Origin of Embrittlement of Metallic Glasses

Abstract

Owing to their glassy nature, metallic glasses demonstrate a toughness that is extremely sensitive to the frozen-in configurational state. This sensitivity gives rise to "annealing embrittlement," which is often severe and in many respects limits the technological advancement of these materials. Here, equilibrium configurations (i.e., "inherent states") of a metallic glass are established around the glass transition, and the configurational properties along with the planestrain fracture toughness are evaluated to associate the intrinsic glass toughness with the inherent state properties and identify the fundamental origin of embrittlement. The established correlations reveal a one-to-one correspondence between toughness and shear modulus continuous over a broad range of inherent states, suggesting that annealing embrittlement is controlled almost solely by an increasing resistance to shear flow. This annealing embrittlement sensitivity is shown to vary substantially between metallic glass compositions, and appears to correlate well with the fragility of the metallic glass.ductility | amorphous metal | fracture toughness U nlike conventional oxide glasses, metallic glasses can be considerably tougher, often approaching values typical of engineering metals. Specifically, the toughness of metallic glasses can vary from values as high as 150-200 MPa·m 1/2 (1, 2), comparable to low-carbon steels, to as low as 2-3 MPa·m 1/2 (3, 4), which is more typical of oxide glasses. But, like any glass, metallic glasses tend to embrittle when annealed around the glass transition, as evidenced by a measurable drop in their toughness and their capacity to deform plastically. This phenomenon, typically referred to as annealing embrittlement, has broad scientific and technological ramifications. From a technological perspective, annealing embrittlement is at least partly responsible for the large variability in toughness observed when the glass is quenched at different cooling rates (5-7), thereby limiting their mechanical performance and engineering reliability. From a physics perspective, the physical origin of annealing embrittlement is still a topic of debate and controversy that is far from being settled, as varying and contradictory theories have been proposed by various groups over the years.Wu and Spaepen (8) made the first attempt to relate the toughness of the metallic glass to its structure. They postulated that annealing embrittlement is associated with a reduction in "free volume" in the glass atomic structure as it is relaxed at incrementally lower temperatures, degrading its capacity for plastic rearrangement and flow. Their concept has been adopted by several other groups (9-11). In recent years, an alternative concept has emerged linking the toughness of the metallic glass to its elastic constants, particularly to the ratio of the bulk to shear modulus (or equivalently to Poisson's ratio). This ratio is used to relate the resistances to cavitation and shear flow, respectively (12). Unlike free volume, the elastic con...