Elsevier

Acta Materialia

Volume 57, Issue 10, June 2009, Pages 3123-3132
Acta Materialia

Influence of equal-channel angular pressing on precipitation in an Al–Zn–Mg–Cu alloy

https://doi.org/10.1016/j.actamat.2009.03.017Get rights and content

Abstract

Processing by equal-channel angular pressing (ECAP) affects the morphology of η precipitates in an Al–Zn–Mg–Cu (Al-7136) alloy. It is shown by transmission electron microscopy that ECAP changes the orientation of precipitates and this influences the atomic configuration and the interfacial energy at the η/α-Al interfaces. Consequently, η precipitates adopt an isotropic growth mode and evolve into equiaxed particles. A three-dimensional atom probe analysis demonstrates that large η precipitates formed in different numbers of ECAP passes are of similar composition. The coalescence of smaller precipitates, rather than the fragmentation of larger precipitates, dominates the precipitate evolution.

Introduction

Ageing treatments and alloy composition engineering are used widely to control the type, size and number density of nanoscale precipitates with the objective of achieving optimum precipitation hardening. In practice, the morphology of nanoscale precipitates is seldom a suitable variable for manipulation using conventional ageing treatments, although the same precipitation phases, such as the η phase in Al–Zn–Mg alloys [1], [2] and the θ phase in Al–Cu alloys [3], may have multiple morphologies. During ageing treatments, the precipitation process follows a fixed transformation sequence dominated by the precipitation kinetics and thermodynamics so that the precipitates generally have a simple orientation relationship with the matrix and this determines their unique morphology, such as rods or platelets [2]. Manipulating the orientation and morphology of nanoscale precipitates has long been a challenge for material scientists such that even microalloying was demonstrated recently to affect the precipitate morphologies and orientations [4].

Processing by severe plastic deformation (SPD) is an effective procedure for producing metals and alloys having ultrafine-grained (UFG) and/or nanocrystalline structures [5], [6], [7]. Of the various SPD techniques currently available, equal-channel angular pressing (ECAP) is especially attractive because it can be scaled-up to produce bulk UFG materials that may be used for structural applications [5], [6], [8]. Early research on the processing of Al and Al alloys by ECAP generally focused on the process of grain refinement [9], [10], [11], [12]. More recently, there has been a growing interest in controlling the precipitation microstructures of Al alloys and thereby achieving a combination of strengthening from both grain refinement and precipitation hardening [13], [14], [15]. The evidence to date suggests that processing by ECAP has a relatively complicated effect on precipitate evolution. Thus, conducting ECAP at room temperature generally suppresses precipitation in Al alloys because no precipitation was reported in as-quenched samples during the process [14], [16] and there was a dissolution of pre-existing θ′ precipitates in the matrix after eight passes of ECAP [13]. On the other hand, when the ECAP is conducted at higher temperatures, such as 473 K, precipitation is promoted in an Al–Zn–Mg alloy [17]. Experiments on an Al–Cu–Mg alloy showed that one pass of ECAP at room temperature initiated a rotation of pre-existing large θ′ precipitates, whereas four passes led to the fragmentation of θ′ plates and the formation of spherical nanoparticles [13]. In an Al–Zn–Mg–Cu (Al-7034) alloy, processing by ECAP at 473 K produced spherical precipitates by fragmentation of the pre-existing larger platelet precipitates [18]. To date, only fragmentation of the larger precipitates has been identified as a mechanism dominating precipitate evolution for materials containing pre-existing large precipitates [13], [18], and accordingly it is not clear at present whether other mechanisms may be involved and, if so, whether they are equally important in precipitate evolution. The results to date have established that the high strains imposed by SPD are effective in altering the precipitate orientations within the matrix. This potentially may provide an effective method for manipulating the precipitate morphology and obtaining unique microstructures that are significantly different from those formed by conventional ageing treatments. Thus, a more complete understanding of the precipitation behaviour during ECAP is an essential prerequisite for successfully manipulating the precipitation microstructures and attaining optimum properties in these materials.

This present investigation was initiated to examine the effect of ECAP on Al-7136 alloy containing a fine and uniform dispersion of precipitates. The specific objective was to influence the early stages of the precipitation process and thereby alter the orientations of any small precipitates, thus effectively manipulating their morphologies. Transmission electron microscopy (TEM) and atom probe tomography (APT) were employed to investigate the evolution of precipitation, the precipitation sequence and the orientation relationships between the precipitates and the matrix. Quantitative APT analysis was used also to provide chemical information on the precipitates and the matrix and the spatial distribution of precipitates during ECAP, and to yield unique information on both the structure and the chemistry of the precipitate evolution [19], [20]. The new information from this research provides additional insight into the precipitation mechanism occurring in SPD processing.

Section snippets

Experimental material and procedures

The Al-7136 alloy was received as extruded rods having the composition shown in Table 1. Prior to ECAP, the rods were cut into short billets having lengths of 64 mm. To evaluate the effect of an ageing treatment, some of these billets were aged at 473 K for various times up to a maximum of 40 min. The remaining samples were processed by ECAP in the as-received condition at a temperature of 473 K for different numbers of passes up to a maximum of eight. Since a time of ∼2 min was needed to heat the

Microstructure of the as-received alloy

The as-received material contained densely distributed ultrafine features with a size of ∼5 nm and dark contrast, as shown in Fig. 1a. Several spherical Al3Zr particles, with diameters of ∼25 nm, are marked with black arrows in the TEM micrograph. The selected area diffraction pattern (SADP) along a <1 1 1>α-Al zone axis (shown in Fig. 1b) contains weak diffractions from precipitates. The weak diffraction at 1/3 of {4 2 2}α-Al, marked with a white arrow, is from GPII zones [26] and the weak

Precipitation during ECAP of the Al–Zn–Mg–Cu alloy

After one pass of ECAP at 473 K, the precipitation microstructure evolves from a mixture of fine coherent GP zones and η′ precipitates in the as-received alloy to a mixture dominated by platelet η but with some remaining η′. In addition, only the η precipitates are present after four and eight passes. This observation shows that η is stable at 473 K in the alloy, which agrees with differential scanning calorimetry measurements showing the thermal stability of η in an Al–Zn–Mg (Al-7050) alloy in

Summary and conclusions

  • 1.

    Processing by ECAP of an Al–Zn–Mg–Cu (Al-7136) alloy at 473 K changes the precipitate orientations in the matrix. The consequent change in interfacial energy at the η/α-Al interfaces promotes η precipitates in an isotropic growth and leads to equiaxed particles in multiple pass samples.

  • 2.

    The precipitate evolution occurring during ECAP at 473 K is about 50 times faster than in conventional ageing treatments at the same temperature.

  • 3.

    The high density of mobile dislocations produced by ECAP promotes the

Acknowledgements

The authors are grateful for scientific and technical input and support from the Australian Microscopy & Microanalysis Research Facility (AMMRF) node at The University of Sydney. This project is in part financially supported by the Australian Research Council (ARC), including ARC Discovery Project (Grant No. DP0772880, Y.B.W. and X.Z.L.), and by the US Army Research Office (Grant No. W911NF-08-1-0201, Z.C.D. and T.G.L.).

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