The effects of thermal processing on the microstructure and properties of selectively laser melted CM247LC nickel-base superalloy

Boswell, John Harold (2018). The effects of thermal processing on the microstructure and properties of selectively laser melted CM247LC nickel-base superalloy. University of Birmingham. M.Sc.

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Abstract

There are many challenges associated with the laser powder bed (LPBF) manufacture of high γ' content superalloys such as CM247LC, including anisotropic material properties, micro-cracking during build and low creep ductility at elevated temperatures.

This thesis investigates the effects of heat treatment on high temperature creep and tensile properties of CM247LC at 950°C.

Cooling rates between 30°C/min and 45°C/min, resulted in serrated grain boundaries lined with γ' and carbides. Solution heat treatment experiments showed that grain growth and recrystallisation plateaued after 1 hour at a temperature of 1265°C.

Growth of micro-cracks during HIP can initially be attributed to ductility drop cracking between 450°C and 700°C, but above this temperature strain age cracking SAC is prevalent.

Creep strain tests at 950°C showed that the best mechanical properties in terms of creep lives and tensile ductility were obtained by heat treatment using a 2 -stage HIP process, 1230°C, 2 hours - 1270°C, 2 hours, fast cool from HIP followed by a single stage age at 871°C for 16 hours.

The principal recommendation from this work is that to improve tensile ductility and creep life of LPBF CM247LC fast cool HIP should replace conventional solution heat treatment followed by gas fan quench (GFQ).

Type of Work:

Thesis (Masters by Research > M.Sc.)

Award Type:

Masters by Research > M.Sc.

Supervisor(s):