It can be said that the pendulum clock is made up of invar because invar's coefficient of thermal expansion is small. So, the length of the pendulum remains unaffected during changes in temperature. Invar is derived from the word invariable. It is an alloy which is made from nickel and iron. It has a low coefficient of thermal expansion. So, there will not be linear expansion of the pendulum due to the rise in temperature. And the time shown in the clock will be accurate. Thus, invar is used in the pendulum of the clock. Invar, also known as Nickel Alloy 36, is a nickel-iron alloy noted for its uniquely low coefficient of thermal expansion. 

A small percentage of boron is added to steel in order to Increase hardenability.