Grain size measurements, particularly in austenitic grains, usually ignore twin boundaries even though twin boundaries contribute to strengthening and have clear structure-property correlations. Isn't this likely to affect calculations made by, say, the Hall-Petch relation? Is there a fundamental reason for ignoring these boundaries?
This depends on the question you are interested in. Nobody is interested in the grain size if there is no correlation to a materials property. And if you are interested in this property you have to ask yourself whether this property is perhaps independent on specific grain boundaries. Electrical properties are possibly sensitive to high-angle boundaries if there are impurities, or they are more resistent. For twin boundaries the match is perfect so that it makes no sense to consider them. This might be similar for several mechanical properties like dislocation movement. Therefore, you need to ask yourself, WHY do you want to know the grain size... If there is no meaning you can skip it. Not everything that is possible is automatically meaningful.
Thank you for your answer Dr. Nolze! I appreciate the logical argument that the method of grain size determination should be application-specific.
Nevertheless, grain size determination by itself is a well established and standardised process, and the ASTM standard itself requires that grain size should exclude twins (a reference to this given below). This leads me to believe that there must be a fundamental reason for neglecting twin (besides specific property-related applications that you suggested).
Your point about electrical properties is well taken, but it was the statement about mechanical properties that is really interesting to me. I wonder if you could tell me a little more about such properties? Thanks in advance.
Reference: https://books.google.co.in/books?id=d2R5dvk9vX4C&pg=PA243&lpg=PA243&dq=artificial+intelligence+for+twin+identification&source=bl&ots=oqP3lA1iIB&sig=19rJ4Z8Xgmq6Cc1YoY5psEjElDU&hl=en&sa=X&ved=0ahUKEwiX_cGb8s7VAhVJMo8KHf5ABloQ6AEIJTAA#v=onepage&q=artificial%20intelligence%20for%20twin%20identification&f=false
Standards are introduced in oder to define a special procedure when it makes sense and shall be applied. There is no direct relationship to a property but only to a measuerment procedure. You cannot apply it simply because it exists (or you can ignore the effect only because a standard does not exist ). You need to think first WHY do you exclude a very special grain boundary. If it does not make any sense to you this ASTM is useless (for your application).
As written in the reference: ...Once twins have been identified, it is POSSIBLE to perform the analysis for structure-property correlations.
Dear Dr. Nolze,
Thank you once again. I agree that the inclusion or exclusion of twin boundaries should depend on the application. My question is not specific to one application, but to the convention.
Perhaps a better question would be : in what cases can(should) twin boundaries be ignored while measuring grain size?The electrical properties example pointed out by you is one case. I am trying to find out mechanical properties where the effect of twins can be neglected.
I am unable to find major properties which fall under this category. Even so, it is very common to find in literature that twins are neglected in grain size measurements, without any explicitly stated reason for this. A few references from cold working-annealing [1,2] and hot working[3] on different material systems are given at the end of the post.
Also, while it is mentioned -even suggested- that twins should not be ignored for structure-property correlations, we see the statement: 'The best correlations are obtained when both types of boundaries are counted. Such a measurement, however, should not be referred to as grain size.'
(Vander Voort,G.F., 'Grain Size Measurement', Practical Applications of Quantitative Metallography, ASTM STP 839 pp. 85-131)
I'm curious to know why this was added -I suppose there would be some common, grain size-related property which prompted it.
Refs:
[1]http://www.sciencedirect.com/science/article/pii/S0921509311007143
[2] https://pure.strath.ac.uk/portal/files/62000247/Konkova_etal_JAC2016_Grain_growth_during_annealing_of_cryogenically_rolled_Cu_307n_brass.pdf
[3] http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2818.2007.01814.x/pdf
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