Observations of grain-boundary phase transformations in an elemental metal

Abstract

While the theory of grain boundary (GB) structure has a long history 1 and the proposition that grain boundaries can undergo phase transformations was made already 50 years ago 2 , 3 , the search for these GB transitions has been in vain. The underlying assumption was that multiple stable and metastable states exist for different grain boundary orientations 4 , 5 , 6 . The terminology complexion was recently proposed to distinguish between those interfacial states that differ in any equilibrium thermodynamic property 7 . Different types of complexions and transitions between them have been characterized mostly in binary or multicomponent systems 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 . Novel simulation schemes have provided a new perspective on the phase behavior of interfaces and showed that grain boundary transitions can occur in a multitude of material systems 20 , 21 , 22 , 23 , 24 . However, their direct experimental observation and transformation kinetics in an elemental metal remained elusive. Here, we show atomic scale grain boundary phase coexistence and transformations at symmetric and asymmetric [11-1] tilt grain boundaries in elemental copper (Cu). We found the coexistence of two different grain boundary structures at Σ19b grain boundaries by atomic resolution imaging. Evolutionary grain boundary structure search and clustering analysis 21 , 25 , 26 finds the same structures and predicts the properties of these GB phases. Furthermore, finite temperature molecular dynamics simulations explore their coexistence and transformation kinetics. Our results demonstrate how grain boundary phases can be kinetically trapped enabling atomic scale room temperature observations. Our work paves the way for atomic scale in situ studies of grain boundary phase transformations in metallic grain boundaries. In the past, only indirect measurements have indicated the existence of interfacial transitions 9 , 15 , 27 , 28 , 29 . Now, their atomic scale role on the influence of abnormal grain growth, non-Arrhenius type diffusion or liquid metal embrittlement can be explored.

Segregation-Induced Nanofaceting Transition at an Asymmetric Tilt Grain Boundary in Copper

“…Both types, non-congruent (faceting) and congruent transitions are referred to as GB phase transformations [5] or synonymously GB complexion transitions [6,7]. However, these transitions are typically studied in perfectly symmetric and planar GBs [8][9][10] and mostly derived from indirect experimental measurements [11,12] or atomistic simulations [13]. Instances of how complex such GB transitions can be and how factors like temperature and chemistry can influence a GB's structure were recently demonstrated by scanning transmission electron microscopy (STEM).…”

Phase transformations observed at the interfaces between crystalline grains in pure metals

“…Another issue is that the large number of possible grain-boundary polymorphs will make it difficult to systematically determine polymorph properties. High-throughput computational methods based on machine learning and big data will be of help here 1,13 . Indeed, Meiners and colleagues' work is a promising example of how the synergistic combination of high-resolution microscopy techniques and computational methods can lead to conceptual breakthroughs in the study of grain boundaries.…”

“…The answer depends on how you define a bird, how you recognize the most basal (earliest diverging) birds in the fossil record, and how you account for the palaeontological and genetic gaps in our knowledge of bird evolution. On page 397, Field et al 1 report the discovery of a fossil bird from 66.7 million years (Myr) ago, during the Late Cretaceous period, the most recent point in time currently known when the first representatives of today's birds evolved. In the process, the authors offer what might be a useful corrective to genetics-based estimates of the timing of bird diversification.…”

“…The transformation of γ-to α-iron that occurs when iron alloyed with carbon is rapidly cooled from a high temperature has long been used to produce hard and strong steels; by contrast, pure iron is soft and ductile. On page 375, Meiners et al 1 report that polymorphic phase transformations can also occur at the interfaces between the tiny crystals that make up most pure metals. This discovery suggests a fresh approach for processing metallic materials to optimize their properties for applications.…”

“…The authors therefore obtained further proof using a machine-learning approach (an evolutionary algorithm) in atomic computer simulations of a grain boundary that had the same geometry as the experimentally observed boundaries. The simulations showed that the pearl phase corresponds to the lowest-energy state of the grain boundary, whereas the domino phase 1 used high-resolution microscopy techniques to investigate grain boundaries in thin copper films deposited on sapphire, and observed two coexisting phases (pictured), which they named domino and pearl phases. The authors also observed these phases in atomic computer simulations (not shown) of a grain boundary that had the same geometry as the ones in the films.…”