An axially-split radial turbine comprising: a forward rotor section, wherein the forward rotor section comprises a forward blade segment and a forward hub section metallurgically bonded together along a first bond line; an aft rotor section wherein the aft rotor section comprises an aft blade segment and an aft hub section metallurgically bonded together along a second bond line, wherein the forward rotor section and the aft rotor section are mechanically and abuttingly coupled to one another, at both the respective blade segments and hub sections, along an annular interface that resides within a plane generally orthogonal to a rotational axis of the axially-split radial turbine; an inner annular cavity (68, 70) fully defined within the axially-split radial turbine, wherein the forward blade segment and the aft blade segment each comprise a cooling air flow passage (65, 67), wherein the cooling air flow passage of the forward blade segment and the cooling air flow passage of the aft blade segment are fluidly connected to the inner annular cavity; a hub cooling air passage between the forward hub section and the aft hub section and fluidly connected to the inner annular cavity, wherein the forward hub section defines a first annular cavity groove and wherein the aft hub section defines a second annular cavity groove, and wherein when the forward rotor section and the aft rotor section are coupled to one another, the first and second annular grooves define the inner annular cavity. a forward rotor section, wherein the forward rotor section comprises a forward blade segment and a forward hub section metallurgically bonded together along a first bond line; an aft rotor section wherein the aft rotor section comprises an aft blade segment and an aft hub section metallurgically bonded together along a second bond line, wherein the forward rotor section and the aft rotor section are mechanically and abuttingly coupled to one another, at both the respective blade segments and hub sections, along an annular interface that resides within a plane generally orthogonal to a rotational axis of the axially-split radial turbine; an inner annular cavity (68, 70) fully defined within the axially-split radial turbine, wherein the forward blade segment and the aft blade segment each comprise a cooling air flow passage (65, 67), wherein the cooling air flow passage of the forward blade segment and the cooling air flow passage of the aft blade segment are fluidly connected to the inner annular cavity; wherein the forward blade segment and the aft blade segment each comprise a cooling air flow passage (65, 67), wherein the cooling air flow passage of the forward blade segment and the cooling air flow passage of the aft blade segment are fluidly connected to the inner annular cavity; a hub cooling air passage between the forward hub section and the aft hub section and fluidly connected to the inner annular cavity, wherein the forward hub section defines a first annular cavity groove and wherein the aft hub section defines a second annular cavity groove, and wherein when the forward rotor section and the aft rotor section are coupled to one another, the first and second annular grooves define the inner annular cavity.