Pulling Metallic Glasses Ductile

Abstract

A major shortcoming of most known bulk metallic glasses (BMGs) is that they lack sufficient ductility, or toughness, when fabricated under conditions resulting in bulk glass formation. To address this, processing techniques to improve ductility that mechanically affect the glass have been developed, however it remains unclear for which BMG formers they work, and by how much. We show here that, instead of manipulating the glass state, an applied strain rate can excite the liquid, and simultaneous cooling results in freezing of the excited liquid into a glass with a higher fictive temperature. Microscopically, straining causes the structure to dilate, hence “pulling” the structure energetically up the potential energy landscape. Upon further cooling, the resulting excited liquid freezes into an excited glass that exhibits enhanced ductility. We use Zr44Ti11Cu10Ni10Be25 to demonstrate how pulling BMGs through this excited liquid cooling methods can lead to a tripling of the bending ductility.

Cooling under applied stress rejuvenates amorphous alloys and enhances their ductility

“…3 (d) ]. Further, it was argued that the effect of rejuvenation is enhanced when the typical timescale associated with the cooling process (i.e., the inverse cooling rate) exceeds the characteristic time of the straining process, which is roughly a few percent divided by the strain rate [46]. For the parameters of the present study, this condition is satisfied for all values of the applied stress, thus leading to rejuvenated states with respect to energy levels obtained upon cooling at…”

“…Similar to the definition used in Ref. [46], the strain along the ẑ direction was computed as follows:…”

Cooling under Applied Stress Rejuvenates Amorphous Alloys and Enhances Their Ductility

“…In general, more slowly cooled glass formers settle at lower energy states, and, under external deformation, exhibit higher yield stress and are more brittle [51]. Remarkably, it was recently demonstrated experimentally that ductility can be enhanced when sufficiently large normal stress is applied during the cooling process, leading to highly rejuvenated states [46]. The key parameters of the cooling under stress protocol include the initial temperature of a glass former, the rate of cooling, and the applied stress.…”

“…On the other hand, amorphous alloys can be mechanically annealed to lower energy states by applying time periodic deformation with an amplitude below the yielding point [39][40][41][42][43], and the decay of the potential energy is accelerated when the loading orientation is periodically alternated in two or three spatial dimensions [44,45]. Most recently, it was discovered experimentally that upon cooling under tension, bulk metallic glass formers become highly rejuvenated and their bending ductility can be tripled [46]. These findings were rationalized by invoking the concept of fictive temperature upon cooling when the characteristic timescale of structural relaxation is comparable to the inverse cooling rate.…”

“…These findings were rationalized by invoking the concept of fictive temperature upon cooling when the characteristic timescale of structural relaxation is comparable to the inverse cooling rate. Moreover, the fictive temperature can be increased by the application of sufficiently high strain rate during freezing, which leads to enhanced ductility [46]. However, despite recent progress, optimization and design of the cooling protocols, including stress-versus strain-controlled deformation as well as the range of annealing temperatures and applied stresses, remain a challenging problem.…”