Invaders from Rigel — Fletcher Pratt

Astronomers had observed the comet for some time, and had predicted its course. It would collide with Earth. There was no hysteria; scientists were apparently convinced that the results wouldn't be dangerous to life.

But Murray Lee woke up with a feeling of overpowering stiffness in every muscle. He turned over in bed and felt his left elbow, which seemed to be aching particularly -- and received the shock of his life. The motion was attended by a creaking clang, and his elbow felt like a complex wheel.

Why -- he was metal all over!