What's a slip system?

I'm not teaching material science this year (2022-23), but my neighboring chemistry teacher is.

He taught the class a decade or so ago when it was just a semester, and he's never taught it as a year-long course before. 

As such, he's been coming to me quite often for help in understanding the concepts, explaining them, and practicing the labs and demonstrations. It's been kind of rewarding to help him understand things better - and in a few cases it's forced me to understand the material science concepts better so I can explain them to him, something I'm used to in teaching students but that isn't as familiar to me in teaching my fellow teachers in a longer form than our week-long summer camps.

Recently, my neighbor came to me asking about slip planes, so I went looking for a video that would directly and clearly label the slip planes in face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close packed (HCP) crystal structures. I wanted a shortish video that would use computer animation to highlight the slip planes and give me a concise, clear marking of where the slip planes are in each type of crystal.

Much to my chagrin, such a video doesn't seem to exist - at least not that I could find with a few days of searching on YouTube.

What I did find, however, was a much deeper understanding of what slip planes and - possibly more appropriately - slip systems are. 

In this first video, I found a very helpful quote as written in the notes...

"Slip plane will be most densely packed highest planar density"

So the slip plane is the plane with the highest packing. For FCC that's the diagonal labeled as (1,1,1) because it connects the vertices of the crystal structure and is the 'triangular' plane that we show with our tennis balls in class. For HCP it's any of the 'horizontal' layers that we show with our tennis balls. For BCC, it's the diagonal from edge to edge in the crystal - but it's not very closely packed, making it a crappy slip plane.

"Slip direction is the one most closely packed with atoms (linear density)"

That means FCC structures have three slip directions for every one of their four slip planes. That give them twelve (4x3) slip systems.

But HCP crystals have only three slip systems - one slip plane times three slip directions.

This is also covered in this video...

...and somewhat in this wikipedia article about slips which includes this illuminating quote...

...however, unlike fcc, there are no truly close-packed planes in the bcc crystal structure. Thus, a slip system in bcc requires heat to activate.

I still want a nice, succinct video that shows all that, but I'll take this combination that has helped me understand slip planes way better than I did before my neighbor asked me to explain them to him.