A bimetallic strip is made of aluminium and steel $({\mathrm{\alpha }}_{\mathrm{Al}}>{\mathrm{\alpha }}_{\mathrm{steel}})$. On heating, the strip will  

As ${\mathrm{\alpha }}_{\mathrm{Al}}>{\mathrm{\alpha }}_{\mathrm{steel}}$, aluminium will expand more. So, it should have larger radius of curvature. Hence, aluminium will be on convex side and steel on concave side.