Activation volume details from nonlinear anelastic deformation of a metallic glass

Abstract

At high stress, the viscosity of a metallic glass is non-Newtonian, and therefore the rate of anelastic stress relaxation is not linear in the applied stress. In this regime, one can obtain information on the details of the activation volume that are not accessible in the linear regime. While bending in the nonlinear regime introduces a complicated stress state, it offers great stability for noninstrumented measurements over many orders of magnitude of time. We have developed a method of controlled sample bending to a strain of up to ∼0.0155 for Al 86.8 Ni 3.7 Y 9.5 metallic glass. Significant nonlinearity of the anelastic strain in the stress was observed, which is mainly associated with the largest and slowest shear transformation zones involved not reaching mechanical equilibrium at the end of the constraining period. Combining nonlinear kinetics under constraint and zero bending moment after constraint removal, the volume of the largest shear transformation zones and the transformation shear strain were obtained independently for the inherent state-their most likely values are 4.8 × 10 −28 m 3 and 0.18, respectively.

Structural Relaxation, Rejuvenation and Plasticity of Metallic Glasses: Microscopic Details from Anelastic Relaxation Spectra

“…Comparison between the two-parameter fit (dotted line) and two-step fit (dashed line) (see Ref. [28]): the latter yields a better fit for the small-strain data than the former. Reproduced from Lei, T.J.; Atzmon, M. Activation volume details from nonlinear anelastic deformation of a metallic glass.…”

“…Deviation from linearity occurs at high strain. Comparison between the two-parameter fit (dotted line) and two-step fit (dashed line) (see Ref [28]…”

What renders bulk metallic glass ductile/brittle? -- new insight from the medium-range order

“…The estimated MRO volume of Ω ST Z = (0.52 ± 0.21) nm 3 for as-cast Vit105 shows an excellent agreement with the literature values of STZ volumes of various brittle MGs Ω ST Z = (0.2 − 0.8) nm 3 (Tab. I) [15][16][17] . On the other hand, at first glance the estimated volumes (V 1 or V 2 ) for as-cast Pd 40 Ni 40 P 20 do not agree with the reported literature value of about Ω ST Z = 6.5 nm 318 .…”

“…Microstructurally, the structure-property relation of brittle/ductile behavior of MGs has been associated with the size (volume) of shear transformation zones (STZs), which carry the deformation. For brittle MGs, several investigations using rate-jump nanoindentation testing 15,16 or bending tests 17 showed relatively small STZ volumes of Ω ST Z = 0.2 − 0.8 nm 3 . On the other hand, the STZ volumes of ductile BMGs were found to be larger, i.e.…”