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I was learning about the charpy impact test and came across the graph on this webiste that shows the impact energy for bcc and fcc structures as a function of temperature. I tried searching for an explanation as to why the graph differs for the two crystal systems but couldn't find any. Some places say it's because of the slip planes but I still didn't understand.

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    $\begingroup$ The question should be "why do some BCC steels show a brittle transition and others do not ?" . Many BCC steels are very tough ; Had one nearly stop the Charpy hammer ( it is one of those things where people in the room look around and say "what just happened ?". $\endgroup$ Oct 25 '20 at 0:36
  • $\begingroup$ @blacksmith37 Oh Okay thanks can you suggest an edit? $\endgroup$ Oct 25 '20 at 5:48
  • $\begingroup$ There is a library full of information . Mostly starting with WW 2 Liberty ship brittle fractures. But some going back to the Boston molasses tank failure of about 1900. The Naval Research Lab started the serious research because of the liberty ships ( Authors : Pellini, Puzak , Goode , Judy, etc ). Now generally under the heading of Linear Elastic Fracture Mechanics and Fracture Toughness. $\endgroup$ Oct 25 '20 at 14:51
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It essentially depends on the fracture mechanisms available to the material at the temperature in question. In general, dislocations are more mobile at higher temperatures, enabling plastic deformation and ductile fracture.

FCC materials have more slip systems, or ways for dislocations to move, than BCC materials. A metal needs five independent slip systems to plastically deform. As you lower the temperature, certain slip systems will be “frozen out”, meaning that it is essentially impossible for dislocations to move according to that slip system. Because BCC metals have fewer slip systems to start with than FCC metals, they become brittle at higher temperatures than FCC metals.

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No one has found the mechanism after over 100 years of investigation; Admittedly I have not followed literature for about 20 years. Some steels have a low transition temperature = good toughness ; some steels have a high transition temperature = poor toughness. There is a library full of techniques to improve toughness , but no basic understanding of the mechanism causing poor toughness in some BCC steels.

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