Lattice rotations in precipitate free zones in an Al-Mg-Si alloy
Introduction
Precipitate free zones (PFZs) along grain boundaries (GBs) in age-hardenable aluminium alloys and their relation to material properties have been studied for many years [[1], [2], [3], [4], [5]]. The review by Vasudévan and Doherty [5] covers the earlier works on the topic and concludes that, while particles and precipitates on GBs are the most important sites for nucleation and growth of intergranular ductile fracture, PFZs also serve their part by localising strain and accelerating void nucleation and growth at the GB particles. PFZs form due to vacancy and/or solute diffusion to grain boundaries [6], where solute form precipitates and particles, while at the same time the precipitation potential in the PFZ is suppressed. Whereas small metastable precipitates that form homogeneously throughout the grain are beneficial to strength [7], these GB precipitates and particles can be detrimental to ductility [5]. Because the PFZ lacks hardening precipitates, strain may localise in these regions and thus promote intergranular fracture. This is especially true for GBs inclined to the loading direction so that the shear stress along their PFZs becomes large [8]. However, since the PFZs typically retain some solute in solid solution [6], they will be stronger than pure aluminium [9]. It is clear that the material behaviour depends on the width and work hardening of the PFZs, the GB precipitation, and the grain interior strength, which makes it challenging to isolate influences from individual parameters. Because of this complexity, both experiments and simulations are necessary in order to elucidate the impact of each parameter. This work is an experimental study of lattice orientations inside PFZs of an Al-Mg-Si alloy after deformation. No attempt is made to provide direct information regarding the influences of different parameters, but rather to provide insight into the strain localisation in the PFZs in an age-hardenable aluminium alloy. Such insight should prove relevant for simulations such as the ones by e.g. Pardoen et al. [10,11] that establish continuum models with assumptions based on microstructural knowledge.
A study by Schwellinger [4] showed that PFZs in Al-Mg-Si alloys may develop misorientations of ∼20° relative to their parent grains when such alloys were strained close to fracture. He observed walls with large dislocation densities at the PFZ boundaries (i.e., the interface between PFZ and precipitate strengthened grain), and suggested that these may serve as void initiation sites when impinged by slip bands from the grain interiors. This suggestion was motivated by observations of Gardner et al. [12] and Wilsdorf [13] who showed how dislocation structures can serve as nucleation sites for voids in pure metals. More recent studies on pure tantalum crystals support this idea, and attribute such void formation to vacancy condensation at dislocation cell boundary block walls [14]. Although void nucleation and growth in precipitate strengthened alloys will be different from that in pure metals, it seems reasonable that dislocation structures should have some impact on ductile fracture.
In addition to Schwellinger [4] there are several studies that have reported on dislocation structures in PFZs of aluminium alloys [[15], [16], [17], [18]]. Styczyńska and Łojkowski [15] observed by in situ TEM straining experiments that dislocation sources at GBs became active before sources inside the grain interiors. This resulted in dislocations bowing out from the GB and becoming pinned at the precipitates at the PFZ boundary, before penetrating into the grain interiors. During cyclic loading, Jain [16] observed dislocation networks within grains that terminated at the outer edges of the PFZs. He also observed dislocation structures inside the PFZs similar to those observed by Schwellinger [4]. Watanabe et al. [17] did not study PFZs specifically, but noted that some PFZs in an Al-Mg-Sc alloy under cyclic loading became misoriented relative to their parent grains. Khadyko et al. [18] observed several different dislocation structures in PFZs and their vicinity in an Al-Mg-Si alloy stretched to fracture in uniaxial tension. In most PFZs, both inside and outside the neck, dislocations spanned the PFZs and appeared almost perpendicular to the GB plane. Inside the neck however, some GB PFZs were misoriented relative to their parent grains, or had dislocation walls along their PFZ boundaries. Khadyko et al. [18] also performed crystal plasticity simulations taking the dislocation storage close to PFZs into account, and observed an increased stress and strain localisation within PFZs compared to a model with no PFZ, as well as a lattice rotation within the PFZs. Such effects were larger for PFZs inclined to the loading axis. Even though misoriented PFZs have been observed in strained aluminium alloys for the last 35 years, and several hypotheses exist regarding their importance, the only work attempting to quantify such misorientations is the one by Schwellinger [4]. With the advent of more advanced techniques, it is now possible to investigate the misorientations in greater detail. Therefore, the aim of the present study is to determine crystal orientations within PFZs along grain boundaries in an age-hardenable aluminium alloy.
Quantification of crystal lattice orientations is possible on the nanoscale by automated crystal orientation mapping [19] of scanning precession electron diffraction (SPED) [20,21] data. This technique is perfect for studying orientations within PFZs because it can be combined with a conventional TEM study of the exact same region. This means that imaging, dislocation analysis, and crystal orientation mapping can be performed from the same GB region and neighbourhood.
