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We have characterized the nanoscale mechanical properties of grain boundary precipitate-free zones (PFZ's) in an AlCuSiGe alloy, using combined nanoindentation and in-situ atomic force microscopy (AFM). These mechanical properties were then correlated to the composition, precipitate distribution and, indirectly, to the vacancy concentration within...
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... result highlights the necessity for some non-equilibrium vacancy concentration in the formation of 00 as well. Figure 5a shows an AFM image of a typical AlCuSiGe grain boundary. Figure 5b shows the same area but in plan view, allowing for easy identification of the width of the precipitate-depleted region, where the Si-Ge PFZ is denoted by an arrow. ...
Context 2
... 5a shows an AFM image of a typical AlCuSiGe grain boundary. Figure 5b shows the same area but in plan view, allowing for easy identification of the width of the precipitate-depleted region, where the Si-Ge PFZ is denoted by an arrow. The depleted zone observed by AFM extends about 500 nm on either side of the grain boundary, in agreement with the TEM results. ...
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Citations
... The obtained hardness was influenced by the solute concentration and precipitate distribution. A decrease in nanohardness with decreasing distance from the GB was also observed in Al-Cu-Si-Ge alloys by Radmilovic et al. [13]. Also here, three different regions adjacent to the GB were defined and correlated to the different diffusivity of Cu, Si and Ge and vacancies. ...
... Based on TEM observations in this work and the findings from previous nanoindentation studies on Al-Mg-Si alloys in peak aged condition [12,14,15], the hardness profile across the GB can be divided into the following regions: (I) PFZ, (II) the transition region, (III) precipitation hardening and (IV) grain interior. Possible strengthening mechanisms in this alloy includes solid solution hardening, GB strengthening and precipitation hardening [13]. β ′′ is the most common precipitate phase existing in the peak hardened state and precipitation strengthening is considered to be the main strengthening mechanism in these alloys [20]. ...
Al-Mg-Si (6xxx series) alloys show excellent mechanical properties due to the precipitates formed during heat treatment. However, heat treatment of these alloys results in a soft precipitation free zone (PFZ) close to grain boundaries that weakens them and promotes fracture, and thereby reduces the ductility of the material. This study provides quantitative insights into the mechanical properties and underlying plasticity behavior of Al-Mg-Si (6xxx series) alloys through combined nanoindentation hardness measurements and in-depth characterization of the microstructure adjacent to the PFZ region and in the grain interior. Experimental nanoindentation, transmission microscopy (TEM) and electron channeling contrast imaging results confirm the weakening effect from PFZ by means of a reduced hardness close to grain boundaries. The nanoindentation hardness mapping also revealed an increase in hardness a few micrometers from the grain boundary with respect to the grain interior. Precipitate quantification from TEM images confirms that the hardness increase is caused by a locally higher density of precipitates.To the authors’ best knowledge, this harder zone has not been recognized nor discussed in previously reported findings. The phenomenon has important implications for the mechanical properties of large-grained (>100 µm) aluminium alloys.
... The obtained hardness was influenced by the solute concentration and precipitate distribution. A decrease in nanohardness with decreasing distance from the GB was also observed in Al-Cu-Si-Ge alloys by Radmilovic et al. [13]. Also here, three different regions adjacent to the GB were defined and correlated to the different diffusivity of Cu, Si and Ge and vacancies. ...
... Based on TEM observations in this work and the findings from previous nanoindentation studies on Al-Mg-Si alloys in peak aged condition [12,14,15], the hardness profile across the GB can be divided into the following regions: (I) PFZ, (II) the transition region, (III) precipitation hardening and (IV) grain interior. Possible strengthening mechanisms in this alloy includes solid solution hardening, GB strengthening and precipitation hardening [13]. β ′′ is the most common precipitate phase existing in the peak hardened state and precipitation strengthening is considered to be the main strengthening mechanism in these alloys [20]. ...
