Deformation Mechanisms of Nanostructured Al Alloys

Abstract

Cryomilling has emerged in recent years as an effective approach to process large amounts of nanostructured Al alloys. In the present review, progress in our understanding of the microstructural characteristics and deformation behavior of nanostructured Al alloys processed via consolidation of cryomilled nanostructured powders is reviewed in an effort to elucidate salient findings and point out additional studies needed. A survey of published papers show that the microstructure of consolidated cryomilled Al alloys is comprised of equiaxed grains in the range of ∼ 50 nm to 200 nm, depending primarily on process parameters. Moreover, in many cases the microstructure consists of a supersaturated solid solution, despite the presence of significant amounts of alloying elements. As far as mechanical properties, the tensile behavior of these nanostructured Al systems is characterized by high strength and low strain hardening. The high strength was primarily attributed to three types of strengthening: grain size effect, solid solution hardening and Orowan strengthening. The low strain hardening or work softening behavior was accompanied with the occurrence of Lüders banding. Recent work suggests that there are two approaches that may be effectively used to address the lack of dislocation activity that typically accompany the presence of nanocrystalline grains < 10–50 nm. The first approach involves selective blending of powders to achieve a microstructure that contains multiple length scales (i.e., nanostructured, 50–200 nm, and submicron, 200–750 nm). The second approach involves thermal annealing in an effort to introduce small amounts of submicron grains for improvement of ductility. Recent related work is discussed herein.

Effect of Dislocation Mechanisms during Extrusion of Nanostructured Aluminum Powder Alloy

“…It has been suggested that such phenomena may involve GB sliding or grain rotation accompanied by shortrange diffusion-assisted healing events. [6] A review of these studies [1][2][3][4][6][7][8][9][10][11][12] shows that the data obtained by different investigators are not completely consistent. Various aspects characterizing these consistencies and limitations are identified.…”

“…In recent years, creep and superplasticity, [1,2] cold rolling and annealing, [3] and the quantitative understanding of the mechanical behavior [4] of nanostructured materials have been the subject of a number of studies.…”

“Bulk” Nanocrystalline Metals: Review of the Current State of the Art and Future Opportunities for Copper and Copper Alloys

“…as a function of grain size, which has been reported in a number of studies. 11 In general, the amount of strengthening for grain sizes smaller than 50 nm makes nanocrystalline materials an attractive candidate system for future applications over its coarsegrained counterparts. Moreover, the ability to use solute segregation to grain boundaries, the introduction and dispersion of second phases, and the grains of secondary elements/phases can lead to even better mechanical properties.…”

Temperature-Dependent Mechanical Response of an UFG Aluminum Alloy at High Rates

“…The room temperature results also show a very weak strain hardening behavior, in contrast to the strain hardening observed for commercially available CG Al-5083 H131(This is demonstrated in [31]). This phenomenon is commonly observed with cryomilled UFG materials [1,32]. For UFG materials, the reduced size of the grain interior limits the amount of dislocation accumulation as mobile dislocations will often reach a grain boundary and annihilate before interacting with another dislocation.…”