Original ArticlesMechanical properties of partially crystallized aluminum based amorphous alloys
Introduction
Since the discovery of amorphous alloys with good ductility in the Al-Ni-Si system (1), numerous rapidly solidified Al-based amorphous alloys have been reported in the Al-Ln (2), Al-M1-M2 (3), and Al-Ln-M1 4, 5 (Ln = lanthanide metal, Ml and M2 = transition metal) systems. Furthermore, it has been reported that these Al-based amorphous alloys exhibit high tensile strength above 1000 MPa even at Al-rich compositions of 84 to 86 at% (6) and the highest strength reaches as high as 1150 MPa in the as-melt spun ribbons (7). It has subsequently been found 8, 9 that a homogeneous dispersion of nanoscale fcc-Al particles within the amorphous matrix can significantly increase the tensile strength upto 1560 MPa which is about 1.5 times as high as that of the corresponding amorphous single phase alloy. However, the strengthening mechanism has not been well explained yet. Kim et al. (9) suggested that the increase of strength is due to an enhancement of the resistance to shear deformation caused by the nanoscale fcc particles which have higher mechanical strength than the amorphous phase with the same composition. Recently, Zhong et al. (10) attributed the hardening of Al-Ni-Y alloys to the solute enrichment of the remaining amorphous phase, but they appear not to have taken into consideration the presence of nanoscale Al particles and the decrease of the volume fraction of the remaining amorphous phase.
In this paper, we present a quantitative description of the change of mechanical properties in Al-Ni-Y systems consisting of amorphous matrix and nanoscale precipitates using a composite model regarding the material as a nanophase composite.
Section snippets
Mixture model
The alloy was modelled as a nanocomposite consisting of particles embedded in a matrix as shown in Figure 1. It would not be necessary to consider the dislocation motion especially in the case of amorphous and nanocrystalline materials since there is hardly any dislocation motion and therefore there is no hardening in both material phases. In this case the rule of mixtures based on the volume fraction of each phase agrees well with the results of finite element analysis of the unit cell model
Results and discussions
Using the mixture model, the hardness behavior of Al-Ni-Y alloys with various alloy compositions has been investigated. According to Inoue et al’s results for a wide range of alloy compositions, the glass formation for Al-Ni-Y system is in the range of 3 to 22%Y and 4 to 33%Ni, that is about 10 to 50% of solute. However, above 20% of solute the amorphous phase makes the material brittle (4). In the model as the volume fraction of fcc-Al particles fAl increases, the average concentration of
Conclusions
In conclusion, the mechanical properties of melt spun and heat treated Al-Ni-Y alloys with fine nanoscale particles embedded in the amorphous matrix have been analyzed using a composite model which uses the rule of mixtures based on the volume fraction of each phase. The nanoscale fcc-Al particles are treated as a perfect material with a theoretical shear strength. The strength of the amorphous matrix is used from the fully amorphous ribbon’s experimental results assuming that the solute
Acknowledgements
One of the authors (HSK) acknowledges support from the British Council through a British Council Fellowship.
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