... The minimum value of 1.35 GPa is achieved in the proximity of grain boundary which increases to the value of 2.3 GPa at the interface of PFZ and the matrix. Other researchers [16,28] have also determined the variation of nanoindentation hardness within PFZ for different aluminium alloys. For instance, Ogura et al. [16] have observed the much lower value of nanoindentation hardness within PFZ as compared to the grain interior and revealed that the hardness values decrease towards the grain boundary. ...
... They attributed the hardness within PFZ only to the solid solution strengthening as no precipitate is formed in the PFZ. Similarly, Radmilovic et al. [28] determined that for the Al-Cu-Si-Ge alloy, the lowest values of nanoindentation hardness (1.6 GPa) were achieved in the solute depletion zone (90 nm) and increased up to 2.8 GPa in the region of fine precipitation. Fig. 6a shows the effect of aging temperature and time on the PFZ width. ...
The present study is focussed on investigating the effect of heat treatment variables on the formation of precipitate free zones (PFZs) in the AZ80 Mg alloy. The effect of solutionizing temperature, quenching media, aging temperature and time on the PFZ development has been systematically examined. Scanning electron microscopy and nanoindentation were employed to characterize the PFZ regions. Electron probe micro analysis (EPMA) was used to examine the role of solute depletion in the formation of these zones. Moreover, the estimation of vacancy concentration profiles near the grain boundary was used to investigate the role of vacancy depletion. The solute depletion is found to be the dominant mechanism in the development of PFZ regions as compared to vacancy.
... The diminished concentration of vacancies adjacent to the grain boundaries are thought to slow precipitation kinetics and lead to a deficiency of b 00 precipitation near the boundaries. The PFZ immediately adjacent to the grain boundary is expected to exhibit yield strengths closer to those for the solid solution matrix in the aged AA6063 alloy than for the peak aged microstructure [10]. ...
The effect of specimen orientation and extrusion seam welds on the fatigue life of specimens milled from a hollow AA6063 aluminum alloy extrusion profile have been characterized in the form of S–N curves. The fatigue behavior of the fully recrystallized AA6063 alloy reported here is compared to previously reported results on a partially recrystallized AA6082 extruded aluminum alloy [Nanninga N, White C, Furu T, Anderson O, Dickson R. Effect of orientation and extrusion welds on the fatigue life of an Al–Mg–Si–Mn alloy. Int J Fatigue 2008; 30(9): 1569–78]. Specimen orientation with respect to the extrusion direction has a large effect on fatigue life, and is attributed to the orientation of extrusion die lines. Transverse specimens exhibit only a fraction of the high cycle fatigue life exhibited by longitudinal specimens. The presence of seam and/or charge welds, oriented both longitudinal and transverse to the loading direction, may also reduce the fatigue life. However, the effects of these welds are not as significant as the effects of orientation alone, and appear to be related to increased surface roughness at the welds. Variations in fatigue life are explained by surface roughness and variations in microstructure. Grain boundary separation was the primary mechanism of fatigue crack initiation in this study. Results from Auger energy spectroscopy and transmission electron microscope analyses suggest that localized strain in precipitate free zones may assist in this behavior.
... Since vacancy concentrations and particle growth kinetics are temperature and rate dependant, low solutionizing temperatures and fast quench rates can minimize the width of PFZs. The variation in hardness from the boundary edge to the bulk of a grain in a Al-Cu-Si-Ge alloy has been measured using nano-indentation [62]. The hardness was reduced by 75% near the grain boundary, within a PFZ. ...
