In a composite bar, two materials with different coefficients of thermal expansion (CTE) are rigidly joined together, which forces both materials to undergo the same change in length — even though each material naturally "wants" to expand or contract by a different amount.
• On cooling, the part with the higher CTE wants to shrink (contract) more       

   than the part with the lower CTE.
• Since both parts are joined together, the high-CTE part is restrained from     

   shrinking as much as it wants by the low-CTE part.
• This restraint effectively stretches it relative to its natural (free) contracted 

   length, which induces tensile stress in the high-CTE part.
• Conversely, the low-CTE part is pulled inward more than it naturally would

   contract, inducing compressive stress in it.
So, on cooling: the high CTE part experiences tensile stress, and the low CTE part experiences compressive stress.