The alloy used in this study is the lean Al-Mg-Si alloy AA6060. When this alloy is heat treated, small metastable precipitates form homogeneously inside the grains through vacancy and solute condensation from a super saturated solid solution and subsequent growth [[22], [23], [24]]. The main hardening precipitate phase in this alloy system is the β″ precipitate, which forms as long needles along the ⟨100⟩ directions of the aluminium matrix in the peak aged condition (T6 temper) [25]. This precipitate phase has a monoclinic unit cell with a = 1.516 nm, b = 0.405 nm, c = 0.674 nm, and angle β = 105.3 [26]. The needles are coherent with the matrix along their length, and strain their local matrix neighbourhood in their lateral dimensions [27]. This alloy has been used in several other studies, including Khadyko et al. [18] and Frodal et al. [28], and serves as a model alloy, which also has extensive industrial applications. We have deformed the alloy in compression in order to examine a wide range of strains, which would not be possible in tension. A general TEM investigation of PFZs of several GBs in specimens compressed to 5%, 10%, 20%, and 50% engineering strains has been conducted to investigate the variety of PFZ microstructures that forms. The study focuses, however, on a detailed SPED and orientation mapping study of crystal orientations inside PFZs of four individual GBs in the specimen compressed to 20%, and how these GB PFZs relate to the general observations.
Section snippets
Materials
The material used in this work was an extruded profile of the AA6060 aluminium alloy. The composition of the alloy is given in Table 1. Small cylinders measuring 9 mm in diameter and 13 mm in length were machined from the profile with their longitudinal direction along the transverse direction (TD) of the profile, see Fig. 1. The cylinders were subjected to a standard T6 heat treatment as illustrated in Fig. 2. The exact same alloy and heat treatment was studied by Frodal et al. [28], where it
Results
TEM studies of the alloy in the undeformed state were performed to establish a reference for the investigation of the deformed states. Fig. 4 presents TEM images of the microstructure of the alloy prior to deformation. The characteristic diffraction contrast of β″ precipitates is clearly seen in the BF image. The precipitates are oriented with their longitudinal axis along ⟨100⟩ directions and appear as needles along [010] and [001], while the ones along [100] are aligned out-of-plane and
Discussion
The purpose of this study is to investigate how PFZs in a material behave when the material is deformed, with special emphasis on strain and crystal orientations. Based on the results obtained, it is clear that some PFZs can develop significantly different orientations than their parent grains, while others do not. PFZs may develop dislocations, walls, bands, or grains.
Regarding the grains that can form in PFZs, it should be noted that the orientation mapping algorithm can be somewhat
Conclusions
A detailed TEM and SPED orientation mapping study of four different HAGBs in the AA6060 aluminium alloy in peak aged condition compressed by 20% engineering strain has been performed along with a more general BF TEM study of specimens compressed to 5%, 10%, 20% and 50% engineering strain. The results show that different PFZs in the same material can develop significantly different crystal orientations than their parent grains. We also show that in regions where such orientation differences do
Acknowledgments
This research was conducted at the Centre for Advanced Structural Analysis (CASA), funded by the Research Council of Norway [grant number 237885] and several public and company partners.
The TEM work was carried out using the NORTEM infrastructure funded by the Research Council of Norway [grant number 197405].
Data Availability
The raw data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons. The processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons.
References (35)
The influence of a precipitate-free zone on the mechanical properties of an Al-Mg-Zn alloy
Acta Metall.
(1968)- et al.
Ductile fracture in the interior of precipitate free zone in an Al-6.0%Zn-2.6%Mg alloy
Acta Metall.
(1976) - et al.
Grain boundary ductile fracture in precipitation hardened aluminum alloys
Acta Metall.
(1987) - et al.
The origin of the grain boundary precipitate free zone
Acta Metall.
(1969) - et al.
The interaction of dislocations and precipitates
Acta Metall.
(1960) - et al.
Quench sensitivity of toughness in an Al alloy: direct observation and analysis of failure initiation at the precipitate-free zone
Acta Mater.
(2008) - et al.
Strengthening mechanisms in an Al–Mg alloy
Mater. Sci. Eng. A
(2010) - et al.
Grain boundary versus transgranular ductile failure
J. Mech. Phys. Solids
(2003) - et al.
Interface controlled plastic flow modelled by strain gradient plasticity theory
C.R. Mec.
(2012) - et al.
Crack initiation at dislocation cell boundaries in the ductile fracture of metals
Mater. Sci. Eng.
(1977)
The ductile fracture of metals: a microstructural viewpoint
Mater. Sci. Eng.
Do voids nucleate at grain boundaries during ductile rupture?
Acta Mater.
Grain boundaries as dislocation sources in a material with precipitate-free zones
Scr. Metall.
Effects of Al3Sc particle size and precipitate-free zones on fatigue behavior and dislocation structure of an aged Al–Mg–Sc alloy
Int. J. Fatigue
Deformation and strain localization in polycrystals with plastically heterogeneous grains
Int. J. Plast.
Automated crystal orientation and phase mapping in TEM
Mater. Charact.
Double conical beam-rocking system for measurement of integrated electron diffraction intensities
Ultramicroscopy
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