Abstract: The structure which supports a rotor-surrounding shroud is composed of a material exhibiting a low coefficient of thermal expansion at low temperatures and a high coefficient of thermal expansion at higher temperatures. In this way, low alpha characteristics persist during periods of low temperature operation to provide large cold clearances and acceptable clearances during transient operation, while high alpha characteristics persist during periods of high temperature operation to provide close clearance control at maximum and part-power, steady-state operating conditions. The two-alpha material also finds use with other relatively rotating parts having close clearance relationship therebetween, such as in the stationary portion of a seal structure.