High temperature low cycle fatigue behaviour of hot isostatically pressed superalloy Udimet 720 LI

Abstract

Nickel base superalloy Udimet 720 LI was processed through a powder metallurgy (PyM) hot isostatic pressing (HIP) route. The effects of this consolidation technique (HIPing) on low cycle fatigue (LCF) properties were evaluated and are reported in this paper. Total strain controlled LCF tests have been conducted in air at 550 C at a nominal strain rate of 6.67610 À 3 s À 1 . The as-HIPed Udimet 720 Li exhibits stable behaviour followed by very mild cyclic hardening at total strain amplitudes + 0.4 -0.5%. However, cyclic hardening has been observed at total strain amplitudes + 0.6 -1.2%. The cyclic hardening is attributed to dynamic strain aging (DSA). The dual slope behaviour in a Coffin-Manson plot is attributed to a change in fracture mechanism.

Influence of HIP processing on microstructure and mechanical properties of superalloy Udimet 720LI

“…The prealloyed powder being the starting material, it was characterised for various important properties, and the results have been reported elsewhere. 11 The powder was subjected to hipping under optimised conditions, and the resulting material with near theoretical density was characterised for microstructure, tensile and SR properties. In order to improve the mechanical properties to the desired level, the as hipped alloy was heat treated.…”

“…In order to substantiate the density measurement, the as polished cross-section of the hipped compact was examined optically and was found to be free from porosity and significant inclusions. 11 The density value and the absence of microporosity in the as hipped material suggest that hipping at 1200uC under a pressure of 120 MPa for 3 h is sufficient for consolidation of superalloy U720LI powder of average particle size ,70 mm to its near theoretical density. Hot isostatic press processing above the c9 solvus results in coarse grain size than subsolvus processing.…”

“…On the other hand, the dark contrast particles with a varied size present at the grain boundaries as well as in the matrix have been identified as the primary and secondary c9-Ni 3 (Ti, Al) precipitates respectively. 11 It is evident from the micrograph that the primary c9 has an elliptical morphology, while the secondary c9 is cuboidal. Precipitation of c9 phase occurs in Ni based superalloys during slow cooling stage of HIP.…”

“…The SEM examination has also revealed the fine dendrites and cellular grain structure of particles. 11 These microstructural features of the powder are directly related to the cooling kinetics encountered by the molten alloy droplets during atomisation. 15 The powder had wide particle size distribution, as shown in Fig.…”

“…The microstructure and tensile properties of the as hipped material were studied first. 11 In order to improve the mechanical properties, the as hipped material was heat treated and evaluated for microstructure, tensile and stress rupture (SR) properties. The structure and properties of as hipped and HIPzheat treated alloy in comparison with those of the conventionally processed wrought material are presented in the present paper.…”