Stretch piece of metal within its elastic limit and it cools down. Does it cool down for the same reason that a gas cools down when expanded? But if the piece of metal is repeatedly stretched and released (like with a spring), I am led to believe that it heats up, although I am not sure. I know that the process of cycling the metal within its elastic limit is not 100% efficient, and this is due to hysteresis. So whether repeatedly stretched or compressed, I would expect the metal will heat up.