A turbine wheel (4) having a hub (29); a multiplicity of turbine wheel blades (30) arranged around the hub (29); a turbine wheel back wall (31) which is arranged on the hub (29) adjacent to an edge region (32) of the turbine wheel blades (30); wherein the wall thickness (W) of the turbine wheel (4) back wall is reduced in regions.

A turbine nozzle for an air cycle machine includes a disk having a central axis and a plurality of airfoils extending from a surface of the disk. The plurality of airfoils is arranged radially about the disk section to direct a flow of a fluid passing thereby. A relief cut adjacent the airfoils, wherein the relief cut provides a channel in a least a portion of the surface of the disk.