Study Of Plastic Deformation Behavior In ZrCuAgAl BMG System

Bulk metallic glasses (BMGs) combine superior mechanical properties such as high strength and large elastic strain with excellent glass forming ability (GFA), making them attractive for certain structural applications. Great success in improving critical diameters of BMGs above 20 mm has been achieved in Zr, Pd, Y, Mg and La-based alloys. However, their main drawback, catastrophic failure through unhindered shear banding, has not been conquered, significantly limiting their structural reliability. Plenty of effort has been made recently to enhance their plasticity. For instance, BMG/crystal composites were found to possess some tensile ductility, since the interaction of glass matrix and second phase crystalline particles effectively inhibit the propagation of shear bands. Pre-straining of BMGs by cold-rolling, compression and surface shot-peening was also found to lead improved plastic strain due to the introduction of shear bands and compressive residual stresses. Interestingly, a number of monolithic BMGs showing remarkable compressive plasticity emerged due to (a) high Poisson's ratio; (b) chemical and microstructural inhomogeneity including phase separation, soft and hard regions with the same composition; (c) in-situ nanocrystallization during deformation; (d) more free volume by cooling faster or minor alloying. These hypotheses have not been fully verified, but it still provides instructive clues to enhance the plasticity of monolithic BMGs.Recently, a series of Zr-(Cu,Ag)-Al BMGs with diameters at least 20 mm was developed, and even 25 g ingots became full glass upon slow cooling in an arc-melting machine in a wide Zr-(Cu,Ag)-Al composition range. These Zr-(Cu,Ag)-Al BMGs still suffer from low plasticity. Therefore, further development of Zr-(Cu,Ag)-Al alloys, simultaneously having good plasticity and relatively high GFA, is desirable. In the present work, a systematic composition tuning in the Zr-(Cu,Ag)-Al alloy system was carried out. After testing more than 100 alloys with fine composition variations, we successfully dig out one composition, which can form 1 mm thick and 10 mm wide BMG plates exhibiting distinct plasticity under both bending and compression. The thermal and mechanical properties, as well as atomic structure for the newly-developed Zr-Cu-Ag-Al BMG are presented. The intrinsic plastic deformability of the new BMG is discussed from the aspects of micro-and atomic structure. In comparison with Zr₄₆Cu_37.6Ag_8.4Al₈, the increase of Zr content in Zr53.8Cu31.6Ag7,oAl7.6 results in the increased portion of Zr-Zr atomic pair in the nearest shell of a pair distribution function. Steep peak profile on the right side of the pair distribution function in Zr_53.8Cu_31.6Ag_7.0A_l7.6 suggests that more atoms are involved upon yielding and the opportunity of forming more shear bands increases. This finding may provide useful guideline for the development of plastic metallic glasses from the structural aspect.