an all metal reusable o - ring sealing arrangement for sealing two concentric tubes in an ultra - high vacuum system . an o - ring of a heat recoverable alloy such as nitinol is concentrically positioned between protruding sealing rings of the concentric tubes . the o - ring is installed between the tubes while in a stressed martensitic state and is made to undergo a thermally induced transformation to an austenitic state . during the transformation the o - ring expands outwardly and contracts inwardly toward a previously sized austenitic configuration , thereby sealing against the protruding sealing rings of the concentric tubes .