Título

Solidification Structure and Phase Transformation During the Heat Treatments of New Ni-Base Superalloys Containing Niobium for High Temperature Tribological Applications

Autoria

FRANCO, H. BERNS, A. FISCHER, S. D.

FRANCO, H. BERNS, A. FISCHER, S. D.

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DOI

10.5151/2594-5327-49v2-361-378

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Resumo

The solidification and microstructure of precipitation hardening NiCr20AlSi superalloys with a Niobium content of ≈ 25% were studied using scanning- and transmission electron microscopy, differential thermal analysis, unidirectional solidification and X-ray diffraction. The carbides identified are: primary NbC, eutectic NbC and M₆C. The former attain considerable size (approximately 30 μm), which may considerably enhance the wear resistance, depending on tribological application. During the heat treatment to precipitate Ni₃(Al,Ti) (γ′) the formation of a narrow diffusion zone producing stable carbides was observed. Also included are the effects of Molybdenum on the transformation and microstructure upon heat treatment, where an intermetallic phase may appear. As part of a program studying hot abrasive wear mechanism of superalloys the hot hardness of the matrix and the primary NbC were measured, using a newly developed equipment. In this device the sample as well as the diamond indenter are heated to evaluate the mechanical behaviour. It was observed, that the matrix has an excellent hardness which is nearly constant up to 700 °C.

Palavras-chave

Ni-base superalloys, solidification, NbC carbides, phase transformation, heat treatment, tribological applications, hot hardness, precipitation hardening, scanning electron microscopy, X-ray diffraction