Evolution of the Mechanical Properties of Aluminum Containing Al₈Fe₂Si Precipitates During Equal Channel Angular Pressing

Abstract

Significant differences in the mechanical properties were reported for ultrafine grain (UFG) metals produced by equal channel angular (ECA) pressing, [1] compared to coarsegrained materials. Refinement of a microstructure is seen to be responsible for behaviours such as superplasticity, [2,3] enhanced flow stress [4,5] and Vickers microhardness, [6,7] significant enhancement of the fatigue limit and the fatigue life. [8] In the ECA technique, the sample is strained essentially by approximately simple shear, when pressed many times through a deviated die; this makes this procedure very anisotropic in nature. It should result that the microstructure and the related mechanical behaviour are anisotropic and are strongly dependant on the pressing cycles and routes. [9±11] The main advantage of ECA is that the technique is easy to set up, using commercially available pressing tools. It allows to produce large UFG metals on a large scale with no real restriction on the master alloys. In a previous paper [12] we reported on recycled aluminium alloys with grain size of~85 lm, containing hard Al 8 Fe 2 Si precipites (approximately 1 vol%) processed by ECA. Analyses were focused on the microstructure anisotropy resulting from the materials flow. This was revealed by the evolution after each pass of the Al 8 Fe 2 Si precipitates distribution observed at a micron scale on three different sample orientations (see experimental section of this article). In good purity aluminium, experiments showed that the strength improved by ECA pressing, depends on the impurities and on the temperature of pressing. [13] For the recycled aluminium processed as described earlier [12] Al 8 Fe 2 Si precipitates are located essentially at grain boundaries to improve both the mechanical behaviour and the thermal stability of this UFG material.Macroscopic anistropy of the various processed samples was easily revealed using Vickers microhardness, H V , measured in the X, Y and Z planes as a function of the number of passes through the die (Fig. 1a). H V is significantly increased after the first pass, being twice as much for N = 1 compared to the master alloy in the Y and Z directions. Notably, though an increase in H V is also significant in the X direction, anisotropy is observed with a lesser value compared to Y and Z.The variation of H V was quite similar on the Y and Z planes whereas on the X plane, values were slightly lower up to N = 2. For N = 3, a slight decrease of the microhardness was observed in the Y and Z planes. For N > 3, the values on the X and Z planes were the same but seemingly different from those on the Y plane. In our preceeding paper, [12] Figure 3 shows anistropy depicted by preferential orientation of the precipitates, accentuated with the number of passes, essentially in the Y plane compared to X and Z. The shear direction becomes closer to the X direction, which is qualitatively consistant in term of easily back-deformation direction, with the lesser H V values observed in the X plane, at least for N < 3.Three mec...

The studies of crystallite size and microstrains in aluminum powder prepared by mechanical milling

“…The same material was deformed by ECAP (Fig. 8) [12,13]. Both methods lead to a grain refinement structure of aluminum but it is clear that mechanical milling gives a crystallite size much refined (≈27 nm) then those given by ECAP (≈200 nm).…”

“…Fig. 8 Evolution of the crystallite size against number of passe determined from ECAP process [12,13].…”

“…Milled powder was characterized, after different processing times, by using the X-ray diffraction peak and the Electron Microscopy (SEM and TEM). To this end we have decided to compare the results concerning the size-grain obtained from mechanical milling, with those obtained by ECAP of the same material [12,13]. …”

Microstructure investigation of equal channel angular pressed aluminium by X‐ray diffraction and scanning electron microscopy

“…ECAP was applied to large bulk metals and alloys to improve their mechanical properties. In fact, a superplastic behaviour [3], an enhanced flow stress [4] and Vickers microhardness [5] were reported for several UFG materials.…”

Mechanical Behaviour of Interstitial Free Steel and Pure Al Processed by ECAP

“…10) what is in agreement with work realised on pure aluminium by. [20] Determination of the scalar parameter a of the change of strain path as well as extrusion process of interstitial free steel and aluminium, allowed us to deduce the following conclusions.…”