Are FCC or BCC Crystal Structures Stronger?

[jojoba]

"Which metals tend to be stronger, those that form FCC or those that form BCC crystals? "
This is the solution provided which doesn't make much sense to me: "You should look at APF and coordination number of each crystal system to base your answer"
Is this even a valid question? Doesn't the answer depend on which direction the material is going to be subjected to a force since "strongness" is a directional property? I thought we should choose a particular direction for both systems and look at planar and linear densities. Hence, the slip phenomenon in the crystal structure with a greater planar density in the "chosen direction" will progress with more ease which suggests that the the material is "stronger" if subjected to a force in the chosen direction.
I would appreciate it if someone could point me to the flaw in my reasoning.

 

[darkelf]

It is a valid question. FCC metals tend to have more slip systems, i.e. slip directions and slip planes than BCC. Yes the APF is important, the atomic packing factor, that is the reason FCC has more slip systems, because of the way the atoms are arranged in the crystal.
Thus FCC metals deform easier than BCC metals and thus they are more ductile. BCC metals are infact stronger than FCC metals. HCP metals are the most brittle.
Calister or Smith explains this if you can get your hands on either of the books.

 

[oswaler]

I understand your confusion with the provided solution. The strength of a metal depends on various factors, including its crystal structure, dislocation density, and alloy composition. Therefore, it is not a straightforward answer to say that one crystal system is inherently stronger than the other.

To answer this question, we need to consider the specific properties of the metals in question. For example, FCC metals tend to be more ductile and have higher deformability due to their closely packed atomic arrangement. On the other hand, BCC metals have a lower ductility and higher strength due to their less closely packed atomic arrangement.

However, as you mentioned, the strength of a metal also depends on the direction in which the force is applied. This is because the atomic arrangement in a crystal structure is not uniform in all directions. For example, in BCC metals, the slip planes and directions are different in different crystallographic orientations, which can affect the material's strength. Therefore, it is essential to consider the direction of force application when comparing the strength of metals with different crystal structures.

In summary, the strength of a metal depends on various factors, including its crystal structure and the direction of force application. Hence, it is not a valid question to ask which crystal structure is inherently stronger, and the answer would depend on the specific properties and conditions of the metals in question.