Michal Demkowicz

Affiliation: Materials Science and Engineering, Texas A&M University, College Station, TX, 77843

Preferential degradation of coherent twin boundaries in Ni and Ni-base alloys

Coherent twin boundaries (CTBs) are frequently thought to be the grain boundaries most
resistant to degradation in FCC metals. I will present two counterexamples to this widely-
held view. In the first, we observe long, corrosion-induced trenches along CTBs in pure
Ni under cathodic charging. These trenches form by the nucleation, growth, and overlap
of conical cavities along symmetric incoherent twin boundary facets on CTBs. In the
second, we use in situ tensile testing in a scanning electron microscope to investigate
intergranular fracture in hydrogen-charged Ni-base alloys and find that CTBs are in fact
preferential sites for crack initiation in these materials. Using a large experimental
database of cracked CTBs and a specially-developed statistical inference method, we find
that the conditions for fracture along these boundaries involve simultaneous mode-I
loading and slip along the CTB plane. Our findings indicate that—far from being
universally degradation-resistant—CTBs may be a material’s weakest links under certain
environmental conditions.

Speaker Bio

Michal J. Demkowicz is an Associate Professor in Materials Science and Engineering at Texas
A&M University. Prior to that, he was an Associate Professor at MIT’s Department of Materials
Science and Engineering. Demkowicz received his PhD in Mechanical Engineering from MIT in
2005, working with Ali S. Argon. During that time, he was an NSF fellow. During 2005-2008, he
was a Director’s Postdoctoral Fellow and technical staff member at Los Alamos National
Laboratory. In 2007, Demkowicz was honored with the LANL Postdoctoral Distinguished
Performance Award. He received the TMS Early Career Faculty Fellow and NSF CAREER awards
in 2012, the MIT Graduate Materials Council outstanding teacher award in 2014 and 2015, and
the Texas A&M University Dean of Engineering Excellence Award in 2018. Demkowicz is an
author on over 100 peer-reviewed publications with more than 2000 citations and a Web of
Science H-factor of 28. His research interests lie in the fundamental physics of metals and metal
composite behavior, especially their mechanical and radiation response.