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.
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.
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.