According to, Freeze-it will cool down to -60°F/-51°C. That is cold enough to make commonly used ferritic steels, e.g. ASTM A36 or SAE 1018 brittle like glass. It is not enough for e.g. BS 4360 grade 50E, which will just laugh off the hammer blow. To make that brittle like glass, you need to cool it down further, to liquid nitrogen (LM) temperatures. There is, as far as I know, no ferritic steel that can withstand LM and remain ductile. That is why storage vessels for LM are built from austenitic (stainless) steels, e.g. SAE 317. The emphasis here is on the crystal structure (face centered cubic) that is implied by being austenitic, not on the rust-resisting property. Ferritic steel is body centered cubic, and as such inherently susceptible to low-temperature embrittlement. But metallurgists know how to deal with it . . .
As to that, a cutting tool only cuts well if it has a sharp edge, and can penetrate deeply. If you use a diamond disk on that material, yes, it will cut the hard alumina spheres without much trouble . . . if it can present sharp diamond edges to the alumina. But before it can get there, it needs to cut through the soft aluminum . . . and that will fill the gaps between the diamonds, protecting the hard spheres from attack by the sharp diamond edges.
The same holds for that hammer and chisel idea. Again, if it wants to cut, the chisel must penetrate deep into the material. Since the cutting edge is wedge-shaped, the soft aluminum needs to be pushed sideways a considerable amount. But there are the alumina spheres in the way, and they move the chisel's cutting force sideways, thus limiting penetration. And that means you'll have a hard time cutting through . . .
By the way, the whole idea is nothing new. I have seen the original research some 30 years ago . . . This particular paper is just a bit racy - to solicit research funds, I would think, or, more likely, to fill the pages of a engineering magazine. To cut it, I would suggest using a bolt cutter, if in rod form, or a guillotine, if in plate form.

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