Microstructure, mechanical properties and cracking behaviour in a γ′-precipitation strengthened nickel-base superalloy fabricated by electron beam melting

The influences of EBM processing and post-processing on microstructure, mechanical properties and cracking behaviour in a γ′ precipitation-strengthened nickel-base superalloy DZ125 are critically assessed. Results show that a solution treating and ageing (STA) is required to obtain γ′ precipitates with a cuboidal shape. The columnar grain width was found to gradually increase from the bottom to the top of the as-EBM sample having a total build height of 80 mm, whereas there was little gradient in γ′ size. The presence of EBM induced intergranular cracks can be closed by hot isostatic pressing (HIP), however reappearance of intergranular cracks with a much wider crack opening width was observed after STA treatment. EBM induced cracks are classified as liquid-state cracking, as the classic dendritic morphology were found on the cracked surfaces. The post-processing induced cracks can be attributed to the effect of γ′ dissolution and re-precipitation of fine γ′ during the cooling stage of solution treatment. The results suggest that HIP is not an effective approach in healing liquid-state cracks in EBM fabricated γ′ precipitation-strengthened superalloys.