Steel grain boundary misorientation close to 60 degrees: is that necessarily a twin boundary? Can it be just a high-angle grain boundary?
In fcc crystals (such as austenitic steels), a coherent twin boundary is defined as a 60 deg misorientation with a {111} boundary plane. By definition, that is a high-angle (60 deg) grain boundary (gb). However it is a low energy gb, as are a lot of low-angle gb.
You can certainly have 60 deg misorientation gb that are not coherent twin boundaries if the gb plane is not {111}, and they would likely be higher energy. In real austenitic steels, a {112} plane is often encountered. They are usually called incoherent twin boundaries.
Last point: when you are saying 60 deg misorientation for twin boundaries, that usually means 60 deg tilt around a direction. If you tilt around a axis, the angle to form the same boundary is 70.53. (see https://www.tf.uni-kiel.de/matwis/amat/def_en/kap_7/backbone/r7_1_1.html)
Gabriel, if I understood correctly, in all cases of 60 deg GB misorientation these are the twin boundaries and the main difference is whether they are coherent or not?
No, a 60 deg GB mis-orientation is not necessarily a twin boundary. A 60 deg (or 180 deg) GB misorientation around a axis is a twin boundary, and the gb plane determines whether it is coherent or not.
From Christian and Mahajan, "The classical definition of twinning requires that the twin and parent (or matrix) lattices are related either by a reflection in some plane or by a rotation of 180° about some axis."
Gabriel Meric de Bellefon : You mentioned "In real austenitic steels, a {112} plane is often encountered. They are usually called incoherent twin boundaries. "
Why is a grain boundary with plane boundary of {112} called incoherent?
In other words, what is the definition of incoherency in grain boundaries?
Incoherent means that there is an atomic mismatch at the boundary.
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