Strips of two metals A and B are firmly joined together as shown in the figure(Before Heating) On heating, A expands more than B does. If this joined strip is heated, then it will appear as

When two different metal strips are rigidly bonded, unequal thermal expansion forces the composite to bend so the differing length changes can be accommodated; this is the operating principle of a bimetallic strip [1]. If metal A expands more than metal B on heating, A must increase its length more. Because the two are fixed together, the composite curves with the more-expandable metal (A) on the outer, convex side and the less-expandable metal (B) on the inner, concave side; thus the strip bends toward B. This behaviour is illustrated by common brass–steel bimetal examples where the higher-expansion metal lies on the outer arc when heated [2].

Sources

1. [1] https://www.sciencedirect.com/topics/engineering/bimetallic-thermometer
2. [2] https://lecdem.physics.umd.edu/i/i1/i1-13.html