Cooperative Effect of Li Content and Equal-Channel Angular Pressing on Microstructure and Mechanical Properties of Al-Mg-Li Alloy

Abstract

This paper investigated the cooperative effects of Li addition and warm Equal Channel Angular Pressing (ECAP) on improving the mechanical properties of an Al-Mg-Li alloy for further weight reduction. The results showed that more Li addition could obviously refine the grains of the as-cast alloy and reduce its density value to only 2.37 g/cm 3 when Li content reached 2.5% (about 12.6% lighter than commercial Al alloys). It also had a pronounced influence on the precipitations in the Al-5.5Mg-xLi-0.1Zr alloy, i.e., the number density and size of Al 2 MgLi and δ'-Al 3 Li phases. The yield strength of the as-cast alloy was increased with increasing Li, but the elongation deteriorated due to primary network intergranular Al 2 MgLi and more δ' particles in the high Li-containing alloy. The warm multi-pass ECAP process was found to simultaneously improve the strength and ductility of the Al-Mg-Li alloys, and greatly alleviate the detrimental effect of Li addition on the elongation, thereby making the ultrafine-grained (UFG) alloys, a good combination of lightweight and high performance, very attractive for aerospace applications for much higher specific strength than both Al matrix composites and Mg alloys.Al 2 MgLi, which normally precipitates along the grain boundaries (GBs) [5][6][7]. As the lightest metal, Li (0.5 g/cm 3 ) has been well known to give the greatest weight reduction effect on Al (2.7 g/cm 3 ). As reported, each weight percent of lithium into Al alloys lowers approximately 3% of density and simultaneously increases 6% of modulus [8]. It is widely accepted that the main strengthening in binary Al-Li alloys and Al-Mg-Li alloys is generally derived from the large volume fraction of the metastable, coherent, and ordered precipitates (δ'-Al 3 Li phase) with a LI 2 (Cu 3 Au) type superlattice structure. δ' is precipitated homogeneously in the form of spherical particles, which is the main reason for high elastic modulus observed in these alloys [9]. Therefore, Li addition provides a very promising way to develop excellent light-weight Al alloys. Nevertheless, previous works rarely investigated the Al-Li alloys with more Li content. This is probably because the δ'-Al 3 Li phase contributes not only to the high value of strength and elastic modulus, but also to the low ductility and poor fracture toughness due to planar slip. δ' phases are known to be easily sheared by dislocations, attributed to their comparatively small interfacial energy (about 10-250 mJ/m 2 ) and low antiphase boundary energy (180 mJ/m 2 ) resulting in strain localization during deformation [10].In the past few decades, there have been many attempts at solving the problem. One approach to alleviate stress concentrations is to decrease the dislocation glide distance by the refinement of the grain size. Severe plastic deformation (SPD) has shown high effectiveness in improving the mechanical properties of Al alloys by greatly refined microstructure [11][12][13]. Among the commonly used SPD processes, equal-channel-angular pressing (ECAP) is w...