A sealing element comprises two distinct materials of different melting points. An outer shell material has a higher melting point than an inner core material. The core material can be a relatively low melting point metal, plastic or polymer. The seal is positioned in the wellbore and, in response to heating to a selected temperature, such as a bottom hole static temperature, the core material melts or softens. Upon melting or softening of the inner material, the outer shell's elastic modulus decreases, allowing longitudinal compression and radial expansion of the sealing element. The inner material can also thermally expand and thereby assist or cause radial expansion of the outer shell. The sealing element can provide a metal-to-metal seal with a casing or tubing. The cross-sectional shape of the outer shell can take various shapes to promote controlled compression, expansion and sealing.