Quasi-static and dynamic compressive deformation behaviors in Zr-based amorphous alloys containing ductile dendrites
Introduction
Since amorphous alloys were developed by conventional casting methods [1], [2], [3], intense advances in these alloys have been made. Particularly, Zr-based amorphous alloys show high amorphous forming ability as well as excellent hardness, stiffness, strength, and corrosion resistance [4], [5], [6], [7], [8], which leads to wide applications to high-performance structural components. For the wider applications, there are problems, typical one of which is the brittle fracture [9], [10], [11], [12], [13]. This brittle fracture can be solved by the fabrication of amorphous alloys containing ductile phases [14], [15], [16], [17], [18], [19], [20]. Fabrication processes of these amorphous alloys include the formation of crystalline dendrites from the amorphous melt [14], [15], [16], casting of reinforcements and amorphous alloys [17], [18], and addition of crystalline particles into the amorphous melt [19], [20]. A Zr-based amorphous alloy, where ductile dendrites of crystalline β phases (structure; bcc) are formed in situ from the amorphous matrix, i.e., an ‘LM2’ alloy (commercial brand name of the Liquidmetal Technologies, Lake Forest, CA, USA, composition; Zr56.2Ti13.8Nb5.0Cu6.9Ni5.6Be12.5 (at%), dendrite size; 6–7 μm, dendrite volume fraction; about 40%), shows the tensile strength of 1470 MPa and ductility of 2.5–3.0% [21]. This result is related with the formation of deformation bands at dendrites and multiple shear bands in the amorphous matrix simultaneously [14], [15], [16], and shows the better properties than those of a monolithic Zr-based amorphous alloy, i.e., an ‘LM1’ alloy (commercial brand name of the Liquidmetal Technologies, Lake Forest, CA, USA, composition; Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 (at%)).
Most of studies on Zr-based amorphous alloys are related to phenomena occurring under a static or quasi-static loading condition, and the deformation behavior occurring under a dynamic loading condition is rarely studied. It is required to obtain the information on dynamic deformation of amorphous alloys so that the alloys can be effectively applied to strategic fields such as defense, aerospace, precision machinery, and automotive industries. Under dynamic loading conditions such as ballistic impact, machining, and high-speed metal forming, the resistance to deformation or fracture is generally lower than under static or quasi-static loading conditions, and the plastic deformation is often highly localized in a narrow region [22], [23], [24], [25], [26]. Qiao et al. [27] investigated the dynamic compressive behavior of Zr-based amorphous alloys, and found that multiple shear bands were not sufficiently formed under a dynamic loading condition, thereby resulting in the lower maximum compressive stresses than those measured under a quasi-static loading condition. Thus, studies on dynamic deformation are essentially needed for the evaluation of alloy designing, microstructural modification, and process control in order to improve dynamic properties, but only limited information is available.
In this study, four Zr-based LM2 alloy plates having different thickness were fabricated by varying cooling rates after a vacuum arc melting. The size of ductile dendrites was varied, while their volume fraction was almost constant. Mechanical properties of the alloy plates were evaluated by conducting quasi-static and dynamic compressive tests. Using a split Hopkinson pressure bar, the dynamic compressive deformation behavior was investigated at a strain rate of about 103 s−1. Deformation mechanisms occurring during quasi-static and dynamic loading conditions were investigated by focusing on how the size of ductile dendrites affected the initiation and propagation of deformation bands or shear bands.
Section snippets
Experimental
An LM2 alloy ingot was arc-melted in a water-cooled copper crucible under a Ti-gettered argon atmosphere to produce a master alloy. Four LM2 alloy plates were made in four water-cooled copper molds, whose inner sizes were 50×15×(3, 5, 7, and 10) mm, by suction casting. These alloy plates of 3, 5, 7, and 10 mm in thickness are referred to as Z3, Z5, Z7, and Z10 alloys, respectively, for convenience. The arc melting and suction casting were conducted in a vacuum chamber, and the direct measurement
Microstructure and hardness
Fig. 2(a) through (d) shows SEM micrographs of the Z3, Z5, Z7, and Z10 alloys. In all the alloys, dendrites are evenly distributed in the amorphous matrix. The volume fraction and size of dendrites are provided in Table 1. The volume fraction of dendrites is almost similar (47–49%) in all the alloys, but their size increases with increasing alloy plate thickness (or with decreasing cooling rate [34]. Fig. 3 shows the EBSD analysis data of the four alloys. Inverse pole figure color maps can be
Discussion
The brittle deformation behavior in amorphous alloys is generally caused by limited plastic deformation mechanisms. This is because the ductility would be restricted without the formation of slips or twins which are essential to plastic deformation in conventional crystalline alloys. Thus, it is essentially needed to beneficially utilize deformation mechanisms of ductile dendrites.
The present amorphous alloys have similar microstructures containing 47–49 vol% of dendrites whose sizes are ranged
Conclusions
Quasi-static and dynamic compressive properties of four Zr-based amorphous alloys whose dendrite size was varied with plate thickness (or cooling rate) were evaluated, and deformation mechanisms related with property improvement were investigated.
- (1)
The present amorphous alloys had similar microstructures containing 47–49 vol% of dendrites whose effective sizes were ranged from 15 μm to 43 μm, but showed the different compressive properties under the quasi-static or dynamic loading. This was
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant (No. 2010-0026981) funded by the Ministry of Education, Science, and Technology, Korea. Authors are grateful to Mr. Hyunmin Kim of POSTECH for his helpful discussion on dynamic compressive properties of the Zr-based amorphous alloys.
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