Microstructure and Mechanical Properties of Nanostructured Al-4Cu Alloy Produced by Mechanical Alloying and Vacuum Hot Pressing

Abstract

Bulk nanostructured Al-4 wt pct Cu alloy with high compression strength (879 MPa) was produced by mechanical alloying followed by vacuum hot pressing (VHP). The hot-pressed compacts were nanostructured with a grain size of 50 nm and were densified to more than 99 pct of the theoretical density. Contributions from different strengthening mechanisms were estimated using simplified models and were compared with the experiment.Ultrafine-grained (UFG) and nanocrystalline (nc) materials are increasingly being studied due to their improved mechanical properties. Bulk UFG or nc metals and alloys are commonly produced by methods based on severe plastic deformation (SPD). These SPD techniques, such as high energy ball milling, [1] equal channel angular pressing, [2] and high pressure torsion, [3] introduce very large plastic strains leading to grain refinement in coarse-grained powder or bulk materials. High energy ball milling can also be used to produce homogeneous supersaturated solid solutions from elemental powder blends. [4] The nc powders produced by high energy ball milling are usually consolidated into bulk by hot pressing, spark plasma sintering, and hot isostatic pressing followed by hot extrusion. [5,6] The Al-Cu alloys are important due to their high strength, which is achieved through precipitation strengthening. Recent work has shown that in precipitation-hardened UFG alloys, simultaneous improvement in strength and ductility could be achieved through SPD followed by annealing and aging. [7] There is very limited literature on the microstructure and mechanical properties of nanostructured Al-Cu alloys. Fogagnolo et al. studied solid solution formation in an Al-4.5 pct Cu alloy during milling and characterized the thermal stability during subsequent heat treatment. [8] The present work aims to synthesize nc Al-4 wt pct Cu binary alloy from elemental Al and Cu powder blend by high energy ball milling and to study the mechanical properties on bulk samples obtained via consolidation of the nc powders.Al-4 wt pct Cu alloy was prepared by mechanical alloying from high-purity (99.9 pct) elemental Al and Cu powders using a Retsch PM400 (RETSCH PM 400, Haan, Germany) high-energy planetary ball mill at 200 rpm and a ball-to-powder weight ratio of 14:1 using a hardened steel vessel and balls (10-mm diameter). Stearic acid (1 wt pct) was added as the process control agent. The loading and unloading of the powders were done in a glove box in argon atmosphere. Samples were taken at regular intervals during milling and analyzed using X-ray diffraction (XRD) with Cu K a radiation. From the XRD peak profile, instrumental broadening and K a2 components were subtracted and then the grain size (crystallite size) and lattice strain were estimated using the Williamson and Hall method. [9] Lattice parameters of Al were calculated from four peaks using the Cohen method. [10] The Cu concentration in solid solution was determined from the values of the lattice parameter using Vegard's law. Hardness variation as a function of milling ...

Microstructure and Mechanical Properties of Oxide-Dispersion Strengthened Al6063 Alloy with Ultra-Fine Grain Structure

“…In other words, the role of the Orowan mechanism on the strengthening became less important as the grain structure of the Al alloy was refined while the volume fraction of the particles was low. In agreement with other reports, [24,51] this finding can be rationalized by the fact that UFG structure limit the length of dislocation pileup and hence the magnitude of stress concentration at the grain boundaries. [24] As a result, a high applied stress is required to promote slip transmission across grain boundaries.…”

“…High-strength bulk nanostructured and UFG materials also are prepared through consolidation of mechanically alloyed powders. [24][25][26] It is known that the mechanical behavior of UFG materials is different than those of CG materials, particularly with respect to the effect of grain size [27] and strain-hardening behavior. [28] Grain-boundary shear, which affects dislocations pile-up, [29] grain-boundary stability of severeplastic deformed materials, [30] dynamic recovery, [31] and dislocation bow-out, [32] have been proposed to describe the mechanical behavior of UFG materials.…”

Phase Stability of a Mechanically Alloyed CoCrCuFeNi High Entropy Alloy

“…[17][18][19][20][21][22][23] Milling variables (e.g., milling time, velocity, and media) influence the alloying, grain refinement, and final mechanical properties of the bulk compacts. [24][25][26] Among the HEA systems, CoCrFeNi have been extensively investigated over the past 13 years. [1][2][3][4][5][6]27] Cu, Al, and Ti are often added separately or together as minor or equivalent amount in CoCrFeNi system to improve the corrosion and mechanical properties.…”

Effect of strengthening mechanisms on cold workability and instantaneous strain hardening behavior during grain refinement of AA 6061-10wt.% TiO2 composite prepared by mechanical alloying

“…It was observed from Table 3 that the grain boundary strengthening of 40 h UFG sintered composite was considerably high by a factor of 2.3 times higher than 1 h CG sintered composite. The increase in strength due to forest dislocation can be estimated by dis =˛MGb 1/2 (5) where˛is a constant of the order of 0.3, M is the Taylor factor (M = 3 for FCC material), dis is the dislocation density [32] and can be determined from XRD analysis using the relation [33]:…”

“…The total strength expected due to the combination of these various strengthening mechanism can be expressed according to superposition laws [32] tot = gs + n gs + n dis + n pt + n disper 1/n (11) where n can vary between 1 (linear superposition) and 2 (Pythagorean superposition). Based upon the microstructural Table 3.…”