Observation of Shear Thickening during Compressive Flow of Mg54Y11Ag7Cu28 in the Supercooled Liquid Region

Abstract

Uniaxial compression tests of Mg 54 Y 11 Ag 7 Cu 28 metallic glass were performed in the supercooled liquid region (SCLR) (468 K) at a strain rate range of 5 9 10 -4 to 2 9 10 -3 s -1 . In the course of testing, shear thickening behavior was observed, where the magnitude of the peak stress increases with increasing initial strain rate. The deformation behavior of the glassy metal is discussed with emphasis on the commonly reported stress overshoot, strain softening behavior, and the previously unreported shear thickening behavior observed.Metallic glasses have been the subject of intensive research following their discovery 50 years ago. The rapid crystallization kinetics of these early metallic glass ribbons prevented the study of the material above the glass transition. In the 1980s and 1990s, metallic glasses with slower crystallization kinetics were discovered; these materials are referred to as bulk metallic glasses (BMGs). [1] These new alloys allowed formation of metallic glasses with increased critical casting diameters, and they also permitted study of amorphous metal behavior in the supercooled liquid region (SCLR) above the glass transition temperature (T g ) but below the crystallization temperature. Of particular interest is the near Newtonian flow of BMGs in the SCLR. Strain rate sensitivity values (m values) between 0.8 and 1 are typically reported for low strain rates in the literature. [2] As a result of the Newtonian behavior, BMGs exhibit excellent superplastic properties. Tensile elongations above T g vary widely depending on testing and material parameters, but elongations in excess of 1000 pct have been observed in multiple alloy systems with some elongations exceeding 10,000 pct. [2][3][4] Furthermore, the strength of a BMG in the supercooled region is nearly an order of magnitude lower than the strength of the same material below T g . [5,6] An example of the low flow stresses in the SCLR is the superplastic blow forming performed on a Zr-based BMG. Parts with exceptional surface finish and feature replication were formed with gas pressures as low as 0.2 MPa. [6] Due to the exceptional formability and superplasticity exhibited by BMGs in the SCLR and their exceptional roomtemperature strengths, [7] BMGs show a great deal of promise in MEMS devices and other small scale, mechanically demanding applications. The most widely studied BMGs, such as Zr-based and Pd-based systems, have high densities and are very expensive. Lower density BMG systems, such as those that are Mg and Ca based, have comparatively low room-temperature fracture toughness. [8] The poor performance at room temperature has limited the amount of work being done on monolithic BMGs from these systems. The present study highlights material behavior discovered during compressive deformation in the SCLR of one Mg-based glass alloy. Uniaxial compression testing on cylindrical samples of Mg 54 Y 11 Ag 7 Cu 28 revealed that this alloy exhibits an m value greater than 1 when deformed in a narrow temperature range and at relatively l...