consisted of coarse elongated grains with significant texture variation from a {100} <110> major component at surface to {110} <112> major component at the center layer. Samples from different locations across thickness and orientations were subjected to monotonic tensile loading at different strain rates in solutionized as well as under aged temper conditions. All specimen showed serrations in stress-strain curves, but a decreased serrated intensity from surface to center of the plate. Detailed microstructural examination and texture analysis suggested that the solute-dislocation interaction and shearing of the δ' precipitates could be the possible reasons for the PLC effect in this alloy. The throughthickness anisotropy in strength and serrated flow behavior of the alloy are attributed to the grain morphology and the texture variation in the material.
Experimental and computational investigation of the dynamic behavior of Al–Cu–Li alloys
A dislocation-density based crystalline plasticity formulation, finite-element techniques, rational crystallographic orientation relations and a new fracture methodology were used to predict the failure modes associated with the high strain rate behavior of high strength Al-Cu-Li alloys. Widely used aluminum alloy 2195 (AA2195) was taken as the representative of Al-Cu-Li alloys. Experimental characterization using Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) were performed to gain insights on microstructural behavior. The alloy aggregate was modeled with representative microstructures that included precipitates, dispersed particles, and different grain boundary (GB) distributions. The new fracture methodology, based on overlapping elements and phantom nodes, was used with a fracture criteria specialized for fracture on different cleavage planes to investigate dynamic crack nucleation and growth. The compressive behavior of AA2195 under high strain rate loading was compared with that of Al-Cu alloy 2139 to further understand the behavior of the AA2195 with the more ductile AA2139. The predictions quantify how local microstructural effects, due to precipitates and dispersed particles, have a dominant effect on crack initiation and growth.
Mechanical behaviour of aluminium-lithium alloys
Sadhana, 2003
Aluminium-lithium alloys hold promise of providing a breakthrough response to the crying need for lightweight alloys for use as structurals in aerospace applications. Considerable worldwide research has gone into developing a range of these alloys over the last three decades. As a result, substantial understanding has been developed of the microstructure-based micromechanisms of strengthening, of fatigue and fracture as well as of anisotropy in mechanical properties. However, these alloys have not yet greatly displaced the conventionally used denser Al alloys on account of their poorer ductility, fracture toughness and low cycle fatigue resistance. This review aims to summarise the work pertaining to study of structure and mechanical properties with a view to indicate the directions that have been and can be pursued to overcome property limitations.
2001
The role of plastic deformation prior to arti®cial aging on the microstructural evolution and mechanical properties of a novel Al± Li±Cu±X alloy designated AF/C 458 was investigated. Induced plastic deformation ranged from a non-stretched or 0% stretch condition to an 8% stretch, with intermediate stretches of 2%, 4% and 6%. Tensile properties, fractography and quantitative precipitate analysis were acquired from specimens that were water quenched from a solution heat treatment, immediately stretched and arti®cially aged at 150°C. Fractography was investigated through scanning electron microscopy (SEM). Quantitative transmission electron microscopy (TEM) determined the variation in precipitate type, number density, size and volume fraction of the major strengthening precipitates Age hardening curves for each level of mechanical stretch illustrated the enhanced aging kinetics of plastically deformed material. Quantitative TEM indicated that increasing amounts of pre-age stretch were found to greatly aect the competitive precipitation kinetics of T 1 and h HH ah H in AF/C 458 augmenting the volume fraction of ®ne matrix T 1 plates and dramatically decreasing the volume fraction of h HH ah H for isochronal treatments. A quantitative microstructural comparison of specimens exhibiting a given strength demonstrated that the imposed level of cold work dictated the density, size and volume fraction of the competing precipitates. The tensile data indicated a trend of increasing ductility for equivalent yield strengths with the increasing amount of pre-age mechanical stretch and therefore shorter arti®cial aging times. The quantitative precipitate data were used with a computer simulation for yield strength determination. The theoretical simulation reported calculated yield strengths in good accord with experimental results and can thus be used to predict the optimum microstructural con®guration for high strength. Ó
Dynamic and static strain aging effects in Fe-modified Ll2 Al3 Ti
Acta Metallurgica et Materialia, 1994
Serrations have been observed in the stress-strain curves of Fe-modified L 12 A13 Ti-based alloys at intermediate temperatures and related to dynamic interactions between mobile dislocations and solute atoms, i.e. dynamic strain aging effects. Serrations appear only at intermediate, not at high or low temperatures, because of the following reasons: at high temperatures the diffusivity of the solute atoms is high enough that the solute atoms and dislocations move together, thereby requiring no break-away of the dislocations. At low temperatures the diffusivity of the solute atoms is so low that no dynamic interaction can occur, but a static interaction is observed, instead. Thus the entire range of yielding phenomena can be consistently explained in terms of interactions between solute atoms and mobile dislocations and requires no assumptions about changes in the nature of dislocation cores. The solutes giving rise to this effect are likely of substitutional type but have not been identified.
