Effect of a controlled volume fraction of dendritic phases on tensile and compressive ductility in La-based metallic glass matrix composites
Introduction
In the past few years, a new class of composite materials has emerged, combining an amorphous metallic matrix with metallic or ceramic reinforcements of various morphologies [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. The first such materials were formed through an ex situ process by which solid crystalline phases were added to the molten matrix [1], [2], [3]. Later, several groups developed in situ composites in which the reinforcement phase nucleates from a solid solution, most often during the process of cooling from the melt [4], [5], [6], [7], [8], [10], [11]. Beginning with the work of Hays et al. [6], these in situ composites have now been developed in several bulk metallic glass (BMG) forming systems. In these composites, the BMG matrix provides extreme strength, while the presence of reinforcing phases can apparently suppress catastrophic failure due to shear localization, and leads to legitimate plastic flow [2], [4], [5], [6], [10]. The resulting unique combination of ceramic-like strength with metal-like ductility is a strong technological motivation for further work in this area.
Although prior works have demonstrated the benefit of coarse crystalline reinforcements in metallic glasses, there has yet to be a systematic study of the role of reinforcement volume fraction on various properties, especially for the new class of in situ composites. Therefore, it remains unclear which properties can be reasonably described using, e.g., rule-of-mixtures type approaches, and which properties require new physical models. The purpose of this paper is to present the first systematic study of volume-fraction effects on the mechanical properties of model in situ BMG composites. Using specimens with a wide range of reinforcement loadings, we examine yield and fracture stresses, Charpy impact response, fracture surfaces, as well as evidence for plastic asymmetry.
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Experimental procedure
Monolithic La62Cu12Ni12Al14 amorphous alloy and La86−yAl14(Cu, Ni)y (y=1–20) alloy composites were prepared by arc-melting a mixture of La (99.9%), Al (99.9%), Ni (99.98%) and Cu (99.999%) in an argon atmosphere. Since pure α-La is known to precipitate from this system when the La content is increased [11], a systematic variation of composition (via a change in y) can produce BMG composites with a chosen volume fraction of reinforcing α phase. The BMG alloy and its composites were prepared by
Microstructure
The scanning electron micrographs in Fig. 1 show characteristic microstructures obtained in this study. The amorphous alloy exhibits a featureless microstructure, while the BMG composite structures consist of a uniformly distributed crystalline dendritic phase and an amorphous matrix. Based on quantitative image analysis, we find that the volume fraction of crystalline phases increases from 0% to ∼50% as the value of y decreases from 24 to 1 along the La86−yAl14(Cu, Ni)y line (see Table 1).
Discussion
In the present system, we have successfully produced in situ BMG composites that differ primarily in their reinforcement volume fractions, with nominally similar microstructural length scales and glass transition temperatures of the matrix. Therefore, it is reasonable to assume that the different mechanical behaviors we observe among these specimens are most dominantly affected by the dendrite volume fraction, and to examine these behaviors systematically on this basis. In the following
Conclusions
A systematic study has been carried out to identify the effect of second phase reinforcements on the mechanical properties of amorphous alloys based on the composition La86−yAl14(Cu, Ni)y (y=1–24). The most salient results of this work are as follows:
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First, we observe that like most composites, these BMG-matrix composites exhibit tensile and compressive yield strengths that generally obey a rule-of-mixtures relationship. Their ductility and impact toughness however exhibit distinctly non-linear
Acknowledgements
This work was supported primarily by the Singapore-MIT Alliance. C.A.S. acknowledges the support of the US Army research office, under contract DAAD19-03-1-0235, although the views expressed in this work are not endorsed by the sponsor. Collaboration with Prof. W. C. Carter of MIT is gratefully acknowledged.
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