**1. Introduction**

Metallic glasses (MGs) or amorphous metals are materials composed of metal components but without crystalline structure [1]. In addition to applications as high-performance structural materials [2], the unique structure of MGs makes them promising candidate materials in many other fields. As one of the important applications, MGs can be used as storage and separation media for hydrogen gas (H2), which is a well-known clean energy source. Zr-based MGs for instance have been observed to absorb large amounts of hydrogen [3] with a content even comparable to the best crystalline hydrogen-storage materials [4] due to their large number of interstitial-like sites and favorable hydrogen–metal chemistry. The high solubility and moderate di ffusivity of hydrogen [5] in the amorphous structure also increases H2 permeability in MGs [6,7]. With high H2 permeability, high strength, and corrosion resistance, MG films can be used as H2 separation membranes in hydrogen selective devices [6,8]. As a critical part of the device, MGs must survive mechanical pressure in the gaseous H2 environment. The performance and stability of MGs in H2 have significant impact on device safety, and therefore the e ffects of hydrogen on the mechanical properties of MGs have received increasing attention.

Although the e ffect of hydrogen in MGs has been studied by several groups in recent years, debates remain on questions such as whether hydrogen is a detrimental [7] or beneficial [9,10] element for MGs, or whether doping hydrogen increases [11] or decreases [12] the strength of MGs. Like in crystalline materials, hydrogen can induce embrittlement in MGs [7,13–15]. In contrast, positive effects of hydrogen on plasticity have also been found in some alloys. For instance, in Zr-based

MGs [9,16–18] which have large negative enthalpies of mixing with hydrogen [19], the hydrogen addition increases both glass-forming ability and ductility, opening a new approach to make both stable and ductile MGs. It has been proposed that hydrogen addition in MGs can introduce local structural heterogeneities which prevent single, catastrophic shear bands and promote the formation of multiple shear bands [16,18,20]. A recent work on elastic heterogeneity actually shows that the spatial correlation of the heterogeneities plays an important role in tuning ductility of MGs [21]. Since hydrogen addition is potentially useful in MG alloy design, the underlying mechanisms of how hydrogen addition improves ductility and stability of MGs deserve further investigation.

In this work, the e ffect of solid-solution hydrogen on pop-in behavior of an MG is studied. A pop-in during spherical indentation results from an abrupt shear event along a shear band trajectory [22] which represents early plasticity and is a smaller version of the shear bands observed in bulk MGs [22,23]. The results are interpreted in terms of the hydrogen-induced changes to the MG structure and potential energy landscape, and we discuss how hydrogen can be used to control shear band formation. A Zr-based MG of similar composition to those used in several studies in the literature [17,24] is used.
