Strain-controlling mechanical behavior in noncrystalline materials. I: Onset of plastic deformation

In noncrystalline materials, a new critical condition is proposed that the controlling mechanism for the onset of plastic deformation depends directly on a critical strain. The yield strain-controlling the initiation of nonlinear plastic deformation, falls within a band of similar large values for all the classes of noncrystalline materials like oxide glasses, metallic glasses, and amorphous polymers, covering a large variation in Young's modulus. It has been shown in Zr-based noncrystalline alloys that heterogeneous and homogeneous plastic deformation initiate at a constant strain independent of the glass and supercooled liquid state. The magnitude of yield stress achieved at the constant critical strain drastically changes depending on the viscoelastic behavior and influences the deformation mechanisms. In glassy amorphous alloys with high elastic modulus, the formation of shear bands leads to their spontaneous growth under high yield strength, thereby no strain rate effect, while amorphous polymers with lower elastic modulus have thermally activated deformation. This strain criterion can also better explain the pressure effect on yield in amorphous polymers but the lack of pressure effect on metallic glasses, and the variation of the slip plane in tension and compression.