Effect of melt temperature on the mechanical properties of bulk metallic glasses
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The effect of melt temperature on the structure and mechanical properties of three Zr-based bulk metallic glasses (BMGs)—Zr62Cu17Ni13Al8, Zr55Cu20Ni10Al10Ti5, and Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit105)—has been studied. The results show that the BMGs cast from higher melt temperature exhibit large plastic strains despite their amorphous structure. The samples become macroscopically brittle when the quenched-in crystals form an interconnected microstructure. In contrast to previous studies, there is no notable effect on the Poisson’s ratio (n) and other elastic constants. I. INTRODUCTION
The amorphous structure of bulk metallic glasses (BMGs) results in a range of attractive properties including, large elastic strain limit, high strength, high corrosion resistance, and potential for shaping precise microparts.1–15 However, a majority of BMGs exhibit no tensile ductility that limits their field of applications. This is because the plastic deformation in metallic glasses is localized in shear bands, and consequently strain softening occurs instead of strain hardening.2,5,10,16 The formation of shear bands is controlled by the geometry of deformation tests, microstructure, and the chemistry of BMGs.10,11,17–23 In crystalline materials, it is known that the materials with a lower shear modulus (m) to bulk modulus (B) ratio (m/B) or higher Poisson’s ratio (n) tend to be more ductile.24 A similar analogy has been used for explaining the plasticity and brittleness of BMGs.25,26 A critical value of m/B (0.41–0.43) or n (0.31–0.32) has been suggested for a transition from tough to brittle behavior for BMGs.25,27 The micrographs of fractured BMGs typically suggest that the plasticity assessed from the compressive stressstrain curves scales with the number of shear bands formed and the shear offset.19,28,29 The alloys with multiple and wavy shear bands exhibit a larger plastic strain than those with fewer and straighter shear bands. Thus, there are primarily two factors that govern the plasticity of BMGs, i.e., ease in forming shear bands and their proliferation. The high Poisson’s ratio is related to the ease in shearing, whereas the multiple shear band formation must be due to the presence of large shear nucleation or pinning sites. The Poisson’s ratio has been thought to a)
Address all correspondence to this author. e-mail: [email protected] Present address: Yale University, BC216, Prospect St. 15, New Haven, Connecticut 06511. DOI: 10.1557/JMR.2009.0272 J. Mater. Res., Vol. 24, No. 7, Jul 2009
be independent of density.30 A decrease in Poisson’s ratio upon annealing has also been observed25,27; however, the magnitude of change is too small to account for the annealing-induced severe embrittlement. The complex dependence of Poisson’s ratio on microstructure or local atomic structure is not yet well understood, and this remains a crucial problem in the ability to predict and optimize the mechanical properties of BMGs composites. Although the Poisson’s ratio can be modified by changing composition,31 recent studies reveal
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