Molecular dynamics simulation of Ga penetration alongΣ5symmetric tilt grain boundaries in an Al bicrystal

Abstract

Liquid metal embrittlement ͑LME͒ is a common feature of systems in which a low melting point liquid metal is in contact with another, higher melting point, polycrystalline metal. While different systems exhibit different LME fracture characteristics, the penetration of nanometer-thick liquid metal films along the grain boundary is one of the hallmarks of the process. We employ EAM potentials optimized for Al-Ga binary alloys in a series of molecular dynamics simulations of an Al bicrystal ͑with a ⌺5 36.9°͑301͒/͓010͔ symmetric tilt boundary͒ in contact with liquid Ga with and without an applied stress. Our simulations clarify the mechanism of LME and how it is affected by applied stresses. The interplay of stress and penetrating Ga atoms leads to the nucleation of a train of dislocations on the grain boundary below the liquid groove root which climbs down the grain boundary at a nearly constant rate. The dislocation climb mechanism and the Ga penetration are coupled. While the dislocations do relax part of the applied stress, the residual stresses keep the grain boundary open, thereby allowing more, fast Ga transport to the penetration front ͑i.e., Ga layer thickening process͒. The coupled Ga transport and "dislocation climb" is the key to the anomalously fast, time-independent penetration of Ga along grain boundaries in Al. The simulations explain a wide range of experimental observations of LME in the Al-Ga literature.

Atomic-scale analysis of liquid-gallium embrittlement of aluminum grain boundaries

“…One of the most widely accepted LME mechanisms suggests that penetration of the liquid metal into grain boundaries (GBs) modifies interfacial energy so as to promote intergranular failure [3,9,10,20,[22][23][24]29,34,35]. Consequently, the reasons for such variation is likely associated with the specific type of liquid environments [10,18,29] tested, whereby some elements act to enhance cohesion in certain systems, while others promote decohesion [28,35].…”

“…However, recent nanoscale experimental studies suggest that one of the primary contributing factors in LME is the formation of intermetallic compounds at GBs [1] and bilayer interfacial phases (e.g., in Ni-Bi) [17]. The lack of a definitive understanding of the mechanistic origin of LME hinders our ability to satisfactorily address LME in important technologies, including in the nuclear-power generation sector, where liquid-metal coolants are in contact with metallic structural components, and in industrial sectors that employ liquid-based joining technologies (e.g., soldering and brazing) [4,9,10,16,19,32].…”

“…High resolution transmission electron microscope (TEM) has provided important insight into LME in Al-Ga [22][23][24]. While Ga does penetrate most GBs in Al, leaving behind Ga layers that range from one to several monolayers [9,[22][23][24]27], in situ TEM studies have shown that low energy boundaries, such as high coincident site density (small ) boundaries, show little segregation [23]. The interplay of the variables that influence embrittlement complicates the separation of which properties affect and do not affect LME.…”

“…Monte Carlo and molecular dynamics (MD) methods have also been widely used to characterize LME [2,5,9,16,23,25,38,39]. While such calculations commonly incorporate a much larger numbers of atoms, they have been limited by their use of empirical descriptions of atomic interactions.…”

“…These metals exhibit a drastic loss of ductility in the presence of certain liquid-metals, such as gallium (Ga), bismuth (Bi), and mercury (Hg) [1][2][3][4][5][6][7][8][9][10]. Understanding the mechanisms behind LME has been of particular interest in both experimental [2,3,5,8,[10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] and simulation [7,9,[26][27][28][29][30][31][32][33] research.…”