Can a Prince Rupert's drop be made of metal instead of glass?
This is rendered a bit of a moot point as most metals already have the the sort of hardness and impact resistance that heat treating imparts on glass anyway so you are essentially approaching the problem from the opposite direction.
However it is certainly true that heat treating can create residual stresses in susceptible metals in a similar fashion, stainless steel with its high coefficient of thermal expansion and low thermal conductivity is particularly susceptible.
Also steels with sufficient carbon content can be dfferentially hardened by quenching so if you quench a thick enough section of high carbon steel you will get a hard surface layer with a softer core. Also the hard section will increase in volume, creating residual stress throughout the whole sample.
There are also various processes for surface hardening of steels, such as case hardening which works by diffusing carbon into the surface of low carbon steels.
The closest real world application is in ball bearings which tend to have hard surfaces and soft cores to combine surface wear resistance with toughness. This isn't exactly the same as a Prince Rupert's drop as you have a change of micro-structure going on.
There is also the complication that molten high carbon steel tends to start to burn in air so making an actual droplet might require an inert atmosphere, equally molten steel is generally a lot less viscous than glass so forming the 'tail' wouldn't work in quite the same way. Having said that you could achieve a similar effect by quenching from solid.
Fully hardened (ie quenched but not tempered) high carbon steels aren't that far away from the brittleness of glass. Indeed it is not unknown for quenched parts to spontaneously crack between quenching and hardening.
Diving deep, given the correct material/component, I was think if it is possible to use a strong external fast changing EM field (electro-magnetic) to control the shape of melting metal and keep it suspended/floating in space, and cool down using water or a high speed jet of liquid air.
this will result in a material as hard as rupert's drop with no tail (no achilles heel). this can be used for military and space exploriation. I was wondering if a similar process has been used (secretly) in military industry already.
----- pioneering towards the Prince Rupert's drop with short or no tail, in glass, and in metail.