Turbinenschaufeln

The material

CMSX-10 is not cast fine-grained, and not directionally solidified. It is grown as one crystal, because grain boundaries are where creep failure begins at the temperatures a turbine blade runs at — so the alloy is made without any.

It carries around 6 % rhenium, twice that of the second-generation alloys it succeeded, and holds the highest creep strength of any production cast nickel single-crystal alloy: roughly 30 °C better than CMSX-4, with fatigue strength two to three times higher. Rhenium is among the rarest metals in industrial use. The blank arrives worth more than most finished parts.

Why machining it is a different problem

On ordinary metals, a machining error shows up immediately as a dimension out of tolerance. Here it can show up years later, in service.

Cutting leaves plastic strain in the surface layer. In a single-crystal alloy that stored strain is a seed: at operating temperature the damaged layer can recrystallize — grow new grains, with boundaries between them — inside a part whose entire purpose was to have none. The blade passes inspection, holds every dimension, and has quietly lost the property it was made for.

So the constraint is not only where the tool goes, but how hard it pushes to get there. Cutting forces have to stay low enough to leave the surface layer undeformed, which rules out the aggressive strategies that make hard materials economic everywhere else.

On top of that the alloy behaves like every nickel superalloy under a tool, only more so: it work-hardens as it is cut, it puts heat into the cutting edge rather than into the chip, and its gamma-prime precipitates and heavy refractory content are abrasive.

Data sheet

Material

CMSX-10 — third-generation single-crystal nickel superalloy

Rhenium content

~6 % by weight

Grain boundaries

none; cast as a single crystal

Blade type

rotating

Application

combined heat and power plant turbine