A swellable metallic material is activated upon functional movement of a tubular member to lock the tubular member in place. In one example, a well tool includes a tubular member moveable within a through bore of a tool housing from a first position to a second position to perform a tool function. A swellable metallic material is captured in a cavity. The tubular member blocks flow to the cavity in the first position and opens the cavity for exposure of the swellable metallic material to an activation fluid in the second position. The swellable metallic material is configured to swell into engagement with the tubular member in response to the exposure to the activation fluid to hold the tubular member in the second position.