Extraordinary Nanocrystalline Pb Whisker Growth from Bi-Mg-Pb Pools in Aluminum Alloy 6026 Moderated through Oriented Attachment

Abstract

The elucidation of spontaneous growth of metal whiskers from metal surfaces is still ongoing, with the mainstream research conducted on Sn whiskers. This work reports on the discovery of Pb whisker growth from Bi-Mg-Pb solid pools found in common machinable aluminum alloy. The whiskers and hillocks display unique morphologies and complex growth that have not been documented beforehand. In contrast to typical understanding of whisker growth, the presented Pb whiskers show a clear nanocrystalline induced growth mechanism, which is a novel concept. Furthermore, the investigated whiskers are also found to be completely composed of nanocrystals throughout their entire length. The performed research gives new insight into nucleation and growth of metal whiskers, which raises new theoretical questions and challenges current theories of spontaneous metal whisker growth. Additionally, this work provides the first microscopic confirmation of recrystallization growth theory of whiskers that relates to oriented attachment of nanocrystals formed within an amorphous metallic matrix. The impact of mechanical stress, generated through Bi oxidation within the pools, is theoretically discussed with relation to the observed whisker and hillock growth. The newly discovered nanocrystalline growth provides a new step towards understanding spontaneous metal whisker growth and possibility of developing nanostructures for potential usage in sensing and electronics applications.

Revealing the Pb Whisker Growth Mechanism from Al-Alloy Surface and Morphological Dependency on Material Stress and Growth Environment

“…However, from the experimental observations at hand, this is clearly not the case. The agglomeration behavior provides support to our proposed theory from a previous paper on oriented attachment growth mode [ 39 ], which allows the reformation of the whisker form and growth in a funnel-like morphology. Furthermore, the other proposed theories cannot explain the observed growth behavior as the diffusion of vacancies of atoms cannot occur beyond the opening of the whisker growth as the pressure change directions prevent overflow of the whisker material into the center of the erupting structure.…”

“…The diffraction investigation of the whisker and faceted hillock reveals that the structures are crystallographically similar and confirm the single crystalline Pb structure throughout the entire structure. The SAED reveals a subset of misoriented subdomains of the structure, which goes in hand with the proposed whisker growth mode through oriented attachment, discussed in our previous paper [ 39 ]. The EDS maps indicate dominant Pb composition in the whisker structure but also shows a slight presence of Bi, which could be a result of the miscibility of up to 15 atm.…”

“…Up to now, the dynamics of the whisker growth are addressed from a several-hour point of view, which is also covered also in our previous publication [ 39 ]. However, for an in-depth understanding of the whisker growth modes and behavior a more time-resolved investigation is required in the seconds-to minute regime.…”

“…In our investigated aluminum alloy system (EN AW 6026), we discovered that Pb whiskers grow from specific Bi-Mg-Pb solid pools in complicated forms and display unique growth behaviors [ 39 ]. Further details about the microstructural characteristics of the investigated material are provided in [ 40 ].…”

“…The transition from a dynamic atomistic to nano building-blocks model allows the tools to explain the different morphologies and structures that are predefined by local stress and the available material from the pool reservoirs. For more details, the reader is encouraged to read the previous work [ 39 ].…”