Deformation of glass forming metallic liquids: Configurational changes and their relation to elastic softening

Abstract

The change in the configurational enthalpy of metallic glass forming liquids induced by mechanical deformation and its effect on elastic softening is assessed. The acoustically measured shear modulus is found to decrease with increasing configurational enthalpy by a dependence similar to one obtained by softening via thermal annealing. This establishes that elastic softening is governed by a unique functional relationship between shear modulus and configurational enthalpy. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2717017͔In the recent work of Johnson and co-workers, 1-4 a link between elastic softening and configurational changes in metallic glass forming liquids has been proposed. The steepness of the viscosity dependence on temperature in the vicinity of the glass transition, i.e., the liquid fragility, has been known to be associated with the stored configurational enthalpy since the early work of Angell and co-workers. [5][6][7] Furthermore, the effect of strain rate on viscosity induced by mechanical deformation has also been linked to changes in configurational enthalpy. 8 However, attributing the deformationally induced softening of liquids to a unique functional relation between shear modulus and stored configurational enthalpy is a concept that has just recently been brought to attention. 3 This concept essentially suggests that conversion of mechanical work into stored configurational enthalpy induces softening via a dependence of shear modulus on configurational enthalpy. The thermodynamic state variable controlling flow in this concept is identified to be the isoconfigurational shear modulus. Experimental validation of earlier concepts based on the "free volume" hypothesis 9 has not been possible, mainly due to the lack of a fundamental thermodynamic definition of free volume. In contrast, the isoconfigurational shear modulus is a thermodynamically well-defined and experimentally accessible property, rendering this concept experimentally verifiable. In the present study, the change in configurational enthalpy induced by mechanical work and its effect on the softening of metallic glass forming liquids is evaluated by means of compressive experiments, ultrasonic measurements, and enthalpy recovery tests.For the loading experiments we utilized cylindrical specimens of Pt 57.2 Ni 5.3 Cu 14.7 P 22.5 ͑Refs. 10 and 11͒ and Pd 43 Ni 10 Cu 27 P 20 , 3,12 which we deformed isothermally at constant strain rates. The loading setup described in Ref. 13 is utilized. Deformation was performed for a period of time sufficient to allow a steady flow stress state in the nonNewtonian regime to be attained. Upon unloading, the specimens were quenched as rapidly as possible to capture the configurational state associated with that flow stress.We assessed the elastic softening induced by mechanical deformation by evaluating the isoconfigurational shear modulus at the high-frequency "solidlike" limit G. We evaluated G of the quenched unloaded specimens ultrasonically, 14 and extrapolated the room temperature m...

Anelastic to Plastic Transition in Metallic Glass-Forming Liquids

“…As established previously [12], these liquids accommodate stress by undergoing an overshooting stress-strain response associated with a peak stress c and, ultimately, at a strain of 0:1 stabilize at a steady flow stress state. As argued previously [15], this uniaxial strain to steady state of 0:1, which roughly corresponds to an inelastic shear strain of 0.14, is observed to be universal for metallic glass-forming liquids and coincides with the crossing of megabasins whose average configurational spacing is estimated to be 4 c 4 0:036 0:14 [8]. This typical stress-strain response is also evident in the present sequential tests shown in Fig.…”

“…In the inset in Fig. 4, we plot the transient G data against the transient h data gathered here, superimposed on steady-state G vs h data gathered previously at 548 K for different strain rates [15]. As seen in the plot, the transient G data obey the same functional relationship with h as the steady-state data.…”

“…Indeed, G has previously been shown to be uniquely and reversibly dependent on temperature [10,11,13] as well as strain rate [9,11]. More precisely, G was found to be a unique decreasing function of h independent of whether h is induced by thermal excitation or mechanical deformation [11,15]. This decreasing dependence of G on h reflects the decreasing curvature of the potential energy density function with increasing configurational energy and has been validated computationally [17].…”

“…Ultrasonic measurements of the highfrequency shear modulus G plotted against the measured inelastic strain. Inset: Transient G vs h data gathered in the present study (ᮀ), together with steady-state G vs h data gathered previously ( ) [15]. (Note that the h datum in Ref.…”

“…The isoconfigurational shear moduli at the experiment temperature were then estimated by extrapolating the room temperature measurements using a measured linear Debye-Grüneisen constant of 15 MPa=K [14]. The change in configurational potential energy at each transient stage was evaluated by measuring the change in recovered enthalpy h. Stored enthalpies were recovered by reheating the deformed specimens through the glass transition using differential scanning calorimetry, and changes referenced to the stored enthalpy of a specimen relaxed at 548 K with no imposed deformation were measured (see [15]). Finally, the ability of the material to undergo anelastic recovery at each transient stage was assessed by measuring the fraction of inelastic strain recovered upon subsequent annealing near T g (see [16]).…”