The Effect of Carbide Morphologies on Elevated Temperature Tensile and Fatigue Behavior of a Modified Single Crystal Ni-Base Superalloy

Abstract

Minor additions of various grain boundary strengthening elements to single crystal Ni-base superalloys have been shown to improve castability, but their effects on mechanical properties are not well understood. A comparative investigation of baseline CMSX-4 and variations modified with carbon, carbon and boron, and carbon and nitrogen has been performed. These modifications have been shown to form interdendritic MC-type carbides with different morphologies. Heat treatments resulted in the breakdown of some carbide structures into small, spherical carbide particles of similar composition to the as-cast features. Tensile and high cycle fatigue (HCF) testing was conducted at 850 o C. The three carbon-containing modifications showed lower tensile strength than the baseline due to cracking of carbides and greater porosity. Fatigue crack initiations for Un-HIPed specimens occurred at pores that were adjacent to carbides in the modifications, which in general had shorter lifetimes than the baseline. HIPing dramatically increased fatigue lifetimes for all variations. Removal of porosity resulted in crack initiations at carbides and highlighted differences in fatigue behavior for varying carbide morphologies. Script carbide networks observed in the carbon only modification exhibited significant cracking that initiated failure, limiting the fatigue lifetime. Blocky carbides in the carbon and nitrogen modification remained intact, which resulted in longer fatigue lifetimes. The intact carbides did experience de-cohesion from the γ/γ' matrix that showed signs of plastic deformation in the vicinity of carbides.

A critical assessment of fatigue crack nucleation and growth models for Ni- and Ni,Fe-based superalloys

“…However, pores can also form in the vicinity of the carbides during solidification, and the fatigue crack nucleation life then decreases. This has been found to occur in single crystal CMSX-4 12 and RR2072. 7 A detailed analysis of pore size and location was conducted in the RR2072 study, which was correlated to the prominence of surface and subsurface fatigue crack nucleation in high or low cycle fatigue.…”

“…In general, internal pores in interdendritic regions are the source of fatigue crack nucleation. [7][8][9][10][11][12] Temperature, alloying and processing can all influence the amount of porosity. For example, alloying elements traditionally used in polycrystalline alloys to increase grain boundary strength have been investigated in single crystal alloys for their impact on castability.…”

“…The hipped versions of the modified CMSX-4 alloys exhibited fatigue lives almost an order of magnitude larger than the material that was not HIPed. 12 After hot isostatically pressing, MC carbides become the location for crack nucleation, where M is generally tantalum, and carbide morphology becomes a contributing factor (Fig. 3 a ).…”

Low cycle fatigue properties and microstructure evolution at 760°C of a single crystal superalloy

“…The crack initiation was located in the microporosity or oxide layer, as shown in Fig.5(e) and (f) (indicated by the short arrow). Such experimental result was demonstrated for the CMSX-4 alloy [20]. Once initiated, the crack would spread perpendicularly to the stress direction at first and then along different planes.…”

Additive Manufacturing of Nickel‐Base Superalloy René N5 through Scanning Laser Epitaxy (SLE) − Material Processing, Microstructures, and Microhardness Properties

“…1 . [30] Cores of the carbide networks were observed to be between the primary dendrite arms. The carbides analyzed were found to be rich in Ta, Hf, and W. The EDS elemental maps for various alloying elements are shown in Figure 7c.…”