Development and characterization of Al–Li alloys
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2006
Increased strength to weight ratio of aluminium-lithium alloys has attracted material scientists to develop these for aerospace applications. But commercial scale production of these alloys has always been slow in view of difficulties encountered during addition of lithium and in ensuring homogeneous billet composition. A new technique of Li addition has been adapted, which gives maximum recovery of Li in the billet. Using this technique, aluminium-lithium alloys of two different grades for aerospace application were cast. Billets were hot forged and rolled to the thickness range of 3-4 mm and heat-treated for different temper conditions. Mechanical properties were evaluated in T6 (solution treated and artificial aged), T8 (solution treated, cold worked and artificial aged) and T4 (solution treated and natural aged) temper conditions. Both alloys exhibit a strong natural aging response. Reversion for short periods at 180 • C results in decrease of strength. With artificial reaging strength reaches above the T4 temper condition level. Characterization was carried out using optical microscope (OM) and scanning electron microscope (SEM). Experimental investigation shows that addition of lithium at high melt temperature gives lower recovery of Li, and use of impure aluminium adversely affects the mechanical properties of the alloy in all temper conditions.
Fatigue of aluminium—lithium alloys
International Materials Reviews, 1992
alloys are a class of low density, high strength, high stiffness monolithic metallic materials that have been identified as prime candidates for replacing 2000 and 7000 series aluminium alloys currently used in commercial and military aircraft. In this review, the cyclic fatigue strength and fatigue crack propagation characteristics of aluminium-lithium alloys are reviewed in detail with emphasis on the underlying micromechanisms associated with crack advance and their implications to damage tolerant design and lifetime computations. Compared with traditional aerospace aluminium alloys, results on the fatigue of binary AI-Li, experimental AI-Li-Cu, and near commercial AI-Li-Cu-Zr and AI-Li-Cu-Mg-Zr systems indicate that alloying with Li degrades the lowcycle fatigue resistance, though high-cycle fatigue behaviour remains comparable. The alloys, however, display superior (long crack) fatigue crack growth properties, resulting from a prominent role of crack tip shielding, principally due to deflected and tortuous crack path morphologies, induced by the shearable nature of coherent b' precipitates, crystallographic texture, and anisotropic grain structures. Environmental fatigue resistance is comparable with 2000 series alloys and better than 7075-type alloys. The accelerated growth of small fatigue cracks, strong anisotropy, poor short-transverse properties, and a sensitivity to compression overloads are the principal disadvantages of AI-Li alloys. IMR/237 * Although this class of alloys contain alloying elements such as copper, magnesium, and zirconium in addition to aluminium and lithium, the term aluminium-lithium (or Al-Li) alloys is generally used to encompass all aluminium alloys containing greater than 0·5 wt-% lithium. International Materials Reviews 1992 Vol. 37 NO.4 153 157 220 Elongation, % 26 33 2·6 *SHT solution heat treatment; CWO cold water quench. to"y and O"u are the yield and ultimate tensile strengths, respectively. ordered (L1 2 structure), metastable, and spherical J' particles, in rx (fcc) AI-Li solid solution matrix. Owing to the low particle/matrix misfit strains ('" -0'120/0), the precipitates tend to remain coherent with the matrix and retain their spherical morphology, even for particle diameters as large as 300 nm. 54 In the naturally aged condition (or underaged tempers), microstructures show fine homogeneous distributions of J' particles (2-5 nm in diameter) in the matrix and at grain boundaries. With increased aging time and/ or temperature, the matrix J' precipitates coarsen, and equilibrium J particles nucleate heterogeneously along grain boundaries, thereby resulting in the formation of Li-depleted