The high cycle fatigue behavior of hollow extruded AA6082 and AA6063 aluminum extrusions has been studied. Hollow extruded aluminum profiles can be processed into intricate shapes, and may be suitable replacements for fatigue critical automotive applications requiring reduced weight. There are several features inherent in hollow aluminum extrusions, such as seam welds, charge welds, microstructural variations and die lines. The effects of such extrusion variables on high cycle fatigue properties were studied by taking specimens from an actual car bumper extrusion. It appears that extrusion die lines create large anisotropy differences in fatigue properties, while welds themselves have little effect on fatigue lives. Removal of die lines greatly increased fatigue properties of AA6082 specimens taken transverse to the extrusion direction. Without die lines, anisotropy in fatigue properties between AA6082 specimens taken longitudinal and transverse to the extrusion direction, was significantly reduced, and properties associated with the orientation of the microstructure appears to be isotropic. A fibrous microstructure for AA6082 specimens showed great improvements in fatigue behavior. The effects of elevated temperatures and exposure of specimens to NaCl solutions was also studied. Exposure to the salt solution greatly reduced the fatigue lives of specimens, while elevated temperatures showed more moderate reductions in fatigue lives.
A novel Al-13.2Zn-2.5Mg-1.2Cu-0.2Zr alloy was prepared by melt spinning and extrusion, and then further strengthened by solution at 470 °C and artificial aging (AA) at 120 °C. A bimodal grain structure, consisting of strong <001> and <111> double fiber textures along extrusion direction, was obtained by homogenization at 450 °C for 30min, followed by hot extrusion at 450 °C. Precipitate-assisted dynamic recrystallization and the pinning effect of undissolved second-phase particles on grain coarsening play a key role in forming the bimodal grain structure together. As-extruded alloy realizes strength-ductility combination mainly due to the existences of bimodal grain structure, hard-oriented <001> and soft-oriented <111> textures and high-density precipitates. Further solution treatment promotes the dissolution of most second-phase particles, and subsequent AA induces the precipitation of higher density nano-sized η′-phase, resulting in a notable increase in strength. However, the coarsening of undissolved second-phase particles and the thickening of precipitate free zones near grain boundary lead to a notable loss in ductility. As a result, the peak-aged alloy shows a higher strength but a lower elongation as compared to the as-extruded alloy.
The mechanisms of formation and growth of L12 precipitate-free zones (PFZs) in the alumina-forming austenitic stainless steel Fe–20Cr–30Ni–2Nb–5Al during creep were studied using scanning electron microscopy and energy dispersive x-ray spectroscopy. The PFZs formed following the dissolution of L12 precipitates due to depletion of nickel and aluminum. Little PFZ growth occurred until the grain boundaries were substantially covered by Laves phase and B2 precipitates large enough to deplete the surrounding grain of nickel and aluminum. This resulted in the dissolution of the L12 precipitates. Additionally, micro-cracks observed in the PFZ at long creep times suggest that the PFZ is ultimately the weakest point in the microstructure, and that fracture initiates there.
Ternary diffusion behavior in Co-Al-V ternary alloys was investigated at 1373 K and 1473 K (1100 °C and 1200 °C) by the solid-state diffusion-couple technique. The extraction and interpolation of diffusion data allows the diffusion properties of Fcc Co-Al-V alloys to be mapped in the composition arrays of Al and V. A full picture of the diffusion properties was then constructed by interpolating all accessible interdiffusivities and impurity diffusivities of Co-Al binary and Co-Al-V ternary with a Redlich–Kister polynomial, in a graphic manner depicting a rapid increase of Al diffusion with increasing Al and a weak decrease with the V addition alone. Further incorporation of a nanoindentation technique enables the nanohardness property of the Co-Al-V fcc alloys to be screened in the Al and V arrays. The hardenability in the Co-Al-V alloy system has been evidenced; specifically, the alloy arrays containing higher contents of V, being solution-and-quenching processed, exhibit more effective strengthening than those with the addition of Al. The discovery of Co-Al-V alloys with comparable nanohardness but differing alloy compositions could facilitate the strengthening design of next generation Co-based alloys.
An extremely fast hardening response with no reduction in peak hardness was obtained in Al–Cu with minor additions of Si and Ge by 8% plastic deformation before artificial aging. The mechanism for the accelerated hardening was determined by detailed characterization with transmission electron microscopy. Plastic deformation was found to enhance the nucleation rate of Si–Ge precipitates, resulting in a higher volume density. Such precipitates catalyzed the formation of θ′ precipitates that are responsible for hardening.
